ALIEN: A Prototype-Based Constraint System

The main objective of the work reported in this article is the use of constraints in graphics and user interfaces. We believe that constraints need to be integrated in a powerful programming paradigm in order to be usable in large-scale systems. The system we present embeds constraints in a prototype-based model. A prototype object (a template) contains internal constraints, and exports slots so that instances of the prototype can be further constrained. Instantiation uses delegation to share constraints between instances. As a consequence, changing a template has an immediate effect on all its instances. The model also features generic templates and skeletons to create complex constrained objects. The system is open and extensible: new constraint types can be defined and alternative solvers can be used. The basic solver uses local propagation.