The JavaFX part
For the JavaFX part of the app, which is of course the UI part, I used Netbeans. It simply consists of a textarea for the input, another textarea for the output and a toolbar to execute the input or clear the input.
Admittedly, the UI could be much nicer, especially with JavaFX. But as said, this was just an experiment and I'm not quite a UI guy.
One thing that struck me, was that JavaFX did not have a textarea component. At least at the time I was building the application. I can't say, if there is one today. Fortunately, I found a blog post on the web (but I can't find it anymore), where the author describes how to wrap a Swing textarea into a JavaFX component. This worked quite nice. Adding some JavaFX binding and some event handling functions to the buttons, and the UI part was working. Doing the design part with JavaFX was real fun.
The Scala Part
I implemented the interpreter part in Scala as a separate project in Eclipse. I also used Maven with maven-scala-plugin. I had some issues with the Eclipse Scala plugin, but the Maven plugin worked quite nice. Scala comes with an
Interpreterclass to interprete Scala code. It would be really simple to execute a whole script at once with the interpreter, but I wanted the application to interprete the code line by line, just as the the Scala console does that comes bundled with the Scala distribution. So, I used Scala's case class pattern matching do accomplish this, see the source code for more details.
Bringing it together
There's nothing special to bring the two parts together as everything runs on the JVM. I bundled the Scala part into a jar with Maven and added it together with the Scala jars to the JavaFX project. Instantiating the Scala classes from JavaFX is nothing more than calling
 Project (Source code and WebStart) at github