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

[Ryan Ingram]

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
<jmaessen at 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_______________________________________________
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