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  ANSI Common Lisp   6 Iteration   6.1 The LOOP Facility

6.1.4 Termination Test Clauses

The repeat construct causes iteration to terminate after a specified number of times. The loop body executes n times, where n is the value of the expression form. The form argument is evaluated one time in the loop prologue. If the expression evaluates to 0 or to a negative number, the loop body is not evaluated.

The constructs always, never, thereis, while, until, and the macro loop-finish allow conditional termination of iteration within a loop.

The constructs always, never, and thereis provide specific values to be returned when a loop terminates. Using always, never, or thereis in a loop with value accumulation clauses that are not into causes an error of type program-error to be signaled (at macro expansion time). Since always, never, and thereis use the return-from special operator to terminate iteration, any finally clause that is supplied is not evaluated when exit occurs due to any of these constructs. In all other respects these constructs behave like the while and until constructs.

The always construct takes one form and terminates the loop if the form ever evaluates to nil; in this case, it returns nil. Otherwise, it provides a default return value of t. If the value of the supplied form is never nil, some other construct can terminate the iteration.

The never construct terminates iteration the first time that the value of the supplied form is non-nil; the loop returns nil. If the value of the supplied form is always nil, some other construct can terminate the iteration. Unless some other clause contributes a return value, the default value returned is t.

The thereis construct terminates iteration the first time that the value of the supplied form is non-nil; the loop returns the value of the supplied form. If the value of the supplied form is always nil, some other construct can terminate the iteration. Unless some other clause contributes a return value, the default value returned is nil.

There are two differences between the thereis and until constructs:

  • The until construct does not return a value or nil based on the value of the supplied form.

  • The until construct executes any finally clause. Since thereis uses the return-from special operator to terminate iteration, any finally clause that is supplied is not evaluated when exit occurs due to thereis.

The while construct allows iteration to continue until the supplied form evaluates to false. The supplied form is reevaluated at the location of the while clause.

The until construct is equivalent to while (not form).... If the value of the supplied form is non-nil, iteration terminates.

Termination-test control constructs can be used anywhere within the loop body. The termination tests are used in the order in which they appear. If an until or while clause causes termination, any clauses that precede it in the source are still evaluated. If the until and while constructs cause termination, control is passed to the loop epilogue, where any finally clauses will be executed.

There are two differences between the never and until constructs:

  • The until construct does not return t or nil based on the value of the supplied form.

  • The until construct does not bypass any finally clauses. Since never uses the return-from special operator to terminate iteration, any finally clause that is supplied is not evaluated when exit occurs due to never.

In most cases it is not necessary to use loop-finish because other loop control clauses terminate the loop. The macro loop-finish is used to provide a normal exit from a nested conditional inside a loop. Since loop-finish transfers control to the loop epilogue, using loop-finish within a finally expression can cause infinite looping.

6.1.4.1  Examples of REPEAT clause
6.1.4.2  Examples of ALWAYS, NEVER, and THEREIS clauses
6.1.4.3  Examples of WHILE and UNTIL clauses


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