I am pretty sure this problem is known, but you should add this code
to the bug report:
http://hackage.haskell.org/trac/ghc/ticket/1496
-- ryan
On Tue, Mar 9, 2010 at 6:54 AM, Jan-Willem Maessen
On Mar 9, 2010, at 5:53 AM, Max Cantor wrote:
Isn't this just an extension of the notion that multi-parameter typeclasses without functional dependencies or type families are dangerous and allow for type-naughtiness?
I wondered the same thing, but came up with an analogous problematic case that *only* uses generalized newtype deriving:
{-# LANGUAGE GeneralizedNewtypeDeriving #-} module Main(main) where import Data.Set
class IsoInt a where stripToInt :: item a -> item Int convFromInt :: item Int -> item a
instance IsoInt Int where stripToInt = id convFromInt = id
newtype Down a = Down a deriving (Eq, Show, IsoInt)
instance Ord a => Ord (Down a) where compare (Down a) (Down b) = compare b a
asSetDown :: Set (Down Int) -> Set (Down Int) asSetDown = id
a1 = toAscList . asSetDown . convFromInt . fromAscList $ [0..10] a2 = toAscList . asSetDown . fromAscList . reverse . convFromInt $ [0..10]
main = do print a1 print a2
-Jan-Willem Maessen_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe