Iterable.where() should be generic and return an iterable with the given type

[Hixie]

The following code is sound but the analyzer can't tell:

new Directory('packages')
    .listSync()
    .where((FileSystemEntity entity) => entity is Directory)
    .where((Directory dir) { // ERROR HERE
      // ...
    });

If where was defined as Iterable<T> where<T extends E>(...) then the above could work (might even work automatically given type inference?).

[pq]

cc @bwilkerson

[alexeieleusis]

I have had to deal with this issue too, but where is about predicates, not types, how should it behave when the predicate is not a test for a type?

[Hixie]

presumably if you don't give the type explicitly, it would just return an Iterable<E>. (Maybe it would have to be annotated with @optionalTypeArgs?)

[lrhn]

I'd rather add a new method to do type-filtering, like Iterable<T> Iterable.of<T>().
The where is used for many other things, and doesn't generally need the type parameter, even if it's declares as <T extends E> so we have a good default if it's omitted.

(I don't know what @optionalTypeArgs is, but I can assure you that it won't be in the platform libraries) :)

[Hixie]

I'd be totally on board with .of<T> too. :-)

(@optionalTypeArgs lets you enable the require all type annotations lint but still have some type annotations be optional. The platform libraries will presumably have to add it anywhere they declare a generic type where sometimes it makes no sense to give a type. For example, the Flutter framework uses it for GlobalKey, since the type argument only makes sense if you're going to use the currentState getter, but if you're using the key with a stateless widget there's no type that could make sense.)

[Hixie]

Just ran into this again. This is definitely going to become a bigger paint point in Dart 2.0.

[srawlins]

Is this now solved with Iterable.cast() and Iterable.retype()?

[eernstg]

The following works with dart2js --preview-dart-2 2.0.0-dev.47.0 and it might be what you're looking for:

main() {
  Iterable<num> xs = [1, 2.2, 3, 4.2];
  xs.whereType<int>()
      .where((int i) => i > 1)
      .forEach((int i) { print(i); }); // Prints '(3)'.
}

The vm prints UnimplementedError: whereType is not yet supported, but it's coming soon.

[a14n]

Once Iterable.whereType will be supported you could add the lint prefer_iterable_whereType :)

[natebosch]

Dart 2.0 contains whereType and cast and the lint prefer_iterable_wheretype. I don't think there is anything else to do here.