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Helsinki University of Technology, 
     Laboratory for Theoretical Computer Science

Modularity in Answer Set Programs


Emilia Oikarinen. Modularity in Answer Set Programs. Doctoral dissertation, TKK Dissertations in Information and Computer Science TKK-ICS-D7, Helsinki University of Technology, Faculty of Information and Natural Sciences, Department of Information and Computer Science, Espoo, Finland, 2008.


Answer set programming (ASP) is an approach to rule-based constraint programming allowing flexible knowledge representation in variety of application areas. The declarative nature of ASP is reflected in problem solving. First, a programmer writes down a logic program the answer sets of which correspond to the solutions of the problem. The answer sets of the program are then computed using a special purpose search engine, an ASP solver. The development of efficient ASP solvers has enabled the use of answer set programming in various application domains such as planning, product configuration, computer aided verification, and bioinformatics.

The topic of this thesis is modularity in answer set programming. While modern programming languages typically provide means to exploit modularity in a number of ways to govern the complexity of programs and their development process, relatively little attention has been paid to modularity in ASP. When designing a module architecture for ASP, it is essential to establish full compositionality of the semantics with respect to the module system. A balance is sought between introducing restrictions that guarantee the compositionality of the semantics and enforce a good programming style in ASP, and avoiding restrictions on the module hierarchy for the sake of flexibility of knowledge representation.

To justify a replacement of a module with another, that is, to be able to guarantee that changes made on the level of modules do not alter the semantics of the program when seen as an entity, a notion of equivalence for modules is provided. In close connection with the development of the compositional module architecture, a transformation from verification of equivalence to search for answer sets is developed. The translation-based approach makes it unnecessary to develop a dedicated tool for the equivalence verification task by allowing the direct use of existing ASP solvers.

Translations and transformations between different problems, program classes, and formalisms are another central theme in the thesis. To guarantee efficiency and soundness of the translation-based approach, certain syntactical and semantical properties of transformations are desirable, in terms of translation time, solution correspondence between the original and the transformed problem, and locality/globality of a particular transformation.

In certain cases a more refined notion of minimality than that inherent in ASP can make program encodings more intuitive. Lifschitz' parallel and prioritized circumscription offer a solution in which certain atoms are allowed to vary or to have fixed values while others are falsified as far as possible according to priority classes. In this thesis a linear and faithful transformation embedding parallel and prioritized circumscription into ASP is provided. This enhances the knowledge representation capabilities of answer set programming by allowing the use of existing ASP solvers for computing parallel and prioritized circumscription.

Suggested BibTeX entry:

    address = {Espoo, Finland},
    author = {Emilia Oikarinen},
    school = {Helsinki University of Technology, Faculty of Information and Natural Sciences, Department of Information and Computer Science},
    title = {Modularity in Answer Set Programs},
    type = {Doctoral dissertation, {TKK} {D}issertations in {I}nformation and {C}omputer {S}cience {TKK-ICS-D7}},
    year = {2008},

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Latest update: 19 January 2010.