[swift-evolution] [Draft] Anonymous Enum Cases

[Matthew Johnson]

+1.  Combining this with overloaded cases will be very nice for some kinds of enums.

Sent from my iPad

> On Mar 8, 2017, at 9:09 PM, Daniel Duan via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Hi all,
> 
> During review for the first revision of SE-0155, Dave Abraham suggested that we should allow enum cases without base names. So here’s a (rough) draft of a proposal for this feature. Please read and share your thoughts!
> 
> # Anonymous Enum Cases
> 
> * Proposal: [SE-NNNN](NNNN-anonymous-enum-cases.md)
> * Authors: [Daniel Duan](https://github.com/dduan)
> * Review Manager: TBD
> * Status: **Awaiting review**
> 
> ## Introduction
> 
> This proposal adds anonymous cases to Swift's enum.
> 
> ## Motivation
> 
> Naming things is a tough task for programmer. This is one merit of features such
> as anonymous function and tuples: users can use them in a limited scope and skip
> the mind burden of coming up with names. We can expand this convenience to enums
> by allowing user to declare, construct and pattern match anonymous cases.
> 
> ## Proposed solution
> 
> Add anonymous cases to Swift's enum.
> 
> ## Detailed design
> 
> Anonymous enums can be declared with this syntax:
> 
> ```swift
> enum Foo {
>    case (Int)
>    case (label0: Int, label1: String)
> }
> ```
> 
> Anonymous case without any associated values is not allowed.
> 
> Anonymous case values can be constructed with constructors with the enum's name
> being their base name. Following the example:
> 
> ```swift
> Foo(42) // constructs `Foo` case corresponding to `case (Int)`
> Foo(label0: 0, labe1: "hello") // value of type Foo, 2nd case.
> Foo(label0: label1:)(0, "hello") // constructs the same value, SE-0155 syntax
> ```
> 
> For pattern matching, rules remain the same as regular cases, execpt, naturally,
> the base name shall be skipped.
> 
> ```swift
> case .(let x) = Foo(42) // x has value 42
> case .(label0: let a, label1: let b) = Foo(label0: 0, label1: "hello") // ok
> ```
> 
> ## Source compatibility
> 
> This is additive. It's fully source compatible with Swift 3.
> 
> ## Effect on ABI stability
> 
> This feature adds new possibilities to symbols exposed by enum declarations.
> 
> ## Effect on API resilience
> 
> ## Alternatives considered
> 
> It may seem "regular" to make user use `_` in place of base name in
> declarations. The author think that's a perspective that only makes sense for
> language designer/compiler implementors. To a user, the syntax chosen in this
> proposal is clear and succinct.
> 
> 
> 
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