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

5 Jun 2008

      Hi ^_^,

Let's say we have the following data type and functions:

data Tab a =      (:↺:)
           |     a :↓:   Tab a
           | Tab a :↙↘: (Tab a,Tab a)
           deriving (Eq, Show, Read)

map f (:↺:)          = (:↺:)
map f (a :↓: t)      = f a :↓: map f t
map f (h :↙↘: (l,r)) = map f h :↙↘: (map f l, map f r)

filter p (:↺:)          = (:↺:)
filter p (a :↓: t)      | p a       = filter p t
                        | otherwise = a :↓: filter p t
filter p (h :↙↘: (l,r)) = filter p h :↙↘: (filter p l, filter p r)

is it possible to automatically derive map and filter?

data Tab a =      (:↺:)
           |     a :↓:   Tab a
           | Tab a :↙↘: (Tab a,Tab a)
           deriving (Eq, Show, Read, Map, Filter)

If not, do you think it might be nice to have something like this in the
future?

Best Regards,
Cetin Sert