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
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