[swift-evolution] [Draft] Automatically deriving Equatable and Hashable for certain value types

[Matthew Johnson]

> On May 28, 2016, at 8:43 PM, Pedro Vieira via swift-evolution <swift-evolution at swift.org> wrote:
> 
> I really think this would be a great addition to Swift. Although, I don't see the need to use a new keyword `deriving` for this feature.
> The following would be enough:
> 
> struct Foo: Equatable, Hashable {
>   ...
> }
> 
> It's explicit and it uses features already in the language. With this, the compiler would generate all the functions needed for `Foo` to conform to `Equatable` and `Hashable` and, in case the developer wants custom behavior on any of those functions, he/she could just write it from scratch and the compiler would use it over the generated one.

The problem with this is that it doesn’t differentiate between synthesized and manual conformance.  You won’t get error messages you otherwise would when you intend to supply manual conformance.  It is also less clear to a reader of the code that the default, compiler synthesized implementation is being generated.

> 
> Michael Peternell via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> escreveu no dia quinta, 26/05/2016 às 10:58:
> Can we just copy&paste the solution from Haskell instead of creating our own? It's just better in every aspect. Deriving `Equatable` and `Hashable` would become
> 
> struct Polygon deriving Equatable, Hashable {
>     ...
> }
> 
> This has several advantages:
> - you don't have to guess wether `Equatable` or `Hashable` should be automatically derived or not.
> - Deriving becomes an explicit choice.
> - If you need a custom `Equatable` implementation (for whatever reason), you can still do it.
> - It doesn't break any code that is unaware of the change
> - It can be extended in future versions of Swift, without introducing any new incompatibilities. For example, `CustomStringConvertible` could be derived just as easily.
> - It is compatible with generics. E.g. `struct Shape<T> deriving Equatable` will make every `Shape<X>` equatable if `X` is equatable. But if `X` is not equatable, `Shape<X>` can be used as well. (Unless `X` is not used, in which case every `Shape<T>` would be equatable. Unless something in the definition of `Shape` makes deriving `Equatable` impossible => this produces an error.)
> - It is proven to work in production.
> 
> -Michael
> 
> Pedro Vieira
> http://pedrovieira.me <http://pedrovieira.me/>
> -- 
> Pedro Vieira
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