[Haskell] FunDeps and MArray

[Scherrer, Chad]

Hi,

I'm trying to use functionally dependent typeclasses to create an
overloaded (+=) function. A simplified example of a first bit of code
is...

class PlusEq a b c | a b -> c where
    (+=) :: a -> b -> c

instance (MArray a e m, Num e, Ix i) => 
    PlusEq (a i e) (a i e) (m ()) where
    (+=) x y = let updateX i = do xi <- readArray x i
                                  yi <- readArray y i
                                  writeArray x i (xi + yi)
               in  sequence_ . map updateX $ indices x

I keep getting this error in GHCi:

Illegal instance declaration for `PlusEq (a i e) (a i e) (m ())'
    (the instance types do not agree with the functional dependencies of
the class)
    In the instance declaration for `PlusEq (a i e) (a i e) (m ())'
Failed, modules loaded: none.

Looking at GHC's documentation for MArray, the definition starts out

class (HasBounds a, Monad m) => MArray a e m where
...

It seems to me if MArray were written using fundeps (something like
MArray a e m | a e -> m) things may work out. Is there a reason it's not
written this way? If so, is there another way to do what I'm trying to
do? Thanks.

Chad Scherrer
Computational Mathematics Group
Pacific Northwest National Laboratory

"Time flies like an arrow; fruit flies like a banana." -- Groucho Marx