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

[Alfonso Acosta]

2008/5/5 Sai Hemanth K <saihemanth at gmail.com>:
> 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.