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diff --git a/chromium/third_party/cygwin/lib/perl5/5.10/pods/perlsyn.pod b/chromium/third_party/cygwin/lib/perl5/5.10/pods/perlsyn.pod deleted file mode 100644 index 44c4f1aff38..00000000000 --- a/chromium/third_party/cygwin/lib/perl5/5.10/pods/perlsyn.pod +++ /dev/null @@ -1,877 +0,0 @@ -=head1 NAME -X<syntax> - -perlsyn - Perl syntax - -=head1 DESCRIPTION - -A Perl program consists of a sequence of declarations and statements -which run from the top to the bottom. Loops, subroutines and other -control structures allow you to jump around within the code. - -Perl is a B<free-form> language, you can format and indent it however -you like. Whitespace mostly serves to separate tokens, unlike -languages like Python where it is an important part of the syntax. - -Many of Perl's syntactic elements are B<optional>. Rather than -requiring you to put parentheses around every function call and -declare every variable, you can often leave such explicit elements off -and Perl will figure out what you meant. This is known as B<Do What I -Mean>, abbreviated B<DWIM>. It allows programmers to be B<lazy> and to -code in a style with which they are comfortable. - -Perl B<borrows syntax> and concepts from many languages: awk, sed, C, -Bourne Shell, Smalltalk, Lisp and even English. Other -languages have borrowed syntax from Perl, particularly its regular -expression extensions. So if you have programmed in another language -you will see familiar pieces in Perl. They often work the same, but -see L<perltrap> for information about how they differ. - -=head2 Declarations -X<declaration> X<undef> X<undefined> X<uninitialized> - -The only things you need to declare in Perl are report formats and -subroutines (and sometimes not even subroutines). A variable holds -the undefined value (C<undef>) until it has been assigned a defined -value, which is anything other than C<undef>. When used as a number, -C<undef> is treated as C<0>; when used as a string, it is treated as -the empty string, C<"">; and when used as a reference that isn't being -assigned to, it is treated as an error. If you enable warnings, -you'll be notified of an uninitialized value whenever you treat -C<undef> as a string or a number. Well, usually. Boolean contexts, -such as: - - my $a; - if ($a) {} - -are exempt from warnings (because they care about truth rather than -definedness). Operators such as C<++>, C<-->, C<+=>, -C<-=>, and C<.=>, that operate on undefined left values such as: - - my $a; - $a++; - -are also always exempt from such warnings. - -A declaration can be put anywhere a statement can, but has no effect on -the execution of the primary sequence of statements--declarations all -take effect at compile time. Typically all the declarations are put at -the beginning or the end of the script. However, if you're using -lexically-scoped private variables created with C<my()>, you'll -have to make sure -your format or subroutine definition is within the same block scope -as the my if you expect to be able to access those private variables. - -Declaring a subroutine allows a subroutine name to be used as if it were a -list operator from that point forward in the program. You can declare a -subroutine without defining it by saying C<sub name>, thus: -X<subroutine, declaration> - - sub myname; - $me = myname $0 or die "can't get myname"; - -Note that myname() functions as a list operator, not as a unary operator; -so be careful to use C<or> instead of C<||> in this case. However, if -you were to declare the subroutine as C<sub myname ($)>, then -C<myname> would function as a unary operator, so either C<or> or -C<||> would work. - -Subroutines declarations can also be loaded up with the C<require> statement -or both loaded and imported into your namespace with a C<use> statement. -See L<perlmod> for details on this. - -A statement sequence may contain declarations of lexically-scoped -variables, but apart from declaring a variable name, the declaration acts -like an ordinary statement, and is elaborated within the sequence of -statements as if it were an ordinary statement. That means it actually -has both compile-time and run-time effects. - -=head2 Comments -X<comment> X<#> - -Text from a C<"#"> character until the end of the line is a comment, -and is ignored. Exceptions include C<"#"> inside a string or regular -expression. - -=head2 Simple Statements -X<statement> X<semicolon> X<expression> X<;> - -The only kind of simple statement is an expression evaluated for its -side effects. Every simple statement must be terminated with a -semicolon, unless it is the final statement in a block, in which case -the semicolon is optional. (A semicolon is still encouraged if the -block takes up more than one line, because you may eventually add -another line.) Note that there are some operators like C<eval {}> and -C<do {}> that look like compound statements, but aren't (they're just -TERMs in an expression), and thus need an explicit termination if used -as the last item in a statement. - -=head2 Truth and Falsehood -X<truth> X<falsehood> X<true> X<false> X<!> X<not> X<negation> X<0> - -The number 0, the strings C<'0'> and C<''>, the empty list C<()>, and -C<undef> are all false in a boolean context. All other values are true. -Negation of a true value by C<!> or C<not> returns a special false value. -When evaluated as a string it is treated as C<''>, but as a number, it -is treated as 0. - -=head2 Statement Modifiers -X<statement modifier> X<modifier> X<if> X<unless> X<while> -X<until> X<foreach> X<for> - -Any simple statement may optionally be followed by a I<SINGLE> modifier, -just before the terminating semicolon (or block ending). The possible -modifiers are: - - if EXPR - unless EXPR - while EXPR - until EXPR - foreach LIST - -The C<EXPR> following the modifier is referred to as the "condition". -Its truth or falsehood determines how the modifier will behave. - -C<if> executes the statement once I<if> and only if the condition is -true. C<unless> is the opposite, it executes the statement I<unless> -the condition is true (i.e., if the condition is false). - - print "Basset hounds got long ears" if length $ear >= 10; - go_outside() and play() unless $is_raining; - -The C<foreach> modifier is an iterator: it executes the statement once -for each item in the LIST (with C<$_> aliased to each item in turn). - - print "Hello $_!\n" foreach qw(world Dolly nurse); - -C<while> repeats the statement I<while> the condition is true. -C<until> does the opposite, it repeats the statement I<until> the -condition is true (or while the condition is false): - - # Both of these count from 0 to 10. - print $i++ while $i <= 10; - print $j++ until $j > 10; - -The C<while> and C<until> modifiers have the usual "C<while> loop" -semantics (conditional evaluated first), except when applied to a -C<do>-BLOCK (or to the deprecated C<do>-SUBROUTINE statement), in -which case the block executes once before the conditional is -evaluated. This is so that you can write loops like: - - do { - $line = <STDIN>; - ... - } until $line eq ".\n"; - -See L<perlfunc/do>. Note also that the loop control statements described -later will I<NOT> work in this construct, because modifiers don't take -loop labels. Sorry. You can always put another block inside of it -(for C<next>) or around it (for C<last>) to do that sort of thing. -For C<next>, just double the braces: -X<next> X<last> X<redo> - - do {{ - next if $x == $y; - # do something here - }} until $x++ > $z; - -For C<last>, you have to be more elaborate: -X<last> - - LOOP: { - do { - last if $x = $y**2; - # do something here - } while $x++ <= $z; - } - -B<NOTE:> The behaviour of a C<my> statement modified with a statement -modifier conditional or loop construct (e.g. C<my $x if ...>) is -B<undefined>. The value of the C<my> variable may be C<undef>, any -previously assigned value, or possibly anything else. Don't rely on -it. Future versions of perl might do something different from the -version of perl you try it out on. Here be dragons. -X<my> - -=head2 Compound Statements -X<statement, compound> X<block> X<bracket, curly> X<curly bracket> X<brace> -X<{> X<}> X<if> X<unless> X<while> X<until> X<foreach> X<for> X<continue> - -In Perl, a sequence of statements that defines a scope is called a block. -Sometimes a block is delimited by the file containing it (in the case -of a required file, or the program as a whole), and sometimes a block -is delimited by the extent of a string (in the case of an eval). - -But generally, a block is delimited