Hello , those rank-2 types wil make me mad :) encodeHelper :: (MRef m r, Binary m a, BitStream m (StringBuffer m r)) => a -> m String encodeHelper x = do h <- newStringBuffer "" stringBufferDefaultCloseFunc put_ h x getStringBuffer h encode x = runST (encodeHelper x) i can't force `encode` to compile, either with or without any type signatures. actually, signature for `encodeHelper` was developed by Hugs, but now i don't know how to proceed further just now tried to force Hugs tell me its type. mission impossible :) DataAltBinaryClass> :t (runST (encodeHelper 1)) ERROR - Cannot justify constraints in application *** Expression : encodeHelper 1 *** Type : ST c [Char] *** Given context : () *** Constraints : (Num b, Binary (ST c) b) Binary class in my library includes monad in its header. May be it's the cause of problems? class Binary m a where -- | Convert value to sequence of bits and write it to the stream put_ :: (BitStream m h) => h -> a -> m () but it's needed because MArray types, for example, can be serialized only in the appropriate monad: instance Binary IO (IOArray a) where ... -- Best regards, Bulat mailto:bulatz@HotPOP.com