Object-Oriented Concurrent Constraint Programming in Oz

1.1 Abstract Oz is a higher-order concurrent constraint programming system under development at DFKI. It combines ideas from logic and concurrent programming in a simple yet expressive language. From logic programming Oz inherits logic variables and logic data structures, which provide for a programming style where partial information about the values of variables is imposed concurrently and incrementally. A novel feature of Oz is the support of higher-order programming without sacriicing that denotation and equality of variables are captured by rst-order logic. Another new feature of Oz are cells, a concurrent construct providing a minimal form of state fully compatible with logic data structures. These two features allow to express objects as procedures with state, avoiding the problems of stream communication, the conventional communication mechanism employed in concurrent logic programming. Based on cells and higher-order programming, Oz readily supports concurrent object-oriented programming including object identity, late method binding, multiple inheritance, \self", \super", batches, synchronous and asynchronous communication. is an attempt to create a high-level concurrent programming language bringing together the merits of logic and object-oriented programming. Our starting point was concurrent constraint programming 14], which brings together ideas from constraint and concurrent logic programming. Constraint logic programming 8, 1], on the one hand, originated with Prolog II 4] and was prompted by the need to integrate numbers and data structures in an operationally eecient, yet logically sound manner. Concurrent logic programming 18], on the other hand, originated with the Relational Language 3] and was promoted by the Japanese Fifth Generation Project, where logic programming was conceived as the basic system programming language and thus had to account for concurrency, synchronization and indeterminism. For this purpose, the