Putting it All Together

For the final example I will exhibit all the techniques shown previously in a single application. We will write a Scheme function which will calculate and return the length of a string.

For the impatient, the quickest way to get started may be to look at this example with the official API Reference to hand.

The only thing explicitly not covered in the previous posts is the use of sexp_procedurep to check that a Scheme function has been defined before trying to call it; just for the sake of printing an error message and letting the user know what is wrong (nothing bad would happen otherwise).


#include <stdio.h>
#include <chibi/eval.h>
int my_str_len(sexp ctx, const char *message)
    int len = -1;
    sexp_gc_var3(arg_sym, arg_val, ret);
    sexp_gc_preserve3(ctx, arg_sym, arg_val, ret);

    arg_val = sexp_c_string(ctx, message, -1);
    arg_sym = sexp_intern(ctx, "arg", -1);
    sexp_env_define(ctx, sexp_context_env(ctx), arg_sym, arg_val);
    ret = sexp_eval_string(ctx, "(my-str-len arg)", -1, NULL);

    if (sexp_integerp(ret)) {
        len = sexp_unbox_fixnum(ret);

    return len;

int is_defined(sexp ctx, const char *sym)
    sexp_gc_preserve1(ctx, ret);

    ret = sexp_eval_string(ctx, sym, -1, NULL);

    int defined = sexp_procedurep(ret);


    return defined;

int main(int argc, char **argv)
    if (argc < 2) {
        printf("Usage: %s [MESSAGE]...\n", argv[0]);
        return 1;

    int i;
    sexp ctx;
    ctx = sexp_make_eval_context(NULL, NULL, NULL, 0, 0);
    sexp_load_standard_env(ctx, NULL, SEXP_SEVEN);

    sexp_gc_preserve1(ctx, file_path);
    file_path = sexp_c_string(ctx, "strlen.ss", -1);
    sexp_load(ctx, file_path, NULL);

    const char *sym = "my-str-len";
    if (is_defined(ctx, sym))
        printf("%s is defined.\n", sym);
    else {
        printf("%s is not defined!\n", sym);
        return 1;

    for (i = 1; i < argc; ++i) {
        const char *msg = argv[i];
        int len = my_str_len(ctx, msg);
        printf("Chibi told me that the length of <%s> is <%d>\n", msg, len);
        printf("C told me that the length of <%s> is <%lu>\n", msg, strlen(msg));



(define (my-str-len s)
  (letrec ((my-str-len-h (lambda (s acc)
    (if (null? s)
      (my-str-len-h (cdr s) (+ 1 acc))))))
  (my-str-len-h (string->list s) 0)))

To build:

$ gcc -Iinclude/ -L. -lchibi-scheme -lm strlen.c -o strlen

Example usage:

$ LD_LIBRARY_PATH=. ./strlen hello something unicode†string
my-str-len is defined.
Chibi told me that the length of <hello> is <5>
C told me that the length of <hello> is <5>
Chibi told me that the length of <something> is <9>
C told me that the length of <something> is <9>
Chibi told me that the length of <unicode†string> is <14>
C told me that the length of <unicode†string> is <16>

Note how in the case of the last string which contains unicode, C gets the wrong answer. ;)