Hello the following program fails to load in Hugs2005 with all Haskell extensions enabled: {-# OPTIONS_GHC -fglasgow-exts -fallow-undecidable-instances #-} class (Monad m) => Stream m h where class (Stream m h) => CharStream m h where vGetChar :: h -> m Char vGetContents :: h -> m String vGetContents h = func where func = do vGetChar h return "" class (Stream m h) => ByteStream m h where vGetByte :: h -> m () instance (CharStream m h) => ByteStream m h where vGetByte h = undefined instance (ByteStream m h) => CharStream m h where vGetChar = undefined message printed is: ERROR file:.\SystemStreamClasses.hs - *** The type checker has reached the cutoff limit while trying to *** determine whether: *** CharStream a b *** can be deduced from: *** () *** This may indicate that the problem is undecidable. However, *** you may still try to increase the cutoff limit using the -c *** option and then try again. (The current setting is -c40) this program compiles ok in GHC 6.4.1. if call to `func` is replaced with its body, then program loads ok: vGetContents h = do vGetChar h return "" can problem with loading my code in Hugs be resolved by giving to `func` some type signmature? i can't replace call with its body because code in real program is much more complicated: vGetContents h = let loop = let func = do c <- vGetChar h next <- loop c `seq` return (c : next) handler e = if isEOFError e then return [] else mError e in mCatch func handler in do firstchar <- vGetChar h rest <- loop return (firstchar : rest) class (Monad m) => MonadHelper m where mError :: IOError -> m a mCatch :: m a -> (IOError -> m a) -> m a class (Show h, MonadHelper m) => Stream m h | h -> m where .... -- Best regards, Bulat mailto:bulatz@HotPOP.com