hoc.y

%{
#include <ctype.h>
#include <stdio.h>
#define YYSTYPE double
%}
%token  NUMBER
%left   '+' '-' 
%left   '*' '/' 
%left   UNARYMINUS
%left   UNARYPLUS
%%
list:
    | list '\n'
    | list expr '\n' { printf("\t%.8g\n", $2); }
    ;
expr: NUMBER    { $$ = $1; }
    | '-' expr %prec UNARYMINUS { $$ = -$2; }
    | '+' expr %prec UNARYMINUS { $$ = +$2; }
    | expr '+' expr { $$ = $1 + $3; }
    | expr '-' expr { $$ = $1 - $3; }
    | expr '*' expr { $$ = $1 * $3; }
    | expr '/' expr { $$ = $1 / $3; }
    | '(' expr ')'  { $$ = $2; }
    ;
%%
char  *progname;
int   lineno = 1;

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

yylex() {
  int c;
  
  while((c = getchar()) == ' ' || c == '\t')
    ;   

  if(c == EOF)
    return 0;

  if(c == '.' || isdigit(c)) {
    ungetc(c, stdin);
    scanf("%lf", &yylval);
    return NUMBER;
  }

  if(c == '\n')
    lineno++;

  return c;
}

yyerror(char *s) {
  warning(s, (char *) 0); 
}

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