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   This manual (19 November 2008) is for GNU Bison (version 2.4.1), the
GNU parser generator.

   Copyright (C) 1988, 1989, 1990, 1991, 1992, 1993, 1995, 1998, 1999,
2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008 Free Software
Foundation, Inc.

     Permission is granted to copy, distribute and/or modify this
     document under the terms of the GNU Free Documentation License,
     Version 1.2 or any later version published by the Free Software
     Foundation; with no Invariant Sections, with the Front-Cover texts
     being "A GNU Manual," and with the Back-Cover Texts as in (a)
     below.  A copy of the license is included in the section entitled
     "GNU Free Documentation License."

     (a) The FSF's Back-Cover Text is: "You have the freedom to copy and
     modify this GNU manual.  Buying copies from the FSF supports it in
     developing GNU and promoting software freedom."

INFO-DIR-SECTION Software development
START-INFO-DIR-ENTRY
* bison: (bison).       GNU parser generator (Yacc replacement).
END-INFO-DIR-ENTRY


File: bison.info,  Node: Top,  Next: Introduction,  Up: (dir)

Bison
*****

This manual (19 November 2008) is for GNU Bison (version 2.4.1), the
GNU parser generator.

   Copyright (C) 1988, 1989, 1990, 1991, 1992, 1993, 1995, 1998, 1999,
2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008 Free Software
Foundation, Inc.

     Permission is granted to copy, distribute and/or modify this
     document under the terms of the GNU Free Documentation License,
     Version 1.2 or any later version published by the Free Software
     Foundation; with no Invariant Sections, with the Front-Cover texts
     being "A GNU Manual," and with the Back-Cover Texts as in (a)
     below.  A copy of the license is included in the section entitled
     "GNU Free Documentation License."

     (a) The FSF's Back-Cover Text is: "You have the freedom to copy and
     modify this GNU manual.  Buying copies from the FSF supports it in
     developing GNU and promoting software freedom."

* Menu:

* Introduction::
* Conditions::
* Copying::             The GNU General Public License says
                          how you can copy and share Bison.

Tutorial sections:
* Concepts::            Basic concepts for understanding Bison.
* Examples::            Three simple explained examples of using Bison.

Reference sections:
* Grammar File::        Writing Bison declarations and rules.
* Interface::           C-language interface to the parser function `yyparse'.
* Algorithm::           How the Bison parser works at run-time.
* Error Recovery::      Writing rules for error recovery.
* Context Dependency::  What to do if your language syntax is too
                          messy for Bison to handle straightforwardly.
* Debugging::           Understanding or debugging Bison parsers.
* Invocation::          How to run Bison (to produce the parser source file).
* Other Languages::     Creating C++ and Java parsers.
* FAQ::                 Frequently Asked Questions
* Table of Symbols::    All the keywords of the Bison language are explained.
* Glossary::            Basic concepts are explained.
* Copying This Manual:: License for copying this manual.
* Index::               Cross-references to the text.

 --- The Detailed Node Listing ---

The Concepts of Bison

* Language and Grammar:: Languages and context-free grammars,
                           as mathematical ideas.
* Grammar in Bison::     How we represent grammars for Bison's sake.
* Semantic Values::      Each token or syntactic grouping can have
                           a semantic value (the value of an integer,
                           the name of an identifier, etc.).
* Semantic Actions::     Each rule can have an action containing C code.
* GLR Parsers::          Writing parsers for general context-free languages.
* Locations Overview::   Tracking Locations.
* Bison Parser::         What are Bison's input and output,
                           how is the output used?
* Stages::               Stages in writing and running Bison grammars.
* Grammar Layout::       Overall structure of a Bison grammar file.

Writing GLR Parsers

* Simple GLR Parsers::     Using GLR parsers on unambiguous grammars.
* Merging GLR Parses::     Using GLR parsers to resolve ambiguities.
* GLR Semantic Actions::   Deferred semantic actions have special concerns.
* Compiler Requirements::  GLR parsers require a modern C compiler.

Examples

* RPN Calc::               Reverse polish notation calculator;
                             a first example with no operator precedence.
* Infix Calc::             Infix (algebraic) notation calculator.
                             Operator precedence is introduced.
* Simple Error Recovery::  Continuing after syntax errors.
* Location Tracking Calc:: Demonstrating the use of @N and @$.
* Multi-function Calc::    Calculator with memory and trig functions.
                             It uses multiple data-types for semantic values.
* Exercises::              Ideas for improving the multi-function calculator.

Reverse Polish Notation Calculator

* Rpcalc Declarations::    Prologue (declarations) for rpcalc.
* Rpcalc Rules::           Grammar Rules for rpcalc, with explanation.
* Rpcalc Lexer::           The lexical analyzer.
* Rpcalc Main::            The controlling function.
* Rpcalc Error::           The error reporting function.
* Rpcalc Generate::        Running Bison on the grammar file.
* Rpcalc Compile::         Run the C compiler on the output code.

Grammar Rules for `rpcalc'

* Rpcalc Input::
* Rpcalc Line::
* Rpcalc Expr::

Location Tracking Calculator: `ltcalc'

* Ltcalc Declarations::    Bison and C declarations for ltcalc.
* Ltcalc Rules::           Grammar rules for ltcalc, with explanations.
* Ltcalc Lexer::           The lexical analyzer.

Multi-Function Calculator: `mfcalc'

* Mfcalc Declarations::    Bison declarations for multi-function calculator.
* Mfcalc Rules::           Grammar rules for the calculator.
* Mfcalc Symbol Table::    Symbol table management subroutines.

Bison Grammar Files

* Grammar Outline::   Overall layout of the grammar file.
* Symbols::           Terminal and nonterminal symbols.
* Rules::             How to write grammar rules.
* Recursion::         Writing recursive rules.
* Semantics::         Semantic values and actions.
* Locations::         Locations and actions.
* Declarations::      All kinds of Bison declarations are described here.
* Multiple Parsers::  Putting more than one Bison parser in one program.

Outline of a Bison Grammar

* Prologue::              Syntax and usage of the prologue.
* Prologue Alternatives:: Syntax and usage of alternatives to the prologue.
* Bison Declarations::    Syntax and usage of the Bison declarations section.
* Grammar Rules::         Syntax and usage of the grammar rules section.
* Epilogue::              Syntax and usage of the epilogue.

Defining Language Semantics

* Value Type::        Specifying one data type for all semantic values.
* Multiple Types::    Specifying several alternative data types.
* Actions::           An action is the semantic definition of a grammar rule.
* Action Types::      Specifying data types for actions to operate on.
* Mid-Rule Actions::  Most actions go at the end of a rule.
                      This says when, why and how to use the exceptional
                        action in the middle of a rule.

Tracking Locations

* Location Type::               Specifying a data type for locations.
* Actions and Locations::       Using locations in actions.
* Location Default Action::     Defining a general way to compute locations.

Bison Declarations

* Require Decl::      Requiring a Bison version.
* Token Decl::        Declaring terminal symbols.
* Precedence Decl::   Declaring terminals with precedence and associativity.
* Union Decl::        Declaring the set of all semantic value types.
* Type Decl::         Declaring the choice of type for a nonterminal symbol.
* Initial Action Decl::  Code run before parsing starts.
* Destructor Decl::   Declaring how symbols are freed.
* Expect Decl::       Suppressing warnings about parsing conflicts.
* Start Decl::        Specifying the start symbol.
* Pure Decl::         Requesting a reentrant parser.
* Push Decl::         Requesting a push parser.
* Decl Summary::      Table of all Bison declarations.

Parser C-Language Interface

* Parser Function::         How to call `yyparse' and what it returns.
* Push Parser Function::    How to call `yypush_parse' and what it returns.
* Pull Parser Function::    How to call `yypull_parse' and what it returns.
* Parser Create Function::  How to call `yypstate_new' and what it returns.
* Parser Delete Function::  How to call `yypstate_delete' and what it returns.
* Lexical::                 You must supply a function `yylex'
                              which reads tokens.
* Error Reporting::         You must supply a function `yyerror'.
* Action Features::         Special features for use in actions.
* Internationalization::    How to let the parser speak in the user's
                              native language.

The Lexical Analyzer Function `yylex'

* Calling Convention::  How `yyparse' calls `yylex'.
* Token Values::        How `yylex' must return the semantic value
                          of the token it has read.
* Token Locations::     How `yylex' must return the text location
                          (line number, etc.) of the token, if the
                          actions want that.
* Pure Calling::        How the calling convention differs in a pure parser
                          (*note A Pure (Reentrant) Parser: Pure Decl.).

The Bison Parser Algorithm

* Lookahead::         Parser looks one token ahead when deciding what to do.
* Shift/Reduce::      Conflicts: when either shifting or reduction is valid.
* Precedence::        Operator precedence works by resolving conflicts.
* Contextual Precedence::  When an operator's precedence depends on context.
* Parser States::     The parser is a finite-state-machine with stack.
* Reduce/Reduce::     When two rules are applicable in the same situation.
* Mystery Conflicts:: Reduce/reduce conflicts that look unjustified.
* Generalized LR Parsing::  Parsing arbitrary context-free grammars.
* Memory Management:: What happens when memory is exhausted.  How to avoid it.

Operator Precedence

* Why Precedence::    An example showing why precedence is needed.
* Using Precedence::  How to specify precedence in Bison grammars.
* Precedence Examples::  How these features are used in the previous example.
* How Precedence::    How they work.

Handling Context Dependencies

* Semantic Tokens::   Token parsing can depend on the semantic context.
* Lexical Tie-ins::   Token parsing can depend on the syntactic context.
* Tie-in Recovery::   Lexical tie-ins have implications for how
                        error recovery rules must be written.

Debugging Your Parser

* Understanding::     Understanding the structure of your parser.
* Tracing::           Tracing the execution of your parser.

Invoking Bison

* Bison Options::     All the options described in detail,
                        in alphabetical order by short options.
* Option Cross Key::  Alphabetical list of long options.
* Yacc Library::      Yacc-compatible `yylex' and `main'.

Parsers Written In Other Languages

* C++ Parsers::                 The interface to generate C++ parser classes
* Java Parsers::                The interface to generate Java parser classes

C++ Parsers

* C++ Bison Interface::         Asking for C++ parser generation
* C++ Semantic Values::         %union vs. C++
* C++ Location Values::         The position and location classes
* C++ Parser Interface::        Instantiating and running the parser
* C++ Scanner Interface::       Exchanges between yylex and parse
* A Complete C++ Example::      Demonstrating their use

A Complete C++ Example

* Calc++ --- C++ Calculator::   The specifications
* Calc++ Parsing Driver::       An active parsing context
* Calc++ Parser::               A parser class
* Calc++ Scanner::              A pure C++ Flex scanner
* Calc++ Top Level::            Conducting the band

Java Parsers

* Java Bison Interface::        Asking for Java parser generation
* Java Semantic Values::        %type and %token vs. Java
* Java Location Values::        The position and location classes
* Java Parser Interface::       Instantiating and running the parser
* Java Scanner Interface::      Specifying the scanner for the parser
* Java Action Features::        Special features for use in actions
* Java Differences::            Differences between C/C++ and Java Grammars
* Java Declarations Summary::   List of Bison declarations used with Java

Frequently Asked Questions

* Memory Exhausted::            Breaking the Stack Limits
* How Can I Reset the Parser::  `yyparse' Keeps some State
* Strings are Destroyed::       `yylval' Loses Track of Strings
* Implementing Gotos/Loops::    Control Flow in the Calculator
* Multiple start-symbols::      Factoring closely related grammars
* Secure?  Conform?::           Is Bison POSIX safe?
* I can't build Bison::         Troubleshooting
* Where can I find help?::      Troubleshouting
* Bug Reports::                 Troublereporting
* More Languages::              Parsers in C++, Java, and so on
* Beta Testing::                Experimenting development versions
* Mailing Lists::               Meeting other Bison users

Copying This Manual

* Copying This Manual::         License for copying this manual.


File: bison.info,  Node: Introduction,  Next: Conditions,  Prev: Top,  Up: Top

Introduction
************

"Bison" is a general-purpose parser generator that converts an
annotated context-free grammar into an LALR(1) or GLR parser for that
grammar.  Once you are proficient with Bison, you can use it to develop
a wide range of language parsers, from those used in simple desk
calculators to complex programming languages.

   Bison is upward compatible with Yacc: all properly-written Yacc
grammars ought to work with Bison with no change.  Anyone familiar with
Yacc should be able to use Bison with little trouble.  You need to be
fluent in C or C++ programming in order to use Bison or to understand
this manual.

   We begin with tutorial chapters that explain the basic concepts of
using Bison and show three explained examples, each building on the
last.  If you don't know Bison or Yacc, start by reading these
chapters.  Reference chapters follow which describe specific aspects of
Bison in detail.

   Bison was written primarily by Robert Corbett; Richard Stallman made
it Yacc-compatible.  Wilfred Hansen of Carnegie Mellon University added
multi-character string literals and other features.

   This edition corresponds to version 2.4.1 of Bison.


File: bison.info,  Node: Conditions,  Next: Copying,  Prev: Introduction,  Up: Top

Conditions for Using Bison
**************************

The distribution terms for Bison-generated parsers permit using the
parsers in nonfree programs.  Before Bison version 2.2, these extra
permissions applied only when Bison was generating LALR(1) parsers in
C.  And before Bison version 1.24, Bison-generated parsers could be
used only in programs that were free software.

   The other GNU programming tools, such as the GNU C compiler, have
never had such a requirement.  They could always be used for nonfree
software.  The reason Bison was different was not due to a special
policy decision; it resulted from applying the usual General Public
License to all of the Bison source code.

   The output of the Bison utility--the Bison parser file--contains a
verbatim copy of a sizable piece of Bison, which is the code for the
parser's implementation.  (The actions from your grammar are inserted
into this implementation at one point, but most of the rest of the
implementation is not changed.)  When we applied the GPL terms to the
skeleton code for the parser's implementation, the effect was to
restrict the use of Bison output to free software.

   We didn't change the terms because of sympathy for people who want to
make software proprietary.  *Software should be free.*  But we
concluded that limiting Bison's use to free software was doing little to
encourage people to make other software free.  So we decided to make the
practical conditions for using Bison match the practical conditions for
using the other GNU tools.

   This exception applies when Bison is generating code for a parser.
You can tell whether the exception applies to a Bison output file by
inspecting the file for text beginning with "As a special
exception...".  The text spells out the exact terms of the exception.


File: bison.info,  Node: Copying,  Next: Concepts,  Prev: Conditions,  Up: Top

GNU GENERAL PUBLIC LICENSE
**************************

                        Version 3, 29 June 2007

     Copyright (C) 2007 Free Software Foundation, Inc. `http://fsf.org/'

     Everyone is permitted to copy and distribute verbatim copies of this
     license document, but changing it is not allowed.

Preamble
========

The GNU General Public License is a free, copyleft license for software
and other kinds of works.

   The licenses for most software and other practical works are designed
to take away your freedom to share and change the works.  By contrast,
the GNU General Public License is intended to guarantee your freedom to
share and change all versions of a program--to make sure it remains
free software for all its users.  We, the Free Software Foundation, use
the GNU General Public License for most of our software; it applies
also to any other work released this way by its authors.  You can apply
it to your programs, too.

   When we speak of free software, we are referring to freedom, not
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   Developers that use the GNU GPL protect your rights with two steps:
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   For the developers' and authors' protection, the GPL clearly explains
that there is no warranty for this free software.  For both users' and
authors' sake, the GPL requires that modified versions be marked as
changed, so that their problems will not be attributed erroneously to
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   Some devices are designed to deny users access to install or run
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   Finally, every program is threatened constantly by software patents.
States should not allow patents to restrict development and use of
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   The precise terms and conditions for copying, distribution and
modification follow.

TERMS AND CONDITIONS
====================

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File: bison.info,  Node: Concepts,  Next: Examples,  Prev: Copying,  Up: Top

1 The Concepts of Bison
***********************

This chapter introduces many of the basic concepts without which the
details of Bison will not make sense.  If you do not already know how to
use Bison or Yacc, we suggest you start by reading this chapter
carefully.

* Menu:

* Language and Grammar:: Languages and context-free grammars,
                           as mathematical ideas.
* Grammar in Bison::     How we represent grammars for Bison's sake.
* Semantic Values::      Each token or syntactic grouping can have
                           a semantic value (the value of an integer,
                           the name of an identifier, etc.).
* Semantic Actions::     Each rule can have an action containing C code.
* GLR Parsers::          Writing parsers for general context-free languages.
* Locations Overview::   Tracking Locations.
* Bison Parser::         What are Bison's input and output,
                           how is the output used?
* Stages::               Stages in writing and running Bison grammars.
* Grammar Layout::       Overall structure of a Bison grammar file.


File: bison.info,  Node: Language and Grammar,  Next: Grammar in Bison,  Up: Concepts

1.1 Languages and Context-Free Grammars
=======================================

In order for Bison to parse a language, it must be described by a
"context-free grammar".  This means that you specify one or more
"syntactic groupings" and give rules for constructing them from their
parts.  For example, in the C language, one kind of grouping is called
an `expression'.  One rule for making an expression might be, "An
expression can be made of a minus sign and another expression".
Another would be, "An expression can be an integer".  As you can see,
rules are often recursive, but there must be at least one rule which
leads out of the recursion.

   The most common formal system for presenting such rules for humans
to read is "Backus-Naur Form" or "BNF", which was developed in order to
specify the language Algol 60.  Any grammar expressed in BNF is a
context-free grammar.  The input to Bison is essentially
machine-readable BNF.

   There are various important subclasses of context-free grammar.
Although it can handle almost all context-free grammars, Bison is
optimized for what are called LALR(1) grammars.  In brief, in these
grammars, it must be possible to tell how to parse any portion of an
input string with just a single token of lookahead.  Strictly speaking,
that is a description of an LR(1) grammar, and LALR(1) involves
additional restrictions that are hard to explain simply; but it is rare
in actual practice to find an LR(1) grammar that fails to be LALR(1).
*Note Mysterious Reduce/Reduce Conflicts: Mystery Conflicts, for more
information on this.

   Parsers for LALR(1) grammars are "deterministic", meaning roughly
that the next grammar rule to apply at any point in the input is
uniquely determined by the preceding input and a fixed, finite portion
(called a "lookahead") of the remaining input.  A context-free grammar
can be "ambiguous", meaning that there are multiple ways to apply the
grammar rules to get the same inputs.  Even unambiguous grammars can be
"nondeterministic", meaning that no fixed lookahead always suffices to
determine the next grammar rule to apply.  With the proper
declarations, Bison is also able to parse these more general
context-free grammars, using a technique known as GLR parsing (for
Generalized LR).  Bison's GLR parsers are able to handle any
context-free grammar for which the number of possible parses of any
given string is finite.

   In the formal grammatical rules for a language, each kind of
syntactic unit or grouping is named by a "symbol".  Those which are
built by grouping smaller constructs according to grammatical rules are
called "nonterminal symbols"; those which can't be subdivided are called
"terminal symbols" or "token types".  We call a piece of input
corresponding to a single terminal symbol a "token", and a piece
corresponding to a single nonterminal symbol a "grouping".

   We can use the C language as an example of what symbols, terminal and
nonterminal, mean.  The tokens of C are identifiers, constants (numeric
and string), and the various keywords, arithmetic operators and
punctuation marks.  So the terminal symbols of a grammar for C include
`identifier', `number', `string', plus one symbol for each keyword,
operator or punctuation mark: `if', `return', `const', `static', `int',
`char', `plus-sign', `open-brace', `close-brace', `comma' and many more.
(These tokens can be subdivided into characters, but that is a matter of
lexicography, not grammar.)

   Here is a simple C function subdivided into tokens:

     int             /* keyword `int' */
     square (int x)  /* identifier, open-paren, keyword `int',
                        identifier, close-paren */
     {               /* open-brace */
       return x * x; /* keyword `return', identifier, asterisk,
                        identifier, semicolon */
     }               /* close-brace */

   The syntactic groupings of C include the expression, the statement,
the declaration, and the function definition.  These are represented in
the grammar of C by nonterminal symbols `expression', `statement',
`declaration' and `function definition'.  The full grammar uses dozens
of additional language constructs, each with its own nonterminal
symbol, in order to express the meanings of these four.  The example
above is a function definition; it contains one declaration, and one
statement.  In the statement, each `x' is an expression and so is `x *
x'.

   Each nonterminal symbol must have grammatical rules showing how it
is made out of simpler constructs.  For example, one kind of C
statement is the `return' statement; this would be described with a
grammar rule which reads informally as follows:

     A `statement' can be made of a `return' keyword, an `expression'
     and a `semicolon'.

There would be many other rules for `statement', one for each kind of
statement in C.

   One nonterminal symbol must be distinguished as the special one which
defines a complete utterance in the language.  It is called the "start
symbol".  In a compiler, this means a complete input program.  In the C
language, the nonterminal symbol `sequence of definitions and
declarations' plays this role.

   For example, `1 + 2' is a valid C expression--a valid part of a C
program--but it is not valid as an _entire_ C program.  In the
context-free grammar of C, this follows from the fact that `expression'
is not the start symbol.

   The Bison parser reads a sequence of tokens as its input, and groups
the tokens using the grammar rules.  If the input is valid, the end
result is that the entire token sequence reduces to a single grouping
whose symbol is the grammar's start symbol.  If we use a grammar for C,
the entire input must be a `sequence of definitions and declarations'.
If not, the parser reports a syntax error.


File: bison.info,  Node: Grammar in Bison,  Next: Semantic Values,  Prev: Language and Grammar,  Up: Concepts

1.2 From Formal Rules to Bison Input
====================================

A formal grammar is a mathematical construct.  To define the language
for Bison, you must write a file expressing the grammar in Bison syntax:
a "Bison grammar" file.  *Note Bison Grammar Files: Grammar File.

   A nonterminal symbol in the formal grammar is represented in Bison
input as an identifier, like an identifier in C.  By convention, it
should be in lower case, such as `expr', `stmt' or `declaration'.

   The Bison representation for a terminal symbol is also called a
"token type".  Token types as well can be represented as C-like
identifiers.  By convention, these identifiers should be upper case to
distinguish them from nonterminals: for example, `INTEGER',
`IDENTIFIER', `IF' or `RETURN'.  A terminal symbol that stands for a
particular keyword in the language should be named after that keyword
converted to upper case.  The terminal symbol `error' is reserved for
error recovery.  *Note Symbols::.

   A terminal symbol can also be represented as a character literal,
just like a C character constant.  You should do this whenever a token
is just a single character (parenthesis, plus-sign, etc.): use that
same character in a literal as the terminal symbol for that token.

   A third way to represent a terminal symbol is with a C string
constant containing several characters.  *Note Symbols::, for more
information.

   The grammar rules also have an expression in Bison syntax.  For
example, here is the Bison rule for a C `return' statement.  The
semicolon in quotes is a literal character token, representing part of
the C syntax for the statement; the naked semicolon, and the colon, are
Bison punctuation used in every rule.

     stmt:   RETURN expr ';'
             ;

*Note Syntax of Grammar Rules: Rules.


File: bison.info,  Node: Semantic Values,  Next: Semantic Actions,  Prev: Grammar in Bison,  Up: Concepts

1.3 Semantic Values
===================

A formal grammar selects tokens only by their classifications: for
example, if a rule mentions the terminal symbol `integer constant', it
means that _any_ integer constant is grammatically valid in that
position.  The precise value of the constant is irrelevant to how to
parse the input: if `x+4' is grammatical then `x+1' or `x+3989' is
equally grammatical.

   But the precise value is very important for what the input means
once it is parsed.  A compiler is useless if it fails to distinguish
between 4, 1 and 3989 as constants in the program!  Therefore, each
token in a Bison grammar has both a token type and a "semantic value".
*Note Defining Language Semantics: Semantics, for details.

   The token type is a terminal symbol defined in the grammar, such as
`INTEGER', `IDENTIFIER' or `',''.  It tells everything you need to know
to decide where the token may validly appear and how to group it with
other tokens.  The grammar rules know nothing about tokens except their
types.

   The semantic value has all the rest of the information about the
meaning of the token, such as the value of an integer, or the name of an
identifier.  (A token such as `','' which is just punctuation doesn't
need to have any semantic value.)

   For example, an input token might be classified as token type
`INTEGER' and have the semantic value 4.  Another input token might
have the same token type `INTEGER' but value 3989.  When a grammar rule
says that `INTEGER' is allowed, either of these tokens is acceptable
because each is an `INTEGER'.  When the parser accepts the token, it
keeps track of the token's semantic value.

   Each grouping can also have a semantic value as well as its
nonterminal symbol.  For example, in a calculator, an expression
typically has a semantic value that is a number.  In a compiler for a
programming language, an expression typically has a semantic value that
is a tree structure describing the meaning of the expression.


File: bison.info,  Node: Semantic Actions,  Next: GLR Parsers,  Prev: Semantic Values,  Up: Concepts

1.4 Semantic Actions
====================

In order to be useful, a program must do more than parse input; it must
also produce some output based on the input.  In a Bison grammar, a
grammar rule can have an "action" made up of C statements.  Each time
the parser recognizes a match for that rule, the action is executed.
*Note Actions::.

   Most of the time, the purpose of an action is to compute the
semantic value of the whole construct from the semantic values of its
parts.  For example, suppose we have a rule which says an expression
can be the sum of two expressions.  When the parser recognizes such a
sum, each of the subexpressions has a semantic value which describes
how it was built up.  The action for this rule should create a similar
sort of value for the newly recognized larger expression.

   For example, here is a rule that says an expression can be the sum of
two subexpressions:

     expr: expr '+' expr   { $$ = $1 + $3; }
             ;

The action says how to produce the semantic value of the sum expression
from the values of the two subexpressions.


File: bison.info,  Node: GLR Parsers,  Next: Locations Overview,  Prev: Semantic Actions,  Up: Concepts

1.5 Writing GLR Parsers
=======================

In some grammars, Bison's standard LALR(1) parsing algorithm cannot
decide whether to apply a certain grammar rule at a given point.  That
is, it may not be able to decide (on the basis of the input read so
far) which of two possible reductions (applications of a grammar rule)
applies, or whether to apply a reduction or read more of the input and
apply a reduction later in the input.  These are known respectively as
"reduce/reduce" conflicts (*note Reduce/Reduce::), and "shift/reduce"
conflicts (*note Shift/Reduce::).

   To use a grammar that is not easily modified to be LALR(1), a more
general parsing algorithm is sometimes necessary.  If you include
`%glr-parser' among the Bison declarations in your file (*note Grammar
Outline::), the result is a Generalized LR (GLR) parser.  These parsers
handle Bison grammars that contain no unresolved conflicts (i.e., after
applying precedence declarations) identically to LALR(1) parsers.
However, when faced with unresolved shift/reduce and reduce/reduce
conflicts, GLR parsers use the simple expedient of doing both,
effectively cloning the parser to follow both possibilities.  Each of
the resulting parsers can again split, so that at any given time, there
can be any number of possible parses being explored.  The parsers
proceed in lockstep; that is, all of them consume (shift) a given input
symbol before any of them proceed to the next.  Each of the cloned
parsers eventually meets one of two possible fates: either it runs into
a parsing error, in which case it simply vanishes, or it merges with
another parser, because the two of them have reduced the input to an
identical set of symbols.

   During the time that there are multiple parsers, semantic actions are
recorded, but not performed.  When a parser disappears, its recorded
semantic actions disappear as well, and are never performed.  When a
reduction makes two parsers identical, causing them to merge, Bison
records both sets of semantic actions.  Whenever the last two parsers
merge, reverting to the single-parser case, Bison resolves all the
outstanding actions either by precedences given to the grammar rules
involved, or by performing both actions, and then calling a designated
user-defined function on the resulting values to produce an arbitrary
merged result.

* Menu:

* Simple GLR Parsers::     Using GLR parsers on unambiguous grammars.
* Merging GLR Parses::     Using GLR parsers to resolve ambiguities.
* GLR Semantic Actions::   Deferred semantic actions have special concerns.
* Compiler Requirements::  GLR parsers require a modern C compiler.


File: bison.info,  Node: Simple GLR Parsers,  Next: Merging GLR Parses,  Up: GLR Parsers

1.5.1 Using GLR on Unambiguous Grammars
---------------------------------------

In the simplest cases, you can use the GLR algorithm to parse grammars
that are unambiguous, but fail to be LALR(1).  Such grammars typically
require more than one symbol of lookahead, or (in rare cases) fall into
the category of grammars in which the LALR(1) algorithm throws away too
much information (they are in LR(1), but not LALR(1), *Note Mystery
Conflicts::).

   Consider a problem that arises in the declaration of enumerated and
subrange types in the programming language Pascal.  Here are some
examples:

     type subrange = lo .. hi;
     type enum = (a, b, c);

The original language standard allows only numeric literals and
constant identifiers for the subrange bounds (`lo' and `hi'), but
Extended Pascal (ISO/IEC 10206) and many other Pascal implementations
allow arbitrary expressions there.  This gives rise to the following
situation, containing a superfluous pair of parentheses:

     type subrange = (a) .. b;

Compare this to the following declaration of an enumerated type with
only one value:

     type enum = (a);

(These declarations are contrived, but they are syntactically valid,
and more-complicated cases can come up in practical programs.)

   These two declarations look identical until the `..' token.  With
normal LALR(1) one-token lookahead it is not possible to decide between
the two forms when the identifier `a' is parsed.  It is, however,
desirable for a parser to decide this, since in the latter case `a'
must become a new identifier to represent the enumeration value, while
in the former case `a' must be evaluated with its current meaning,
which may be a constant or even a function call.

   You could parse `(a)' as an "unspecified identifier in parentheses",
to be resolved later, but this typically requires substantial
contortions in both semantic actions and large parts of the grammar,
where the parentheses are nested in the recursive rules for expressions.

   You might think of using the lexer to distinguish between the two
forms by returning different tokens for currently defined and undefined
identifiers.  But if these declarations occur in a local scope, and `a'
is defined in an outer scope, then both forms are possible--either
locally redefining `a', or using the value of `a' from the outer scope.
So this approach cannot work.

   A simple solution to this problem is to declare the parser to use
the GLR algorithm.  When the GLR parser reaches the critical state, it
merely splits into two branches and pursues both syntax rules
simultaneously.  Sooner or later, one of them runs into a parsing
error.  If there is a `..' token before the next `;', the rule for
enumerated types fails since it cannot accept `..' anywhere; otherwise,
the subrange type rule fails since it requires a `..' token.  So one of
the branches fails silently, and the other one continues normally,
performing all the intermediate actions that were postponed during the
split.

   If the input is syntactically incorrect, both branches fail and the
parser reports a syntax error as usual.

   The effect of all this is that the parser seems to "guess" the
correct branch to take, or in other words, it seems to use more
lookahead than the underlying LALR(1) algorithm actually allows for.
In this example, LALR(2) would suffice, but also some cases that are
not LALR(k) for any k can be handled this way.

   In general, a GLR parser can take quadratic or cubic worst-case time,
and the current Bison parser even takes exponential time and space for
some grammars.  In practice, this rarely happens, and for many grammars
it is possible to prove that it cannot happen.  The present example
contains only one conflict between two rules, and the type-declaration
context containing the conflict cannot be nested.  So the number of
branches that can exist at any time is limited by the constant 2, and
the parsing time is still linear.

   Here is a Bison grammar corresponding to the example above.  It
parses a vastly simplified form of Pascal type declarations.

     %token TYPE DOTDOT ID

     %left '+' '-'
     %left '*' '/'

     %%

     type_decl : TYPE ID '=' type ';'
          ;

     type : '(' id_list ')'
          | expr DOTDOT expr
          ;

     id_list : ID
          | id_list ',' ID
          ;

     expr : '(' expr ')'
          | expr '+' expr
          | expr '-' expr
          | expr '*' expr
          | expr '/' expr
          | ID
          ;

   When used as a normal LALR(1) grammar, Bison correctly complains
about one reduce/reduce conflict.  In the conflicting situation the
parser chooses one of the alternatives, arbitrarily the one declared
first.  Therefore the following correct input is not recognized:

     type t = (a) .. b;

   The parser can be turned into a GLR parser, while also telling Bison
to be silent about the one known reduce/reduce conflict, by adding
these two declarations to the Bison input file (before the first `%%'):

     %glr-parser
     %expect-rr 1

No change in the grammar itself is required.  Now the parser recognizes
all valid declarations, according to the limited syntax above,
transparently.  In fact, the user does not even notice when the parser
splits.

   So here we have a case where we can use the benefits of GLR, almost
without disadvantages.  Even in simple cases like this, however, there
are at least two potential problems to beware.  First, always analyze
the conflicts reported by Bison to make sure that GLR splitting is only
done where it is intended.  A GLR parser splitting inadvertently may
cause problems less obvious than an LALR parser statically choosing the
wrong alternative in a conflict.  Second, consider interactions with
the lexer (*note Semantic Tokens::) with great care.  Since a split
parser consumes tokens without performing any actions during the split,
the lexer cannot obtain information via parser actions.  Some cases of
lexer interactions can be eliminated by using GLR to shift the
complications from the lexer to the parser.  You must check the
remaining cases for correctness.

   In our example, it would be safe for the lexer to return tokens
based on their current meanings in some symbol table, because no new
symbols are defined in the middle of a type declaration.  Though it is
possible for a parser to define the enumeration constants as they are
parsed, before the type declaration is completed, it actually makes no
difference since they cannot be used within the same enumerated type
declaration.


File: bison.info,  Node: Merging GLR Parses,  Next: GLR Semantic Actions,  Prev: Simple GLR Parsers,  Up: GLR Parsers

1.5.2 Using GLR to Resolve Ambiguities
--------------------------------------

Let's consider an example, vastly simplified from a C++ grammar.

     %{
       #include <stdio.h>
       #define YYSTYPE char const *
       int yylex (void);
       void yyerror (char const *);
     %}

     %token TYPENAME ID

     %right '='
     %left '+'

     %glr-parser

     %%

     prog :
          | prog stmt   { printf ("\n"); }
          ;

     stmt : expr ';'  %dprec 1
          | decl      %dprec 2
          ;

     expr : ID               { printf ("%s ", $$); }
          | TYPENAME '(' expr ')'
                             { printf ("%s <cast> ", $1); }
          | expr '+' expr    { printf ("+ "); }
          | expr '=' expr    { printf ("= "); }
          ;

     decl : TYPENAME declarator ';'
                             { printf ("%s <declare> ", $1); }
          | TYPENAME declarator '=' expr ';'
                             { printf ("%s <init-declare> ", $1); }
          ;

     declarator : ID         { printf ("\"%s\" ", $1); }
          | '(' declarator ')'
          ;

This models a problematic part of the C++ grammar--the ambiguity between
certain declarations and statements.  For example,

     T (x) = y+z;

parses as either an `expr' or a `stmt' (assuming that `T' is recognized
as a `TYPENAME' and `x' as an `ID').  Bison detects this as a
reduce/reduce conflict between the rules `expr : ID' and `declarator :
ID', which it cannot resolve at the time it encounters `x' in the
example above.  Since this is a GLR parser, it therefore splits the
problem into two parses, one for each choice of resolving the
reduce/reduce conflict.  Unlike the example from the previous section
(*note Simple GLR Parsers::), however, neither of these parses "dies,"
because the grammar as it stands is ambiguous.  One of the parsers
eventually reduces `stmt : expr ';'' and the other reduces `stmt :
decl', after which both parsers are in an identical state: they've seen
`prog stmt' and have the same unprocessed input remaining.  We say that
these parses have "merged."

   At this point, the GLR parser requires a specification in the
grammar of how to choose between the competing parses.  In the example
above, the two `%dprec' declarations specify that Bison is to give
precedence to the parse that interprets the example as a `decl', which
implies that `x' is a declarator.  The parser therefore prints

     "x" y z + T <init-declare>

   The `%dprec' declarations only come into play when more than one
parse survives.  Consider a different input string for this parser:

     T (x) + y;

This is another example of using GLR to parse an unambiguous construct,
as shown in the previous section (*note Simple GLR Parsers::).  Here,
there is no ambiguity (this cannot be parsed as a declaration).
However, at the time the Bison parser encounters `x', it does not have
enough information to resolve the reduce/reduce conflict (again,
between `x' as an `expr' or a `declarator').  In this case, no
precedence declaration is used.  Again, the parser splits into two, one
assuming that `x' is an `expr', and the other assuming `x' is a
`declarator'.  The second of these parsers then vanishes when it sees
`+', and the parser prints

     x T <cast> y +

   Suppose that instead of resolving the ambiguity, you wanted to see
all the possibilities.  For this purpose, you must merge the semantic
actions of the two possible parsers, rather than choosing one over the
other.  To do so, you could change the declaration of `stmt' as follows:

     stmt : expr ';'  %merge <stmtMerge>
          | decl      %merge <stmtMerge>
          ;

and define the `stmtMerge' function as:

     static YYSTYPE
     stmtMerge (YYSTYPE x0, YYSTYPE x1)
     {
       printf ("<OR> ");
       return "";
     }

with an accompanying forward declaration in the C declarations at the
beginning of the file:

     %{
       #define YYSTYPE char const *
       static YYSTYPE stmtMerge (YYSTYPE x0, YYSTYPE x1);
     %}

With these declarations, the resulting parser parses the first example
as both an `expr' and a `decl', and prints

     "x" y z + T <init-declare> x T <cast> y z + = <OR>

   Bison requires that all of the productions that participate in any
particular merge have identical `%merge' clauses.  Otherwise, the
ambiguity would be unresolvable, and the parser will report an error
during any parse that results in the offending merge.


File: bison.info,  Node: GLR Semantic Actions,  Next: Compiler Requirements,  Prev: Merging GLR Parses,  Up: GLR Parsers

1.5.3 GLR Semantic Actions
--------------------------

By definition, a deferred semantic action is not performed at the same
time as the associated reduction.  This raises caveats for several
Bison features you might use in a semantic action in a GLR parser.

   In any semantic action, you can examine `yychar' to determine the
type of the lookahead token present at the time of the associated
reduction.  After checking that `yychar' is not set to `YYEMPTY' or
`YYEOF', you can then examine `yylval' and `yylloc' to determine the
lookahead token's semantic value and location, if any.  In a
nondeferred semantic action, you can also modify any of these variables
to influence syntax analysis.  *Note Lookahead Tokens: Lookahead.

   In a deferred semantic action, it's too late to influence syntax
analysis.  In this case, `yychar', `yylval', and `yylloc' are set to
shallow copies of the values they had at the time of the associated
reduction.  For this reason alone, modifying them is dangerous.
Moreover, the result of modifying them is undefined and subject to
change with future versions of Bison.  For example, if a semantic
action might be deferred, you should never write it to invoke
`yyclearin' (*note Action Features::) or to attempt to free memory
referenced by `yylval'.

   Another Bison feature requiring special consideration is `YYERROR'
(*note Action Features::), which you can invoke in a semantic action to
initiate error recovery.  During deterministic GLR operation, the
effect of `YYERROR' is the same as its effect in an LALR(1) parser.  In
a deferred semantic action, its effect is undefined.

   Also, see *Note Default Action for Locations: Location Default
Action, which describes a special usage of `YYLLOC_DEFAULT' in GLR
parsers.


File: bison.info,  Node: Compiler Requirements,  Prev: GLR Semantic Actions,  Up: GLR Parsers

1.5.4 Considerations when Compiling GLR Parsers
-----------------------------------------------

The GLR parsers require a compiler for ISO C89 or later.  In addition,
they use the `inline' keyword, which is not C89, but is C99 and is a
common extension in pre-C99 compilers.  It is up to the user of these
parsers to handle portability issues.  For instance, if using Autoconf
and the Autoconf macro `AC_C_INLINE', a mere

     %{
       #include <config.h>
     %}

will suffice.  Otherwise, we suggest

     %{
       #if __STDC_VERSION__ < 199901 && ! defined __GNUC__ && ! defined inline
        #define inline
       #endif
     %}


File: bison.info,  Node: Locations Overview,  Next: Bison Parser,  Prev: GLR Parsers,  Up: Concepts

1.6 Locations
=============

Many applications, like interpreters or compilers, have to produce
verbose and useful error messages.  To achieve this, one must be able
to keep track of the "textual location", or "location", of each
syntactic construct.  Bison provides a mechanism for handling these
locations.

