CHR 2009

Sixth International Workshop on Constraint Handling Rules

Pasadena, California at the occasion of ICLP 2009

July 15th


The Constraint Handling Rules (CHR) language has become a major declarative specification formalism and implementation language for constraint reasoning 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.  Its clean semantics facilitates program design, analysis, and transformation.  See the CHR website for more information.

Previous Workshops on Constraint Handling Rules were organized in May 2004 in Ulm (Germany), in October 2005 in Sitges (Spain) at ICLP, in July 2006 in Venice (Italy) at ICALP, in September 2007 in Porto (Portgual) at ICLP, and in July 2008 in Hagenberg (Austria) at RTA.

Topics of Interest

The workshop calls for full papers and short papers describing ongoing work on any aspect of CHR and related approaches. The following topics are relevant (this list is non-exhaustive):

  • (Logical) Algorithms
  • Applications
  • Comparisons with Related Approaches
  • Constraint Solvers
  • Critical Assessment
  • Expressivity and Complexity
  • Implementations and Optimization
  • Language Extensions (Types, Modules,...)
  • Program Analysis
  • Program Transformation and Generation
  • Programming Environments (Debugging)
  • Programming Pearls
  • Programming Tools
  • Retractable Constraints
  • Semantics
  • System Descriptions

The call for papers can be found here.

Invited Talk

The workshop will feature an invited talk on the topic "First steps towards a lingua franca for computer science: Rule-based Approaches in CHR" by Thom Frühwirth.


The notion of "rule" is ubiqitous in computer science, from theoretical formalisms to practical programming languages. Matured rule-based programming experiences a renaissance due to its applications in areas such as business rules, semantic web, computational biology, medical diagnosis, software verification, and security.

We will embed rule-based approaches into the Constraint Handling Rules (CHR) language by simple source-to-source transformations. We also consider the other direction of embeddings. This gives us the possibility to compare and analyse the different approaches.

   Classical Rule-Based Systems
          * Production Rule Systems
          * Event-Condition-Action Rules
   Rewriting- and Graph-based Formalisms
          * Term Rewriting Systems and Graph Transformation Systems
          * Chemical Abstract Machine and Multiset Transformation
          * Petri Nets
   Constraint- and Logic-based Programming
          * Deductive Databases and Logical Algorithms
          * Prolog and Constraint Logic Programming
          * Concurrent Constraint Programming

In this talk, we will highlight the commonalities of the transformations and what of their features may suggest themselves as extensions of CHR - as indeed they have already been proposed in the literature:

          * rule priorities
          * negation-as-absence
          * disjunction and search
          * diagrammatic notation

On the other hand, a simple fragment of CHR can cover rule-based approaches without constraints.

Recommended External Talk

We also recommend the following invited talk by Mike Elston, Scientific Software and Systems Ltd. which is held at the CULP workshop.

From Prolog to Porsche: experiences developing a large scale financial application in Prolog.


Work on the Y2K bug ten years ago exposed widespread dissatisfaction with the software applications available to New Zealand stockbrokers. Sensing an opportunity and having successfully applied Prolog to optimization problems in agriculture, Scientific Software and Systems Ltd developed SecuritEase, a comprehensive stock broking dealing and settlement system. We quickly determined that the main problem with the incumbent systems was their lack of flexibility and consequent inability to accommodate the demands of increasing competition and globalization. Noting that the response to these challenges by our competitors was to outsource development to lower wage economies, SSS resolved instead to use Prolog to increase development productivity through the creation of a set of domain specific tools. Using these tools, SSS analysts and programmers worked together to create, refine and deploy the SecuritEase system. Within five years SSS had displaced its competitors to become the dominant supplier in New Zealand. Many lessons were learned on the way including: the importance of Microsoft Windows and just how difficult it is to achieve 24x7 reliability. This paper describes how Prolog allowed the development of three key tools: a forward chaining user interface system, a productive and efficient relational database interface, and an approach to quickly building financial messaging interfaces. It also describes how the many aspects of a large scale Prolog-based project were managed including finding, training and retaining staff, achieving reliability, Prolog performance, the role of open source software, the need for constant refactoring, and the benefits of a single language approach. Finally it describes how the application is now being adopted by much larger companies outside New Zealand and the particular challenges associated with justifying Prolog to a sceptical

Accepted Papers


The workshop proceedings have been published as a TR and are available here.

Workshop Program

09:30 - 10:20 Invited talk: Thom Frühwirth
First steps towards a lingua franca for computer science: Rule-based Approaches in CHR

10:20 - 10:50 Coffee break

10:50 - 12:05 Session 1

  • Frank Raiser and Thom Frühwirth.
    Operational Equivalence of Graph Transformation Systems.
  • Frank Raiser, Hariolf Betz and Thom Frühwirth.
    Equivalence of CHR States Revisited.
  • Peter Van Weert, Leslie De Koninck and Jon Sneyers.
    A Proposal for a Next Generation of CHR.

12:05 - 12:40 Discussion Session: the future of CHR research

12:40 - 14:40 Lunch break

14:40 - 15:55 Session 2:

  • Thierry Martinez.
    On connections between CHR and LCC.
  • Jon Sneyers, Wannes Meert and Joost Vennekens.
    CHRiSM: Chance Rules induce Statistical Models.
  • Marcos Aurelio Almeida da Silva and Jacques Robin.
    Extending CHR with objects under a variety of inheritance and world-closure assumptions.

Registration and Travel Information

For registration and travel information please see the corresponding pages of the ICLP conference.

Program Committee

  • Hariolf Betz, Universität Ulm (Germany)
  • Henning Christiansen, Roskilde University (Denmark)
  • François Fages, INRIA Rocquencourt (France)
  • Thom Frühwirth, Universität Ulm (Germany)
  • Rémy Haemmerlé, Universidad Politécnica de Madrid (Spain)
  • Leslie De Koninck, Katholieke Universiteit Leuven (Belgium)
  • Edmund S. Lam, National University of Singapore (Singapore)
  • Eric Monfroy, Université de Nantes (France)
  • Jacques Robin, Universidade Federal de Pernambuco (Brazil)
  • Beata Sarna-Starosta, LogicBlox Inc., Atlanta (USA)
  • Tom Schrijvers, Katholieke Universiteit Leuven (Belgium)
  • Peter J. Stuckey, NICTA Victoria Laboratory (Australia)

Workshop Coordinators

Frank Raiser
Faculty of Engineering and Computer Science
Ulm University, Germany

Jon Sneyers
Department of Computer Science
K.U.Leuven, Belgium