[swift-evolution] Proposal: Allow explicit type parameter specification in generic function call
David Hart
david at hartbit.com
Wed Nov 30 00:40:37 CST 2016
For example, here is a design pattern I would find clearer with explicit type information:
let cat = factory.get<Cat>()
vs
let cat: Cat = factory.get()
Having the explicit type information in angle brackets allows us to move the type information closer to where it make sense.
> On 29 Nov 2016, at 23:52, Douglas Gregor via swift-evolution <swift-evolution at swift.org> wrote:
>
>
>> On Nov 29, 2016, at 8:00 AM, Derrick Ho via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>
>> I don't think the angle brackets adds any additional benefit than adding the type information as a parameter. Adding Angle brackets will just make it more crowded….
>
> Adding the type information as a parameter effectively disables type inference for that parameter, because you have to pass the parameter explicitly. In some cases, like the unsafeBitCast function I pointed out, that is a good thing. In other cases, type inference might do the right thing for most callers, but specific call sites want the extra control provided by explicitly specifying generic arguments. That’s how I see this proposal: not as a replacement for the metatype parameter idiom that unsafeBitCast uses, but as a way to be more explicit at particular call sites when type inference either fails (e.g., due to lack of contextual type information), produces a result different than what is desired, or is sufficiently complicated that the call site requires more documentation.
>
>> plus, the syntax just seems too much like c++
>
> Lots of languages use angle brackets like this; Swift has it for types already, so I find this an odd criticism.
>
> - Doug
>
>> On Tue, Nov 29, 2016 at 8:43 AM Goffredo Marocchi via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> I think this is a case where the angle bran jets is both more readable and terse without losing context... opinions and all :).
>>
>> Sent from my iPhone
>>
>> On 29 Nov 2016, at 09:47, Andrew Trick via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>
>>>
>>>> On Nov 28, 2016, at 10:11 PM, Douglas Gregor via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>>
>>>>>
>>>>> On Nov 21, 2016, at 3:05 PM, Ramiro Feria Purón via swift-evolution <swift-evolution at swift.org <mailto: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…
>>>
>>> Angle brackets in function calls are hideous. This is objectively more clear and much prettier IMO:
>>>
>>> unsafeBitCast(something, to: Int)
>>>
>>>> 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.
>>>
>>> +1 for required type parameters being normal parameters.
>>>
>>> I think the case mentioned in the proposal reads much better as:
>>>
>>> processAll(in: vehicles, as: Bicycle, condition: aboveSpeedLimit)
>>>
>>> If angle brackets can be limited to generic definitions and type names, that’s a great accomplishment.
>>>
>>> -Andy
>>>
>>>>
>>>> And, as Dave notes, it’s effectively syntactic sugar, so it belongs in Swift 4 stage 2.
>>>>
>>>> - Doug
>>>>
>>>>
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