Valid recursion leads to runtime error

[jvoigtlaender]

Edited by @evancz

The following program leads to a runtime error ("g is not a function").

import Html

g = h

h n =
  if n < 2 then 1 else g (n - 1)

main = Html.text (toString (h 10))

With the 0.18 compiler, it produces this JavaScript:

var _user$project$Main$g = _user$project$Main$h;
var _user$project$Main$h = function (n) {
	return (_elm_lang$core$Native_Utils.cmp(n, 2) < 0) ? 1 : _user$project$Main$g(n - 1);
};
var _user$project$Main$main = _elm_lang$virtual_dom$Native_VirtualDom.staticProgram(
	_elm_lang$html$Html$text(
		_elm_lang$core$Basics$toString(
			_user$project$Main$h(10))));

If the var definitions for _user$project$Main$g and _user$project$Main$h were reordered, everything would be fine. It is unclear how to detect this in general, so no concrete steps are proposed to fix this right now.

---

The following two programs are equivalent, but do run successfully:

import Html

h n =
  if n < 2 then 1 else g (n - 1)

g = h

main = Html.text (toString (h 10))

import Html

h n =
  if n < 2 then 1 else h (n - 1)

main = Html.text (toString (h 10))

[Deleted user]

Reproduced on elm 0.17. The issue is that the order of definition is incorrect.

RuntimeError Elm

-- leads to a runtime error "g is not a function"
import Html
g = h
h n =
    if n < 2 then
        1
    else
        g (n - 1)

main = Html.text (toString (h 10))

produces:

var _user$project$Main$g = _user$project$Main$h;
var _user$project$Main$h = function (n) {
    return (_elm_lang$core$Native_Utils.cmp(n, 2) < 0) ? 1 : _user$project$Main$g(n - 1);
};
var _user$project$Main$main = {
    main: _elm_lang$html$Html$text(
        _elm_lang$core$Basics$toString(
            _user$project$Main$h(10)))
};

Valid Elm

import Html
h n =
  if n < 2 then
      1
  else
      g (n - 1)
g = h

main = Html.text (toString (h 10))

produces:

var _user$project$Main$h = function (n) {
    return (_elm_lang$core$Native_Utils.cmp(n, 2) < 0) ? 1 : _user$project$Main$g(n - 1);
};
var _user$project$Main$g = _user$project$Main$h;
var _user$project$Main$main = {
    main: _elm_lang$html$Html$text(
        _elm_lang$core$Basics$toString(
            _user$project$Main$h(10)))
};

[evancz]

Added all of these to #1377 so it can be addressed alongside a bunch of similar issues.

Thanks for the report!

[evancz]

With latest version of the compiler, it generates this:

var user$project$Main$h = function (n) {
	return (_Utils_cmp(n, 2) < 0) ? 1 : user$project$Main$g(n - 1);
};
var user$project$Main$g = user$project$Main$h;
var user$project$Main$main = _VirtualDom_staticProgram(
	elm_lang$html$Html$text(
		elm_lang$core$Basics$toString(
			user$project$Main$h(10))));

So the program runs as expected. Thanks for the report!