by curly brackets, also known as braces. -We will call this syntactic construct a BLOCK. - -The following compound statements may be used to control flow: - - if (EXPR) BLOCK - if (EXPR) BLOCK else BLOCK - if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK - LABEL while (EXPR) BLOCK - LABEL while (EXPR) BLOCK continue BLOCK - LABEL until (EXPR) BLOCK - LABEL until (EXPR) BLOCK continue BLOCK - LABEL for (EXPR; EXPR; EXPR) BLOCK - LABEL foreach VAR (LIST) BLOCK - LABEL foreach VAR (LIST) BLOCK continue BLOCK - LABEL BLOCK continue BLOCK - -Note that, unlike C and Pascal, these are defined in terms of BLOCKs, -not statements. This means that the curly brackets are I<required>--no -dangling statements allowed. If you want to write conditionals without -curly brackets there are several other ways to do it. The following -all do the same thing: - - if (!open(FOO)) { die "Can't open $FOO: $!"; } - die "Can't open $FOO: $!" unless open(FOO); - open(FOO) or die "Can't open $FOO: $!"; # FOO or bust! - open(FOO) ? 'hi mom' : die "Can't open $FOO: $!"; - # a bit exotic, that last one - -The C<if> statement is straightforward. Because BLOCKs are always -bounded by curly brackets, there is never any ambiguity about which -C<if> an C<else> goes with. If you use C<unless> in place of C<if>, -the sense of the test is reversed. - -The C<while> statement executes the block as long as the expression is -L<true|/"Truth and Falsehood">. -The C<until> statement executes the block as long as the expression is -false. -The LABEL is optional, and if present, consists of an identifier followed -by a colon. The LABEL identifies the loop for the loop control -statements C<next>, C<last>, and C<redo>. -If the LABEL is omitted, the loop control statement -refers to the innermost enclosing loop. This may include dynamically -looking back your call-stack at run time to find the LABEL. Such -desperate behavior triggers a warning if you use the C<use warnings> -pragma or the B<-w> flag. - -If there is a C<continue> BLOCK, it is always executed just before the -conditional is about to be evaluated again. Thus it can be used to -increment a loop variable, even when the loop has been continued via -the C<next> statement. - -=head2 Loop Control -X<loop control> X<loop, control> X<next> X<last> X<redo> X<continue> - -The C<next> command starts the next iteration of the loop: - - LINE: while (<STDIN>) { - next LINE if /^#/; # discard comments - ... - } - -The C<last> command immediately exits the loop in question. The -C<continue> block, if any, is not executed: - - LINE: while (<STDIN>) { - last LINE if /^$/; # exit when done with header - ... - } - -The C<redo> command restarts the loop block without evaluating the -conditional again. The C<continue> block, if any, is I<not> executed. -This command is normally used by programs that want to lie to themselves -about what was just input. - -For example, when processing a file like F</etc/termcap>. -If your input lines might end in backslashes to indicate continuation, you -want to skip ahead and get the next record. - - while (<>) { - chomp; - if (s/\\$//) { - $_ .= <>; - redo unless eof(); - } - # now process $_ - } - -which is Perl short-hand for the more explicitly written version: - - LINE: while (defined($line = <ARGV>)) { - chomp($line); - if ($line =~ s/\\$//) { - $line .= <ARGV>; - redo LINE unless eof(); # not eof(ARGV)! - } - # now process $line - } - -Note that if there were a C<continue> block on the above code, it would -get executed only on lines discarded by the regex (since redo skips the -continue block). A continue block is often used to reset line counters -or C<?pat?> one-time matches: - - # inspired by :1,$g/fred/s//WILMA/ - while (<>) { - ?(fred)? && s//WILMA $1 WILMA/; - ?(barney)? && s//BETTY $1 BETTY/; - ?