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? Bob On 5 Jun 2008, at 10:28, Miguel Mitrofanov wrote:
Well, it's certainly not possible for "filter", at least, not without additional hints to the compiler. For example, consider this type:
data Weird a = A | B a (Weird a) (Weird a)
filter p A = A filter p (B x w1 w2) | p x = B x (filter p w1) (filter p w2) | otherwise = ?????
On 5 Jun 2008, at 12:03, Cetin Sert wrote:
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 _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
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