Friday, December 21, 2007

Java and higher order generics

Funny coincidence: yesterday in work I was trying to find a way to express something simple enough in Java: I want a method map that takes a generic collection and return a collection of the same type (that is, passing a List should return a List, passing a Set should return a Set) with a different generic type. In Java you can have:

public <T1,T2> Collection<T2> map(Collection<T1> c,Mapper<T1,T2>

Where Mapper is an interface defining the method T2 map(T1 o);
There is no way I think in Java to express that if you pass a subclass of collection you get the same subclass back.

I tried things like <T1,T2,C extends Collection,C1 extends C
<T1>,C2 extends C<T2> and other crazy things that my compiler didn't appreciate.

You have to duplicate methods, and that's what we've done in work:

public <T1,T2> List<T2> map(List<T1> c,Mapper<T1,T2>
public <T1,T2> Set<T2> map(Set<T1> c,Mapper<T1,T2>

And of course a colleague needed a Vector because he was using an old library still using Vectors...
And then today I found that blog post, that point to that PDF document. Apparently you can do this kind of things in Scala and Haskell... Do I need to convince my company to move to Scala?

Friday, November 30, 2007

Haskell design patterns are (probably) needed

I was reading that post, and thinking: well when I start a Haskell project (for fun, yes, I don't use Haskell in work) I do wonder how best to design it. Yes I know about higher order functions, generics and such, but I don't think these low level constructs are enough to express, on their own, the design of the system as the design patterns from the GoF do. I would love to see patterns on how to allow extensibility, separation of concerns and such in Haskell. Years of experience in Java mean that I see a lot of things in terms of objects, inheritance vs composition, interfaces, etc, and not all of that apply in Haskell. I see a lot of low level algorithms in Haskell, but few software design papers (papers like this one are useful, we need more of them).

Monday, November 26, 2007

Structure of a functional Java, er, method

A method in FLOJ (remember, the functional Java based language I'm toying with) has a simple structure:
- a method definition 100% similar to a normal Java method: visibility generic return-type type parameters.
There are no exceptions, of course, since exceptions are things Best Done in Monads, once I've figured out
out I'll implement monads.
- a series of assignments of the form x=e where x is a variable name, and e an expression
- a return expression

For example:

public static String[] testMap(){

Since the return expression is always the last expression, there is no need for a return keyword, but it's accepted by the parser. In the exemple above, the method returns the list of String TOTO and TITI.
Note also that the assignments do not allow nor require the type definition of the left hand side of the assignment. The type is inferred from the
return type of the right hand side expression (in the exemple above, a is a list of String). It's not as powerful as the full blown Haskell type inference but at least it saves some typing.

Of course, the generated Java code will add a sneaky final keyword to all the assignments, to enforce the fact that there's no variables in a functional language.

All in all, that structure makes the code easy to read and to understand: the sequence of calculations and the final expression yielding the method result.

I don't think my FLOJ language will ever be a full blown industrial-strength language, but I know that by implementing my own functional language I will learn a lot
more about functional languages than by just using one. Just implementing method pointers, type inference, combinator parsers and such, I understand more the why and how of Haskell.

Friday, October 26, 2007

A simple functional structure in Java

I have progressed a little bit in creating a functional language on top of Java, code name FLOJ (Functional Language On Java (-:). For the moment I'm only working on single files (I have a working parser), on the simple language constructs.
It's really interesting, because looking at very simple things make you think in depth about some issues that you may not really consider in Java proper. For example, this is the definition of a very simple structure:


public class Simple(String data,int port){


So you can see that the package system is Java's and class definition are very similar. There are no constructor, so. Since null are not allowed (nulls are a huge source of bugs, I'll provide a Maybe object or something like that later),
the structure must have all its data filled in. If you want different operations to be done, you just create static functions that take parameters and create what you need, no need for several constructors. So the list of fields is entered straight after the name of the class.
Simple is a structure with a string and an int. It's easy enough to generate the constructor in Java:


public class Simple{
public final String data;

public final int port;

public Simple(final String _data, final int _port){
if(_data==null){throw new NullPointerException("_data");};

The fields are immutable of course, so we don't bother with a getXXX() method, we just expose the field.
And you can then add setters that return a new object :

... public Simple setData(final String _data){
if(_data==null){throw new NullPointerException("_data");}
if (_data.equals(data)){return this;}
return new,port);

public Simple setPort(final int _port){
if (_port==port){return this;}
return new,_port);

Note the null handling is not ideal, left for later. Note also we avoid superfluous creations if the data will not change.

