Dear Erica: How do you create `Never` callbacks?

Vivien Leroy writes: I tried using Never for my API module callbacks but couldn’t really figure out how to use it when my callbacks contains guards. Here’s an example:

Capture d’écran 2016-09-06 à 17.15.59

Do you have any hints on how to properly use Never for such case?

While you don’t really need a Never return type for completion methods, especially those that don’t exit the application, you can work around guard conditions by skillful use of do clauses.

You might create a Never-based completion type:

typealias CompletionNever = () -> Never

func functionWithNeverCompletion(completion: CompletionNever) 
    // ... do interesting things ...

Guard conditions require you to exit scope. Never mean you cannot  return from your function. So, instead, add a simple do-scope layer:

functionWithNeverCompletion {
    alldone: do {
        guard somecondition else { break alldone }

This ensures you can perform all the standard guard conditions, including Boolean tests, optional binding, availability checks, and so forth, while maintaining the “no return” integrity of the handler.

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