Hello John, Monday, March 20, 2006, 2:49:14 PM, you wrote: JM> Is it possible to create a RULES that fires only if a type has a given JM> class constraint? something like:
snub :: Ord a => [a] -> [a] snub xs = f Set.empty xs where f _ [] = [] f (x:xs) set | x `Set.member` set = f xs | otherwise = x:f xs (Set.insert x set)
{-# RULES "nub/snub" Ord a => forall a xs . nub (xs::[a]) = snub xs #-}
the following compiles but i really never tested whether it works :) {-# RULES "freeze/IOUArray" forall (x :: (forall s e i . (Unboxed e, HasDefaultValue e) => IOUArray i e)) . freeze x = freezeIOUArray x "thaw/IOUArray" forall (x :: (forall e i . (Unboxed e, HasDefaultValue e) => UArray i e)) . thaw x = thawIOUArray x "unsafeFreeze/IOUArray" forall (x :: (forall s e i . (Unboxed e, HasDefaultValue e) => IOUArray i e)) . unsafeFreeze x = unsafeFreezeIOUArray x "unsafeThaw/IOUArray" forall (x :: (forall e i . (Unboxed e, HasDefaultValue e) => UArray i e)) . unsafeThaw x = unsafeThawIOUArray x #-} see http://freearc.narod.ru/ArrayRef.tar.gz for the whole story -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com