Hi! I believe dependent types is the right term for my problem. I was trying to solve it with GADTs but I was not successful so I am turning to infinite (lazy) wisdom of Haskell Cafe. I am attaching example code where I had to define MaybePacket data type which combines different types of Packets I would like to allow over Line. The problem is that there is a correlation between Line type and MaybePacket type and I would like to tell Haskell about that. But I am not sure how. Because now compiler, for example, warns me of a non-exhaustive pattern even if some MaybePacket value is not possible for given Line. Somehow I would like to have a getFromFirstLine function which would based on type of given Line return Maybe (Packet i) (for Line) or Maybe AnyPacket (for LineAny). So that this would be enforced and type checked. Best regards and thanks for any help Mitar