Hello Pieter, Saturday, June 21, 2008, 2:04:10 AM, you wrote:
for me, it seems just like you directly translated OOP classes into Haskell that is the wrong way. you may look into http://haskell.org/haskellwiki/OOP_vs_type_classes and ghc user manual which discuss functional dependencies on the example of collection classes > 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 -- Best regards, Bulat mailto:[EMAIL PROTECTED] _______________________________________________ Haskell-Cafe mailing list [email protected] http://www.haskell.org/mailman/listinfo/haskell-cafe
