QueueList's cast and retype do not adhere to the List methods' documentation

[srawlins]

I'd like to say that QueueList's cast and retype do not adhere to the contract laid out by List.cast and List.retype but that may be up for debate. In any case, List.cast's documentation says:

Returns a view of this list as a list of R instances, if necessary.

List.retype's documentation says, similarly:

Returns a view of this list as a list of R instances.

(Ignoring for now what "if necessary" must mean...) I believe it is intended that the return value be the same list (just a different view of it), not a new list. For example, let's play with List:

void main() {
  var a = new List<num>()..addAll([1,2,3]);
  var b = a.retype<int>();
  b.add(4); // add 4 to the view.
  a.add(5); // add 5 to the original.
  b.remove(3); // remove 3 from the view.
  print('a: $a'); //=> [1, 2, 4, 5]
  print('b: $b'); //=> [1, 2, 4, 5]
}

They are the same list. Now let's do the same for QueueList:

import 'package:collection/collection.dart';

void main() {
  var c = new QueueList<num>()..addAll([1,2,3]);
  var d = c.retype<int>();
  d.add(4);
  print('c: $c'); //=> {1, 2, 3} // Not the same list!
  print('d: $d'); //=> {1, 2, 3, null, null, null, null, null, 4} // Duck?
  c.add(5);
  print('c: $c'); //=> {1, 2, 3, 5}
  print('d: $d'); //=> {1, 2, 3, null, null, null, null, null, 4}
}

I have not checked the other cast and retype implementations yet, but when I do, I presume I'll just add on to this bug, rather than filing a bug for each class.

[matanlurey]

I believe it is intended that the return value be the same list (just a different view of it), not a new list.

I don't think so (@leafpetersen @lrhn), even though I wish that was the case - I think .cast tries to re-use the same list, and basically invokes .retype if not possible, making this OK.

I.e. this is just a convenience for calling List.castFrom:

https://github.com/dart-lang/sdk/blob/f1ebe2bd5cfcb6b522e5b4fd406cdabb1a2d2091/sdk/lib/collection/list.dart#L332-L335

Let's look at the implementation of DoubleLinkedQueue.cast:

https://github.com/dart-lang/sdk/blob/f1ebe2bd5cfcb6b522e5b4fd406cdabb1a2d2091/sdk/lib/collection/queue.dart#L359-L362

Unfortunately there is no way in Dart to really "re-type" something, so unlike Dart 1's as List<String>, list.cast<String>() may return a new list if necessary to fulfill that cast.

[lrhn]

Both cast or retype are really intended to return a view of the original collection, one that also forwards mutating methods. The "if necessary" means that cast returns the original list if it already implements the desired interface, possibly by implementing a subtype, where retype always wraps so the return type is exact. It should not create a new collection that isn't linked to the original.

The QueueList class has the extra issue of needing to implement both List.cast and Queue.cast, so it needs to return something that is both a List and a Queue. I have a class ready for that in the private parts of the SDK, because one of the plans for Dart 2 was to make ListQueue implement List as well, but that doesn't help this package.

[matanlurey]

Let me give this a shot, thanks @srawlins and @lrhn.