[swift-evolution] Equality of enum with payload

[Marc Knaup]

Just stumbled upon the question whether enums/structs should automatically
conform to Equatable when the conformance is declared indirectly through
another protocol. This can be unexpected for the developer when declaring
conformance to a protocol of another developer.

protocol YourProtocol: Equatable {}


// implicitly declares conformance to Equatable - should equality operator
be generated or not?
enum MyEnum: YourProtocol {}

// better require explicit declaration to automatically create equality
operator and avoid unexpected behavior
enum MyEnum: Equatable, YourProtocol {}


// same for Hashable
enum MyEnum: Hashable {} // should raise a error since Equatable isn't
satisfied

// must also be explicit to automatically create equality operator
enum MyEnum: Equatable, Hashable {}

On Fri, Dec 11, 2015 at 6:09 PM, Jonathan Hise Kaldma via swift-evolution <
swift-evolution at swift.org> wrote:

> +1
>
> Would love to see this.
>
> /Jonathan
>
> 11 dec. 2015 kl. 17:37 skrev Slava Pestov via swift-evolution <
> swift-evolution at swift.org>:
>
> This could be implemented by extending the derived conformance logic in
> Sema. Right now it only derives Equatable for enums without payload cases,
> but it would be relatively straightforward to synthesize the obvious
> Equatable conformance if all payloads are themselves Equatable, or tuples
> of Equatable types. You would then just write
>
> extension MyEnumWithPayload : Equatable {}
>
> Ditto for Hashable.
>
> Slava
>
> On Dec 11, 2015, at 8:15 AM, Marc Knaup via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> It's a similar discussion with simple structs which could automatically be
> equatable.
> And for Hashable.
> It's difficult to draw a line where that makes sense and where not.
>
> In any case I'd prefer to not have recursive equality automatically.
>
>    - It can easily lead to unexpected behavior when you forget to
>    implement an own equality operator in order to remove variables from the
>    equation which do not affect the equality.
>    - You could add another variable to an existing enum (or struct) which
>    is now automatically compared for equality without you noticing and which
>    might be wrong.
>    - You cannot opt out of that behavior.
>
> But I agree that the current implementations of the equality operator for
> enums are awful.
> Maybe we can make the enum (or struct) conform to a special protocol which
> allows it to automatically generate the equality operator (and even default
> hashValue).
>
> Something like this:
>
> protocol DefaultEquatable: Equatable {}
> protocol DefaultHashable: Hashable {}
>
> enum MyEnumWithPayload: DefaultEquatable, DefaultHashable {
>     case One(payload: String)
>     case Two(payload: String)
> }
>
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