nlp: A Compiler for Nested Logic Programming

nlp is a compiler for nested logic programming under answer set semantics. It is designed as a front-end translating nested logic programs into disjunctive ones, whose answer sets are then computable by disjunctive logic programming systems, like dlv or gnt. nlp offers different translations: One is polynomial but necessitates the introduction of new atoms, another is exponential in the worst case but avoids extending the language. We report experimental results, comparing the translations on several classes of benchmark problems. 1 Nested Logic Programs and their Compilation Nested logic programs allow for arbitrarily nested formulas in heads and bodies of logic program rules under answer set semantics [4]. Nested expressions can be formed using conjunction, disjunction, and the negation-as-failure operator in an unrestricted fashion. Previous results [4] show that nested logic programs can be transformed into standard (unnested) disjunctive logic programs in an elementary way, applying the negation-as-failure operator to body literals only. This is of great practical relevance since it allows us to evaluate nested logic programs by means of off-the-shelf disjunctive logic programming systems, like dlv [3] and gnt [2]. However, it turns out that this straightforward transformation results in an exponential blow-up in the worst-case, despite the fact that complexity results indicate a polynomial translation among both formalisms. In [5], we provide such a polynomial translations of nested logic programs into disjunctive ones. This translation introduces new atoms reflecting the structure of the original program; in the sequel, we refer to it as the structural translation. Likewise, we refer to the possibly exponential one as being language-preserving.