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callCC in 2.7.0

12 messages · Hadley Wickham, Gabor Grothendieck, Luke Tierney +1 more

Original
Gabor Grothendieck
# 
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?
Hadley Wickham
# 
It facilitates a completely different style of programming - see
http://en.wikipedia.org/wiki/Continuation-passing_style

--
http://had.co.nz/
Gabor Grothendieck
# 
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.
On Sun, Mar 30, 2008 at 12:04 PM, <h.wickham at gmail.com> wrote:
Gabor Grothendieck
# 
Also in trying it out again it seems that its not like
on.exit but more like return:

F <- function(f) { f(10); print(2); f(20); 3}
callCC(F)

acts the same as:

F <- function() { return(10); print(2); f(20); 3}
F()

and there is no documented way to restart F
at the point it left off so I assume it can't.

On Sun, Mar 30, 2008 at 12:34 PM, Gabor Grothendieck
<ggrothendieck at gmail.com> wrote:
Luke Tierney
#
On Sun, 30 Mar 2008, Gabor Grothendieck wrote:

            

      
      
      
    

        
Yes -- if you can point out what in the documentation ever gave the
idea it might be like on.exit then we can fix the documentation.

            

      
      
      
    

        
The documentation descriped this as a downward-only version -- that is
standard terminology for a callCC that produces continuations that are
no longer valid after the callCC call exits.

Not sure why your original question was about "callCC in 2.7.0" -- I
believe callCC was added in 2.5.0, the same time as codetools became
recommended, and hasn't changed since.  It is not a function every R
user needs to have in their repertoire, but it can be very useful in
some situations.

luke

            

      
      
      
    

        

      
      
    

        

  
    

      
      
    

  


  


      

    

    

      
      

        


  

        

      Gabor Grothendieck
    

    

      
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I came across this in googling for continuations and was surprised when
I found it in R 2.7.0 and since I had not come across it before  I assumed
it was added just now.

Can you explain how its intended to be used with an example
that is more realistic than in the example section.

        
On Sun, Mar 30, 2008 at 1:14 PM, Luke Tierney <luke at stat.uiowa.edu> wrote:

            

      
      
      
    

        

      
      
    

            

      
      
      
    

  
  


  


      

    

    

      
      

        


  

        

      Luke Tierney
    

    

      
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On Sun, 30 Mar 2008, Gabor Grothendieck wrote:

        

            

      
      
      
    

        
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

            

      
      
      
    

        

      
      
    

        

  
    

      
      
    

  


  


      

    

    

      
      

        


  

        

      Gabor Grothendieck
    

    

      
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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:

            

      
      
      
    

        

      
      
    

            

      
      
      
    

  
  


  


      

    

    

      
      

        


  

        

      Gabor Grothendieck
    

    

      
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Sorry it should be as follows:

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

Now, how do we transform that to use callCC?


On Sun, Mar 30, 2008 at 1:42 PM, Gabor Grothendieck

        
<ggrothendieck at gmail.com> wrote:

            

      
      
      
    

        

      
      
    

            

      
      
      
    

  
  


  


      

    

    

      
      

        


  

        

      Luke Tierney
    

    

      
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On Sun, 30 Mar 2008, Gabor Grothendieck wrote:

        

            

      
      
      
    

        
What makes you think callCC has anything to contribute here?

The Wikipedia article Hadley cited says that using CPS effectively is
difficult without tail call optimization, which R does not have and
can't, at least not without some restrictions, because the stack is
avialable via sys.xyz functions.  It does _not_ say that having
explicit continuations implies having tail call optimization.

If you want to, you can rewrite your fib in CPS, something like

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

and you can then use callCC to bridge between the nonCPS and CPS
world, e.g. with

     fib <- function(i) callCC(function(k) fibCPS(k, i, 0, 1))

This will grow the stack just like any other recursion in R.  A
(minor) difference compared to your original fib is that the final
exit happens in a single jump rather than a sequence of returns.

The point of adding the current downward-only callCC to R is to
provide a clean, lexically scoped non-local exit mechanism for exiting
from complex, possibly recursive function calls. Dylan provides this
as part of its 'block' construct (also as an exit or continuation
function); Common Lisp has 'block'/'return-from' constructs that use
lexically scoped block name symbols.

For a slightly more realistic example than the ones currently in the
help page: Here is some code that implements a simple binary tree data
structure, and a function that visits the nodes of the tree and calls
a specified function on the value in each node:

     mkTree <- function(value, left, right)
 	list(isLeaf = FALSE, value = value, left = left, right = right)
     mkLeaf <- function(value) list(isLeaf = TRUE, value = value)

     visit <- function(node, fun) {
 	if (node$isLeaf) fun(node$value)
 	else {
 	    visit(node$left, fun)
 	    visit(node$right, fun)
 	    fun(node$value)
 	}
     }

A simple example tree:

     x <- mkTree(1, mkTree(2, mkLeaf(3), mkLeaf(4)),
                    mkTree(5, mkLeaf(6), mkLeaf(7)))

You can use visit() to print out the node values with

     > visit(x, print)
     [1] 3
     [1] 4
     [1] 2
     [1] 6
     [1] 7
     [1] 5
     [1] 1
     >

If you want to use a function like visit() to traverse the tree but
want the traversal to stop when some condition is met then you can
either rewrite visit to allow for this, making it more complicated, or
you can use a non-local exit.  That is where callCC comes in.  To
print all values until you find one equal to 7 you can use

     > callCC(function(exit) {
     +     fun <- function(value) {
     +         if (value == 7) exit(NULL)
     +         else print(value)
     +     }
     +     visit(x, fun)
     + })
     [1] 3
     [1] 4
     [1] 2
     [1] 6
     NULL
     >

One can also imagine situations where this might be useful in a
function passed to an optimizer like optim.  Given that our 'break'
only breaks out of one loop level, callCC can also be useful for
breaking out of a set of nested loops.

I first wrote this particular version of callCC when prototyping the
tryCatch mechanism in pure R code where one needs a means to jump from
a point where a condition is signaled to the point where the handler
is established.  The current tryCatch implementation does things
differently because it needs to integrate with the error handling at
the C level. Currently I use callCC in the constantFold function in
codetools (which is why callCC was added when codetools became
recommended).  This use is similar to the tree example.

luke

            

      
      
      
    

        

      
      
    

            

      
      
      
    

        

  
    

      
      
    

  


  


      

    

    

      
      

        


  

        

      Gabor Grothendieck
    

    

      
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Thanks.  So its intended to jump straight out of deeply nested
calls without having to manage the unwinding.

        
On Sun, Mar 30, 2008 at 4:22 PM, Luke Tierney <luke at stat.uiowa.edu> wrote:

            

      
      
      
    

        

      
      
    

            

      
      
      
    

  
  


  


      

    

    

      
      

        


  

        

      f.jamitzky
    

    

      
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callcc is similar to the "yield" keyword in python and c#
it lets you define e.g. a generator of lists of numbers.

        
Gabor Grothendieck wrote: