List<T>.map loses type information when passing to constructor

[passsy]

It seems like dart is losing the type when calling a generic constructor inside map (and reduce). Is this intended? Is there a workaround?

I did not expect this from a statically typed language.

class Generic<T> {
  final T thing;
  Generic(this.thing);
}

void main() {
    print(Generic("test").runtimeType);
    // Expected: Generic<String>
    // Actual: Generic<String>

    var list = ["a", 1, new Text("hello")];

    list.forEach((it) => print(it.runtimeType));
	// Expected: String, int, Text
    // Actual: String, int, Text

    list.map((it) => Generic(it)).forEach((it) => print(it.runtimeType));
    // Expected: Generic<String>, Generic<int>, Generic<Text>
    // Actual: Generic<Object>, Generic<Object>, Generic<Object>
}

Running with
Dart version 2.0.0-dev.41.0.flutter-2f68e82526

[matanlurey]

Your list type is a List<Object>.

Dart doesn't have structural typing, so:

['a', 1, new Text('hello')]

...'s closest shared interface is Object - or WAI.

There is no way to currently say "this is a list of different known static types".

[matanlurey]

That being said, to be clever, you could write a custom map function.

NOTE: This requires Dart2 semantics.

Iterable<Object> map3<A, B, C>(Iterable<Object> elements) sync* {
  for (final e in elements) {
    if (e is A) {
      yield new Generic(e);
    }
   if (e is B) {
      yield new Generic(e);
    }
   if (e is C) {
      yield new Generic(e);
    }
  }
}

main() {
  var list = ["a", 1, new Text("hello")];
  map3<String, int, Text>(list).forEach((it) => it.runtimeType);
}

... should work as you intended. Obviously it's not a great general solution.

[dgrove]

This is currently WAI - we don't know if we'll implement union types (#4938) at some point.