use Perl

Oh well, I could say the same, read my post; I addressed that concern too :) unfoldr actually allows to return empty lists. Here's a hacked quote from haskell description:

The unfoldr function is a `dual' to foldr: while foldr reduces a list to a summary value, unfoldr builds a list from a seed value. The function takes the element and returns empty list if it is done producing the list or returns (@a,$b), in which case, @a is a appended to the list and $b is used as the next element in a recursive call.

As to the problem as you address it that you need to return two lists, I don't think there is a point to do so, at least for unfoldr, because it transforms a scalar to a list, not a list to a list. A list and a scalar is just fine.

I played a bit with the concept, and found an issue I can't resolve, possibly you can help me here. The problem concerns whether $_ is an alias or a copy to the parameter passed. If it is a copy then you can't modify it unless it is a reference. If it is an alias you can't modify it if it is read-only. You may ask why should it be modified at all, and here comes the classic unfoldr:

unfoldr { $_ ? ($_, $_ - 1) : () } 10;

10,9,8,7,6,5,4,3,2,1

which allows to pass arbitrary values to the callback . Now back to your example, if I want to send values other than the original parameter to myself (e.g. callback), then I can't. Also, I can't send an "undef" back to me that way. That seems too much of a limitation.

So a question. Why not a good old unfoldr?

unfoldr { @$_ ? ( pop @$_, $_ ) : () } [1..10];

10,9,8,7,6,5,4,3,2,1