   Each token has a semantic value.  In a similar fashion, each token
has an associated location, but the type of locations is the same for
all tokens and groupings.  Moreover, the output parser is equipped with
a default data structure for storing locations (*note Locations::, for
more details).

   Like semantic values, locations can be reached in actions using a
dedicated set of constructs.  In the example above, the location of the
whole grouping is `@$', while the locations of the subexpressions are
`@1' and `@3'.

   When a rule is matched, a default action is used to compute the
semantic value of its left hand side (*note Actions::).  In the same
way, another default action is used for locations.  However, the action
for locations is general enough for most cases, meaning there is
usually no need to describe for each rule how `@$' should be formed.
When building a new location for a given grouping, the default behavior
of the output parser is to take the beginning of the first symbol, and
the end of the last symbol.


File: bison.info,  Node: Bison Parser,  Next: Stages,  Prev: Locations Overview,  Up: Concepts

1.7 Bison Output: the Parser File
=================================

When you run Bison, you give it a Bison grammar file as input.  The
output is a C source file that parses the language described by the
grammar.  This file is called a "Bison parser".  Keep in mind that the
Bison utility and the Bison parser are two distinct programs: the Bison
utility is a program whose output is the Bison parser that becomes part
of your program.

   The job of the Bison parser is to group tokens into groupings
according to the grammar rules--for example, to build identifiers and
operators into expressions.  As it does this, it runs the actions for
the grammar rules it uses.

   The tokens come from a function called the "lexical analyzer" that
you must supply in some fashion (such as by writing it in C).  The Bison
parser calls the lexical analyzer each time it wants a new token.  It
doesn't know what is "inside" the tokens (though their semantic values
may reflect this).  Typically the lexical analyzer makes the tokens by
parsing characters of text, but Bison does not depend on this.  *Note
The Lexical Analyzer Function `yylex': Lexical.

   The Bison parser file is C code which defines a function named
`yyparse' which implements that grammar.  This function does not make a
complete C program: you must supply some additional functions.  One is
the lexical analyzer.  Another is an error-reporting function which the
parser calls to report an error.  In addition, a complete C program must
start with a function called `main'; you have to provide this, and
arrange for it to call `yyparse' or the parser will never run.  *Note
Parser C-Language Interface: Interface.

   Aside from the token type names and the symbols in the actions you
write, all symbols defined in the Bison parser file itself begin with
`yy' or `YY'.  This includes interface functions such as the lexical
analyzer function `yylex', the error reporting function `yyerror' and
the parser function `yyparse' itself.  This also includes numerous
identifiers used for internal purposes.  Therefore, you should avoid
using C identifiers starting with `yy' or `YY' in the Bison grammar
file except for the ones defined in this manual.  Also, you should
avoid using the C identifiers `malloc' and `free' for anything other
than their usual meanings.

   In some cases the Bison parser file includes system headers, and in
those cases your code should respect the identifiers reserved by those
headers.  On some non-GNU hosts, `<alloca.h>', `<malloc.h>',
`<stddef.h>', and `<stdlib.h>' are included as needed to declare memory
allocators and related types.  `<libintl.h>' is included if message
translation is in use (*note Internationalization::).  Other system
headers may be included if you define `YYDEBUG' to a nonzero value
(*note Tracing Your Parser: Tracing.).


File: bison.info,  Node: Stages,  Next: Grammar Layout,  Prev: Bison Parser,  Up: Concepts

1.8 Stages in Using Bison
=========================

The actual language-design process using Bison, from grammar
specification to a working compiler or interpreter, has these parts:

  1. Formally specify the grammar in a form recognized by Bison (*note
     Bison Grammar Files: Grammar File.).  For each grammatical rule in
     the language, describe the action that is to be taken when an
     instance of that rule is recognized.  The action is described by a
     sequence of C statements.

  2. Write a lexical analyzer to process input and pass tokens to the
     parser.  The lexical analyzer may be written by hand in C (*note
     The Lexical Analyzer Function `yylex': Lexical.).  It could also
     be produced using Lex, but the use of Lex is not discussed in this
     manual.

  3. Write a controlling function that calls the Bison-produced parser.

  4. Write error-reporting routines.

   To turn this source code as written into a runnable program, you
must follow these steps:

  1. Run Bison on the grammar to produce the parser.

  2. Compile the code output by Bison, as well as any other source
     files.

  3. Link the object files to produce the finished product.


File: bison.info,  Node: Grammar Layout,  Prev: Stages,  Up: Concepts

1.9 The Overall Layout of a Bison Grammar
=========================================

The input file for the Bison utility is a "Bison grammar file".  The
general form of a Bison grammar file is as follows:

     %{
     PROLOGUE
     %}

     BISON DECLARATIONS

     %%
     GRAMMAR RULES
     %%
     EPILOGUE

The `%%', `%{' and `%}' are punctuation that appears in every Bison
grammar file to separate the sections.

   The prologue may define types and variables used in the actions.
You can also use preprocessor commands to define macros used there, and
use `#include' to include header files that do any of these things.
You need to declare the lexical analyzer `yylex' and the error printer
`yyerror' here, along with any other global identifiers used by the
actions in the grammar rules.

   The Bison declarations declare the names of the terminal and
nonterminal symbols, and may also describe operator precedence and the
data types of semantic values of various symbols.

   The grammar rules define how to construct each nonterminal symbol
from its parts.

   The epilogue can contain any code you want to use.  Often the
definitions of functions declared in the prologue go here.  In a simple
program, all the rest of the program can go here.


File: bison.info,  Node: Examples,  Next: Grammar File,  Prev: Concepts,  Up: Top

2 Examples
**********

Now we show and explain three sample programs written using Bison: a
reverse polish notation calculator, an algebraic (infix) notation
calculator, and a multi-function calculator.  All three have been tested
under BSD Unix 4.3; each produces a usable, though limited, interactive
desk-top calculator.

   These examples are simple, but Bison grammars for real programming
languages are written the same way.  You can copy these examples into a
source file to try them.

* Menu:

* RPN Calc::               Reverse polish notation calculator;
                             a first example with no operator precedence.
* Infix Calc::             Infix (algebraic) notation calculator.
                             Operator precedence is introduced.
* Simple Error Recovery::  Continuing after syntax errors.
* Location Tracking Calc:: Demonstrating the use of @N and @$.
* Multi-function Calc::    Calculator with memory and trig functions.
                             It uses multiple data-types for semantic values.
* Exercises::              Ideas for improving the multi-function calculator.


File: bison.info,  Node: RPN Calc,  Next: Infix Calc,  Up: Examples

2.1 Reverse Polish Notation Calculator
======================================

The first example is that of a simple double-precision "reverse polish
notation" calculator (a calculator using postfix operators).  This
example provides a good starting point, since operator precedence is
not an issue.  The second example will illustrate how operator
precedence is handled.

   The source code for this calculator is named `rpcalc.y'.  The `.y'
extension is a convention used for Bison input files.

* Menu:

* Rpcalc Declarations::    Prologue (declarations) for rpcalc.
* Rpcalc Rules::           Grammar Rules for rpcalc, with explanation.
* Rpcalc Lexer::           The lexical analyzer.
* Rpcalc Main::            The controlling function.
* Rpcalc Error::           The error reporting function.
* Rpcalc Generate::        Running Bison on the grammar file.
* Rpcalc Compile::         Run the C compiler on the output code.


File: bison.info,  Node: Rpcalc Declarations,  Next: Rpcalc Rules,  Up: RPN Calc

2.1.1 Declarations for `rpcalc'
-------------------------------

Here are the C and Bison declarations for the reverse polish notation
calculator.  As in C, comments are placed between `/*...*/'.

     /* Reverse polish notation calculator.  */

     %{
       #define YYSTYPE double
       #include <math.h>
       int yylex (void);
       void yyerror (char const *);
     %}

     %token NUM

     %% /* Grammar rules and actions follow.  */

   The declarations section (*note The prologue: Prologue.) contains two
preprocessor directives and two forward declarations.

   The `#define' directive defines the macro `YYSTYPE', thus specifying
the C data type for semantic values of both tokens and groupings (*note
Data Types of Semantic Values: Value Type.).  The Bison parser will use
whatever type `YYSTYPE' is defined as; if you don't define it, `int' is
the default.  Because we specify `double', each token and each
expression has an associated value, which is a floating point number.

   The `#include' directive is used to declare the exponentiation
function `pow'.

   The forward declarations for `yylex' and `yyerror' are needed
because the C language requires that functions be declared before they
are used.  These functions will be defined in the epilogue, but the
parser calls them so they must be declared in the prologue.

   The second section, Bison declarations, provides information to Bison
about the token types (*note The Bison Declarations Section: Bison
Declarations.).  Each terminal symbol that is not a single-character
literal must be declared here.  (Single-character literals normally
don't need to be declared.)  In this example, all the arithmetic
operators are designated by single-character literals, so the only
terminal symbol that needs to be declared is `NUM', the token type for
numeric constants.


File: bison.info,  Node: Rpcalc Rules,  Next: Rpcalc Lexer,  Prev: Rpcalc Declarations,  Up: RPN Calc

2.1.2 Grammar Rules for `rpcalc'
--------------------------------

Here are the grammar rules for the reverse polish notation calculator.

     input:    /* empty */
             | input line
     ;

     line:     '\n'
             | exp '\n'      { printf ("\t%.10g\n", $1); }
     ;

     exp:      NUM           { $$ = $1;           }
             | exp exp '+'   { $$ = $1 + $2;      }
             | exp exp '-'   { $$ = $1 - $2;      }
             | exp exp '*'   { $$ = $1 * $2;      }
             | exp exp '/'   { $$ = $1 / $2;      }
              /* Exponentiation */
             | exp exp '^'   { $$ = pow ($1, $2); }
              /* Unary minus    */
             | exp 'n'       { $$ = -$1;          }
     ;
     %%

   The groupings of the rpcalc "language" defined here are the
expression (given the name `exp'), the line of input (`line'), and the
complete input transcript (`input').  Each of these nonterminal symbols
has several alternate rules, joined by the vertical bar `|' which is
read as "or".  The following sections explain what these rules mean.

   The semantics of the language is determined by the actions taken
when a grouping is recognized.  The actions are the C code that appears
inside braces.  *Note Actions::.

   You must specify these actions in C, but Bison provides the means for
passing semantic values between the rules.  In each action, the
pseudo-variable `$$' stands for the semantic value for the grouping
that the rule is going to construct.  Assigning a value to `$$' is the
main job of most actions.  The semantic values of the components of the
rule are referred to as `$1', `$2', and so on.

* Menu:

* Rpcalc Input::
* Rpcalc Line::
* Rpcalc Expr::


File: bison.info,  Node: Rpcalc Input,  Next: Rpcalc Line,  Up: Rpcalc Rules

2.1.2.1 Explanation of `input'
..............................

Consider the definition of `input':

     input:    /* empty */
             | input line
     ;

   This definition reads as follows: "A complete input is either an
empty string, or a complete input followed by an input line".  Notice
that "complete input" is defined in terms of itself.  This definition
is said to be "left recursive" since `input' appears always as the
leftmost symbol in the sequence.  *Note Recursive Rules: Recursion.

   The first alternative is empty because there are no symbols between
the colon and the first `|'; this means that `input' can match an empty
string of input (no tokens).  We write the rules this way because it is
legitimate to type `Ctrl-d' right after you start the calculator.  It's
conventional to put an empty alternative first and write the comment
`/* empty */' in it.

   The second alternate rule (`input line') handles all nontrivial
input.  It means, "After reading any number of lines, read one more
line if possible."  The left recursion makes this rule into a loop.
Since the first alternative matches empty input, the loop can be
executed zero or more times.

   The parser function `yyparse' continues to process input until a
grammatical error is seen or the lexical analyzer says there are no more
input tokens; we will arrange for the latter to happen at end-of-input.


File: bison.info,  Node: Rpcalc Line,  Next: Rpcalc Expr,  Prev: Rpcalc Input,  Up: Rpcalc Rules

2.1.2.2 Explanation of `line'
.............................

Now consider the definition of `line':

     line:     '\n'
             | exp '\n'  { printf ("\t%.10g\n", $1); }
     ;

   The first alternative is a token which is a newline character; this
means that rpcalc accepts a blank line (and ignores it, since there is
no action).  The second alternative is an expression followed by a
newline.  This is the alternative that makes rpcalc useful.  The
semantic value of the `exp' grouping is the value of `$1' because the
`exp' in question is the first symbol in the alternative.  The action
prints this value, which is the result of the computation the user
asked for.

   This action is unusual because it does not assign a value to `$$'.
As a consequence, the semantic value associated with the `line' is
uninitialized (its value will be unpredictable).  This would be a bug if
that value were ever used, but we don't use it: once rpcalc has printed
the value of the user's input line, that value is no longer needed.


File: bison.info,  Node: Rpcalc Expr,  Prev: Rpcalc Line,  Up: Rpcalc Rules

2.1.2.3 Explanation of `expr'
.............................

The `exp' grouping has several rules, one for each kind of expression.
The first rule handles the simplest expressions: those that are just
numbers.  The second handles an addition-expression, which looks like
two expressions followed by a plus-sign.  The third handles
subtraction, and so on.

     exp:      NUM
             | exp exp '+'     { $$ = $1 + $2;    }
             | exp exp '-'     { $$ = $1 - $2;    }
             ...
             ;

   We have used `|' to join all the rules for `exp', but we could
equally well have written them separately:

     exp:      NUM ;
     exp:      exp exp '+'     { $$ = $1 + $2;    } ;
     exp:      exp exp '-'     { $$ = $1 - $2;    } ;
             ...

   Most of the rules have actions that compute the value of the
expression in terms of the value of its parts.  For example, in the
rule for addition, `$1' refers to the first component `exp' and `$2'
refers to the second one.  The third component, `'+'', has no meaningful
associated semantic value, but if it had one you could refer to it as
`$3'.  When `yyparse' recognizes a sum expression using this rule, the
sum of the two subexpressions' values is produced as the value of the
entire expression.  *Note Actions::.

   You don't have to give an action for every rule.  When a rule has no
action, Bison by default copies the value of `$1' into `$$'.  This is
what happens in the first rule (the one that uses `NUM').

   The formatting shown here is the recommended convention, but Bison
does not require it.  You can add or change white space as much as you
wish.  For example, this:

     exp   : NUM | exp exp '+' {$$ = $1 + $2; } | ... ;

means the same thing as this:

     exp:      NUM
             | exp exp '+'    { $$ = $1 + $2; }
             | ...
     ;

The latter, however, is much more readable.


File: bison.info,  Node: Rpcalc Lexer,  Next: Rpcalc Main,  Prev: Rpcalc Rules,  Up: RPN Calc

2.1.3 The `rpcalc' Lexical Analyzer
-----------------------------------

The lexical analyzer's job is low-level parsing: converting characters
or sequences of characters into tokens.  The Bison parser gets its
tokens by calling the lexical analyzer.  *Note The Lexical Analyzer
Function `yylex': Lexical.

   Only a simple lexical analyzer is needed for the RPN calculator.
This lexical analyzer skips blanks and tabs, then reads in numbers as
`double' and returns them as `NUM' tokens.  Any other character that
isn't part of a number is a separate token.  Note that the token-code
for such a single-character token is the character itself.

   The return value of the lexical analyzer function is a numeric code
which represents a token type.  The same text used in Bison rules to
stand for this token type is also a C expression for the numeric code
for the type.  This works in two ways.  If the token type is a
character literal, then its numeric code is that of the character; you
can use the same character literal in the lexical analyzer to express
the number.  If the token type is an identifier, that identifier is
defined by Bison as a C macro whose definition is the appropriate
number.  In this example, therefore, `NUM' becomes a macro for `yylex'
to use.

   The semantic value of the token (if it has one) is stored into the
global variable `yylval', which is where the Bison parser will look for
it.  (The C data type of `yylval' is `YYSTYPE', which was defined at
the beginning of the grammar; *note Declarations for `rpcalc': Rpcalc
Declarations.)

   A token type code of zero is returned if the end-of-input is
encountered.  (Bison recognizes any nonpositive value as indicating
end-of-input.)

   Here is the code for the lexical analyzer:

     /* The lexical analyzer returns a double floating point
        number on the stack and the token NUM, or the numeric code
        of the character read if not a number.  It skips all blanks
        and tabs, and returns 0 for end-of-input.  */

     #include <ctype.h>

     int
     yylex (void)
     {
       int c;

       /* Skip white space.  */
       while ((c = getchar ()) == ' ' || c == '\t')
         ;
       /* Process numbers.  */
       if (c == '.' || isdigit (c))
         {
           ungetc (c, stdin);
           scanf ("%lf", &yylval);
           return NUM;
         }
       /* Return end-of-input.  */
       if (c == EOF)
         return 0;
       /* Return a single char.  */
       return c;
     }


File: bison.info,  Node: Rpcalc Main,  Next: Rpcalc Error,  Prev: Rpcalc Lexer,  Up: RPN Calc

2.1.4 The Controlling Function
------------------------------

In keeping with the spirit of this example, the controlling function is
kept to the bare minimum.  The only requirement is that it call
`yyparse' to start the process of parsing.

     int
     main (void)
     {
       return yyparse ();
     }


File: bison.info,  Node: Rpcalc Error,  Next: Rpcalc Generate,  Prev: Rpcalc Main,  Up: RPN Calc

2.1.5 The Error Reporting Routine
---------------------------------

When `yyparse' detects a syntax error, it calls the error reporting
function `yyerror' to print an error message (usually but not always
`"syntax error"').  It is up to the programmer to supply `yyerror'
(*note Parser C-Language Interface: Interface.), so here is the
definition we will use:

     #include <stdio.h>

     /* Called by yyparse on error.  */
     void
     yyerror (char const *s)
     {
       fprintf (stderr, "%s\n", s);
     }

   After `yyerror' returns, the Bison parser may recover from the error
and continue parsing if the grammar contains a suitable error rule
(*note Error Recovery::).  Otherwise, `yyparse' returns nonzero.  We
have not written any error rules in this example, so any invalid input
will cause the calculator program to exit.  This is not clean behavior
for a real calculator, but it is adequate for the first example.


File: bison.info,  Node: Rpcalc Generate,  Next: Rpcalc Compile,  Prev: Rpcalc Error,  Up: RPN Calc

2.1.6 Running Bison to Make the Parser
--------------------------------------

Before running Bison to produce a parser, we need to decide how to
arrange all the source code in one or more source files.  For such a
simple example, the easiest thing is to put everything in one file.  The
definitions of `yylex', `yyerror' and `main' go at the end, in the
epilogue of the file (*note The Overall Layout of a Bison Grammar:
Grammar Layout.).

   For a large project, you would probably have several source files,
and use `make' to arrange to recompile them.

   With all the source in a single file, you use the following command
to convert it into a parser file:

     bison FILE.y

In this example the file was called `rpcalc.y' (for "Reverse Polish
CALCulator").  Bison produces a file named `FILE.tab.c', removing the
`.y' from the original file name.  The file output by Bison contains
the source code for `yyparse'.  The additional functions in the input
file (`yylex', `yyerror' and `main') are copied verbatim to the output.


File: bison.info,  Node: Rpcalc Compile,  Prev: Rpcalc Generate,  Up: RPN Calc

2.1.7 Compiling the Parser File
-------------------------------

Here is how to compile and run the parser file:

     # List files in current directory.
     $ ls
     rpcalc.tab.c  rpcalc.y

     # Compile the Bison parser.
     # `-lm' tells compiler to search math library for `pow'.
     $ cc -lm -o rpcalc rpcalc.tab.c

     # List files again.
     $ ls
     rpcalc  rpcalc.tab.c  rpcalc.y

   The file `rpcalc' now contains the executable code.  Here is an
example session using `rpcalc'.

     $ rpcalc
     4 9 +
     13
     3 7 + 3 4 5 *+-
     -13
     3 7 + 3 4 5 * + - n              Note the unary minus, `n'
     13
     5 6 / 4 n +
     -3.166666667
     3 4 ^                            Exponentiation
     81
     ^D                               End-of-file indicator
     $


File: bison.info,  Node: Infix Calc,  Next: Simple Error Recovery,  Prev: RPN Calc,  Up: Examples

2.2 Infix Notation Calculator: `calc'
=====================================

We now modify rpcalc to handle infix operators instead of postfix.
Infix notation involves the concept of operator precedence and the need
for parentheses nested to arbitrary depth.  Here is the Bison code for
`calc.y', an infix desk-top calculator.

     /* Infix notation calculator.  */

     %{
       #define YYSTYPE double
       #include <math.h>
       #include <stdio.h>
       int yylex (void);
       void yyerror (char const *);
     %}

     /* Bison declarations.  */
     %token NUM
     %left '-' '+'
     %left '*' '/'
     %left NEG     /* negation--unary minus */
     %right '^'    /* exponentiation */

     %% /* The grammar follows.  */
     input:    /* empty */
             | input line
     ;

     line:     '\n'
             | exp '\n'  { printf ("\t%.10g\n", $1); }
     ;

     exp:      NUM                { $$ = $1;         }
             | exp '+' exp        { $$ = $1 + $3;    }
             | exp '-' exp        { $$ = $1 - $3;    }
             | exp '*' exp        { $$ = $1 * $3;    }
             | exp '/' exp        { $$ = $1 / $3;    }
             | '-' exp  %prec NEG { $$ = -$2;        }
             | exp '^' exp        { $$ = pow ($1, $3); }
             | '(' exp ')'        { $$ = $2;         }
     ;
     %%

The functions `yylex', `yyerror' and `main' can be the same as before.

   There are two important new features shown in this code.

   In the second section (Bison declarations), `%left' declares token
types and says they are left-associative operators.  The declarations
`%left' and `%right' (right associativity) take the place of `%token'
which is used to declare a token type name without associativity.
(These tokens are single-character literals, which ordinarily don't
need to be declared.  We declare them here to specify the
associativity.)

   Operator precedence is determined by the line ordering of the
declarations; the higher the line number of the declaration (lower on
the page or screen), the higher the precedence.  Hence, exponentiation
has the highest precedence, unary minus (`NEG') is next, followed by
`*' and `/', and so on.  *Note Operator Precedence: Precedence.

   The other important new feature is the `%prec' in the grammar
section for the unary minus operator.  The `%prec' simply instructs
Bison that the rule `| '-' exp' has the same precedence as `NEG'--in
this case the next-to-highest.  *Note Context-Dependent Precedence:
Contextual Precedence.

   Here is a sample run of `calc.y':

     $ calc
     4 + 4.5 - (34/(8*3+-3))
     6.880952381
     -56 + 2
     -54
     3 ^ 2
     9


File: bison.info,  Node: Simple Error Recovery,  Next: Location Tracking Calc,  Prev: Infix Calc,  Up: Examples

2.3 Simple Error Recovery
=========================

Up to this point, this manual has not addressed the issue of "error
recovery"--how to continue parsing after the parser detects a syntax
error.  All we have handled is error reporting with `yyerror'.  Recall
that by default `yyparse' returns after calling `yyerror'.  This means
that an erroneous input line causes the calculator program to exit.
Now we show how to rectify this deficiency.

   The Bison language itself includes the reserved word `error', which
may be included in the grammar rules.  In the example below it has been
added to one of the alternatives for `line':

     line:     '\n'
             | exp '\n'   { printf ("\t%.10g\n", $1); }
             | error '\n' { yyerrok;                  }
     ;

   This addition to the grammar allows for simple error recovery in the
event of a syntax error.  If an expression that cannot be evaluated is
read, the error will be recognized by the third rule for `line', and
parsing will continue.  (The `yyerror' function is still called upon to
print its message as well.)  The action executes the statement
`yyerrok', a macro defined automatically by Bison; its meaning is that
error recovery is complete (*note Error Recovery::).  Note the
difference between `yyerrok' and `yyerror'; neither one is a misprint.

   This form of error recovery deals with syntax errors.  There are
other kinds of errors; for example, division by zero, which raises an
exception signal that is normally fatal.  A real calculator program
must handle this signal and use `longjmp' to return to `main' and
resume parsing input lines; it would also have to discard the rest of
the current line of input.  We won't discuss this issue further because
it is not specific to Bison programs.


File: bison.info,  Node: Location Tracking Calc,  Next: Multi-function Calc,  Prev: Simple Error Recovery,  Up: Examples

2.4 Location Tracking Calculator: `ltcalc'
==========================================

This example extends the infix notation calculator with location
tracking.  This feature will be used to improve the error messages.  For
the sake of clarity, this example is a simple integer calculator, since
most of the work needed to use locations will be done in the lexical
analyzer.

* Menu:

* Ltcalc Declarations::    Bison and C declarations for ltcalc.
* Ltcalc Rules::           Grammar rules for ltcalc, with explanations.
* Ltcalc Lexer::           The lexical analyzer.


File: bison.info,  Node: Ltcalc Declarations,  Next: Ltcalc Rules,  Up: Location Tracking Calc

2.4.1 Declarations for `ltcalc'
-------------------------------

The C and Bison declarations for the location tracking calculator are
the same as the declarations for the infix notation calculator.

     /* Location tracking calculator.  */

     %{
       #define YYSTYPE int
       #include <math.h>
       int yylex (void);
       void yyerror (char const *);
     %}

     /* Bison declarations.  */
     %token NUM

     %left '-' '+'
     %left '*' '/'
     %left NEG
     %right '^'

     %% /* The grammar follows.  */

Note there are no declarations specific to locations.  Defining a data
type for storing locations is not needed: we will use the type provided
by default (*note Data Types of Locations: Location Type.), which is a
four member structure with the following integer fields: `first_line',
`first_column', `last_line' and `last_column'.  By conventions, and in
accordance with the GNU Coding Standards and common practice, the line
and column count both start at 1.


File: bison.info,  Node: Ltcalc Rules,  Next: Ltcalc Lexer,  Prev: Ltcalc Declarations,  Up: Location Tracking Calc

2.4.2 Grammar Rules for `ltcalc'
--------------------------------

Whether handling locations or not has no effect on the syntax of your
language.  Therefore, grammar rules for this example will be very close
to those of the previous example: we will only modify them to benefit
from the new information.

   Here, we will use locations to report divisions by zero, and locate
the wrong expressions or subexpressions.

     input   : /* empty */
             | input line
     ;

     line    : '\n'
             | exp '\n' { printf ("%d\n", $1); }
     ;

     exp     : NUM           { $$ = $1; }
             | exp '+' exp   { $$ = $1 + $3; }
             | exp '-' exp   { $$ = $1 - $3; }
             | exp '*' exp   { $$ = $1 * $3; }
             | exp '/' exp
                 {
                   if ($3)
                     $$ = $1 / $3;
                   else
                     {
                       $$ = 1;
                       fprintf (stderr, "%d.%d-%d.%d: division by zero",
                                @3.first_line, @3.first_column,
                                @3.last_line, @3.last_column);
                     }
                 }
             | '-' exp %prec NEG     { $$ = -$2; }
             | exp '^' exp           { $$ = pow ($1, $3); }
             | '(' exp ')'           { $$ = $2; }

   This code shows how to reach locations inside of semantic actions, by
using the pseudo-variables `@N' for rule components, and the
pseudo-variable `@$' for groupings.

   We don't need to assign a value to `@$': the output parser does it
automatically.  By default, before executing the C code of each action,
`@$' is set to range from the beginning of `@1' to the end of `@N', for
a rule with N components.  This behavior can be redefined (*note
Default Action for Locations: Location Default Action.), and for very
specific rules, `@$' can be computed by hand.


File: bison.info,  Node: Ltcalc Lexer,  Prev: Ltcalc Rules,  Up: Location Tracking Calc

2.4.3 The `ltcalc' Lexical Analyzer.
------------------------------------

Until now, we relied on Bison's defaults to enable location tracking.
The next step is to rewrite the lexical analyzer, and make it able to
feed the parser with the token locations, as it already does for
semantic values.

   To this end, we must take into account every single character of the
input text, to avoid the computed locations of being fuzzy or wrong:

     int
     yylex (void)
     {
       int c;

       /* Skip white space.  */
       while ((c = getchar ()) == ' ' || c == '\t')
         ++yylloc.last_column;

       /* Step.  */
       yylloc.first_line = yylloc.last_line;
       yylloc.first_column = yylloc.last_column;

       /* Process numbers.  */
       if (isdigit (c))
         {
           yylval = c - '0';
           ++yylloc.last_column;
           while (isdigit (c = getchar ()))
             {
               ++yylloc.last_column;
               yylval = yylval * 10 + c - '0';
             }
           ungetc (c, stdin);
           return NUM;
         }

       /* Return end-of-input.  */
       if (c == EOF)
         return 0;

       /* Return a single char, and update location.  */
       if (c == '\n')
         {
           ++yylloc.last_line;
           yylloc.last_column = 0;
         }
       else
         ++yylloc.last_column;
       return c;
     }

   Basically, the lexical analyzer performs the same processing as
before: it skips blanks and tabs, and reads numbers or single-character
tokens.  In addition, it updates `yylloc', the global variable (of type
`YYLTYPE') containing the token's location.

   Now, each time this function returns a token, the parser has its
number as well as its semantic value, and its location in the text.
The last needed change is to initialize `yylloc', for example in the
controlling function:

     int
     main (void)
     {
       yylloc.first_line = yylloc.last_line = 1;
       yylloc.first_column = yylloc.last_column = 0;
       return yyparse ();
     }

   Remember that computing locations is not a matter of syntax.  Every
character must be associated to a location update, whether it is in
valid input, in comments, in literal strings, and so on.


File: bison.info,  Node: Multi-function Calc,  Next: Exercises,  Prev: Location Tracking Calc,  Up: Examples

2.5 Multi-Function Calculator: `mfcalc'
=======================================

Now that the basics of Bison have been discussed, it is time to move on
to a more advanced problem.  The above calculators provided only five
functions, `+', `-', `*', `/' and `^'.  It would be nice to have a
calculator that provides other mathematical functions such as `sin',
`cos', etc.

   It is easy to add new operators to the infix calculator as long as
they are only single-character literals.  The lexical analyzer `yylex'
passes back all nonnumeric characters as tokens, so new grammar rules
suffice for adding a new operator.  But we want something more
flexible: built-in functions whose syntax has this form:

     FUNCTION_NAME (ARGUMENT)

At the same time, we will add memory to the calculator, by allowing you
to create named variables, store values in them, and use them later.
Here is a sample session with the multi-function calculator:

     $ mfcalc
     pi = 3.141592653589
     3.1415926536
     sin(pi)
     0.0000000000
     alpha = beta1 = 2.3
     2.3000000000
     alpha
     2.3000000000
     ln(alpha)
     0.8329091229
     exp(ln(beta1))
     2.3000000000
     $

   Note that multiple assignment and nested function calls are
permitted.

* Menu:

* Mfcalc Declarations::    Bison declarations for multi-function calculator.
* Mfcalc Rules::           Grammar rules for the calculator.
* Mfcalc Symbol Table::    Symbol table management subroutines.


File: bison.info,  Node: Mfcalc Declarations,  Next: Mfcalc Rules,  Up: Multi-function Calc

2.5.1 Declarations for `mfcalc'
-------------------------------

Here are the C and Bison declarations for the multi-function calculator.

     %{
       #include <math.h>  /* For math functions, cos(), sin(), etc.  */
       #include "calc.h"  /* Contains definition of `symrec'.  */
       int yylex (void);
       void yyerror (char const *);
     %}
     %union {
       double    val;   /* For returning numbers.  */
       symrec  *tptr;   /* For returning symbol-table pointers.  */
     }
     %token <val>  NUM        /* Simple double precision number.  */
     %token <tptr> VAR FNCT   /* Variable and Function.  */
     %type  <val>  exp

     %right '='
     %left '-' '+'
     %left '*' '/'
     %left NEG     /* negation--unary minus */
     %right '^'    /* exponentiation */
     %% /* The grammar follows.  */

   The above grammar introduces only two new features of the Bison
language.  These features allow semantic values to have various data
types (*note More Than One Value Type: Multiple Types.).

   The `%union' declaration specifies the entire list of possible types;
this is instead of defining `YYSTYPE'.  The allowable types are now
double-floats (for `exp' and `NUM') and pointers to entries in the
symbol table.  *Note The Collection of Value Types: Union Decl.

   Since values can now have various types, it is necessary to
associate a type with each grammar symbol whose semantic value is used.
These symbols are `NUM', `VAR', `FNCT', and `exp'.  Their declarations
are augmented with information about their data type (placed between
angle brackets).

   The Bison construct `%type' is used for declaring nonterminal
symbols, just as `%token' is used for declaring token types.  We have
not used `%type' before because nonterminal symbols are normally
declared implicitly by the rules that define them.  But `exp' must be
declared explicitly so we can specify its value type.  *Note
Nonterminal Symbols: Type Decl.


File: bison.info,  Node: Mfcalc Rules,  Next: Mfcalc Symbol Table,  Prev: Mfcalc Declarations,  Up: Multi-function Calc

2.5.2 Grammar Rules for `mfcalc'
--------------------------------

Here are the grammar rules for the multi-function calculator.  Most of
them are copied directly from `calc'; three rules, those which mention
`VAR' or `FNCT', are new.

     input:   /* empty */
             | input line
     ;

     line:
               '\n'
             | exp '\n'   { printf ("\t%.10g\n", $1); }
             | error '\n' { yyerrok;                  }
     ;

     exp:      NUM                { $$ = $1;                         }
             | VAR                { $$ = $1->value.var;              }
             | VAR '=' exp        { $$ = $3; $1->value.var = $3;     }
             | FNCT '(' exp ')'   { $$ = (*($1->value.fnctptr))($3); }
             | exp '+' exp        { $$ = $1 + $3;                    }
             | exp '-' exp        { $$ = $1 - $3;                    }
             | exp '*' exp        { $$ = $1 * $3;                    }
             | exp '/' exp        { $$ = $1 / $3;                    }
             | '-' exp  %prec NEG { $$ = -$2;                        }
             | exp '^' exp        { $$ = pow ($1, $3);               }
             | '(' exp ')'        { $$ = $2;                         }
     ;
     /* End of grammar.  */
     %%


File: bison.info,  Node: Mfcalc Symbol Table,  Prev: Mfcalc Rules,  Up: Multi-function Calc

2.5.3 The `mfcalc' Symbol Table
-------------------------------

The multi-function calculator requires a symbol table to keep track of
the names and meanings of variables and functions.  This doesn't affect
the grammar rules (except for the actions) or the Bison declarations,
but it requires some additional C functions for support.

   The symbol table itself consists of a linked list of records.  Its
definition, which is kept in the header `calc.h', is as follows.  It
provides for either functions or variables to be placed in the table.

     /* Function type.  */
     typedef double (*func_t) (double);

     /* Data type for links in the chain of symbols.  */
     struct symrec
     {
       char *name;  /* name of symbol */
       int type;    /* type of symbol: either VAR or FNCT */
       union
       {
         double var;      /* value of a VAR */
         func_t fnctptr;  /* value of a FNCT */
       } value;
       struct symrec *next;  /* link field */
     };

     typedef struct symrec symrec;

     /* The symbol table: a chain of `struct symrec'.  */
     extern symrec *sym_table;

     symrec *putsym (char const *, int);
     symrec *getsym (char const *);

   The new version of `main' includes a call to `init_table', a
function that initializes the symbol table.  Here it is, and
`init_table' as well:

     #include <stdio.h>

     /* Called by yyparse on error.  */
     void
     yyerror (char const *s)
     {
       printf ("%s\n", s);
     }

     struct init
     {
       char const *fname;
       double (*fnct) (double);
     };

     struct init const arith_fncts[] =
     {
       "sin",  sin,
       "cos",  cos,
       "atan", atan,
       "ln",   log,
       "exp",  exp,
       "sqrt", sqrt,
       0, 0
     };

     /* The symbol table: a chain of `struct symrec'.  */
     symrec *sym_table;

     /* Put arithmetic functions in table.  */
     void
     init_table (void)
     {
       int i;
       symrec *ptr;
       for (i = 0; arith_fncts[i].fname != 0; i++)
         {
           ptr = putsym (arith_fncts[i].fname, FNCT);
           ptr->value.fnctptr = arith_fncts[i].fnct;
         }
     }

     int
     main (void)
     {
       init_table ();
       return yyparse ();
     }

   By simply editing the initialization list and adding the necessary
include files, you can add additional functions to the calculator.

   Two important functions allow look-up and installation of symbols in
the symbol table.  The function `putsym' is passed a name and the type
(`VAR' or `FNCT') of the object to be installed.  The object is linked
to the front of the list, and a pointer to the object is returned.  The
function `getsym' is passed the name of the symbol to look up.  If
found, a pointer to that symbol is returned; otherwise zero is returned.

     symrec *
     putsym (char const *sym_name, int sym_type)
     {
       symrec *ptr;
       ptr = (symrec *) malloc (sizeof (symrec));
       ptr->name = (char *) malloc (strlen (sym_name) + 1);
       strcpy (ptr->name,sym_name);
       ptr->type = sym_type;
       ptr->value.var = 0; /* Set value to 0 even if fctn.  */
       ptr->next = (struct symrec *)sym_table;
       sym_table = ptr;
       return ptr;
     }

     symrec *
     getsym (char const *sym_name)
     {
       symrec *ptr;
       for (ptr = sym_table; ptr != (symrec *) 0;
            ptr = (symrec *)ptr->next)
         if (strcmp (ptr->name,sym_name) == 0)
           return ptr;
       return 0;
     }

   The function `yylex' must now recognize variables, numeric values,
and the single-character arithmetic operators.  Strings of alphanumeric
characters with a leading letter are recognized as either variables or
functions depending on what the symbol table says about them.

   The string is passed to `getsym' for look up in the symbol table.  If
the name appears in the table, a pointer to its location and its type
(`VAR' or `FNCT') is returned to `yyparse'.  If it is not already in
the table, then it is installed as a `VAR' using `putsym'.  Again, a
pointer and its type (which must be `VAR') is returned to `yyparse'.

   No change is needed in the handling of numeric values and arithmetic
operators in `yylex'.

     #include <ctype.h>

     int
     yylex (void)
     {
       int c;

       /* Ignore white space, get first nonwhite character.  */
       while ((c = getchar ()) == ' ' || c == '\t');

       if (c == EOF)
         return 0;

       /* Char starts a number => parse the number.         */
       if (c == '.' || isdigit (c))
         {
           ungetc (c, stdin);
           scanf ("%lf", &yylval.val);
           return NUM;
         }

       /* Char starts an identifier => read the name.       */
       if (isalpha (c))
         {
           symrec *s;
           static char *symbuf = 0;
           static int length = 0;
           int i;

           /* Initially make the buffer long enough
              for a 40-character symbol name.  */
           if (length == 0)
             length = 40, symbuf = (char *)malloc (length + 1);

           i = 0;
           do
             {
               /* If buffer is full, make it bigger.        */
               if (i == length)
                 {
                   length *= 2;
                   symbuf = (char *) realloc (symbuf, length + 1);
                 }
               /* Add this character to the buffer.         */
               symbuf[i++] = c;
               /* Get another character.                    */
               c = getchar ();
             }
           while (isalnum (c));

           ungetc (c, stdin);
           symbuf[i] = '\0';

           s = getsym (symbuf);
           if (s == 0)
             s = putsym (symbuf, VAR);
           yylval.tptr = s;
           return s->type;
         }

       /* Any other character is a token by itself.        */
       return c;
     }

   This program is both powerful and flexible.  You may easily add new
functions, and it is a simple job to modify this code to install
predefined variables such as `pi' or `e' as well.


File: bison.info,  Node: Exercises,  Prev: Multi-function Calc,  Up: Examples

2.6 Exercises
=============

  1. Add some new functions from `math.h' to the initialization list.

  2. Add another array that contains constants and their values.  Then
     modify `init_table' to add these constants to the symbol table.
     It will be easiest to give the constants type `VAR'.

