Hi guys,
It would be awesome if with this evolution, we could also auto-write equality
operators for enum with equatable payloads.
Today, considering this enum
enum SimpleEnum {
case case1
case case2
}
the condition SimpleEnum.case1 == SimpleEnum.case1 compiles and return true.
But if the enum has a payload
enum EnumWithPayload {
case integer(value: Int)
case float(value: Float)
}
the condition EnumWithPayload.integer(value: 1) ==
EnumWithPayload.integer(value: 1) does not compile: operator == does not exist
for that type.
Hand-writing this missing equality operator turns out to be pretty fastidious
and error prone, notably because you might prefer to not use a “default” case,
to take advantage of the completeness check of the compiler.
So you have to write n * (n-1) cases and that can be a lot of cases !
Jerome
> Good point.
>
> The real push here is that when the programmer *does* declare a type
> Equatable and the op == has an obvious implementation, that the programmer
> shouldn’t have to manually implement it him/herself. This would apply only to
> types that have been declared Equatable and that consist of exclusively
> properties which have all also been declared Equatable.
>
> This would work much like the Haskell Eq class.
>
> > On Sep 12, 2016, at 12:03 PM, Robert Widmann<devteam.codafi at
> > gmail.com>wrote:
> >
> > Please be careful when wording this proposal. You want derived
> > conformances, but don't obscure that message with the claim that every type
> > admits a useful Equatable instance. It is most certainly not the case that
> > every value type has a useful (read [mostly]: decidable) equality. A few
> > counterexamples, the type of lazy streams
> > (https://github.com/typelift/Swiftz/blob/swift-develop/Swiftz/Stream.swift#L24<https://github.com/typelift/Swiftz/blob/swift-develop/Swiftz/Stream.swift#L24>)
> > requires infinite space to evaluate a useful answer. The type of functions
> > [without a modulus of continuity] also don't admit a useful, or even
> > canonical, equality (in Swift at least).
> >
> > ~Robert Widmann
> >
> > 2016/09/10 8:24、Daniel Tartaglia via swift-evolution<swift-evolution at
> > swift.org<mailto:swift-evolution at swift.org>>のメッセージ:
> >
> > > Now that Swift 3 is out the door, I’m going to float this proposal again…
> > >
> > > Given that every value type should be equatable (rational here):
> > > https://www.andrewcbancroft.com/2015/07/01/every-swift-value-type-should-be-equatable/<https://www.andrewcbancroft.com/2015/07/01/every-swift-value-type-should-be-equatable/>
> > > And that many, if not most, value types consist of properties that are
> > > value types.
> > > Then the language should make it easy to conform to the Equatable
> > > protocol.
> > >
> > > In other words, if I declare my value type as Equatable, and it is
> > > exclusively composed of value types that are already equatable, then
> > > implementing the actual == function should be optional (or maybe even
> > > forbidden.)
> > >
> > > Implementing == in such cases is tedious boilerplate that the compiler
> > > should be able to infer on its own.
> > >
> > > Does anybody want to help me write up an official proposal?
>
>
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