[swift-evolution] Equatability for enums with associated values

[Slava Pestov]

> On Jan 13, 2017, at 2:30 PM, David Sweeris via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 
> On Jan 13, 2017, at 15:10, Anton Zhilin via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
> 
>> That seems pretty close to Rust’s derive. Why not invent a similar syntax in Swift? Otherwise it will make us look through all the sources to make sure deriving is used.
>> 
>> enum Option: @derive Equatable {
>>     ...
>> }
>> Also, we can get better looking compilation errors, like:
>> 
>> ERROR at line 1, col 14: could not derive Equatable for Option
>> enum Option: @derive Equatable {
>>              ^~~~~~~~~~~~~~~~~
> 
> I think that idea came up once before... can't remember where, though, or what we thought of it at the time.
> 
> As far as reducing enum boilerplate, what about borrowing the generic system's syntax and looking at it from the switch's PoV?
> func == (lhs: MyEnum, rhs: MyEnum) -> Bool {
>     switch <c is MyEnum> (lhs, rhs) {
>     case (c(let lVal), c(let rVal)): // both lhs and rhs are "c" and the same case
>         return lVal == rVal //syntax error if `==` isn't defined for the associated value types of every case
>     default: return false
>     }
> }

I think initially, we would like to implement deriving these witnesses directly in the compiler, instead of trying to come up with a metaprogramming syntax for them.

Slava

> 
> - Dave Sweeris
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