[swift-evolution] Equatability for enums with associated values

David Sweeris davesweeris at mac.com
Fri Jan 13 14:31:38 CST 2017

Sent from my iPhone
> On Jan 13, 2017, at 13:51, 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?

I think it makes more sense to come up with some syntax for reducing that kind of boilerplate code in general. In "pseudo-english", I'd write the == function as "if lhs and rhs are the same case && their associated values are the equal, return true, else return false".

What about doing something with the reflection system? Isn't that supposed to get overhauled for Swift 4? I'm not sure what the performance implications would be, though.

- Dave Sweeris 
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20170113/0aa3cb15/attachment.html>

More information about the swift-evolution mailing list