[swift-evolution] [Discussion] Enum Leading Dot Prefixes

[T.J. Usiyan]

+1 and I would not object to requiring the leading dot for enum
declarations.

On Fri, Feb 12, 2016 at 9:23 AM, Daniel Steinberg via swift-evolution <
swift-evolution at swift.org> wrote:

> +1 for consistency reasons
>
> On Feb 11, 2016, at 10:00 PM, Erica Sadun via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> https://gist.github.com/erica/e0b8a3a22ab716a19db4
>
> Requiring Leading Dot Prefixes for Enum Instance Member Implementations
>
>    - Proposal: TBD
>    - Author(s): Erica Sadun <http://github.com/erica>, Chris Lattner
>    <https://github.com/lattner>
>    - Status: TBD
>    - Review manager: TBD
>
> <https://gist.github.com/erica/e0b8a3a22ab716a19db4#introduction>
> Introduction
>
> Enumeration cases are essentially static not instance type members. Unlike
> static members in structures and classes, enumeration cases can be
> mentioned in initializers and instance methods without referencing a fully
> qualified type. This makes little sense. In no other case can an instance
> implementation directly access a static member. This proposal introduces a
> rule that requires leading dots or fully qualified references
> (EnumType.caseMember) to provide a more consistent developer experience to
> clearly disambiguate static cases from instance members.
> <https://gist.github.com/erica/e0b8a3a22ab716a19db4#motivation>Motivation
>
> Swift infers the enclosing type for a case on a developer's behalf when
> the use is unambiguously of a single enumeration type. Inference enables
> you to craft switch statements like this:
>
> switch Coin() {case .Heads: print("Heads")case .Tails: print("Tails")
> }
>
> A leading dot has become a conventional shorthand for "enumeration case"
> across the language. When used internally in enum implementations, a
> leading dot is not required, nor is a type name to access the static case
> member. The following code is legal in Swift.
>
> enum Coin {
>     case Heads, Tails
>     func printMe() {
>         switch self {
>         case Heads: print("Heads")  // no leading dot
>         case .Tails: print("Tails") // leading dot
>         }
>
>         if self == Heads {          // no leading dot
>             print("This is a head")
>         }
>
>         if self == .Tails {         // leading dot
>             print("This is a tail")
>         }
>     }
>
>     init() {
>         let cointoss = arc4random_uniform(2) == 0
>         self = cointoss ? .Heads : Tails // mix and match leading dots
>     }
> }
>
> This quirk produces a language inconsistency that can confuse developers
> and contravenes the guiding *Principle of Least Astonishment*. We propose
> to mandate a leading dot. This will bring case mentions into lock-step with
> the conventions used to reference them outside of enumeration type
> implementations.
> <https://gist.github.com/erica/e0b8a3a22ab716a19db4#detail-design>Detail
> Design
>
> Under this rule, the compiler will require a leading dot for all case
> members. The change will not affect other static members, which require
> fully qualified references from instance methods and infer self from
> static methods.
>
> enum Coin {
>     case Heads, Tails
>     static func doSomething() { print("Something") }
>     static func staticFunc() { doSomething() } // does not require leading dot
>     static func staticFunc2() { let foo = .Tails } // requires leading dot
>     func instanceFunc() { self.dynamicType.doSomething() } // requires full qualification
>     func otherFunc() { if self == .Heads ... } // requires leading dot, also initializers
>
>     /// ...
> }
>
>
> <https://gist.github.com/erica/e0b8a3a22ab716a19db4#alternatives-considered>Alternatives
> Considered
> Other than leaving the status quo, the language could force instance
> members to refer to cases using a fully qualified type, as with other
> static members.
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