Towards an Open Architecture for IcDl

We extend LDl to allow programs to call external procedures and vice versa. This extension allows the modularization of L Dl, since external predicates are equivalent to external procedures written in L3 DL. External predicates are viewed as infinite relations so that the traditional semantics of logic programs remain ap plicable. To avoid computing infinite relations, wellformedness conditions for programs in extended L Df? are given. The traditional optimization framework can still be used; it is only necessary to define a new set of cost functions capable of handling the infinite relations. The problem of interfacing L3 DL programs with external procedures-exchanging complex objects and returning multiple solutions-is discussed. Thus, we provide a general framework to allow logic programs to interact with external procedures without sacrificing amenities such as optimization, safety, etc. This approach forms the basis for the implementation of externals and modules in the LDL compiler and optimizer at MCC.