Provide a 'Void' type for use in generics

[DartBot]

This issue was originally filed by ross.m.smit...@gmail.com

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It is often the case where a method or getter in a generic Class<T> has a return value of T. When constructing specializations of this type, it is desirable in many cases to apply a 'void' type constraint to T.

Consider, for example, a Future<T> which does not compute a value, but which rather marks the asynchronous completion of an operation. The use case of having a Future<void> was discussed in this forum thread:

https://groups.google.com/a/dartlang.org/group/misc/browse_thread/thread/f03889a626eda9ab/2b5397a6d92e3b7a?lnk=gst&amp;q=Void+as+a+type&amp;pli=1#­2b5397a6d92e3b7a

Another use case that I have is for an IEvent<T>, where T represents the event arguments. In the case of an event which does not have any information to pass to its observers (other than the fact that it happened), it would be quite useful for me to apply a 'void' type constraint here.

[iposva-google]

Added Area-Language, Triaged labels.

[gbracha]

It's not so simple. In reality, void is not a type. We get into very strange issues if we allow it as a type argument. What about a method that takes T as an argument? What does this mean if T = void? What is List<void>?.

The poster child for this is Future, but in fact, Future with no type arguments serves perfectly well. Similarly, IEVent could use Dynamic or Object.

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Set owner to @gbracha.
Added Accepted label.

[munificent]

It's not so simple. In reality, void is not a type.

Given that void functions in Dart return "null", isn't it just a unit type with the single value "null"?

What about a method that takes T as an argument? What does this mean if T = void? What is List<void>?.

Wouldn't that be equivalent to a List<Null>?

Future with no type arguments serves perfectly well.

It works OK, but it doesn't let the API designer communicate a distinction between a future of "anything" and a future of "nothing". Given how loose we are with other stuff, that probably doesn't matter too much, but it does feel strange.

[gbracha]

isn' it a unit type with the single value "null"?

No it isn't (I've had this conversation before - I should dig up the discussion).

[gbracha]

Removed Type-Defect label.
Added Type-Enhancement label.

[gbracha]

Added WontFix label.