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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