So far, so good.

Then I decided to implement equals, so that equals is based on the data and not pointer equality:

public boolean equals(final java.lang.Object o){
if (this==o){return true;}
if (o instanceof{ co=(;
if (!{return false;}
if (this.port!=co.port){return false;}
return true;
return false;

And with this of course the problem of overloaded equals method raises its head. If I want to allow subclasses to add more fields, my equals method won't work for subclasses.
If I check the exact class name in equals, subclasses that only add/overload methods will not be equals to superclasses.
So what can I do? I haven't decided. I could forbid subclasses to define more data fields (that's I think what you have in Haskell: you inherit functions from your type classes but no data).
I could also make the equals method final and not generate it for subclasses, but equality would then ignore the additional fields.
Another option would be to check the class name in that equals method and generate the proper equals method for all subclasses, but that may cause problems if we then extends the FLOJ class by a Java Class (which will be possible since we generate Java code).
But maybe it should be so, that a class is never equals to its subclasses? Returning equals between two instances that have the same data but different behavior (one instance has an overloaded method) is probably an error in the OO world...

Anyway, creating your own language is fun, I'm learning a lot and maybe I'll even end up with something usable!! I'll keep you posted on my progress.

Thursday, October 11, 2007

Generator (with yield) in Java

For some reason I was thinking about generators and the yield function (as found in Python). All this functional talk about infinite lists and such I suppose... So I had a go at implementing one in Java, which tied in nicely with my current reflexions about a java-based functional language and the need to provide concurrency out of the box. And another occasion to dabble with wait and notify in Java! After I've written it I found a similar looking implementation (but without the standard Iterator/Iterable interface support) and something on Google code that use bytecode intrumentation (why oh why). But still I like mine, that you can find here (and the unit test is here).
Basically the work is done in another thread and we call wait() when we do a yield. We conform to the Iterator and the Iterable interfaces, and we provide a stop method to actually stop the thread (say for infinite generators). Otherwise if the generator has not finished the thread goes on waiting forever. I've thought about trying to stop the thread when the generator is not used, but I don't know if it even possible with things like SoftReferences (The thread having a pointer to its runnable...).

Tuesday, October 09, 2007

My own Java-based functional language

I've been toying for a while with the idea of creating my own Java based functional language, if only to be able to see first hand the challenges in creating a functional language. The language I envision has the following characteristics:
- syntax close to Java: contrary to JHaskell or CAL, I want something that looks like Java, not something totally different
- Java type system: use the normal Java OO type system: classes with at most one superclass, interfaces. Maybe I'll run into issues but for the moment...
- pure functional and monadic: what I love in Haskell is the clear delimitation between what's purely functional and what's subject to side effects (IO, etc...). I'd like to have that in my language. I know about Scala, but Scala lets you mix functional and imperative code, I want something pure (and I don't like to add new very un-Java like keywords like def)!
- Java integration: there is no point of being implemented in Java if we can't reuse the wealth of Java code and libraries out there. Probably I'll need a way to distinguish somehow functional Java code (like the String methods) and not functional code, and wrap access to non functional code in a monad.

Now that's an ambitious program!! I've started some parsing and conversion to Java source files (hey, if Java can do some of the type checking for me...), but it's going to be a long road...
Of course, if such a language already exists, I'd be delighted to hear about it!

Wednesday, September 19, 2007

Parser Combinator in Java

Pffuii, I'm quite busy these days, hence the posting frequency has dropped. I'm in the process of playing with a functional Java based language (more on that in later posts, hopefully) and I decided to have a go at a little parser combinator in Java.
The end result works (I can parse things like Java class declaration) and looks a lot like parsec, but there is a lot of Java cruft that I'd like to remove, things that are hidden with the wonderful magic (I don't understand it well enough to call it otherwise) of Haskell monads.
For example:

try {
Character c=new Try(new Letter()).parse(input);
} catch (JCParseException jpe){

This code tries to match a letter in the input at its current position (the input variable) and does not consume any input if it fails. I don't like having to explicitly tell the combinator to parse the input, and I don't like using exceptions to indicate a parsing failure. Now, anybody has any great ideas?

