[Proposal]: Nested Generics

[martindrlik]

Nested Generics

• Proposed
• Prototype: Not Started
• Implementation: Not Started
• Specification: Not Started

Summary

Type figured out from the nested generics without directly specifying.

Motivation

No need to specify type that can be figured out from nested generics. I believe that example helps me clarify the idea. For instance MyHandler can implement generic interface IHandler<TRequest, TResponse> and in case that TRequest implements IRequest<TResponse> it might be possible to find out TResponse type from IRequest. With nested generics we could define IHandler<TRequest<TResponse>> and, while implementing, we can provide only request type, response type is figured out from request:

class MyResponse
class MyRequest : IRequest<MyResponse>
class MyHandler : IHandler<MyRequest> // response type is figured out without specifying

Detailed design

Detailed code example:

class MyHandler : IHandler<MyRequest> {
    // Note that MyResponse is figured out from MyRequest "nested generic"
    ValueTask<MyResponse> SendAsync(MyRequest request) { ... }
}

interface IHandler<TRequest<TResponse>> // "nested generics"
    where TRequest : IRequest<TResponse> {
    ValueTask<TResponse> SendAsync(TRequest request);
}

class MyResponse { ... }
class MyRequest : IRequest<MyResponse> { ... }
...
interface IRequest<TResponse> { ... }

Drawbacks

Alternatives

Currently I need to explicitly specify all types even if some would be possible to figure out:

class MyResponse
class MyRequest : IRequest<MyResponse>
class Handler : IHandler<MyRequest, MyResponse> // MyResponse already mentioned for MyRequest

Unresolved questions

Design meetings

[Frassle]

I think this is a duplicate of #339