[Jscheme-devel] Re: JScheme Q

[Ken A. <kan...@bb...>]

At 02:59 PM 4/23/2002, you wrote:
>is there a Scheme equivalent to CL's (return) ?
>
>I have loops I want to terminate early (i.e., at a certain condition being
>met), where I'd prefer for simplicity to use DOLIST rather than DO...
>
>is that appropriate in scheme?

Good question.  I keep meaning to write a tutorial for Scheme for Lispers.
The general way to return something is with call/cc 
(call-with-current-continuation)


 > (define (the-first p xs)
     (call/cc
      (lambda (return)
        (for-each* (lambda (x) (if (p x) (return x))) xs))))
{jsint.Closure the-first[2] (p xs)}
 > (the-first odd? #(2 3 4 5 6))
3
 >

Here's a version of dolist that lets you use return:

(define-macro (dolist iters . body)
   ;; Like Common Lisp's (dolist).
   (let ((var (car iters))
         (items (cadr iters))
         (result (or (and (pair? (cddr iters)) (caddr iters)) '())))
     `(call/cc
       (lambda (return)
         (let <loop> ((.items. ,items))
              (if (null? .items.) ,result
                  (let ((,var (car .items.)))
                    ,@body
                    (<loop> (cdr .items.)))))))))

 > {jsint.Macro ??[1,n] (iters . body)}
 > (dolist (x '(2 3 4 5 6))
     (if (odd? x) (return x)))
3
 >