(define (copy-string string)
  (let ((data (blob->u8vector (string->blob string))))
    ((foreign-lambda* (c-pointer char) ((u8vector src) (int size))
       "char *dest = malloc((size + 1) * sizeof(char));
        memcpy(dest, src, size * sizeof(char));
        dest[size] = '\\0';
        C_return(dest);")
     data (u8vector-length data))))

(define completion-index 0)
(define completion-candidates #("foo" "bar" "baz" "💩")) ; <- utf-8 test

(define-external (readline_completer ((const c-string) prefix) (int state)) (c-pointer char)
  (when (zero? state)
    (set! completion-index 0)) ; initialization
  (if (< completion-index (vector-length completion-candidates))
      (let ((candidate (vector-ref completion-candidates completion-index)))
        (set! completion-index (add1 completion-index))
        (copy-string candidate))
      #f))

(define (enable-completer!)
  ((foreign-lambda* void ()
     "rl_completion_entry_function = readline_completer;")))

(define (disable-completer!)
  ((foreign-lambda* void ()
     "rl_completion_entry_function = NULL;")))