  3. Make the program report an error if the user refers to an
     uninitialized variable in any way except to store a value in it.


File: bison.info,  Node: Grammar File,  Next: Interface,  Prev: Examples,  Up: Top

3 Bison Grammar Files
*********************

Bison takes as input a context-free grammar specification and produces a
C-language function that recognizes correct instances of the grammar.

   The Bison grammar input file conventionally has a name ending in
`.y'.  *Note Invoking Bison: Invocation.

* Menu:

* Grammar Outline::   Overall layout of the grammar file.
* Symbols::           Terminal and nonterminal symbols.
* Rules::             How to write grammar rules.
* Recursion::         Writing recursive rules.
* Semantics::         Semantic values and actions.
* Locations::         Locations and actions.
* Declarations::      All kinds of Bison declarations are described here.
* Multiple Parsers::  Putting more than one Bison parser in one program.


File: bison.info,  Node: Grammar Outline,  Next: Symbols,  Up: Grammar File

3.1 Outline of a Bison Grammar
==============================

A Bison grammar file has four main sections, shown here with the
appropriate delimiters:

     %{
       PROLOGUE
     %}

     BISON DECLARATIONS

     %%
     GRAMMAR RULES
     %%

     EPILOGUE

   Comments enclosed in `/* ... */' may appear in any of the sections.
As a GNU extension, `//' introduces a comment that continues until end
of line.

* Menu:

* Prologue::              Syntax and usage of the prologue.
* Prologue Alternatives:: Syntax and usage of alternatives to the prologue.
* Bison Declarations::    Syntax and usage of the Bison declarations section.
* Grammar Rules::         Syntax and usage of the grammar rules section.
* Epilogue::              Syntax and usage of the epilogue.


File: bison.info,  Node: Prologue,  Next: Prologue Alternatives,  Up: Grammar Outline

3.1.1 The prologue
------------------

The PROLOGUE section contains macro definitions and declarations of
functions and variables that are used in the actions in the grammar
rules.  These are copied to the beginning of the parser file so that
they precede the definition of `yyparse'.  You can use `#include' to
get the declarations from a header file.  If you don't need any C
declarations, you may omit the `%{' and `%}' delimiters that bracket
this section.

   The PROLOGUE section is terminated by the first occurrence of `%}'
that is outside a comment, a string literal, or a character constant.

   You may have more than one PROLOGUE section, intermixed with the
BISON DECLARATIONS.  This allows you to have C and Bison declarations
that refer to each other.  For example, the `%union' declaration may
use types defined in a header file, and you may wish to prototype
functions that take arguments of type `YYSTYPE'.  This can be done with
two PROLOGUE blocks, one before and one after the `%union' declaration.

     %{
       #define _GNU_SOURCE
       #include <stdio.h>
       #include "ptypes.h"
     %}

     %union {
       long int n;
       tree t;  /* `tree' is defined in `ptypes.h'. */
     }

     %{
       static void print_token_value (FILE *, int, YYSTYPE);
       #define YYPRINT(F, N, L) print_token_value (F, N, L)
     %}

     ...

   When in doubt, it is usually safer to put prologue code before all
Bison declarations, rather than after.  For example, any definitions of
feature test macros like `_GNU_SOURCE' or `_POSIX_C_SOURCE' should
appear before all Bison declarations, as feature test macros can affect
the behavior of Bison-generated `#include' directives.


File: bison.info,  Node: Prologue Alternatives,  Next: Bison Declarations,  Prev: Prologue,  Up: Grammar Outline

3.1.2 Prologue Alternatives
---------------------------

(The prologue alternatives described here are experimental.  More user
feedback will help to determine whether they should become permanent
features.)

   The functionality of PROLOGUE sections can often be subtle and
inflexible.  As an alternative, Bison provides a %code directive with
an explicit qualifier field, which identifies the purpose of the code
and thus the location(s) where Bison should generate it.  For C/C++,
the qualifier can be omitted for the default location, or it can be one
of `requires', `provides', `top'.  *Note %code: Decl Summary.

   Look again at the example of the previous section:

     %{
       #define _GNU_SOURCE
       #include <stdio.h>
       #include "ptypes.h"
     %}

     %union {
       long int n;
       tree t;  /* `tree' is defined in `ptypes.h'. */
     }

     %{
       static void print_token_value (FILE *, int, YYSTYPE);
       #define YYPRINT(F, N, L) print_token_value (F, N, L)
     %}

     ...

Notice that there are two PROLOGUE sections here, but there's a subtle
distinction between their functionality.  For example, if you decide to
override Bison's default definition for `YYLTYPE', in which PROLOGUE
section should you write your new definition?  You should write it in
the first since Bison will insert that code into the parser source code
file _before_ the default `YYLTYPE' definition.  In which PROLOGUE
section should you prototype an internal function, `trace_token', that
accepts `YYLTYPE' and `yytokentype' as arguments?  You should prototype
it in the second since Bison will insert that code _after_ the
`YYLTYPE' and `yytokentype' definitions.

   This distinction in functionality between the two PROLOGUE sections
is established by the appearance of the `%union' between them.  This
behavior raises a few questions.  First, why should the position of a
`%union' affect definitions related to `YYLTYPE' and `yytokentype'?
Second, what if there is no `%union'?  In that case, the second kind of
PROLOGUE section is not available.  This behavior is not intuitive.

   To avoid this subtle `%union' dependency, rewrite the example using a
`%code top' and an unqualified `%code'.  Let's go ahead and add the new
`YYLTYPE' definition and the `trace_token' prototype at the same time:

     %code top {
       #define _GNU_SOURCE
       #include <stdio.h>

       /* WARNING: The following code really belongs
        * in a `%code requires'; see below.  */

       #include "ptypes.h"
       #define YYLTYPE YYLTYPE
       typedef struct YYLTYPE
       {
         int first_line;
         int first_column;
         int last_line;
         int last_column;
         char *filename;
       } YYLTYPE;
     }

     %union {
       long int n;
       tree t;  /* `tree' is defined in `ptypes.h'. */
     }

     %code {
       static void print_token_value (FILE *, int, YYSTYPE);
       #define YYPRINT(F, N, L) print_token_value (F, N, L)
       static void trace_token (enum yytokentype token, YYLTYPE loc);
     }

     ...

In this way, `%code top' and the unqualified `%code' achieve the same
functionality as the two kinds of PROLOGUE sections, but it's always
explicit which kind you intend.  Moreover, both kinds are always
available even in the absence of `%union'.

   The `%code top' block above logically contains two parts.  The first
two lines before the warning need to appear near the top of the parser
source code file.  The first line after the warning is required by
`YYSTYPE' and thus also needs to appear in the parser source code file.
However, if you've instructed Bison to generate a parser header file
(*note %defines: Decl Summary.), you probably want that line to appear
before the `YYSTYPE' definition in that header file as well.  The
`YYLTYPE' definition should also appear in the parser header file to
override the default `YYLTYPE' definition there.

   In other words, in the `%code top' block above, all but the first two
lines are dependency code required by the `YYSTYPE' and `YYLTYPE'
definitions.  Thus, they belong in one or more `%code requires':

     %code top {
       #define _GNU_SOURCE
       #include <stdio.h>
     }

     %code requires {
       #include "ptypes.h"
     }
     %union {
       long int n;
       tree t;  /* `tree' is defined in `ptypes.h'. */
     }

     %code requires {
       #define YYLTYPE YYLTYPE
       typedef struct YYLTYPE
       {
         int first_line;
         int first_column;
         int last_line;
         int last_column;
         char *filename;
       } YYLTYPE;
     }

     %code {
       static void print_token_value (FILE *, int, YYSTYPE);
       #define YYPRINT(F, N, L) print_token_value (F, N, L)
       static void trace_token (enum yytokentype token, YYLTYPE loc);
     }

     ...

Now Bison will insert `#include "ptypes.h"' and the new `YYLTYPE'
definition before the Bison-generated `YYSTYPE' and `YYLTYPE'
definitions in both the parser source code file and the parser header
file.  (By the same reasoning, `%code requires' would also be the
appropriate place to write your own definition for `YYSTYPE'.)

   When you are writing dependency code for `YYSTYPE' and `YYLTYPE', you
should prefer `%code requires' over `%code top' regardless of whether
you instruct Bison to generate a parser header file.  When you are
writing code that you need Bison to insert only into the parser source
code file and that has no special need to appear at the top of that
file, you should prefer the unqualified `%code' over `%code top'.
These practices will make the purpose of each block of your code
explicit to Bison and to other developers reading your grammar file.
Following these practices, we expect the unqualified `%code' and `%code
requires' to be the most important of the four PROLOGUE alternatives.

   At some point while developing your parser, you might decide to
provide `trace_token' to modules that are external to your parser.
Thus, you might wish for Bison to insert the prototype into both the
parser header file and the parser source code file.  Since this
function is not a dependency required by `YYSTYPE' or `YYLTYPE', it
doesn't make sense to move its prototype to a `%code requires'.  More
importantly, since it depends upon `YYLTYPE' and `yytokentype', `%code
requires' is not sufficient.  Instead, move its prototype from the
unqualified `%code' to a `%code provides':

     %code top {
       #define _GNU_SOURCE
       #include <stdio.h>
     }

     %code requires {
       #include "ptypes.h"
     }
     %union {
       long int n;
       tree t;  /* `tree' is defined in `ptypes.h'. */
     }

     %code requires {
       #define YYLTYPE YYLTYPE
       typedef struct YYLTYPE
       {
         int first_line;
         int first_column;
         int last_line;
         int last_column;
         char *filename;
       } YYLTYPE;
     }

     %code provides {
       void trace_token (enum yytokentype token, YYLTYPE loc);
     }

     %code {
       static void print_token_value (FILE *, int, YYSTYPE);
       #define YYPRINT(F, N, L) print_token_value (F, N, L)
     }

     ...

Bison will insert the `trace_token' prototype into both the parser
header file and the parser source code file after the definitions for
`yytokentype', `YYLTYPE', and `YYSTYPE'.

   The above examples are careful to write directives in an order that
reflects the layout of the generated parser source code and header
files: `%code top', `%code requires', `%code provides', and then
`%code'.  While your grammar files may generally be easier to read if
you also follow this order, Bison does not require it.  Instead, Bison
lets you choose an organization that makes sense to you.

   You may declare any of these directives multiple times in the
grammar file.  In that case, Bison concatenates the contained code in
declaration order.  This is the only way in which the position of one
of these directives within the grammar file affects its functionality.

   The result of the previous two properties is greater flexibility in
how you may organize your grammar file.  For example, you may organize
semantic-type-related directives by semantic type:

     %code requires { #include "type1.h" }
     %union { type1 field1; }
     %destructor { type1_free ($$); } <field1>
     %printer { type1_print ($$); } <field1>

     %code requires { #include "type2.h" }
     %union { type2 field2; }
     %destructor { type2_free ($$); } <field2>
     %printer { type2_print ($$); } <field2>

You could even place each of the above directive groups in the rules
section of the grammar file next to the set of rules that uses the
associated semantic type.  (In the rules section, you must terminate
each of those directives with a semicolon.)  And you don't have to
worry that some directive (like a `%union') in the definitions section
is going to adversely affect their functionality in some
counter-intuitive manner just because it comes first.  Such an
organization is not possible using PROLOGUE sections.

   This section has been concerned with explaining the advantages of
the four PROLOGUE alternatives over the original Yacc PROLOGUE.
However, in most cases when using these directives, you shouldn't need
to think about all the low-level ordering issues discussed here.
Instead, you should simply use these directives to label each block of
your code according to its purpose and let Bison handle the ordering.
`%code' is the most generic label.  Move code to `%code requires',
`%code provides', or `%code top' as needed.


File: bison.info,  Node: Bison Declarations,  Next: Grammar Rules,  Prev: Prologue Alternatives,  Up: Grammar Outline

3.1.3 The Bison Declarations Section
------------------------------------

The BISON DECLARATIONS section contains declarations that define
terminal and nonterminal symbols, specify precedence, and so on.  In
some simple grammars you may not need any declarations.  *Note Bison
Declarations: Declarations.


File: bison.info,  Node: Grammar Rules,  Next: Epilogue,  Prev: Bison Declarations,  Up: Grammar Outline

3.1.4 The Grammar Rules Section
-------------------------------

The "grammar rules" section contains one or more Bison grammar rules,
and nothing else.  *Note Syntax of Grammar Rules: Rules.

   There must always be at least one grammar rule, and the first `%%'
(which precedes the grammar rules) may never be omitted even if it is
the first thing in the file.


File: bison.info,  Node: Epilogue,  Prev: Grammar Rules,  Up: Grammar Outline

3.1.5 The epilogue
------------------

The EPILOGUE is copied verbatim to the end of the parser file, just as
the PROLOGUE is copied to the beginning.  This is the most convenient
place to put anything that you want to have in the parser file but
which need not come before the definition of `yyparse'.  For example,
the definitions of `yylex' and `yyerror' often go here.  Because C
requires functions to be declared before being used, you often need to
declare functions like `yylex' and `yyerror' in the Prologue, even if
you define them in the Epilogue.  *Note Parser C-Language Interface:
Interface.

   If the last section is empty, you may omit the `%%' that separates it
from the grammar rules.

   The Bison parser itself contains many macros and identifiers whose
names start with `yy' or `YY', so it is a good idea to avoid using any
such names (except those documented in this manual) in the epilogue of
the grammar file.


File: bison.info,  Node: Symbols,  Next: Rules,  Prev: Grammar Outline,  Up: Grammar File

3.2 Symbols, Terminal and Nonterminal
=====================================

"Symbols" in Bison grammars represent the grammatical classifications
of the language.

   A "terminal symbol" (also known as a "token type") represents a
class of syntactically equivalent tokens.  You use the symbol in grammar
rules to mean that a token in that class is allowed.  The symbol is
represented in the Bison parser by a numeric code, and the `yylex'
function returns a token type code to indicate what kind of token has
been read.  You don't need to know what the code value is; you can use
the symbol to stand for it.

   A "nonterminal symbol" stands for a class of syntactically
equivalent groupings.  The symbol name is used in writing grammar rules.
By convention, it should be all lower case.

   Symbol names can contain letters, digits (not at the beginning),
underscores and periods.  Periods make sense only in nonterminals.

   There are three ways of writing terminal symbols in the grammar:

   * A "named token type" is written with an identifier, like an
     identifier in C.  By convention, it should be all upper case.  Each
     such name must be defined with a Bison declaration such as
     `%token'.  *Note Token Type Names: Token Decl.

   * A "character token type" (or "literal character token") is written
     in the grammar using the same syntax used in C for character
     constants; for example, `'+'' is a character token type.  A
     character token type doesn't need to be declared unless you need to
     specify its semantic value data type (*note Data Types of Semantic
     Values: Value Type.), associativity, or precedence (*note Operator
     Precedence: Precedence.).

     By convention, a character token type is used only to represent a
     token that consists of that particular character.  Thus, the token
     type `'+'' is used to represent the character `+' as a token.
     Nothing enforces this convention, but if you depart from it, your
     program will confuse other readers.

     All the usual escape sequences used in character literals in C can
     be used in Bison as well, but you must not use the null character
     as a character literal because its numeric code, zero, signifies
     end-of-input (*note Calling Convention for `yylex': Calling
     Convention.).  Also, unlike standard C, trigraphs have no special
     meaning in Bison character literals, nor is backslash-newline
     allowed.

   * A "literal string token" is written like a C string constant; for
     example, `"<="' is a literal string token.  A literal string token
     doesn't need to be declared unless you need to specify its semantic
     value data type (*note Value Type::), associativity, or precedence
     (*note Precedence::).

     You can associate the literal string token with a symbolic name as
     an alias, using the `%token' declaration (*note Token
     Declarations: Token Decl.).  If you don't do that, the lexical
     analyzer has to retrieve the token number for the literal string
     token from the `yytname' table (*note Calling Convention::).

     *Warning*: literal string tokens do not work in Yacc.

     By convention, a literal string token is used only to represent a
     token that consists of that particular string.  Thus, you should
     use the token type `"<="' to represent the string `<=' as a token.
     Bison does not enforce this convention, but if you depart from
     it, people who read your program will be confused.

     All the escape sequences used in string literals in C can be used
     in Bison as well, except that you must not use a null character
     within a string literal.  Also, unlike Standard C, trigraphs have
     no special meaning in Bison string literals, nor is
     backslash-newline allowed.  A literal string token must contain
     two or more characters; for a token containing just one character,
     use a character token (see above).

   How you choose to write a terminal symbol has no effect on its
grammatical meaning.  That depends only on where it appears in rules and
on when the parser function returns that symbol.

   The value returned by `yylex' is always one of the terminal symbols,
except that a zero or negative value signifies end-of-input.  Whichever
way you write the token type in the grammar rules, you write it the
same way in the definition of `yylex'.  The numeric code for a
character token type is simply the positive numeric code of the
character, so `yylex' can use the identical value to generate the
requisite code, though you may need to convert it to `unsigned char' to
avoid sign-extension on hosts where `char' is signed.  Each named token
type becomes a C macro in the parser file, so `yylex' can use the name
to stand for the code.  (This is why periods don't make sense in
terminal symbols.)  *Note Calling Convention for `yylex': Calling
Convention.

   If `yylex' is defined in a separate file, you need to arrange for the
token-type macro definitions to be available there.  Use the `-d'
option when you run Bison, so that it will write these macro definitions
into a separate header file `NAME.tab.h' which you can include in the
other source files that need it.  *Note Invoking Bison: Invocation.

   If you want to write a grammar that is portable to any Standard C
host, you must use only nonnull character tokens taken from the basic
execution character set of Standard C.  This set consists of the ten
digits, the 52 lower- and upper-case English letters, and the
characters in the following C-language string:

     "\a\b\t\n\v\f\r !\"#%&'()*+,-./:;<=>?[\\]^_{|}~"

   The `yylex' function and Bison must use a consistent character set
and encoding for character tokens.  For example, if you run Bison in an
ASCII environment, but then compile and run the resulting program in an
environment that uses an incompatible character set like EBCDIC, the
resulting program may not work because the tables generated by Bison
will assume ASCII numeric values for character tokens.  It is standard
practice for software distributions to contain C source files that were
generated by Bison in an ASCII environment, so installers on platforms
that are incompatible with ASCII must rebuild those files before
compiling them.

   The symbol `error' is a terminal symbol reserved for error recovery
(*note Error Recovery::); you shouldn't use it for any other purpose.
In particular, `yylex' should never return this value.  The default
value of the error token is 256, unless you explicitly assigned 256 to
one of your tokens with a `%token' declaration.


File: bison.info,  Node: Rules,  Next: Recursion,  Prev: Symbols,  Up: Grammar File

3.3 Syntax of Grammar Rules
===========================

A Bison grammar rule has the following general form:

     RESULT: COMPONENTS...
             ;

where RESULT is the nonterminal symbol that this rule describes, and
COMPONENTS are various terminal and nonterminal symbols that are put
together by this rule (*note Symbols::).

   For example,

     exp:      exp '+' exp
             ;

says that two groupings of type `exp', with a `+' token in between, can
be combined into a larger grouping of type `exp'.

   White space in rules is significant only to separate symbols.  You
can add extra white space as you wish.

   Scattered among the components can be ACTIONS that determine the
semantics of the rule.  An action looks like this:

     {C STATEMENTS}

This is an example of "braced code", that is, C code surrounded by
braces, much like a compound statement in C.  Braced code can contain
any sequence of C tokens, so long as its braces are balanced.  Bison
does not check the braced code for correctness directly; it merely
copies the code to the output file, where the C compiler can check it.

   Within braced code, the balanced-brace count is not affected by
braces within comments, string literals, or character constants, but it
is affected by the C digraphs `<%' and `%>' that represent braces.  At
the top level braced code must be terminated by `}' and not by a
digraph.  Bison does not look for trigraphs, so if braced code uses
trigraphs you should ensure that they do not affect the nesting of
braces or the boundaries of comments, string literals, or character
constants.

   Usually there is only one action and it follows the components.
*Note Actions::.

   Multiple rules for the same RESULT can be written separately or can
be joined with the vertical-bar character `|' as follows:

     RESULT:    RULE1-COMPONENTS...
             | RULE2-COMPONENTS...
             ...
             ;

They are still considered distinct rules even when joined in this way.

   If COMPONENTS in a rule is empty, it means that RESULT can match the
empty string.  For example, here is how to define a comma-separated
sequence of zero or more `exp' groupings:

     expseq:   /* empty */
             | expseq1
             ;

     expseq1:  exp
             | expseq1 ',' exp
             ;

It is customary to write a comment `/* empty */' in each rule with no
components.


File: bison.info,  Node: Recursion,  Next: Semantics,  Prev: Rules,  Up: Grammar File

3.4 Recursive Rules
===================

A rule is called "recursive" when its RESULT nonterminal appears also
on its right hand side.  Nearly all Bison grammars need to use
recursion, because that is the only way to define a sequence of any
number of a particular thing.  Consider this recursive definition of a
comma-separated sequence of one or more expressions:

     expseq1:  exp
             | expseq1 ',' exp
             ;

Since the recursive use of `expseq1' is the leftmost symbol in the
right hand side, we call this "left recursion".  By contrast, here the
same construct is defined using "right recursion":

     expseq1:  exp
             | exp ',' expseq1
             ;

Any kind of sequence can be defined using either left recursion or right
recursion, but you should always use left recursion, because it can
parse a sequence of any number of elements with bounded stack space.
Right recursion uses up space on the Bison stack in proportion to the
number of elements in the sequence, because all the elements must be
shifted onto the stack before the rule can be applied even once.  *Note
The Bison Parser Algorithm: Algorithm, for further explanation of this.

   "Indirect" or "mutual" recursion occurs when the result of the rule
does not appear directly on its right hand side, but does appear in
rules for other nonterminals which do appear on its right hand side.

   For example:

     expr:     primary
             | primary '+' primary
             ;

     primary:  constant
             | '(' expr ')'
             ;

defines two mutually-recursive nonterminals, since each refers to the
other.


File: bison.info,  Node: Semantics,  Next: Locations,  Prev: Recursion,  Up: Grammar File

3.5 Defining Language Semantics
===============================

The grammar rules for a language determine only the syntax.  The
semantics are determined by the semantic values associated with various
tokens and groupings, and by the actions taken when various groupings
are recognized.

   For example, the calculator calculates properly because the value
associated with each expression is the proper number; it adds properly
because the action for the grouping `X + Y' is to add the numbers
associated with X and Y.

* Menu:

* Value Type::        Specifying one data type for all semantic values.
* Multiple Types::    Specifying several alternative data types.
* Actions::           An action is the semantic definition of a grammar rule.
* Action Types::      Specifying data types for actions to operate on.
* Mid-Rule Actions::  Most actions go at the end of a rule.
                      This says when, why and how to use the exceptional
                        action in the middle of a rule.


File: bison.info,  Node: Value Type,  Next: Multiple Types,  Up: Semantics

3.5.1 Data Types of Semantic Values
-----------------------------------

In a simple program it may be sufficient to use the same data type for
the semantic values of all language constructs.  This was true in the
RPN and infix calculator examples (*note Reverse Polish Notation
Calculator: RPN Calc.).

   Bison normally uses the type `int' for semantic values if your
program uses the same data type for all language constructs.  To
specify some other type, define `YYSTYPE' as a macro, like this:

     #define YYSTYPE double

`YYSTYPE''s replacement list should be a type name that does not
contain parentheses or square brackets.  This macro definition must go
in the prologue of the grammar file (*note Outline of a Bison Grammar:
Grammar Outline.).


File: bison.info,  Node: Multiple Types,  Next: Actions,  Prev: Value Type,  Up: Semantics

3.5.2 More Than One Value Type
------------------------------

In most programs, you will need different data types for different kinds
of tokens and groupings.  For example, a numeric constant may need type
`int' or `long int', while a string constant needs type `char *', and
an identifier might need a pointer to an entry in the symbol table.

   To use more than one data type for semantic values in one parser,
Bison requires you to do two things:

   * Specify the entire collection of possible data types, either by
     using the `%union' Bison declaration (*note The Collection of
     Value Types: Union Decl.), or by using a `typedef' or a `#define'
     to define `YYSTYPE' to be a union type whose member names are the
     type tags.

   * Choose one of those types for each symbol (terminal or
     nonterminal) for which semantic values are used.  This is done for
     tokens with the `%token' Bison declaration (*note Token Type
     Names: Token Decl.)  and for groupings with the `%type' Bison
     declaration (*note Nonterminal Symbols: Type Decl.).


File: bison.info,  Node: Actions,  Next: Action Types,  Prev: Multiple Types,  Up: Semantics

3.5.3 Actions
-------------

An action accompanies a syntactic rule and contains C code to be
executed each time an instance of that rule is recognized.  The task of
most actions is to compute a semantic value for the grouping built by
the rule from the semantic values associated with tokens or smaller
groupings.

   An action consists of braced code containing C statements, and can be
placed at any position in the rule; it is executed at that position.
Most rules have just one action at the end of the rule, following all
the components.  Actions in the middle of a rule are tricky and used
only for special purposes (*note Actions in Mid-Rule: Mid-Rule
Actions.).

   The C code in an action can refer to the semantic values of the
components matched by the rule with the construct `$N', which stands for
the value of the Nth component.  The semantic value for the grouping
being constructed is `$$'.  Bison translates both of these constructs
into expressions of the appropriate type when it copies the actions
into the parser file.  `$$' is translated to a modifiable lvalue, so it
can be assigned to.

   Here is a typical example:

     exp:    ...
             | exp '+' exp
                 { $$ = $1 + $3; }

This rule constructs an `exp' from two smaller `exp' groupings
connected by a plus-sign token.  In the action, `$1' and `$3' refer to
the semantic values of the two component `exp' groupings, which are the
first and third symbols on the right hand side of the rule.  The sum is
stored into `$$' so that it becomes the semantic value of the
addition-expression just recognized by the rule.  If there were a
useful semantic value associated with the `+' token, it could be
referred to as `$2'.

   Note that the vertical-bar character `|' is really a rule separator,
and actions are attached to a single rule.  This is a difference with
tools like Flex, for which `|' stands for either "or", or "the same
action as that of the next rule".  In the following example, the action
is triggered only when `b' is found:

     a-or-b: 'a'|'b'   { a_or_b_found = 1; };

   If you don't specify an action for a rule, Bison supplies a default:
`$$ = $1'.  Thus, the value of the first symbol in the rule becomes the
value of the whole rule.  Of course, the default action is valid only
if the two data types match.  There is no meaningful default action for
an empty rule; every empty rule must have an explicit action unless the
rule's value does not matter.

   `$N' with N zero or negative is allowed for reference to tokens and
groupings on the stack _before_ those that match the current rule.
This is a very risky practice, and to use it reliably you must be
certain of the context in which the rule is applied.  Here is a case in
which you can use this reliably:

     foo:      expr bar '+' expr  { ... }
             | expr bar '-' expr  { ... }
             ;

     bar:      /* empty */
             { previous_expr = $0; }
             ;

   As long as `bar' is used only in the fashion shown here, `$0' always
refers to the `expr' which precedes `bar' in the definition of `foo'.

   It is also possible to access the semantic value of the lookahead
token, if any, from a semantic action.  This semantic value is stored
in `yylval'.  *Note Special Features for Use in Actions: Action
Features.


File: bison.info,  Node: Action Types,  Next: Mid-Rule Actions,  Prev: Actions,  Up: Semantics

3.5.4 Data Types of Values in Actions
-------------------------------------

If you have chosen a single data type for semantic values, the `$$' and
`$N' constructs always have that data type.

   If you have used `%union' to specify a variety of data types, then
you must declare a choice among these types for each terminal or
nonterminal symbol that can have a semantic value.  Then each time you
use `$$' or `$N', its data type is determined by which symbol it refers
to in the rule.  In this example,

     exp:    ...
             | exp '+' exp
                 { $$ = $1 + $3; }

`$1' and `$3' refer to instances of `exp', so they all have the data
type declared for the nonterminal symbol `exp'.  If `$2' were used, it
would have the data type declared for the terminal symbol `'+'',
whatever that might be.

   Alternatively, you can specify the data type when you refer to the
value, by inserting `<TYPE>' after the `$' at the beginning of the
reference.  For example, if you have defined types as shown here:

     %union {
       int itype;
       double dtype;
     }

then you can write `$<itype>1' to refer to the first subunit of the
rule as an integer, or `$<dtype>1' to refer to it as a double.


File: bison.info,  Node: Mid-Rule Actions,  Prev: Action Types,  Up: Semantics

3.5.5 Actions in Mid-Rule
-------------------------

Occasionally it is useful to put an action in the middle of a rule.
These actions are written just like usual end-of-rule actions, but they
are executed before the parser even recognizes the following components.

   A mid-rule action may refer to the components preceding it using
`$N', but it may not refer to subsequent components because it is run
before they are parsed.

   The mid-rule action itself counts as one of the components of the
rule.  This makes a difference when there is another action later in
the same rule (and usually there is another at the end): you have to
count the actions along with the symbols when working out which number
N to use in `$N'.

   The mid-rule action can also have a semantic value.  The action can
set its value with an assignment to `$$', and actions later in the rule
can refer to the value using `$N'.  Since there is no symbol to name
the action, there is no way to declare a data type for the value in
advance, so you must use the `$<...>N' construct to specify a data type
each time you refer to this value.

   There is no way to set the value of the entire rule with a mid-rule
action, because assignments to `$$' do not have that effect.  The only
way to set the value for the entire rule is with an ordinary action at
the end of the rule.

   Here is an example from a hypothetical compiler, handling a `let'
statement that looks like `let (VARIABLE) STATEMENT' and serves to
create a variable named VARIABLE temporarily for the duration of
STATEMENT.  To parse this construct, we must put VARIABLE into the
symbol table while STATEMENT is parsed, then remove it afterward.  Here
is how it is done:

     stmt:   LET '(' var ')'
                     { $<context>$ = push_context ();
                       declare_variable ($3); }
             stmt    { $$ = $6;
                       pop_context ($<context>5); }

As soon as `let (VARIABLE)' has been recognized, the first action is
run.  It saves a copy of the current semantic context (the list of
accessible variables) as its semantic value, using alternative
`context' in the data-type union.  Then it calls `declare_variable' to
add the new variable to that list.  Once the first action is finished,
the embedded statement `stmt' can be parsed.  Note that the mid-rule
action is component number 5, so the `stmt' is component number 6.

   After the embedded statement is parsed, its semantic value becomes
the value of the entire `let'-statement.  Then the semantic value from
the earlier action is used to restore the prior list of variables.  This
removes the temporary `let'-variable from the list so that it won't
appear to exist while the rest of the program is parsed.

   In the above example, if the parser initiates error recovery (*note
Error Recovery::) while parsing the tokens in the embedded statement
`stmt', it might discard the previous semantic context `$<context>5'
without restoring it.  Thus, `$<context>5' needs a destructor (*note
Freeing Discarded Symbols: Destructor Decl.).  However, Bison currently
provides no means to declare a destructor specific to a particular
mid-rule action's semantic value.

   One solution is to bury the mid-rule action inside a nonterminal
symbol and to declare a destructor for that symbol:

     %type <context> let
     %destructor { pop_context ($$); } let

     %%

     stmt:  let stmt
                    { $$ = $2;
                      pop_context ($1); }
            ;

     let:   LET '(' var ')'
                    { $$ = push_context ();
                      declare_variable ($3); }
            ;

Note that the action is now at the end of its rule.  Any mid-rule
action can be converted to an end-of-rule action in this way, and this
is what Bison actually does to implement mid-rule actions.

   Taking action before a rule is completely recognized often leads to
conflicts since the parser must commit to a parse in order to execute
the action.  For example, the following two rules, without mid-rule
actions, can coexist in a working parser because the parser can shift
the open-brace token and look at what follows before deciding whether
there is a declaration or not:

     compound: '{' declarations statements '}'
             | '{' statements '}'
             ;

But when we add a mid-rule action as follows, the rules become
nonfunctional:

     compound: { prepare_for_local_variables (); }
               '{' declarations statements '}'
             | '{' statements '}'
             ;

Now the parser is forced to decide whether to run the mid-rule action
when it has read no farther than the open-brace.  In other words, it
must commit to using one rule or the other, without sufficient
information to do it correctly.  (The open-brace token is what is called
the "lookahead" token at this time, since the parser is still deciding
what to do about it.  *Note Lookahead Tokens: Lookahead.)

   You might think that you could correct the problem by putting
identical actions into the two rules, like this:

     compound: { prepare_for_local_variables (); }
               '{' declarations statements '}'
             | { prepare_for_local_variables (); }
               '{' statements '}'
             ;

But this does not help, because Bison does not realize that the two
actions are identical.  (Bison never tries to understand the C code in
an action.)

   If the grammar is such that a declaration can be distinguished from a
statement by the first token (which is true in C), then one solution
which does work is to put the action after the open-brace, like this:

     compound: '{' { prepare_for_local_variables (); }
               declarations statements '}'
             | '{' statements '}'
             ;

Now the first token of the following declaration or statement, which
would in any case tell Bison which rule to use, can still do so.

   Another solution is to bury the action inside a nonterminal symbol
which serves as a subroutine:

     subroutine: /* empty */
               { prepare_for_local_variables (); }
             ;

     compound: subroutine
               '{' declarations statements '}'
             | subroutine
               '{' statements '}'
             ;

Now Bison can execute the action in the rule for `subroutine' without
deciding which rule for `compound' it will eventually use.


File: bison.info,  Node: Locations,  Next: Declarations,  Prev: Semantics,  Up: Grammar File

3.6 Tracking Locations
======================

Though grammar rules and semantic actions are enough to write a fully
functional parser, it can be useful to process some additional
information, especially symbol locations.

   The way locations are handled is defined by providing a data type,
and actions to take when rules are matched.

* Menu:

* Location Type::               Specifying a data type for locations.
* Actions and Locations::       Using locations in actions.
* Location Default Action::     Defining a general way to compute locations.


File: bison.info,  Node: Location Type,  Next: Actions and Locations,  Up: Locations

3.6.1 Data Type of Locations
----------------------------

Defining a data type for locations is much simpler than for semantic
values, since all tokens and groupings always use the same type.

   You can specify the type of locations by defining a macro called
`YYLTYPE', just as you can specify the semantic value type by defining
a `YYSTYPE' macro (*note Value Type::).  When `YYLTYPE' is not defined,
Bison uses a default structure type with four members:

     typedef struct YYLTYPE
     {
       int first_line;
       int first_column;
       int last_line;
       int last_column;
     } YYLTYPE;

   At the beginning of the parsing, Bison initializes all these fields
to 1 for `yylloc'.


File: bison.info,  Node: Actions and Locations,  Next: Location Default Action,  Prev: Location Type,  Up: Locations

3.6.2 Actions and Locations
---------------------------

Actions are not only useful for defining language semantics, but also
for describing the behavior of the output parser with locations.

   The most obvious way for building locations of syntactic groupings
is very similar to the way semantic values are computed.  In a given
rule, several constructs can be used to access the locations of the
elements being matched.  The location of the Nth component of the right
hand side is `@N', while the location of the left hand side grouping is
`@$'.

   Here is a basic example using the default data type for locations:

     exp:    ...
             | exp '/' exp
                 {
                   @$.first_column = @1.first_column;
                   @$.first_line = @1.first_line;
                   @$.last_column = @3.last_column;
                   @$.last_line = @3.last_line;
                   if ($3)
                     $$ = $1 / $3;
                   else
                     {
                       $$ = 1;
                       fprintf (stderr,
                                "Division by zero, l%d,c%d-l%d,c%d",
                                @3.first_line, @3.first_column,
                                @3.last_line, @3.last_column);
                     }
                 }

   As for semantic values, there is a default action for locations that
is run each time a rule is matched.  It sets the beginning of `@$' to
the beginning of the first symbol, and the end of `@$' to the end of the
last symbol.

   With this default action, the location tracking can be fully
automatic.  The example above simply rewrites this way:

     exp:    ...
             | exp '/' exp
                 {
                   if ($3)
                     $$ = $1 / $3;
                   else
                     {
                       $$ = 1;
                       fprintf (stderr,
                                "Division by zero, l%d,c%d-l%d,c%d",
                                @3.first_line, @3.first_column,
                                @3.last_line, @3.last_column);
                     }
                 }

   It is also possible to access the location of the lookahead token,
if any, from a semantic action.  This location is stored in `yylloc'.
*Note Special Features for Use in Actions: Action Features.


File: bison.info,  Node: Location Default Action,  Prev: Actions and Locations,  Up: Locations

3.6.3 Default Action for Locations
----------------------------------

Actually, actions are not the best place to compute locations.  Since
locations are much more general than semantic values, there is room in
the output parser to redefine the default action to take for each rule.
The `YYLLOC_DEFAULT' macro is invoked each time a rule is matched,
before the associated action is run.  It is also invoked while
processing a syntax error, to compute the error's location.  Before
reporting an unresolvable syntactic ambiguity, a GLR parser invokes
`YYLLOC_DEFAULT' recursively to compute the location of that ambiguity.

   Most of the time, this macro is general enough to suppress location
dedicated code from semantic actions.

   The `YYLLOC_DEFAULT' macro takes three parameters.  The first one is
the location of the grouping (the result of the computation).  When a
rule is matched, the second parameter identifies locations of all right
hand side elements of the rule being matched, and the third parameter
is the size of the rule's right hand side.  When a GLR parser reports
an ambiguity, which of multiple candidate right hand sides it passes to
`YYLLOC_DEFAULT' is undefined.  When processing a syntax error, the
second parameter identifies locations of the symbols that were
discarded during error processing, and the third parameter is the
number of discarded symbols.

   By default, `YYLLOC_DEFAULT' is defined this way:

     # define YYLLOC_DEFAULT(Current, Rhs, N)                                \
         do                                                                  \
           if (N)                                                            \
             {                                                               \
               (Current).first_line   = YYRHSLOC(Rhs, 1).first_line;         \
               (Current).first_column = YYRHSLOC(Rhs, 1).first_column;       \
               (Current).last_line    = YYRHSLOC(Rhs, N).last_line;          \
               (Current).last_column  = YYRHSLOC(Rhs, N).last_column;        \
             }                                                               \
           else                                                              \
             {                                                               \
               (Current).first_line   = (Current).last_line   =              \
                 YYRHSLOC(Rhs, 0).last_line;                                 \
               (Current).first_column = (Current).last_column =              \
                 YYRHSLOC(Rhs, 0).last_column;                               \
             }                                                               \
         while (0)

   where `YYRHSLOC (rhs, k)' is the location of the Kth symbol in RHS
when K is positive, and the location of the symbol just before the
reduction when K and N are both zero.

   When defining `YYLLOC_DEFAULT', you should consider that:

   * All arguments are free of side-effects.  However, only the first
     one (the result) should be modified by `YYLLOC_DEFAULT'.

   * For consistency with semantic actions, valid indexes within the
     right hand side range from 1 to N.  When N is zero, only 0 is a
     valid index, and it refers to the symbol just before the reduction.
     During error processing N is always positive.

   * Your macro should parenthesize its arguments, if need be, since the
     actual arguments may not be surrounded by parentheses.  Also, your
     macro should expand to something that can be used as a single
     statement when it is followed by a semicolon.


File: bison.info,  Node: Declarations,  Next: Multiple Parsers,  Prev: Locations,  Up: Grammar File

3.7 Bison Declarations
======================

The "Bison declarations" section of a Bison grammar defines the symbols
used in formulating the grammar and the data types of semantic values.
*Note Symbols::.

   All token type names (but not single-character literal tokens such as
`'+'' and `'*'') must be declared.  Nonterminal symbols must be
declared if you need to specify which data type to use for the semantic
value (*note More Than One Value Type: Multiple Types.).

   The first rule in the file also specifies the start symbol, by
default.  If you want some other symbol to be the start symbol, you
must declare it explicitly (*note Languages and Context-Free Grammars:
Language and Grammar.).

