UArray

[Simon Marlow]

Frederik Eaton wrote:
> I was trying to track down the problem with this piece of code:
> 
>     instance (Elt k, IxB a, Eq a, Eq k) => Num (Vector k a) where
>         (+) = vmap2 (+)
>         (-) = vmap2 (-)
>         (*) = vmap2 (*)
>         negate = vmap negate
>         signum = vmap signum
>         abs = vmap abs
>         fromInteger n = vconst $ fromInteger n
> 
>     Vector2.hs:246:0:
>         Could not deduce (Eq (UArray a k)) from the context (Elt k, IxB a, Eq a, Eq k)
>           arising from the superclasses of an instance declaration at Vector2.hs:246:0
>         Probable fix:
>           add (Eq (UArray a k)) to the instance declaration superclass context
>           or add an instance declaration for (Eq (UArray a k))
>         In the instance declaration for `Num (Vector k a)'
> 
> So it appears that here is another barrier to using UArray
> polymorphically. Am I the first user of this interface?
> 
> In Data/Array/Base.hs it says:
> 
>     eqUArray :: (IArray UArray e, Ix i, Eq e) => UArray i e -> UArray i e -> Bool
>     eqUArray arr1@(UArray l1 u1 _) arr2@(UArray l2 u2 _) =
>         if rangeSize (l1,u1) == 0 then rangeSize (l2,u2) == 0 else
>         l1 == l2 && u1 == u2 &&
>         and [unsafeAt arr1 i == unsafeAt arr2 i | i <- [0 .. rangeSize (l1,u1) - 1]]
> 
>     ...
>     instance Ix ix => Eq (UArray ix Bool) where
>         (==) = eqUArray
> 
>     instance Ix ix => Eq (UArray ix Bool) where
>         (==) = eqUArray
> 
>     instance Ix ix => Eq (UArray ix Char) where
>         (==) = eqUArray
> 
>     instance Ix ix => Eq (UArray ix Int) where
>         (==) = eqUArray
> 
>     instance Ix ix => Eq (UArray ix Word) where
>         (==) = eqUArray
>     ...
>     etc.
> 
> Why define separate instances when one could just define one
> equivalent one:
> 
>     instance (Ix ix, Eq e, IArray UArray e) => Eq (UArray ix e)
>             (==) = eqUArray

That version certainly compiles without any extra options.  It's not 
H98, but then this library requires MPTC anyway (but not much else, I 
think).  I think Hugs will accept it too.  It certainly looks like an 
improvement, so I'll commit it.  Thanks!

Cheers,
	Simon