[swift-evolution] [Discussion] Enum Leading Dot Prefixes
Daniel Steinberg
daniel at dimsumthinking.com
Fri Feb 12 08:23:38 CST 2016
+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 <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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> swift-evolution at swift.org
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