Re: Zipping two sequences together with only cons, empty, foldr

[Kevin Atkinson]

Lennart Augustsson wrote:
> 
> Kevin Atkinson wrote:
> 
> > Lennart Augustsson wrote:
> >
> > > No, it will not be as efficient.  foldr is not the right primitive for making
> > > functions on lists.  You want the more general
> > >   recurse :: (a -> c a -> b -> b) -> b -> c a -> b
> >
> > Could you give me some refrence on how that function is used as this is
> > the first time I herd of it.
> 
> Well, I just made up the name since it's not a standard function (it should be :-).
> It is useful for functions doing primitive recursion.
> 
> > What exactly does it do?  What would its defination for a list be.
> > How would you define a foldl, foldr, foldl1, foldr1, zip, etc.. with it.
> 
> I think the type should (almost) explain what it does :), let me exemplfy for lists
> recurse c n [] = n
> recurse c n (x:xs) = c x xs (recurse c n xs)
> 
> So it's like foldr, except that the cons function gets the tail of the list,
> not just the head.  With it you can define an efficient tail function.

Ok so how would you define, a foldl, foldr, foldl1, foldr1, and zip
using it?

I remember seeing it before and I could not figure out how to define a
foldl using it.

-- 
Kevin Atkinson
[EMAIL PROTECTED]
http://metalab.unc.edu/kevina/