Analyses, optimizations and extensions of constraint handling rules: ph.d. summary

• Authors: Tom Schrijvers
• Venue: ICLP'05 Proceedings of the 21st international conference on Logic Programming
• Year: 2005

This is a summary of the Ph.D. thesis of Tom Schrijvers [4]. Constraint Handling Rules (CHR) [3] is a rule-based language commonly embedded in a host language. It combines elements of Constraint Logic Programming and term rewriting. Several implementations of CHR exist: in Prolog, Haskell, Java and HAL. Typical applications of CHR are in the area of constraint solving, but currently CHR is also used in a wider range of applications, such as type checking, natural language processing and multi-agent systems. In this work we contribute program analyses, program optimizations and extensions of the CHR language. For the optimized compilation of CHR we present several new optimizations: code specialization for ground constraints, anti-monotonic delay avoidance, hashtable constraint stores and a new late storage optimization. These and other optimizations have been implemented in a new state-of-the-art CHR system: the K.U.Leuven CHR system [5] which is currently available in SWI-Prolog [10], XSB [9] and hProlog [2]. Abstract interpretation is a general technique for program analysis [1]. We propose a framework of abstract interpretation for the CHR language [7], in particular for the formulation of analyses that drive program optimization. This frameworks allows for the uniform formulation of program analyses as well as easier improvements and combinations of existing analyses. We also evaluate analyses for theoretical properties, confluence and time complexity, on a practical case study to investigate their relevance. We contribute two extensions to the expressivity of CHR. The first extension comprises the integration of CHR with tabled execution [8]. Tabled execution avoids many forms of non-termination and is useful for automatic program optimization through the dynamic reuse of previous computations. The second extension automatically provides implication checking functionality to constraint solvers written in CHR [6]. Implication checking is an essential building block for formulating complex constraints in terms of basic constraints and for composing constraint solvers.