Invoking methods from abstract classes

[ebelevics]

I'm using method from abstact class
typedef Converters<View, Data> = FutureOr<Data?> Function(View? view);

but after using it I receive for this example
type '(String?) => String?' is not a subtype of type '(dynamic) => dynamic'

why is this happening? For some reason I thought Dart 2.15 would solve this. Otherwise I need to replace typedef with abstract class and then extending with ordinary class which does work, but it makes code much more boilery, for exact same effect.

[mraleph]

You need to provide a more complete reproduction of the problem. I'd guess you did not properly instantiate the type somewhere so Converters ends up being (dynamic) => dynamic - but what exactly happened is hard to say without seeing more code.

[ebelevics]

here is small example to produce the problem

typedef Converters<ViewT, DataT> = FutureOr<DataT?> Function(ViewT? view);

abstract class AbstractControl<ViewT, DataT> {
  late ViewT? initValue;
  final Converters<ViewT?, DataT?> convertToData;
  AbstractControl({this.initValue, required this.convertToData});
}

class Control<ViewT, DataT> extends AbstractControl<ViewT, DataT> {
  Control({
    ViewT? initValue,
    required FutureOr<DataT?> Function(ViewT? val) convertToData,
  }) : super(initValue: initValue, convertToData: convertToData);
}

main() {
  final Map<String, AbstractControl> controls = {
    "firstName": Control<String, String>(
      initValue: "abc",
      convertToData: (val) => val,
    ),
    "lastName": Control<String, String>(
      initValue: "xyz",
      convertToData: (val) => "test + $val",
    ),
  };

  controls.forEach((key, control) {
    final data = control.convertToData(control.initValue);
  });
}

while this works:

import 'dart:async';

abstract class AbstractConverter<ViewT, DataT> {
  FutureOr<DataT?> viewToData(ViewT? val);
}

class Converter<V, D> extends AbstractConverter<V, D> {
  final FutureOr<D?> Function(V? val) converter;
  Converter(this.converter);

  @override
  FutureOr<D?> viewToData(V? val) => converter(val);
}

abstract class AbstractControl<ViewT, DataT> {
  late ViewT? initValue;
  final Converter<ViewT?, DataT?> convertToData;
  AbstractControl({this.initValue, required this.convertToData});
}

class Control<ViewT, DataT> extends AbstractControl<ViewT, DataT> {
  Control({
    ViewT? initValue,
    required FutureOr<DataT?> Function(ViewT? val) convertToData,
  }) : super(initValue: initValue, convertToData: Converter(convertToData));
}

main() {
  final Map<String, AbstractControl> controls = {
    "firstName": Control<String, String>(
      initValue: "abc",
      convertToData: (val) => val,
    ),
    "lastName": Control<String, String>(
      initValue: "xyz",
      convertToData: (val) => "test + $val",
    ),
  };

  controls.forEach((key, control) {
    final data = control.convertToData.viewToData(control.initValue);
    print(data);
  });
}

[mraleph]

Yeah this is a known problem. It is reduced to:

class C<T> {
  void Function(T) f;
  C(this.f);
}

void main() {
  C<dynamic> o = C<String>((String v) { });
  o.f("smth");  // throws: type '(String) => Null' is not a subtype of type '(dynamic) => void'
}

This is a corner case in the type-system: generics are covariant, but T is used in the contravariant position in void Function(T) making it unsound unless there is a dynamic check which verifies a match between declared and actual types in runtime (or alternatively there is a dynamic type check at the entry to the closure - which is something we want to avoid).

So

void main() {
  C<dynamic> o = C<String>((String v) { });
  o.f("smth");
}

is compiled as

void main() {
  C<dynamic> o = C<String>((String v) { });
  (o.f as void Function(dynamic))("smth");
}

Which should explain the error.

See dart-lang/language#1137 for the deeper discussion.