[swift-evolution] Variadics through tuples

[Vladimir.S]

Yes, I expect that with the implemented proposal SE-0110 your code will not 
compile because `moveTo` has type `(Int, Int)->Void` and not required 
`((Int, Int))->Void`. You'll need to change the moveTo to accept tuple 
argument or use some function/operator to transform argument list 
parametrized function to function with tuple argument.

For example (this compiles now and IMO should after SE-0110 implemented, if 
accepted):

//function application operator
infix operator => {}
func =><In, Out>(a: In, b: (In) -> Out) -> Out {
     return try b(a)
}

infix operator ?=> {}
func ?=><In, Out>(a: In?, b: (In) -> Out?) -> Out? {
     if let unwrapped = a {
         return try b(unwrapped)
     }
     return nil
}

//some defined functions

func defaultDestination() -> (x: Int, y: Int) {return (1,1) }
func loadedDestination() -> (x: Int, y: Int)? {return nil }


// ---- Variant 1 ----
func moveTo1(point: (x: Int, y: Int)) {print("move1 to: ", point.x, point.y)}


// ---- Variant 2 ----
func moveTo2(x: Int, y: Int) {print("move2 to: ", x, y)}

func tupleize<T,U,V>(_ f: (T,U)->V ) -> ( ((T, U))->V ) {
     return { tu in return f(tu.0, tu.1) }
}


//actual code

defaultDestination() => moveTo1
(loadedDestination() ?=> moveTo1) ?? print("load1 failed")

defaultDestination() => tupleize(moveTo2)
(loadedDestination() ?=> tupleize(moveTo2)) ?? print("load2 failed")


If we'll have such `tupleize` as built-in operator, then things will be 
even better. Don't see any problem here just like you, for example, don't 
expect function of (Int, (Int, String))->() will be accepted where (Int, 
Int, String)->() is required an so on. List of function arguments is not 
tuple at these days and so IMO only explicit conversion can exist to accept 
second when first is required and vice-versa. But I'm storng +1 to have 
such handy convertion operator.


On 01.07.2016 17:16, James Froggatt via swift-evolution wrote:
> Currently, the following code is allowed:
>
> //function application operator
> infix operator => {}
> func =><In, Out>(a: In, b: In -> Out) -> Out {
>     return try b(a)
> }
>
> infix operator ?=> {}
> func ?=><In, Out>(a: In?, b: In -> Out?) -> Out? {
>     if let unwrapped = a {
>         return try b(a)
>     }
>     return nil
> }
>
> //some defined functions
>
> func defaultDestination() -> (x: Int, y: Int)
> func loadedDestination() -> (x: Int, y: Int)?
> func moveTo(x: Int, y: Int)
>
> //actual code
>
> defaultDestination() => moveTo
> loadedDestination() ?=> moveTo ?? print("load failed")
>
> //code without functional chaining
>
> let point = defaultDestination()
> moveTo(x: point.x, y: point.y)
>
> if let loaded = loadedDestination() {
>     moveTo(x: loaded.x, y: loaded.y)
> } else {
>     print("load failed")
> }
>
> I'm expecting this to stop working at some point in Swift 3's development, since it is related to tuple splat. I've heard talk of tuple splat returning in the future through an operator, with the stand-in syntax:
>
> moveTo(*defaultDestination())
>
> So, how would a functional chaining operator work under these conditions? It would require a second variadic splat operator:
>
> defaultDestination() => *moveTo(x:y:)
>
> The motivation for removing the standard form of tuple splat is that the calling syntax looks like an overload. But in this case, an explicit splat operator doesn't add any clarity, since the function being referred to can be unambiguous.
>
>
>
> So, my question is whether this is worth removing full support for in the first place. This behaviour can be make to fit Swift 3's distinction of parameter lists and tuples, by applying specific rules to the existing behaviour to create a lightweight variadics system:
>
> takesAClosure<T>(_: (T) -> ()) //closure explicitly takes a single parameter
>
> takesAClosure<T>(_: T -> ()) //closure takes any number of parameters
>
> takesAClosure<T>(_: T, _: T -> ()) //closure takes any number of parameters, but must have a parameter list which can be directly represented as a tuple
>
>
>
> Either way, I'm hoping whatever syntax ends up chosen for variadics is nearly as simple to use as tuple splat has been, tuples are (literally) made for this purpose. I'll be disappointed to see this feature removed.
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