Created by Thom Frühwirth in 1991, the CHR language has become a major specification and implementation language for constraint-based algorithms and applications. Algorithms are often specified using inference rules, rewrite rules, sequents, proof rules, or logical axioms that can be directly written in CHR. Based on first order predicate logic, the clean semantics of CHR facilitates non-trivial program analysis and transformation. About a dozen implementations of CHR exist in Prolog, Haskell, and Java. CHR is also available as WebCHR for online experimentation with more than 40 constraint solvers. More than 100 projects use CHR.

More info: http://www.cs.kuleuven.ac.be/~dtai/projects/CHR/

Interactive demo: http://chr.informatik.uni-ulm.de/~webchr/