callCC in 2.7.0

OK. Can you show code to implement the tail recursive version of
fib using callCC in R, say.

Here it is transformed to tail recursive style:

fib <- function(i, a = 0, b = 1) {
 if (i == 0) a else fib(i-1, b, a+b)

Now, how do I add callCC to all this so that the fib call
presumably does not create a new stack instance?


On Sun, Mar 30, 2008 at 1:31 PM, Luke Tierney <luke at stat.uiowa.edu> wrote:

On Sun, 30 Mar 2008, Gabor Grothendieck wrote:

I think the only relationship to that is the name since
it does not appear to allow one to leave a function
in the middle of its processing and re-enter it back
at that point -- which is what would be needed.

The article conflates basic CPS with having first class continuations
as in Scheme. The discussion about compilers and tail calls only
requires downward-only continuations of the kind provided by R's
current callCC.  The user interface and coroutine discussion requires
continuations that can be run outside of their creating context.  The
most sophisticated variant, as provided in Scheme, also allows
continuations to be run more than once.  I don't think any of the
examples in the Wikipedia article need that, but there is some
interesting work on using that to model web browsing behavior.

At any rate, there is plenty of precedent for using callCC as the name
for the construct here even when the continuation is no longer valid
outside of the creating callCC call. So the relationship is more than
just the name.

luke

On Sun, Mar 30, 2008 at 12:04 PM,  <h.wickham at gmail.com> wrote:

Would anyone like to explain if callCC in R 2.7.0 gives
anything that on.exit does not already provide?

It seems that the exit condition once defined cannot
be added to overridden whereas with on.exit multiple
on.exit's add additional on.exits rather than being ignored.

Is this important?

It facilitates a completely different style of programming - see
http://en.wikipedia.org/wiki/Continuation-passing_style

--
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--
Luke Tierney
Chair, Statistics and Actuarial Science
Ralph E. Wareham Professor of Mathematical Sciences
University of Iowa                  Phone:             319-335-3386
Department of Statistics and        Fax:               319-335-3017
   Actuarial Science
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