VISUAL CONSTRAINT RULES

Visual rule-based languages have been used quite successfully to progr am graphical simulations. They all use rewrite rules, which have the o ften-mentioned advantage that a program can supposedly be extended sim ply by adding a few more rules. In practice, however, the rules tend t o depend on each other, and instead of just adding rules, existing rul es need to be changed. Visual constraint rules combine ideas from rule -based programming and constraint programming to create declarative fo rward-chaining-like rules that can be used in a more modular way to su pport iterative programming. Libraries of visual descriptions can be b uilt and reused to compose complex behavior, which makes exploring the space of possible descriptions of simulations easier. This is valuabl e 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 imp ortant in simulations, yet are not supported by similar systems. (C) 1 997 Academic Press Limited.