[swift-evolution] Equatability for enums with associated values

[Jonathan Hull]

+1, with the caveat that you should still have to explicitly mark it Equatable.

I think the “when all their associated values were Equatable” is the technical issue holding this type of thing up.  The ability to spell that type of thing is on the generics roadmap, but I don’t know when it will actually happen.  There seem to be a lot of things on hold because of it.

Thanks,
Jon

> On Jan 13, 2017, at 11:51 AM, Adam Shin via swift-evolution <swift-evolution at swift.org> wrote:
> 
> When using enums with associated values, it's often necessary to check for equality between two enum objects in some way. That can lead to boilerplate code like this:
> 
> enum Option {
>     case foo(String)
>     case bar(Int)
> 	case zip
> }
> 
> func ==(lhs: Option, rhs: Option) -> Bool {
>     switch (lhs, rhs) {
>     case (.foo(let a), .foo(let b)) where a == b: return true
>     case (.bar(let a), .bar(let b)) where a == b: return true
>     case (.zip, .zip): return true
>     default: return false
>     }
> }
> 
> ..which results in code duplication and opens the door to potential logic errors.
> 
> Instead, what if enums with associated values were automatically Equatable when all their associated values were Equatable? That would remove the need for such boilerplate code.
> 
> The Swift language guide states that custom classes and structs don't receive a default implementation of the == operator because the compiler can't guess what "equality" means for them. However, I think this could make sense for enums. An enum case, even with associated values, seems closer to a value itself than an object with logic and state.
> 
> I'd be interested to hear any thoughts on this. Would this be a beneficial addition?
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