Function with multiple closure arguments gives error “Expression resolved to an unused function”

Question

I have followed a tutorial on Swift completion handlers to write this function. Whenever I attempt to use the closure, I receive an error, "Expression resolves to an unused function"

func completion<Result>(onResult: @escaping (Result) -> Void, 
onError: @escaping (Error) -> Void) -> ((Result?, Error?) -> Void) {
return { (maybeResult, maybeError) in
    if let result = maybeResult {
        onResult(result)
    } else if let error = maybeError {
        onError(error)
    } else {
        onError(SplitError.NoResultFound)
    }
}
}

How it's being called

    completion(onResult: { (j) in
        print(j)
    }) { (e) in
        print(e)
    }

error: Expression resolves to an unused function

Answer 1

The generic function completion returns a closure which is intended to use as completion handler to some async functions.

For example, assume you have some async method like this:

func someAsyncFunc(completion: @escaping (Data?, Error?)->Void) {
    //...
}

So, to use the closure, you may need to store the returned closure to some closure variable and use it later:

    let myCompletionHandler = completion(onResult: { (j: Data) in
        print(j)
    }) { (e) in
        print(e)
    }

    someAsyncFunc(completion: myCompletionHandler)

Or else, you can write it directly as a parameter:

    someAsyncFunc(completion: completion(onResult: { (j) in
        print(j)
    }) { (e) in
        print(e)
    })

Anyway, I cannot be sure if this might really be an improvement and you should better find a better tutorial.

Answer 2

That looks ok to me, you can always try

completion(onResult: { (j) in print(j)}, 
           onError: { (e) in print(e) })

Answer 3

Your function returns a closure. What you haven't done is used the closure. Do this:

completion(onResult: { (j) in
    print(j)
}) { (e) in
    print(e)
}()

Note the parenthesis at the end. Or to be more clear:

let completionClosure = completion(onResult: { (j) in
    print(j)
}) { (e) in
    print(e)
}
completionClosure()