passing generic function literals

[Andersmholmgren]

I have the following

typedef BuiltListProperty<T> BuiltListPropertyFactory<T>(
    Stream<ListMutation<T>> mutations);

BuiltListProperty<Object /*=T*/ > _defaultBuiltListPropertyFactory /*<T>*/ (
        Stream<ListMutation /*<T>*/ > mutations) =>
    new BuiltListProperty<Object /*=T*/ >(mutations);

class BuiltListProperty<E> {
  BuiltListProperty /*<R>*/ map /*<R>*/ (/*=R*/ f(E element),
          [BuiltListPropertyFactory /*<R>*/ builtListFactory =
              _defaultBuiltListPropertyFactory /*<R>*/]) => ...
}

The analyzer complains with

ERROR: Type check failed: _defaultBuiltListPropertyFactory ((Stream<ListMutation>) → BuiltListProperty) is not of type (Stream<ListMutation>) → BuiltListProperty ([frentity] lib/src/list_property/built_list_property.dart:64)

[jmesserly]

Nice example! A few things could be going on here:

• we don't have instantiated tear-off support, so _defaultBuiltListPropertyFactory /*<R>*/ doesn't parse the <R>
• we're not doing downward inference for instantiated tear-offs. Otherwise we'd push the <R> down to _defaultBuiltListPropertyFactory.

The error I'm seeing makes a little more sense:

[error] Type check failed: _defaultBuiltListPropertyFactory (<T>(dynamic) → BuiltListProperty<T>) is not of type (dynamic) → BuiltListProperty<R>

Basically it ends up with the uninstantiated generic function type. So the error message itself is correct, but, it's no good that you can't express what you're trying to express here. I'll see if I can find the bug about instantiated tear-offs & if not, open a new one.

[jmesserly]

The other bug is dart-lang/sdk#25175. We thought perhaps we could wait to implement instantiated tear-offs, but this example is a good one for why that doesn't work. :)

It's ugly, but a workaround in the short term might be:

typedef BuiltListProperty<T> BuiltListPropertyFactory<T>(
    Stream<ListMutation<T>> mutations);

class BuiltListProperty<E> {
  BuiltListProperty /*<R>*/ map /*<R>*/ (/*=R*/ f(E element),
          [BuiltListPropertyFactory /*<R>*/ builtListFactory]) {
    if (builtListFactory == null) {
      builtListFactory = (Stream<ListMutation /*<R>*/ > mutations) =>
          new BuiltListProperty<Object /*=R*/ >(mutations);
    }
  }
}

[jmesserly]

believe this should be fixed now. Strong mode will infer it.