Aggregates in CHR

We propose an extension of the Constraint Handling Rules language with aggregates like sum, count, findall, and min in the heads of rules. We define the semantics of aggregate expressions formally and informally. Our prototype implementation allows nested aggregate expressions over guarded conjunctions of constraints, using either an ondemand or an incremental computation strategy. Case studies demonstrate that by using aggregates, the program size can be reduced significantly, with an acceptable constant run-time overhead.