Re: [Haskell-cafe] automatically deriving Map and Filter on datatypes etc.

On 5 Jun 2008, at 1:39 AM, Thomas Davie wrote:

Even deriving an instance of Functor seems rather implausable, what should it do for

data Wierd a b = Nil | A a (Wierd a b) | B b (Wierd a b)

Should fmap's function argument operate on 'a's, 'b's, or both?

class Functor (f :: * -> *) where ... so, 'b's. jcc PS Why isn't Functor derivable?