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