"Simon Marlow"
I'll confess I haven't looked into your stuff in any great detail, but I'm wondering whether Hal's idea can be achieved without any extra machinery. Here's my best shot; it's not entirely satisfactory because there's an extra dummy type parameter to the STUPairArray type constructor, and you need -fallow-undecidable-instances, but it seems to do the job:
module PairArray where
import Data.Array.Base import Control.Monad.ST
data STUPairArray a b s i t = STUPairArray (STUArray s i a) (STUArray s i b)
instance HasBounds (STUPairArray a b s) where bounds (STUPairArray left right) = bounds left
instance (MArray (STUArray s) a m, MArray (STUArray s) b m) => MArray (STUPairArray a b s) (a,b) m where
newArray (l,u) (a,b) = do lft <- newArray (l,u) a rgt <- newArray (l,u) b return (STUPairArray lft rgt)
unsafeRead (STUPairArray lft rgt) i = do a <- unsafeRead lft i b <- unsafeRead rgt i return (a,b)
unsafeWrite (STUPairArray lft rgt) i (a,b) = do unsafeWrite lft i a unsafeWrite rgt i b
Not too far from how I do it. Only I use functional dependencies instead of undecidable instances. Cheers, Manuel