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

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

Best regards, Steffen