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_______________________________________________
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