parser_grammer.y.bak

%{
    #include <stdio.h>
    #include <stdlib.h>
    #include <math.h>
    int yylex(void);
    void yyerror (char const *);
%}

%union {
    int intval;
    float floatval;
    char *stringval;
}

%token <intval> INT
%token <floatval> FLOAT
%token <stringval> STRING

%type <intval> exp

%%
input:
    /* empty */
    | input line
    ;

line:
    '\n'
    | exp '\n' { printf("%.10g\n", $1); }
    ;

exp:
    INT         { $$ = $1; }
    | exp '+' exp   { $$ = $1 + $3; }
    | exp '-' exp   { $$ = $1 - $3; }
    | exp '*' exp   { $$ = $1 * $3; }
    | exp '/' exp   { $$ = $1 / $3; }
    | exp 'n'       { $$ = -$1; }
    ;

%%

int main(void){
    yyparse();
}

void yyerror(const char *s){
    printf("There's an error %s", s);
    exit(-1);
}