[swift-evolution] Equatability for enums with associated values

[Tony Allevato]

Such a proposed syntax doesn't solve the general problem though, which is
that comparing two enum values requires enumerating all of the cases to
test whether they are (1) the same and (2) have the same associated values,
if any. The desire here is to get rid of the boilerplate that users must
write to implement simple equality (and hashability, in the case of my
proposal draft), similarly to how enums without associated values already
get Equatable and Hashable for free.


On Fri, Jan 13, 2017 at 3:37 PM Derrick Ho via swift-evolution <
swift-evolution at swift.org> wrote:

> I think it is better to create a syntax for getting the associated values
> and then comparing them.
>
> enum Option {
> case foo(String)
> case bar(Int)
> case zip
> }
>
> let op = Option.foo("hello")
> let bb = Option.foo("world")
>
> // proposed tuple-like syntax
>
> op.foo.0 // returns Optional("hello")
>
> // then we can compare values directly
>
> if op.foo.0 == bb.foo.0 {
> // ...
> }
>
> On Fri, Jan 13, 2017 at 5:44 PM Slava Pestov via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> 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> 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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