How FutureOr<T> works?

[guangmingzizai]

I want to define a type like FutureOr, for example:

@pragma("vm:entry-point")
abstract class PromiseOr<T> {
  // Private generative constructor, so that it is not subclassable, mixable, or
  // instantiable.
  PromiseOr._() {
    throw new UnsupportedError("PromiseOr can't be instantiated");
  }
}

The code is identical to FutureOr, but it doesn't work.

[eernstg]

FutureOr is a built-in type which is basically a union type for the special case where the union has the form Future<T> | T for some T. You can't get the same special treatment of any type that a non-system library can define.

You might have some plans to create your own fork of the Dart system and SDK (I can see that you're using @pragma("vm:entry-point")), but you would then need to introduce a treatment of types like PromiseOr which is similar to the many special exceptions with FutureOr in the common front end and the analyzer, and you'd probably need to make substantial changes to additional tools such as dart2js and DDC. So you can't do this in Dart, and even if you plan to create your own "MyDart" variant of the language it would be a highly non-trivial exercise.

We may get union types in general in the future, but for that it's better to keep it in one issue: dart-lang/language#145.

Because this is a repository where issues with the current implementation are handled (not proposals for new language design), and because of the shared goal, I'll close this issue. Please follow up as needed on dart-lang/language#145.

[AsteriskZuo]

sdk/runtime/vm/class_id.h
sdk/runtime/vm/object.h
FutureOr implement in C++ native code.
FutureOr define in object.h
I hope this is helpful.
I am also a curious person.