Add extensions on non-function typedefs

[sebastianbuechler]

In the original issue for introducing typedefs for non-function types, I saw questions about how extensions would work with this new feature. However, it seems that this did not make the cut yet.

The purpose of this could be something like this to parse more conveniently json lists of objects:

typedef Cars= List<Car>;

extension on Cars{
  Cars fromJson(List json) {
    final Cars cars = [];
    for (final car in json) {
      cars.add(Car.fromJson(car as Map<String, dynamic>));
    }

    return cars;
  }
}

Original comment:
That's how it works already with extensions on functions, so likely.

typedef VoidCallback = void Function();

extension on VoidCallback {
  void foo() {}
}

void main() {
  void Function() test = () {};

  test.foo();
}

Originally posted by @rrousselGit in #65 (comment)

[eernstg]

The non-function type alias feature does work with extensions:

class Car {}

typedef Cars = List<Car>;

extension on Cars {
  void foo() {
    print('This is foo!');
  }
}

void main() {
  <Car>[].foo(); // Compiles, runs, and prints.
}

However, it looks like your example is intended to make fromJson a static method. You don't show any invocations of fromJson, so we can't tell, but it would in any case be a static method on the extension, so you would have to give the extension a named (say, E) and then you would be able to call it as E.fromJson(...).

If you wish to be able to declare static methods in an extension and call them as if they had been static methods on something (for instance, if the extension is on T where T is a class, they could be treated as static methods on T), then the right issue could be #723 (but there may be several similar ones).

[sebastianbuechler]

The non-function type alias feature does work with extensions:

class Car {}

typedef Cars = List<Car>;

extension on Cars {
  void foo() {
    print('This is foo!');
  }
}

void main() {
  <Car>[].foo(); // Compiles, runs, and prints.
}

However, it looks like your example is intended to make fromJson a static method. You don't show any invocations of fromJson, so we can't tell, but it would in any case be a static method on the extension, so you would have to give the extension a named (say, E) and then you would be able to call it as E.fromJson(...).

If you wish to be able to declare static methods in an extension and call them as if they had been static methods on something (for instance, if the extension is on T where T is a class, they could be treated as static methods on T), then the right issue could be #723 (but there may be several similar ones).

You're right. I first did a static method and then tried to call Cars.fromJson(json). With the naming of the extension (i.e. CarsParser) I was able to achieve it as you proposed. Thank you very much!