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+# Copyright (c) 2013-2014 Sandstorm Development Group, Inc. and contributors
+# Licensed under the MIT License:
+#
+# Permission is hereby granted, free of charge, to any person obtaining a copy
+# of this software and associated documentation files (the "Software"), to deal
+# in the Software without restriction, including without limitation the rights
+# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+# copies of the Software, and to permit persons to whom the Software is
+# furnished to do so, subject to the following conditions:
+#
+# The above copyright notice and this permission notice shall be included in
+# all copies or substantial portions of the Software.
+#
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+# THE SOFTWARE.
+
+@0x85150b117366d14b;
+
+interface Calculator {
+  # A "simple" mathematical calculator, callable via RPC.
+  #
+  # But, to show off Cap'n Proto, we add some twists:
+  #
+  # - You can use the result from one call as the input to the next
+  #   without a network round trip.  To accomplish this, evaluate()
+  #   returns a `Value` object wrapping the actual numeric value.
+  #   This object may be used in a subsequent expression.  With
+  #   promise pipelining, the Value can actually be used before
+  #   the evaluate() call that creates it returns!
+  #
+  # - You can define new functions, and then call them.  This again
+  #   shows off pipelining, but it also gives the client the
+  #   opportunity to define a function on the client side and have
+  #   the server call back to it.
+  #
+  # - The basic arithmetic operators are exposed as Functions, and
+  #   you have to call getOperator() to obtain them from the server.
+  #   This again demonstrates pipelining -- using getOperator() to
+  #   get each operator and then using them in evaluate() still
+  #   only takes one network round trip.
+
+  evaluate @0 (expression :Expression) -> (value :Value);
+  # Evaluate the given expression and return the result.  The
+  # result is returned wrapped in a Value interface so that you
+  # may pass it back to the server in a pipelined request.  To
+  # actually get the numeric value, you must call read() on the
+  # Value -- but again, this can be pipelined so that it incurs
+  # no additional latency.
+
+  struct Expression {
+    # A numeric expression.
+
+    union {
+      literal @0 :Float64;
+      # A literal numeric value.
+
+      previousResult @1 :Value;
+      # A value that was (or, will be) returned by a previous
+      # evaluate().
+
+      parameter @2 :UInt32;
+      # A parameter to the function (only valid in function bodies;
+      # see defFunction).
+
+      call :group {
+        # Call a function on a list of parameters.
+        function @3 :Function;
+        params @4 :List(Expression);
+      }
+    }
+  }
+
+  interface Value {
+    # Wraps a numeric value in an RPC object.  This allows the value
+    # to be used in subsequent evaluate() requests without the client
+    # waiting for the evaluate() that returns the Value to finish.
+
+    read @0 () -> (value :Float64);
+    # Read back the raw numeric value.
+  }
+
+  defFunction @1 (paramCount :Int32, body :Expression)
+              -> (func :Function);
+  # Define a function that takes `paramCount` parameters and returns the
+  # evaluation of `body` after substituting these parameters.
+
+  interface Function {
+    # An algebraic function.  Can be called directly, or can be used inside
+    # an Expression.
+    #
+    # A client can create a Function that runs on the server side using
+    # `defFunction()` or `getOperator()`.  Alternatively, a client can
+    # implement a Function on the client side and the server will call back
+    # to it.  However, a function defined on the client side will require a
+    # network round trip whenever the server needs to call it, whereas
+    # functions defined on the server and then passed back to it are called
+    # locally.
+
+    call @0 (params :List(Float64)) -> (value :Float64);
+    # Call the function on the given parameters.
+  }
+
+  getOperator @2 (op :Operator) -> (func :Function);
+  # Get a Function representing an arithmetic operator, which can then be
+  # used in Expressions.
+
+  enum Operator {
+    add @0;
+    subtract @1;
+    multiply @2;
+    divide @3;
+  }
+}