Abstract. We propose an extension of Constraint Handling Rules (CHR) with aggregates such as sum, count, findall, and min. This new feature significantly improves the conciseness and expressiveness of the language. In this paper, we describe an implementation based on source-to-source transformations to CHR (extended with some low-level compiler directives). We allow user-defined aggregates and nested aggregate expressions over arbitrary guarded conjunctions of constraints. Both an on-demand and an incremental aggregate computation strategy are supported. 1

Abstract. We extend the Constraint Handling Rules language with aggregates such as sum, count, findall, and min. The proposed extension features nested aggregate expressions over guarded conjunctions of constraints, a series of predefined aggregates, and application-tailored user-defined aggregates. We formally define the operational semantics of aggregates, and show how incremental aggregate computation facilitates efficient implementations. Case studies demonstrate that language support for aggregates significantly reduces program size, thus improving readability and maintainability considerably. 1

Introduction Constraint Handling Rules (CHR) [1, 4] is a powerful, yet elegant committed-choice CLP language, consisting of multi-headed, guarded multiset rewrite rules. Originally designed for the implementation of constraint solvers, CHR has matured towards a general purpose language, used in a wide range of application domains, including natural language processing, multi-agent systems,