> On Nov 30, 2016, at 4:09 PM, Dave Abrahams via swift-evolution
> <swift-evolution@swift.org> wrote:
>
>
> on Mon Nov 28 2016, Douglas Gregor <swift-evolution@swift.org
> <mailto:swift-evolution@swift.org>> wrote:
>
>>> On Nov 21, 2016, at 3:05 PM, Ramiro Feria Purón via swift-evolution
>> <swift-evolution@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.
>
> Yeah, but IMO ideally we'd have a way to inhibit deduction of some
> generic type parameters.
Well, I’d say we already have it: it’s the pass-a-metatype approach already
used by unsafeBitCast, and I think usages of that API read really, really well
as it is.
> I might even be willing to inhibit deduction,
> by default, of all generic function type parameters that don't appear in
> the parameter list.
I’m not a fan of this for the reasons I and Ramiro outlined.
- Doug
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