Error typing Stream.fromIterable

[nex3]

When I analyze the following code in strong mode:

import 'dart:async';

Stream<int> getStream() {
  return new Stream.fromIterable([1, 2, 3].map((i) => i));
}

I see this error:

[error] Type check failed: new Stream.fromIterable([1, 2, 3].map((i) => i)) (Stream<dynamic>) is not of type Stream<int> because [1, 2, 3].map((i) => i) cannot be typed as Iterable<int>

Is this to do with the in-progress generic method support?

[jmesserly]

Is this to do with the in-progress generic method support?

Hmmm, I think we might need more powerful inference, e.g. parameter inference (#203).

Inference correctly pushed <int> into the new Stream.fromIterable but it failed to push <int> into .map((i) => i). map is a generic method that takes a function of T -> R, so it needs to see the return type R is int, push that to the parameter type T, which lets it conclude the target of the method is an Iterable<int> and then push that type to the list literal [1, 2, 3].

One interesting thing to try would be .where((i) => true) instead of map, because it's easier to infer where because the target and return type are both Iterable<T>, so you don't need to understand that (i) => i is the identity function

[vsmenon]

That's a nice example. For now, you'll need to type i in the closure as int.

@leafpetersen 's planning on downward inference for generic methods, which might make that unnecessary.

[leafpetersen]

Right. map needs to be made a generic method, otherwise the return type of the map is List<dynamic>, which can't be typed as Iterable<int> as required by the context there. Vijay added some ad-hoc inference for map and company, but I think it runs separately from (and before) the downwards inference, and so I don't think it will fire here, since at the time that it runs the type of the closure is still dynamic->dynamic. With generic methods and downwards inference, this should work just fine. I think Vijay is correct that if you type the parameter as int, the static type of the closure will be int -> int, which will allow his special case inference for map to fire.

[vsmenon]

Merging this into #300