• Videos of the talks
  • Program HCVS 2021
  • HCVS registration: All participants (including authors, (invited) speakers) need to register to attend the workshop. Deadlines (which is within a week) and associated fees (for workshop only):
    • Early registration: Until 15 March 2021 (25 EUR)
    • Late registration: From 16 March 2021 (45 EUR)
    • Registration webpage:
    • For issues concerning registration please contact directly to the ETAPS organizer since we have no control over it.

Horn Clauses for Verification and Synthesis

Many Program Verification and Synthesis problems of interest can be modeled directly using Horn clauses and many recent advances in the CLP and CAV communities have centered around efficiently solving problems presented as Horn clauses.

This series of workshops aims to bring together researchers working in the two communities of Constraint/Logic Programming (e.g., ICLP and CP), Program Verification (e.g., CAV, TACAS, and VMCAI), and Automated Deduction (e.g., CADE, IJCAR), on the topic of Horn clause based analysis, verification, and synthesis.

Horn clauses for verification and synthesis have been advocated by these communities in different times and from different perspectives and HCVS is organized to stimulate interaction and a fruitful exchange and integration of experiences.

The workshop follows seven previous meetings: HCVS 2020 in Dublin, Ireland (ETAPS 2020), HCVS 2019 in Prague, Czech Republic (ETAPS 2019), HCVS 2018 in Oxford, UK (CAV, ICLP and IJCAR at FLoC 2018), HCVS 2017 in Gothenburg, Sweden (CADE), HCVS 2016 in Eindhoven, The Netherlands (ETAPS), HCVS 2015 in San Francisco, CA, USA (CAV), and HCVS 2014 in Vienna, Austria (VSL).

Accepted Papers

Regular Papers

Presentation only Papers

Contributed Papers

Aims and Scope

Topics of interest include, but are not limited to the use of Horn clauses, constraints, and related formalisms in the following areas:

  • Analysis and verification of programs and systems of various kinds (e.g., imperative, object-oriented, functional, logic, higher-order, concurrent, transition systems, petri-nets, smart contracts)
  • Program synthesis
  • Program testing
  • Program transformation
  • Constraint solving
  • Type systems
  • Machine learning and automated reasoning
  • CHC encoding of analysis and verification problems
  • Resource analysis
  • Case studies and tools
  • Challenging problems
We solicit regular papers describing theory and implementation of Horn-clause based analysis and tool descriptions. We also solicit extended abstracts describing work-in-progress, as well as presentations covering previously published results that are of interest to the workshop.

CHC Competition

HCVS 2021 will host the 4th competition on constraint Horn clauses ( CHC-COMP ), which will compare state-of-the-art tools for CHC solving for performance and effectiveness on a set of publicly available benchmarks. A report on the 4th CHC-COMP will be part of the workshop's proceedings. The report also contains tool descriptions of the participating solvers.

Invited speakers

  • Samir Genaim. Termination analysis of programs with multiphase control-flow
  • Programs with multiphase control-flow are programs where the execution passes through several (implicit) phases. Proving termination of such programs (or inferring corresponding runtime bounds) is often challenging since it requires reasoning on these phases separately. This talk overviews techniques for proving termination of such programs, in particular: (1) using multiphase ranking functions, where we will discuss theoretical aspects of such ranking functions for several kinds of program representations and their surprising relation to proving non-termination; and (2) using partial evaluation of Horn clauses as a general-purpose technique for control-flow refinement that simplifies the control-flow allowing, among other things, to prove termination with simpler ranking functions.

  • Naoki Kobayashi. An Overview of the HFL Model Checking Project
  • We give an overview of our project on automated program verification based on higher-order fixpoint logic (HFL). HFL, originally proposed by Viswanathan and Viswanathan, is a higher-order extension of the modal mu-calculus. HFL(Z), an extension of HFL with integers, can also be viewed as an extension of Constrained Horn Clauses (CHCs) with higher-order predicates and alternating fixpoints. After a gentle introduction to HFL(Z), we show how a wide range of program verification problems can naturally be reduced to the HFL(Z) model checking problem, and how the HFL(Z) model checking problem can be solved by using a combination of various techniques such as higher-order model checking, refinement type inference, fold/unfold transformations, and CHC solving.

Program Chairs

Program Committee


Submission has to be done in one of the following formats:

  • Regular papers (up to 12 pages plus bibliography in EPTCS format), which should present previously unpublished work (completed or in progress), including descriptions of research, tools, and applications.
  • Tool papers (up to 4 pages in EPTCS format), including the papers written by the CHC-COMP participants, which can outline the theoretical framework, the architecture, the usage, and experiments of the tool.
  • Extended abstracts (up to 3 pages in EPTCS format), which describe work in progress or aim to initiate discussions.
  • Presentation-only papers, i.e., papers already submitted or presented at a conference or another workshop. Such papers can be submitted in any format, and will not be included in the workshop post-proceedings.
  • Posters that are of interest to the workshop
All submitted papers will be refereed by the program committee and will be selected for inclusion in accordance with the referee reports. Accepted regular papers and extended abstracts will be published electronically as a volume in the Electronic Proceedings in Theoretical Computer Science (EPTCS) series, see (provided that enough regular papers are accepted).

Papers must be submitted through the EasyChair system using the web page: