[Rd] callCC in 2.7.0

[Luke Tierney]

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for the construct here even when the continuation is no longer valid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
>>
>> --
>> http://had.co.nz/
>>
>
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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
241 Schaeffer Hall                  email:      luke at stat.uiowa.edu
Iowa City, IA 52242                 WWW:  http://www.stat.uiowa.edu