Passing curried instance method as closure parameter that accepts instance method?

Using Swift
1

Hi! I've been reading through the (excellent) posts on Doug's Compiler Corner[1] from @Douglas_Gregor . I learn new things! I did not have much experience with "curried instance methods"[2]:

struct Point {
  var x: Int
  var y: Int
  
  func flippedOverXAxis() -> Point { ... }
  func flippedOverYAxis() -> Point { ... }
}

let memberFunction = Point.flippedOverXAxis

var p = Point(x: 1, y: 2)
p = memberFunction(p)()

I have a situation with a function that accepts a closure that operates on a Point instance:

func f(_: (Point) -> Point) { }

f({ point in Point.flippedOverXAxis(point)() })

This works… but I am wondering if there is some way to pass that curried instance method directly without the ceremony of wrapping it in a closure and passing a point instance:

f(Point.flippedOverXAxis)

This fails:

18 | f({ point in Point.flippedOverXAxis(point)() })
19 | 
20 | f(Point.flippedOverXAxis)
   |         `- error: cannot convert value of type '@Sendable () -> Point' to expected argument type '(Point) -> Point'
   |   `- error: instance member 'flippedOverXAxis' cannot be used on type 'Point'; did you mean to use a value of this type instead?
21 |

Is there any way to pass that curried method directly to a closure that expects an "instance" method? I am unblocked (because I can always wrap the curried method in a closure)… but it would be interesting if there was a shortcut there that worked. Thanks!

  1. https://www.douggregor.net ↩︎

  2. Swift for C++ Practitioners, Part 10: Operator Overloading | Doug's Compiler Corner ↩︎

2

You forgot to include the parameters in the instance method signature.

func f(_: (Point) -> () -> Point) { }

The generic form looks like this:

func f<Instance, each Parameter, Output>(
  _: (Instance) -> (repeat each Parameter) -> Output
) { }

f(Point.flippedOverXAxis)
3 Likes
3

Forever unhappy that we didn't get time to implement SE-0042.

6 Likes
5

@Slava_Pestov, @Quedlinbug: I am trying out @Quedlinbug's generic function above.

But the following code fails to compile (on Xcode 15.4).

f (Foo (), p: 1, 2, 3, 4, 5, o: Foo.bar)

struct Foo {
    func bar <each U>(_ u: repeat each U) -> Foo {
        let v = (repeat each u)
        print (#function, v)
        return self
    }
}

func f <Instance, each Parameter, Output> (
    _ i:Instance,
    p: repeat each Parameter,
    o v: (Instance) -> (repeat each Parameter) -> Output
) {
    print (#function)
    _ = v (i) (repeat each p)
}
Compilation Errors 
Command SwiftCompile failed with a nonzero exit code
error: compile command failed due to signal 11 (use -v to see invocation)
6

Crashes in Xcode 16b3 as well.

7

Sure… but if we are changing the function signature then that's not exactly my original question. If we were stuck with the original function (because it came from a library or other source we don't control)… then we still need some kind of ceremony to forward that to the original function:

func f(_ body: (Point) -> Point) { }

func g(_ body: (Point) -> () -> Point) {
  f({ point in body(point)() })
}

g(Point.flippedOverXAxis)

Unless I am missing something… there's no shortcut to get directly to the compact version without the SE-0042 changes (as referenced by @jrose)

8

If I change Foo.bar to a regular method (I think this is what the orginal discussion was about), it works fine.

I suspect the crash is because variadic generic function can't be used as closure. Below is a simpler example to reproduce the crash:

struct Foo { 
    func bar <each U>(_ u: repeat each U) -> Foo { 
        let v = (repeat each u)
        print (#function, v)
        return self
    } 
}

func f(_ i: Foo, p1: Int, p2: Int, c: (Foo) -> (Int, Int) -> Foo) {
    c(i)(p1, p2)
}

f(Foo(), p1: 1, p2: 2, c: Foo.bar)
1 Like
9

What you are describing is converting a method to a property*.

func convertMethodToProperty<Instance, Property>(
  _ method: @escaping (Instance) -> () -> Property
) -> (Instance) -> Property {
  { method($0)() }
}

Even disregarding the need for @escaping, is that an ergonomic improvement?

f { $0.flippedOverXAxis() }
f(convertMethodToProperty(Point.flippedOverXAxis))

* While it wouldn't work for the article, flippedOverXAxis should be a property. Change it to

var flippedOverXAxis: Point

and then you have what you want:

f(\.flippedOverXAxis)

That would not do it. You want (Point) -> Point, but that would provide (Point, Void) -> Point.

1 Like