[swift-evolution] Proposal: Allow explicit type parameter specification in generic function call
Douglas Gregor
dgregor at apple.com
Wed Nov 30 22:53:37 CST 2016
> On Nov 30, 2016, at 4:05 PM, Ramiro Feria Purón <ramiro.feria.puron at gmail.com> wrote:
>
> Formally, perhaps the proposal should include the following:
>
> 1. Missing type parameters are assumed to be implicit and trailing (as in the particular case of all type parameters missing).
Yeah, this makes sense. Presumably this means that one could also write
foo<>(1)
where everything is still deduced. In C++, this syntax exists and means that non-template functions named “foo” are ignored.
> 2. _ is allowed as a placeholder when specifying explicit parameters in a function call. The corresponding parameter is assumed to be implicit.
This could be a more general feature that works wherever a type is specified and deduction works. For example, today you can do, e.g,
let d1: Dictionary = [ “Hello” : 2017 ]
and we’ll infer the type arguments. It wouldn’t be unreasonable to allow
let d2: Dictionary<_, Int32> = [ “Hello”: 2017 ]
or
let d3: [ _: Int32 ] = [ “Hello”: 2017 ]
or
let d4 = [ “Hello”: 2017 ] as [ _: Int32 ]
to partially specify the types of the result. I think this is a separate-but-interesting proposal (that is out of scope for Swift 4 stage 1).
>
> In addition to this, and following the discussion on the unsafeBitCast example, it might be a good idea to incorporate to the proposal adding an (optional) explicit keyword for type parameters in generic function definitions:
>
> func unsafeBitCast<explicit U, T>(_ x: T) -> U
>
> foo(unsafeBitCast(something)) // error: explicit type parameter required
> foo(unsafeBitCast<U>(something)) // ok
As I noted in my reply to Dave, I personally don’t want this: I feel that the idiom used by unsafeBitCast produces more readable code with already-existing features.
- Doug
> Regards,
> Ramiro
>
>
>
>
> On Thu, 1 Dec 2016 at 01:36 Ramiro Feria Purón <ramiro.feria.puron at gmail.com <mailto:ramiro.feria.puron at gmail.com>> wrote:
> Or perhaps in a more "swifty" way, use _ for implicit type parameters, i.e.:
>
> f<_,T>()
>
> On Thu, 1 Dec 2016 at 01:13 Ramiro Feria Purón <ramiro.feria.puron at gmail.com <mailto:ramiro.feria.puron at gmail.com>> wrote:
> Douglas,
>
> Regarding the question on the restriction for type parameters to appear on the signature, the answer is remain. The proposal does not intend this restriction to be lifted.
>
> One might expect to find a few legitimate cases where having it lifted would be handy or desirable. They seem to emerge often, for example, while developing components using Core Data. From a real case scenario:
>
> protocol Activable { var dateLastActive: Date { get } }
>
> class A: NSManagedObject {}
> class B: NSManagedObject {}
> //..
> class Z: NSManagedObject {}
>
> extension A: Activable { var dateLastActive: Date { return Date() } }
> extension B: Activable { var dateLastActive: Date { return Date() } }
> //..
> extension Z: Activable { var dateLastActive: Date { return Date() } }
>
>
>
> func deleteInactiveObjects<T: NSManagedObject>(since date: Date, inContext context: NSManagedObjectContext) where T: Activable {
> //..
> }
>
> // for the sake of the example
> let context = NSManagedObjectContext(concurrencyType: .privateQueueConcurrencyType)
> let yesterday = Date()
> let oneWeekAgo = Date()
>
>
> deleteInactiveObjects<A>(since: yesterday, inContext: context)
> deleteInactiveObjects<B>(since: oneWeekAgo, inContext: context)
> //..
>
> (here again, as you mention, the parameter affects how the function operates, yet it is not part of the signature)
>
> If the restriction was lifted, however, it would also be in detriment of the educational value of the proposal (apart from your arguments above). When defining a generic function, it feels natural to expect all of the type parameters to be present in the generic function signature. Relaxing this rule could be perceived by the novice as an invitation to an obscure design. In the best case, it would generate doubts about its actual intent.
>
> From a pedagogical perspective, the proposal aims to save Swift from disappointment when this topic is brought to discussion, say whether at the end of a Programming or a Compilers undergraduate course -- albeit fully understanding (or implementing) the current alternatives could be an excellent exercise for the class.
>
> From the (primary) language and development perspective, I don't think it could be expressed in a better way than in your lines, which are truly appreciated:
>
> "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."
>
> Finally, we should also consider the possibility of being explicit about some but not all type parameters. If allowed, something like "only trailing type parameters could be missing" would be necessary to avoid ambiguity.
>
> All the best,
> Ramiro
>
>
> On Tue, 29 Nov 2016 at 17:11 Douglas Gregor <dgregor at apple.com <mailto:dgregor at apple.com>> 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...
>
> 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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