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

Robert van Herk wrote:

Sorry, this is the compiler error I get:

No instances for (KeyHasValue MyKeyVal k' v', Datasource.Tools.FakePrelude.TypeEq Float k' z, Datasource' z [MyKeyVal] [MyKeyVal] Float Int) When I am trying to do

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

Robert

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Subject: Re: Allowing duplicate instances in GHC 6.4 From: Robert van Herk Date: Thu, 31 Mar 2005 16:49:07 +0200 To: glasgow-haskell-users@haskell.org

To: glasgow-haskell-users@haskell.org

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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

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); } 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.