Visual Constraint Rules

Visual rule-based languages have been used quite successfully to program graphical simulations. They all use rewrite rules, which have the often-mentioned advantage that a program can supposedly be extended simply by adding a few more rules. In practice however, the rules tend to depend on each other, and instead of just adding rules, existing rules need to be changed. Visual constraint rules combine ideas from rule-based programming and constraint programming to create declarative forward-chaining-like rules that can be used in a more modular way to support iterative programming. Libraries of visual descriptions can be built and reused to compose complex behavior, which makes exploring the space of possible descriptions of simulations easier. This is valuable for the intended educational use of Cartoonist, a visual programming environment to build simulations. Furthermore, constraint rules also provide a way to describe a variety of parallel behaviors that are important in simulations, yet are not supported by similar systems.