Defglobal Construct in AI

• With the defglobal construct, global variables can be defined, set and accessed within the CLIPS environment.

• Global variables can be accessed as part of the pattern-matching process, but changing them does not invoke the pattern  matching process.

• The bind function is used to set the value of global variables.

• Global variables are reset to their original value when the reset command is performed or when bind is called for the global with no values.

• Global variables can be removed by using the clear command or the underfglobal command.

•  Syntax 

              (defglobal [<defmodule-name>] <global-assignment>*)

              <global-assignment> ::= <global-variables> = <expression>

              <global-variable> : := ?* <symbol>*

• There may be multiple defglobal constructs and any number of global variables may be defined  in each defglobal statement.

• The optional <defmodule -name> indicates the module in which the defglobal will be defined. If none is specified, the globals will be placed in the current module.

• Global variables may be used anyplace that a local variable could be used (with two exceptions).
• Global variables may not be used as a parameter variable for a deffunction, defmethod, or message-handler.
• Global variables may not be used in the same way that a local variables is used on the LHS of a rule to bind a value. 
• Therefore, the following rule is illegal

          ( fact ?*x*)

• Note that this rule will not necessarily be updated when  the value of ?*x* is changed. For example, if ?*x* is 4 and the fact (fact 3) is added, then the rule is not satisfied. If the value of ?*x* is now changed to 2, the rule will not be activated.