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test_and_setval(+VarName, ?Old, ?New)

Test whether a non-logical variable has value Old and if so, set it to New.

VarName: An atom
Old: A term
New: A term

Description

VarName must be the name of a non-logical variable (not an array element). If the current value of this variable is not identical to Old, the predicate fails. If it is identical, the variable's value gets changed to the value New. Test and set is done as an atomic operation in a multithreaded setting.

Modes and Determinism

test_and_setval(+, ?, ?) is semidet

Modules

This predicate is sensitive to its module context (tool predicate, see @/2).

Fail Conditions

Fails it the current value of the variable is not identical to Old

Exceptions

(4) instantiation fault: VarName is not ground
(5) type error: VarName not an atom
(41) array or global variable does not exist: VarName is not the name of a non-logical variable.

Examples

Success:
     ?- setval(k, 3), test_and_setval(k, 3, 5).

     wait_for_lock :-
	    ( test_and_setval(lock, 0, 1) ->
		true
	    ;
	        wait_for_lock
	    ).

Fail:
      setval(k, 1), test_and_setval(k, 3, 5).