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

20 Jun 2008


      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

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

Dan Weston