How to create typealias for completion handlers with named arguments

Question

What I have so far is this:

1. I've defined typealias completion handler

   typealias UserCompletionHandler = (_ object: User?, _ error: ApiError?) -> Void
   

2. And I've created a service function that is using this typealias

   func login(email: String, password: String, completion: UserCompletionHandler) {
      // ...
      // this part here handles API call and parsing logic
      // ...
   
      completion(user, nil)
   }
   

What I want to achieve is to have more readable completion callback with parameters by introducing named arguments. Idea is to end up with this:

   completion(object: user, error: nil)

Or even better to make error parameter optional, so I can just call

   completion(object: user)

Issue is that I can't find a way to change typealias definition to achieve this.

Braided

Answer 1

Apparently this is not possible. You can find the explanation behind this choice in the swift evolution proposal: 0111-remove-arg-label-type-significance.md

Function types may only be defined in terms of the types of the formal parameters and the return value. Writing out a function type containing argument labels will be prohibited

Brume

Answer 2

Not sure if this answer is ideal, but you could use a tuple as your input argument:

typealias UserCompletionHandler = ((object: User?, error: ApiError?)) -> Void

and the usage would look like this:

completion((object: user, error: nil))

Yoshida

I like this approach. It does have extra braces around it, but it does give cleaner naming and improves readability of the handler usage. Thanks Edward! – Braided