A helper function to make error propagation for asynchronous operations easier in Swift.

asyncMap.swift

/// `asyncMap` helps you with error propagation for asynchronous tasks.
/// - Parameters:
///   - onFailure: A closure that will be called in an error condition if in an error state.
///   - onSuccess: A closure that will be called with your success value if succesful.
/// - Returns: A closure that will call `onError` if it is passed a failing result, and that will execute `onSuccess` if it is successful.
func asyncMap<Success, NeverSuccess, Error: Swift.Error>(
    _ onFailure: @escaping ((Result<NeverSuccess, Error>) -> Void),
    _ onSuccess: @escaping (Success) -> Void
) -> ((Result<Success, Error>) -> Void) {
    return { result in
        switch result {
        case let .success(s):
            onSuccess(s)
        case let .failure(e):
            onFailure(.failure(e))
        }
    }
}

example.swift

func complexAsyncOperation(done: @escaping (Result<Void, Error>) -> Void) {
    complexThing1(asyncMap(done) { str in
        print("got string: \(str)")
        complexThing2(asyncMap(done) { num in
            print("got numbers: \(num)")
            done(.success(()))
        })
    })
}

///

func complexThing1(_ done: @escaping (Result<String, Error>) -> Void) {
    // trivial example of async task returning String
    DispatchQueue.main.async {
        done(.success("abc"))
    }
}

func complexThing2(_ done: @escaping (Result<Int, Error>) -> Void) {
    // trivial example of async task returning Int
    DispatchQueue.main.async {
        done(.success(123))
    }
}