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

[Bulat Ziganshin]

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 at gmail.com