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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.  */
     
     %{
       #include <stdio.h>
       #include <math.h>
       int yylex (void);
       void yyerror (char const *);
     %}
     
     %define api.value.type {double}
     %token NUM
     
     %% /* Grammar rules and actions follow.  */

The declarations section (see The prologue) contains two preprocessor directives and two forward declarations.

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 tokens and their types (see The Bison Declarations Section).

The %define directive defines the variable api.value.type, thus specifying the C data type for semantic values of both tokens and groupings (see Data Types of Semantic Values). The Bison parser will use whatever type api.value.type 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. C code can use YYSTYPE to refer to the value api.value.type.

Each terminal symbol that is not a single-character literal must be declared. (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.