A.12 Adding Built-in Predicates

A.12 Adding Built-in Predicates

Common Prolog provides several special forms for adding new predicates written in Lisp. Each one is described below, with an example.


(defdetpred <
name
> <
num args
> <
body
>)

Defines a simple predicate that just runs lisp code and doesn't have to unify any variables. Arguments are referenced with: (special-arg <argnum> ) . Succeeds by default. If a failure case arises, use:(detpred-fail <name> <num args> ) .

(defdetpred integer 1

  (unless (integerp (special-arg 0))

    (go fail)))

(defdetunipred <name
> <num args
> <unifier1
 unifier2
>

              <aux-vars
> <body
>)

defdetunipred is used when the defined predicate needs to unify values with arguments (or unify in general). The body is executed and, if successful, (that is, detpred-fail has not been called) unification is performed on the two unifiers. (If more than two items need to be unified, cons up lists of items to unify).

(defdetunipred arg 3 (temp1 temp2)

         (temp1 temp2 index term value)

         (setf index (special-arg 0)

               term (special-arg 1)

               value (special-arg 2))

         (unless (and (numberp index)

                      (plusp index)

                      (or (and (term-p term)

                               (< index (length term)))

                          (and (consp term)

                               (< index 3))))

                 (detpred-fail arg 3))

         (if (consp term)

             (setf temp1 (if (= index 1)

                             (car term)

                             (cdr term)))

            (setf temp1 (term-ref term index)))

         (setf temp2 value))

KnowledgeWorks and Prolog User Guide - 4 Apr 2005