%{
  #include <stdio.h>
  #include "Expr.h"
  int yylex(void);
  void yyerror (char const * s) { fprintf(stderr, "%s\n", s); }
%}
%union {
  long int intval;
  char *   string;
  Expr     expr;
}
%token <intval> TOK_NUMBER
%token <string> TOK_ID
%token TOK_IF TOK_THEN TOK_ELSE
%type <expr> expr term factor
%start input
%%
factor : TOK_NUMBER         { $$ = exprInt($1); }
       | TOK_ID             { $$ = exprVar($1); }
       | '(' expr ')'       { $$ = $2; }

term : factor
     | term '*' factor  { $$ = exprBin("*", $1, $3); }
     | term '/' factor  { $$ = exprBin("/", $1, $3); }

expr : term
     | expr '+' term  { $$ = exprBin("+", $1, $3); }
     | expr '-' term  { $$ = exprBin("-", $1, $3); }

input : /* empty */ | input line          /* line-by-line processing */

line : '\n'                {}                        /* empty lines allowed */
     | expr '\n'           { printf(" = %ld\n", exprEval($1)); }
%%
int main ( void ) { return yyparse(); }