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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 and not a reference). 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.

Modes and Determinism

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
(5) type error
VarName is the name of a reference
(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).

See Also

decval / 1, incval / 1, variable / 1, setval / 2, getval / 2