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:Bulat.Ziganshin@gmail.com