Re: [Haskell-cafe] generalized newtype deriving allows the definition of otherwise undefinable functions

9 Mar 2010


      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