Tail Evaluation

All of MzScheme's built-in functions and syntax support proper tail-recursion. When a new primitive procedure or syntax is added to MzScheme, special care must be taken to ensure that tail recursion is handled properly. Specifically, when the final return value of a function is the result of an application, then scheme_tail_apply should be used instead of scheme_apply. When scheme_tail_apply is called, it postpones the procedure application until control returns to the Scheme evaluation loop.

For example, consider the following implementation of a thunk-or primitive, which takes any number of thunks and performs or on the results of the thunks, evaluating only as many thunks as necessary.

static Scheme_Object *
thunk_or (int argc, Scheme_Object **argv)
{
  int i;
  Scheme_Object *v;

  if (!argc)
    return scheme_false;

  for (i = 0; i < argc - 1; i++)
    if (SCHEME_FALSEP((v = _scheme_apply(argv[i], 0, NULL))))
      return v;

  return scheme_tail_apply(argv[argc - 1], 0, NULL);
}

This thunk-or properly implements tail-recursion: if the final thunk is applied, then the result of thunk-or is the result of that application, so scheme_tail_apply is used for the final application.