Friday, August 10, 2007

The beauty of Haskell Combinators

Pffiuu, it's been a while since I've posted here, but don't think I've been idle. In addition to my day job, I'm still hacking away at Haskell. And still loving it!! I just wanted today to express my pleasure (yes, pleasure) at working with some cool APIs and libraries that are out there.
I'm doing a bit of research into language parsing and I'm using Parsec for parsing and the HughesPJ Pretty Printing library (included with GHC) to dump the parsed AST and check that I can output what I parse and reparse it into a equivalent structure. I have to say, working with combinators (monadic or otherwise) is actually fun and easy. I've developed code parser before, for example using some Java parser generators, but coding your parser directly in your favorite functional language is really sweet. And of course, I like the way that similar concepts are expressed the same in the two libraries, even down to the function names: parens() in Parsec will parse something surrounded but parentheses and parens() in HughesPJ will write something in between parenthesis.
I had also a exhilarating ah ah moment when I discovered in the Parsec doc makeTokenParser that suddenly made it so easy to accommodate with spaces and pesky littls things of this caliber.
Thanks to all contributors to these libraries for making my functional programming evenings fun!

Friday, June 01, 2007

A very dumb neural network in Haskell

Continuing my implementations of Michael Negnevitsky book on Artifical Intelligence in Haskell, I have tried to code a three layer neural network to implement exclusive or (Intelligence indeed...).

A neuron is made of its input weight and its threshold, and its output is calculated thus (apologies to all of you who gave me some tips about parameter order and newtype declaration, I haven't changed the code yet to follow your good advice!):

type Neuron= ([Value],Value)
type Value = Float

neuronOutput :: [Value] -> Neuron -> Value
neuronOutput inputs (weights,threshold) =
tot=(foldl (+) 0 (zipWith (*) inputs weights)) - threshold
1 / (1 + ((2.7182) ** (-tot)))

A three layer network whose input layer does nothing but fire its input to the midle layer can be modelled by to lists of neurons, and I use a TestCase type to keep track of inputs and expected outputs
type Network = ([Neuron],[Neuron])
type TestCase = ([Value],[Value])

Running the whole network with some input values is easy:

exec :: Network -> [Value] -> ([Value],[Value])
exec (hidden,output) inputs =
hiddenOutputs = map (neuronOutput inputs) hidden
outputOutputs = map (neuronOutput hiddenOutputs) output

The meat is all in one function, that calculates the output of the network for a given test and adapts the weights accordingly, first for the output layer then for the hidden layer:


step :: (Network,Value) -> TestCase -> (Network,Value)
step ((hidden,output),err) (inputs,expected)=
(hiddenOutputs,outputOutputs)=exec (hidden,output) inputs
errorOutputs= zipWith (-) expected outputOutputs
gradientOutputs=zipWith (\o d->o*(1-o)*d) outputOutputs errorOutputs
newOutputs=zipWith (\(ws,t) g -> ( zipWith (\h w->w+ (defaultLearningRate*h*g)) hiddenOutputs ws ,t+(defaultLearningRate*g*(-1)))) output gradientOutputs
weightsByHidden= transpose (map fst output)
gradientHidden =zipWith (\o ws-> o * (1-o) * (foldl (+) 0 (zipWith (*) ws gradientOutputs))) hiddenOutputs weightsByHidden
newHidden=zipWith (\(ws,t) g -> ( zipWith (\h w->w+ (defaultLearningRate*h*g)) inputs ws ,t+(defaultLearningRate*g*(-1)))) hidden gradientHidden
newErr=err + (foldl (+) 0 (map (^2) errorOutputs))
in ((newHidden,newOutputs),newErr)

The result is a new adapted network and the sum of squared errors.

An epoch is a set of tests:

epoch :: Network -> [TestCase] -> (Network,Value)
epoch network= foldl step (network,0)

We can then repeat epoch till the sum of squared errors for an epoch is less than the threshold for convergence:


run :: Network -> [TestCase] -> Int -> (Network,Int,Value)
run network allInputs epochNb=
let (newNetwork,delta) = epoch network allInputs
in (?) (delta <= errorConvergence || epochNb>225)
(newNetwork, epochNb,delta)
(run newNetwork allInputs (epochNb+1))

(?) :: Bool -> a -> a -> a
(?) True a _ = a
(?) False _ a = a

(Is there a standard function for (?)? I got tired of if then else...)

