Infer Generic Type In Completion Block

Question

I have a function:

static func requestArray<T>(completion: @escaping (_ result: Result<[T], Error>) -> ()) {

}

That is called like this:

MyClass.requestArray() { result in
    switch result {
        case .success(let array):
            break
        case .failure(let error):
            break
        }
}

However, this doesn't work because it can't infer the generic type T. How can I alter my code so that it does recognise what type T should be? (Assume array should be of type [String] )

Answer 1

It's generally more convenient if the API moves the type into the signature, in order to simplify the closure:

static func requestArray<T>(of: T.Type, completion: @escaping (_ result: Result<[T], Error>) -> ()) { }

When done this way, the call becomes:

MyClass.requestArray(of: String.self) { result in
    switch result {
        case .success(let array):
            break
        case .failure(let error):
            break
        }
}

This approach also works nicely when T is a return type. See Codeable for a good example of this approach in methods like decode(_;from:) .

Answer 2

You have explicitly declare the type of result in the completion handler when you call the function:

MyClass.requestArray() { (result: Result<[String], Error>) in
    switch result {
    case .success(let array):
        break
    case .failure(let error):
        break
    }
}