[swift-evolution] Treating an Enum's Cases as Its Subtypes

[Matthew Johnson]

> On Feb 20, 2017, at 3:22 PM, Joe Groff <jgroff at apple.com> wrote:
> 
>> 
>> On Feb 20, 2017, at 1:04 PM, Matthew Johnson <matthew at anandabits.com <mailto:matthew at anandabits.com>> wrote:
>> 
>>> 
>>> On Feb 20, 2017, at 2:38 PM, Joe Groff <jgroff at apple.com <mailto:jgroff at apple.com>> wrote:
>>> 
>>>> 
>>>> On Feb 20, 2017, at 7:32 AM, Matthew Johnson via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>> 
>>>>> 
>>>>> On Feb 20, 2017, at 12:40 AM, Niels Andriesse via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>>> 
>>>>> I'd like to discuss the possibility of treating the cases of a given enum as if they are subtypes of that enum. This seems like a natural thing to do because enum cases (especially when they have associated values) effectively define a closed set of subtypes.
>>>>> 
>>>>> Doing so would allow for constructions such as the following:
>>>>> 
>>>>> enum Foo {
>>>>>   case a(name: String)
>>>>> }
>>>>> 
>>>>> func isA(foo: Foo) -> Bool {
>>>>>   // The old way:
>>>>>   if case .a = foo { return true }
>>>>>   return false
>>>>>   // The new way:
>>>>>   return foo is .a
>>>>> }
>>>>> 
>>>>> func printNameIfFooIsA(foo: Foo) -> Bool {
>>>>>   // The old way:
>>>>>   if case let .a(name) = foo {
>>>>>     print(name)
>>>>>   }
>>>>>   // The new way (1):
>>>>>   if let a = foo as? .a {
>>>>>     print(a.name <http://a.name/>)
>>>>>   }
>>>>>   // The new way (2):
>>>>>   if let name = (foo as? .a)?.name {
>>>>>     print(name)
>>>>>   }
>>>>> }
>>>>> 
>>>>> Treating an enum's cases as its subtypes would make enums easier to work with because handling them would be syntactically the same as handling other types.
>>>>> 
>>>>> The pattern matching capabilities of enums wouldn't be affected by this proposal.
>>>>> 
>>>>> Multiple other proposals have already attempted to simplify enum handling (they have particularly focused on getting rid of "if case" and adding the ability to treat enum case tests as expressions), but none of the solutions presented in those proposals have worked out so far.
>>>>> 
>>>>> I believe that this could be the right solution to multiple enum-related problems that have been brought up repeatedly.
>>>> 
>>>> I would like to see enum cases treated as subtypes of the enum type.  This is an interesting way to refer to the type of a case.  Unfortunately I don’t think it will work if we accept the proposal to give cases a compound name.  If we do that the name of this case becomes `a(name:)` which is not a valid type name.
>>> 
>>> I think there are definitely places where having cases be a subtype of an enum make sense, but I don't think it makes sense for *all* cases to be subtypes. For example, with "biased" containers like Optional and Result, it makes sense for the "right" side to be a subtype and the "wrong" side to be explicitly constructed, IMO.  If the types of cases overlap, it would also be *ambiguous* which case ought to be constructed when the payload is converted to the enum type
>> 
>> Identical case types would definitely be a problem but I don’t think overlapping case types are always a problem.  I imagine this conversion working the same as any other ordinary overload resolution for ad-hoc overloads.
> 
> Conversions happen at runtime too. `0 as Any as? Either<Int, Int>` wouldn't have any way to tell what `Either` to form if both arms of the Either were subtype candidates. An Either<T, U> in <T, U> context can end up being bound to Either<Int, Int> at runtime and interacting with runtime casts that way.

Hmm.  This is unfortunate.

In cases where T and U overlap and form a linear hierarchy but are not identical couldn’t the runtime determine the most direct path and choose that?

If the compiler prohibited cases with exactly the same types like `Either<Int, Int>` from being expressed statically how do these types end up getting formed dynamically?  Is there any way those operations could be failable?


> 
>> 
>>> —remember that enums are sums, not unions, and that's important for composability and uniform behavior with generics. 
>> 
>> I’ve always thought of enums as nominal discriminated unions.  Maybe I’m using the wrong terminology.  Can you elaborate on the difference between sums and unions?  When you say union are you talking about the kind of thing some people have brought up in the past where any members in common are automatically made available on the union type?
> 
> Sums maintain structure whereas unions collapse it. As a sum, Optional<T> maintains its shape even when T = Optional<U>. If it were a union, T u Nil u Nil would collapse to T u Nil, losing the distinction between the inner and outer nil and leading to problems in APIs that use the outer nil to communicate meaning about some outer structure, such as asking for the `first` element of a collection of Optionals.

Got it.  This is certainly a problem for `Optional`.

But sometimes this behavior of collapsing the syntactic specification to a canonical sum type would be very useful.  What is the reason we can’t have something syntactic type expressions like `Int | String`, `Int | String | String, `String | Int | String | Int`, etc all collapse to the same canonical structural sum type:

enum {
   sub case int(Int), string(String)
}

This is how I’ve been thinking about those syntactic types.  We already allow existential types to be formed using syntax that collapses to a canonical type:

typealias Existential1 = Protocol1 & Protocol2
typealias Existential2 = Protocol2 & Existential1 & Protocol 3 & Protocol1
typealias Existential3 = Existential1 & Protocol3

In this example Existential1 and Existential3 are different names for the same type.

Is there a reason we can’t have similar syntax that collapses to a similarly canonical sum type?  If we’re going to allow case subtypes this feels to me like a very natural and useful direction.  

If we don’t allow it there are two problems: people have to invent a largely meaningless name for the enum and it is incompatible with any other similarly structured enum.  Neither is a significant problem but they do add (seemingly) unnecessary friction to the language.

I wouldn’t expect these to be widely used - they would play a similar role as tuples - but they would be very appreciated where they are used.

> 
> -Joe

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