[Haskell-cafe] casting a into Maybe a

If I have the following type signature transMit :: Serialize a => Socket -> POSIXTime -> KEY -> Maybe a -> TPSQ -> TMap a -> IO () And the function is called with transMit s now key newmsgs q m where newmsgs is whatever type a I get but _not_ a Maybe a then I get the error Could not deduce (a ~ Maybe a) from the context (Serialize a) Can I somehow when I call transmit cast newmsgs into a Maybe newmsgs or so so that the function call fits the type signature? --Joerg