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.
_______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users