[swift-evolution] Proposal: Allow explicit type parameter specification in generic function call

[Douglas Gregor]

> On Nov 21, 2016, at 3:05 PM, Ramiro Feria Purón via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Problem:
> 
> Currently, it is not possible to be explicit about the generic parameters (type parameters) in a generic function call. Type parameters are inferred from actual parameters:
> 
> func f<T>(_ t: T) {
>     
>     //..
> }
> 
> f(5)            // T inferred to be Int
> f("xzcvzxcvx")  // T inferred to be string 
> 
> If no type parameter is involved in the formal parameters, the type parameter needs to be used somehow as part of the return type. For example:
> 
> func g<T>(_ x: Int) -> [T] {
>     
>     var result: [T] = []
>     
>     //..
>     
>     return result
> }
> 
> In such cases, the type parameters must be inferrable from the context:
> 
> g(7)                            // Error: T cannot be inferred
> let array = g(7)                // Error: T cannot be inferred
> let array: [String] = g(7)      // Ok: T inferred to be String
> let array = g<String>(7)        // Error: Cannot explicitly specialise generic function
> 
> 
> 
> Proposed Solution:
> 
> Allow explicit type parameters in generic function call:
> 
> let _ = g<String>(7)            // Ok
> 
> 
> 
> Motivation:
> 
> Consider the following contrived example:
> 
> class Vehicle {
>     var currentSpeed = 0
>     //..
> }
> 
> class Bicycle: Vehicle {
>     //..
> }
> 
> class Car: Vehicle {
>     //..
> }
> 
> @discardableResult
> func processAll<T: Vehicle>(in vehicles: [Vehicle], condition: (Vehicle) -> Bool) -> [T] {
>     
>     var processed: [T] = []
>     
>     for vehicle in vehicles {
>         guard let t = vehicle as? T, condition(vehicle) else { continue }
>         //..
>         processed.append(t)
>     }
>     
>     return processed
> }
> 
> func aboveSpeedLimit(vehicle: Vehicle) -> Bool {
>     return vehicle.currentSpeed >= 100
> }
> 
> 
> let processedVehicles = processAll(in: vehicles, condition: aboveSpeedLimit)        // Uh, T inferred to be Vehicle!
> let processedCars: [Car] = processAll(in: vehicles, condition: aboveSpeedLimit)     // T inferred to be Car
> processAll<Bicycle>(in: vehicles, condition: aboveSpeedLimit)                       // This should be allowed under this proposal
> 
> 
> Notes:
> 
> If necessary, the (real life) Swift code that lead to the proposal could be shared.

This seems completely reasonable to me. I had always expected us to implement this feature, but we never got around to it, and it wasn’t a high priority because one can always use type inference. Additionally, there were a few places where we originally thought we wanted this feature, but prefer the more-explicit form where the user is required to explicitly pass along a metatype. unsafeBitCast is one such case:

	func unsafeBitCast<T, U>(_ x: T, to: U.Type) -> U

Even if we had the ability to provide explicit type arguments, we would *not* want to change this signature to

	func unsafeBitCast<U, T>(_ x: T) -> U     // bad idea

because while it makes the correct usage slightly cleaner:

	unsafeBitCast<Int>(something)	// slightly prettier, but...

it would enable type inference to go wild with unsafe casts:

	foo(unsafeBitCast(something))	// just cast it to.. whatever	

which is… not great.

I’d like one bit of clarification in the proposal. Right now, one is not permitted to have a type parameter in a generic function that isn’t used somewhere in its signature, e.g.,

	func f<T>() -> Void { … }   // error: T is not part of the signature of f()

This restriction is obvious in today’s Swift, because there is absolutely no way one could ever use this function. With your proposed extension, it would be possible to use this function. Does the restriction remain or is it lifted?

Personally, I’d like the restriction to stay, because it feels like such functions fall into the same camp as unsafeBitCast: if the type parameter affects how the function operates but is *not* part of its signature, then it should be expressed like a normal parameter (of a metatype). It also helps provide better diagnostics when changing a generic function to no longer require one of its type parameters.

And, as Dave notes, it’s effectively syntactic sugar, so it belongs in Swift 4 stage 2.

	- Doug


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