Monday, December 04, 2006


What is it ? well, Pointillism is a style of painting I quite like, but Pointillist is simply the (working) name of a small graph library I'm working on. The previous post was discussing about the simulation software I wrote, which uses a simple graph view. I decided to clean up a bit that view, put it in a framework and commit it somewhere (likely on the étoilé repository). Not quite done yet, but it's shaping well, and it's now quite a bit more generic:

One of my "I will do it one day" project is a simple graph calculator... So in order to properly test the graph framework, I created a class using the StepTalk scripting framework to express functions (written in Smalltalk), which actually makes me rather close to have such a program :-P

for the curious, the code of the functions is:

Red function:

y := -0.5.
((x < 0.5) and: (x > -0.5)) ifTrue: [y := 1].
(x < -1) ifTrue: [ y := 0.25].
( x < -1.5) ifTrue: [ y := -2 ].
(x > 1.5) ifTrue: [ y := 2].
y := y + 1.5.

Green function:
x sin - 1 / x

Blue function:

|y mod|
y := -1.
mod := x mod: 2.
(mod = 0) ifTrue: [ y := 1 ].
y := y - 2.
^ y

StepTalk is really cool btw -- Easy to integrate, and works both on OSX and GNUstep. Here is the code I use here:

id environment = [STEnvironment environmentWithDefaultDescription];
id conversation = [STConversation conversationWithEnvironment: environment
language: @"Smalltalk"];


id number = [NSNumber numberWithFloat: x];
[environment setObject: number forName: @"x"];
[conversation interpretScript: @"^ (x tan) / (x atan)"];
id result = [conversation result];

[values addObject: result];

values beeing a NSMutableArray containing the numbers returned by the steptalk function (the ^ in smalltalk is equivalent to a "return" statement in C). As you can see, difficult to have a scripting framework simpler than that to work with!

You can see that in the environment object I put an "external" (to the script) object, number, and this object can be accessed from the script using the given name (here, x). You could, instead of getting the script result, get the value of specific objects you put in the environment.

Anyway, it's very simple to add StepTalk support to your program, and recent versions even add nifty UI widgets to play with scripts.. also, StepTalk is not "just" a Smalltalk scripting framework : it can actually use other languages. On GNUstep for instance there's an Io bundle, so you could use Io instead of Smalltalk, etc. (it's actually fairly easy to add languages to StepTalk).

Note also that StepTalk automatically bridge the numbers in the script to actual NSNumber instances... which is how I decided to implement the math functions, and this actually demonstrate a very cool Objective-C feature; NSNumber doesn't have thoses sin,cos,mod.. math functions; so in another programming language we'd be stuck. A "normal" solution could be to modify StepTalk so that it would use something else than NSNumber (say, a custom class of ours), or reversely, to add those functions to NSNumber and recompile -- wait! you could do that with GNUstep (because we have the source), but certainly not with Cocoa on OSX. So how comes it works ?

Well, Objective-C has this great feature: categories. A Category lets you add new code to an existing class, at runtime. So that's simply what I did here -- I created a category on NSNumber with my specific math functions. Without needing to have access to the NSNumber source, or to recompile Foundation. Isn't it great ?

1 comment:

Anonymous said...

Is this going to compete with Narrative and GraphX?