[Haskell-cafe] What's wrong with the classes/insances?

21 Jun 2008

      HI,

What 's wrong with this:

type Id = String

class Catalog a where
    listItems :: a -> IO [String]
    getItem :: a -> Id -> IO (Maybe String)

class Item a where
    getCatalog :: Catalog catalog => a -> catalog

data Catalog c => Content c = Content {auteur :: String, inhoud::
String, catalog::c}

instance Catalog c => Item (Content c) where
   getCatalog (Content  _ _ c) = c

I get this as error from ghci:

    Couldn't match expected type `catalog' against inferred type `c'
      `catalog' is a rigid type variable bound by
                the type signature for `getCatalog'
                  at
../Sites/liberaleswebsite/www.liberales.be/cgi-bin/Test.hs:16:26
      `c' is a rigid type variable bound by
          the instance declaration
            at ../Sites/liberaleswebsite/www.liberales.be/cgi-bin/Test.hs:20:17
    In the expression: c
    In the definition of `getCatalog': getCatalog (Content _ _ c) = c
    In the definition for method `getCatalog'
Failed, modules loaded: none.

thanks in advance,

P

-- 
Pieter Laeremans <pieter@laeremans.org>

"The future is here. It's just not evenly distributed yet." W. Gibson