Babelsberg/JS

Overview

Babelsberg/JS is a JavaScript implementation of the Babelsberg family of object constraint programming languages, which augments standard JavaScript with the ability to state constraints that should be automatically maintained. Here is a simple example:

z3 = new ServerZ3()
z3.reset()
a = lively.morphic.Morph.makeRectangle(0, 0, 10, 10)
b = lively.morphic.Morph.makeCircle(pt(0,0), 10)
always: {
    solver: z3
    a.getPosition().dist(b.getPosition()) == 200
}
a.getPosition() // lively.pt(0.5,-200.0)
b.getPosition() // lively.pt(0.0,0.0)

a.setPosition(pt(100, 100))
a.getPosition() // lively.pt(100.0,100.0)
b.getPosition() // lively.pt(99.9,300.0)

It allows the programmer to write constraints using the always primitive that you always want to hold using existing object-oriented abstractions. (The above example uses the methods getPosition and dist.) The system will then attempt to satisfy the constraints at the time they are asserted, and maintain them as further statements perturb the system. The extent to which the system is able to keep constraints satisfied depends on the solver that is used. This implementation provides Z3, DeltaBlue, and Cassowary.

We have used this to implement electrical simulations, a simulation of the Lively Engine, and some graphical layouting examples. The implementation is available to try at lively-web.org. At any given time it may be broken, though, because the code is changing fairly often.

Download

A JavaScript/LivelyKernel implementation of Babelsberg licensed under 3-clause BSD

Download minified version of Babelsberg/JS. (includes Prototype.js)

If you already use Prototype.js you can download a smaller version of Babelsberg/JS without Prototype.js.

Examples

A page with additional examples is at lively-kernel.org.

Publications

Tim Felgentreff, Alan Borning, and Robert Hirschfeld, “Babelsberg: Specifying and Solving Constraints on Object Behavior,” Journal of Object Technology 13:4, 2014 This paper describes the general Babelsberg framework and the Ruby implementation, but is a good starting point.
Tim Felgentreff, Alan Borning, Jens Lincke, Robert Hirschfeld, Yoshiki Ohshima, Bert Freudenberg, and Robert Krahn, “Babelsberg/JS - A Browser-based Implementation of an Object Constraint Language,” European Conference on Object-oriented Programming (ECOOP), 2014.
Tim Felgentreff, Todd Millstein, and Alan Borning, “Developing a Formal Semantics for Babelsberg: A Step-by-Step Approach,” Technical Report, Viewpoints Research Institute, TR-2014-002a, 2015.

Documentation

API Reference

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Babelsberg-js