There is a Zip class in category-extras 's Control.Functor.Zip on hackage
that covers this use-case.
http://hackage.haskell.org/packages/archive/category-extras/latest/doc/html/...
It can basically be viewed as the ap of an Applicative functor chosen to be
the left inverse of a genericly definable 'unzip'. Though, a Zippable
functor isn't necessarily Applicative, because there is no reason it needs
to support pure -- a lot of zippable functors are comonads after all.
I wrote a short blog post on this:
http://comonad.com/reader/2008/zipping-and-unzipping-functors/
and one on the less powerful dual operations (less powerful because while
every Haskell Functor is strong, much fewer are costrong):
http://comonad.com/reader/2008/cozipping/
-Edward Kmett
On Thu, Jul 16, 2009 at 5:56 PM, Job Vranish
I was needing a way to zip generic data structures together today and was very annoyed to find that there is no Zippable class, or variant there of.
So I made my own:
class (Foldable f, Functor f) => Zippable f where fmaps :: (Foldable g) => g (a -> b) -> f a -> f b fmaps' :: [a -> b] -> f a -> f b -- to save a step on instance implementation zipWith :: (a -> b -> c) -> f a -> f b -> f c zip :: f a -> f b -> f (a, b) unzip :: f (a, b) -> (f a, f b)
fmaps fs a = fmaps' (toList fs) a fmaps' fs a = fmaps fs a zipWith f a b = fmaps (fmap f a) b zip = zipWith (,) unzip a = (fmap fst a, fmap snd a)
instance Zippable [] where fmaps' (fx:fs) (x:xs) = fx x : fmaps' fs xs fmaps' _ _ = []
--The fmaps function is also quite handy as a replacment for zipWith3, zipWith4, etc... --For example:
x = [1, 3, 5, 7, 3] y = [6, 9, 3, 1, 4] z = [2, 4, 0, 8, 2] test = fmap (,,) x `fmaps` y `fmaps` z -- > [(1,6,2),(3,9,4),(5,3,0),(7,1,8),(3,4,2)]
--you can also throw in a functor instance to remove the dependency on the Functor class, but it -- might not be worth it: instance (Zippable f) => Functor f where fmap f a = fmaps (repeat f) a
Is there any good reason that there isn't something like this in the standard libraries? Or, as far as I can tell, on hackage? If not, then maybe I'll stick it on hackage.
- Job Vranish
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