All five evaluate test-form each time through the iteration and decide, based on the resulting value, whether to terminate the loop. They differ in what happens after they terminate the loop—if they do—and how they decide.

The loop keywords while and introduce the “mild” termination clauses. When they decide to terminate the loop, control passes to the epilogue, skipping the rest of the loop body. The epilogue can then return a value or do whatever it wants to finish the loop. A while clause terminates the loop the first time the test form is false; until, conversely, stops it the first time the test form is true.

The other three clauses—always, never, and thereis--terminate the loop with extreme prejudice; they immediately return from the loop, skipping not only any subsequent loop clauses but also the epilogue. They also provide a default value for the loop even when they don’t cause the loop to terminate. However, if the loop is not terminated by one of these termination tests, the epilogue is run and can return a value other than the default provided by the termination clauses.

Because these clauses provide their own return values, they can’t be combined with accumulation clauses unless the accumulation clause has an into subclause. The compiler (or interpreter) should signal an error at compile time if they are.The always and never clauses return only boolean values, so they’re most useful when you need to use a loop expression as part of a predicate. You can use to check that the test form is true on every iteration of the loop. Conversely, never tests that the test form evaluates to **NIL** on every iteration. If the test form fails (returning **NIL** in an always clause or non-**NIL** in a never clause), the loop is immediately terminated, returning **NIL**. If the loop runs to completion, the default value of **T** is provided.

Equivalently you could write the following:

A thereis clause is used to test whether the test form is ever true. As soon as the test form returns a non-**NIL** value, the loop is terminated, returning that value. If the loop runs to completion, the thereis clause provides a default return value of .