[Haskell-cafe] Why is there no Zippable class? Would this work?
jvranish at gmail.com
Thu Jul 16 17:56:39 EDT 2009
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
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,
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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