On Sat, 2008-06-21 at 00:04 +0200, Pieter Laeremans wrote:
HI,
What 's wrong with this:
It looks like you think Haskell is an OO language or want it to be. It is not.
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
This means class Item a where getCatalog :: forall catalog. Catalog catalog => a -> catalog That means, given some Item a, I can create a value of -any- type that is an instance of Catalog. There is no well-defined function that could do that.
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