Invert guard let scoping

Evolution Pitches
,
10

So this is admittedly wonky, but map is another option:

guard error.map({print($0)}) == nil else {
    print("Guarded")
    return
}

Or with your original example:

class Network {
    let session: URLSession = .shared
    func callServer(request: URLRequest, completion: @escaping (Result<Data,Error>) -> Void) {
        let task = session.dataTask(with: request) {  (data, response, error) in
            guard error.map({
                completion(.failure($0))
                print("ERROR: \($0)")
                return Void.self
            }) == nil else {
            return
            }
        // The rest of your function
        }
    }
}

There are usually plenty of ways to work around the issue (I usually use switch) but it does seem natural for something like this to exist. Personally I would prefer to keep guard with it's current semantics. It can already be a bit confusing to reason about. Adding a case where a let could be available in the typical scope or inside else seems problematic.

How about escape?

escape if let error = error {
// return is required
    return
}