Friday, January 22, 2010

Continuations plugin for Scala 2.8 beta

The first Scala 2.8 beta is due to be released soon. Unfortunately the continuations code won't be merged in time for the beta. In the meantime if you want to try out the continuations features then you'll need to build the continuations compiler plugin and support library yourself. In preparation for the beta release I thought I'd post some updated instructions for building the continuations plugin.

Building the plugin

These instructions are based on a recent beta release candidate, but they should also work fine for the actual beta. The process for building the plugin is pretty much as I described in my previous post. The only real change is that you now need to download an additional JAR from the Maven repository, because the JAR is no longer included in the standard Scala distribution.

First download the Scala 2.8 beta. If you want to use a newer release then you'll of course need to update the URLs.

$ wget http://www.scala-lang.org/downloads/distrib/files/scala-2.8.0.Beta1-RC8.tgz
$ tar xzf scala-2.8.0.Beta1-RC8.tgz
$ wget http://scala-tools.org/repo-releases/org/scala-lang/scala-partest/2.8.0.Beta1-RC8/scala-partest-2.8.0.Beta1-RC8.jar -O scala-2.8.0.Beta1-RC8/lib/scala-partest.jar
$ export SCALA_HOME=`pwd`/scala-2.8.0.Beta1-RC8

Next get the source for the continuations plugin.

$ svn co http://lampsvn.epfl.ch/svn-repos/scala/compiler-plugins/continuations/trunk continuations

Then build it and run the tests.

$ cd continuations
$ ANT_OPTS=-Xmx512m ant test

Compiling and running a program that uses continuations

Set an environment variable to point to the continuations code.

$ export CONT_HOME=.../continuations

Here's a short example program that uses continuations.

$ cat > example.scala
import scala.continuations.ControlContext._

object Example {
  def main(args: Array[String]) {
    reset {
      println("first")
      shift { k: (Unit => Unit) => k(k(())) }
      println("last")
    }
  }
}

To compile it you'll need to load the continuations plugin into the compiler.

$ $SCALA_HOME/bin/scalac -Xplugin:$CONT_HOME/build/pack/selectivecps-plugin.jar -classpath $CONT_HOME/build/build.library example.scala

To run it just make sure you include the continuations runtime library.

$ $SCALA_HOME/bin/scala -classpath $CONT_HOME/build/build.library:. Example
first
last
last

Enjoy!

No comments: