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  6 Iteration   6.1 The LOOP Facility   6.1.1 Overview of the Loop Facility   6.1.1.5 Summary of Loop Clauses

6.1.1.5.3 Summary of Termination Test Clauses

The for and as constructs provide a termination test that is determined by the iteration control clause.

The repeat construct causes termination after a specified number of iterations. (It uses an internal variable to keep track of the number of iterations.)

The while construct takes one form, a test, and terminates the iteration if the test evaluates to false. A while clause is equivalent to the expression (if (not test) (loop-finish)).

The until construct is the inverse of while; it terminates the iteration if the test evaluates to any non-nil value. An until clause is equivalent to the expression (if test (loop-finish)).

The always construct takes one form and terminates the loop if the form ever evaluates to false; in this case, the loop form returns nil. Otherwise, it provides a default return value of t.

The never construct takes one form and terminates the loop if the form ever evaluates to true; in this case, the loop form returns nil. Otherwise, it provides a default return value of t.

The thereis construct takes one form and terminates the loop if the form ever evaluates to a non-nil object; in this case, the loop form returns that object. Otherwise, it provides a default return value of nil.

If multiple termination test clauses are specified, the loop form terminates if any are satisfied.

For more information, see Section 6.1.4 Termination Test Clauses.


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