hoc.y

%{
#define YYSTYPE double /* data type of yacc stack */
%}
%token NUMBER
%left '+' '-'   /* left associative, same precedence */
%left '*' '/'   /* left assoc., higher precedence */
%left UNARYMINUS
%%
list:     /* nothing */
        | list '\n'
        | list expr '\n'    { printf("\t%.8g\n", $2); }
        ;
expr:     NUMBER        { $$ = $1; }
        | '-' expr %prec UNARYMINUS { $$ = -$2; }
        | expr '+' expr { $$ = $1 + $3; }
        | expr '-' expr { $$ = $1 - $3; }
        | expr '*' expr { $$ = $1 * $3; }
        | expr '/' expr { $$ = $1 / $3; }
        | '(' expr ')'  { $$ = $2; }
        ;
%%
        /* end of grammer */

#include <stdio.h>
#include <ctype.h>
char    *progname;  /* for error message */
int     lineno = 1;

main(argc, argv)
        char *argv[];
{
        progname = argv[0];
        yyparse();
}

yylex()
{
        int c;

        while ((c=getchar()) == ' ' || c == '\t')
                ;
        if (c == EOF)
                return 0;
        if (c == '.' || isdigit(c)) {   /* number */
                ungetc(c, stdin);
                scanf("%1f", &yylval);
                return NUMBER;
        }
        if (c == '\n')
                lineno++;
        return c;
}

yyerror(s)      /* called for yacc syntax error */
        char *s;
{
        warning(s, (char *) 0);
}

warning(s, t)   /* print warning message */
        char *s, *t;
{
        fprintf(stderr, "%s: %s", progname, s);
        if (t)
                fprintf(stderr, " %s", t);
        fprintf(stderr, " near line %d\n", lineno);
}

$ ./hoc
-2-3
        -1.0630702e-314
2*5
        0
1/0
        inf
0/1
        0
1+1
        1.0527088e-314
^C