Suggestion: Allow classes to implement type aliases

[Elephant-Vessel]

The implements-keyword in TS is used to guide the programmer of a class to ensure that the class is compatible with a certain (structural) type.

The interface-keyword in TS defines a type, that except being a regular type, can be used design-time as a tool to ensure that a class is compatible with a (structural) type.

Types other than those defined by interface or class can not be used with the implements-keyword.

I suggest to drop this constraint to allow authors of classes to use (almost*) any type to guide the implementation of a class.

* Except types consisting of primitives (string, bool, number...), as these are nominal and cannot be implemented by a class.

(Sorry if this already have been discussed somewhere, but I can't find anything.)

[RyanCavanaugh]

I don't see any reason not to

[mhegazy]

What does it mean to implement a union type?

[Elephant-Vessel]

That would mean that the programmer at design time is required to make the class implement (at least) either of the constituents of the union type, so that it will fulfill the contract of the union type.

[mhegazy]

Do you have a scenario in mind where this would be useful?

[Elephant-Vessel]

As classes and types are clearly separated concepts in structurally types languages, I'd personally would like to use these respective conceptual tools in encapsulated forms. I would like to be able to directly define types (via type aliases) that I can use throughout my code, without being forced to use the old-school class/type-intertwined concept of interfaces.

I don't see why it would be required for a type to be declared as either an interface or a class for us to use it as a way to guide design-time implementation of classes since TS is structural.

So for me, this is more about having a cleaner conceptual model rather than being able to technically doing this or that easier. That is important for me as a developer, it makes my brain happier.

[Elephant-Vessel]

What do you guys think? Do you think it's a reasonable proposal?

[RyanCavanaugh]

If the target is something we would have let you implement anyway, I don't see why not. But there may be some complexity hiding behind the scenes; we'll have to discuss first.

[aluanhaddad]

I think the most interesting things to implement would be function types but there's no way of doing that with the classes especially given that the [[call]]able classes proposal was dropped by TC39.

Still I do find myself wanting to implement arbitrary type declarations from time to time. It would be useful.