All Manuals > KnowledgeWorks and Prolog User Guide > 6 Advanced Topics > 6.1 Control Flow > 6.1.1 Meta Rule Protocol

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Meta-rules can also be used to provide an explanation facility. A full implementation of the explanation facility described here is included among the examples distributed with KnowledgeWorks, and is given also in Explanation Facility

Suppose we have a rule about truck scheduling of the form

(defrule allocate-truck-to-load :forward
          (load ?l size ?s truck nil destination 
           ?d location ?loc)
          (test (not (eq ?d ?loc)))
          (truck ?t capacity ?c load nil location ?loc)
          (test (> ?c ?s))
          (assert (truck ?t load ?l))
          (assert (load ?l truck ?t)))

and we wish to add an explanation by entering a form like

(defexplain allocate-truck-to-load
             :why ("~S has not reached its destination
                   ~S and ~ does not have a truck
                   allocated, ~ ~S does not have a load
                   allocated, and ~ with capacity ~S is
                   able to carry the load, ~ and both
                   are at the same place ~S"
                   ?l ?d ?t ?c ?loc)
             :what ("~S is scheduled to carry ~S to ~S"
                    ?t ?l ?d)
             :because ("A customer requires ~S to be
                       moved to ~S" ?l ?d))

where the :why form explains why the rule is allowed to fire, the :what form explains what the rule does and the :because gives the ultimate reason for firing the rule.

The stages in the implementation are as follows:

(defun add-instantiation (inst)
  (let ((explain-info
         (gethash (inst-rulename inst) 
       (when explain-info
         (do-the-rest explain-info 
            (inst-bindings inst))))))
(defrule explain-context :backward
   (instantiation ?inst)
   ((add-explanation ?inst))
   (fire-rule ?inst)

KnowledgeWorks and Prolog User Guide (Windows version) - 6 Dec 2011