2.5 Variable initializations

2.5.1 Sequential and parallel initialization

By default, thewith construct uses sequential initialization of variables; that is, one variable is assigned a value before the next expression is evaluated. However, by using the loop keywordand to join severalwith clauses, you can force parallel initialization; that is, all of the specified expressions are evaluated, and the results are bound to the respective variables simultaneously.

Use sequential binding when you want the initialization of some variables to depend on the values of previously bound variables. For example, suppose you want to bind the variablesa,b, andc in sequence:

> (loop with a = 1 
        with b = (+ a 2) 
        with c = (+ b 3)
        return (list a b c))
(1 3 6)

The execution of the above loop is equivalent to the execution of the following code:

(let* ((a 1)
       (b (+ a 2))
       (c (+ b 3)))
  (block nil
    (tagbody
        next-loop (return (list a b c))
       (go next-loop)
     end-loop)))

If you are not depending on the value of previously bound variables for the initialization of other local variables, you can use anand clause to force the bindings to occur in parallel:

> (loop with a = 1 
        and b = 2 
        and c = 3
        return (list a b c))
(1 2 3)

The execution of the above loop is equivalent to the execution of the following code:

(let ((a 1)
      (b 2)
      (c 3))
  (block nil
    (tagbody
        next-loop (return (list a b c))
                  (go next-loop)
                  end-loop)))

The Loop Facility - 9 SEP 1996

Generated with Harlequin WebMaker