Logic Programming with Mode-directed Preferences

Preference logic programming (PLP) is an extension of constraint logic programming for declaratively specifying problems requiring optimization or comparison and selection among alternative solutions to a query. PLP essentially separates the programming of a problem itself from the criteria specification of its solution selection. In this paper we give a precise formalization for the syntax and semantics of PLP based on the Herbrand model theory. This paper also presents an elegant and easy method of specifying and executing preference logic programs in terms of tabled Prolog. The method introduces a formal predicate mode declaration for designating certain predicates as optimization predicates, and stating the criteria for determining their optimal solutions via preference rules. A tabled Prolog, incorporated with the flexible mode declaration, provides an easy implementation vehicle for programming with preferences. Automatic transformation is applied to embed the preferences into the problem specification for efficient evaluation. We show that the procedural semantics of a preference logic program is equivalent to its declarative semantics.