I think the problem is here:
> getCatalog :: Catalog catalog => a -> catalog
This wants to constrain the result of getCatalog to be an instance of
Catalog, but this only works for function arguments, not results. The
following code does typecheck, though I have no idea what is does or if
it is what you want:
type Id = String
class Catalog a where
listItems :: a -> IO [String]
getItem :: a -> Id -> IO (Maybe String)
class Catalog q => Item q a where
getCatalog :: a -> q
data Content d = MkContent {auteur :: String,
inhoud :: String,
catalog :: d}
instance Catalog c => Item c (Content c) where
getCatalog (MkContent _ _ e) = e
Pieter Laeremans wrote:
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
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