Mixing Constraints and Objects: a Case Study in Automatic Harmonization

We propose an extension of Smalltalk with f ini te-domain constraint sat isfact ion mechanisms. Our system, called BackTalk, allows the definition of constraints over arbitrary Smalltalk objects, and implements efficient algorithms for constraints satisfaction. We exemplify the use of BackTalk on a problem known to be complex, automatic harmonization. We outline several previous attempts to solve the problem with similar mechanisms, and stress on their inefficiencies, mainly the lack of structure of domain objects. We propose to solve the problem by a combination of constraints and objects that fully benefits from object structures. This is achieved in practice by separating the constraint satisfaction process in two steps. By comparison, our system yields excellent results, both in term of efficiency and readability. We discuss the generality of our approach to problems involving numerous and heterogeneous object structures. Key-words: constraints, finite-domain constraint satisfaction, embedded constraint systems, automatic harmonization. 1. Yet an other object + constraint