(homer)? && s//MARGE $1 MARGE/; - } continue { - print "$ARGV $.: $_"; - close ARGV if eof(); # reset $. - reset if eof(); # reset ?pat? - } - -If the word C<while> is replaced by the word C<until>, the sense of the -test is reversed, but the conditional is still tested before the first -iteration. - -The loop control statements don't work in an C<if> or C<unless>, since -they aren't loops. You can double the braces to make them such, though. - - if (/pattern/) {{ - last if /fred/; - next if /barney/; # same effect as "last", but doesn't document as well - # do something here - }} - -This is caused by the fact that a block by itself acts as a loop that -executes once, see L<"Basic BLOCKs">. - -The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer -available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>. - -=head2 For Loops -X<for> X<foreach> - -Perl's C-style C<for> loop works like the corresponding C<while> loop; -that means that this: - - for ($i = 1; $i < 10; $i++) { - ... - } - -is the same as this: - - $i = 1; - while ($i < 10) { - ... - } continue { - $i++; - } - -There is one minor difference: if variables are declared with C<my> -in the initialization section of the C<for>, the lexical scope of -those variables is exactly the C<for> loop (the body of the loop -and the control sections). -X<my> - -Besides the normal array index looping, C<for> can lend itself -to many other interesting applications. Here's one that avoids the -problem you get into if you explicitly test for end-of-file on -an interactive file descriptor causing your program to appear to -hang. -X<eof> X<end-of-file> X<end of file> - - $on_a_tty = -t STDIN && -t STDOUT; - sub prompt { print "yes? " if $on_a_tty } - for ( prompt(); <STDIN>; prompt() ) { - # do something - } - -Using C<readline> (or the operator form, C<< <EXPR> >>) as the -conditional of a C<for> loop is shorthand for the following. This -behaviour is the same as a C<while> loop conditional. -X<readline> X<< <> >> - - for ( prompt(); defined( $_ = <STDIN> ); prompt() ) { - # do something - } - -=head2 Foreach Loops -X<for> X<foreach> - -The C<foreach> loop iterates over a normal list value and sets the -variable VAR to be each element of the list in turn. If the variable -is preceded with the keyword C<my>, then it is lexically scoped, and -is therefore visible only within the loop. Otherwise, the variable is -implicitly local to the loop and regains its former value upon exiting -the loop. If the variable was previously declared with C<my>, it uses -that variable instead of the global one, but it's still localized to -the loop. This implicit localisation occurs I<only> in a C<foreach> -loop. -X<my> X<local> - -The C<foreach> keyword is actually a synonym for the C<for> keyword, so -you can use C<foreach> for readability or C<for> for brevity. (Or because -the Bourne shell is more familiar to you than I<csh>, so writing C<for> -comes more naturally.) If VAR is omitted, C<$_> is set to each value. -X<$_> - -If any element of LIST is an lvalue, you can modify it by modifying -VAR inside the loop. Conversely, if any element of LIST is NOT an -lvalue, any attempt to modify that element will fail. In other words, -the C<foreach> loop index variable is an implicit alias for each item -in the list that you're looping over. -X<alias> - -If any part of LIST is an array, C<foreach> will get very confused if -you add or remove elements within the loop body, for example with -C<splice>. So don't do that. -X<splice> - -C<foreach> probably won't do what you expect if VAR is a tied or other -special variable. Don't do that either. - -Examples: - - for (@ary) { s/foo/bar/ } - - for my $elem (@elements) { - $elem *= 2; - } - - for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') { - print $count, "\n"; sleep(1); - } - - for (1..15) { print "Merry Christmas\n"; } - - foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) { - print "Item: $item\n"; - } - -Here's how a C programmer might code up a particular algorithm in Perl: - - for (my $i = 0; $i < @ary1; $i++) { - for (my $j = 0; $j < @ary2; $j++) { - if ($ary1[$i] > $ary2[$j]) { - last; # can't go to outer :-( - } - $ary1[$i] += $ary2[$j]; - } - # this is where that last takes me - } - -Whereas here's how a Perl programmer more comfortable with the idiom might -do it: - - OUTER: for my $wid (@ary1) { - INNER: for my $jet (@ary2) { - next OUTER if $wid > $jet; - $wid += $jet; - } - } - -See how much easier this is? It's cleaner, safer, and faster. It's -cleaner because it's less noisy. It's safer because if code gets added -between the inner and outer loops later on, the new code won't be -accidentally executed. The C<next> explicitly iterates the other loop -rather than merely terminating the inner one. And it's