Constraint Handling Rules (CHR) is a declarative programming language extension introduced in 1991 by Thom Frühwirth. Originally designed for developing (prototypes of) constraint programming systems, CHR is increasingly used as a high-level general-purpose programming language. Typical application domains of CHR are abduction, multi-agent systems, natural language processing, compilation, scheduling, spatial-temporal reasoning, testing and verification, and type systems.
Although CHR is Turing complete, it is not commonly used as a programming language in its own right. Rather, it is used to extend a host language with constraints. Current host languages include Prolog, Java and Haskell. Prolog is by far the most popular host language and CHR is included in many Prolog implementations, including SICStus and SWI-Prolog.
A CHR program, sometimes called a constraint handler, is a sequence of guarded rules for simplification, propagation, and "simpagation" (a mix of simplification and propagation) of conjunctions of constraints. The CHR constraint store is a multi-set. In contrast to Prolog, the rules are multi-headed and are executed in a committed-choice manner using a forward chaining algorithm.
Read more about Constraint Handling Rules: Confluence, Example Program
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