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

[Ramiro Feria Purón]

Inferring the type of an expression from its surrounding context is not
necessarily an ideal feature in a strongly typed programming language.

The goal here is to be able to be explicit about the type, in an elegant
and clear way, especially when there is no (need for a) context.

Note that the <Type> "noise" would only be necessary when the type could
not be inferred from the actual parameters (and if the current surrounding
context type inference was removed, which is not part of the proposal).

The line of code from the Motivation that does not compile is:

processAll<Bicycle>(in: vehicles, condition: aboveSpeedLimit)
        // This should be allowed under this proposal

2016-11-22 10:35 GMT+11:00 Adrian Zubarev <adrian.zubarev at devandartist.com>:

> I’m not sure what are you trying to solve here. Your code from the
> motivation compiles just fine in my Playground.
>
> About the g function:
>
> g(7) as [String]
> let _: [String] = g(7)
>
> Personally I don’t like to see <Type> noise on functions in Swift.
>
>
>
> --
> Adrian Zubarev
> Sent with Airmail
>
> Am 22. November 2016 um 00:06:02, Ramiro Feria Purón via swift-evolution (
> swift-evolution at swift.org) schrieb:
>
> *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.
>
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution
>
>
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