[r6rs-discuss] [Formal] last-element behavior of for-each

From: AndrevanTonder <andre>
Date: Fri Oct 6 18:58:45 2006

On Fri, 6 Oct 2006, Per Bothner wrote:

> I'm having problems seeing the purpose of the function, and also
> which lists would be affected by tail-call semantics. (I'm not
> not very good with complex recursion, and this is effectively
> two nested cursive functions.) Could you explain?

Sorry, what I meant to write (but didn't) was the following:

(define (traverse-tree tree)
   (for-each (lambda (node)
               (if (list? node)
                   (traverse-tree node)
                   (display node)))
             tree))

(traverse-tree '((1 2 3) (3 4 5) (((((6))))) (7)))

    ===> 1234567

(traverse-tree '(((((((((((((((((1))))))))))))))))))

    ===> 1

Here, for example, the sequence of examples

   (1), ((1)), (((1))), ...

will all require the same space to run under the proposed r6rs semantics. For
e.g. a balanced binary tree, the maximum amount of memory used
during traversal will not be affected, but the average amount over the time
of execution will be about half with r6rs-semantics (only the leftmost branch
would take the maximum memory, and after that the memory use just gets less and
less).

Cheers
Andre
Received on Fri Oct 06 2006 - 18:56:27 UTC