Loop variable referenced from inner function

[lowentropy]

When I have a loop which produces inner functions referencing the iterator, those functions will refer not to their own copy of the iterator, but to an variable defined in the outer scope. The following test illustrates the problem:

test "iterator scope", ->

  actual = []
  expected = [1, 2, 3]
  funs = []

  for x in expected
    funs.push -> actual.push x

  funs[0]()
  funs[1]()
  funs[2]()

  eq actual, expected  

This produces the following javascript using coffee version 1.0.0:

(function() {
  test("iterator scope", function() {
    var actual, expected, funs, x, _i, _len;
    actual = [];
    expected = [1, 2, 3];
    funs = [];
    for (_i = 0, _len = expected.length; _i < _len; _i++) {
      x = expected[_i];
      funs.push(function() {
        return actual.push(x);
      });
    }
    funs[0]();
    funs[1]();
    funs[2]();
    return eq(actual, expected);
  });
}).call(this);

The loop variable is re-bound to each element of the array and is shared by all the functions produced. When they are called, they all use the last value assigned (in this case, the 'actual' array ends up being [3, 3, 3], instead of the expected [1, 2, 3]).

In previous versions of Coffeescript, the code would actually produce something like this:

(function() {
  test("iterator scope", function() {
    var actual, expected, funs, _i, _len, _fn;
    actual = [];
    expected = [1, 2, 3];
    funs = [];
    _fn = function(x) {
      funs.push(function() {
        return actual.push(x);
      });
    };
    for (_i = 0, _len = expected.length; _i < _len; _i++) {
      _fn(expected[_i]);
    }
    funs[0]();
    funs[1]();
    funs[2]();
    return eq(actual, expected);
  });
}).call(this);

Which works correctly.

[michaelficarra]

Yep, auto-closure-wrapping was removed just before 1.0 because it was broken. A proper fix is possible, but it's a very difficult patch. See #959 for the suggestions (and here's mine).

edit: Also, see the first sentence of the change log for 1.0.0.

[lowentropy]

Thanks!

[michaelficarra]

Oh yeah, I never told you how to fix it. Here:

for x in expected
  do (x) ->
    funs.push -> actual.push x