[swift-evolution] [draft] Compound Names For Enum Cases

[Joe Groff]

> On Jan 19, 2017, at 1:47 PM, Douglas Gregor via swift-evolution <swift-evolution at swift.org> wrote:
> 
> This looks totally reasonable to me. A couple of comments:
> 
> 1) Because this proposal is breaking the link between the associated value of an enum case and tuple types, I think it should spell out the rules that switch statements will use when matching an enum value against a a case with an associated value. Some kind of rules fell out of them being treated as tuple types, but they might not be what we want.

I was about to bring up the same. Right now, an enum pattern works like .<identifier> <tuple-pattern>, where the <tuple-pattern> then recursively matches the payload tuple. In this model, it seems like we'd want to treat it more like .<identifier>(<pattern>, <pattern>, ...). Similar to how we lost "tuple splatting" to forward a bunch of arguments, we'd have to decide whether we lose the ability to match all parts of the payload into a tuple. I also don't think we currently enforce matching argument labels, so you can match a `case foo(x: Int, y: Int)` with a `.foo(let q, let z)` or `.foo(apples: let x, bananas: let y)` pattern. We should probably tighten that up as part of this proposal as well.

-Joe

> 2) I wouldn’t blame you if you wanted to slip in default arguments for associated values here, because this is really making enum cases with associated values much more function-like
> 
> 	- Doug
> 
>> On Jan 19, 2017, at 10:37 AM, Daniel Duan via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> Hi all,
>> 
>> Here’s a short proposal for fixing an inconsistency in Swift’s enum. Please share you feedback :)
>> 
>> (Updating/rendered version: https://github.com/dduan/swift-evolution/blob/compound-names-for-enum-cases/proposals/NNNN-Compound-Names-For-Enum-Cases.md)
>> 
>> 
>> ## Introduction
>> 
>> Argument labels are part of its function's declaration name. An enum case
>> declares a function that can be used to construct enum values. For cases with
>> associated values, their labels should be part of the constructor name, similar
>> to "normal" function and methods. In Swift 3, however, this is not true. This
>> proposal aim to change that.
>> 
>> ## Motivation
>> 
>> After SE-0111, Swift function's fully qualified name consists of its base name
>> and all argument labels. As a example, one can invoke a function with its
>> fully name:
>> 
>> ```swift
>> func f(x: Int, y: Int) {}
>> 
>> f(x: y:)(0, 0) // Okay, this is equivalent to f(x: 0, y: 0)
>> ```
>> 
>> This, however, is not true when enum cases with associated value were
>> constructed:
>> 
>> ```swift
>> enum Foo {
>>     case bar(x: Int, y: Int)
>> }
>> 
>> Foo.bar(x: y:)(0, 0) // Does not compile as of Swift 3
>> ```
>> 
>> Here, the declared name for the case is `foo`; it has a tuple with two labeled
>> fields as its associated value. `x` and `y` aren't part of the case name. This
>> inconsistency may surprise some users.
>> 
>> Using tuple to implement associated value also limits us from certain layout
>> optimizations as each payload need to be a tuple first, as opposed to simply be
>> unique to the enum.
>> 
>> ## Proposed solution
>> 
>> Include labels in enum case's declaration name. In the last example, `bar`'s
>> full name would become `bar(x:y:)`, `x` and `y` will no longer be labels in a
>> tuple. The compiler may also stop using tuple to represent associated values.
>> 
>> ## Detailed design
>> 
>> When labels are present in enum cases, they are now part of case's declared name
>> instead of being labels for fields in a tuple. In details, when constructing an
>> enum value with the case name, label names must either be supplied in the
>> argument list it self, or as part of the full name.
>> 
>> ```swift
>> Foo.bar(x: 0, y: 0) // Okay, the Swift 3 way.
>> Foo.bar(x: y:)(0, 0) // Equivalent to the previous line.
>> Foo.bar(x: y:)(x: 0, y: 0) // This would be an error, however.
>> ```
>> 
>> Note that since the labels aren't part of a tuple, they no longer participate in
>> type checking, similar to functions:
>> 
>> ```swift
>> let f = Foo.bar // f has type (Int, Int) -> Foo
>> f(0, 0) // Okay!
>> f(x: 0, y: 0) // Won't compile.
>> ```
>> 
>> ## Source compatibility
>> 
>> Since type-checking rules on labeled tuple is stricter than that on function
>> argument labels, existing enum value construction by case name remain valid.
>> This change is source compatible with Swift 3.
>> 
>> ## Effect on ABI stability and resilience
>> 
>> This change introduces compound names for enum cases, which affects their
>> declaration's name mangling.
>> 
>> The compiler may also choose to change enum payload's representation from tuple.
>> This may open up more space for improving enum's memory layout.
>> 
>> ## Alternatives considered
>> 
>> Keep current behaviors, which means we live with the inconsistency.
>> 
>> _______________________________________________
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>> swift-evolution at swift.org
>> https://lists.swift.org/mailman/listinfo/swift-evolution
> 
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