Friday, March 30, 2007

A Haskell GUI in Flex

In an earlier post I mentioned a little Haskell game I developed where you had to navigate in a maze. I used HGL as the GUI framework, and noted it wasn't very clear what was the functional GUI framework of choice for Haskell. So I decided to give Flex a go and to hook it up with my Haskell back-end. I'm quite please with the result, I have to say.
The setup is as follows:
1) I've implemented a very simple single-threaded HTTP server in Haskell, using as an inspiration code that I found here and there (using Parsec for parsing HTTP messages, etc.). The server can server static HTML and SWF files as well as having a dynamic component: you can register Haskell methods that take a JSON object as a parameter, some internal state and return a new JSON object and the new state (yes I know, there is a monad for that...). I used the code here for JSON in Haskell. I thus expose a method that generates a Maze and a method to move to a cell, validating that there is no wall in between the current cell and the target cell and checking to see if we reached the exit.
2) I use Flex and ActionScript to create the GUI. The compiled SWF file will be served by the Haskell server and will call it back through asynchronous calls. I was a bit taken aback when I realized that Flex doesn't come with JSON support out of the box, but luckily there's a library for it. So you connect via your browser to a page on the server that embeds the SWF, Flex ask for the maze data to the server and displays it, and let you navigate using the keyboards or the mouse.
And that's it! So I have a nice, platform independent GUI and functional business code. I was thinking, who needs a special GUI framework for the desktop when an application could be made easily with a very lightweight local HTTP server and a web based interface? I even thought that Flex was a bit too much: why do I need a GUI technology that requires compilation when I can do nice stuff with pure HTML/Javascript frameworks like Dojo? So I can use a UI technology that is purely for UIs, and a functional language with the goodies Haskell provide for all the back-end, computation-heavy work.
The code for Haskell is here (start from Framework.hs), the Flex code is here.

Friday, March 23, 2007

Dynamic class behaviour in Java

In a previous post, I was using annotations to generate method pointers in Java. The functions objects were generated in a separate class file. However, if you suppose that a modern IDE would be able to generate all that code for you when you create a method, you could have the function object in the same class file as the method code itself. Since Java allows you to have a field and a method with the same name, everything works fine.
In that previous post the function object referenced the actual method in the original class file. What if we did the opposite, that is to say, the actual code is in the function object, and the method merely delegates? Here is a trivial example:

public class StringHelperDynamic {

public String toUpper(String a){

public Function1 toUpper = new Function1 (this){
public java.lang.String run(java.lang.String a) {
return a.toUpperCase();


Here, we have a toUpper method that takes a String and return a String, and a toUpper function object that we can pass around, perform currying on, etc... the interesting twist of this architecture is that the toUpper field can be final or not. If not, we can then change it at runtime, and hence change the behaviour of the class at runtime, while still calling the same methods. Who needs a dynamic scripting language (-: ? To the silly example:

StringHelperDynamic me=new StringHelperDynamic();
me.toUpper=new Function1(me){
public java.lang.String run(java.lang.String a) {
return a.toLowerCase();


OK, breaking the contract of the method is NOT what this is about, but you get the gist of it. The code for the Function1 class can be found here.

Friday, March 16, 2007

VList in Java

Two days ago I was reading this article about LISP like lists implemented in Java,
which reminded me I had written an implementation of a VList in Java. It can be found
here, along with a unit test.I make no garantee this is the best or fastest implementation possible!

It implements: get(n), first (head in Haskell), rest() (tail in Haskell), size(), iterator() and map(Mapper), where Mapper is the mapping interface I mentioned earlier.

It's using Java Generics of course. Am I right in thinking there is no way with Java generics to get rid of the compilation warnings and of the necessity of passing the Class object in the code that creates a new array of the generic type?

Friday, March 09, 2007

Java properties with no language changes (oh no!)

There's been a lot of discussions on the web about Java and the support for bean properties. People would like to have a mechanism that relies less on java bean conventions (reflection on method names, find getters and setters) and more on explicit language constructs. The goal is to simplify bean and property handling, have stronger type checks, be able to reference the property and not its values, etc... Most proposals involve additions to the Java syntax. I don't want to add to the confusion, but I just played around with what we have now in Java 5.
What we could have is instead of defining classes directly with fields in them, we could have them reference Property objects instead. Property objects carry the type information and the value itself. An inheritance hierarchy can provide for read-only, write-only and read-write hierarchy. Using public final fields we can have an access syntax that is not the same as the current getter and setter methods, but pretty close.

public class PersonBean {
public final IFullProperty name=new NonNullProperty();

public final IReadProperty id;

public final IFullProperty email=new FullProperty(){
public void set(String value) {
if (value.indexOf('@')==-1){
throw new IllegalArgumentException("email does not contain @");

public PersonBean(int id,String name){; ReadOnlyProperty(id);

Of course, the generic madness means the type information has to be typed twice, but when generics syntax is cleaned up it'd be a lot more readable. This bean says that it has a changeable name property, a String that cannot be null. It has an id integer property that cannot be changed. Finally it has an email property that has ad hoc constraint checking. The Property interfaces and its sub interfaces define a get() and a set() method so accessing the fields becomes:

bean.getName() ->
bean.setName(name) ->

The property change listener can be attached to the property object so that the bean doesn't have to have any line of code related to listeners. Everything can be done via static methods:

static void addPropertyChangeListener(Object bean,PropertyChangeListener pcl);

to add a listener on all properties

static void addPropertyChangeListener(Object bean,PropertyChangeListener pcl,IProperty p);

to add a listener on a specific property. Note that since we're taking a property object, and not a property name for example we cannot register a listener on a non existing property. The property does not have a back link to its bean (to avoid disgracious constructors with this), so we need to have both the bean and the propert.

If anybody's interested, all the code with unit tests is available here.