HEX-Programs with Nested Program Calls ⋆ Thomas Eiter, Thomas Krennwallner, and Christoph Redl Institut f ¨ ur Informationssysteme, Technische Universit¨ at Wien Favoritenstraße 9-11, A-1040 Vienna, Austria {eiter,tkren,redl}@kr.tuwien.ac.at Abstract. Answer-Set Programming (ASP) is an established declarative program- ming paradigm. However, classical ASP lacks subprogram calls as in procedural programming, and access to external computations (akin to remote procedure calls) in general. This feature is desired for increasing modularity and—assuming proper access in place—(meta-)reasoning over subprogram results. While HEX- programs extend classical ASP with external source access, they do not support calls of (sub-)programs upfront. We present nested HEX-programs, which extend HEX-programs to serve the desired feature in a user-friendly manner. Notably, the answer sets of called sub-programs can be individually accessed. This is partic- ularly useful for applications that need to reason over answer sets like belief set merging, user-defined aggregate functions, or preferences of answer sets. We will further present a novel method for rapid prototyping of external sources by the use of nested programs. 1 Introduction Answer-Set Programming, based on [8], has been established as an important declarative programming formalism [3]. However, a shortcoming of classical ASP is the lack of means for modularity, i.e., dividing programs into several interacting components. Even though reasoners such as DLV, CLASP, and DLVHEX allow to partition programs into several files, they are still viewed as a single monolithic set of rules. On top of that, passing input to selected (sub-)programs is not possible upfront. In procedural programming, the idea of calling subprograms and processing their output is in permanent use. Also in functional programming such modularity is popular. This helps reducing development time (e.g., by using third-party libraries), the length of source code, and, last but not least, makes code human-readable. Reading, understanding, and debugging a typical size application written in a monolithic program is cumbersome. Modular extensions of ASP have been considered [9, 5] with the aim of building an overall answer set from program modules; however, multiple results of subprograms (as typical for ASP) are respected, and no reasoning about such results is supported. XASP [11] is an SMODELS interface for XSB-Prolog. This system is related to our work but less expressive, as it is designed for host programs under well-founded semantics. Moreover, our system allows the seamless integration of queries over subprograms with other external sources. Both is important for some applications, e.g., for the MELD ⋆ This research has been supported by the Austrian Science Fund (FWF) project P20840, P20841, and P24090, and by the Vienna Science and Technology Fund (WWTF) project ICT 08-020.