A lispy arithmetic REPL

lispcalc.y

%{
  #include <stdio.h> // include print functions
  #include <math.h> // include `pow' function
  #include <ctype.h> // include `isdigit' function
  int yylex (void); // declare lexing function
  void yyerror (char const *); // declare yyerror function
%}

%define api.value.type {double} // delcare all tokens and non-terminals to have type `double'
%token NUM // declare NUM as a terminal

%%

input:
  %empty
| input line
;

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

expression:
  NUM
| '(' '+' expression expression ')' { $$ = $3 + $4; }
| '(' '-' expression expression ')' { $$ = $3 - $4; }
| '(' '*' expression expression ')' { $$ = $3 * $4; }
| '(' '/' expression expression ')' { $$ = $3 / $4; }
;

%%

int yylex (void)
{
  int c;

  while ((c = getchar ()) == ' ' || c == '\t')
    continue;

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

  if (c == EOF)
    return 0;

  return c;
}

int main (void)
{ return yyparse(); }

void yyerror (char const *s)
{
  fprintf (stderr, "%s\n", s);
  yyparse();
}

Makefile

all:
	/usr/local/Cellar/bison/3.0.4/bin/bison lispcalc.y && gcc -lm -o lispcalc lispcalc.tab.c