Common Lisp relations: an extension of Lisp for logic programming

The author presents a set of relational constructs that support logic programming in the Common Lisp framework. These constructs support locally defined and first-class relations, an interface between function and relation-calling logical variables as first-class Lisp objects, and the ability to define relations over Common Lisp structures. The relational programming constructs are amenable to standard Prolog compilation techniques, thus making possible high-speed performance. A relation is an executable specification defined in terms of a sequence of clause-expressions. Each clause-expression specifies a list of argument patterns that determine the selection and local bindings of the clause as well as a sequence of goals that must be satisfied for the clause to succeed. Sequences of clause-expressions are used for defining relations and single lambda-expressions are used for defining functions. As an analog to the usual function-calling execution mode of Common Lisp, a relation-calling mechanism is provided. The semantics of passing values between relations and functions is described