* Menu:

* Require Decl::      Requiring a Bison version.
* Token Decl::        Declaring terminal symbols.
* Precedence Decl::   Declaring terminals with precedence and associativity.
* Union Decl::        Declaring the set of all semantic value types.
* Type Decl::         Declaring the choice of type for a nonterminal symbol.
* Initial Action Decl::  Code run before parsing starts.
* Destructor Decl::   Declaring how symbols are freed.
* Expect Decl::       Suppressing warnings about parsing conflicts.
* Start Decl::        Specifying the start symbol.
* Pure Decl::         Requesting a reentrant parser.
* Push Decl::         Requesting a push parser.
* Decl Summary::      Table of all Bison declarations.


File: bison.info,  Node: Require Decl,  Next: Token Decl,  Up: Declarations

3.7.1 Require a Version of Bison
--------------------------------

You may require the minimum version of Bison to process the grammar.  If
the requirement is not met, `bison' exits with an error (exit status
63).

     %require "VERSION"


File: bison.info,  Node: Token Decl,  Next: Precedence Decl,  Prev: Require Decl,  Up: Declarations

3.7.2 Token Type Names
----------------------

The basic way to declare a token type name (terminal symbol) is as
follows:

     %token NAME

   Bison will convert this into a `#define' directive in the parser, so
that the function `yylex' (if it is in this file) can use the name NAME
to stand for this token type's code.

   Alternatively, you can use `%left', `%right', or `%nonassoc' instead
of `%token', if you wish to specify associativity and precedence.
*Note Operator Precedence: Precedence Decl.

   You can explicitly specify the numeric code for a token type by
appending a nonnegative decimal or hexadecimal integer value in the
field immediately following the token name:

     %token NUM 300
     %token XNUM 0x12d // a GNU extension

It is generally best, however, to let Bison choose the numeric codes for
all token types.  Bison will automatically select codes that don't
conflict with each other or with normal characters.

   In the event that the stack type is a union, you must augment the
`%token' or other token declaration to include the data type
alternative delimited by angle-brackets (*note More Than One Value
Type: Multiple Types.).

   For example:

     %union {              /* define stack type */
       double val;
       symrec *tptr;
     }
     %token <val> NUM      /* define token NUM and its type */

   You can associate a literal string token with a token type name by
writing the literal string at the end of a `%token' declaration which
declares the name.  For example:

     %token arrow "=>"

For example, a grammar for the C language might specify these names with
equivalent literal string tokens:

     %token  <operator>  OR      "||"
     %token  <operator>  LE 134  "<="
     %left  OR  "<="

Once you equate the literal string and the token name, you can use them
interchangeably in further declarations or the grammar rules.  The
`yylex' function can use the token name or the literal string to obtain
the token type code number (*note Calling Convention::).  Syntax error
messages passed to `yyerror' from the parser will reference the literal
string instead of the token name.

   The token numbered as 0 corresponds to end of file; the following
line allows for nicer error messages referring to "end of file" instead
of "$end":

     %token END 0 "end of file"


File: bison.info,  Node: Precedence Decl,  Next: Union Decl,  Prev: Token Decl,  Up: Declarations

3.7.3 Operator Precedence
-------------------------

Use the `%left', `%right' or `%nonassoc' declaration to declare a token
and specify its precedence and associativity, all at once.  These are
called "precedence declarations".  *Note Operator Precedence:
Precedence, for general information on operator precedence.

   The syntax of a precedence declaration is nearly the same as that of
`%token': either

     %left SYMBOLS...

or

     %left <TYPE> SYMBOLS...

   And indeed any of these declarations serves the purposes of `%token'.
But in addition, they specify the associativity and relative precedence
for all the SYMBOLS:

   * The associativity of an operator OP determines how repeated uses
     of the operator nest: whether `X OP Y OP Z' is parsed by grouping
     X with Y first or by grouping Y with Z first.  `%left' specifies
     left-associativity (grouping X with Y first) and `%right'
     specifies right-associativity (grouping Y with Z first).
     `%nonassoc' specifies no associativity, which means that `X OP Y
     OP Z' is considered a syntax error.

   * The precedence of an operator determines how it nests with other
     operators.  All the tokens declared in a single precedence
     declaration have equal precedence and nest together according to
     their associativity.  When two tokens declared in different
     precedence declarations associate, the one declared later has the
     higher precedence and is grouped first.

   For backward compatibility, there is a confusing difference between
the argument lists of `%token' and precedence declarations.  Only a
`%token' can associate a literal string with a token type name.  A
precedence declaration always interprets a literal string as a
reference to a separate token.  For example:

     %left  OR "<="         // Does not declare an alias.
     %left  OR 134 "<=" 135 // Declares 134 for OR and 135 for "<=".


File: bison.info,  Node: Union Decl,  Next: Type Decl,  Prev: Precedence Decl,  Up: Declarations

3.7.4 The Collection of Value Types
-----------------------------------

The `%union' declaration specifies the entire collection of possible
data types for semantic values.  The keyword `%union' is followed by
braced code containing the same thing that goes inside a `union' in C.

   For example:

     %union {
       double val;
       symrec *tptr;
     }

This says that the two alternative types are `double' and `symrec *'.
They are given names `val' and `tptr'; these names are used in the
`%token' and `%type' declarations to pick one of the types for a
terminal or nonterminal symbol (*note Nonterminal Symbols: Type Decl.).

   As an extension to POSIX, a tag is allowed after the `union'.  For
example:

     %union value {
       double val;
       symrec *tptr;
     }

specifies the union tag `value', so the corresponding C type is `union
value'.  If you do not specify a tag, it defaults to `YYSTYPE'.

   As another extension to POSIX, you may specify multiple `%union'
declarations; their contents are concatenated.  However, only the first
`%union' declaration can specify a tag.

   Note that, unlike making a `union' declaration in C, you need not
write a semicolon after the closing brace.

   Instead of `%union', you can define and use your own union type
`YYSTYPE' if your grammar contains at least one `<TYPE>' tag.  For
example, you can put the following into a header file `parser.h':

     union YYSTYPE {
       double val;
       symrec *tptr;
     };
     typedef union YYSTYPE YYSTYPE;

and then your grammar can use the following instead of `%union':

     %{
     #include "parser.h"
     %}
     %type <val> expr
     %token <tptr> ID


File: bison.info,  Node: Type Decl,  Next: Initial Action Decl,  Prev: Union Decl,  Up: Declarations

3.7.5 Nonterminal Symbols
-------------------------

When you use `%union' to specify multiple value types, you must declare
the value type of each nonterminal symbol for which values are used.
This is done with a `%type' declaration, like this:

     %type <TYPE> NONTERMINAL...

Here NONTERMINAL is the name of a nonterminal symbol, and TYPE is the
name given in the `%union' to the alternative that you want (*note The
Collection of Value Types: Union Decl.).  You can give any number of
nonterminal symbols in the same `%type' declaration, if they have the
same value type.  Use spaces to separate the symbol names.

   You can also declare the value type of a terminal symbol.  To do
this, use the same `<TYPE>' construction in a declaration for the
terminal symbol.  All kinds of token declarations allow `<TYPE>'.


File: bison.info,  Node: Initial Action Decl,  Next: Destructor Decl,  Prev: Type Decl,  Up: Declarations

3.7.6 Performing Actions before Parsing
---------------------------------------

Sometimes your parser needs to perform some initializations before
parsing.  The `%initial-action' directive allows for such arbitrary
code.

 -- Directive: %initial-action { CODE }
     Declare that the braced CODE must be invoked before parsing each
     time `yyparse' is called.  The CODE may use `$$' and `@$' --
     initial value and location of the lookahead -- and the
     `%parse-param'.

   For instance, if your locations use a file name, you may use

     %parse-param { char const *file_name };
     %initial-action
     {
       @$.initialize (file_name);
     };


File: bison.info,  Node: Destructor Decl,  Next: Expect Decl,  Prev: Initial Action Decl,  Up: Declarations

3.7.7 Freeing Discarded Symbols
-------------------------------

During error recovery (*note Error Recovery::), symbols already pushed
on the stack and tokens coming from the rest of the file are discarded
until the parser falls on its feet.  If the parser runs out of memory,
or if it returns via `YYABORT' or `YYACCEPT', all the symbols on the
stack must be discarded.  Even if the parser succeeds, it must discard
the start symbol.

   When discarded symbols convey heap based information, this memory is
lost.  While this behavior can be tolerable for batch parsers, such as
in traditional compilers, it is unacceptable for programs like shells or
protocol implementations that may parse and execute indefinitely.

   The `%destructor' directive defines code that is called when a
symbol is automatically discarded.

 -- Directive: %destructor { CODE } SYMBOLS
     Invoke the braced CODE whenever the parser discards one of the
     SYMBOLS.  Within CODE, `$$' designates the semantic value
     associated with the discarded symbol, and `@$' designates its
     location.  The additional parser parameters are also available
     (*note The Parser Function `yyparse': Parser Function.).

     When a symbol is listed among SYMBOLS, its `%destructor' is called
     a per-symbol `%destructor'.  You may also define a per-type
     `%destructor' by listing a semantic type tag among SYMBOLS.  In
     that case, the parser will invoke this CODE whenever it discards
     any grammar symbol that has that semantic type tag unless that
     symbol has its own per-symbol `%destructor'.

     Finally, you can define two different kinds of default
     `%destructor's.  (These default forms are experimental.  More user
     feedback will help to determine whether they should become
     permanent features.)  You can place each of `<*>' and `<>' in the
     SYMBOLS list of exactly one `%destructor' declaration in your
     grammar file.  The parser will invoke the CODE associated with one
     of these whenever it discards any user-defined grammar symbol that
     has no per-symbol and no per-type `%destructor'.  The parser uses
     the CODE for `<*>' in the case of such a grammar symbol for which
     you have formally declared a semantic type tag (`%type' counts as
     such a declaration, but `$<tag>$' does not).  The parser uses the
     CODE for `<>' in the case of such a grammar symbol that has no
     declared semantic type tag.

For example:

     %union { char *string; }
     %token <string> STRING1
     %token <string> STRING2
     %type  <string> string1
     %type  <string> string2
     %union { char character; }
     %token <character> CHR
     %type  <character> chr
     %token TAGLESS

     %destructor { } <character>
     %destructor { free ($$); } <*>
     %destructor { free ($$); printf ("%d", @$.first_line); } STRING1 string1
     %destructor { printf ("Discarding tagless symbol.\n"); } <>

guarantees that, when the parser discards any user-defined symbol that
has a semantic type tag other than `<character>', it passes its
semantic value to `free' by default.  However, when the parser discards
a `STRING1' or a `string1', it also prints its line number to `stdout'.
It performs only the second `%destructor' in this case, so it invokes
`free' only once.  Finally, the parser merely prints a message whenever
it discards any symbol, such as `TAGLESS', that has no semantic type
tag.

   A Bison-generated parser invokes the default `%destructor's only for
user-defined as opposed to Bison-defined symbols.  For example, the
parser will not invoke either kind of default `%destructor' for the
special Bison-defined symbols `$accept', `$undefined', or `$end' (*note
Bison Symbols: Table of Symbols.), none of which you can reference in
your grammar.  It also will not invoke either for the `error' token
(*note error: Table of Symbols.), which is always defined by Bison
regardless of whether you reference it in your grammar.  However, it
may invoke one of them for the end token (token 0) if you redefine it
from `$end' to, for example, `END':

     %token END 0

   Finally, Bison will never invoke a `%destructor' for an unreferenced
mid-rule semantic value (*note Actions in Mid-Rule: Mid-Rule Actions.).
That is, Bison does not consider a mid-rule to have a semantic value if
you do not reference `$$' in the mid-rule's action or `$N' (where N is
the RHS symbol position of the mid-rule) in any later action in that
rule.  However, if you do reference either, the Bison-generated parser
will invoke the `<>' `%destructor' whenever it discards the mid-rule
symbol.


   "Discarded symbols" are the following:

   * stacked symbols popped during the first phase of error recovery,

   * incoming terminals during the second phase of error recovery,

   * the current lookahead and the entire stack (except the current
     right-hand side symbols) when the parser returns immediately, and

   * the start symbol, when the parser succeeds.

   The parser can "return immediately" because of an explicit call to
`YYABORT' or `YYACCEPT', or failed error recovery, or memory exhaustion.

   Right-hand side symbols of a rule that explicitly triggers a syntax
error via `YYERROR' are not discarded automatically.  As a rule of
thumb, destructors are invoked only when user actions cannot manage the
memory.


File: bison.info,  Node: Expect Decl,  Next: Start Decl,  Prev: Destructor Decl,  Up: Declarations

3.7.8 Suppressing Conflict Warnings
-----------------------------------

Bison normally warns if there are any conflicts in the grammar (*note
Shift/Reduce Conflicts: Shift/Reduce.), but most real grammars have
harmless shift/reduce conflicts which are resolved in a predictable way
and would be difficult to eliminate.  It is desirable to suppress the
warning about these conflicts unless the number of conflicts changes.
You can do this with the `%expect' declaration.

   The declaration looks like this:

     %expect N

   Here N is a decimal integer.  The declaration says there should be N
shift/reduce conflicts and no reduce/reduce conflicts.  Bison reports
an error if the number of shift/reduce conflicts differs from N, or if
there are any reduce/reduce conflicts.

   For normal LALR(1) parsers, reduce/reduce conflicts are more
serious, and should be eliminated entirely.  Bison will always report
reduce/reduce conflicts for these parsers.  With GLR parsers, however,
both kinds of conflicts are routine; otherwise, there would be no need
to use GLR parsing.  Therefore, it is also possible to specify an
expected number of reduce/reduce conflicts in GLR parsers, using the
declaration:

     %expect-rr N

   In general, using `%expect' involves these steps:

   * Compile your grammar without `%expect'.  Use the `-v' option to
     get a verbose list of where the conflicts occur.  Bison will also
     print the number of conflicts.

   * Check each of the conflicts to make sure that Bison's default
     resolution is what you really want.  If not, rewrite the grammar
     and go back to the beginning.

   * Add an `%expect' declaration, copying the number N from the number
     which Bison printed.  With GLR parsers, add an `%expect-rr'
     declaration as well.

   Now Bison will warn you if you introduce an unexpected conflict, but
will keep silent otherwise.


File: bison.info,  Node: Start Decl,  Next: Pure Decl,  Prev: Expect Decl,  Up: Declarations

3.7.9 The Start-Symbol
----------------------

Bison assumes by default that the start symbol for the grammar is the
first nonterminal specified in the grammar specification section.  The
programmer may override this restriction with the `%start' declaration
as follows:

     %start SYMBOL


File: bison.info,  Node: Pure Decl,  Next: Push Decl,  Prev: Start Decl,  Up: Declarations

3.7.10 A Pure (Reentrant) Parser
--------------------------------

A "reentrant" program is one which does not alter in the course of
execution; in other words, it consists entirely of "pure" (read-only)
code.  Reentrancy is important whenever asynchronous execution is
possible; for example, a nonreentrant program may not be safe to call
from a signal handler.  In systems with multiple threads of control, a
nonreentrant program must be called only within interlocks.

   Normally, Bison generates a parser which is not reentrant.  This is
suitable for most uses, and it permits compatibility with Yacc.  (The
standard Yacc interfaces are inherently nonreentrant, because they use
statically allocated variables for communication with `yylex',
including `yylval' and `yylloc'.)

   Alternatively, you can generate a pure, reentrant parser.  The Bison
declaration `%define api.pure' says that you want the parser to be
reentrant.  It looks like this:

     %define api.pure

   The result is that the communication variables `yylval' and `yylloc'
become local variables in `yyparse', and a different calling convention
is used for the lexical analyzer function `yylex'.  *Note Calling
Conventions for Pure Parsers: Pure Calling, for the details of this.
The variable `yynerrs' becomes local in `yyparse' in pull mode but it
becomes a member of yypstate in push mode.  (*note The Error Reporting
Function `yyerror': Error Reporting.).  The convention for calling
`yyparse' itself is unchanged.

   Whether the parser is pure has nothing to do with the grammar rules.
You can generate either a pure parser or a nonreentrant parser from any
valid grammar.


File: bison.info,  Node: Push Decl,  Next: Decl Summary,  Prev: Pure Decl,  Up: Declarations

3.7.11 A Push Parser
--------------------

(The current push parsing interface is experimental and may evolve.
More user feedback will help to stabilize it.)

   A pull parser is called once and it takes control until all its input
is completely parsed.  A push parser, on the other hand, is called each
time a new token is made available.

   A push parser is typically useful when the parser is part of a main
event loop in the client's application.  This is typically a
requirement of a GUI, when the main event loop needs to be triggered
within a certain time period.

   Normally, Bison generates a pull parser.  The following Bison
declaration says that you want the parser to be a push parser (*note
%define api.push_pull: Decl Summary.):

     %define api.push_pull "push"

   In almost all cases, you want to ensure that your push parser is also
a pure parser (*note A Pure (Reentrant) Parser: Pure Decl.).  The only
time you should create an impure push parser is to have backwards
compatibility with the impure Yacc pull mode interface.  Unless you know
what you are doing, your declarations should look like this:

     %define api.pure
     %define api.push_pull "push"

   There is a major notable functional difference between the pure push
parser and the impure push parser.  It is acceptable for a pure push
parser to have many parser instances, of the same type of parser, in
memory at the same time.  An impure push parser should only use one
parser at a time.

   When a push parser is selected, Bison will generate some new symbols
in the generated parser.  `yypstate' is a structure that the generated
parser uses to store the parser's state.  `yypstate_new' is the
function that will create a new parser instance.  `yypstate_delete'
will free the resources associated with the corresponding parser
instance.  Finally, `yypush_parse' is the function that should be
called whenever a token is available to provide the parser.  A trivial
example of using a pure push parser would look like this:

     int status;
     yypstate *ps = yypstate_new ();
     do {
       status = yypush_parse (ps, yylex (), NULL);
     } while (status == YYPUSH_MORE);
     yypstate_delete (ps);

   If the user decided to use an impure push parser, a few things about
the generated parser will change.  The `yychar' variable becomes a
global variable instead of a variable in the `yypush_parse' function.
For this reason, the signature of the `yypush_parse' function is
changed to remove the token as a parameter.  A nonreentrant push parser
example would thus look like this:

     extern int yychar;
     int status;
     yypstate *ps = yypstate_new ();
     do {
       yychar = yylex ();
       status = yypush_parse (ps);
     } while (status == YYPUSH_MORE);
     yypstate_delete (ps);

   That's it. Notice the next token is put into the global variable
`yychar' for use by the next invocation of the `yypush_parse' function.

   Bison also supports both the push parser interface along with the
pull parser interface in the same generated parser.  In order to get
this functionality, you should replace the `%define api.push_pull
"push"' declaration with the `%define api.push_pull "both"'
declaration.  Doing this will create all of the symbols mentioned
earlier along with the two extra symbols, `yyparse' and `yypull_parse'.
`yyparse' can be used exactly as it normally would be used.  However,
the user should note that it is implemented in the generated parser by
calling `yypull_parse'.  This makes the `yyparse' function that is
generated with the `%define api.push_pull "both"' declaration slower
than the normal `yyparse' function.  If the user calls the
`yypull_parse' function it will parse the rest of the input stream.  It
is possible to `yypush_parse' tokens to select a subgrammar and then
`yypull_parse' the rest of the input stream.  If you would like to
switch back and forth between between parsing styles, you would have to
write your own `yypull_parse' function that knows when to quit looking
for input.  An example of using the `yypull_parse' function would look
like this:

     yypstate *ps = yypstate_new ();
     yypull_parse (ps); /* Will call the lexer */
     yypstate_delete (ps);

   Adding the `%define api.pure' declaration does exactly the same
thing to the generated parser with `%define api.push_pull "both"' as it
did for `%define api.push_pull "push"'.


File: bison.info,  Node: Decl Summary,  Prev: Push Decl,  Up: Declarations

3.7.12 Bison Declaration Summary
--------------------------------

Here is a summary of the declarations used to define a grammar:

 -- Directive: %union
     Declare the collection of data types that semantic values may have
     (*note The Collection of Value Types: Union Decl.).

 -- Directive: %token
     Declare a terminal symbol (token type name) with no precedence or
     associativity specified (*note Token Type Names: Token Decl.).

 -- Directive: %right
     Declare a terminal symbol (token type name) that is
     right-associative (*note Operator Precedence: Precedence Decl.).

 -- Directive: %left
     Declare a terminal symbol (token type name) that is
     left-associative (*note Operator Precedence: Precedence Decl.).

 -- Directive: %nonassoc
     Declare a terminal symbol (token type name) that is nonassociative
     (*note Operator Precedence: Precedence Decl.).  Using it in a way
     that would be associative is a syntax error.

 -- Directive: %type
     Declare the type of semantic values for a nonterminal symbol
     (*note Nonterminal Symbols: Type Decl.).

 -- Directive: %start
     Specify the grammar's start symbol (*note The Start-Symbol: Start
     Decl.).

 -- Directive: %expect
     Declare the expected number of shift-reduce conflicts (*note
     Suppressing Conflict Warnings: Expect Decl.).


In order to change the behavior of `bison', use the following
directives:

 -- Directive: %code {CODE}
     This is the unqualified form of the `%code' directive.  It inserts
     CODE verbatim at a language-dependent default location in the
     output(1).

     For C/C++, the default location is the parser source code file
     after the usual contents of the parser header file.  Thus, `%code'
     replaces the traditional Yacc prologue, `%{CODE%}', for most
     purposes.  For a detailed discussion, see *Note Prologue
     Alternatives::.

     For Java, the default location is inside the parser class.

     (Like all the Yacc prologue alternatives, this directive is
     experimental.  More user feedback will help to determine whether
     it should become a permanent feature.)

 -- Directive: %code QUALIFIER {CODE}
     This is the qualified form of the `%code' directive.  If you need
     to specify location-sensitive verbatim CODE that does not belong
     at the default location selected by the unqualified `%code' form,
     use this form instead.

     QUALIFIER identifies the purpose of CODE and thus the location(s)
     where Bison should generate it.  Not all values of QUALIFIER are
     available for all target languages:

        * requires 

             * Language(s): C, C++

             * Purpose: This is the best place to write dependency code
               required for `YYSTYPE' and `YYLTYPE'.  In other words,
               it's the best place to define types referenced in
               `%union' directives, and it's the best place to override
               Bison's default `YYSTYPE' and `YYLTYPE' definitions.

             * Location(s): The parser header file and the parser
               source code file before the Bison-generated `YYSTYPE'
               and `YYLTYPE' definitions.

        * provides 

             * Language(s): C, C++

             * Purpose: This is the best place to write additional
               definitions and declarations that should be provided to
               other modules.

             * Location(s): The parser header file and the parser
               source code file after the Bison-generated `YYSTYPE',
               `YYLTYPE', and token definitions.

        * top 

             * Language(s): C, C++

             * Purpose: The unqualified `%code' or `%code requires'
               should usually be more appropriate than `%code top'.
               However, occasionally it is necessary to insert code
               much nearer the top of the parser source code file.  For
               example:

                    %code top {
                      #define _GNU_SOURCE
                      #include <stdio.h>
                    }

             * Location(s): Near the top of the parser source code file.

        * imports 

             * Language(s): Java

             * Purpose: This is the best place to write Java import
               directives.

             * Location(s): The parser Java file after any Java package
               directive and before any class definitions.

     (Like all the Yacc prologue alternatives, this directive is
     experimental.  More user feedback will help to determine whether
     it should become a permanent feature.)

     For a detailed discussion of how to use `%code' in place of the
     traditional Yacc prologue for C/C++, see *Note Prologue
     Alternatives::.

 -- Directive: %debug
     In the parser file, define the macro `YYDEBUG' to 1 if it is not
     already defined, so that the debugging facilities are compiled.
   *Note Tracing Your Parser: Tracing.

 -- Directive: %define VARIABLE
 -- Directive: %define VARIABLE "VALUE"
     Define a variable to adjust Bison's behavior.  The possible
     choices for VARIABLE, as well as their meanings, depend on the
     selected target language and/or the parser skeleton (*note
     %language: Decl Summary, *note %skeleton: Decl Summary.).

     Bison will warn if a VARIABLE is defined multiple times.

     Omitting `"VALUE"' is always equivalent to specifying it as `""'.

     Some VARIABLEs may be used as Booleans.  In this case, Bison will
     complain if the variable definition does not meet one of the
     following four conditions:

       1. `"VALUE"' is `"true"'

       2. `"VALUE"' is omitted (or is `""').  This is equivalent to
          `"true"'.

       3. `"VALUE"' is `"false"'.

       4. VARIABLE is never defined.  In this case, Bison selects a
          default value, which may depend on the selected target
          language and/or parser skeleton.

     Some of the accepted VARIABLEs are:

        * api.pure 

             * Language(s): C

             * Purpose: Request a pure (reentrant) parser program.
               *Note A Pure (Reentrant) Parser: Pure Decl.

             * Accepted Values: Boolean

             * Default Value: `"false"'

        * api.push_pull 

             * Language(s): C (LALR(1) only)

             * Purpose: Requests a pull parser, a push parser, or both.
               *Note A Push Parser: Push Decl.  (The current push
               parsing interface is experimental and may evolve.  More
               user feedback will help to stabilize it.)

             * Accepted Values: `"pull"', `"push"', `"both"'

             * Default Value: `"pull"'

        * lr.keep_unreachable_states 

             * Language(s): all

             * Purpose: Requests that Bison allow unreachable parser
               states to remain in the parser tables.  Bison considers
               a state to be unreachable if there exists no sequence of
               transitions from the start state to that state.  A state
               can become unreachable during conflict resolution if
               Bison disables a shift action leading to it from a
               predecessor state.  Keeping unreachable states is
               sometimes useful for analysis purposes, but they are
               useless in the generated parser.

             * Accepted Values: Boolean

             * Default Value: `"false"'

             * Caveats:

                  * Unreachable states may contain conflicts and may
                    use rules not used in any other state.  Thus,
                    keeping unreachable states may induce warnings that
                    are irrelevant to your parser's behavior, and it
                    may eliminate warnings that are relevant.  Of
                    course, the change in warnings may actually be
                    relevant to a parser table analysis that wants to
                    keep unreachable states, so this behavior will
                    likely remain in future Bison releases.

                  * While Bison is able to remove unreachable states,
                    it is not guaranteed to remove other kinds of
                    useless states.  Specifically, when Bison disables
                    reduce actions during conflict resolution, some
                    goto actions may become useless, and thus some
                    additional states may become useless.  If Bison
                    were to compute which goto actions were useless and
                    then disable those actions, it could identify such
                    states as unreachable and then remove those states.
                    However, Bison does not compute which goto actions
                    are useless.

        * namespace 

             * Languages(s): C++

             * Purpose: Specifies the namespace for the parser class.
               For example, if you specify:

                    %define namespace "foo::bar"

               Bison uses `foo::bar' verbatim in references such as:

                    foo::bar::parser::semantic_type

               However, to open a namespace, Bison removes any leading
               `::' and then splits on any remaining occurrences:

                    namespace foo { namespace bar {
                      class position;
                      class location;
                    } }

             * Accepted Values: Any absolute or relative C++ namespace
               reference without a trailing `"::"'.  For example,
               `"foo"' or `"::foo::bar"'.

             * Default Value: The value specified by `%name-prefix',
               which defaults to `yy'.  This usage of `%name-prefix' is
               for backward compatibility and can be confusing since
               `%name-prefix' also specifies the textual prefix for the
               lexical analyzer function.  Thus, if you specify
               `%name-prefix', it is best to also specify `%define
               namespace' so that `%name-prefix' _only_ affects the
               lexical analyzer function.  For example, if you specify:

                    %define namespace "foo"
                    %name-prefix "bar::"

               The parser namespace is `foo' and `yylex' is referenced
               as `bar::lex'.


 -- Directive: %defines
     Write a header file containing macro definitions for the token type
     names defined in the grammar as well as a few other declarations.
     If the parser output file is named `NAME.c' then this file is
     named `NAME.h'.

     For C parsers, the output header declares `YYSTYPE' unless
     `YYSTYPE' is already defined as a macro or you have used a
     `<TYPE>' tag without using `%union'.  Therefore, if you are using
     a `%union' (*note More Than One Value Type: Multiple Types.) with
     components that require other definitions, or if you have defined
     a `YYSTYPE' macro or type definition (*note Data Types of Semantic
     Values: Value Type.), you need to arrange for these definitions to
     be propagated to all modules, e.g., by putting them in a
     prerequisite header that is included both by your parser and by
     any other module that needs `YYSTYPE'.

     Unless your parser is pure, the output header declares `yylval' as
     an external variable.  *Note A Pure (Reentrant) Parser: Pure Decl.

     If you have also used locations, the output header declares
     `YYLTYPE' and `yylloc' using a protocol similar to that of the
     `YYSTYPE' macro and `yylval'.  *Note Tracking Locations: Locations.

     This output file is normally essential if you wish to put the
     definition of `yylex' in a separate source file, because `yylex'
     typically needs to be able to refer to the above-mentioned
     declarations and to the token type codes.  *Note Semantic Values
     of Tokens: Token Values.

     If you have declared `%code requires' or `%code provides', the
     output header also contains their code.  *Note %code: Decl Summary.

 -- Directive: %defines DEFINES-FILE
     Same as above, but save in the file DEFINES-FILE.

 -- Directive: %destructor
     Specify how the parser should reclaim the memory associated to
     discarded symbols.  *Note Freeing Discarded Symbols: Destructor
     Decl.

 -- Directive: %file-prefix "PREFIX"
     Specify a prefix to use for all Bison output file names.  The
     names are chosen as if the input file were named `PREFIX.y'.

 -- Directive: %language "LANGUAGE"
     Specify the programming language for the generated parser.
     Currently supported languages include C, C++, and Java.  LANGUAGE
     is case-insensitive.

     This directive is experimental and its effect may be modified in
     future releases.

 -- Directive: %locations
     Generate the code processing the locations (*note Special Features
     for Use in Actions: Action Features.).  This mode is enabled as
     soon as the grammar uses the special `@N' tokens, but if your
     grammar does not use it, using `%locations' allows for more
     accurate syntax error messages.

 -- Directive: %name-prefix "PREFIX"
     Rename the external symbols used in the parser so that they start
     with PREFIX instead of `yy'.  The precise list of symbols renamed
     in C parsers is `yyparse', `yylex', `yyerror', `yynerrs',
     `yylval', `yychar', `yydebug', and (if locations are used)
     `yylloc'.  If you use a push parser, `yypush_parse',
     `yypull_parse', `yypstate', `yypstate_new' and `yypstate_delete'
     will also be renamed.  For example, if you use `%name-prefix
     "c_"', the names become `c_parse', `c_lex', and so on.  For C++
     parsers, see the `%define namespace' documentation in this section.
     *Note Multiple Parsers in the Same Program: Multiple Parsers.

 -- Directive: %no-lines
     Don't generate any `#line' preprocessor commands in the parser
     file.  Ordinarily Bison writes these commands in the parser file
     so that the C compiler and debuggers will associate errors and
     object code with your source file (the grammar file).  This
     directive causes them to associate errors with the parser file,
     treating it an independent source file in its own right.

 -- Directive: %output "FILE"
     Specify FILE for the parser file.

 -- Directive: %pure-parser
     Deprecated version of `%define api.pure' (*note %define: Decl
     Summary.), for which Bison is more careful to warn about
     unreasonable usage.

 -- Directive: %require "VERSION"
     Require version VERSION or higher of Bison.  *Note Require a
     Version of Bison: Require Decl.

 -- Directive: %skeleton "FILE"
     Specify the skeleton to use.

     If FILE does not contain a `/', FILE is the name of a skeleton
     file in the Bison installation directory.  If it does, FILE is an
     absolute file name or a file name relative to the directory of the
     grammar file.  This is similar to how most shells resolve commands.

 -- Directive: %token-table
     Generate an array of token names in the parser file.  The name of
     the array is `yytname'; `yytname[I]' is the name of the token
     whose internal Bison token code number is I.  The first three
     elements of `yytname' correspond to the predefined tokens `"$end"',
     `"error"', and `"$undefined"'; after these come the symbols
     defined in the grammar file.

     The name in the table includes all the characters needed to
     represent the token in Bison.  For single-character literals and
     literal strings, this includes the surrounding quoting characters
     and any escape sequences.  For example, the Bison single-character
     literal `'+'' corresponds to a three-character name, represented
     in C as `"'+'"'; and the Bison two-character literal string `"\\/"'
     corresponds to a five-character name, represented in C as
     `"\"\\\\/\""'.

     When you specify `%token-table', Bison also generates macro
     definitions for macros `YYNTOKENS', `YYNNTS', and `YYNRULES', and
     `YYNSTATES':

    `YYNTOKENS'
          The highest token number, plus one.

    `YYNNTS'
          The number of nonterminal symbols.

    `YYNRULES'
          The number of grammar rules,

    `YYNSTATES'
          The number of parser states (*note Parser States::).

 -- Directive: %verbose
     Write an extra output file containing verbose descriptions of the
     parser states and what is done for each type of lookahead token in
     that state.  *Note Understanding Your Parser: Understanding, for
     more information.

 -- Directive: %yacc
     Pretend the option `--yacc' was given, i.e., imitate Yacc,
     including its naming conventions.  *Note Bison Options::, for more.

   ---------- Footnotes ----------

   (1) The default location is actually skeleton-dependent;   writers
of non-standard skeletons however should choose the default location
consistently with the behavior of the standard Bison skeletons.


File: bison.info,  Node: Multiple Parsers,  Prev: Declarations,  Up: Grammar File

3.8 Multiple Parsers in the Same Program
========================================

Most programs that use Bison parse only one language and therefore
contain only one Bison parser.  But what if you want to parse more than
one language with the same program?  Then you need to avoid a name
conflict between different definitions of `yyparse', `yylval', and so
on.

   The easy way to do this is to use the option `-p PREFIX' (*note
Invoking Bison: Invocation.).  This renames the interface functions and
variables of the Bison parser to start with PREFIX instead of `yy'.
You can use this to give each parser distinct names that do not
conflict.

   The precise list of symbols renamed is `yyparse', `yylex',
`yyerror', `yynerrs', `yylval', `yylloc', `yychar' and `yydebug'.  If
you use a push parser, `yypush_parse', `yypull_parse', `yypstate',
`yypstate_new' and `yypstate_delete' will also be renamed.  For
example, if you use `-p c', the names become `cparse', `clex', and so
on.

   *All the other variables and macros associated with Bison are not
renamed.* These others are not global; there is no conflict if the same
name is used in different parsers.  For example, `YYSTYPE' is not
renamed, but defining this in different ways in different parsers causes
no trouble (*note Data Types of Semantic Values: Value Type.).

   The `-p' option works by adding macro definitions to the beginning
of the parser source file, defining `yyparse' as `PREFIXparse', and so
on.  This effectively substitutes one name for the other in the entire
parser file.


File: bison.info,  Node: Interface,  Next: Algorithm,  Prev: Grammar File,  Up: Top

4 Parser C-Language Interface
*****************************

The Bison parser is actually a C function named `yyparse'.  Here we
describe the interface conventions of `yyparse' and the other functions
that it needs to use.

   Keep in mind that the parser uses many C identifiers starting with
`yy' and `YY' for internal purposes.  If you use such an identifier
(aside from those in this manual) in an action or in epilogue in the
grammar file, you are likely to run into trouble.

* Menu:

* Parser Function::         How to call `yyparse' and what it returns.
* Push Parser Function::    How to call `yypush_parse' and what it returns.
* Pull Parser Function::    How to call `yypull_parse' and what it returns.
* Parser Create Function::  How to call `yypstate_new' and what it returns.
* Parser Delete Function::  How to call `yypstate_delete' and what it returns.
* Lexical::                 You must supply a function `yylex'
                              which reads tokens.
* Error Reporting::         You must supply a function `yyerror'.
* Action Features::         Special features for use in actions.
* Internationalization::    How to let the parser speak in the user's
                              native language.


File: bison.info,  Node: Parser Function,  Next: Push Parser Function,  Up: Interface

4.1 The Parser Function `yyparse'
=================================

You call the function `yyparse' to cause parsing to occur.  This
function reads tokens, executes actions, and ultimately returns when it
encounters end-of-input or an unrecoverable syntax error.  You can also
write an action which directs `yyparse' to return immediately without
reading further.

 -- Function: int yyparse (void)
     The value returned by `yyparse' is 0 if parsing was successful
     (return is due to end-of-input).

     The value is 1 if parsing failed because of invalid input, i.e.,
     input that contains a syntax error or that causes `YYABORT' to be
     invoked.

     The value is 2 if parsing failed due to memory exhaustion.

   In an action, you can cause immediate return from `yyparse' by using
these macros:

 -- Macro: YYACCEPT
     Return immediately with value 0 (to report success).

 -- Macro: YYABORT
     Return immediately with value 1 (to report failure).

   If you use a reentrant parser, you can optionally pass additional
parameter information to it in a reentrant way.  To do so, use the
declaration `%parse-param':

 -- Directive: %parse-param {ARGUMENT-DECLARATION}
     Declare that an argument declared by the braced-code
     ARGUMENT-DECLARATION is an additional `yyparse' argument.  The
     ARGUMENT-DECLARATION is used when declaring functions or
     prototypes.  The last identifier in ARGUMENT-DECLARATION must be
     the argument name.

   Here's an example.  Write this in the parser:

     %parse-param {int *nastiness}
     %parse-param {int *randomness}

Then call the parser like this:

     {
       int nastiness, randomness;
       ...  /* Store proper data in `nastiness' and `randomness'.  */
       value = yyparse (&nastiness, &randomness);
       ...
     }

In the grammar actions, use expressions like this to refer to the data:

     exp: ...    { ...; *randomness += 1; ... }


File: bison.info,  Node: Push Parser Function,  Next: Pull Parser Function,  Prev: Parser Function,  Up: Interface

4.2 The Push Parser Function `yypush_parse'
===========================================

(The current push parsing interface is experimental and may evolve.
More user feedback will help to stabilize it.)

   You call the function `yypush_parse' to parse a single token.  This
function is available if either the `%define api.push_pull "push"' or
`%define api.push_pull "both"' declaration is used.  *Note A Push
Parser: Push Decl.

 -- Function: int yypush_parse (yypstate *yyps)
     The value returned by `yypush_parse' is the same as for yyparse
     with the following exception.  `yypush_parse' will return
     YYPUSH_MORE if more input is required to finish parsing the
     grammar.


File: bison.info,  Node: Pull Parser Function,  Next: Parser Create Function,  Prev: Push Parser Function,  Up: Interface

4.3 The Pull Parser Function `yypull_parse'
===========================================

(The current push parsing interface is experimental and may evolve.
More user feedback will help to stabilize it.)

   You call the function `yypull_parse' to parse the rest of the input
stream.  This function is available if the `%define api.push_pull
"both"' declaration is used.  *Note A Push Parser: Push Decl.

 -- Function: int yypull_parse (yypstate *yyps)
     The value returned by `yypull_parse' is the same as for `yyparse'.


File: bison.info,  Node: Parser Create Function,  Next: Parser Delete Function,  Prev: Pull Parser Function,  Up: Interface

4.4 The Parser Create Function `yystate_new'
============================================

(The current push parsing interface is experimental and may evolve.
More user feedback will help to stabilize it.)

   You call the function `yypstate_new' to create a new parser instance.
This function is available if either the `%define api.push_pull "push"'
or `%define api.push_pull "both"' declaration is used.  *Note A Push
Parser: Push Decl.

 -- Function: yypstate *yypstate_new (void)
     The fuction will return a valid parser instance if there was
     memory available or 0 if no memory was available.  In impure mode,
     it will also return 0 if a parser instance is currently allocated.


File: bison.info,  Node: Parser Delete Function,  Next: Lexical,  Prev: Parser Create Function,  Up: Interface

4.5 The Parser Delete Function `yystate_delete'
===============================================

(The current push parsing interface is experimental and may evolve.
More user feedback will help to stabilize it.)

   You call the function `yypstate_delete' to delete a parser instance.
function is available if either the `%define api.push_pull "push"' or
`%define api.push_pull "both"' declaration is used.  *Note A Push
Parser: Push Decl.

