[swift-evolution] [Pitch] KeyPath based map, flatMap, filter

Benjamin Herzog mail at benchr.de
Sun Jul 9 09:36:27 CDT 2017

In Scala you can implement an apply method which makes it possible to call an object just like a function. Example:

case class Foo(x: Int) {
	def apply(y: Int) = x + y

val foo = Foo(3)
val bar = foo(4)

That is similar to what you suggested to have a possibility to convert an object to a closure getting called. And I totally see the point for this! I think using a keyword or special name like apply is not a good idea because it's not obvious what it does and it also makes it possible to just call the method with its name: foo.apply(4).

However, having a protocol is kinda hard because it's not possible to have a flexible parameter list. Maybe having a method without a name? Swift example:

class Foo {
    var x: Int
    init(x: Int) { self.x = x }

    func (y: Int) -> Int {
        return self.x + y

let foo = Foo(x: 3)
let bar = foo(y: 4)

I actually like that, would be like an anonymous function. It would also be possible to have multiple of those defined for one object (which would have to be unambiguous of course).

So getting back to KeyPath, it could look like this:

class KeyPath<Root, Value> {
    func (_ root: Root) -> Value {
        return root[keyPath: self]

I see that this would be a much bigger change and would not justify the syntactic sugar for map, flatMap, etc. But it would still be a nice addition to the Swift programming language, especially for KeyPath, transformers etc.

What do you think?

Benjamin Herzog

> On 9. Jul 2017, at 13:09, Karl Wagner <razielim at gmail.com> wrote:
> I agree that it should be completely implicit.
> KeyPaths are simply chains of partially-applied properties and subscripts. At the same time, it was noted in the KeyPath proposal that a similar mechanism might be used to model chains of partially-applied functions. I think that having both types be convertible to a closure would be sensible.
> In fact, you could argue for a general-purpose “Executable” protocol which would allow any conforming object to be implicitly used as a function/closure. Command-style objects such as predictes and transformers would also benefit from such a feature.
> - Karl

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