YACC 实例分析

%{
#include <string.h>
#include <math.h>
#include "symbol.h"
%}

%union {
    double dval;
    struct symtab *symp;
}
%token <symp> NAME
%token <dval> NUMBER
%left '-' '+'
%left '*' '/'
%nonassoc UMINUS

%type <dval> expression
%%
statement_list: statement '\n'
    |   statement_list statement '\n'
    ;

statement:  NAME '=' expression { $1->value = $3;  printf("(%s) = (%g)\n", $1->name, $1->value); }
    |   expression      { printf("= %g\n", $1); } 
    ;

expression: expression '+' expression { $$ = $1 + $3; }
    |   expression '-' expression { $$ = $1 - $3; }
    |   expression '*' expression { $$ = $1 * $3; }


    |   expression '/' expression
                {   if($3 == 0.0)
                        yyerror("divide by zero");
                    else
                        $$ = $1 / $3;
                }
    |   '-' expression %prec UMINUS { $$ = -$2; }
    |   '(' expression ')'  { $$ = $2; }
    |   NUMBER          { $$ = $1;   }
    |   NAME            { $$ = $1->value; }
    |   NAME '(' expression ')' {
            if($1->funcptr)
                $$ = ($1->funcptr)($3);
            else {
                printf("%s not a function\n", $1->name);
                $$ = 0.0;
            }
        }
    ;
%%