Re: [Haskell-cafe] Why is there no Zippable class? Would this work?

Yeah I tried applicative, but saw that the <*> operator didn't do what I want with lists, and started looking elsewhere. I didn't even see the ZipList! Actually the other problem is that the data structure that I'm using won't support pure, so no Applicative :( Though for a generic zip, Applicative may be the better general purpose way to go. I didn't see TypeCompose and category-extras either those look pretty sweet :) Those would have worked. But I think I in my case, my version is a bit more general. Thanks for the input! It has been very enlightening. :) Hmmm I also should have pulled the zips out of the typeclass: class (Foldable 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 fmaps fs a = fmaps' (toList fs) a fmaps' fs a = fmaps fs a zipWith :: (Zippable f) => (a -> b -> c) -> f a -> f b -> f c zipWith f a b = fmaps (fmap f a) b zip :: (Zippable f) => f a -> f b -> f (a, b) zip a b = zipWith (,) a b unzip :: (Functor f) => f (a, b) -> (f a, f b) unzip a = (fmap fst a, fmap snd a) instance Zippable [] where fmaps' (fx:fs) (x:xs) = fx x : fmaps' fs xs fmaps' _ _ = [] On Thu, Jul 16, 2009 at 8:40 PM, Ryan Ingram wrote:

(I'm going to play fast and loose with constructors for this post, treating MyList and ZipList as if they were [])

On Thu, Jul 16, 2009 at 4:10 PM, Dan Weston wrote:

...

-- different from [], sum rather than product instance Applicative MyList where pure x = x ::: Nil (<*>) (f ::: fs) (x ::: xs) = f x ::: (fs <*> xs) (<*>) _ _ = Nil

Unfortunately, this instance doesn't fulfill this Applicative law: pure id <*> f = f

pure id <*> [1,2,3] = [id] <*> [1,2,3] = [id 1] = [1]

Fortunately, the solution already exists in Control.Applicative:

...

-- | Lists, but with an 'Applicative' functor based on zipping, so that -- -- @f '<$>' 'ZipList' xs1 '<*>' ... '<*>' 'ZipList' xsn = 'ZipList' (zipWithn f xs1 ... xsn)@ -- newtype ZipList a = ZipList { getZipList :: [a] }

instance Functor ZipList where fmap f (ZipList xs) = ZipList (map f xs)

instance Applicative ZipList where pure x = ZipList (repeat x) ZipList fs <*> ZipList xs = ZipList (zipWith id fs xs)

In this case:

pure id <*> [1,2,3] = [id, id, ...] <*> [1,2,3] = [id 1, id 2, id 3] = [1,2,3]

-- ryan