Then I test my code with the same start network as in the book:

test = do
let n = ([([0.5,0.4],0.8),([0.9,1.0],-0.1)],[([-1.2,1.1],0.3)])
let (n',e,err) = run n [([1,1],[0]),([0,1],[1]),([1,0],[1]),([0,0],[0])] 1
print (n',e,err)
return ()

And I get a mixed feeling about the result: the network does converge, and then it does work, but after something like 54000 epochs, while in the book it supposedly take 224! The book gives the result for the first test and I'm getting exactly the same thing, but it looks like my network learns a lot slower than it should. I would suppose I have a subtle error somewhere, but for the life of me I can't find it... If it jumps at anybody, please let me know. My hope now is that when refactoring the code and following the books advice on improving the performance I will find the bug!!

I have tried with random networks:

network :: Int -> Int -> Int-> IO Network
network inputNb hiddenNb outputNb= do
a<-randomNeurons inputNb hiddenNb
b<-randomNeurons hiddenNb outputNb
return (a,b)

randomNeurons:: Int -> Int -> IO [Neuron]
randomNeurons nbInput nbNeurons= replicateM nbNeurons (randomNeuron nbInput)

randomNeuron:: Int -> IO Neuron
randomNeuron nb= do
w<-replicateM nb (randomWeight nb)
t<-randomWeight nb
return (w,t)

randomWeight:: Int -> IO Value
randomWeight nbInput= do
let interval= randomR ((-2.4)/(fromIntegral nbInput),2.4/(fromIntegral nbInput))
getStdRandom interval

But I don't get better results...

Friday, May 25, 2007

A perceptron in Haskell

After reading Michael Negnevitsky book on Artifical Intelligence, I started playing with some of the algorithms he gives in Haskell. So my first endeavour is writing a simple perceptron (a one neuron neural network) to perform AND or OR logical operations. Next I'll start a full multi layer network to do much more complex operations like exclusive OR (-:.
Once again I was struck with the simplicity of the Haskell code necessary to implement such functionality (and I'm no Haskell expert yet). You think about how you going to implement it, the type checker tells you when you're doing something stupid, and by the time everything is written and compiles it kinda works.
The core method is calculating the output of the neuron, given the inputs, the weight of each input and the threshold value for that neuron:

neuronOutput :: [Float] -> [Float] -> Float -> Float
neuronOutput inputs weights threshold =
tot=(foldl (+) 0 (zipWith (*) inputs weights)) - threshold
if tot >= 0
then 1
else 0

Using zipWith we multiply each input with its weigth, calculate the sum with fold and remove the threshold. A negative result yields 0, else it yields 1.

Adjusting the weights to adapt to the error of a test case is no more difficult:


adjustWeights :: [Float] -> [Float] -> Float -> Float -> [Float]
adjustWeights inputs origWeights expected actual =
e=expected - actual
delta (i,w) = w + (defaultLearningRate * i * e)
map delta (zip inputs origWeights)

We modify the weight by applying to each weight a delta that is dependant on the input weight, the error ratio and the learning rat

A single step is made of calculating the output of the neuron and adapting the weights, given the expected value:


step :: [Float] -> [Float] -> Float -> [Float]
step inputs weights expected=
o = neuronOutput inputs weights defaultThreshold
adjustWeights inputs weights expected o

Then an epoch applies the step function to each different input sets. The hardest part is calculating the average of all deltas to find if the weights have changed at all during the evaluation of all the test cases

epoch :: [([Float],Float)] -> [Float] -> ([Float],Float)
epoch allInputs weights=
f w (inputs,expected) = step inputs w expected
newWeights=foldl f weights allInputs
delta= (foldl (+) 0 (map abs (zipWith (-) newWeights weights))) / (fromIntegral $ length weights)
in (newWeights,delta)

A training run is only launching the epoch function repeatedly till the delta is zero:

run :: [([Float],Float)] -> [Float] -> Int -> ([Float],Int)
run allInputs weights epochNb=
let (newWeights,delta) = epoch allInputs weights
in if delta == 0
(newWeights, epochNb)
run allInputs newWeights (epochNb+1)

