swift_unwrapnil

/// checks a value is not nil else throws a custom error
public extension Future where Expectation: OptionalType {
    
    func unwrapnil(or error: @autoclosure @escaping () -> Error) ->
        Future<Expectation.WrappedType> {
            
            return map(to: Expectation.WrappedType.self) { optional in
                guard optional.wrapped != nil else {
                    throw error()
                }
                
                return optional.wrapped!
            }
    }
}