Re: [swift-evolution] [Pitch] Allow explicit specialization of generic functions

[Mark Lacey via swift-evolution]

> On May 25, 2016, at 4:17 PM, David Hart via swift-evolution 
> <swift-evolution@swift.org> wrote:
> 
> Hello,
> 
> This is a new pitch to allow explicitly specializing generic functions. 
> Notice that potential ambiguity with initialisers and how I’m currently 
> trying to avoid it. Please let me know what you think!

Hi David,

I’m wondering if the only motivation here is to be more explicit about the 
types involved so you can avoid having to rely on type inference and the code 
is more clear, or if you also have the expectation that we’ll generate a 
type-specialized version of the body of the function in question which you 
would expect to be more efficient than the fully generic version?

Do you imagine this supporting a scenario where only some of the type 
parameters are given explicitly and others are inferred, or would this only be 
supported in the case where all type parameters were given explicitly?

What if some of those type parameters were themselves generics? For example:
  func callee<T : SignedInteger, U : AnotherProtocol>(x: T, y: U) { … }
  func caller<T : AnotherProtocol>(x: T) {
    let f = bar<Int, T> // supported
    f(3, x)
  }

Mark

> 
> David
> 
> Allow explicit specialization of generic functions
> 
> Proposal: SE-XXXX 
> <https://github.com/apple/swift-evolution/blob/master/proposals/XXXX-allow-explicit-specialization-generic-functions.md>
> Author: David Hart <https://github.com/hartbit>, Douglas Gregor 
> <https://github.com/DougGregor>
> Status: TBD
> Review manager: TBD
>  
> <https://github.com/hartbit/swift-evolution/tree/allow-explicit-types-generic-functions#introduction>Introduction
> 
> This proposal allows bypassing the type inference engine and explicitly 
> specializing type arguments of generic functions. 
> 
>  
> <https://github.com/hartbit/swift-evolution/tree/allow-explicit-types-generic-functions#motivation>Motivation
> 
> In Swift, generic type parameters are inferred by the argument or return 
> value types as follows:
> 
> func foo<T>(t: T) { ... }
> 
> foo(5) // infers T = Int
> There exists certain scenarios when a programmer wants to explicitly 
> specialize a generic function. Swift does not allow it, so we resort to 
> giving hints to the inference engine:
> 
> let f1 = foo as ((Int) -> Void)
> let f2: (Int) -> Void = foo
> let f3 = foo<Int> // error: Cannot explicitly specialize a generic function
> 
> func bar<T>() -> T { ... }
> 
> let b1 = bar() as Int
> let b2: Int = bar()
> let b3 = bar<Int>() // error: Cannot explicitly specialize a generic function
> This behaviour is not very consistent with generic types which allow 
> specialization:
> 
> let array: Array<Int> = Array<Int>(arrayLiteral: 1, 2, 3)
> Therefore, this proposal seeks to make the above errors valid specializations:
> 
> let f3 = foo<Int> // explicitly specialized to (Int) -> Void 
> let b3 = bar<Int>() // explicitly specialized to () -> Int 
> An ambiguous scenario arrises when we wish to specialize initializer 
> functions:
> 
> struct Foo<T: RawRepresentable where T.RawValue == String> {
>     let storage: T
> 
>     init<U: CustomStringConvertible>(_ value: U) {
>         storage = T(rawValue: value.description)!
>     }
> }
> 
> enum Bar: String, CustomStringConvertible {
>     case foobar = "foo"
> 
>     var description: String {
>         return self.rawValue
>     }
> }
> 
> let a = Foo<Bar>(Bar.foobar)
> Does this specialization specialize the struct's or the initializer's generic 
> type? The proposal solves this ambiguity by requiring initializer generic 
> type specialization to use the init syntax:
> 
> let a = Foo<Bar>.init<Bar>(Bar.foobar)
>  
> <https://github.com/hartbit/swift-evolution/tree/allow-explicit-types-generic-functions#detailed-design>Detailed
>  Design
> 
> Function calls are fairly straight forward and have their grammar modified as 
> follows:
> 
> function-call-expression → postfix-expression­ generic-argument-clause­opt 
> parenthesized-expression
> 
> function-call-expression → postfix-expression generic-argument-clause­opt 
> ­parenthesized-expression­opt ­trailing-closure­
> 
> To allow initializers to be called with explicit specialization, we need to 
> use the Initializer Expression. Its grammar is modified to:
> 
> initializer-expression → postfix-expression­ . ­init­ 
> generic-argument-clause­opt
> 
> initializer-expression → postfix-expression­ . ­init­ 
> generic-argument-clause­opt ( ­argument-names­ )
> 
>  
> <https://github.com/hartbit/swift-evolution/tree/allow-explicit-types-generic-functions#impact-on-existing-code>Impact
>  on Existing Code
> 
> This proposal is purely additive and will have no impact on existing code.
> 
>  
> <https://github.com/hartbit/swift-evolution/tree/allow-explicit-types-generic-functions#alternatives-considered>Alternatives
>  Considered
> 
> Not adopting this proposal for Swift.
> _______________________________________________
> swift-evolution mailing list
> swift-evolution@swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution


_______________________________________________
swift-evolution mailing list
swift-evolution@swift.org
https://lists.swift.org/mailman/listinfo/swift-evolution