Re: Oops [Fwd: Re: Allowing duplicate instances in GHC 6.4]

[Keean Schupke]

Some more fixes...
Keean Schupke wrote:
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);
                                     }

   _dsread (JoinedDS l r) k = _dsread' (typeEq (undefined::k') k) l r k
class Datasource' z l r k v | l r k -> v where

class Datasource' z l r k v | z 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);

The type says the return type of Datasource' is v where v is the type 
resturned from _dsread so:

  _dsread' _ (l,r) k = _dsread l k
The types are determined by the instance... (I don't understand why you 
are trying to return
hTrue????

   _dsread :: s -> k -> v
and for Datasource'
   _dsread :: z -> l -> r -> k -> 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.
See change above!
Also note type of fundep for Datasource should now be:
class Datasource s k v | s -> k v where ...
   Keean.
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