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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.

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