This paper describes the Oz Explorer and its implementation. The Explorer is a visual constraint programming tool intended to support the development of constraint programs. It uses the search tree of a constraint problem as its central metaphor. Exploration and visualization of the search tree are userdriven and interactive. The constraints of any node in the tree are available first-class: predefined or user-defined procedures can be used to display or analyze them. The Explorer is a fast and memory efficient tool intended for the development of real-world constraint programs. The Explorer is implemented in Oz using first-class computation spaces. There is no fixed search strategy in Oz. Instead, first-class computation spaces allow to program search engines. The Explorer is one particular example of a user-guided search engine. The use of recomputation to trade space for time makes it possible to solve large real-world problems, which would use too much memory otherwise.

Solutions to combinatorial search problems can benefit from custom-made constraint-based inference engines that go beyond depthfirst search. Several constraint programming systems support the programming of such inference engines through programming abstractions. For example, the Mozart system for Oz comes with several engines, extended in dimensions such as interaction, visualization, and optimization. However, so far such extensions are monolithic in their software design, not catering for systematic reuse of components. We present an object-oriented modular architecture for building inference engines that achieves high reusability and supports rapid prototyping of search algorithms and their extensions. For the sake of clarity, we present the architecture in the setting of a C++ constraint programming library. The SearchToolKit, a search library for Oz based on the presented architecture, provides evidence for the practicality of the design.

This paper shows the Oz Solver, a tool designed to support the development of constraint programs. The Solver allows the user to interactively solve a constraint problem. As the problem's search tree is explored the Solver incrementally draws its graphical presentation. The user has access to the computation state at any node in the tree. Although search is controlled interactively by the user, implementing the Solver does not require any other primitives than those provided by Oz. It is planned to include the Oz Solver in the next release of the DFKI Oz system. Keywords---Constraint Programming, Programming Environment, Debugging, Encapsulated Search, Graphical User Interfaces. 1. Introduction Oz [13], [4], [10], [14] is a concurrent constraint language developed at DFKI supporting higher-order functional programming, concurrent object-oriented programming, and constraint programming. This paper presents the Oz Solver, a tool designed to support the development of constraint program...