Lennart Augustsson wrote:
>
> Kevin Atkinson wrote:
>
> > Ok you haskell experts. I have an interesting challenge (or maybe just
> > a question if you have seen it before).
> >
> > Is it possible to zip two sequences together with just:
> >
> > cons :: a -> c a -> c a
> > empty :: c
> > foldr :: (a -> b -> b) -> b -> c a -> b
> >
> > And if so how would one do so.
> >
> > Remember you may ONLY use the three functions given above and NOTHING
> > else. Creating a list with "foldr (:) []" is also not allowed.
>
> With only that you definitely cannot do it since zip involes pairs.
That was not very well worded. I meant to using only the above three
functions to manipulate the sequence.
> Assuming that
> you allow pairs and lambda expressions you can do it like this:
>
> caseList xs n c =
> fst (foldr (\ x (_, xs) -> (\ n c -> c x xs, x `cons` xs))
> (\ n c -> n, empty) xs) n c
>
> zip =
> foldr (\ a g ys -> caseList ys empty ( \ b bs ->(a,b) `cons` g bs))
> (\ _ -> empty)
That is a heck of a lot of lambda's. What exactly is going on here and
if the container was a list would it be as efficient as the straight
forward zip defined in the prelude.
--
Kevin Atkinson
[EMAIL PROTECTED]
http://metalab.unc.edu/kevina/