faster because -Perl executes a C<foreach> statement more rapidly than it would the -equivalent C<for> loop. - -=head2 Basic BLOCKs -X<block> - -A BLOCK by itself (labeled or not) is semantically equivalent to a -loop that executes once. Thus you can use any of the loop control -statements in it to leave or restart the block. (Note that this is -I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief -C<do{}> blocks, which do I<NOT> count as loops.) The C<continue> -block is optional. - -The BLOCK construct can be used to emulate case structures. - - SWITCH: { - if (/^abc/) { $abc = 1; last SWITCH; } - if (/^def/) { $def = 1; last SWITCH; } - if (/^xyz/) { $xyz = 1; last SWITCH; } - $nothing = 1; - } - -Such constructs are quite frequently used, because older versions -of Perl had no official C<switch> statement. - -=head2 Switch statements -X<switch> X<case> X<given> X<when> X<default> - -Starting from Perl 5.10, you can say - - use feature "switch"; - -which enables a switch feature that is closely based on the -Perl 6 proposal. - -The keywords C<given> and C<when> are analogous -to C<switch> and C<case> in other languages, so the code -above could be written as - - given($_) { - when (/^abc/) { $abc = 1; } - when (/^def/) { $def = 1; } - when (/^xyz/) { $xyz = 1; } - default { $nothing = 1; } - } - -This construct is very flexible and powerful. For example: - - use feature ":5.10"; - given($foo) { - when (undef) { - say '$foo is undefined'; - } - - when ("foo") { - say '$foo is the string "foo"'; - } - - when ([1,3,5,7,9]) { - say '$foo is an odd digit'; - continue; # Fall through - } - - when ($_ < 100) { - say '$foo is numerically less than 100'; - } - - when (\&complicated_check) { - say 'complicated_check($foo) is true'; - } - - default { - die q(I don't know what to do with $foo); - } - } - -C<given(EXPR)> will assign the value of EXPR to C<$_> -within the lexical scope of the block, so it's similar to - - do { my $_ = EXPR; ... } - -except that the block is automatically broken out of by a -successful C<when> or an explicit C<break>. - -Most of the power comes from implicit smart matching: - - when($foo) - -is exactly equivalent to - - when($_ ~~ $foo) - -In fact C<when(EXPR)> is treated as an implicit smart match most of the -time. The exceptions are that when EXPR is: - -=over 4 - -=item * - -a subroutine or method call - -=item * - -a regular expression match, i.e. C</REGEX/> or C<$foo =~ /REGEX/>, -or a negated regular expression match C<$foo !~ /REGEX/>. - -=item * - -a comparison such as C<$_ E<lt> 10> or C<$x eq "abc"> -(or of course C<$_ ~~ $c>) - -=item * - -C<defined(...)>, C<exists(...)>, or C<eof(...)> - -=item * - -A negated expression C<!(...)> or C<not (...)>, or a logical -exclusive-or C<(...) xor (...)>. - -=back - -then the value of EXPR is used directly as a boolean. -Furthermore: - -=over 4 - -=item o - -If EXPR is C<... && ...> or C<... and ...>, the test -is applied recursively to both arguments. If I<both> -arguments pass the test, then the argument is treated -as boolean. - -=item o - -If EXPR is C<... || ...> or C<... or ...>, the test -is applied recursively to the first argument. - -=back - -These rules look complicated, but usually they will do what -you want. For example you could write: - - when (/^\d+$/ && $_ < 75) { ... } - -Another useful shortcut is that, if you use a literal array -or hash as the argument to C<when>, it is turned into a -reference. So C<given(@foo)> is the same as C<given(\@foo)>, -for example. - -C<default> behaves exactly like C<when(1 == 1)>, which is -to say that it always matches. - -See L</"Smart matching in detail"> for more information -on smart matching. - -=head3 Breaking out - -You can use the C<break> keyword to break out of the enclosing -C<given> block. Every C<when> block is implicitly ended with -a C<break>. - -=head3 Fall-through - -You can use the C<continue> keyword to fall through from one -case to the next: - - given($foo) { - when (/x/) { say '$foo contains an x'; continue } - when (/y/) { say '$foo contains a y' } - default { say '$foo does not contain a y' } - } - -=head3 Switching in a loop - -Instead of using C<given()>, you can use a C<foreach()> loop. -For example, here's one way to count how many times a particular -string occurs in an