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

[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