Better completion blocks by using higher order functions

Completion.swift

func completion<Result>(onResult: @escaping (Result) -> Void, onError: @escaping (Error) -> Void) -> ((Result?, Error?) -> Void) {
    return { (maybeResult, maybeError) in
        if let result = maybeResult {
            onResult(result)
        } else if let error = maybeError {
            onError(error)
        } else {
            onError(SplitError.NoResultFound)
        }
    }
}

// Use where a completion block will give a `Bool` value indicating it was successful or not.
func completion(onSuccess: @escaping () -> Void, onFailure: @escaping () -> Void) -> ((Bool) -> Void) {
    return { success in
        if (success) {
            onSuccess()
        } else {
            onFailure()
        }
    }
}

enum SplitError: Error {
    case NoResultFound
}