Allowing duplicate instances in GHC 6.4

[Robert van Herk]

Hi Keean,

First of all, thank you for your answers. I have tried your solution 
using TypeEq.

instance (Datasource l k' v', TypeEq k k' z, Datasource' z l r k v) =>
  Datasource (JoinedDS l r) k v where
  _dsread (JoinedDS refl refr) k = do { l <- readIORef refl;
                                        r <- readIORef 
refr;                                     
                                        (z,l,r,v) <- _dsread' (l,r) k;
                                        writeIORef refl l;
                                        writeIORef refr r;
                                        return (JoinedDS refl refr, v);
                                      }

class Datasource' z l r k v | l r k -> v where
  _dsread'  :: (l,r) -> k -> IO (z,l,r,Maybe v)
  _dswrite' :: (l,r) -> k -> v -> IO (z,l,r)
instance Datasource l k v => Datasource' HTrue  l r k v where
  _dsread' (l,r) k = do { (l,v) <- _dsread l k;
                          return (hTrue, l, r, v);
                        }
instance Datasource r k v => Datasource' HFalse l r k v where
  _dsread' (l,r) k = do { (r,v) <- _dsread r k;
                          return (hFalse, l, r, v);
                        }

This compiles.

I cannot, however, include type z in the fundep of Datasource', since 
this conflicts with Datasource ds k v | ds k -> v. Furthermore, I do not 
understand how the key and value types of my right datasource (r k v) is 
bound to the instance of Datasource (JoinedDS l r) k v, since in the 
premisse (Datasource l k' v', TypeEq k k' z, Datasource' z l r k v), 
nothing is said about Datasource r k'' v''. However, I could be wrong in 
this, since Datasource r k v is in the premisse of instance Datasource r 
k v => Datsource' HFalse l r k v.

However, my problem is, that when I use this code:

do {joined <- createJoinedDS' x y;
      (joined,(v::Maybe Int)) <- _dsread joined (2.0::Float);
     }

{- | Easy constructor -}
createJoinedDS :: (IORef left) -> (IORef right) -> JoinedDS left right
createJoinedDS left right = JoinedDS left right
createJoinedDS' :: left -> right -> IO (JoinedDS left right)
createJoinedDS' l r = do { left <- newIORef l;
                           right <- newIORef r;
                           return (createJoinedDS left right);
                         }

the compiler will complain:

  Could not deduce (Datasource' z1 l r k v)
      from the context (Datasource (JoinedDS l r) k v,
                        Datasource l k' v',
                        TypeEq k k' z,
                        Datasource' z l r k v)
      arising from use of `_dsread''

It seems to be the case that it cannot decide on the type of z.

Would you know how to solve this?

Regards,

Robert