Swift generic enum as a parameter for escaping closure that is a parameter for function

Question

protocol A {}
extension Int: A {}
extension String: A {}

enum Result<T> {
    case success(T)
    case error(Error)
}

typealias BaseFunc = (Result<A>) -> ()

func foo(r: Result<Int>) {}
func bar(r: Result<String>) {}

var f: BaseFunc = foo

Compiler says cannot convert value of type '(Result<Int>) -> ()' to specified type 'BaseFunc' (aka '(Result<A>) -> ()')

Is what I'm trying to do possible in Swift? Maybe there is another way to achieve similar result?

@iWheelBuy answered correctly for this question. However I formulated it wrong. What I want is something like this:

protocol A {}
extension Int: A {}
extension String: A {}

enum Result<T> {
    case success(T)
    case error(Error)
}

typealias BaseFunc<T> = (Result<T>) -> ()

func foo(r: Result<Int>) {}
func bar(r: Result<String>) {}

var f: BaseFunc<A>
if arc4random_uniform(10) < 5 {
    f = foo
} else {
    f = bar
}

Which results in: cannot assign value of type '(Result<Int>) -> ()' to type '(Result<A>) -> ()'

A `(Result) -> Void` is not a `(Result) -> Void`. Say your code complied; I could say `f(.success("foobar"))`. We just passed a `String` to something expecting an `Int`. But in any case, [generics are invariant](https://stackoverflow.com/q/41976844/2976878), so the opposite would be just as illegal (but is technically sound because of the value semantics of `Result`). — Hamish, Sep 01 '17 at 10:53

score 2 · Accepted Answer · answered Sep 01 '17 at 10:50

2

Have you tried this way?

typealias BaseFunc<B:A> = (Result<B>) -> ()

Your typealias doesn't have a generic parameter

answered Sep 01 '17 at 10:50

iWheelBuy

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You are absolutely correct. However when I tried to follow your advice in my more specific example it still fails. protocol A {} extension Int: A {} extension String: A {} enum Result { case success(T) case error(Error) } typealias BaseFunc = (Result) -> () func foo(r: Result) {} func bar(r: Result) {} var f: BaseFunc if arc4random_uniform(10) < 5 { f = foo } else { f = bar } – Serge Rykovski Sep 01 '17 at 16:27
1

@SergeRykovski [My comment above](https://stackoverflow.com/questions/45997818/swift-generic-enum-as-a-parameter-for-escaping-closure-that-is-a-parameter-for-f#comment78957302_45997818) still applies to your new example. A `(Result) -> ()` is *not* a `(Result) -> Void`. – Hamish Sep 01 '17 at 18:29
@SergeRykovski can you please also tell us the way you want to use VAR F after it is set? – iWheelBuy Sep 01 '17 at 20:11
@iWheelBuy In real life scenario there is a bunch of functions with similar signatures - the only difference between that is the actual class that is wrapped in Result (all of those classes conform to A). There's an API that returns Result (I don't always know actual class that's wrapped in result, however I intend to use only in context of conformance to A). So following functional style I'd like to have a function that returns function (depending on some rules). I have an API that returns Result (depending on some other rules). In the end I want to call returned function with result. – Serge Rykovski Sep 04 '17 at 12:40

Swift generic enum as a parameter for escaping closure that is a parameter for function

1 Answers1