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 <jmaes...@alum.mit.edu> wrote: > > 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 > _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
