ErrorOnNil.swift

extension SignalProtocol where Value: OptionalProtocol {
    func errorOnNil(_ error: Error) -> Signal<Value.Wrapped, Error> {
        return attemptMap {
            switch $0.optional {
            case let unwrapped?:
                return .success(unwrapped)
            case nil:
                return .failure(error)
            }
        }
    }
}

extension SignalProducerProtocol where Value: OptionalProtocol {
    func errorOnNil(_ error: Error) -> SignalProducer<Value.Wrapped, Error> {
        return lift { $0.errorOnNil(error) }
    }
}