Tree iterator with continuations?

dir.listFiles foreach { f => ... } is indeed the problem here. Capturing a continuation via shift requires that the call path up to the closest surrounding reset consists of cps-transformed methods. The standard foreach, map and friends, however, know nothing about continuations, so a function passed to foreach cannot simply interrupt the iteration by calling shift (issuing a compiler error in this case is on my to-do list).

The easiest way to achieve the desired behavior is to import scala.continuations.Loops._ and then use (in your case) dir.listFiles.toList.suspendable foreach { f => ... }. Making .suspendable work for all Iterables is another item on my to-do list, but for the moment the toList is required.

scala.continuations.Loops also contains a method loopWhile that you can use as a replacement for regular while loops.

Hope this helps,
- Tiark

On 24.11.2009, at 19:56, Cay Horstmann wrote:

> I am trying to get a basic tree iterator example to work with Scala
> continuations, and I can't figure out why it isn't working. Here is my
> code, stripped down to the essentials:
>
> import scala.continuations._
> import scala.continuations.ControlContext._
> import java.io._
>
> object Example {
> var k1 : (Unit => Unit) = null
>
> def processFile(f : File) : Unit @cps[Unit, Unit] = {
> println("Processing " + f)
> shift { k : (Unit => Unit) => {
> println("In shift")
> k1 = k
> }
> }
> }
>
> def processDirectory(dir : File) : Unit @cps[Unit, Unit] = {
> println("Entering " + dir)
> dir.listFiles foreach { f =>
> if (f.isDirectory)
> processDirectory(f) else
> processFile(f)
> }
> }
>
> def main(args : Array[String]) {
> reset {
> // processFile(new File("."))
> processDirectory(new File("."))
> }
> k1()
> }
> }
>
> If you call processFile in main, "Processing..." is printed then the
> shift is entered and the message "In shift" is printed.
>
> But if you instead run processDirectory, which calls processFile, then
> "Processing..." is printed, but then the shift is not entered.
>
> Why would it make a difference that processFile is called from another
> method? Ok, from a foreach inside another method?
>
> I can't figure out for my life why that would be so. It must have to
> do with the CPS transform, but it is entirely opaque to me what is
> going on, and more importantly, what I am supposed to do if I want to
> capture the continuation in a loop so that I can later get back to it.
> (When I replace the foreach with a while, I get a cryptic comment
> about not being in a cps context.)
>
> Previously, I used the tree iterator example in Scheme and ML. But my
> students already know Scala, so I would be very grateful for any help
> getting this to work in Scala.
>
> Thanks,
>
> Cay

Thanks very much--that helps a lot!!! Now it works like I thought it
would. Is there any documentation besides
http://www.scala-lang.org/node/2096 and your ICFP'09 paper?

And yes, a compiler warning would be much appreciated :-)

Cheers,

Cay

Glad it works ;-) Regardings docs, here are some blog posts that might be helpful:

http://blog.richdougherty.com/2009/02/delimited-continuations-in-scala_2...
http://blog.richdougherty.com/2009/03/goto-in-scala.html

http://ahardcodersday.blogspot.com/2009/08/control-flows-in-heaven-with-...
http://ahardcodersday.blogspot.com/2009/08/control-flows-in-heaven-with-...

http://dcsobral.blogspot.com/2009/07/delimited-continuations-explained-i...

http://stackoverflow.com/questions/1512930/what-are-scala-continuations-...

Also, be sure to have a look at the code examples:

http://lampsvn.epfl.ch/trac/scala/browser/compiler-plugins/continuations...

Besides the paper and what's in svn, there is currently no other "official" documentation.

Cheers,
- Tiark

On 25.11.2009, at 01:17, Cay Horstmann wrote:

> Thanks very much--that helps a lot!!! Now it works like I thought it
> would. Is there any documentation besides
> http://www.scala-lang.org/node/2096 and your ICFP'09 paper?
>
> And yes, a compiler warning would be much appreciated :-)
>
> Cheers,
>
> Cay

Thanks very much! That's very helpful. I have another issue that I
can't wrap my head around. I made an example with polling a mouse

 var k1 : (Point => Unit) = null
 var p1 : Point = null

 def getPoint() : Point = {
   val sema = new Semaphore(0)
   reset {
     p1 = shift { k : (Point => Unit) => k1 = k; sema.acquire() }
     sema.release()
   }
   p1
 }

where the mouse listener calls k1(event.getPoint). It all works fine.
But when I move

 var p1 : Point = null

inside getPoint, then I get a nasty error

error: type mismatch;
 found   : java.awt.Point @scala.continuations.docps
@scala.continuations.cps[Unit,Unit]
 required: java.awt.Point @scala.continuations.cps[java.awt.Point,Unit]
     shift { k : (Point => Unit) => k1 = k; sema.acquire() }
           ^
When I tag the getPoint method with @cps[Point, Unit], I get this error:

error: illegal answer type modification:
scala.continuations.cps[java.awt.Point,Unit] andThen
scala.continuations.cps[java.awt.Point,Unit]
 def main(args : Array[String]) {

Is there a way of returning the result of the shift without resorting
to a global variable?

Thanks,

Cay