Hello, everybody! I am trying to develop some sort of library, which supposed to sign into a WEB service, then perform some requests with it. Initially I designed methods in the following way data DServError = InvalidCredentials | InvalidRequest | ... newtype Result a = Result { getOpResult :: Either DServError a } data DSession = Session { ... } data DLoginResponse = LoginResponse { currentSession :: DSession, ... } login :: String -> String -> IO ( Result LoginResponse ) servRequest1 :: DSession -> ParamType1 -> ParamType2 -> ... -> IO ( Result DServResponse ) Now I want to be able of doing something like authenticatedReq = do loginResponse <- login "username" "password" let session = currentSession loginResponse servRequest1 session ... ... ... servRequest2 session ... ... ... ... so if login succeeds - I will be able to extract Right data from the Either response ( with explicit or implicit usage of getOpResult), if any of operations within "do" block will fail with DServError - then an error should be reported. I think the solution for this may be using Control.Exception and it's try/catch? Or may be there's some trick available for Either? I looked at EitherT, and it seems that I have to wrap every invocation into EitherT and then chain them with >>/>>= -- Eugene Dzhurinsky