We generate the initial weights in the IO monad, passing the number of weights needed:

initialWeights :: Int -> IO [Float]
initialWeights nb = do
let interval= randomR (-0.5,0.5)
(replicateM nb (getStdRandom interval))

Putting it all together:

test :: IO()
test = do
w<-initialWeights 2
let (ws,i)=run [([0,0],0),([0,1],0),([1,0],0),([1,1],1)] w 1
print (ws,i)
return ()

test find out the proper weights to implement AND and print them out, with the number of epochs needed to reach the result. You can then call neuronOutput with the resulting weights and your input: you have a AND logical gate!

As usual, any pointer to how to improve the code is welcome!

Wednesday, May 16, 2007

Javascript and Hibernate, hand in hand

Some time ago I had written a little prototype that was quite interesting, but I never pursued it. Maybe I should try to get some people interested and develop it further...
The idea was to use Rhino to have a Javascript-based framework on the server, thus using the same language on the server and on the client, like ASP did in the good old days. I was also thinking of having the source code for custom applications stored in the database, instead of having it in source files, but that's another story. That explains why the examples below are about "applications" and "components"
The first component is Hibernate connectivity. My framework lets you:
1. Dynamically configure Hibernate mapping for Javascript defined classes. For example, if you wanted to define a Web Application with several attributes and a reference to the components that it defines:

function Application(){

Note that storage() is a function of the Function object which takes a Javascript structure that mimicks the Hibernate XML.

2. Access Hibernate functionality through a "hibernate" host object. For example, a JSUnit method testing the addition of a new "Application" object:

var appId;
var appUrlName;

function testAdd(){
var app=new Application();
app.displayName="TestAppJS"+new Date();

The hibernate object lets you list objects, get a particular object, modify or create instances, etc. I have implemented a Hibernate EntityTuplizer and an subclass of Rhino's NativeArray that encapsulate a PersistentList, so that multiples references can be seen as Javascript arrays.

3. Query the database either through the HQL language:

var apps=hibernate.query("from Application where urlName=?",["TestAppJS"]);

or through a pure Javascript function based mechanism:

var apps=hibernate.query("Application",function(app){
return app.urlName=="TestAppJS";}

That I think is very cool. In Rhino I can access the source code of the function that is passed as the second argument to query, and transform it into HQL. I haven't implemented as much as I could, of course, but it works with simple queries. At least there is no other syntax to learn to do a query, it's basically the equivalent of a filter function in functional language.

Then I had started to develop a web framework (probably the millioneth Java web framework by now) with Javascript objects to generate HTML and a mapping between urls and javascript functions, and where one URL could chain several functions, but I haven't gone very far.

Now if somebody feels there's some value in any of that I can sure post the code and we can move it forward. Anything that can show that Javascript is a cool language to run in a JVM in a worthwhile enterprise (-:

Friday, May 11, 2007

Poor man's JavaFX Script is, er... Javascript

Sun has release its own RIA framework called Java FX. It provide JavaFX Script, which is a language based on Java whose goal is to develop quickly graphical applications with the power of Java. Is it touted as competition to Flex that I have mentioned before. It is safe to say that it hasn't been received with a great deal of enthusiasm. The fact is, I don't want to learn another language again to do my Web 2.0 application. And I do believe that Fielding has a point when he reflects that applets break visibility and raise the barrier of entry for web developers. Also, I'm usually all in favor of static typing and compilation, but for GUIs I'm not too sure. When you want to tweak your borders or your layouts you probably happy enough with a quick cycle of change code/refresh page. So why don't we use Javascript and Java Applets together instead of inventing yet another technology?
Ok, it's time you get over your irrational distrust of Javascript. A multi paradigm (object, functional) language available for free in every browser!! And no, I don't suggest we go on using the HTML DOM or things like that. That's why we need applets.
Applets? I must be mad! Applets have not worked on the web. But what I suggest is not to develop one applet for each RIA application, but only develop one Applet that everybody could use, much like a plugin, that would bridge the gap between the JavaScript world and the Java/Swing world. If it has been done, please let me know. As for me, I've put together a prototype in an hour that works fine:
  1. The JavaScript code constructs an object tree that represents the GUI.
  2. The object tree is serialized to JSON and passed on to the applet
  3. The applet deserializes the JSON and constructs a Swing GUI by translating the object tree
  4. The object tree can contain handlers that are the names of Javascript functions
  5. The applet calls the javascript functions on Swing events
  6. The JavaScript event handlers update the GUI tree and notify the applet with the changes.
A very simple example:


