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

[David Hart]

I personally don't really see the advantage of those deduction operators. I would prefer writing:

let plus1: (Int, Int) -> Int = +
let plus2 = + as ((Int, Int) -> Int)

> On 26 May 2016, at 06:11, Callionica (Swift) <swift-callionica at callionica.com> wrote:
> 
> I have an alternative you might like to consider: type deduction operators
> 
> The input type deduction operator lets you put one or more types in front of a function to guide type deduction based on the parameters
> 
> The output type deduction operator lets you put a type after a function to guide type deduction based on the return type
> 
> This is a library-only solution that lets you not only select a specialization for a generic function, but also choose an overload from an overload set
> 
> It's up to the user whether they use input, output, or both type deduction ops and up to them how many types they supply for input. For example, when you know that the overloads or generic functions you're choosing from have two parameters of the same type, you only need to provide a single type to trigger the correct type deduction (shown below with operator+).
> 
> Here's the basic idea (the specific symbol used is just what I use, could be changed):
> 
> infix operator >>> { associativity left }
> 
> // Input type deduction operator
> func >>> <In, Out>(deduce: In.Type, fn: In -> Out) -> In -> Out {
>     return fn
> }
> 
> // Add versions for functions with 2-5 parameters
> func >>> <In, In2, Out>(deduce: In.Type, fn: (In, In2) -> Out) -> (In, In2) -> Out {
>     return fn
> }
> 
> // Add versions for 2-5 inputs
> func >>> <In, In2, Out>(deduce: (In.Type, In2.Type), fn: (In, In2) -> Out) -> (In, In2) -> Out {
>     return fn
> }
> 
> // Output type deduction operator
> func >>> <In, Out>(fn: In -> Out, deduce: Out.Type) -> In -> Out {
>     return fn
> }
> 
> let plus1 = Int.self >>> (+)
> let plus2 = (Int.self, Int.self) >>> (+) 
> 
> -- Callionica
> 
> 
> 
> 
> 
> 
>> On Wed, May 25, 2016 at 4:17 PM, David Hart via swift-evolution <swift-evolution at 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!
>> 
>> David
>> 
>> Allow explicit specialization of generic functions
>> Proposal: SE-XXXX
>> Author: David Hart, Douglas Gregor
>> Status: TBD
>> Review manager: TBD
>> Introduction
>> 
>> This proposal allows bypassing the type inference engine and explicitly specializing type arguments of generic functions. 
>> 
>> 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)
>> 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 )
>> 
>> Impact on Existing Code
>> 
>> This proposal is purely additive and will have no impact on existing code.
>> 
>> Alternatives Considered
>> 
>> Not adopting this proposal for Swift.
>> 
>> _______________________________________________
>> swift-evolution mailing list
>> swift-evolution at swift.org
>> https://lists.swift.org/mailman/listinfo/swift-evolution
> 
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