Re: [Haskell-cafe] Question related to Multi Param Type Classes

2008/5/5 Sai Hemanth K :

Hi,

I declared a multi param type class as below: class MyString m c where zIndex :: m -> Int -> c zLength :: m -> Int zPack :: [c] -> m zEquals :: c ->c -> Bool zWrap :: [c] -> (m,AnExistingDataType)

I infer from your questions that you probably missed setting a functional dependency, that is

class MyString m c | m -> c where zIndex :: m -> Int -> c zLength :: m -> Int zPack :: [c] -> m zEquals :: c ->c -> Bool zWrap :: [c] -> (m,AnExistingDataType)

which means, "c is univocaly determined by m". You'll need -XFunctionalDependencies to make it work in GHC.