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(cl)Iteration Clauses

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Iteration Clauses
-----------------

Aside from `for' clauses, there are several other loop clauses that
control the way the loop operates.  They might be used by themselves,
or in conjunction with one or more `for' clauses.

`repeat INTEGER'
     This clause simply counts up to the specified number using an
     internal temporary variable.  The loops

          (loop repeat n do ...)
          (loop for temp to n do ...)

     are identical except that the second one forces you to choose a
     name for a variable you aren't actually going to use.

`while CONDITION'
     This clause stops the loop when the specified condition (any Lisp
     expression) becomes `nil'.  For example, the following two loops
     are equivalent, except for the implicit `nil' block that surrounds
     the second one:

          (while COND FORMS...)
          (loop while COND do FORMS...)

`until CONDITION'
     This clause stops the loop when the specified condition is true,
     i.e., non-`nil'.

`always CONDITION'
     This clause stops the loop when the specified condition is `nil'.
     Unlike `while', it stops the loop using `return nil' so that the
     `finally' clauses are not executed.  If all the conditions were
     non-`nil', the loop returns `t':

          (if (loop for size in size-list always (> size 10))
              (some-big-sizes)
            (no-big-sizes))

`never CONDITION'
     This clause is like `always', except that the loop returns `t' if
     any conditions were false, or `nil' otherwise.

`thereis CONDITION'
     This clause stops the loop when the specified form is non-`nil';
     in this case, it returns that non-`nil' value.  If all the values
     were `nil', the loop returns `nil'.