Getting Values Out of Scheme

This is perhaps one of the most useful things to do. Once we can lookup values defined in a Scheme file and pull them into C, we can use Chibi Scheme for configuration files instead of using XML, JSON or writing custom file parsing code.

luckynum.c

#include <stdio.h>
#include <chibi/eval.h>

int main()
{
  sexp ctx;
  ctx = sexp_make_eval_context(NULL, NULL, NULL, 0, 0);
  sexp_load_standard_env(ctx, NULL, SEXP_SEVEN);
  //sexp_load_standard_ports(ctx, NULL, stdin, stdout, stderr, 0);

  sexp_gc_var2(file_path, ret);
  sexp_gc_preserve2(ctx, file_path, ret);

  file_path = sexp_c_string(ctx, "luckynum.ss", -1);
  sexp_load(ctx, file_path, NULL);

  ret = sexp_eval_string(ctx, "lucky-number", -1, NULL);

  int lucky_number = -1;
  if (sexp_integerp(ret)) {
    lucky_number = sexp_unbox_fixnum(ret);
  }

  printf("It looks like the lucky number is (%d)\n", lucky_number);

  sexp_gc_release2(ctx);
  sexp_destroy_context(ctx);
}

luckynum.ss

(import (chibi))
(define lucky-number 666)

A few things to note here:

• those familiar with Lisp may already understand this; in sexp_eval_string(..., "lucky-number", ...) we are asking the Scheme interpreter to evaluate the variable name, which will return us its value. If we change lucky-number to be a function within luckynum.ss then we would need to wrap it in parenthesis in order to execute it (and get back the return value of the function).

• we do not need sexp_load_standard_ports for this one because Scheme is not doing any IO.

• you may need to link against the standard maths library (-lm)

With the call to sexp_integerp(ret) we check that ret is an integer. We then use sexp_unbox_fixnum(ret) to get the value as a C int. The “p” at the end stands for “predicate”, possibly because it is convention in Scheme to name functions that return boolean values with a “?” at the end. Since C does not support this syntax I guess the Chibi Scheme developers decided to use a “p” instead. In the documentation you will find several other predicate and conversion functions for different types. In addition to int I will cover strings later on.