[swift-evolution] Pitch: Automatically deriving Equatable/Hashable for more value types

[Brent Royal-Gordon]

> On May 4, 2017, at 3:01 PM, Xiaodi Wu via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Hmm, I can see the appeal of automatically deriving Equatable and Hashable conformance, but I'd like that to be opt-in. That is, types should declare that they are Equatable or Hashable to begin with. It wouldn't have to take extra syntax, as compiler magic could effectively synthesize default implementations for == and/or hashValue when all members are themselves Equatable or Hashable, respectively.

Another benefit is that the problem you're currently having with recursion goes away: from outside the type, you merely need to check if conformance is declared.

Explicit with no special syntactic marker is definitely the way to go. It would work just like Codable is slated to.

> With such a scheme, consideration can be made to accommodating classes too.


I would think only final classes could participate in this, since a subclassable class would need to allow subclasses to override equality, and you can't override a static `==` operator method.

(My time is not unlimited right now, but I'd be willing to help with either the proposal or its implementation. This would be a great thing to get into Swift 4.)

-- 
Brent Royal-Gordon
Architechies

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