Allow type arguments in class literals

[DartBot]

This issue was originally filed by @seaneagan

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We have class literals e.g. List, but they do not allow using generics, e.g. List<String>. One use case is issue #10407, being able to do:

expect(x, isA(List<String>));

instead of just:

expect(x, isA(List));

[Edit Mar 2021 eernstg: Closing this issue as a duplicate of dart-lang/language#123. This one is older indeed, but today dart-lang/language#123 is used to discuss this feature proposal.]

[iposva-google]

Added Area-Language, Triaged labels.

[lrhn]

Parsing could be harder in that case. What is:
  x < y > -1
?
It's silly code, because the Type object likely won't have a minus operator (yet? There has been talk about making static methods into instance methods on the type object).

[DartBot]

This comment was originally written by @seaneagan

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issue #1321 says chained comparisons will not be supported, and as noted there, in this case I think the associated syntax should not be allowed in the context of comparisons, since it leads to calling a relational operator on a boolean which doesn't makes sens. In which case:

x < y > -1

would not represent any comparisons, but rather calling the - operator on the class literal x<y> with argument 1.

[DartBot]

This comment was originally written by @seaneagan

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On second thought, that syntax is useful for DSLs, so it should be allowed. In which case, maybe the best we can do is:

to fix issue #2626 which would allow:

typedef StringList = List<String>;

...

expect(x, isA(StringList));

and then maybe sweeten it up a little bit, by allowing inline typedefs:

expect(x, isA(typedef List<String>));

[gbracha]

On the one hand, one would like to have literals for all types. On the other hand, this is a rat hole (as anything related to generics invariably is).

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Set owner to @gbracha.
Removed Type-Defect label.
Added Type-Enhancement, Accepted labels.

[gbracha]

Issue #18758 has been merged into this issue.

[gbracha]

Issue #18757 has been merged into this issue.

[gbracha]

Issue #20027 has been merged into this issue.

[kevmoo]

There is a request for this feature in Angular: angular/angular#1691

[kevmoo]

Issue #23221 has been merged into this issue.

[rrousselGit]

I made a S/O question about this recently: https://stackoverflow.com/questions/52891537/how-to-get-generic-type

The answer provides a pretty good solution:

Type typeOf<T>() => T;

main() {
  Type type = typeOf<MyClass<int>>();
  print(type);
}

Kudos to Xavier

[Levi-Lesches]

Any updates? I'm discussing adding macros on dart-lang/language#1482, and one of the big issues (I suppose for dart:mirrors as well) is that there's no way to specify "here's the type I want, make it a return type of this function". For example:

Macro getMacro() {
  final Type type = getType();  // some other function to return a type
  return Macro<type>();  // ERROR: The name 'type' isn't a type so it can't be used as a type argument. 
}

Among other uses.

[eernstg]

@Levi-Lesches, I think your comment belongs somewhere else. This issue requests the ability to use a parameterized type (that is, a term of the form <typeName> <typeArguments>, e.g., List<int>) as an expression.

The language proposal to add this feature is dart-lang/language#123, and the standard workaround was shown by @rrousselGit here.

I'll close this issue because it is a language feature proposal, and they live in the language repository today, but if we were to transfer this issue to the language repository then it would be a duplicate of dart-lang/language#123.

[Levi-Lesches]

Ah okay I misunderstood. I realized anyway that by allowing a variable type as a generic, there could be massive problems with type-safety.