On Mon, Mar 08, 2010 at 11:32:16PM +0100, Wolfgang Jeltsch wrote:

Generalized newtype deriving doesn’t just allow otherwise undefinable functions to be defined. It probably also allows for faster function implementations. For example, with the above conv method, you could probably convert a list of some type [t] into a list of type [Wrapped t] in O(1) time. If you would code this conversion by hand, it would take O(n) time, of course.

So you can! wrapList :: [a] -> [Wrapped a] wrapList xs = conv xs

wrapList "Hello" [Wrap 'H',Wrap 'e',Wrap 'l',Wrap 'l',Wrap 'o']

This is quite interesting. And perhaps very, very bad. Take this example:

newtype Down a = Down a deriving (Iso a, Show, Eq)

instance Ord a => Ord (Down a) where compare (Down x) (Down y) = compare y x

downSet :: Set.Set a -> Set.Set (Down a) downSet ss = conv ss

xs = "abcdef"

sxs1 = downSet (Set.fromList xs) sxs2 = Set.fromList (map Down xs)

Set.toAscList sxs1 [Down 'a',Down 'b',Down 'c',Down 'd',Down 'e',Down 'f'] Set.toAscList sxs2 [Down 'f',Down 'e',Down 'd',Down 'c',Down 'b',Down 'a'] We have been able to break the invarients of 'Set' using newtype deriving of a completely unrelated class 'Iso'. It seems that generalized newtype deriving may break type classes in a big way. John -- John Meacham - ⑆repetae.net⑆john⑈ - http://notanumber.net/