Scala for Sling @ Jazzon 09

26 06 2009

Yesterday I gave a presentation at Jazoon 09 about using Scala for scripting RESTful web applications with Apache Sling.

In the session I showed how to take advantage of Scala to create RESTful web applications with Apache Sling. I demonstrated how to uses its DSL capability and support for XML literals to create type safe web site templates. In contrast to conventional web site template mechanisms (e.g. JSP), this does not rely on a pre-processor but rather uses pure Scala code.

There are Session slides and support material available here. The support material contains a fully workable demo application. A Scala scripting bundle for Sling is also included.

Puzzle: implement this (solution)

24 06 2009

Well, I wasn’t aware of Ticket #1737 when I was trying to find a solution to the problem from my previous post. Thanks to Jorge Ortiz for pointing this out. However, I reviewed my approach to solving this and didn’t find sever limitations. Maybe someone else does…

When I initially stumbled on this, I remembered that existential types where introduced into Scala for coping with Java’s raw types. But there is an additional twist here, we need to tell the compiler that our MyIterator implementation actually ‘is an instance of a raw type’. So combining existential types with self types led me to the following solution:

class MyIterator extends Iterator2 {
  this: java.util.Iterator[_] =>
  def hasNext = true
  def remove = throw new Error  
  def next = "infinity"   

We can now safely use instances of MyIterator.

  def test1(it: MyIterator) = {
  def test2(it: java.util.Iterator[_]) = {
  val it = new MyIterator
  val v: String =

The approach using existential types in combination with self types makes sure that values returned from the next method always are typed correctly.

Puzzle: implement this

19 06 2009

This is something I stumbled on recently when trying to implement javax.jcr.NodeIterator in Scala.

Assume you are using a library which exports an Iterator2 interface:

public interface Iterator2 extends java.util.Iterator {}

Note that Iterator is a raw type and Iterator2 does not take any type parameters. So how would you implement Iterator2 in Scala?

Here is a start:

class MyIterator extends Iterator2 {
  def hasNext = false
  def remove = throw new Error
  def next: Nothing = throw new Error

But if the next method should return an actual value, what would be it’s return type? It turn’s out that any other type than Nothing results in a compiler error:

error overriding method next in trait Iterator of type ()E;
method next has incompatible type ()Any

So how would you implement Iterator2?