 -- Function: void yypstate_delete (yypstate *yyps)
     This function will reclaim the memory associated with a parser
     instance.  After this call, you should no longer attempt to use
     the parser instance.


File: bison.info,  Node: Lexical,  Next: Error Reporting,  Prev: Parser Delete Function,  Up: Interface

4.6 The Lexical Analyzer Function `yylex'
=========================================

The "lexical analyzer" function, `yylex', recognizes tokens from the
input stream and returns them to the parser.  Bison does not create
this function automatically; you must write it so that `yyparse' can
call it.  The function is sometimes referred to as a lexical scanner.

   In simple programs, `yylex' is often defined at the end of the Bison
grammar file.  If `yylex' is defined in a separate source file, you
need to arrange for the token-type macro definitions to be available
there.  To do this, use the `-d' option when you run Bison, so that it
will write these macro definitions into a separate header file
`NAME.tab.h' which you can include in the other source files that need
it.  *Note Invoking Bison: Invocation.

* Menu:

* Calling Convention::  How `yyparse' calls `yylex'.
* Token Values::        How `yylex' must return the semantic value
                          of the token it has read.
* Token Locations::     How `yylex' must return the text location
                          (line number, etc.) of the token, if the
                          actions want that.
* Pure Calling::        How the calling convention differs in a pure parser
                          (*note A Pure (Reentrant) Parser: Pure Decl.).


File: bison.info,  Node: Calling Convention,  Next: Token Values,  Up: Lexical

4.6.1 Calling Convention for `yylex'
------------------------------------

The value that `yylex' returns must be the positive numeric code for
the type of token it has just found; a zero or negative value signifies
end-of-input.

   When a token is referred to in the grammar rules by a name, that name
in the parser file becomes a C macro whose definition is the proper
numeric code for that token type.  So `yylex' can use the name to
indicate that type.  *Note Symbols::.

   When a token is referred to in the grammar rules by a character
literal, the numeric code for that character is also the code for the
token type.  So `yylex' can simply return that character code, possibly
converted to `unsigned char' to avoid sign-extension.  The null
character must not be used this way, because its code is zero and that
signifies end-of-input.

   Here is an example showing these things:

     int
     yylex (void)
     {
       ...
       if (c == EOF)    /* Detect end-of-input.  */
         return 0;
       ...
       if (c == '+' || c == '-')
         return c;      /* Assume token type for `+' is '+'.  */
       ...
       return INT;      /* Return the type of the token.  */
       ...
     }

This interface has been designed so that the output from the `lex'
utility can be used without change as the definition of `yylex'.

   If the grammar uses literal string tokens, there are two ways that
`yylex' can determine the token type codes for them:

   * If the grammar defines symbolic token names as aliases for the
     literal string tokens, `yylex' can use these symbolic names like
     all others.  In this case, the use of the literal string tokens in
     the grammar file has no effect on `yylex'.

   * `yylex' can find the multicharacter token in the `yytname' table.
     The index of the token in the table is the token type's code.  The
     name of a multicharacter token is recorded in `yytname' with a
     double-quote, the token's characters, and another double-quote.
     The token's characters are escaped as necessary to be suitable as
     input to Bison.

     Here's code for looking up a multicharacter token in `yytname',
     assuming that the characters of the token are stored in
     `token_buffer', and assuming that the token does not contain any
     characters like `"' that require escaping.

          for (i = 0; i < YYNTOKENS; i++)
            {
              if (yytname[i] != 0
                  && yytname[i][0] == '"'
                  && ! strncmp (yytname[i] + 1, token_buffer,
                                strlen (token_buffer))
                  && yytname[i][strlen (token_buffer) + 1] == '"'
                  && yytname[i][strlen (token_buffer) + 2] == 0)
                break;
            }

     The `yytname' table is generated only if you use the
     `%token-table' declaration.  *Note Decl Summary::.


File: bison.info,  Node: Token Values,  Next: Token Locations,  Prev: Calling Convention,  Up: Lexical

4.6.2 Semantic Values of Tokens
-------------------------------

In an ordinary (nonreentrant) parser, the semantic value of the token
must be stored into the global variable `yylval'.  When you are using
just one data type for semantic values, `yylval' has that type.  Thus,
if the type is `int' (the default), you might write this in `yylex':

       ...
       yylval = value;  /* Put value onto Bison stack.  */
       return INT;      /* Return the type of the token.  */
       ...

   When you are using multiple data types, `yylval''s type is a union
made from the `%union' declaration (*note The Collection of Value
Types: Union Decl.).  So when you store a token's value, you must use
the proper member of the union.  If the `%union' declaration looks like
this:

     %union {
       int intval;
       double val;
       symrec *tptr;
     }

then the code in `yylex' might look like this:

       ...
       yylval.intval = value; /* Put value onto Bison stack.  */
       return INT;            /* Return the type of the token.  */
       ...


File: bison.info,  Node: Token Locations,  Next: Pure Calling,  Prev: Token Values,  Up: Lexical

4.6.3 Textual Locations of Tokens
---------------------------------

If you are using the `@N'-feature (*note Tracking Locations:
Locations.) in actions to keep track of the textual locations of tokens
and groupings, then you must provide this information in `yylex'.  The
function `yyparse' expects to find the textual location of a token just
parsed in the global variable `yylloc'.  So `yylex' must store the
proper data in that variable.

   By default, the value of `yylloc' is a structure and you need only
initialize the members that are going to be used by the actions.  The
four members are called `first_line', `first_column', `last_line' and
`last_column'.  Note that the use of this feature makes the parser
noticeably slower.

   The data type of `yylloc' has the name `YYLTYPE'.


File: bison.info,  Node: Pure Calling,  Prev: Token Locations,  Up: Lexical

4.6.4 Calling Conventions for Pure Parsers
------------------------------------------

When you use the Bison declaration `%define api.pure' to request a
pure, reentrant parser, the global communication variables `yylval' and
`yylloc' cannot be used.  (*Note A Pure (Reentrant) Parser: Pure Decl.)
In such parsers the two global variables are replaced by pointers
passed as arguments to `yylex'.  You must declare them as shown here,
and pass the information back by storing it through those pointers.

     int
     yylex (YYSTYPE *lvalp, YYLTYPE *llocp)
     {
       ...
       *lvalp = value;  /* Put value onto Bison stack.  */
       return INT;      /* Return the type of the token.  */
       ...
     }

   If the grammar file does not use the `@' constructs to refer to
textual locations, then the type `YYLTYPE' will not be defined.  In
this case, omit the second argument; `yylex' will be called with only
one argument.

   If you wish to pass the additional parameter data to `yylex', use
`%lex-param' just like `%parse-param' (*note Parser Function::).

 -- Directive: lex-param {ARGUMENT-DECLARATION}
     Declare that the braced-code ARGUMENT-DECLARATION is an additional
     `yylex' argument declaration.

   For instance:

     %parse-param {int *nastiness}
     %lex-param   {int *nastiness}
     %parse-param {int *randomness}

results in the following signature:

     int yylex   (int *nastiness);
     int yyparse (int *nastiness, int *randomness);

   If `%define api.pure' is added:

     int yylex   (YYSTYPE *lvalp, int *nastiness);
     int yyparse (int *nastiness, int *randomness);

and finally, if both `%define api.pure' and `%locations' are used:

     int yylex   (YYSTYPE *lvalp, YYLTYPE *llocp, int *nastiness);
     int yyparse (int *nastiness, int *randomness);


File: bison.info,  Node: Error Reporting,  Next: Action Features,  Prev: Lexical,  Up: Interface

4.7 The Error Reporting Function `yyerror'
==========================================

The Bison parser detects a "syntax error" or "parse error" whenever it
reads a token which cannot satisfy any syntax rule.  An action in the
grammar can also explicitly proclaim an error, using the macro
`YYERROR' (*note Special Features for Use in Actions: Action Features.).

   The Bison parser expects to report the error by calling an error
reporting function named `yyerror', which you must supply.  It is
called by `yyparse' whenever a syntax error is found, and it receives
one argument.  For a syntax error, the string is normally
`"syntax error"'.

   If you invoke the directive `%error-verbose' in the Bison
declarations section (*note The Bison Declarations Section: Bison
Declarations.), then Bison provides a more verbose and specific error
message string instead of just plain `"syntax error"'.

   The parser can detect one other kind of error: memory exhaustion.
This can happen when the input contains constructions that are very
deeply nested.  It isn't likely you will encounter this, since the Bison
parser normally extends its stack automatically up to a very large
limit.  But if memory is exhausted, `yyparse' calls `yyerror' in the
usual fashion, except that the argument string is `"memory exhausted"'.

   In some cases diagnostics like `"syntax error"' are translated
automatically from English to some other language before they are
passed to `yyerror'.  *Note Internationalization::.

   The following definition suffices in simple programs:

     void
     yyerror (char const *s)
     {
       fprintf (stderr, "%s\n", s);
     }

   After `yyerror' returns to `yyparse', the latter will attempt error
recovery if you have written suitable error recovery grammar rules
(*note Error Recovery::).  If recovery is impossible, `yyparse' will
immediately return 1.

   Obviously, in location tracking pure parsers, `yyerror' should have
an access to the current location.  This is indeed the case for the GLR
parsers, but not for the Yacc parser, for historical reasons.  I.e., if
`%locations %define api.pure' is passed then the prototypes for
`yyerror' are:

     void yyerror (char const *msg);                 /* Yacc parsers.  */
     void yyerror (YYLTYPE *locp, char const *msg);  /* GLR parsers.   */

   If `%parse-param {int *nastiness}' is used, then:

     void yyerror (int *nastiness, char const *msg);  /* Yacc parsers.  */
     void yyerror (int *nastiness, char const *msg);  /* GLR parsers.   */

   Finally, GLR and Yacc parsers share the same `yyerror' calling
convention for absolutely pure parsers, i.e., when the calling
convention of `yylex' _and_ the calling convention of `%define
api.pure' are pure.  I.e.:

     /* Location tracking.  */
     %locations
     /* Pure yylex.  */
     %define api.pure
     %lex-param   {int *nastiness}
     /* Pure yyparse.  */
     %parse-param {int *nastiness}
     %parse-param {int *randomness}

results in the following signatures for all the parser kinds:

     int yylex (YYSTYPE *lvalp, YYLTYPE *llocp, int *nastiness);
     int yyparse (int *nastiness, int *randomness);
     void yyerror (YYLTYPE *locp,
                   int *nastiness, int *randomness,
                   char const *msg);

The prototypes are only indications of how the code produced by Bison
uses `yyerror'.  Bison-generated code always ignores the returned
value, so `yyerror' can return any type, including `void'.  Also,
`yyerror' can be a variadic function; that is why the message is always
passed last.

   Traditionally `yyerror' returns an `int' that is always ignored, but
this is purely for historical reasons, and `void' is preferable since
it more accurately describes the return type for `yyerror'.

   The variable `yynerrs' contains the number of syntax errors reported
so far.  Normally this variable is global; but if you request a pure
parser (*note A Pure (Reentrant) Parser: Pure Decl.)  then it is a
local variable which only the actions can access.


File: bison.info,  Node: Action Features,  Next: Internationalization,  Prev: Error Reporting,  Up: Interface

4.8 Special Features for Use in Actions
=======================================

Here is a table of Bison constructs, variables and macros that are
useful in actions.

 -- Variable: $$
     Acts like a variable that contains the semantic value for the
     grouping made by the current rule.  *Note Actions::.

 -- Variable: $N
     Acts like a variable that contains the semantic value for the Nth
     component of the current rule.  *Note Actions::.

 -- Variable: $<TYPEALT>$
     Like `$$' but specifies alternative TYPEALT in the union specified
     by the `%union' declaration.  *Note Data Types of Values in
     Actions: Action Types.

 -- Variable: $<TYPEALT>N
     Like `$N' but specifies alternative TYPEALT in the union specified
     by the `%union' declaration.  *Note Data Types of Values in
     Actions: Action Types.

 -- Macro: YYABORT;
     Return immediately from `yyparse', indicating failure.  *Note The
     Parser Function `yyparse': Parser Function.

 -- Macro: YYACCEPT;
     Return immediately from `yyparse', indicating success.  *Note The
     Parser Function `yyparse': Parser Function.

 -- Macro: YYBACKUP (TOKEN, VALUE);
     Unshift a token.  This macro is allowed only for rules that reduce
     a single value, and only when there is no lookahead token.  It is
     also disallowed in GLR parsers.  It installs a lookahead token
     with token type TOKEN and semantic value VALUE; then it discards
     the value that was going to be reduced by this rule.

     If the macro is used when it is not valid, such as when there is a
     lookahead token already, then it reports a syntax error with a
     message `cannot back up' and performs ordinary error recovery.

     In either case, the rest of the action is not executed.

 -- Macro: YYEMPTY
     Value stored in `yychar' when there is no lookahead token.

 -- Macro: YYEOF
     Value stored in `yychar' when the lookahead is the end of the input
     stream.

 -- Macro: YYERROR;
     Cause an immediate syntax error.  This statement initiates error
     recovery just as if the parser itself had detected an error;
     however, it does not call `yyerror', and does not print any
     message.  If you want to print an error message, call `yyerror'
     explicitly before the `YYERROR;' statement.  *Note Error
     Recovery::.

 -- Macro: YYRECOVERING
     The expression `YYRECOVERING ()' yields 1 when the parser is
     recovering from a syntax error, and 0 otherwise.  *Note Error
     Recovery::.

 -- Variable: yychar
     Variable containing either the lookahead token, or `YYEOF' when the
     lookahead is the end of the input stream, or `YYEMPTY' when no
     lookahead has been performed so the next token is not yet known.
     Do not modify `yychar' in a deferred semantic action (*note GLR
     Semantic Actions::).  *Note Lookahead Tokens: Lookahead.

 -- Macro: yyclearin;
     Discard the current lookahead token.  This is useful primarily in
     error rules.  Do not invoke `yyclearin' in a deferred semantic
     action (*note GLR Semantic Actions::).  *Note Error Recovery::.

 -- Macro: yyerrok;
     Resume generating error messages immediately for subsequent syntax
     errors.  This is useful primarily in error rules.  *Note Error
     Recovery::.

 -- Variable: yylloc
     Variable containing the lookahead token location when `yychar' is
     not set to `YYEMPTY' or `YYEOF'.  Do not modify `yylloc' in a
     deferred semantic action (*note GLR Semantic Actions::).  *Note
     Actions and Locations: Actions and Locations.

 -- Variable: yylval
     Variable containing the lookahead token semantic value when
     `yychar' is not set to `YYEMPTY' or `YYEOF'.  Do not modify
     `yylval' in a deferred semantic action (*note GLR Semantic
     Actions::).  *Note Actions: Actions.

 -- Value: @$
     Acts like a structure variable containing information on the
     textual location of the grouping made by the current rule.  *Note
     Tracking Locations: Locations.


 -- Value: @N
     Acts like a structure variable containing information on the
     textual location of the Nth component of the current rule.  *Note
     Tracking Locations: Locations.


File: bison.info,  Node: Internationalization,  Prev: Action Features,  Up: Interface

4.9 Parser Internationalization
===============================

A Bison-generated parser can print diagnostics, including error and
tracing messages.  By default, they appear in English.  However, Bison
also supports outputting diagnostics in the user's native language.  To
make this work, the user should set the usual environment variables.
*Note The User's View: (gettext)Users.  For example, the shell command
`export LC_ALL=fr_CA.UTF-8' might set the user's locale to French
Canadian using the UTF-8 encoding.  The exact set of available locales
depends on the user's installation.

   The maintainer of a package that uses a Bison-generated parser
enables the internationalization of the parser's output through the
following steps.  Here we assume a package that uses GNU Autoconf and
GNU Automake.

  1. Into the directory containing the GNU Autoconf macros used by the
     package--often called `m4'--copy the `bison-i18n.m4' file
     installed by Bison under `share/aclocal/bison-i18n.m4' in Bison's
     installation directory.  For example:

          cp /usr/local/share/aclocal/bison-i18n.m4 m4/bison-i18n.m4

  2. In the top-level `configure.ac', after the `AM_GNU_GETTEXT'
     invocation, add an invocation of `BISON_I18N'.  This macro is
     defined in the file `bison-i18n.m4' that you copied earlier.  It
     causes `configure' to find the value of the `BISON_LOCALEDIR'
     variable, and it defines the source-language symbol `YYENABLE_NLS'
     to enable translations in the Bison-generated parser.

  3. In the `main' function of your program, designate the directory
     containing Bison's runtime message catalog, through a call to
     `bindtextdomain' with domain name `bison-runtime'.  For example:

          bindtextdomain ("bison-runtime", BISON_LOCALEDIR);

     Typically this appears after any other call `bindtextdomain
     (PACKAGE, LOCALEDIR)' that your package already has.  Here we rely
     on `BISON_LOCALEDIR' to be defined as a string through the
     `Makefile'.

  4. In the `Makefile.am' that controls the compilation of the `main'
     function, make `BISON_LOCALEDIR' available as a C preprocessor
     macro, either in `DEFS' or in `AM_CPPFLAGS'.  For example:

          DEFS = @DEFS@ -DBISON_LOCALEDIR='"$(BISON_LOCALEDIR)"'

     or:

          AM_CPPFLAGS = -DBISON_LOCALEDIR='"$(BISON_LOCALEDIR)"'

  5. Finally, invoke the command `autoreconf' to generate the build
     infrastructure.


File: bison.info,  Node: Algorithm,  Next: Error Recovery,  Prev: Interface,  Up: Top

5 The Bison Parser Algorithm
****************************

As Bison reads tokens, it pushes them onto a stack along with their
semantic values.  The stack is called the "parser stack".  Pushing a
token is traditionally called "shifting".

   For example, suppose the infix calculator has read `1 + 5 *', with a
`3' to come.  The stack will have four elements, one for each token
that was shifted.

   But the stack does not always have an element for each token read.
When the last N tokens and groupings shifted match the components of a
grammar rule, they can be combined according to that rule.  This is
called "reduction".  Those tokens and groupings are replaced on the
stack by a single grouping whose symbol is the result (left hand side)
of that rule.  Running the rule's action is part of the process of
reduction, because this is what computes the semantic value of the
resulting grouping.

   For example, if the infix calculator's parser stack contains this:

     1 + 5 * 3

and the next input token is a newline character, then the last three
elements can be reduced to 15 via the rule:

     expr: expr '*' expr;

Then the stack contains just these three elements:

     1 + 15

At this point, another reduction can be made, resulting in the single
value 16.  Then the newline token can be shifted.

   The parser tries, by shifts and reductions, to reduce the entire
input down to a single grouping whose symbol is the grammar's
start-symbol (*note Languages and Context-Free Grammars: Language and
Grammar.).

   This kind of parser is known in the literature as a bottom-up parser.

* Menu:

* Lookahead::         Parser looks one token ahead when deciding what to do.
* Shift/Reduce::      Conflicts: when either shifting or reduction is valid.
* Precedence::        Operator precedence works by resolving conflicts.
* Contextual Precedence::  When an operator's precedence depends on context.
* Parser States::     The parser is a finite-state-machine with stack.
* Reduce/Reduce::     When two rules are applicable in the same situation.
* Mystery Conflicts:: Reduce/reduce conflicts that look unjustified.
* Generalized LR Parsing::  Parsing arbitrary context-free grammars.
* Memory Management:: What happens when memory is exhausted.  How to avoid it.


File: bison.info,  Node: Lookahead,  Next: Shift/Reduce,  Up: Algorithm

5.1 Lookahead Tokens
====================

The Bison parser does _not_ always reduce immediately as soon as the
last N tokens and groupings match a rule.  This is because such a
simple strategy is inadequate to handle most languages.  Instead, when a
reduction is possible, the parser sometimes "looks ahead" at the next
token in order to decide what to do.

   When a token is read, it is not immediately shifted; first it
becomes the "lookahead token", which is not on the stack.  Now the
parser can perform one or more reductions of tokens and groupings on
the stack, while the lookahead token remains off to the side.  When no
more reductions should take place, the lookahead token is shifted onto
the stack.  This does not mean that all possible reductions have been
done; depending on the token type of the lookahead token, some rules
may choose to delay their application.

   Here is a simple case where lookahead is needed.  These three rules
define expressions which contain binary addition operators and postfix
unary factorial operators (`!'), and allow parentheses for grouping.

     expr:     term '+' expr
             | term
             ;

     term:     '(' expr ')'
             | term '!'
             | NUMBER
             ;

   Suppose that the tokens `1 + 2' have been read and shifted; what
should be done?  If the following token is `)', then the first three
tokens must be reduced to form an `expr'.  This is the only valid
course, because shifting the `)' would produce a sequence of symbols
`term ')'', and no rule allows this.

   If the following token is `!', then it must be shifted immediately so
that `2 !' can be reduced to make a `term'.  If instead the parser were
to reduce before shifting, `1 + 2' would become an `expr'.  It would
then be impossible to shift the `!' because doing so would produce on
the stack the sequence of symbols `expr '!''.  No rule allows that
sequence.

   The lookahead token is stored in the variable `yychar'.  Its
semantic value and location, if any, are stored in the variables
`yylval' and `yylloc'.  *Note Special Features for Use in Actions:
Action Features.


File: bison.info,  Node: Shift/Reduce,  Next: Precedence,  Prev: Lookahead,  Up: Algorithm

5.2 Shift/Reduce Conflicts
==========================

Suppose we are parsing a language which has if-then and if-then-else
statements, with a pair of rules like this:

     if_stmt:
               IF expr THEN stmt
             | IF expr THEN stmt ELSE stmt
             ;

Here we assume that `IF', `THEN' and `ELSE' are terminal symbols for
specific keyword tokens.

   When the `ELSE' token is read and becomes the lookahead token, the
contents of the stack (assuming the input is valid) are just right for
reduction by the first rule.  But it is also legitimate to shift the
`ELSE', because that would lead to eventual reduction by the second
rule.

   This situation, where either a shift or a reduction would be valid,
is called a "shift/reduce conflict".  Bison is designed to resolve
these conflicts by choosing to shift, unless otherwise directed by
operator precedence declarations.  To see the reason for this, let's
contrast it with the other alternative.

   Since the parser prefers to shift the `ELSE', the result is to attach
the else-clause to the innermost if-statement, making these two inputs
equivalent:

     if x then if y then win (); else lose;

     if x then do; if y then win (); else lose; end;

   But if the parser chose to reduce when possible rather than shift,
the result would be to attach the else-clause to the outermost
if-statement, making these two inputs equivalent:

     if x then if y then win (); else lose;

     if x then do; if y then win (); end; else lose;

   The conflict exists because the grammar as written is ambiguous:
either parsing of the simple nested if-statement is legitimate.  The
established convention is that these ambiguities are resolved by
attaching the else-clause to the innermost if-statement; this is what
Bison accomplishes by choosing to shift rather than reduce.  (It would
ideally be cleaner to write an unambiguous grammar, but that is very
hard to do in this case.)  This particular ambiguity was first
encountered in the specifications of Algol 60 and is called the
"dangling `else'" ambiguity.

   To avoid warnings from Bison about predictable, legitimate
shift/reduce conflicts, use the `%expect N' declaration.  There will be
no warning as long as the number of shift/reduce conflicts is exactly N.
*Note Suppressing Conflict Warnings: Expect Decl.

   The definition of `if_stmt' above is solely to blame for the
conflict, but the conflict does not actually appear without additional
rules.  Here is a complete Bison input file that actually manifests the
conflict:

     %token IF THEN ELSE variable
     %%
     stmt:     expr
             | if_stmt
             ;

     if_stmt:
               IF expr THEN stmt
             | IF expr THEN stmt ELSE stmt
             ;

     expr:     variable
             ;


File: bison.info,  Node: Precedence,  Next: Contextual Precedence,  Prev: Shift/Reduce,  Up: Algorithm

5.3 Operator Precedence
=======================

Another situation where shift/reduce conflicts appear is in arithmetic
expressions.  Here shifting is not always the preferred resolution; the
Bison declarations for operator precedence allow you to specify when to
shift and when to reduce.

* Menu:

* Why Precedence::    An example showing why precedence is needed.
* Using Precedence::  How to specify precedence in Bison grammars.
* Precedence Examples::  How these features are used in the previous example.
* How Precedence::    How they work.


File: bison.info,  Node: Why Precedence,  Next: Using Precedence,  Up: Precedence

5.3.1 When Precedence is Needed
-------------------------------

Consider the following ambiguous grammar fragment (ambiguous because the
input `1 - 2 * 3' can be parsed in two different ways):

     expr:     expr '-' expr
             | expr '*' expr
             | expr '<' expr
             | '(' expr ')'
             ...
             ;

Suppose the parser has seen the tokens `1', `-' and `2'; should it
reduce them via the rule for the subtraction operator?  It depends on
the next token.  Of course, if the next token is `)', we must reduce;
shifting is invalid because no single rule can reduce the token
sequence `- 2 )' or anything starting with that.  But if the next token
is `*' or `<', we have a choice: either shifting or reduction would
allow the parse to complete, but with different results.

   To decide which one Bison should do, we must consider the results.
If the next operator token OP is shifted, then it must be reduced first
in order to permit another opportunity to reduce the difference.  The
result is (in effect) `1 - (2 OP 3)'.  On the other hand, if the
subtraction is reduced before shifting OP, the result is
`(1 - 2) OP 3'.  Clearly, then, the choice of shift or reduce should
depend on the relative precedence of the operators `-' and OP: `*'
should be shifted first, but not `<'.

   What about input such as `1 - 2 - 5'; should this be `(1 - 2) - 5'
or should it be `1 - (2 - 5)'?  For most operators we prefer the
former, which is called "left association".  The latter alternative,
"right association", is desirable for assignment operators.  The choice
of left or right association is a matter of whether the parser chooses
to shift or reduce when the stack contains `1 - 2' and the lookahead
token is `-': shifting makes right-associativity.


File: bison.info,  Node: Using Precedence,  Next: Precedence Examples,  Prev: Why Precedence,  Up: Precedence

5.3.2 Specifying Operator Precedence
------------------------------------

Bison allows you to specify these choices with the operator precedence
declarations `%left' and `%right'.  Each such declaration contains a
list of tokens, which are operators whose precedence and associativity
is being declared.  The `%left' declaration makes all those operators
left-associative and the `%right' declaration makes them
right-associative.  A third alternative is `%nonassoc', which declares
that it is a syntax error to find the same operator twice "in a row".

   The relative precedence of different operators is controlled by the
order in which they are declared.  The first `%left' or `%right'
declaration in the file declares the operators whose precedence is
lowest, the next such declaration declares the operators whose
precedence is a little higher, and so on.


File: bison.info,  Node: Precedence Examples,  Next: How Precedence,  Prev: Using Precedence,  Up: Precedence

5.3.3 Precedence Examples
-------------------------

In our example, we would want the following declarations:

     %left '<'
     %left '-'
     %left '*'

   In a more complete example, which supports other operators as well,
we would declare them in groups of equal precedence.  For example,
`'+'' is declared with `'-'':

     %left '<' '>' '=' NE LE GE
     %left '+' '-'
     %left '*' '/'

(Here `NE' and so on stand for the operators for "not equal" and so on.
We assume that these tokens are more than one character long and
therefore are represented by names, not character literals.)


File: bison.info,  Node: How Precedence,  Prev: Precedence Examples,  Up: Precedence

5.3.4 How Precedence Works
--------------------------

The first effect of the precedence declarations is to assign precedence
levels to the terminal symbols declared.  The second effect is to assign
precedence levels to certain rules: each rule gets its precedence from
the last terminal symbol mentioned in the components.  (You can also
specify explicitly the precedence of a rule.  *Note Context-Dependent
Precedence: Contextual Precedence.)

   Finally, the resolution of conflicts works by comparing the
precedence of the rule being considered with that of the lookahead
token.  If the token's precedence is higher, the choice is to shift.
If the rule's precedence is higher, the choice is to reduce.  If they
have equal precedence, the choice is made based on the associativity of
that precedence level.  The verbose output file made by `-v' (*note
Invoking Bison: Invocation.) says how each conflict was resolved.

   Not all rules and not all tokens have precedence.  If either the
rule or the lookahead token has no precedence, then the default is to
shift.


File: bison.info,  Node: Contextual Precedence,  Next: Parser States,  Prev: Precedence,  Up: Algorithm

5.4 Context-Dependent Precedence
================================

Often the precedence of an operator depends on the context.  This sounds
outlandish at first, but it is really very common.  For example, a minus
sign typically has a very high precedence as a unary operator, and a
somewhat lower precedence (lower than multiplication) as a binary
operator.

   The Bison precedence declarations, `%left', `%right' and
`%nonassoc', can only be used once for a given token; so a token has
only one precedence declared in this way.  For context-dependent
precedence, you need to use an additional mechanism: the `%prec'
modifier for rules.

   The `%prec' modifier declares the precedence of a particular rule by
specifying a terminal symbol whose precedence should be used for that
rule.  It's not necessary for that symbol to appear otherwise in the
rule.  The modifier's syntax is:

     %prec TERMINAL-SYMBOL

and it is written after the components of the rule.  Its effect is to
assign the rule the precedence of TERMINAL-SYMBOL, overriding the
precedence that would be deduced for it in the ordinary way.  The
altered rule precedence then affects how conflicts involving that rule
are resolved (*note Operator Precedence: Precedence.).

   Here is how `%prec' solves the problem of unary minus.  First,
declare a precedence for a fictitious terminal symbol named `UMINUS'.
There are no tokens of this type, but the symbol serves to stand for its
precedence:

     ...
     %left '+' '-'
     %left '*'
     %left UMINUS

   Now the precedence of `UMINUS' can be used in specific rules:

     exp:    ...
             | exp '-' exp
             ...
             | '-' exp %prec UMINUS


File: bison.info,  Node: Parser States,  Next: Reduce/Reduce,  Prev: Contextual Precedence,  Up: Algorithm

5.5 Parser States
=================

The function `yyparse' is implemented using a finite-state machine.
The values pushed on the parser stack are not simply token type codes;
they represent the entire sequence of terminal and nonterminal symbols
at or near the top of the stack.  The current state collects all the
information about previous input which is relevant to deciding what to
do next.

   Each time a lookahead token is read, the current parser state
together with the type of lookahead token are looked up in a table.
This table entry can say, "Shift the lookahead token."  In this case,
it also specifies the new parser state, which is pushed onto the top of
the parser stack.  Or it can say, "Reduce using rule number N."  This
means that a certain number of tokens or groupings are taken off the
top of the stack, and replaced by one grouping.  In other words, that
number of states are popped from the stack, and one new state is pushed.

   There is one other alternative: the table can say that the lookahead
token is erroneous in the current state.  This causes error processing
to begin (*note Error Recovery::).


File: bison.info,  Node: Reduce/Reduce,  Next: Mystery Conflicts,  Prev: Parser States,  Up: Algorithm

5.6 Reduce/Reduce Conflicts
===========================

A reduce/reduce conflict occurs if there are two or more rules that
apply to the same sequence of input.  This usually indicates a serious
error in the grammar.

   For example, here is an erroneous attempt to define a sequence of
zero or more `word' groupings.

     sequence: /* empty */
                     { printf ("empty sequence\n"); }
             | maybeword
             | sequence word
                     { printf ("added word %s\n", $2); }
             ;

     maybeword: /* empty */
                     { printf ("empty maybeword\n"); }
             | word
                     { printf ("single word %s\n", $1); }
             ;

The error is an ambiguity: there is more than one way to parse a single
`word' into a `sequence'.  It could be reduced to a `maybeword' and
then into a `sequence' via the second rule.  Alternatively,
nothing-at-all could be reduced into a `sequence' via the first rule,
and this could be combined with the `word' using the third rule for
`sequence'.

   There is also more than one way to reduce nothing-at-all into a
`sequence'.  This can be done directly via the first rule, or
indirectly via `maybeword' and then the second rule.

   You might think that this is a distinction without a difference,
because it does not change whether any particular input is valid or
not.  But it does affect which actions are run.  One parsing order runs
the second rule's action; the other runs the first rule's action and
the third rule's action.  In this example, the output of the program
changes.

   Bison resolves a reduce/reduce conflict by choosing to use the rule
that appears first in the grammar, but it is very risky to rely on
this.  Every reduce/reduce conflict must be studied and usually
eliminated.  Here is the proper way to define `sequence':

     sequence: /* empty */
                     { printf ("empty sequence\n"); }
             | sequence word
                     { printf ("added word %s\n", $2); }
             ;

   Here is another common error that yields a reduce/reduce conflict:

     sequence: /* empty */
             | sequence words
             | sequence redirects
             ;

     words:    /* empty */
             | words word
             ;

     redirects:/* empty */
             | redirects redirect
             ;

The intention here is to define a sequence which can contain either
`word' or `redirect' groupings.  The individual definitions of
`sequence', `words' and `redirects' are error-free, but the three
together make a subtle ambiguity: even an empty input can be parsed in
infinitely many ways!

   Consider: nothing-at-all could be a `words'.  Or it could be two
`words' in a row, or three, or any number.  It could equally well be a
`redirects', or two, or any number.  Or it could be a `words' followed
by three `redirects' and another `words'.  And so on.

   Here are two ways to correct these rules.  First, to make it a
single level of sequence:

     sequence: /* empty */
             | sequence word
             | sequence redirect
             ;

   Second, to prevent either a `words' or a `redirects' from being
empty:

     sequence: /* empty */
             | sequence words
             | sequence redirects
             ;

     words:    word
             | words word
             ;

     redirects:redirect
             | redirects redirect
             ;


File: bison.info,  Node: Mystery Conflicts,  Next: Generalized LR Parsing,  Prev: Reduce/Reduce,  Up: Algorithm

5.7 Mysterious Reduce/Reduce Conflicts
======================================

Sometimes reduce/reduce conflicts can occur that don't look warranted.
Here is an example:

     %token ID

     %%
     def:    param_spec return_spec ','
             ;
     param_spec:
                  type
             |    name_list ':' type
             ;
     return_spec:
                  type
             |    name ':' type
             ;
     type:        ID
             ;
     name:        ID
             ;
     name_list:
                  name
             |    name ',' name_list
             ;

   It would seem that this grammar can be parsed with only a single
token of lookahead: when a `param_spec' is being read, an `ID' is a
`name' if a comma or colon follows, or a `type' if another `ID'
follows.  In other words, this grammar is LR(1).

   However, Bison, like most parser generators, cannot actually handle
all LR(1) grammars.  In this grammar, two contexts, that after an `ID'
at the beginning of a `param_spec' and likewise at the beginning of a
`return_spec', are similar enough that Bison assumes they are the same.
They appear similar because the same set of rules would be active--the
rule for reducing to a `name' and that for reducing to a `type'.  Bison
is unable to determine at that stage of processing that the rules would
require different lookahead tokens in the two contexts, so it makes a
single parser state for them both.  Combining the two contexts causes a
conflict later.  In parser terminology, this occurrence means that the
grammar is not LALR(1).

   In general, it is better to fix deficiencies than to document them.
But this particular deficiency is intrinsically hard to fix; parser
generators that can handle LR(1) grammars are hard to write and tend to
produce parsers that are very large.  In practice, Bison is more useful
as it is now.

   When the problem arises, you can often fix it by identifying the two
parser states that are being confused, and adding something to make them
look distinct.  In the above example, adding one rule to `return_spec'
as follows makes the problem go away:

     %token BOGUS
     ...
     %%
     ...
     return_spec:
                  type
             |    name ':' type
             /* This rule is never used.  */
             |    ID BOGUS
             ;

   This corrects the problem because it introduces the possibility of an
additional active rule in the context after the `ID' at the beginning of
`return_spec'.  This rule is not active in the corresponding context in
a `param_spec', so the two contexts receive distinct parser states.  As
long as the token `BOGUS' is never generated by `yylex', the added rule
cannot alter the way actual input is parsed.

   In this particular example, there is another way to solve the
problem: rewrite the rule for `return_spec' to use `ID' directly
instead of via `name'.  This also causes the two confusing contexts to
have different sets of active rules, because the one for `return_spec'
activates the altered rule for `return_spec' rather than the one for
`name'.

     param_spec:
                  type
             |    name_list ':' type
             ;
     return_spec:
                  type
             |    ID ':' type
             ;

   For a more detailed exposition of LALR(1) parsers and parser
generators, please see: Frank DeRemer and Thomas Pennello, Efficient
Computation of LALR(1) Look-Ahead Sets, `ACM Transactions on
Programming Languages and Systems', Vol. 4, No. 4 (October 1982), pp.
615-649 `http://doi.acm.org/10.1145/69622.357187'.


File: bison.info,  Node: Generalized LR Parsing,  Next: Memory Management,  Prev: Mystery Conflicts,  Up: Algorithm

5.8 Generalized LR (GLR) Parsing
================================

Bison produces _deterministic_ parsers that choose uniquely when to
reduce and which reduction to apply based on a summary of the preceding
input and on one extra token of lookahead.  As a result, normal Bison
handles a proper subset of the family of context-free languages.
Ambiguous grammars, since they have strings with more than one possible
sequence of reductions cannot have deterministic parsers in this sense.
The same is true of languages that require more than one symbol of
lookahead, since the parser lacks the information necessary to make a
decision at the point it must be made in a shift-reduce parser.
Finally, as previously mentioned (*note Mystery Conflicts::), there are
languages where Bison's particular choice of how to summarize the input
seen so far loses necessary information.

   When you use the `%glr-parser' declaration in your grammar file,
Bison generates a parser that uses a different algorithm, called
Generalized LR (or GLR).  A Bison GLR parser uses the same basic
algorithm for parsing as an ordinary Bison parser, but behaves
differently in cases where there is a shift-reduce conflict that has not
been resolved by precedence rules (*note Precedence::) or a
reduce-reduce conflict.  When a GLR parser encounters such a situation,
it effectively _splits_ into a several parsers, one for each possible
shift or reduction.  These parsers then proceed as usual, consuming
tokens in lock-step.  Some of the stacks may encounter other conflicts
and split further, with the result that instead of a sequence of states,
a Bison GLR parsing stack is what is in effect a tree of states.

   In effect, each stack represents a guess as to what the proper parse
is.  Additional input may indicate that a guess was wrong, in which case
the appropriate stack silently disappears.  Otherwise, the semantics
actions generated in each stack are saved, rather than being executed
immediately.  When a stack disappears, its saved semantic actions never
get executed.  When a reduction causes two stacks to become equivalent,
their sets of semantic actions are both saved with the state that
results from the reduction.  We say that two stacks are equivalent when
they both represent the same sequence of states, and each pair of
corresponding states represents a grammar symbol that produces the same
segment of the input token stream.

   Whenever the parser makes a transition from having multiple states
to having one, it reverts to the normal LALR(1) parsing algorithm,
after resolving and executing the saved-up actions.  At this
transition, some of the states on the stack will have semantic values
that are sets (actually multisets) of possible actions.  The parser
tries to pick one of the actions by first finding one whose rule has
the highest dynamic precedence, as set by the `%dprec' declaration.
Otherwise, if the alternative actions are not ordered by precedence,
but there the same merging function is declared for both rules by the
`%merge' declaration, Bison resolves and evaluates both and then calls
the merge function on the result.  Otherwise, it reports an ambiguity.

   It is possible to use a data structure for the GLR parsing tree that
permits the processing of any LALR(1) grammar in linear time (in the
size of the input), any unambiguous (not necessarily LALR(1)) grammar in
quadratic worst-case time, and any general (possibly ambiguous)
context-free grammar in cubic worst-case time.  However, Bison currently
uses a simpler data structure that requires time proportional to the
length of the input times the maximum number of stacks required for any
prefix of the input.  Thus, really ambiguous or nondeterministic
grammars can require exponential time and space to process.  Such badly
behaving examples, however, are not generally of practical interest.
Usually, nondeterminism in a grammar is local--the parser is "in doubt"
only for a few tokens at a time.  Therefore, the current data structure
should generally be adequate.  On LALR(1) portions of a grammar, in
particular, it is only slightly slower than with the default Bison
parser.

   For a more detailed exposition of GLR parsers, please see: Elizabeth
Scott, Adrian Johnstone and Shamsa Sadaf Hussain, Tomita-Style
Generalised LR Parsers, Royal Holloway, University of London,
Department of Computer Science, TR-00-12,
`http://www.cs.rhul.ac.uk/research/languages/publications/tomita_style_1.ps',
(2000-12-24).


File: bison.info,  Node: Memory Management,  Prev: Generalized LR Parsing,  Up: Algorithm

5.9 Memory Management, and How to Avoid Memory Exhaustion
=========================================================

The Bison parser stack can run out of memory if too many tokens are
shifted and not reduced.  When this happens, the parser function
`yyparse' calls `yyerror' and then returns 2.

   Because Bison parsers have growing stacks, hitting the upper limit
usually results from using a right recursion instead of a left
recursion, *Note Recursive Rules: Recursion.

   By defining the macro `YYMAXDEPTH', you can control how deep the
parser stack can become before memory is exhausted.  Define the macro
with a value that is an integer.  This value is the maximum number of
tokens that can be shifted (and not reduced) before overflow.

   The stack space allowed is not necessarily allocated.  If you
specify a large value for `YYMAXDEPTH', the parser normally allocates a
small stack at first, and then makes it bigger by stages as needed.
This increasing allocation happens automatically and silently.
Therefore, you do not need to make `YYMAXDEPTH' painfully small merely
to save space for ordinary inputs that do not need much stack.

   However, do not allow `YYMAXDEPTH' to be a value so large that
arithmetic overflow could occur when calculating the size of the stack
space.  Also, do not allow `YYMAXDEPTH' to be less than `YYINITDEPTH'.

   The default value of `YYMAXDEPTH', if you do not define it, is 10000.

   You can control how much stack is allocated initially by defining the
macro `YYINITDEPTH' to a positive integer.  For the C LALR(1) parser,
this value must be a compile-time constant unless you are assuming C99
or some other target language or compiler that allows variable-length
arrays.  The default is 200.

   Do not allow `YYINITDEPTH' to be greater than `YYMAXDEPTH'.

   Because of semantical differences between C and C++, the LALR(1)
parsers in C produced by Bison cannot grow when compiled by C++
compilers.  In this precise case (compiling a C parser as C++) you are
suggested to grow `YYINITDEPTH'.  The Bison maintainers hope to fix
this deficiency in a future release.


File: bison.info,  Node: Error Recovery,  Next: Context Dependency,  Prev: Algorithm,  Up: Top

6 Error Recovery
****************

It is not usually acceptable to have a program terminate on a syntax
error.  For example, a compiler should recover sufficiently to parse the
rest of the input file and check it for errors; a calculator should
accept another expression.

   In a simple interactive command parser where each input is one line,
it may be sufficient to allow `yyparse' to return 1 on error and have
the caller ignore the rest of the input line when that happens (and
then call `yyparse' again).  But this is inadequate for a compiler,
because it forgets all the syntactic context leading up to the error.
A syntax error deep within a function in the compiler input should not
cause the compiler to treat the following line like the beginning of a
source file.

   You can define how to recover from a syntax error by writing rules to
recognize the special token `error'.  This is a terminal symbol that is
always defined (you need not declare it) and reserved for error
handling.  The Bison parser generates an `error' token whenever a
syntax error happens; if you have provided a rule to recognize this
token in the current context, the parse can continue.

   For example:

     stmnts:  /* empty string */
             | stmnts '\n'
             | stmnts exp '\n'
             | stmnts error '\n'

   The fourth rule in this example says that an error followed by a
newline makes a valid addition to any `stmnts'.

   What happens if a syntax error occurs in the middle of an `exp'?  The
error recovery rule, interpreted strictly, applies to the precise
sequence of a `stmnts', an `error' and a newline.  If an error occurs in
the middle of an `exp', there will probably be some additional tokens
and subexpressions on the stack after the last `stmnts', and there will
be tokens to read before the next newline.  So the rule is not
applicable in the ordinary way.

   But Bison can force the situation to fit the rule, by discarding
part of the semantic context and part of the input.  First it discards
states and objects from the stack until it gets back to a state in
which the `error' token is acceptable.  (This means that the
subexpressions already parsed are discarded, back to the last complete
`stmnts'.)  At this point the `error' token can be shifted.  Then, if
the old lookahead token is not acceptable to be shifted next, the
parser reads tokens and discards them until it finds a token which is
acceptable.  In this example, Bison reads and discards input until the
next newline so that the fourth rule can apply.  Note that discarded
symbols are possible sources of memory leaks, see *Note Freeing
Discarded Symbols: Destructor Decl, for a means to reclaim this memory.

   The choice of error rules in the grammar is a choice of strategies
for error recovery.  A simple and useful strategy is simply to skip the
rest of the current input line or current statement if an error is
detected:

     stmnt: error ';'  /* On error, skip until ';' is read.  */

   It is also useful to recover to the matching close-delimiter of an
opening-delimiter that has already been parsed.  Otherwise the
close-delimiter will probably appear to be unmatched, and generate
another, spurious error message:

     primary:  '(' expr ')'
             | '(' error ')'
             ...
             ;

   Error recovery strategies are necessarily guesses.  When they guess
wrong, one syntax error often leads to another.  In the above example,
the error recovery rule guesses that an error is due to bad input
within one `stmnt'.  Suppose that instead a spurious semicolon is
inserted in the middle of a valid `stmnt'.  After the error recovery
rule recovers from the first error, another syntax error will be found
straightaway, since the text following the spurious semicolon is also
an invalid `stmnt'.

   To prevent an outpouring of error messages, the parser will output
no error message for another syntax error that happens shortly after
the first; only after three consecutive input tokens have been
successfully shifted will error messages resume.

   Note that rules which accept the `error' token may have actions, just
as any other rules can.

   You can make error messages resume immediately by using the macro
`yyerrok' in an action.  If you do this in the error rule's action, no
error messages will be suppressed.  This macro requires no arguments;
`yyerrok;' is a valid C statement.

   The previous lookahead token is reanalyzed immediately after an
error.  If this is unacceptable, then the macro `yyclearin' may be used
to clear this token.  Write the statement `yyclearin;' in the error
rule's action.  *Note Special Features for Use in Actions: Action
Features.

   For example, suppose that on a syntax error, an error handling
routine is called that advances the input stream to some point where
parsing should once again commence.  The next symbol returned by the
lexical scanner is probably correct.  The previous lookahead token
ought to be discarded with `yyclearin;'.

   The expression `YYRECOVERING ()' yields 1 when the parser is
recovering from a syntax error, and 0 otherwise.  Syntax error
diagnostics are suppressed while recovering from a syntax error.


File: bison.info,  Node: Context Dependency,  Next: Debugging,  Prev: Error Recovery,  Up: Top

7 Handling Context Dependencies
*******************************

The Bison paradigm is to parse tokens first, then group them into larger
syntactic units.  In many languages, the meaning of a token is affected
by its context.  Although this violates the Bison paradigm, certain
techniques (known as "kludges") may enable you to write Bison parsers
for such languages.

* Menu:

* Semantic Tokens::   Token parsing can depend on the semantic context.
* Lexical Tie-ins::   Token parsing can depend on the syntactic context.
* Tie-in Recovery::   Lexical tie-ins have implications for how
                        error recovery rules must be written.

   (Actually, "kludge" means any technique that gets its job done but is
neither clean nor robust.)


File: bison.info,  Node: Semantic Tokens,  Next: Lexical Tie-ins,  Up: Context Dependency

7.1 Semantic Info in Token Types
================================

The C language has a context dependency: the way an identifier is used
depends on what its current meaning is.  For example, consider this:

     foo (x);

   This looks like a function call statement, but if `foo' is a typedef
name, then this is actually a declaration of `x'.  How can a Bison
parser for C decide how to parse this input?

   The method used in GNU C is to have two different token types,
`IDENTIFIER' and `TYPENAME'.  When `yylex' finds an identifier, it
looks up the current declaration of the identifier in order to decide
which token type to return: `TYPENAME' if the identifier is declared as
a typedef, `IDENTIFIER' otherwise.

   The grammar rules can then express the context dependency by the
choice of token type to recognize.  `IDENTIFIER' is accepted as an
expression, but `TYPENAME' is not.  `TYPENAME' can start a declaration,
but `IDENTIFIER' cannot.  In contexts where the meaning of the
identifier is _not_ significant, such as in declarations that can
shadow a typedef name, either `TYPENAME' or `IDENTIFIER' is
accepted--there is one rule for each of the two token types.

   This technique is simple to use if the decision of which kinds of
identifiers to allow is made at a place close to where the identifier is
parsed.  But in C this is not always so: C allows a declaration to
redeclare a typedef name provided an explicit type has been specified
earlier:

     typedef int foo, bar;
     int baz (void)
     {
       static bar (bar);      /* redeclare `bar' as static variable */
       extern foo foo (foo);  /* redeclare `foo' as function */
       return foo (bar);
     }

   Unfortunately, the name being declared is separated from the
declaration construct itself by a complicated syntactic structure--the
"declarator".

   As a result, part of the Bison parser for C needs to be duplicated,
with all the nonterminal names changed: once for parsing a declaration
in which a typedef name can be redefined, and once for parsing a
declaration in which that can't be done.  Here is a part of the
duplication, with actions omitted for brevity:

     initdcl:
               declarator maybeasm '='
               init
             | declarator maybeasm
             ;

     notype_initdcl:
               notype_declarator maybeasm '='
               init
             | notype_declarator maybeasm
             ;

Here `initdcl' can redeclare a typedef name, but `notype_initdcl'
cannot.  The distinction between `declarator' and `notype_declarator'
is the same sort of thing.

   There is some similarity between this technique and a lexical tie-in
(described next), in that information which alters the lexical analysis
is changed during parsing by other parts of the program.  The
difference is here the information is global, and is used for other
purposes in the program.  A true lexical tie-in has a special-purpose
flag controlled by the syntactic context.


File: bison.info,  Node: Lexical Tie-ins,  Next: Tie-in Recovery,  Prev: Semantic Tokens,  Up: Context Dependency

7.2 Lexical Tie-ins
===================

One way to handle context-dependency is the "lexical tie-in": a flag
which is set by Bison actions, whose purpose is to alter the way tokens
are parsed.

   For example, suppose we have a language vaguely like C, but with a
special construct `hex (HEX-EXPR)'.  After the keyword `hex' comes an
expression in parentheses in which all integers are hexadecimal.  In
particular, the token `a1b' must be treated as an integer rather than
as an identifier if it appears in that context.  Here is how you can do
it:

     %{
       int hexflag;
       int yylex (void);
       void yyerror (char const *);
     %}
     %%
     ...
     expr:   IDENTIFIER
             | constant
             | HEX '('
                     { hexflag = 1; }
               expr ')'
                     { hexflag = 0;
                        $$ = $4; }
             | expr '+' expr
                     { $$ = make_sum ($1, $3); }
             ...
             ;

     constant:
               INTEGER
             | STRING
             ;

Here we assume that `yylex' looks at the value of `hexflag'; when it is
nonzero, all integers are parsed in hexadecimal, and tokens starting
with letters are parsed as integers if possible.

   The declaration of `hexflag' shown in the prologue of the parser file
is needed to make it accessible to the actions (*note The Prologue:
Prologue.).  You must also write the code in `yylex' to obey the flag.


File: bison.info,  Node: Tie-in Recovery,  Prev: Lexical Tie-ins,  Up: Context Dependency

7.3 Lexical Tie-ins and Error Recovery
======================================

Lexical tie-ins make strict demands on any error recovery rules you
have.  *Note Error Recovery::.

   The reason for this is that the purpose of an error recovery rule is
to abort the parsing of one construct and resume in some larger
construct.  For example, in C-like languages, a typical error recovery
rule is to skip tokens until the next semicolon, and then start a new
statement, like this:

     stmt:   expr ';'
             | IF '(' expr ')' stmt { ... }
             ...
             error ';'
                     { hexflag = 0; }
             ;

   If there is a syntax error in the middle of a `hex (EXPR)'
construct, this error rule will apply, and then the action for the
completed `hex (EXPR)' will never run.  So `hexflag' would remain set
for the entire rest of the input, or until the next `hex' keyword,
causing identifiers to be misinterpreted as integers.

   To avoid this problem the error recovery rule itself clears
`hexflag'.

   There may also be an error recovery rule that works within
expressions.  For example, there could be a rule which applies within
parentheses and skips to the close-parenthesis:

     expr:   ...
             | '(' expr ')'
                     { $$ = $2; }
             | '(' error ')'
             ...

   If this rule acts within the `hex' construct, it is not going to
abort that construct (since it applies to an inner level of parentheses
within the construct).  Therefore, it should not clear the flag: the
rest of the `hex' construct should be parsed with the flag still in
effect.

   What if there is an error recovery rule which might abort out of the
`hex' construct or might not, depending on circumstances?  There is no
way you can write the action to determine whether a `hex' construct is
being aborted or not.  So if you are using a lexical tie-in, you had
better make sure your error recovery rules are not of this kind.  Each
rule must be such that you can be sure that it always will, or always
won't, have to clear the flag.


File: bison.info,  Node: Debugging,  Next: Invocation,  Prev: Context Dependency,  Up: Top

8 Debugging Your Parser
***********************

Developing a parser can be a challenge, especially if you don't
understand the algorithm (*note The Bison Parser Algorithm:
Algorithm.).  Even so, sometimes a detailed description of the automaton
can help (*note Understanding Your Parser: Understanding.), or tracing
the execution of the parser can give some insight on why it behaves
improperly (*note Tracing Your Parser: Tracing.).

* Menu:

* Understanding::     Understanding the structure of your parser.
* Tracing::           Tracing the execution of your parser.


File: bison.info,  Node: Understanding,  Next: Tracing,  Up: Debugging

8.1 Understanding Your Parser
=============================

As documented elsewhere (*note The Bison Parser Algorithm: Algorithm.)
Bison parsers are "shift/reduce automata".  In some cases (much more
frequent than one would hope), looking at this automaton is required to
tune or simply fix a parser.  Bison provides two different
representation of it, either textually or graphically (as a DOT file).

   The textual file is generated when the options `--report' or
`--verbose' are specified, see *Note Invoking Bison: Invocation.  Its
name is made by removing `.tab.c' or `.c' from the parser output file
name, and adding `.output' instead.  Therefore, if the input file is
`foo.y', then the parser file is called `foo.tab.c' by default.  As a
consequence, the verbose output file is called `foo.output'.

   The following grammar file, `calc.y', will be used in the sequel:

     %token NUM STR
     %left '+' '-'
     %left '*'
     %%
     exp: exp '+' exp
        | exp '-' exp
        | exp '*' exp
        | exp '/' exp
        | NUM
        ;
     useless: STR;
     %%

   `bison' reports:

     calc.y: warning: 1 nonterminal and 1 rule useless in grammar
     calc.y:11.1-7: warning: nonterminal useless in grammar: useless
     calc.y:11.10-12: warning: rule useless in grammar: useless: STR
     calc.y: conflicts: 7 shift/reduce

   When given `--report=state', in addition to `calc.tab.c', it creates
a file `calc.output' with contents detailed below.  The order of the
output and the exact presentation might vary, but the interpretation is
the same.

   The first section includes details on conflicts that were solved
thanks to precedence and/or associativity:

     Conflict in state 8 between rule 2 and token '+' resolved as reduce.
     Conflict in state 8 between rule 2 and token '-' resolved as reduce.
     Conflict in state 8 between rule 2 and token '*' resolved as shift.
...


The next section lists states that still have conflicts.

     State 8 conflicts: 1 shift/reduce
     State 9 conflicts: 1 shift/reduce
     State 10 conflicts: 1 shift/reduce
     State 11 conflicts: 4 shift/reduce

The next section reports useless tokens, nonterminal and rules.  Useless
nonterminals and rules are removed in order to produce a smaller parser,
but useless tokens are preserved, since they might be used by the
scanner (note the difference between "useless" and "unused" below):

     Nonterminals useless in grammar:
        useless

     Terminals unused in grammar:
        STR

     Rules useless in grammar:
     #6     useless: STR;

The next section reproduces the exact grammar that Bison used:

     Grammar

       Number, Line, Rule
         0   5 $accept -> exp $end
         1   5 exp -> exp '+' exp
         2   6 exp -> exp '-' exp
         3   7 exp -> exp '*' exp
         4   8 exp -> exp '/' exp
         5   9 exp -> NUM

and reports the uses of the symbols:

     Terminals, with rules where they appear

     $end (0) 0
     '*' (42) 3
     '+' (43) 1
     '-' (45) 2
     '/' (47) 4
     error (256)
     NUM (258) 5

     Nonterminals, with rules where they appear

     $accept (8)
         on left: 0
     exp (9)
         on left: 1 2 3 4 5, on right: 0 1 2 3 4

Bison then proceeds onto the automaton itself, describing each state
with it set of "items", also known as "pointed rules".  Each item is a
production rule together with a point (marked by `.') that the input
cursor.

     state 0

         $accept  ->  . exp $   (rule 0)

         NUM         shift, and go to state 1

         exp         go to state 2

   This reads as follows: "state 0 corresponds to being at the very
beginning of the parsing, in the initial rule, right before the start
symbol (here, `exp').  When the parser returns to this state right
after having reduced a rule that produced an `exp', the control flow
jumps to state 2.  If there is no such transition on a nonterminal
symbol, and the lookahead is a `NUM', then this token is shifted on the
parse stack, and the control flow jumps to state 1.  Any other
lookahead triggers a syntax error."

   Even though the only active rule in state 0 seems to be rule 0, the
report lists `NUM' as a lookahead token because `NUM' can be at the
beginning of any rule deriving an `exp'.  By default Bison reports the
so-called "core" or "kernel" of the item set, but if you want to see
more detail you can invoke `bison' with `--report=itemset' to list all
the items, include those that can be derived:

     state 0

         $accept  ->  . exp $   (rule 0)
         exp  ->  . exp '+' exp   (rule 1)
         exp  ->  . exp '-' exp   (rule 2)
         exp  ->  . exp '*' exp   (rule 3)
         exp  ->  . exp '/' exp   (rule 4)
         exp  ->  . NUM   (rule 5)

         NUM         shift, and go to state 1

         exp         go to state 2

In the state 1...

     state 1

         exp  ->  NUM .   (rule 5)

         $default    reduce using rule 5 (exp)

the rule 5, `exp: NUM;', is completed.  Whatever the lookahead token
(`$default'), the parser will reduce it.  If it was coming from state
0, then, after this reduction it will return to state 0, and will jump
to state 2 (`exp: go to state 2').

     state 2

         $accept  ->  exp . $   (rule 0)
         exp  ->  exp . '+' exp   (rule 1)
         exp  ->  exp . '-' exp   (rule 2)
         exp  ->  exp . '*' exp   (rule 3)
         exp  ->  exp . '/' exp   (rule 4)

         $           shift, and go to state 3
         '+'         shift, and go to state 4
         '-'         shift, and go to state 5
         '*'         shift, and go to state 6
         '/'         shift, and go to state 7

In state 2, the automaton can only shift a symbol.  For instance,
because of the item `exp -> exp . '+' exp', if the lookahead if `+', it
will be shifted on the parse stack, and the automaton control will jump
to state 4, corresponding to the item `exp -> exp '+' . exp'.  Since
there is no default action, any other token than those listed above
will trigger a syntax error.

   The state 3 is named the "final state", or the "accepting state":

     state 3

         $accept  ->  exp $ .   (rule 0)

         $default    accept

the initial rule is completed (the start symbol and the end of input
were read), the parsing exits successfully.

   The interpretation of states 4 to 7 is straightforward, and is left
to the reader.

     state 4

         exp  ->  exp '+' . exp   (rule 1)

         NUM         shift, and go to state 1

         exp         go to state 8

     state 5

         exp  ->  exp '-' . exp   (rule 2)

         NUM         shift, and go to state 1

         exp         go to state 9

     state 6

         exp  ->  exp '*' . exp   (rule 3)

         NUM         shift, and go to state 1

         exp         go to state 10

     state 7

         exp  ->  exp '/' . exp   (rule 4)

         NUM         shift, and go to state 1

         exp         go to state 11

   As was announced in beginning of the report, `State 8 conflicts: 1
shift/reduce':

     state 8

         exp  ->  exp . '+' exp   (rule 1)
         exp  ->  exp '+' exp .   (rule 1)
         exp  ->  exp . '-' exp   (rule 2)
         exp  ->  exp . '*' exp   (rule 3)
         exp  ->  exp . '/' exp   (rule 4)

         '*'         shift, and go to state 6
         '/'         shift, and go to state 7

         '/'         [reduce using rule 1 (exp)]
         $default    reduce using rule 1 (exp)

   Indeed, there are two actions associated to the lookahead `/':
either shifting (and going to state 7), or reducing rule 1.  The
conflict means that either the grammar is ambiguous, or the parser lacks
information to make the right decision.  Indeed the grammar is
ambiguous, as, since we did not specify the precedence of `/', the
sentence `NUM + NUM / NUM' can be parsed as `NUM + (NUM / NUM)', which
corresponds to shifting `/', or as `(NUM + NUM) / NUM', which
corresponds to reducing rule 1.

   Because in LALR(1) parsing a single decision can be made, Bison
arbitrarily chose to disable the reduction, see *Note Shift/Reduce
Conflicts: Shift/Reduce.  Discarded actions are reported in between
square brackets.

   Note that all the previous states had a single possible action:
either shifting the next token and going to the corresponding state, or
reducing a single rule.  In the other cases, i.e., when shifting _and_
reducing is possible or when _several_ reductions are possible, the
lookahead is required to select the action.  State 8 is one such state:
if the lookahead is `*' or `/' then the action is shifting, otherwise
the action is reducing rule 1.  In other words, the first two items,
corresponding to rule 1, are not eligible when the lookahead token is
`*', since we specified that `*' has higher precedence than `+'.  More
generally, some items are eligible only with some set of possible
lookahead tokens.  When run with `--report=lookahead', Bison specifies
these lookahead tokens:

     state 8

         exp  ->  exp . '+' exp   (rule 1)
         exp  ->  exp '+' exp .  [$, '+', '-', '/']   (rule 1)
         exp  ->  exp . '-' exp   (rule 2)
         exp  ->  exp . '*' exp   (rule 3)
         exp  ->  exp . '/' exp   (rule 4)

         '*'         shift, and go to state 6
         '/'         shift, and go to state 7

         '/'         [reduce using rule 1 (exp)]
         $default    reduce using rule 1 (exp)

   The remaining states are similar:

     state 9

         exp  ->  exp . '+' exp   (rule 1)
         exp  ->  exp . '-' exp   (rule 2)
         exp  ->  exp '-' exp .   (rule 2)
         exp  ->  exp . '*' exp   (rule 3)
         exp  ->  exp . '/' exp   (rule 4)

         '*'         shift, and go to state 6
         '/'         shift, and go to state 7

         '/'         [reduce using rule 2 (exp)]
         $default    reduce using rule 2 (exp)

     state 10

         exp  ->  exp . '+' exp   (rule 1)
         exp  ->  exp . '-' exp   (rule 2)
         exp  ->  exp . '*' exp   (rule 3)
         exp  ->  exp '*' exp .   (rule 3)
         exp  ->  exp . '/' exp   (rule 4)

         '/'         shift, and go to state 7

         '/'         [reduce using rule 3 (exp)]
         $default    reduce using rule 3 (exp)

     state 11

         exp  ->  exp . '+' exp   (rule 1)
         exp  ->  exp . '-' exp   (rule 2)
         exp  ->  exp . '*' exp   (rule 3)
         exp  ->  exp . '/' exp   (rule 4)
         exp  ->  exp '/' exp .   (rule 4)

         '+'         shift, and go to state 4
         '-'         shift, and go to state 5
         '*'         shift, and go to state 6
         '/'         shift, and go to state 7

         '+'         [reduce using rule 4 (exp)]
         '-'         [reduce using rule 4 (exp)]
         '*'         [reduce using rule 4 (exp)]
         '/'         [reduce using rule 4 (exp)]
         $default    reduce using rule 4 (exp)

Observe that state 11 contains conflicts not only due to the lack of
precedence of `/' with respect to `+', `-', and `*', but also because
the associativity of `/' is not specified.


File: bison.info,  Node: Tracing,  Prev: Understanding,  Up: Debugging

8.2 Tracing Your Parser
=======================

If a Bison grammar compiles properly but doesn't do what you want when
it runs, the `yydebug' parser-trace feature can help you figure out why.

   There are several means to enable compilation of trace facilities:

the macro `YYDEBUG'
     Define the macro `YYDEBUG' to a nonzero value when you compile the
     parser.  This is compliant with POSIX Yacc.  You could use
     `-DYYDEBUG=1' as a compiler option or you could put `#define
     YYDEBUG 1' in the prologue of the grammar file (*note The
     Prologue: Prologue.).

the option `-t', `--debug'
     Use the `-t' option when you run Bison (*note Invoking Bison:
     Invocation.).  This is POSIX compliant too.

the directive `%debug'
     Add the `%debug' directive (*note Bison Declaration Summary: Decl
     Summary.).  This is a Bison extension, which will prove useful
     when Bison will output parsers for languages that don't use a
     preprocessor.  Unless POSIX and Yacc portability matter to you,
     this is the preferred solution.

   We suggest that you always enable the debug option so that debugging
is always possible.

   The trace facility outputs messages with macro calls of the form
`YYFPRINTF (stderr, FORMAT, ARGS)' where FORMAT and ARGS are the usual
`printf' format and variadic arguments.  If you define `YYDEBUG' to a
nonzero value but do not define `YYFPRINTF', `<stdio.h>' is
automatically included and `YYFPRINTF' is defined to `fprintf'.

   Once you have compiled the program with trace facilities, the way to
request a trace is to store a nonzero value in the variable `yydebug'.
You can do this by making the C code do it (in `main', perhaps), or you
can alter the value with a C debugger.

   Each step taken by the parser when `yydebug' is nonzero produces a
line or two of trace information, written on `stderr'.  The trace
messages tell you these things:

   * Each time the parser calls `yylex', what kind of token was read.

   * Each time a token is shifted, the depth and complete contents of
     the state stack (*note Parser States::).

   * Each time a rule is reduced, which rule it is, and the complete
     contents of the state stack afterward.

   To make sense of this information, it helps to refer to the listing
file produced by the Bison `-v' option (*note Invoking Bison:
Invocation.).  This file shows the meaning of each state in terms of
positions in various rules, and also what each state will do with each
possible input token.  As you read the successive trace messages, you
can see that the parser is functioning according to its specification in
the listing file.  Eventually you will arrive at the place where
something undesirable happens, and you will see which parts of the
grammar are to blame.

   The parser file is a C program and you can use C debuggers on it,
but it's not easy to interpret what it is doing.  The parser function
is a finite-state machine interpreter, and aside from the actions it
executes the same code over and over.  Only the values of variables
show where in the grammar it is working.

   The debugging information normally gives the token type of each token
read, but not its semantic value.  You can optionally define a macro
named `YYPRINT' to provide a way to print the value.  If you define
`YYPRINT', it should take three arguments.  The parser will pass a
standard I/O stream, the numeric code for the token type, and the token
value (from `yylval').

   Here is an example of `YYPRINT' suitable for the multi-function
calculator (*note Declarations for `mfcalc': Mfcalc Declarations.):

     %{
       static void print_token_value (FILE *, int, YYSTYPE);
       #define YYPRINT(file, type, value) print_token_value (file, type, value)
     %}

     ... %% ... %% ...

     static void
     print_token_value (FILE *file, int type, YYSTYPE value)
     {
       if (type == VAR)
         fprintf (file, "%s", value.tptr->name);
       else if (type == NUM)
         fprintf (file, "%d", value.val);
     }


File: bison.info,  Node: Invocation,  Next: Other Languages,  Prev: Debugging,  Up: Top

9 Invoking Bison
****************

The usual way to invoke Bison is as follows:

     bison INFILE

   Here INFILE is the grammar file name, which usually ends in `.y'.
The parser file's name is made by replacing the `.y' with `.tab.c' and
removing any leading directory.  Thus, the `bison foo.y' file name
yields `foo.tab.c', and the `bison hack/foo.y' file name yields
`foo.tab.c'.  It's also possible, in case you are writing C++ code
instead of C in your grammar file, to name it `foo.ypp' or `foo.y++'.
Then, the output files will take an extension like the given one as
input (respectively `foo.tab.cpp' and `foo.tab.c++').  This feature
takes effect with all options that manipulate file names like `-o' or
`-d'.

   For example :

     bison -d INFILE.YXX
   will produce `infile.tab.cxx' and `infile.tab.hxx', and

     bison -d -o OUTPUT.C++ INFILE.Y
   will produce `output.c++' and `outfile.h++'.

   For compatibility with POSIX, the standard Bison distribution also
contains a shell script called `yacc' that invokes Bison with the `-y'
option.

* Menu:

* Bison Options::     All the options described in detail,
                        in alphabetical order by short options.
* Option Cross Key::  Alphabetical list of long options.
* Yacc Library::      Yacc-compatible `yylex' and `main'.


File: bison.info,  Node: Bison Options,  Next: Option Cross Key,  Up: Invocation

9.1 Bison Options
=================

Bison supports both traditional single-letter options and mnemonic long
option names.  Long option names are indicated with `--' instead of
`-'.  Abbreviations for option names are allowed as long as they are
unique.  When a long option takes an argument, like `--file-prefix',
connect the option name and the argument with `='.

   Here is a list of options that can be used with Bison, alphabetized
by short option.  It is followed by a cross key alphabetized by long
option.

Operations modes:
`-h'
`--help'
     Print a summary of the command-line options to Bison and exit.

`-V'
`--version'
     Print the version number of Bison and exit.

`--print-localedir'
     Print the name of the directory containing locale-dependent data.

`--print-datadir'
     Print the name of the directory containing skeletons and XSLT.

`-y'
`--yacc'
     Act more like the traditional Yacc command.  This can cause
     different diagnostics to be generated, and may change behavior in
     other minor ways.  Most importantly, imitate Yacc's output file
     name conventions, so that the parser output file is called
     `y.tab.c', and the other outputs are called `y.output' and
     `y.tab.h'.  Also, if generating an LALR(1) parser in C, generate
     `#define' statements in addition to an `enum' to associate token
     numbers with token names.  Thus, the following shell script can
     substitute for Yacc, and the Bison distribution contains such a
     script for compatibility with POSIX:

          #! /bin/sh
          bison -y "$@"

     The `-y'/`--yacc' option is intended for use with traditional Yacc
     grammars.  If your grammar uses a Bison extension like
     `%glr-parser', Bison might not be Yacc-compatible even if this
     option is specified.

`-W'
`--warnings'
     Output warnings falling in CATEGORY.  CATEGORY can be one of:
    `midrule-values'
          Warn about mid-rule values that are set but not used within
          any of the actions of the parent rule.  For example, warn
          about unused `$2' in:

               exp: '1' { $$ = 1; } '+' exp { $$ = $1 + $4; };

          Also warn about mid-rule values that are used but not set.
          For example, warn about unset `$$' in the mid-rule action in:

                exp: '1' { $1 = 1; } '+' exp { $$ = $2 + $4; };

          These warnings are not enabled by default since they
          sometimes prove to be false alarms in existing grammars
          employing the Yacc constructs `$0' or `$-N' (where N is some
          positive integer).

    `yacc'
          Incompatibilities with POSIX Yacc.

    `all'
          All the warnings.

    `none'
          Turn off all the warnings.

    `error'
          Treat warnings as errors.

     A category can be turned off by prefixing its name with `no-'.  For
     instance, `-Wno-syntax' will hide the warnings about unused
     variables.

Tuning the parser:

`-t'
`--debug'
     In the parser file, define the macro `YYDEBUG' to 1 if it is not
     already defined, so that the debugging facilities are compiled.
     *Note Tracing Your Parser: Tracing.

`-L LANGUAGE'
`--language=LANGUAGE'
     Specify the programming language for the generated parser, as if
     `%language' was specified (*note Bison Declaration Summary: Decl
     Summary.).  Currently supported languages include C, C++, and Java.
     LANGUAGE is case-insensitive.

     This option is experimental and its effect may be modified in
     future releases.

`--locations'
     Pretend that `%locations' was specified.  *Note Decl Summary::.

`-p PREFIX'
`--name-prefix=PREFIX'
     Pretend that `%name-prefix "PREFIX"' was specified.  *Note Decl
     Summary::.

`-l'
`--no-lines'
     Don't put any `#line' preprocessor commands in the parser file.
     Ordinarily Bison puts them in the parser file so that the C
     compiler and debuggers will associate errors with your source
     file, the grammar file.  This option causes them to associate
     errors with the parser file, treating it as an independent source
     file in its own right.

`-S FILE'
`--skeleton=FILE'
     Specify the skeleton to use, similar to `%skeleton' (*note Bison
     Declaration Summary: Decl Summary.).

     If FILE does not contain a `/', FILE is the name of a skeleton
     file in the Bison installation directory.  If it does, FILE is an
     absolute file name or a file name relative to the current working
     directory.  This is similar to how most shells resolve commands.

`-k'
`--token-table'
     Pretend that `%token-table' was specified.  *Note Decl Summary::.

Adjust the output:

`--defines[=FILE]'
     Pretend that `%defines' was specified, i.e., write an extra output
     file containing macro definitions for the token type names defined
     in the grammar, as well as a few other declarations.  *Note Decl
     Summary::.

`-d'
     This is the same as `--defines' except `-d' does not accept a FILE
     argument since POSIX Yacc requires that `-d' can be bundled with
     other short options.

`-b FILE-PREFIX'
`--file-prefix=PREFIX'
     Pretend that `%file-prefix' was specified, i.e., specify prefix to
     use for all Bison output file names.  *Note Decl Summary::.

`-r THINGS'
`--report=THINGS'
     Write an extra output file containing verbose description of the
     comma separated list of THINGS among:

    `state'
          Description of the grammar, conflicts (resolved and
          unresolved), and LALR automaton.

    `lookahead'
          Implies `state' and augments the description of the automaton
          with each rule's lookahead set.

    `itemset'
          Implies `state' and augments the description of the automaton
          with the full set of items for each state, instead of its
          core only.

`--report-file=FILE'
     Specify the FILE for the verbose description.

`-v'
`--verbose'
     Pretend that `%verbose' was specified, i.e., write an extra output
     file containing verbose descriptions of the grammar and parser.
     *Note Decl Summary::.

`-o FILE'
`--output=FILE'
     Specify the FILE for the parser file.

     The other output files' names are constructed from FILE as
     described under the `-v' and `-d' options.

`-g[FILE]'
`--graph[=FILE]'
     Output a graphical representation of the LALR(1) grammar automaton
     computed by Bison, in Graphviz (http://www.graphviz.org/) DOT
     (http://www.graphviz.org/doc/info/lang.html) format.  `FILE' is
     optional.  If omitted and the grammar file is `foo.y', the output
     file will be `foo.dot'.

`-x[FILE]'
`--xml[=FILE]'
     Output an XML report of the LALR(1) automaton computed by Bison.
     `FILE' is optional.  If omitted and the grammar file is `foo.y',
     the output file will be `foo.xml'.  (The current XML schema is
     experimental and may evolve.  More user feedback will help to
     stabilize it.)


File: bison.info,  Node: Option Cross Key,  Next: Yacc Library,  Prev: Bison Options,  Up: Invocation

9.2 Option Cross Key
====================

Here is a list of options, alphabetized by long option, to help you find
the corresponding short option.

Long Option                Short Option
------------------------------------------------- 
`--debug'                  `-t'
`--defines=[FILE]'         
`--file-prefix=PREFIX'     `-b' PREFIX
`--graph=[FILE]'           `-g' [FILE]
`--help'                   `-h'
`--language=LANGUAGE'      `-L' LANGUAGE
`--locations'              
`--name-prefix=PREFIX'     `-p' PREFIX
`--no-lines'               `-l'
`--output=FILE'            `-o' FILE
`--print-datadir'          
`--print-localedir'        
`--report-file=FILE'       
`--report=THINGS'          `-r' THINGS
`--skeleton=FILE'          `-S' FILE
`--token-table'            `-k'
`--verbose'                `-v'
`--version'                `-V'
`--warnings'               `-W'
`--xml=[FILE]'             `-x' [FILE]
`--yacc'                   `-y'


File: bison.info,  Node: Yacc Library,  Prev: Option Cross Key,  Up: Invocation

9.3 Yacc Library
================

The Yacc library contains default implementations of the `yyerror' and
`main' functions.  These default implementations are normally not
useful, but POSIX requires them.  To use the Yacc library, link your
program with the `-ly' option.  Note that Bison's implementation of the
Yacc library is distributed under the terms of the GNU General Public
License (*note Copying::).

   If you use the Yacc library's `yyerror' function, you should declare
`yyerror' as follows:

     int yyerror (char const *);

   Bison ignores the `int' value returned by this `yyerror'.  If you
use the Yacc library's `main' function, your `yyparse' function should
have the following type signature:

     int yyparse (void);


File: bison.info,  Node: Other Languages,  Next: FAQ,  Prev: Invocation,  Up: Top

10 Parsers Written In Other Languages
*************************************

* Menu:

* C++ Parsers::                 The interface to generate C++ parser classes
* Java Parsers::                The interface to generate Java parser classes


File: bison.info,  Node: C++ Parsers,  Next: Java Parsers,  Up: Other Languages

10.1 C++ Parsers
================

* Menu:

* C++ Bison Interface::         Asking for C++ parser generation
* C++ Semantic Values::         %union vs. C++
* C++ Location Values::         The position and location classes
* C++ Parser Interface::        Instantiating and running the parser
* C++ Scanner Interface::       Exchanges between yylex and parse
* A Complete C++ Example::      Demonstrating their use


File: bison.info,  Node: C++ Bison Interface,  Next: C++ Semantic Values,  Up: C++ Parsers

10.1.1 C++ Bison Interface
--------------------------

The C++ LALR(1) parser is selected using the skeleton directive,
`%skeleton "lalr1.c"', or the synonymous command-line option
`--skeleton=lalr1.c'.  *Note Decl Summary::.

   When run, `bison' will create several entities in the `yy' namespace.  Use
the `%define namespace' directive to change the namespace name, see
*Note Decl Summary::.  The various classes are generated in the
following files:

`position.hh'
`location.hh'
     The definition of the classes `position' and `location', used for
     location tracking.  *Note C++ Location Values::.

`stack.hh'
     An auxiliary class `stack' used by the parser.

`FILE.hh'
`FILE.cc'
     (Assuming the extension of the input file was `.yy'.)  The
     declaration and implementation of the C++ parser class.  The
     basename and extension of these two files follow the same rules as
     with regular C parsers (*note Invocation::).

     The header is _mandatory_; you must either pass `-d'/`--defines'
     to `bison', or use the `%defines' directive.

   All these files are documented using Doxygen; run `doxygen' for a
complete and accurate documentation.


File: bison.info,  Node: C++ Semantic Values,  Next: C++ Location Values,  Prev: C++ Bison Interface,  Up: C++ Parsers

10.1.2 C++ Semantic Values
--------------------------

The `%union' directive works as for C, see *Note The Collection of
Value Types: Union Decl.  In particular it produces a genuine
`union'(1), which have a few specific features in C++.
   - The type `YYSTYPE' is defined but its use is discouraged: rather
     you should refer to the parser's encapsulated type
     `yy::parser::semantic_type'.

   - Non POD (Plain Old Data) types cannot be used.  C++ forbids any
     instance of classes with constructors in unions: only _pointers_
     to such objects are allowed.

   Because objects have to be stored via pointers, memory is not
reclaimed automatically: using the `%destructor' directive is the only
means to avoid leaks.  *Note Freeing Discarded Symbols: Destructor Decl.

   ---------- Footnotes ----------

   (1) In the future techniques to allow complex types within
pseudo-unions (similar to Boost variants) might be implemented to
alleviate these issues.


File: bison.info,  Node: C++ Location Values,  Next: C++ Parser Interface,  Prev: C++ Semantic Values,  Up: C++ Parsers

10.1.3 C++ Location Values
--------------------------

When the directive `%locations' is used, the C++ parser supports
location tracking, see *Note Locations Overview: Locations.  Two
auxiliary classes define a `position', a single point in a file, and a
`location', a range composed of a pair of `position's (possibly
spanning several files).

 -- Method on position: std::string* file
     The name of the file.  It will always be handled as a pointer, the
     parser will never duplicate nor deallocate it.  As an experimental
     feature you may change it to `TYPE*' using `%define filename_type
     "TYPE"'.

 -- Method on position: unsigned int line
     The line, starting at 1.

 -- Method on position: unsigned int lines (int HEIGHT = 1)
     Advance by HEIGHT lines, resetting the column number.

 -- Method on position: unsigned int column
     The column, starting at 0.

 -- Method on position: unsigned int columns (int WIDTH = 1)
     Advance by WIDTH columns, without changing the line number.

 -- Method on position: position& operator+= (position& POS, int WIDTH)
 -- Method on position: position operator+ (const position& POS, int
          WIDTH)
 -- Method on position: position& operator-= (const position& POS, int
          WIDTH)
 -- Method on position: position operator- (position& POS, int WIDTH)
     Various forms of syntactic sugar for `columns'.

 -- Method on position: position operator<< (std::ostream O, const
          position& P)
     Report P on O like this: `FILE:LINE.COLUMN', or `LINE.COLUMN' if
     FILE is null.

 -- Method on location: position begin
 -- Method on location: position end
     The first, inclusive, position of the range, and the first beyond.

 -- Method on location: unsigned int columns (int WIDTH = 1)
 -- Method on location: unsigned int lines (int HEIGHT = 1)
     Advance the `end' position.

 -- Method on location: location operator+ (const location& BEGIN,
          const location& END)
 -- Method on location: location operator+ (const location& BEGIN, int
          WIDTH)
 -- Method on location: location operator+= (const location& LOC, int
          WIDTH)
     Various forms of syntactic sugar.

 -- Method on location: void step ()
     Move `begin' onto `end'.


File: bison.info,  Node: C++ Parser Interface,  Next: C++ Scanner Interface,  Prev: C++ Location Values,  Up: C++ Parsers

10.1.4 C++ Parser Interface
---------------------------

The output files `OUTPUT.hh' and `OUTPUT.cc' declare and define the
parser class in the namespace `yy'.  The class name defaults to
`parser', but may be changed using `%define parser_class_name "NAME"'.
The interface of this class is detailed below.  It can be extended
using the `%parse-param' feature: its semantics is slightly changed
since it describes an additional member of the parser class, and an
additional argument for its constructor.

 -- Type of parser: semantic_value_type
 -- Type of parser: location_value_type
     The types for semantics value and locations.

 -- Method on parser:  parser (TYPE1 ARG1, ...)
     Build a new parser object.  There are no arguments by default,
     unless `%parse-param {TYPE1 ARG1}' was used.

 -- Method on parser: int parse ()
     Run the syntactic analysis, and return 0 on success, 1 otherwise.

 -- Method on parser: std::ostream& debug_stream ()
 -- Method on parser: void set_debug_stream (std::ostream& O)
     Get or set the stream used for tracing the parsing.  It defaults to
     `std::cerr'.

 -- Method on parser: debug_level_type debug_level ()
 -- Method on parser: void set_debug_level (debug_level L)
     Get or set the tracing level.  Currently its value is either 0, no
     trace, or nonzero, full tracing.

 -- Method on parser: void error (const location_type& L, const
          std::string& M)
     The definition for this member function must be supplied by the
     user: the parser uses it to report a parser error occurring at L,
     described by M.


File: bison.info,  Node: C++ Scanner Interface,  Next: A Complete C++ Example,  Prev: C++ Parser Interface,  Up: C++ Parsers

10.1.5 C++ Scanner Interface
----------------------------

The parser invokes the scanner by calling `yylex'.  Contrary to C
parsers, C++ parsers are always pure: there is no point in using the
`%define api.pure' directive.  Therefore the interface is as follows.

 -- Method on parser: int yylex (semantic_value_type& YYLVAL,
          location_type& YYLLOC, TYPE1 ARG1, ...)
     Return the next token.  Its type is the return value, its semantic
     value and location being YYLVAL and YYLLOC.  Invocations of
     `%lex-param {TYPE1 ARG1}' yield additional arguments.


File: bison.info,  Node: A Complete C++ Example,  Prev: C++ Scanner Interface,  Up: C++ Parsers

10.1.6 A Complete C++ Example
-----------------------------

This section demonstrates the use of a C++ parser with a simple but
complete example.  This example should be available on your system,
ready to compile, in the directory "../bison/examples/calc++".  It
focuses on the use of Bison, therefore the design of the various C++
classes is very naive: no accessors, no encapsulation of members etc.
We will use a Lex scanner, and more precisely, a Flex scanner, to
demonstrate the various interaction.  A hand written scanner is
actually easier to interface with.

* Menu:

* Calc++ --- C++ Calculator::   The specifications
* Calc++ Parsing Driver::       An active parsing context
* Calc++ Parser::               A parser class
* Calc++ Scanner::              A pure C++ Flex scanner
* Calc++ Top Level::            Conducting the band


File: bison.info,  Node: Calc++ --- C++ Calculator,  Next: Calc++ Parsing Driver,  Up: A Complete C++ Example

10.1.6.1 Calc++ -- C++ Calculator
.................................

Of course the grammar is dedicated to arithmetics, a single expression,
possibly preceded by variable assignments.  An environment containing
possibly predefined variables such as `one' and `two', is exchanged
with the parser.  An example of valid input follows.

     three := 3
     seven := one + two * three
     seven * seven


File: bison.info,  Node: Calc++ Parsing Driver,  Next: Calc++ Parser,  Prev: Calc++ --- C++ Calculator,  Up: A Complete C++ Example

10.1.6.2 Calc++ Parsing Driver
..............................

To support a pure interface with the parser (and the scanner) the
technique of the "parsing context" is convenient: a structure
containing all the data to exchange.  Since, in addition to simply
launch the parsing, there are several auxiliary tasks to execute (open
the file for parsing, instantiate the parser etc.), we recommend
transforming the simple parsing context structure into a fully blown
"parsing driver" class.

   The declaration of this driver class, `calc++-driver.hh', is as
follows.  The first part includes the CPP guard and imports the
required standard library components, and the declaration of the parser
class.

     #ifndef CALCXX_DRIVER_HH
     # define CALCXX_DRIVER_HH
     # include <string>
     # include <map>
     # include "calc++-parser.hh"

Then comes the declaration of the scanning function.  Flex expects the
signature of `yylex' to be defined in the macro `YY_DECL', and the C++
parser expects it to be declared.  We can factor both as follows.

     // Tell Flex the lexer's prototype ...
     # define YY_DECL                                        \
       yy::calcxx_parser::token_type                         \
       yylex (yy::calcxx_parser::semantic_type* yylval,      \
              yy::calcxx_parser::location_type* yylloc,      \
              calcxx_driver& driver)
     // ... and declare it for the parser's sake.
     YY_DECL;

The `calcxx_driver' class is then declared with its most obvious
members.

     // Conducting the whole scanning and parsing of Calc++.
     class calcxx_driver
     {
     public:
       calcxx_driver ();
       virtual ~calcxx_driver ();

       std::map<std::string, int> variables;

       int result;

To encapsulate the coordination with the Flex scanner, it is useful to
have two members function to open and close the scanning phase.

       // Handling the scanner.
       void scan_begin ();
       void scan_end ();
       bool trace_scanning;

Similarly for the parser itself.

       // Run the parser.  Return 0 on success.
       int parse (const std::string& f);
       std::string file;
       bool trace_parsing;

To demonstrate pure handling of parse errors, instead of simply dumping
them on the standard error output, we will pass them to the compiler
driver using the following two member functions.  Finally, we close the
class declaration and CPP guard.

       // Error handling.
       void error (const yy::location& l, const std::string& m);
       void error (const std::string& m);
     };
     #endif // ! CALCXX_DRIVER_HH

   The implementation of the driver is straightforward.  The `parse'
member function deserves some attention.  The `error' functions are
simple stubs, they should actually register the located error messages
and set error state.

     #include "calc++-driver.hh"
     #include "calc++-parser.hh"

     calcxx_driver::calcxx_driver ()
       : trace_scanning (false), trace_parsing (false)
     {
       variables["one"] = 1;
       variables["two"] = 2;
     }

     calcxx_driver::~calcxx_driver ()
     {
     }

     int
     calcxx_driver::parse (const std::string &f)
     {
       file = f;
       scan_begin ();
       yy::calcxx_parser parser (*this);
       parser.set_debug_level (trace_parsing);
       int res = parser.parse ();
       scan_end ();
       return res;
     }

     void
     calcxx_driver::error (const yy::location& l, const std::string& m)
     {
       std::cerr << l << ": " << m << std::endl;
     }

     void
     calcxx_driver::error (const std::string& m)
     {
       std::cerr << m << std::endl;
     }


File: bison.info,  Node: Calc++ Parser,  Next: Calc++ Scanner,  Prev: Calc++ Parsing Driver,  Up: A Complete C++ Example

10.1.6.3 Calc++ Parser
......................

The parser definition file `calc++-parser.yy' starts by asking for the
C++ LALR(1) skeleton, the creation of the parser header file, and
specifies the name of the parser class.  Because the C++ skeleton
changed several times, it is safer to require the version you designed
the grammar for.

     %skeleton "lalr1.cc"                          /*  -*- C++ -*- */
     %require "2.4.1"
     %defines
     %define parser_class_name "calcxx_parser"

Then come the declarations/inclusions needed to define the `%union'.
Because the parser uses the parsing driver and reciprocally, both
cannot include the header of the other.  Because the driver's header
needs detailed knowledge about the parser class (in particular its
inner types), it is the parser's header which will simply use a forward
declaration of the driver.  *Note %code: Decl Summary.

     %code requires {
     # include <string>
     class calcxx_driver;
     }

The driver is passed by reference to the parser and to the scanner.
This provides a simple but effective pure interface, not relying on
global variables.

     // The parsing context.
     %parse-param { calcxx_driver& driver }
     %lex-param   { calcxx_driver& driver }

Then we request the location tracking feature, and initialize the first
location's file name.  Afterwards new locations are computed relatively
to the previous locations: the file name will be automatically
propagated.

     %locations
     %initial-action
     {
       // Initialize the initial location.
       @$.begin.filename = @$.end.filename = &driver.file;
     };

Use the two following directives to enable parser tracing and verbose
error messages.

     %debug
     %error-verbose

Semantic values cannot use "real" objects, but only pointers to them.

     // Symbols.
     %union
     {
       int          ival;
       std::string *sval;
     };

The code between `%code {' and `}' is output in the `*.cc' file; it
needs detailed knowledge about the driver.

     %code {
     # include "calc++-driver.hh"
     }

The token numbered as 0 corresponds to end of file; the following line
allows for nicer error messages referring to "end of file" instead of
"$end".  Similarly user friendly named are provided for each symbol.
Note that the tokens names are prefixed by `TOKEN_' to avoid name
clashes.

     %token        END      0 "end of file"
     %token        ASSIGN     ":="
     %token <sval> IDENTIFIER "identifier"
     %token <ival> NUMBER     "number"
     %type  <ival> exp

To enable memory deallocation during error recovery, use `%destructor'.

     %printer    { debug_stream () << *$$; } "identifier"
     %destructor { delete $$; } "identifier"

     %printer    { debug_stream () << $$; } <ival>

The grammar itself is straightforward.

     %%
     %start unit;
     unit: assignments exp  { driver.result = $2; };

     assignments: assignments assignment {}
                | /* Nothing.  */        {};

     assignment:
          "identifier" ":=" exp
            { driver.variables[*$1] = $3; delete $1; };

     %left '+' '-';
     %left '*' '/';
     exp: exp '+' exp   { $$ = $1 + $3; }
        | exp '-' exp   { $$ = $1 - $3; }
        | exp '*' exp   { $$ = $1 * $3; }
        | exp '/' exp   { $$ = $1 / $3; }
        | "identifier"  { $$ = driver.variables[*$1]; delete $1; }
        | "number"      { $$ = $1; };
     %%

Finally the `error' member function registers the errors to the driver.

     void
     yy::calcxx_parser::error (const yy::calcxx_parser::location_type& l,
                               const std::string& m)
     {
       driver.error (l, m);
     }


File: bison.info,  Node: Calc++ Scanner,  Next: Calc++ Top Level,  Prev: Calc++ Parser,  Up: A Complete C++ Example

10.1.6.4 Calc++ Scanner
.......................

The Flex scanner first includes the driver declaration, then the
parser's to get the set of defined tokens.

     %{                                            /* -*- C++ -*- */
     # include <cstdlib>
     # include <errno.h>
     # include <limits.h>
     # include <string>
     # include "calc++-driver.hh"
     # include "calc++-parser.hh"

     /* Work around an incompatibility in flex (at least versions
        2.5.31 through 2.5.33): it generates code that does
        not conform to C89.  See Debian bug 333231
        <http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=333231>.  */
     # undef yywrap
     # define yywrap() 1

     /* By default yylex returns int, we use token_type.
        Unfortunately yyterminate by default returns 0, which is
        not of token_type.  */
     #define yyterminate() return token::END
     %}

Because there is no `#include'-like feature we don't need `yywrap', we
don't need `unput' either, and we parse an actual file, this is not an
interactive session with the user.  Finally we enable the scanner
tracing features.

     %option noyywrap nounput batch debug

Abbreviations allow for more readable rules.

     id    [a-zA-Z][a-zA-Z_0-9]*
     int   [0-9]+
     blank [ \t]

The following paragraph suffices to track locations accurately.  Each
time `yylex' is invoked, the begin position is moved onto the end
position.  Then when a pattern is matched, the end position is advanced
of its width.  In case it matched ends of lines, the end cursor is
adjusted, and each time blanks are matched, the begin cursor is moved
onto the end cursor to effectively ignore the blanks preceding tokens.
Comments would be treated equally.

     %{
     # define YY_USER_ACTION  yylloc->columns (yyleng);
     %}
     %%
     %{
       yylloc->step ();
     %}
     {blank}+   yylloc->step ();
     [\n]+      yylloc->lines (yyleng); yylloc->step ();

The rules are simple, just note the use of the driver to report errors.
It is convenient to use a typedef to shorten
`yy::calcxx_parser::token::identifier' into `token::identifier' for
instance.

     %{
       typedef yy::calcxx_parser::token token;
     %}
                /* Convert ints to the actual type of tokens.  */
     [-+*/]     return yy::calcxx_parser::token_type (yytext[0]);
     ":="       return token::ASSIGN;
     {int}      {
       errno = 0;
       long n = strtol (yytext, NULL, 10);
       if (! (INT_MIN <= n && n <= INT_MAX && errno != ERANGE))
         driver.error (*yylloc, "integer is out of range");
       yylval->ival = n;
       return token::NUMBER;
     }
     {id}       yylval->sval = new std::string (yytext); return token::IDENTIFIER;
     .          driver.error (*yylloc, "invalid character");
     %%

Finally, because the scanner related driver's member function depend on
the scanner's data, it is simpler to implement them in this file.

     void
     calcxx_driver::scan_begin ()
     {
       yy_flex_debug = trace_scanning;
       if (file == "-")
         yyin = stdin;
       else if (!(yyin = fopen (file.c_str (), "r")))
         {
           error (std::string ("cannot open ") + file);
           exit (1);
         }
     }

     void
     calcxx_driver::scan_end ()
     {
       fclose (yyin);
     }


File: bison.info,  Node: Calc++ Top Level,  Prev: Calc++ Scanner,  Up: A Complete C++ Example

10.1.6.5 Calc++ Top Level
.........................

The top level file, `calc++.cc', poses no problem.

     #include <iostream>
     #include "calc++-driver.hh"

     int
     main (int argc, char *argv[])
     {
       calcxx_driver driver;
       for (++argv; argv[0]; ++argv)
         if (*argv == std::string ("-p"))
           driver.trace_parsing = true;
         else if (*argv == std::string ("-s"))
           driver.trace_scanning = true;
         else if (!driver.parse (*argv))
           std::cout << driver.result << std::endl;
     }


File: bison.info,  Node: Java Parsers,  Prev: C++ Parsers,  Up: Other Languages

10.2 Java Parsers
=================

* Menu:

* Java Bison Interface::        Asking for Java parser generation
* Java Semantic Values::        %type and %token vs. Java
* Java Location Values::        The position and location classes
* Java Parser Interface::       Instantiating and running the parser
* Java Scanner Interface::      Specifying the scanner for the parser
* Java Action Features::        Special features for use in actions
* Java Differences::            Differences between C/C++ and Java Grammars
* Java Declarations Summary::   List of Bison declarations used with Java


File: bison.info,  Node: Java Bison Interface,  Next: Java Semantic Values,  Up: Java Parsers

10.2.1 Java Bison Interface
---------------------------

(The current Java interface is experimental and may evolve.  More user
feedback will help to stabilize it.)

   The Java parser skeletons are selected using the `%language "Java"'
directive or the `-L java'/`--language=java' option.

   When generating a Java parser, `bison BASENAME.y' will create a
single Java source file named `BASENAME.java'.  Using an input file
without a `.y' suffix is currently broken.  The basename of the output
file can be changed by the `%file-prefix' directive or the
`-p'/`--name-prefix' option.  The entire output file name can be
changed by the `%output' directive or the `-o'/`--output' option.  The
output file contains a single class for the parser.

   You can create documentation for generated parsers using Javadoc.

   Contrary to C parsers, Java parsers do not use global variables; the
state of the parser is always local to an instance of the parser class.
Therefore, all Java parsers are "pure", and the `%pure-parser' and
`%define api.pure' directives does not do anything when used in Java.

   Push parsers are currently unsupported in Java and `%define
api.push_pull' have no effect.

   GLR parsers are currently unsupported in Java.  Do not use the
`glr-parser' directive.

   No header file can be generated for Java parsers.  Do not use the
`%defines' directive or the `-d'/`--defines' options.

   Currently, support for debugging and verbose errors are always
compiled in.  Thus the `%debug' and `%token-table' directives and the
`-t'/`--debug' and `-k'/`--token-table' options have no effect.  This
may change in the future to eliminate unused code in the generated
parser, so use `%debug' and `%verbose-error' explicitly if needed.
Also, in the future the `%token-table' directive might enable a public
interface to access the token names and codes.


File: bison.info,  Node: Java Semantic Values,  Next: Java Location Values,  Prev: Java Bison Interface,  Up: Java Parsers

10.2.2 Java Semantic Values
---------------------------

There is no `%union' directive in Java parsers.  Instead, the semantic
values' types (class names) should be specified in the `%type' or
`%token' directive:

     %type <Expression> expr assignment_expr term factor
     %type <Integer> number

   By default, the semantic stack is declared to have `Object' members,
which means that the class types you specify can be of any class.  To
improve the type safety of the parser, you can declare the common
superclass of all the semantic values using the `%define stype'
directive.  For example, after the following declaration:

     %define stype "ASTNode"

any `%type' or `%token' specifying a semantic type which is not a
subclass of ASTNode, will cause a compile-time error.

   Types used in the directives may be qualified with a package name.
Primitive data types are accepted for Java version 1.5 or later.  Note
that in this case the autoboxing feature of Java 1.5 will be used.
Generic types may not be used; this is due to a limitation in the
implementation of Bison, and may change in future releases.

   Java parsers do not support `%destructor', since the language adopts
garbage collection.  The parser will try to hold references to semantic
values for as little time as needed.

   Java parsers do not support `%printer', as `toString()' can be used
to print the semantic values.  This however may change (in a
backwards-compatible way) in future versions of Bison.


File: bison.info,  Node: Java Location Values,  Next: Java Parser Interface,  Prev: Java Semantic Values,  Up: Java Parsers

10.2.3 Java Location Values
---------------------------

When the directive `%locations' is used, the Java parser supports
location tracking, see *Note Locations Overview: Locations.  An
auxiliary user-defined class defines a "position", a single point in a
file; Bison itself defines a class representing a "location", a range
composed of a pair of positions (possibly spanning several files).  The
location class is an inner class of the parser; the name is `Location'
by default, and may also be renamed using `%define location_type
"CLASS-NAME'.

   The location class treats the position as a completely opaque value.
By default, the class name is `Position', but this can be changed with
`%define position_type "CLASS-NAME"'.  This class must be supplied by
the user.

 -- Instance Variable of Location: Position begin
 -- Instance Variable of Location: Position end
     The first, inclusive, position of the range, and the first beyond.

 -- Constructor on Location:  Location (Position LOC)
     Create a `Location' denoting an empty range located at a given
     point.

 -- Constructor on Location:  Location (Position BEGIN, Position END)
     Create a `Location' from the endpoints of the range.

 -- Method on Location: String toString ()
     Prints the range represented by the location.  For this to work
     properly, the position class should override the `equals' and
     `toString' methods appropriately.


File: bison.info,  Node: Java Parser Interface,  Next: Java Scanner Interface,  Prev: Java Location Values,  Up: Java Parsers

10.2.4 Java Parser Interface
----------------------------

The name of the generated parser class defaults to `YYParser'.  The
`YY' prefix may be changed using the `%name-prefix' directive or the
`-p'/`--name-prefix' option.  Alternatively, use `%define
parser_class_name "NAME"' to give a custom name to the class.  The
interface of this class is detailed below.

   By default, the parser class has package visibility.  A declaration
`%define public' will change to public visibility.  Remember that,
according to the Java language specification, the name of the `.java'
file should match the name of the class in this case.  Similarly, you
can use `abstract', `final' and `strictfp' with the `%define'
declaration to add other modifiers to the parser class.

   The Java package name of the parser class can be specified using the
`%define package' directive.  The superclass and the implemented
interfaces of the parser class can be specified with the `%define
extends' and `%define implements' directives.

   The parser class defines an inner class, `Location', that is used
for location tracking (see *Note Java Location Values::), and a inner
interface, `Lexer' (see *Note Java Scanner Interface::).  Other than
these inner class/interface, and the members described in the interface
below, all the other members and fields are preceded with a `yy' or
`YY' prefix to avoid clashes with user code.

   The parser class can be extended using the `%parse-param' directive.
Each occurrence of the directive will add a `protected final' field to
the parser class, and an argument to its constructor, which initialize
them automatically.

   Token names defined by `%token' and the predefined `EOF' token name
are added as constant fields to the parser class.

 -- Constructor on YYParser:  YYParser (LEX_PARAM, ..., PARSE_PARAM,
          ...)
     Build a new parser object with embedded `%code lexer'.  There are
     no parameters, unless `%parse-param's and/or `%lex-param's are
     used.

 -- Constructor on YYParser:  YYParser (Lexer LEXER, PARSE_PARAM, ...)
     Build a new parser object using the specified scanner.  There are
     no additional parameters unless `%parse-param's are used.

     If the scanner is defined by `%code lexer', this constructor is
     declared `protected' and is called automatically with a scanner
     created with the correct `%lex-param's.

 -- Method on YYParser: boolean parse ()
     Run the syntactic analysis, and return `true' on success, `false'
     otherwise.

 -- Method on YYParser: boolean recovering ()
     During the syntactic analysis, return `true' if recovering from a
     syntax error.  *Note Error Recovery::.

 -- Method on YYParser: java.io.PrintStream getDebugStream ()
 -- Method on YYParser: void setDebugStream (java.io.printStream O)
     Get or set the stream used for tracing the parsing.  It defaults to
     `System.err'.

 -- Method on YYParser: int getDebugLevel ()
 -- Method on YYParser: void setDebugLevel (int L)
     Get or set the tracing level.  Currently its value is either 0, no
     trace, or nonzero, full tracing.


File: bison.info,  Node: Java Scanner Interface,  Next: Java Action Features,  Prev: Java Parser Interface,  Up: Java Parsers

10.2.5 Java Scanner Interface
-----------------------------

There are two possible ways to interface a Bison-generated Java parser
with a scanner: the scanner may be defined by `%code lexer', or defined
elsewhere.  In either case, the scanner has to implement the `Lexer'
inner interface of the parser class.

   In the first case, the body of the scanner class is placed in `%code
lexer' blocks.  If you want to pass parameters from the parser
constructor to the scanner constructor, specify them with `%lex-param';
they are passed before `%parse-param's to the constructor.

   In the second case, the scanner has to implement the `Lexer'
interface, which is defined within the parser class (e.g.,
`YYParser.Lexer').  The constructor of the parser object will then
accept an object implementing the interface; `%lex-param' is not used
in this case.

   In both cases, the scanner has to implement the following methods.

 -- Method on Lexer: void yyerror (Location LOC, String MSG)
     This method is defined by the user to emit an error message.  The
     first parameter is omitted if location tracking is not active.
     Its type can be changed using `%define location_type "CLASS-NAME".'

 -- Method on Lexer: int yylex ()
     Return the next token.  Its type is the return value, its semantic
     value and location are saved and returned by the ther methods in
     the interface.

     Use `%define lex_throws' to specify any uncaught exceptions.
     Default is `java.io.IOException'.

 -- Method on Lexer: Position getStartPos ()
 -- Method on Lexer: Position getEndPos ()
     Return respectively the first position of the last token that
     `yylex' returned, and the first position beyond it.  These methods
     are not needed unless location tracking is active.

     The return type can be changed using `%define position_type
     "CLASS-NAME".'

 -- Method on Lexer: Object getLVal ()
     Return the semantical value of the last token that yylex returned.

     The return type can be changed using `%define stype "CLASS-NAME".'


File: bison.info,  Node: Java Action Features,  Next: Java Differences,  Prev: Java Scanner Interface,  Up: Java Parsers

10.2.6 Special Features for Use in Java Actions
-----------------------------------------------

The following special constructs can be uses in Java actions.  Other
analogous C action features are currently unavailable for Java.

   Use `%define throws' to specify any uncaught exceptions from parser
actions, and initial actions specified by `%initial-action'.

 -- Variable: $N
     The semantic value for the Nth component of the current rule.
     This may not be assigned to.  *Note Java Semantic Values::.

 -- Variable: $<TYPEALT>N
     Like `$N' but specifies a alternative type TYPEALT.  *Note Java
     Semantic Values::.

 -- Variable: $$
     The semantic value for the grouping made by the current rule.  As a
     value, this is in the base type (`Object' or as specified by
     `%define stype') as in not cast to the declared subtype because
     casts are not allowed on the left-hand side of Java assignments.
     Use an explicit Java cast if the correct subtype is needed.  *Note
     Java Semantic Values::.

 -- Variable: $<TYPEALT>$
     Same as `$$' since Java always allow assigning to the base type.
     Perhaps we should use this and `$<>$' for the value and `$$' for
     setting the value but there is currently no easy way to distinguish
     these constructs.  *Note Java Semantic Values::.

 -- Variable: @N
     The location information of the Nth component of the current rule.
     This may not be assigned to.  *Note Java Location Values::.

 -- Variable: @$
     The location information of the grouping made by the current rule.
     *Note Java Location Values::.

 -- Statement: return YYABORT;
     Return immediately from the parser, indicating failure.  *Note
     Java Parser Interface::.

 -- Statement: return YYACCEPT;
     Return immediately from the parser, indicating success.  *Note
     Java Parser Interface::.

 -- Statement: return YYERROR;
     Start error recovery without printing an error message.  *Note
     Error Recovery::.

 -- Statement: return YYFAIL;
     Print an error message and start error recovery.  *Note Error
     Recovery::.

 -- Function: boolean recovering ()
     Return whether error recovery is being done. In this state, the
     parser reads token until it reaches a known state, and then
     restarts normal operation.  *Note Error Recovery::.

 -- Function: protected void yyerror (String msg)
 -- Function: protected void yyerror (Position pos, String msg)
 -- Function: protected void yyerror (Location loc, String msg)
     Print an error message using the `yyerror' method of the scanner
     instance in use.


File: bison.info,  Node: Java Differences,  Next: Java Declarations Summary,  Prev: Java Action Features,  Up: Java Parsers

10.2.7 Differences between C/C++ and Java Grammars
--------------------------------------------------

The different structure of the Java language forces several differences
between C/C++ grammars, and grammars designed for Java parsers.  This
section summarizes these differences.

   * Java lacks a preprocessor, so the `YYERROR', `YYACCEPT', `YYABORT'
     symbols (*note Table of Symbols::) cannot obviously be macros.
     Instead, they should be preceded by `return' when they appear in
     an action.  The actual definition of these symbols is opaque to
     the Bison grammar, and it might change in the future.  The only
     meaningful operation that you can do, is to return them.  See
     *note Java Action Features::.

     Note that of these three symbols, only `YYACCEPT' and `YYABORT'
     will cause a return from the `yyparse' method(1).

   * Java lacks unions, so `%union' has no effect.  Instead, semantic
     values have a common base type: `Object' or as specified by
     `%define stype'.  Angle backets on `%token', `type', `$N' and `$$'
     specify subtypes rather than fields of an union.  The type of
     `$$', even with angle brackets, is the base type since Java casts
     are not allow on the left-hand side of assignments.  Also, `$N'
     and `@N' are not allowed on the left-hand side of assignments. See
     *note Java Semantic Values:: and *note Java Action Features::.

   * The prolog declarations have a different meaning than in C/C++
     code.
    `%code imports'
          blocks are placed at the beginning of the Java source code.
          They may include copyright notices.  For a `package'
          declarations, it is suggested to use `%define package'
          instead.

    unqualified `%code'
          blocks are placed inside the parser class.

    `%code lexer'
          blocks, if specified, should include the implementation of the
          scanner.  If there is no such block, the scanner can be any
          class that implements the appropriate interface (see *note
          Java Scanner Interface::).

     Other `%code' blocks are not supported in Java parsers.  In
     particular, `%{ ... %}' blocks should not be used and may give an
     error in future versions of Bison.

     The epilogue has the same meaning as in C/C++ code and it can be
     used to define other classes used by the parser _outside_ the
     parser class.

   ---------- Footnotes ----------

   (1) Java parsers include the actions in a separate method than
`yyparse' in order to have an intuitive syntax that corresponds to
these C macros.


File: bison.info,  Node: Java Declarations Summary,  Prev: Java Differences,  Up: Java Parsers

10.2.8 Java Declarations Summary
--------------------------------

This summary only include declarations specific to Java or have special
meaning when used in a Java parser.

 -- Directive: %language "Java"
     Generate a Java class for the parser.

 -- Directive: %lex-param {TYPE NAME}
     A parameter for the lexer class defined by `%code lexer' _only_,
     added as parameters to the lexer constructor and the parser
     constructor that _creates_ a lexer.  Default is none.  *Note Java
     Scanner Interface::.

 -- Directive: %name-prefix "PREFIX"
     The prefix of the parser class name `PREFIXParser' if `%define
     parser_class_name' is not used.  Default is `YY'.  *Note Java
     Bison Interface::.

 -- Directive: %parse-param {TYPE NAME}
     A parameter for the parser class added as parameters to
     constructor(s) and as fields initialized by the constructor(s).
     Default is none.  *Note Java Parser Interface::.

 -- Directive: %token <TYPE> TOKEN ...
     Declare tokens.  Note that the angle brackets enclose a Java
     _type_.  *Note Java Semantic Values::.

 -- Directive: %type <TYPE> NONTERMINAL ...
     Declare the type of nonterminals.  Note that the angle brackets
     enclose a Java _type_.  *Note Java Semantic Values::.

 -- Directive: %code { CODE ... }
     Code appended to the inside of the parser class.  *Note Java
     Differences::.

 -- Directive: %code imports { CODE ... }
     Code inserted just after the `package' declaration.  *Note Java
     Differences::.

 -- Directive: %code lexer { CODE ... }
     Code added to the body of a inner lexer class within the parser
     class.  *Note Java Scanner Interface::.

 -- Directive: %% CODE ...
     Code (after the second `%%') appended to the end of the file,
     _outside_ the parser class.  *Note Java Differences::.

 -- Directive: %{ CODE ... %}
     Not supported.  Use `%code import' instead.  *Note Java
     Differences::.

 -- Directive: %define abstract
     Whether the parser class is declared `abstract'.  Default is false.
     *Note Java Bison Interface::.

 -- Directive: %define extends "SUPERCLASS"
     The superclass of the parser class.  Default is none.  *Note Java
     Bison Interface::.

 -- Directive: %define final
     Whether the parser class is declared `final'.  Default is false.
     *Note Java Bison Interface::.

 -- Directive: %define implements "INTERFACES"
     The implemented interfaces of the parser class, a comma-separated
     list.  Default is none.  *Note Java Bison Interface::.

 -- Directive: %define lex_throws "EXCEPTIONS"
     The exceptions thrown by the `yylex' method of the lexer, a
     comma-separated list.  Default is `java.io.IOException'.  *Note
     Java Scanner Interface::.

 -- Directive: %define location_type "CLASS"
     The name of the class used for locations (a range between two
     positions).  This class is generated as an inner class of the
     parser class by `bison'.  Default is `Location'.  *Note Java
     Location Values::.

 -- Directive: %define package "PACKAGE"
     The package to put the parser class in.  Default is none.  *Note
     Java Bison Interface::.

 -- Directive: %define parser_class_name "NAME"
     The name of the parser class.  Default is `YYParser' or
     `NAME-PREFIXParser'.  *Note Java Bison Interface::.

 -- Directive: %define position_type "CLASS"
     The name of the class used for positions. This class must be
     supplied by the user.  Default is `Position'.  *Note Java Location
     Values::.

 -- Directive: %define public
     Whether the parser class is declared `public'.  Default is false.
     *Note Java Bison Interface::.

 -- Directive: %define stype "CLASS"
     The base type of semantic values.  Default is `Object'.  *Note
     Java Semantic Values::.

 -- Directive: %define strictfp
     Whether the parser class is declared `strictfp'.  Default is false.
     *Note Java Bison Interface::.

 -- Directive: %define throws "EXCEPTIONS"
     The exceptions thrown by user-supplied parser actions and
     `%initial-action', a comma-separated list.  Default is none.
     *Note Java Parser Interface::.


File: bison.info,  Node: FAQ,  Next: Table of Symbols,  Prev: Other Languages,  Up: Top

11 Frequently Asked Questions
*****************************

Several questions about Bison come up occasionally.  Here some of them
are addressed.

* Menu:

* Memory Exhausted::            Breaking the Stack Limits
* How Can I Reset the Parser::  `yyparse' Keeps some State
* Strings are Destroyed::       `yylval' Loses Track of Strings
* Implementing Gotos/Loops::    Control Flow in the Calculator
* Multiple start-symbols::      Factoring closely related grammars
* Secure?  Conform?::           Is Bison POSIX safe?
* I can't build Bison::         Troubleshooting
* Where can I find help?::      Troubleshouting
* Bug Reports::                 Troublereporting
* More Languages::              Parsers in C++, Java, and so on
* Beta Testing::                Experimenting development versions
* Mailing Lists::               Meeting other Bison users


File: bison.info,  Node: Memory Exhausted,  Next: How Can I Reset the Parser,  Up: FAQ

11.1 Memory Exhausted
=====================

     My parser returns with error with a `memory exhausted'
     message.  What can I do?

   This question is already addressed elsewhere, *Note Recursive Rules:
Recursion.


File: bison.info,  Node: How Can I Reset the Parser,  Next: Strings are Destroyed,  Prev: Memory Exhausted,  Up: FAQ

11.2 How Can I Reset the Parser
===============================

The following phenomenon has several symptoms, resulting in the
following typical questions:

     I invoke `yyparse' several times, and on correct input it works
     properly; but when a parse error is found, all the other calls fail
     too.  How can I reset the error flag of `yyparse'?

or

     My parser includes support for an `#include'-like feature, in
     which case I run `yyparse' from `yyparse'.  This fails
     although I did specify `%define api.pure'.

   These problems typically come not from Bison itself, but from
Lex-generated scanners.  Because these scanners use large buffers for
speed, they might not notice a change of input file.  As a
demonstration, consider the following source file, `first-line.l':


%{
#include <stdio.h>
#include <stdlib.h>
%}
%%
.*\n    ECHO; return 1;
%%
int
yyparse (char const *file)
{
  yyin = fopen (file, "r");
  if (!yyin)
    exit (2);
  /* One token only.  */
  yylex ();
  if (fclose (yyin) != 0)
    exit (3);
  return 0;
}

int
main (void)
{
  yyparse ("input");
  yyparse ("input");
  return 0;
}

If the file `input' contains


input:1: Hello,
input:2: World!

then instead of getting the first line twice, you get:

     $ flex -ofirst-line.c first-line.l
     $ gcc  -ofirst-line   first-line.c -ll
     $ ./first-line
     input:1: Hello,
     input:2: World!

   Therefore, whenever you change `yyin', you must tell the
Lex-generated scanner to discard its current buffer and switch to the
new one.  This depends upon your implementation of Lex; see its
documentation for more.  For Flex, it suffices to call
`YY_FLUSH_BUFFER' after each change to `yyin'.  If your Flex-generated
scanner needs to read from several input streams to handle features
like include files, you might consider using Flex functions like
`yy_switch_to_buffer' that manipulate multiple input buffers.

   If your Flex-generated scanner uses start conditions (*note Start
conditions: (flex)Start conditions.), you might also want to reset the
scanner's state, i.e., go back to the initial start condition, through
a call to `BEGIN (0)'.


File: bison.info,  Node: Strings are Destroyed,  Next: Implementing Gotos/Loops,  Prev: How Can I Reset the Parser,  Up: FAQ

11.3 Strings are Destroyed
==========================

     My parser seems to destroy old strings, or maybe it loses track of
     them.  Instead of reporting `"foo", "bar"', it reports
     `"bar", "bar"', or even `"foo\nbar", "bar"'.

   This error is probably the single most frequent "bug report" sent to
Bison lists, but is only concerned with a misunderstanding of the role
of the scanner.  Consider the following Lex code:


%{
#include <stdio.h>
char *yylval = NULL;
%}
%%
.*    yylval = yytext; return 1;
\n    /* IGNORE */
%%
int
main ()
{
  /* Similar to using $1, $2 in a Bison action.  */
  char *fst = (yylex (), yylval);
  char *snd = (yylex (), yylval);
  printf ("\"%s\", \"%s\"\n", fst, snd);
  return 0;
}

   If you compile and run this code, you get:

     $ flex -osplit-lines.c split-lines.l
     $ gcc  -osplit-lines   split-lines.c -ll
     $ printf 'one\ntwo\n' | ./split-lines
     "one
     two", "two"

this is because `yytext' is a buffer provided for _reading_ in the
action, but if you want to keep it, you have to duplicate it (e.g.,
using `strdup').  Note that the output may depend on how your
implementation of Lex handles `yytext'.  For instance, when given the
Lex compatibility option `-l' (which triggers the option `%array') Flex
generates a different behavior:

     $ flex -l -osplit-lines.c split-lines.l
     $ gcc     -osplit-lines   split-lines.c -ll
     $ printf 'one\ntwo\n' | ./split-lines
     "two", "two"


File: bison.info,  Node: Implementing Gotos/Loops,  Next: Multiple start-symbols,  Prev: Strings are Destroyed,  Up: FAQ

11.4 Implementing Gotos/Loops
=============================

     My simple calculator supports variables, assignments, and functions,
     but how can I implement gotos, or loops?

   Although very pedagogical, the examples included in the document blur
the distinction to make between the parser--whose job is to recover the
structure of a text and to transmit it to subsequent modules of the
program--and the processing (such as the execution) of this structure.
This works well with so called straight line programs, i.e., precisely
those that have a straightforward execution model: execute simple
instructions one after the others.

   If you want a richer model, you will probably need to use the parser
to construct a tree that does represent the structure it has recovered;
this tree is usually called the "abstract syntax tree", or "AST" for
short.  Then, walking through this tree, traversing it in various ways,
will enable treatments such as its execution or its translation, which
will result in an interpreter or a compiler.

   This topic is way beyond the scope of this manual, and the reader is
invited to consult the dedicated literature.


