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

[Douglas Gregor]

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.
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 <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.
> 
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution

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