Inlining External Sources in Answer Set Programs

HEX-programs are an extension of answer set programming (ASP) towards external sources. To this end, external atoms provide a bidirectional interface between the program and an external source. The traditional evaluation algorithm for HEX-programs is based on guessing truth values of external atoms and verifying them by explicit calls of the external source. The approach was optimized by techniques that reduce the number of necessary verification calls or speed them up, but the remaining external calls are still expensive. In this paper we present an alternative evaluation approach based on inlining of external atoms, motivated by existing but less general approaches for specialized formalisms such as DL-programs. External atoms are then compiled away such that no verification calls are necessary. The approach is implemented in the dlvhex reasoner. Experiments show a significant performance gain. Besides performance improvements, we further exploit inlining for extending previous notions of programs equivalence from ASP to HEX-programs, including strong equivalence, uniform equivalence and 〈H,B〉-equivalence. Based on this extended equivalence notion we finally characterize also inconsistency of programs wrt. extensions. Since well-known ASP extensions (such as constraint ASP) amount to special cases of HEX, the results are interesting beyond the particular formalism. 1Institut für Logic and Computation, Technische Universität Wien, Favoritenstraße 9-11, A-1040 Vienna, Austria; email: [email protected]. Acknowledgements: This article is an extension of preliminary work presented at AAAI 2017 (Redl 2017b; Redl 2017c). This work has been supported by the Austrian Science Fund (FWF) Grant P27730. Copyright c © 2018 by the authors