File: bison.info,  Node: Multiple start-symbols,  Next: Secure? Conform?,  Prev: Implementing Gotos/Loops,  Up: FAQ

11.5 Multiple start-symbols
===========================

     I have several closely related grammars, and I would like to share their
     implementations.  In fact, I could use a single grammar but with
     multiple entry points.

   Bison does not support multiple start-symbols, but there is a very
simple means to simulate them.  If `foo' and `bar' are the two pseudo
start-symbols, then introduce two new tokens, say `START_FOO' and
`START_BAR', and use them as switches from the real start-symbol:

     %token START_FOO START_BAR;
     %start start;
     start: START_FOO foo
          | START_BAR bar;

   These tokens prevents the introduction of new conflicts.  As far as
the parser goes, that is all that is needed.

   Now the difficult part is ensuring that the scanner will send these
tokens first.  If your scanner is hand-written, that should be
straightforward.  If your scanner is generated by Lex, them there is
simple means to do it: recall that anything between `%{ ... %}' after
the first `%%' is copied verbatim in the top of the generated `yylex'
function.  Make sure a variable `start_token' is available in the
scanner (e.g., a global variable or using `%lex-param' etc.), and use
the following:

       /* Prologue.  */
     %%
     %{
       if (start_token)
         {
           int t = start_token;
           start_token = 0;
           return t;
         }
     %}
       /* The rules.  */


File: bison.info,  Node: Secure? Conform?,  Next: I can't build Bison,  Prev: Multiple start-symbols,  Up: FAQ

11.6 Secure?  Conform?
======================

     Is Bison secure?  Does it conform to POSIX?

   If you're looking for a guarantee or certification, we don't provide
it.  However, Bison is intended to be a reliable program that conforms
to the POSIX specification for Yacc.  If you run into problems, please
send us a bug report.


File: bison.info,  Node: I can't build Bison,  Next: Where can I find help?,  Prev: Secure? Conform?,  Up: FAQ

11.7 I can't build Bison
========================

     I can't build Bison because `make' complains that
     `msgfmt' is not found.
     What should I do?

   Like most GNU packages with internationalization support, that
feature is turned on by default.  If you have problems building in the
`po' subdirectory, it indicates that your system's internationalization
support is lacking.  You can re-configure Bison with `--disable-nls' to
turn off this support, or you can install GNU gettext from
`ftp://ftp.gnu.org/gnu/gettext/' and re-configure Bison.  See the file
`ABOUT-NLS' for more information.


File: bison.info,  Node: Where can I find help?,  Next: Bug Reports,  Prev: I can't build Bison,  Up: FAQ

11.8 Where can I find help?
===========================

     I'm having trouble using Bison.  Where can I find help?

   First, read this fine manual.  Beyond that, you can send mail to
<help-bison@gnu.org>.  This mailing list is intended to be populated
with people who are willing to answer questions about using and
installing Bison.  Please keep in mind that (most of) the people on the
list have aspects of their lives which are not related to Bison (!), so
you may not receive an answer to your question right away.  This can be
frustrating, but please try not to honk them off; remember that any
help they provide is purely voluntary and out of the kindness of their
hearts.


File: bison.info,  Node: Bug Reports,  Next: More Languages,  Prev: Where can I find help?,  Up: FAQ

11.9 Bug Reports
================

     I found a bug.  What should I include in the bug report?

   Before you send a bug report, make sure you are using the latest
version.  Check `ftp://ftp.gnu.org/pub/gnu/bison/' or one of its
mirrors.  Be sure to include the version number in your bug report.  If
the bug is present in the latest version but not in a previous version,
try to determine the most recent version which did not contain the bug.

   If the bug is parser-related, you should include the smallest grammar
you can which demonstrates the bug.  The grammar file should also be
complete (i.e., I should be able to run it through Bison without having
to edit or add anything).  The smaller and simpler the grammar, the
easier it will be to fix the bug.

   Include information about your compilation environment, including
your operating system's name and version and your compiler's name and
version.  If you have trouble compiling, you should also include a
transcript of the build session, starting with the invocation of
`configure'.  Depending on the nature of the bug, you may be asked to
send additional files as well (such as `config.h' or `config.cache').

   Patches are most welcome, but not required.  That is, do not
hesitate to send a bug report just because you can not provide a fix.

   Send bug reports to <bug-bison@gnu.org>.


File: bison.info,  Node: More Languages,  Next: Beta Testing,  Prev: Bug Reports,  Up: FAQ

11.10 More Languages
====================

     Will Bison ever have C++ and Java support?  How about INSERT YOUR
     FAVORITE LANGUAGE HERE?

   C++ and Java support is there now, and is documented.  We'd love to
add other languages; contributions are welcome.


File: bison.info,  Node: Beta Testing,  Next: Mailing Lists,  Prev: More Languages,  Up: FAQ

11.11 Beta Testing
==================

     What is involved in being a beta tester?

   It's not terribly involved.  Basically, you would download a test
release, compile it, and use it to build and run a parser or two.  After
that, you would submit either a bug report or a message saying that
everything is okay.  It is important to report successes as well as
failures because test releases eventually become mainstream releases,
but only if they are adequately tested.  If no one tests, development is
essentially halted.

   Beta testers are particularly needed for operating systems to which
the developers do not have easy access.  They currently have easy
access to recent GNU/Linux and Solaris versions.  Reports about other
operating systems are especially welcome.


File: bison.info,  Node: Mailing Lists,  Prev: Beta Testing,  Up: FAQ

11.12 Mailing Lists
===================

     How do I join the help-bison and bug-bison mailing lists?

   See `http://lists.gnu.org/'.


File: bison.info,  Node: Table of Symbols,  Next: Glossary,  Prev: FAQ,  Up: Top

Appendix A Bison Symbols
************************

 -- Variable: @$
     In an action, the location of the left-hand side of the rule.
     *Note Locations Overview: Locations.

 -- Variable: @N
     In an action, the location of the N-th symbol of the right-hand
     side of the rule.  *Note Locations Overview: Locations.

 -- Variable: $$
     In an action, the semantic value of the left-hand side of the rule.
     *Note Actions::.

 -- Variable: $N
     In an action, the semantic value of the N-th symbol of the
     right-hand side of the rule.  *Note Actions::.

 -- Delimiter: %%
     Delimiter used to separate the grammar rule section from the Bison
     declarations section or the epilogue.  *Note The Overall Layout of
     a Bison Grammar: Grammar Layout.

 -- Delimiter: %{CODE%}
     All code listed between `%{' and `%}' is copied directly to the
     output file uninterpreted.  Such code forms the prologue of the
     input file.  *Note Outline of a Bison Grammar: Grammar Outline.

 -- Construct: /*...*/
     Comment delimiters, as in C.

 -- Delimiter: :
     Separates a rule's result from its components.  *Note Syntax of
     Grammar Rules: Rules.

 -- Delimiter: ;
     Terminates a rule.  *Note Syntax of Grammar Rules: Rules.

 -- Delimiter: |
     Separates alternate rules for the same result nonterminal.  *Note
     Syntax of Grammar Rules: Rules.

 -- Directive: <*>
     Used to define a default tagged `%destructor' or default tagged
     `%printer'.

     This feature is experimental.  More user feedback will help to
     determine whether it should become a permanent feature.

     *Note Freeing Discarded Symbols: Destructor Decl.

 -- Directive: <>
     Used to define a default tagless `%destructor' or default tagless
     `%printer'.

     This feature is experimental.  More user feedback will help to
     determine whether it should become a permanent feature.

     *Note Freeing Discarded Symbols: Destructor Decl.

 -- Symbol: $accept
     The predefined nonterminal whose only rule is `$accept: START
     $end', where START is the start symbol.  *Note The Start-Symbol:
     Start Decl.  It cannot be used in the grammar.

 -- Directive: %code {CODE}
 -- Directive: %code QUALIFIER {CODE}
     Insert CODE verbatim into output parser source.  *Note %code: Decl
     Summary.

 -- Directive: %debug
     Equip the parser for debugging.  *Note Decl Summary::.

 -- Directive: %debug
     Equip the parser for debugging.  *Note Decl Summary::.

 -- Directive: %define DEFINE-VARIABLE
 -- Directive: %define DEFINE-VARIABLE VALUE
     Define a variable to adjust Bison's behavior.  *Note %define: Decl
     Summary.

 -- Directive: %defines
     Bison declaration to create a header file meant for the scanner.
     *Note Decl Summary::.

 -- Directive: %defines DEFINES-FILE
     Same as above, but save in the file DEFINES-FILE.  *Note Decl
     Summary::.

 -- Directive: %destructor
     Specify how the parser should reclaim the memory associated to
     discarded symbols.  *Note Freeing Discarded Symbols: Destructor
     Decl.

 -- Directive: %dprec
     Bison declaration to assign a precedence to a rule that is used at
     parse time to resolve reduce/reduce conflicts.  *Note Writing GLR
     Parsers: GLR Parsers.

 -- Symbol: $end
     The predefined token marking the end of the token stream.  It
     cannot be used in the grammar.

 -- Symbol: error
     A token name reserved for error recovery.  This token may be used
     in grammar rules so as to allow the Bison parser to recognize an
     error in the grammar without halting the process.  In effect, a
     sentence containing an error may be recognized as valid.  On a
     syntax error, the token `error' becomes the current lookahead
     token.  Actions corresponding to `error' are then executed, and
     the lookahead token is reset to the token that originally caused
     the violation.  *Note Error Recovery::.

 -- Directive: %error-verbose
     Bison declaration to request verbose, specific error message
     strings when `yyerror' is called.

 -- Directive: %file-prefix "PREFIX"
     Bison declaration to set the prefix of the output files.  *Note
     Decl Summary::.

 -- Directive: %glr-parser
     Bison declaration to produce a GLR parser.  *Note Writing GLR
     Parsers: GLR Parsers.

 -- Directive: %initial-action
     Run user code before parsing.  *Note Performing Actions before
     Parsing: Initial Action Decl.

 -- Directive: %language
     Specify the programming language for the generated parser.  *Note
     Decl Summary::.

 -- Directive: %left
     Bison declaration to assign left associativity to token(s).  *Note
     Operator Precedence: Precedence Decl.

 -- Directive: %lex-param {ARGUMENT-DECLARATION}
     Bison declaration to specifying an additional parameter that
     `yylex' should accept.  *Note Calling Conventions for Pure
     Parsers: Pure Calling.

 -- Directive: %merge
     Bison declaration to assign a merging function to a rule.  If
     there is a reduce/reduce conflict with a rule having the same
     merging function, the function is applied to the two semantic
     values to get a single result.  *Note Writing GLR Parsers: GLR
     Parsers.

 -- Directive: %name-prefix "PREFIX"
     Bison declaration to rename the external symbols.  *Note Decl
     Summary::.

 -- Directive: %no-lines
     Bison declaration to avoid generating `#line' directives in the
     parser file.  *Note Decl Summary::.

 -- Directive: %nonassoc
     Bison declaration to assign nonassociativity to token(s).  *Note
     Operator Precedence: Precedence Decl.

 -- Directive: %output "FILE"
     Bison declaration to set the name of the parser file.  *Note Decl
     Summary::.

 -- Directive: %parse-param {ARGUMENT-DECLARATION}
     Bison declaration to specifying an additional parameter that
     `yyparse' should accept.  *Note The Parser Function `yyparse':
     Parser Function.

 -- Directive: %prec
     Bison declaration to assign a precedence to a specific rule.
     *Note Context-Dependent Precedence: Contextual Precedence.

 -- Directive: %pure-parser
     Deprecated version of `%define api.pure' (*note %define: Decl
     Summary.), for which Bison is more careful to warn about
     unreasonable usage.

 -- Directive: %require "VERSION"
     Require version VERSION or higher of Bison.  *Note Require a
     Version of Bison: Require Decl.

 -- Directive: %right
     Bison declaration to assign right associativity to token(s).
     *Note Operator Precedence: Precedence Decl.

 -- Directive: %skeleton
     Specify the skeleton to use; usually for development.  *Note Decl
     Summary::.

 -- Directive: %start
     Bison declaration to specify the start symbol.  *Note The
     Start-Symbol: Start Decl.

 -- Directive: %token
     Bison declaration to declare token(s) without specifying
     precedence.  *Note Token Type Names: Token Decl.

 -- Directive: %token-table
     Bison declaration to include a token name table in the parser file.
     *Note Decl Summary::.

 -- Directive: %type
     Bison declaration to declare nonterminals.  *Note Nonterminal
     Symbols: Type Decl.

 -- Symbol: $undefined
     The predefined token onto which all undefined values returned by
     `yylex' are mapped.  It cannot be used in the grammar, rather, use
     `error'.

 -- Directive: %union
     Bison declaration to specify several possible data types for
     semantic values.  *Note The Collection of Value Types: Union Decl.

 -- Macro: YYABORT
     Macro to pretend that an unrecoverable syntax error has occurred,
     by making `yyparse' return 1 immediately.  The error reporting
     function `yyerror' is not called.  *Note The Parser Function
     `yyparse': Parser Function.

     For Java parsers, this functionality is invoked using `return
     YYABORT;' instead.

 -- Macro: YYACCEPT
     Macro to pretend that a complete utterance of the language has been
     read, by making `yyparse' return 0 immediately.  *Note The Parser
     Function `yyparse': Parser Function.

     For Java parsers, this functionality is invoked using `return
     YYACCEPT;' instead.

 -- Macro: YYBACKUP
     Macro to discard a value from the parser stack and fake a lookahead
     token.  *Note Special Features for Use in Actions: Action Features.

 -- Variable: yychar
     External integer variable that contains the integer value of the
     lookahead token.  (In a pure parser, it is a local variable within
     `yyparse'.)  Error-recovery rule actions may examine this variable.
     *Note Special Features for Use in Actions: Action Features.

 -- Variable: yyclearin
     Macro used in error-recovery rule actions.  It clears the previous
     lookahead token.  *Note Error Recovery::.

 -- Macro: YYDEBUG
     Macro to define to equip the parser with tracing code.  *Note
     Tracing Your Parser: Tracing.

 -- Variable: yydebug
     External integer variable set to zero by default.  If `yydebug' is
     given a nonzero value, the parser will output information on input
     symbols and parser action.  *Note Tracing Your Parser: Tracing.

 -- Macro: yyerrok
     Macro to cause parser to recover immediately to its normal mode
     after a syntax error.  *Note Error Recovery::.

 -- Macro: YYERROR
     Macro to pretend that a syntax error has just been detected: call
     `yyerror' and then perform normal error recovery if possible
     (*note Error Recovery::), or (if recovery is impossible) make
     `yyparse' return 1.  *Note Error Recovery::.

     For Java parsers, this functionality is invoked using `return
     YYERROR;' instead.

 -- Function: yyerror
     User-supplied function to be called by `yyparse' on error.  *Note
     The Error Reporting Function `yyerror': Error Reporting.

 -- Macro: YYERROR_VERBOSE
     An obsolete macro that you define with `#define' in the prologue
     to request verbose, specific error message strings when `yyerror'
     is called.  It doesn't matter what definition you use for
     `YYERROR_VERBOSE', just whether you define it.  Using
     `%error-verbose' is preferred.

 -- Macro: YYINITDEPTH
     Macro for specifying the initial size of the parser stack.  *Note
     Memory Management::.

 -- Function: yylex
     User-supplied lexical analyzer function, called with no arguments
     to get the next token.  *Note The Lexical Analyzer Function
     `yylex': Lexical.

 -- Macro: YYLEX_PARAM
     An obsolete macro for specifying an extra argument (or list of
     extra arguments) for `yyparse' to pass to `yylex'.  The use of this
     macro is deprecated, and is supported only for Yacc like parsers.
     *Note Calling Conventions for Pure Parsers: Pure Calling.

 -- Variable: yylloc
     External variable in which `yylex' should place the line and column
     numbers associated with a token.  (In a pure parser, it is a local
     variable within `yyparse', and its address is passed to `yylex'.)
     You can ignore this variable if you don't use the `@' feature in
     the grammar actions.  *Note Textual Locations of Tokens: Token
     Locations.  In semantic actions, it stores the location of the
     lookahead token.  *Note Actions and Locations: Actions and
     Locations.

 -- Type: YYLTYPE
     Data type of `yylloc'; by default, a structure with four members.
     *Note Data Types of Locations: Location Type.

 -- Variable: yylval
     External variable in which `yylex' should place the semantic value
     associated with a token.  (In a pure parser, it is a local
     variable within `yyparse', and its address is passed to `yylex'.)
     *Note Semantic Values of Tokens: Token Values.  In semantic
     actions, it stores the semantic value of the lookahead token.
     *Note Actions: Actions.

 -- Macro: YYMAXDEPTH
     Macro for specifying the maximum size of the parser stack.  *Note
     Memory Management::.

 -- Variable: yynerrs
     Global variable which Bison increments each time it reports a
     syntax error.  (In a pure parser, it is a local variable within
     `yyparse'. In a pure push parser, it is a member of yypstate.)
     *Note The Error Reporting Function `yyerror': Error Reporting.

 -- Function: yyparse
     The parser function produced by Bison; call this function to start
     parsing.  *Note The Parser Function `yyparse': Parser Function.

 -- Function: yypstate_delete
     The function to delete a parser instance, produced by Bison in
     push mode; call this function to delete the memory associated with
     a parser.  *Note The Parser Delete Function `yypstate_delete':
     Parser Delete Function.  (The current push parsing interface is
     experimental and may evolve.  More user feedback will help to
     stabilize it.)

 -- Function: yypstate_new
     The function to create a parser instance, produced by Bison in
     push mode; call this function to create a new parser.  *Note The
     Parser Create Function `yypstate_new': Parser Create Function.
     (The current push parsing interface is experimental and may evolve.
     More user feedback will help to stabilize it.)

 -- Function: yypull_parse
     The parser function produced by Bison in push mode; call this
     function to parse the rest of the input stream.  *Note The Pull
     Parser Function `yypull_parse': Pull Parser Function.  (The
     current push parsing interface is experimental and may evolve.
     More user feedback will help to stabilize it.)

 -- Function: yypush_parse
     The parser function produced by Bison in push mode; call this
     function to parse a single token.  *Note The Push Parser Function
     `yypush_parse': Push Parser Function.  (The current push parsing
     interface is experimental and may evolve.  More user feedback will
     help to stabilize it.)

 -- Macro: YYPARSE_PARAM
     An obsolete macro for specifying the name of a parameter that
     `yyparse' should accept.  The use of this macro is deprecated, and
     is supported only for Yacc like parsers.  *Note Calling
     Conventions for Pure Parsers: Pure Calling.

 -- Macro: YYRECOVERING
     The expression `YYRECOVERING ()' yields 1 when the parser is
     recovering from a syntax error, and 0 otherwise.  *Note Special
     Features for Use in Actions: Action Features.

 -- Macro: YYSTACK_USE_ALLOCA
     Macro used to control the use of `alloca' when the C LALR(1)
     parser needs to extend its stacks.  If defined to 0, the parser
     will use `malloc' to extend its stacks.  If defined to 1, the
     parser will use `alloca'.  Values other than 0 and 1 are reserved
     for future Bison extensions.  If not defined, `YYSTACK_USE_ALLOCA'
     defaults to 0.

     In the all-too-common case where your code may run on a host with a
     limited stack and with unreliable stack-overflow checking, you
     should set `YYMAXDEPTH' to a value that cannot possibly result in
     unchecked stack overflow on any of your target hosts when `alloca'
     is called.  You can inspect the code that Bison generates in order
     to determine the proper numeric values.  This will require some
     expertise in low-level implementation details.

 -- Type: YYSTYPE
     Data type of semantic values; `int' by default.  *Note Data Types
     of Semantic Values: Value Type.


File: bison.info,  Node: Glossary,  Next: Copying This Manual,  Prev: Table of Symbols,  Up: Top

Appendix B Glossary
*******************

Backus-Naur Form (BNF; also called "Backus Normal Form")
     Formal method of specifying context-free grammars originally
     proposed by John Backus, and slightly improved by Peter Naur in
     his 1960-01-02 committee document contributing to what became the
     Algol 60 report.  *Note Languages and Context-Free Grammars:
     Language and Grammar.

Context-free grammars
     Grammars specified as rules that can be applied regardless of
     context.  Thus, if there is a rule which says that an integer can
     be used as an expression, integers are allowed _anywhere_ an
     expression is permitted.  *Note Languages and Context-Free
     Grammars: Language and Grammar.

Dynamic allocation
     Allocation of memory that occurs during execution, rather than at
     compile time or on entry to a function.

Empty string
     Analogous to the empty set in set theory, the empty string is a
     character string of length zero.

Finite-state stack machine
     A "machine" that has discrete states in which it is said to exist
     at each instant in time.  As input to the machine is processed, the
     machine moves from state to state as specified by the logic of the
     machine.  In the case of the parser, the input is the language
     being parsed, and the states correspond to various stages in the
     grammar rules.  *Note The Bison Parser Algorithm: Algorithm.

Generalized LR (GLR)
     A parsing algorithm that can handle all context-free grammars,
     including those that are not LALR(1).  It resolves situations that
     Bison's usual LALR(1) algorithm cannot by effectively splitting
     off multiple parsers, trying all possible parsers, and discarding
     those that fail in the light of additional right context.  *Note
     Generalized LR Parsing: Generalized LR Parsing.

Grouping
     A language construct that is (in general) grammatically divisible;
     for example, `expression' or `declaration' in C.  *Note Languages
     and Context-Free Grammars: Language and Grammar.

Infix operator
     An arithmetic operator that is placed between the operands on
     which it performs some operation.

Input stream
     A continuous flow of data between devices or programs.

Language construct
     One of the typical usage schemas of the language.  For example,
     one of the constructs of the C language is the `if' statement.
     *Note Languages and Context-Free Grammars: Language and Grammar.

Left associativity
     Operators having left associativity are analyzed from left to
     right: `a+b+c' first computes `a+b' and then combines with `c'.
     *Note Operator Precedence: Precedence.

Left recursion
     A rule whose result symbol is also its first component symbol; for
     example, `expseq1 : expseq1 ',' exp;'.  *Note Recursive Rules:
     Recursion.

Left-to-right parsing
     Parsing a sentence of a language by analyzing it token by token
     from left to right.  *Note The Bison Parser Algorithm: Algorithm.

Lexical analyzer (scanner)
     A function that reads an input stream and returns tokens one by
     one.  *Note The Lexical Analyzer Function `yylex': Lexical.

Lexical tie-in
     A flag, set by actions in the grammar rules, which alters the way
     tokens are parsed.  *Note Lexical Tie-ins::.

Literal string token
     A token which consists of two or more fixed characters.  *Note
     Symbols::.

Lookahead token
     A token already read but not yet shifted.  *Note Lookahead Tokens:
     Lookahead.

LALR(1)
     The class of context-free grammars that Bison (like most other
     parser generators) can handle; a subset of LR(1).  *Note
     Mysterious Reduce/Reduce Conflicts: Mystery Conflicts.

LR(1)
     The class of context-free grammars in which at most one token of
     lookahead is needed to disambiguate the parsing of any piece of
     input.

Nonterminal symbol
     A grammar symbol standing for a grammatical construct that can be
     expressed through rules in terms of smaller constructs; in other
     words, a construct that is not a token.  *Note Symbols::.

Parser
     A function that recognizes valid sentences of a language by
     analyzing the syntax structure of a set of tokens passed to it
     from a lexical analyzer.

Postfix operator
     An arithmetic operator that is placed after the operands upon
     which it performs some operation.

Reduction
     Replacing a string of nonterminals and/or terminals with a single
     nonterminal, according to a grammar rule.  *Note The Bison Parser
     Algorithm: Algorithm.

Reentrant
     A reentrant subprogram is a subprogram which can be in invoked any
     number of times in parallel, without interference between the
     various invocations.  *Note A Pure (Reentrant) Parser: Pure Decl.

Reverse polish notation
     A language in which all operators are postfix operators.

Right recursion
     A rule whose result symbol is also its last component symbol; for
     example, `expseq1: exp ',' expseq1;'.  *Note Recursive Rules:
     Recursion.

Semantics
     In computer languages, the semantics are specified by the actions
     taken for each instance of the language, i.e., the meaning of each
     statement.  *Note Defining Language Semantics: Semantics.

Shift
     A parser is said to shift when it makes the choice of analyzing
     further input from the stream rather than reducing immediately some
     already-recognized rule.  *Note The Bison Parser Algorithm:
     Algorithm.

Single-character literal
     A single character that is recognized and interpreted as is.
     *Note From Formal Rules to Bison Input: Grammar in Bison.

Start symbol
     The nonterminal symbol that stands for a complete valid utterance
     in the language being parsed.  The start symbol is usually listed
     as the first nonterminal symbol in a language specification.
     *Note The Start-Symbol: Start Decl.

Symbol table
     A data structure where symbol names and associated data are stored
     during parsing to allow for recognition and use of existing
     information in repeated uses of a symbol.  *Note Multi-function
     Calc::.

Syntax error
     An error encountered during parsing of an input stream due to
     invalid syntax.  *Note Error Recovery::.

Token
     A basic, grammatically indivisible unit of a language.  The symbol
     that describes a token in the grammar is a terminal symbol.  The
     input of the Bison parser is a stream of tokens which comes from
     the lexical analyzer.  *Note Symbols::.

Terminal symbol
     A grammar symbol that has no rules in the grammar and therefore is
     grammatically indivisible.  The piece of text it represents is a
     token.  *Note Languages and Context-Free Grammars: Language and
     Grammar.


File: bison.info,  Node: Copying This Manual,  Next: Index,  Prev: Glossary,  Up: Top

Appendix C Copying This Manual
******************************

                      Version 1.2, November 2002

     Copyright (C) 2000,2001,2002 Free Software Foundation, Inc.
     51 Franklin St, Fifth Floor, Boston, MA  02110-1301, USA

     Everyone is permitted to copy and distribute verbatim copies
     of this license document, but changing it is not allowed.

  0. PREAMBLE

     The purpose of this License is to make a manual, textbook, or other
     functional and useful document "free" in the sense of freedom: to
     assure everyone the effective freedom to copy and redistribute it,
     with or without modifying it, either commercially or
     noncommercially.  Secondarily, this License preserves for the
     author and publisher a way to get credit for their work, while not
     being considered responsible for modifications made by others.

     This License is a kind of "copyleft", which means that derivative
     works of the document must themselves be free in the same sense.
     It complements the GNU General Public License, which is a copyleft
     license designed for free software.

     We have designed this License in order to use it for manuals for
     free software, because free software needs free documentation: a
     free program should come with manuals providing the same freedoms
     that the software does.  But this License is not limited to
     software manuals; it can be used for any textual work, regardless
     of subject matter or whether it is published as a printed book.
     We recommend this License principally for works whose purpose is
     instruction or reference.

  1. APPLICABILITY AND DEFINITIONS

     This License applies to any manual or other work, in any medium,
     that contains a notice placed by the copyright holder saying it
     can be distributed under the terms of this License.  Such a notice
     grants a world-wide, royalty-free license, unlimited in duration,
     to use that work under the conditions stated herein.  The
     "Document", below, refers to any such manual or work.  Any member
     of the public is a licensee, and is addressed as "you".  You
     accept the license if you copy, modify or distribute the work in a
     way requiring permission under copyright law.

     A "Modified Version" of the Document means any work containing the
     Document or a portion of it, either copied verbatim, or with
     modifications and/or translated into another language.

     A "Secondary Section" is a named appendix or a front-matter section
     of the Document that deals exclusively with the relationship of the
     publishers or authors of the Document to the Document's overall
     subject (or to related matters) and contains nothing that could
     fall directly within that overall subject.  (Thus, if the Document
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     of legal, commercial, philosophical, ethical or political position
     regarding them.

     The "Invariant Sections" are certain Secondary Sections whose
     titles are designated, as being those of Invariant Sections, in
     the notice that says that the Document is released under this
     License.  If a section does not fit the above definition of
     Secondary then it is not allowed to be designated as Invariant.
     The Document may contain zero Invariant Sections.  If the Document
     does not identify any Invariant Sections then there are none.

     The "Cover Texts" are certain short passages of text that are
     listed, as Front-Cover Texts or Back-Cover Texts, in the notice
     that says that the Document is released under this License.  A
     Front-Cover Text may be at most 5 words, and a Back-Cover Text may
     be at most 25 words.

     A "Transparent" copy of the Document means a machine-readable copy,
     represented in a format whose specification is available to the
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     copy that is not "Transparent" is called "Opaque".

     Examples of suitable formats for Transparent copies include plain
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     standard-conforming simple HTML, PostScript or PDF designed for
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     produced by some word processors for output purposes only.

     The "Title Page" means, for a printed book, the title page itself,
     plus such following pages as are needed to hold, legibly, the
     material this License requires to appear in the title page.  For
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     Page" means the text near the most prominent appearance of the
     work's title, preceding the beginning of the body of the text.

     A section "Entitled XYZ" means a named subunit of the Document
     whose title either is precisely XYZ or contains XYZ in parentheses
     following text that translates XYZ in another language.  (Here XYZ
     stands for a specific section name mentioned below, such as
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     To "Preserve the Title" of such a section when you modify the
     Document means that it remains a section "Entitled XYZ" according
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     The Document may include Warranty Disclaimers next to the notice
     which states that this License applies to the Document.  These
     Warranty Disclaimers are considered to be included by reference in
     this License, but only as regards disclaiming warranties: any other
     implication that these Warranty Disclaimers may have is void and
     has no effect on the meaning of this License.

  2. VERBATIM COPYING

     You may copy and distribute the Document in any medium, either
     commercially or noncommercially, provided that this License, the
     copyright notices, and the license notice saying this License
     applies to the Document are reproduced in all copies, and that you
     add no other conditions whatsoever to those of this License.  You
     may not use technical measures to obstruct or control the reading
     or further copying of the copies you make or distribute.  However,
     you may accept compensation in exchange for copies.  If you
     distribute a large enough number of copies you must also follow
     the conditions in section 3.

     You may also lend copies, under the same conditions stated above,
     and you may publicly display copies.

  3. COPYING IN QUANTITY

     If you publish printed copies (or copies in media that commonly
     have printed covers) of the Document, numbering more than 100, and
     the Document's license notice requires Cover Texts, you must
     enclose the copies in covers that carry, clearly and legibly, all
     these Cover Texts: Front-Cover Texts on the front cover, and
     Back-Cover Texts on the back cover.  Both covers must also clearly
     and legibly identify you as the publisher of these copies.  The
     front cover must present the full title with all words of the
     title equally prominent and visible.  You may add other material
     on the covers in addition.  Copying with changes limited to the
     covers, as long as they preserve the title of the Document and
     satisfy these conditions, can be treated as verbatim copying in
     other respects.

     If the required texts for either cover are too voluminous to fit
     legibly, you should put the first ones listed (as many as fit
     reasonably) on the actual cover, and continue the rest onto
     adjacent pages.

     If you publish or distribute Opaque copies of the Document
     numbering more than 100, you must either include a
     machine-readable Transparent copy along with each Opaque copy, or
     state in or with each Opaque copy a computer-network location from
     which the general network-using public has access to download
     using public-standard network protocols a complete Transparent
     copy of the Document, free of added material.  If you use the
     latter option, you must take reasonably prudent steps, when you
     begin distribution of Opaque copies in quantity, to ensure that
     this Transparent copy will remain thus accessible at the stated
     location until at least one year after the last time you
     distribute an Opaque copy (directly or through your agents or
     retailers) of that edition to the public.

     It is requested, but not required, that you contact the authors of
     the Document well before redistributing any large number of
     copies, to give them a chance to provide you with an updated
     version of the Document.

  4. MODIFICATIONS

     You may copy and distribute a Modified Version of the Document
     under the conditions of sections 2 and 3 above, provided that you
     release the Modified Version under precisely this License, with
     the Modified Version filling the role of the Document, thus
     licensing distribution and modification of the Modified Version to
     whoever possesses a copy of it.  In addition, you must do these
     things in the Modified Version:

       A. Use in the Title Page (and on the covers, if any) a title
          distinct from that of the Document, and from those of
          previous versions (which should, if there were any, be listed
          in the History section of the Document).  You may use the
          same title as a previous version if the original publisher of
          that version gives permission.

       B. List on the Title Page, as authors, one or more persons or
          entities responsible for authorship of the modifications in
          the Modified Version, together with at least five of the
          principal authors of the Document (all of its principal
          authors, if it has fewer than five), unless they release you
          from this requirement.

       C. State on the Title page the name of the publisher of the
          Modified Version, as the publisher.

       D. Preserve all the copyright notices of the Document.

       E. Add an appropriate copyright notice for your modifications
          adjacent to the other copyright notices.

       F. Include, immediately after the copyright notices, a license
          notice giving the public permission to use the Modified
          Version under the terms of this License, in the form shown in
          the Addendum below.

       G. Preserve in that license notice the full lists of Invariant
          Sections and required Cover Texts given in the Document's
          license notice.

       H. Include an unaltered copy of this License.

       I. Preserve the section Entitled "History", Preserve its Title,
          and add to it an item stating at least the title, year, new
          authors, and publisher of the Modified Version as given on
          the Title Page.  If there is no section Entitled "History" in
          the Document, create one stating the title, year, authors,
          and publisher of the Document as given on its Title Page,
          then add an item describing the Modified Version as stated in
          the previous sentence.

       J. Preserve the network location, if any, given in the Document
          for public access to a Transparent copy of the Document, and
          likewise the network locations given in the Document for
          previous versions it was based on.  These may be placed in
          the "History" section.  You may omit a network location for a
          work that was published at least four years before the
          Document itself, or if the original publisher of the version
          it refers to gives permission.

       K. For any section Entitled "Acknowledgements" or "Dedications",
          Preserve the Title of the section, and preserve in the
          section all the substance and tone of each of the contributor
          acknowledgements and/or dedications given therein.

       L. Preserve all the Invariant Sections of the Document,
          unaltered in their text and in their titles.  Section numbers
          or the equivalent are not considered part of the section
          titles.

       M. Delete any section Entitled "Endorsements".  Such a section
          may not be included in the Modified Version.

       N. Do not retitle any existing section to be Entitled
          "Endorsements" or to conflict in title with any Invariant
          Section.

       O. Preserve any Warranty Disclaimers.

     If the Modified Version includes new front-matter sections or
     appendices that qualify as Secondary Sections and contain no
     material copied from the Document, you may at your option
     designate some or all of these sections as invariant.  To do this,
     add their titles to the list of Invariant Sections in the Modified
     Version's license notice.  These titles must be distinct from any
     other section titles.

     You may add a section Entitled "Endorsements", provided it contains
     nothing but endorsements of your Modified Version by various
     parties--for example, statements of peer review or that the text
     has been approved by an organization as the authoritative
     definition of a standard.

     You may add a passage of up to five words as a Front-Cover Text,
     and a passage of up to 25 words as a Back-Cover Text, to the end
     of the list of Cover Texts in the Modified Version.  Only one
     passage of Front-Cover Text and one of Back-Cover Text may be
     added by (or through arrangements made by) any one entity.  If the
     Document already includes a cover text for the same cover,
     previously added by you or by arrangement made by the same entity
     you are acting on behalf of, you may not add another; but you may
     replace the old one, on explicit permission from the previous
     publisher that added the old one.

     The author(s) and publisher(s) of the Document do not by this
     License give permission to use their names for publicity for or to
     assert or imply endorsement of any Modified Version.

  5. COMBINING DOCUMENTS

     You may combine the Document with other documents released under
     this License, under the terms defined in section 4 above for
     modified versions, provided that you include in the combination
     all of the Invariant Sections of all of the original documents,
     unmodified, and list them all as Invariant Sections of your
     combined work in its license notice, and that you preserve all
     their Warranty Disclaimers.

     The combined work need only contain one copy of this License, and
     multiple identical Invariant Sections may be replaced with a single
     copy.  If there are multiple Invariant Sections with the same name
     but different contents, make the title of each such section unique
     by adding at the end of it, in parentheses, the name of the
     original author or publisher of that section if known, or else a
     unique number.  Make the same adjustment to the section titles in
     the list of Invariant Sections in the license notice of the
     combined work.

     In the combination, you must combine any sections Entitled
     "History" in the various original documents, forming one section
     Entitled "History"; likewise combine any sections Entitled
     "Acknowledgements", and any sections Entitled "Dedications".  You
     must delete all sections Entitled "Endorsements."

  6. COLLECTIONS OF DOCUMENTS

     You may make a collection consisting of the Document and other
     documents released under this License, and replace the individual
     copies of this License in the various documents with a single copy
     that is included in the collection, provided that you follow the
     rules of this License for verbatim copying of each of the
     documents in all other respects.

     You may extract a single document from such a collection, and
     distribute it individually under this License, provided you insert
     a copy of this License into the extracted document, and follow
     this License in all other respects regarding verbatim copying of
     that document.

  7. AGGREGATION WITH INDEPENDENT WORKS

     A compilation of the Document or its derivatives with other
     separate and independent documents or works, in or on a volume of
     a storage or distribution medium, is called an "aggregate" if the
     copyright resulting from the compilation is not used to limit the
     legal rights of the compilation's users beyond what the individual
     works permit.  When the Document is included in an aggregate, this
     License does not apply to the other works in the aggregate which
     are not themselves derivative works of the Document.

     If the Cover Text requirement of section 3 is applicable to these
     copies of the Document, then if the Document is less than one half
     of the entire aggregate, the Document's Cover Texts may be placed
     on covers that bracket the Document within the aggregate, or the
     electronic equivalent of covers if the Document is in electronic
     form.  Otherwise they must appear on printed covers that bracket
     the whole aggregate.

  8. TRANSLATION

     Translation is considered a kind of modification, so you may
     distribute translations of the Document under the terms of section
     4.  Replacing Invariant Sections with translations requires special
     permission from their copyright holders, but you may include
     translations of some or all Invariant Sections in addition to the
     original versions of these Invariant Sections.  You may include a
     translation of this License, and all the license notices in the
     Document, and any Warranty Disclaimers, provided that you also
     include the original English version of this License and the
     original versions of those notices and disclaimers.  In case of a
     disagreement between the translation and the original version of
     this License or a notice or disclaimer, the original version will
     prevail.

     If a section in the Document is Entitled "Acknowledgements",
     "Dedications", or "History", the requirement (section 4) to
     Preserve its Title (section 1) will typically require changing the
     actual title.

  9. TERMINATION

     You may not copy, modify, sublicense, or distribute the Document
     except as expressly provided for under this License.  Any other
     attempt to copy, modify, sublicense or distribute the Document is
     void, and will automatically terminate your rights under this
     License.  However, parties who have received copies, or rights,
     from you under this License will not have their licenses
     terminated so long as such parties remain in full compliance.

 10. FUTURE REVISIONS OF THIS LICENSE

     The Free Software Foundation may publish new, revised versions of
     the GNU Free Documentation License from time to time.  Such new
     versions will be similar in spirit to the present version, but may
     differ in detail to address new problems or concerns.  See
     `http://www.gnu.org/copyleft/'.

     Each version of the License is given a distinguishing version
     number.  If the Document specifies that a particular numbered
     version of this License "or any later version" applies to it, you
     have the option of following the terms and conditions either of
     that specified version or of any later version that has been
     published (not as a draft) by the Free Software Foundation.  If
     the Document does not specify a version number of this License,
     you may choose any version ever published (not as a draft) by the
     Free Software Foundation.

ADDENDUM: How to use this License for your documents
====================================================

To use this License in a document you have written, include a copy of
the License in the document and put the following copyright and license
notices just after the title page:

       Copyright (C)  YEAR  YOUR NAME.
       Permission is granted to copy, distribute and/or modify this document
       under the terms of the GNU Free Documentation License, Version 1.2
       or any later version published by the Free Software Foundation;
       with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
       Texts.  A copy of the license is included in the section entitled ``GNU
       Free Documentation License''.

   If you have Invariant Sections, Front-Cover Texts and Back-Cover
Texts, replace the "with...Texts." line with this:

         with the Invariant Sections being LIST THEIR TITLES, with
         the Front-Cover Texts being LIST, and with the Back-Cover Texts
         being LIST.

   If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the
situation.

   If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of
free software license, such as the GNU General Public License, to
permit their use in free software.


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Index
*****