First class generic function in swift?

Question

Swift has first class functions that can be passed as arguments.

func a() {
}

func b(x: ()) {
}

// Pass a to b…
b(a)

Swift has generic functions.

func generic<T>(x: T) {
}

But, does Swift let me pass a generic function as an argument to another function?

let anIntService = Service<Int>()
let astringService = Service<String>()

func attach<T>(service: Service<T>, to value: T) {
  // Perform type safe attaching of `value` to `service`.
}

func doAttaching(attach: (Service<T>, to T)->Void) {
  attach(anIntService, to: 42)
  attach(aStringService, to: "Hello World!")
}

doAttaching(attach)

…Or does it only let me pass a specific instantiation of a generic function?

If this is possible, please illustrate the syntax for defining a function that accepts a generic function as an argument.

If this isn't supported, a workaround is to define the generic function as a method of a struct, or something, and pass an instead of that. This is not ideal though, as the consuming function doesn't get such a nice calling syntax, they need to do:

func doAttaching(attach: Attacher) {
  attacher.attach(anIntService, to: 42)
  attacher.attach(aStringService, to: "Hello World")
}

score 4 · Answer 1 · answered May 13 '16 at 23:53

4

This feature is commonly called Higher-Kinded Types (HKT), and is not currently supported in Swift.

It has, however, been discussed on the swift-evolution mailing list.

answered May 13 '16 at 23:53

jtbandes

115,675
35
233
266

Hah! Really? :-) That's funny, because I'd read about higher kinded types in the past a few times, and had absolutely no idea what they were talking about. Now I know! Super – it'd be lovely if it makes a future version. Thank you for the reply. – Benjohn May 14 '16 at 07:22

score 0 · Answer 2 · answered May 13 '16 at 20:41

0

Yes, here is an example:

func identity<T>(param: T) -> T {
    return param
}

func callFunction<U>(function: U->U, paramater: U) -> U {
    return function(paramater)
}

let param = 123

let result = callFunction(identity, paramater: param);

print(result)

(You can run it here)

answered May 13 '16 at 20:41

Alexander

59,041
12
98
151

2

Thanks for the reply, but I don't think this does what I need. So, the called function in your example, `callFunction`, still isn't getting the generic function. It's getting a specific instance of it, determined by the type of the `parameter` that you call with – in this case `123`, so an Int. I want to pass the `identity` in your example as a _fully generic_ function to `callFunction`. The definition of `callFunction` can then invoke its `function` argument with multiple types. – Benjohn May 13 '16 at 21:24
Oh I see, sorry, I misunderstood the post :( – Alexander May 15 '16 at 15:52
No problem! Thanks anyway :-) – Benjohn May 15 '16 at 15:56

score 0 · Accepted Answer · answered Jan 13 '22 at 09:14

I realised, five years after asking this question, Swift supports something a little like this which can be useful via callAsFunction.

Here's an example:


final class Service<T> {
    // Implementation here
}

final class Attacher {
    // Important state here.
    var state = Void()
    
    func callAsFunction<T>(service: Service<T>, to: T) {
        // Implementation here.
    }
}

func doAttaching(attach: Attacher) {
    attach(service: Service<Int>(), to: 42)
    attach(service: Service<String>(), to: "Hello World!")
}

A Repeater can be passed in to generic code and provide useful behaviour.

I'm going to mark my own question as the right answer, which feels pretty duchey, but it has been 5 years. — Benjohn, Jan 13 '22 at 09:19

First class generic function in swift?

3 Answers3