Loop behavior altered by continue and return

[TrevorBurnham]

Following up issue 948:

On the current master,

for i in [1, 2, 3]
  j = i
  func1 = -> console.log "i is #{i} and j is #{j}"
  setTimeout func1, 0

gives you

i is 1 and j is 1
i is 2 and j is 2
i is 3 and j is 3

but if you add a continue or return statement anywhere in the loop, the behavior is radically changed, e.g.:

for i in [1, 2, 3]
  j = i
  func1 = -> console.log "i is #{i} and j is #{j}"
  setTimeout func1, 0
  continue

gives you

i is 3 and j is 3
i is 3 and j is 3
i is 3 and j is 3

as would the same code with, say, return if false in place of continue.

Let's look at the compiled output without the continue/return:

var i, _fn, _i, _len, _ref;
_ref = [1, 2, 3];
_fn = function(i) {
  var func1, j;
  j = i;
  func1 = function() {
    return console.log("i is " + i + " and j is " + j);
  };
  setTimeout(func1, 0);
};
for (_i = 0, _len = _ref.length; _i < _len; _i++) {
  i = _ref[_i];
  _fn(i);
}

Currently, the whole _fn thing isn't done if there's a continue or return anywhere in the loop.

One solution would be to replace _fn(i); with retVal = _fn(i); retVal ? return retVal[0] : 0;, and then compile continue to return, return to return [void 0];, and return value to return [value]. (Edit: This proposal wasn't tested and needs more work; see satyr's comment below.)

[satyr]

And break to...?

retVal ? return retVal[0] : 0

Syntax error.

[TrevorBurnham]

Good points both. Do you have an alternative solution in mind?

[satyr]

an alternative solution

Remove this implicit transformation. (I'm glad Coco did.)

[jashkenas]

I'm afraid that this inconsistency is a necessary part of the implicit transformation. As long as we have functions-in-loops that close over their variables, we'll have to have this as well.

The current situation with pure statements like continue, break, and return is that they cancel the closure wrapping, because they have no meaning inside of it.

[jashkenas]

This is a re-hashing of issues #423, and #728. Please take a peek at those.

[odf]

I have an alternative alternative solution: remove continue, break and return from the language. I think fixing the scope issues in loops is more important than keeping obsolete control flow hacks alive indefinitely. ;-)

Failing that, just document the issue and leave things as they are.

[jashkenas]

Thanks for wrapping all of this up in #959, Trevor. Closing this ticket in favor of that one.