Am Dienstag, 9. März 2010 07:24:35 schrieb Steffen Schuldenzucker: > On 03/08/2010 10:45 PM, Wolfgang Jeltsch wrote: > > The point is, of course, that such conversions are not only possible for > > binary operations but for arbitrary values and that these conversions are > > done by a single generic function conv. I don’t think it would be > > possible to implement conv without generalized newtype deriving. > > > > Any thoughts? > > Hi Wolfgang, > > it's not exactly the same, but... > > > import Control.Applicative > > > > newtype Wrapped a = Wrap a deriving Show > > > > instance Functor Wrapped where > > fmap f (Wrap x) = Wrap $ f x > > > > instance Applicative Wrapped where > > pure = Wrap > > (Wrap f) <*> (Wrap x) = Wrap $ f x > > > > convBinOp :: (a -> a -> a) -> (Wrapped a -> Wrapped a -> Wrapped a) > > convBinOp op x y = pure op <*> x <*> y
I think this is fundamentally different. As I said above: > The point is, of course, that such conversions are not only possible for > binary operations but for arbitrary values and that these conversions are > done by a single generic function conv. Your applicative functor Wrapped allows conversions only for n-ary functions, so, for example, John Meachem’s trick to break the invariant of Set doesn’t work. On the other hand, you need a separate conversion function for each arity (pure, fmap, liftA2, liftA3, …) whereas generalized newtype deriving allows you to use the same conversion function for all arities. Best wishes, Wolfgang _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
