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
>
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