Calling a Scheme Function with Arguments

This is perhaps one of the last great pieces of the puzzle: passing a value from C into Scheme.

The way I do this is a little round about, and I am not sure if there is a better way so you have been warned.

To pass a value from C to Scheme we first use sexp_intern to create a new Scheme variable. We then assign it a value with sexp_env_define. And from there we can then use our new Scheme variable as a function argument (even though it is more like a global variable really).


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

int main()
  const int num = 4;
  sexp ctx;
  ctx = sexp_make_eval_context(NULL, NULL, NULL, 0, 0);
  sexp_load_standard_env(ctx, NULL, SEXP_SEVEN);

  sexp_gc_var4(file_path, arg_sym, arg_val, ret);
  sexp_gc_preserve4(ctx, file_path, arg_sym, arg_val, ret);

  arg_val = sexp_make_fixnum(num);
  arg_sym = sexp_intern(ctx, "arg", -1);
  sexp_env_define(ctx, sexp_context_env(ctx), arg_sym, arg_val);

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

  printf("Asking Scheme for the square of (%d)...\n", num);
  ret = sexp_eval_string(ctx, "(square arg)", -1, NULL);
  int result = -1;
  if (sexp_integerp(ret)) {
    result = sexp_unbox_fixnum(ret);

  printf("...Scheme told us (%d)\n", result);


(import (chibi))
(define (square a)
  (* a a))

Here we use sexp_make_fixnum to assign the int value to an sexpression (arg_val). Then we use sexp_intern to create a new symbol within the Scheme context called arg. Lastly we use sexp_env_define to bind the int value arg_val to the arg symbol (referred to as arg_sym on the C side).

We can now do sexp_eval_string(ctx, "(square arg)" to call the Scheme square function with our int argument.