Failed type check: type Function is not assignable to type Iterator

[DartBot]

This issue was originally filed by [email protected]

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What steps will reproduce the problem?
I don't know what is the problem with the following program, as
similar pattern works in Java:

bool check(Iterable iter) {
  return iter !== null;
}

class _Iterable<E> implements Iterable<E> {
  final Iterator<E> _iterator;
  _Iterable(this._iterator);

  Iterator<E> iterator() {
    return _iterator;
  }
}

Iterable copy(Iterable iterable) {
  return new _Iterable(iterable.iterator);
}

main() {
  Expect.isTrue(check(copy([0, 1])));
}

You may try the test program here http://try.dartlang.org/s/zq05

What is the expected output? What do you see instead?
Expected: program should run without a problem
Actual:
Dartboard: Failed type check: type Function is not assignable to type Iterator
Dart editor: Unhandled exception:
type '() => Iterator<Dynamic>' is not a subtype of type 'Iterator<Dynamic>' of '_iterator@28152909'.

What version of the product are you using? On what operating system?
Version 0.1.0.201204131908, Build 6557
Dart SDK version 6478
Windows 7 Home Premium, 64 bit

Please provide any additional information below.

[DartBot]

This comment was originally written by [email protected]

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Iterable copy(Iterable iterable) {
return new _Iterable(iterable.iterator);

Here's the problem. You aren't calling the iterator function, you are closurizing it. If you write iterable.iterator, you get back a function that, when called, produces an iterator. Here, you want to produce the iterator immediately, so you must write iterable.iterator(). This isn't the first (and certainly not last) occurence of this "issue" (I personally call it the closurization trap).

Oh, and btw, creating an Iterable that is backed by an Iterator is a terrible idea, because you can iterate over it only once, yet you usually expect that you can iterate over an Iterable as many times as you want. Here's a reference why it wasn't added to Guava: http://code.google.com/p/guava-libraries/wiki/IdeaGraveyard#A_method_to_view_an_iterator_as_an_iterable

P.S.: I think that this would be better suited to the [email protected] mailing list than to the issue tracker.

[kasperl]

Not really an actionable issue for us. Marking as invalid.

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Added Invalid label.