array: - - my $count = 0; - for (@array) { - when ("foo") { ++$count } - } - print "\@array contains $count copies of 'foo'\n"; - -On exit from the C<when> block, there is an implicit C<next>. -You can override that with an explicit C<last> if you're only -interested in the first match. - -This doesn't work if you explicitly specify a loop variable, -as in C<for $item (@array)>. You have to use the default -variable C<$_>. (You can use C<for my $_ (@array)>.) - -=head3 Smart matching in detail - -The behaviour of a smart match depends on what type of thing -its arguments are. It is always commutative, i.e. C<$a ~~ $b> -behaves the same as C<$b ~~ $a>. The behaviour is determined -by the following table: the first row that applies, in either -order, determines the match behaviour. - - - $a $b Type of Match Implied Matching Code - ====== ===== ===================== ============= - (overloading trumps everything) - - Code[+] Code[+] referential equality $a == $b - Any Code[+] scalar sub truth $b->($a) - - Hash Hash hash keys identical [sort keys %$a]~~[sort keys %$b] - Hash Array hash slice existence grep {exists $a->{$_}} @$b - Hash Regex hash key grep grep /$b/, keys %$a - Hash Any hash entry existence exists $a->{$b} - - Array Array arrays are identical[*] - Array Regex array grep grep /$b/, @$a - Array Num array contains number grep $_ == $b, @$a - Array Any array contains string grep $_ eq $b, @$a - - Any undef undefined !defined $a - Any Regex pattern match $a =~ /$b/ - Code() Code() results are equal $a->() eq $b->() - Any Code() simple closure truth $b->() # ignoring $a - Num numish[!] numeric equality $a == $b - Any Str string equality $a eq $b - Any Num numeric equality $a == $b - - Any Any string equality $a eq $b - - - + - this must be a code reference whose prototype (if present) is not "" - (subs with a "" prototype are dealt with by the 'Code()' entry lower down) - * - that is, each element matches the element of same index in the other - array. If a circular reference is found, we fall back to referential - equality. - ! - either a real number, or a string that looks like a number - -The "matching code" doesn't represent the I<real> matching code, -of course: it's just there to explain the intended meaning. Unlike -C<grep>, the smart match operator will short-circuit whenever it can. - -=head3 Custom matching via overloading - -You can change the way that an object is matched by overloading -the C<~~> operator. This trumps the usual smart match semantics. -See L<overload>. - -=head3 Differences from Perl 6 - -The Perl 5 smart match and C<given>/C<when> constructs are not -absolutely identical to their Perl 6 analogues. The most visible -difference is that, in Perl 5, parentheses are required around -the argument to C<given()> and C<when()>. Parentheses in Perl 6 -are always optional in a control construct such as C<if()>, -C<while()>, or C<when()>; they can't be made optional in Perl -5 without a great deal of potential confusion, because Perl 5 -would parse the expression - - given $foo { - ... - } - -as though the argument to C<given> were an element of the hash -C<%foo>, interpreting the braces as hash-element syntax. - -The table of smart matches is not identical to that proposed by the -Perl 6 specification, mainly due to the differences between Perl 6's -and Perl 5's data models. - -In Perl 6, C<when()> will always do an implicit smart match -with its argument, whilst it is convenient in Perl 5 to -suppress this implicit smart match in certain situations, -as documented above. (The difference is largely because Perl 5 -does not, even internally, have a boolean type.) - -=head2 Goto -X<goto> - -Although not for the faint of heart, Perl does support a C<goto> -statement. There are three forms: C<goto>-LABEL, C<goto>-EXPR, and -C<goto>-&NAME. A loop's LABEL is not actually a valid target for -a C<goto>; it's just the name of the loop. - -The C<goto>-LABEL form finds the statement labeled with LABEL and resumes -execution there. It may not be used to go into any construct that -requires initialization, such as a subroutine or a C<foreach> loop. It -also can't be used to go into a construct that is optimized away. It -can be used to go almost anywhere else within the dynamic scope, -including out of subroutines, but