<script language="Javascript">

var helloButton={type:"button","id":"helloButton","text":"hello","action":onClickHello}
var frame={"contents":

function onClickHello(){

function startJS(){
var msg=document.getElementById("JSApplet").setContent(toJSON(frame));
if (msg){

function updateJS(objs){
var msg=document.getElementById("JSApplet").updateContent(toJSON(objs));
if (msg){

toJSON=function(arg) {

switch (typeof arg) {
case 'object':
if (arg) {
if (arg.constructor == Array) {
var o = '';
for (var i = 0; i < arg.length; ++i) {
var v = toJSON(arg[i]);
if (o) {
o += ',';
if (v != null) {
o += v;
} else {
o += 'null,';
return '[' + o + ']';
} else if (typeof arg.toString != 'undefined') {
var o = '';
for (var i in arg) {
var v = toJSON(arg[i]);
if (v != null) {
if (o) {
o += ',';
o += toJSON(i) + ':' + v;
return '{' + o + '}';
} else {
return 'null';
case 'unknown':
case 'undefined':
case 'function':
case 'string':
return '"' + arg.replace(/(["\\])/g, '\\$1') + '"';
return String(arg);

<body onload="startJS();">
<applet id="JSApplet" alt="JSApplet" code="fr/moresmau/jp/jsapplet/JSApplet.class" codebase="bin" mayscript="mayscript" height="100" width="100"></applet>


This web page creates a simple "Hello" button that when clicked changed its text to upperCase(). It demonstrate the two ways communication between the Javascript code and the applet (note how the function is serialized in JSON just with its name, that's the only thing we need). So the Javascript can deal with a model that is a slightly abstracted Swing model (no more hair pulling between different browsers). I stress again, the applet is a generic applet that deals with the link between the JSON objects and the Swing layer. Of course the applet could offer other services: simple API to call back the server (AJAX from the applet), etc...
Then the applet could be packaged with a cut down version of the JRE (only the classes really needed) as a plugin to do rich Swing application from JavaScript...
And voilĂ ! A scriptable RIA platform!

Friday, April 20, 2007

Haskell code, Java UI

It's been while since I've posted anything, mainly because work crises and holidays have kept me busy or away from my keyboard. I'm still playing with Haskell and user interfaces, I've now developed the same Maze game in HGL, Flex with a Haskell web back end, and WXHaskell. I've looked at trying to implement some declarative syntax that would simplify WXHaskell code but haven't gone very far, I think I'm trying to run in Haskell before I can walk straight!
Anyway, I was looking at Business Objects' Quark framework based on the CAL language. You can find more details here, but basically CAL is a Haskell-inspired impure functional language implemented in Java, that can use Java classes and methods. The impurity comes from not using monads to access Java, if I understand well, which means that you can declare pure functional methods that in fact access Java objects, that can do all kind of stateful voodoo under the covers. While I'm not sure this is really A Good Thing, having Haskell and Java intertwined brings some interesting ideas in the realm of Haskell UIs. Swing and Java are now mature, reasonably fast and cross platform. If I can get my pure Haskell code to talk to a Java GUI, I could get quite happy indeed...

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.

Friday, February 02, 2007

Haskell User Interface???

It's been a long time since I've last posted, the fault to Christmas and holidays. In the meantime though I have done some more Haskell. I have used Prim's algorithms described here to generate a random maze. That forced me to use Random to generate the maze and HGL to display it, and allow the user to navigate from the entrance to the exit with direction keys. Works great, but I was looking at Haskell UIs to see what I could use, and there doesn't seem to be a consensus on a functional GUI library in Haskell. HGL is fairly low level and limited, wxwidgets ports have an imperative air about them and some of the ground breaking frameworks still seem to be experimental (and to be honest, I don't think I'm ready to deal with arrow-based frameworks yet, I'm still struggling with the monads...). So maybe the solution is elsewhere? I've seen in the Java world people talk about Flex, has anybody done anything integrating a Flex front end with a Haskell back-end?