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The Hexlite Solver

Lightweight and Efficient Evaluation of HEX Programs

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Logics in Artificial Intelligence (JELIA 2019)

Part of the book series: Lecture Notes in Computer Science ((LNAI,volume 11468))

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Abstract

\(\textsc {hexlite}\) is a lightweight solver for the \(\textsc {hex}\) formalism which integrates Answer Set Programming (ASP) with external computations. The main goal of \(\textsc {hexlite}\) is efficiency and simplicity, both in implementation as well as in installation of the system. We define the Pragmatic \(\textsc {hex}\) Fragment which permits to partition external computations into two kinds: those that can be evaluated during the program instantiation phase, and those that need to be evaluated during the answer set search phase. \(\textsc {hexlite}\) is written in \(\textsc {python}\) and suitable for evaluating this fragment with external computations that are realized in \(\textsc {python}\). Most performance-critical tasks are delegated to the \(\textsc {python}\) module of \(\textsc {clingo}\). We demonstrate that the Pragmatic \(\textsc {hex}\) Fragment is sufficient for many use cases and that it permits \(\textsc {hexlite}\) to have superior performance compared to the \(\textsc {dlvhex}\) system in relevant application scenarios.

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Notes

  1. 1.

    https://github.com/hexhex/hexlite.

  2. 2.

    Formally, this is the set \(\{ y_i(v_1,\ldots ,v_{t_i}) \in I \}\).

  3. 3.

    \(\textsc {dlvhex}\) replaces them with an ordinary replacement atom, guesses truth of replacement atoms with extra rules, and accepts only answer set candidates I where guessed truth values correspond with external computations wrt. I, see Sect. 4.

  4. 4.

    In that case \(m=0\) so the replacement atom does not include \(X_1,\ldots ,X_m\).

  5. 5.

    A propagator is a program module that interfaces with the search process of \(\textsc {clasp}\) and can (a) infer truth values and (b) add ground clauses based on a partial model.

  6. 6.

    The FLP check implemented in \(\textsc {hexlite}\) is described in Proposition 1 in [8]. The FLP check can be deactivated if it is not required (this is another example of Programmer Responsibility). The custom \(\textsc {python}\) propagator is re-used in the FLP checker.

  7. 7.

    https://bitbucket.org/knowlp/asp-fo-abduction.

  8. 8.

    To permit a fairer comparison, we used only objective functions \(\textsc {card}\) and \(\textsc {coh}\) (\(\textsc {dlvhex}\) is incompatible with objective function \(\textsc {wa}\)) and we removed all facts of the form comment(\(\ldots \)). which served only an informational purpose (\(\textsc {dlvhex}\) is significantly slower if these facts are included).

  9. 9.

    This generalizes the termination mechanism for reasoning as it was implemented in the original \(\textsc {accel}\) reasoner [28].

  10. 10.

    Dataset retrieved from https://old.datahub.io/dataset/colinda.

  11. 11.

    We used git hash 5a1ee06d from git@github.com:hexhex/core.git because the stable version 2.5.0 performed significantly worse.

  12. 12.

    Git hash d0e7896eb from git@github.com:hexhex/hexlite.git.

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Acknowledgements

We are grateful to Stefano Germano, Tobias Kaminski, Christoph Redl, Antonius Weinzierl and the anonymous reviewers for feedback about the \(\textsc {hexlite}\) system and this manuscript. This work has received funding from the Austrian Federal Ministry of Transport, Innovation and Technology (BMVIT) under grant agreement 861263 (DynaCon), and from the European Union’s Horizon 2020 research and innovation programme under grant agreement 825619 (AI4EU).

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Authors and Affiliations

  1. Institut für Logic and Computation, Knowledge-Based Systems Group, Technische Universität Wien, Vienna, Austria

    Peter Schüller

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  1. Peter Schüller
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Correspondence to Peter Schüller .

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Editors and Affiliations

  1. University of Calabria, Rende, Italy

    Francesco Calimeri

  2. University of Calabria, Rende, Italy

    Nicola Leone

  3. Mathematics and Computer Science, University of Calabria, Rende, Italy

    Marco Manna

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Schüller, P. (2019). The Hexlite Solver. In: Calimeri, F., Leone, N., Manna, M. (eds) Logics in Artificial Intelligence. JELIA 2019. Lecture Notes in Computer Science(), vol 11468. Springer, Cham. https://doi.org/10.1007/978-3-030-19570-0_39

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  • DOI: https://doi.org/10.1007/978-3-030-19570-0_39

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