On Sat, Dec 3, 2016 at 2:32 PM, David Feuer <david.feuer@gmail.com> wrote:

You can also sift monomorphic containers using my class, which should probably be called MonoSiftable.

data IntList = Cons !Int IntList | Nil

instance Siftable Int IntList where
sift _ Nil = Nil
sift p (Cons x xs)
| p x = Cons x (sift p xs)
| otherwise = sift p xs

You can also use it with contra-variant or invariant type constructors, e.g.,

instance Siftable a (a -> Bool) where

sift f g = \x -> f x && g x

Dave Menendez <dave@zednenem.com>
<http://www.eyrie.org/~zednenem/>