The modify action allows the user to modify template facts on the fact-list. Only one fact may be modified with a single modify statement. The modification of a fact is equivalent to retracting the present fact and asserting the modified fact. Therefore, any facts receiving logical support from a template fact are retracted (assuming no other support) when the template fact is modified and the new template fact loses any logical support that it previously had.
Syntax
(modify <fact-specifier> <RHS-slot>*)
The term <fact-specifier> includes variables bound on the LHS to fact-addresses as described in section 5.4.1.8 or the fact-index of the desired fact (e.g. 3 for the fact labeled f3). Note that the fact-index generally is not known during the execution of a program, so facts usually are modified by binding them on the LHS of a rule. Static deftemplate checking is not performed when a fact-index is used as the <facts-pecifier> since the deftemplate being referenced is usually ambiguous. Only variables or fact indices may be used in a modify. External functions may not be called. The value returned by this function is the fact-address of the newly modified fact. If the assertion of the newly modified fact causes an error, or if an identical copy of the newly modified fact already exists in the factlist, then the symbol FALSE is returned.
Example
(defrule change-valve-status ?f1<-(status (valve open)) ?f2<-(close-valve) => (retract ?f2) (modify ?f1 (valve closed)))