Section 4: Deffacts Construct

With the deffacts construct, a list of facts can be defined which are automatically asserted whenever the reset command is performed. Facts as?serted through deffacts may be retracted or pattern-matched like any other fact. The initial fact-list, including any defined deffacts, is always reconstructed after a reset command.

Syntax

(deffacts <deffacts-name> [<comment>] | <RHS-pattern>*)

Redefining a currently existing deffacts causes the previous deffacts with the same name to be removed even if the new definition has errors in it. There may be multiple deffacts constructs and any number of facts (either ordered or deftemplate) may be asserted into the initial fact-list by each deffacts construct.

Dynamic expressions may be included in a fact by embedding the expression directly within the fact. All such expressions are evaluated when CLIPS is reset.

Example

(deffacts startup "Refrigerator Status"
  (refrigerator light on)
  (refrigerator door open)
  (refrigerator temp (get-temp)))

Upon startup and after a clear command, CLIPS automatically constructs the following deftemplate and deffacts.

(deftemplate initial-fact)
(deffacts initial-fact
(initial-fact))

In prior versions of CLIPS, the initial-fact deffacts provided a convenient method for starting the execution of a system: rules without conditional elements had an (initial-fact) pattern automatically added to their LHS. When a reset command was issued, the assertion of the (intial-fact) would activate these rules. Starting with version 6.3 of CLIPS, rules without conditional elements are automatically matched without the need for the (initial-fact) assertion. Usage of the initial-fact is now deprecated. Programs should not rely on the assertion of this fact when a reset is performed.