Common Prolog provides several special forms for adding new predicates written in Lisp. Each one is described below, with an example.
The syntax of this form is:
(defdetpred <name> <num-args> <body>)
which defines a simple predicate that just runs Lisp code and does not have to unify any variables. Arguments are referenced with: (special-arg <num>). The body succeeds by default, but if a failure case arises, use: (detpred-fail <name> <num-args>).
For example:
(defdetpred my-integer 1 (unless (integerp (special-arg 0)) (detpred-fail my-integer 1)))
The syntax of this form is:
(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).
For example:
(defdetunipred my-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 my-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 (Windows version) - 18 Feb 2025 15:37:34