it's usually better to use some other -construct such as C<last> or C<die>. The author of Perl has never felt the -need to use this form of C<goto> (in Perl, that is--C is another matter). - -The C<goto>-EXPR form expects a label name, whose scope will be resolved -dynamically. This allows for computed C<goto>s per FORTRAN, but isn't -necessarily recommended if you're optimizing for maintainability: - - goto(("FOO", "BAR", "GLARCH")[$i]); - -The C<goto>-&NAME form is highly magical, and substitutes a call to the -named subroutine for the currently running subroutine. This is used by -C<AUTOLOAD()> subroutines that wish to load another subroutine and then -pretend that the other subroutine had been called in the first place -(except that any modifications to C<@_> in the current subroutine are -propagated to the other subroutine.) After the C<goto>, not even C<caller()> -will be able to tell that this routine was called first. - -In almost all cases like this, it's usually a far, far better idea to use the -structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of -resorting to a C<goto>. For certain applications, the catch and throw pair of -C<eval{}> and die() for exception processing can also be a prudent approach. - -=head2 PODs: Embedded Documentation -X<POD> X<documentation> - -Perl has a mechanism for intermixing documentation with source code. -While it's expecting the beginning of a new statement, if the compiler -encounters a line that begins with an equal sign and a word, like this - - =head1 Here There Be Pods! - -Then that text and all remaining text up through and including a line -beginning with C<=cut> will be ignored. The format of the intervening -text is described in L<perlpod>. - -This allows you to intermix your source code -and your documentation text freely, as in - - =item snazzle($) - - The snazzle() function will behave in the most spectacular - form that you can possibly imagine, not even excepting - cybernetic pyrotechnics. - - =cut back to the compiler, nuff of this pod stuff! - - sub snazzle($) { - my $thingie = shift; - ......... - } - -Note that pod translators should look at only paragraphs beginning -with a pod directive (it makes parsing easier), whereas the compiler -actually knows to look for pod escapes even in the middle of a -paragraph. This means that the following secret stuff will be -ignored by both the compiler and the translators. - - $a=3; - =secret stuff - warn "Neither POD nor CODE!?" - =cut back - print "got $a\n"; - -You probably shouldn't rely upon the C<warn()> being podded out forever. -Not all pod translators are well-behaved in this regard, and perhaps -the compiler will become pickier. - -One may also use pod directives to quickly comment out a section -of code. - -=head2 Plain Old Comments (Not!) -X<comment> X<line> X<#> X<preprocessor> X<eval> - -Perl can process line directives, much like the C preprocessor. Using -this, one can control Perl's idea of filenames and line numbers in -error or warning messages (especially for strings that are processed -with C<eval()>). The syntax for this mechanism is the same as for most -C preprocessors: it matches the regular expression - - # example: '# line 42 "new_filename.plx"' - /^\# \s* - line \s+ (\d+) \s* - (?:\s("?)([^"]+)\2)? \s* - $/x - -with C<$1> being the line number for the next line, and C<$3> being -the optional filename (specified with or without quotes). - -There is a fairly obvious gotcha included with the line directive: -Debuggers and profilers will only show the last source line to appear -at a particular line number in a given file. Care should be taken not -to cause line number collisions in code you'd like to debug later. - -Here are some examples that you should be able to type into your command -shell: - - % perl - # line 200 "bzzzt" - # the `#' on the previous line must be the first char on line - die 'foo'; - __END__ - foo at bzzzt line 201. - - % perl - # line 200 "bzzzt" - eval qq[\n#line 2001 ""\ndie 'foo']; print $@; - __END__ - foo at - line 2001. - - % perl - eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@; - __END__ - foo at foo bar line 200. - - % perl - # line 345 "goop" - eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'"; - print $@; - __END__ - foo at goop line 345. - -=cut |