Re: while break and continue

[Matthias Koeppe <mkoeppe@mail.math.uni-magdeburg.de>]

Kevin Ryde <user42@zip.com.au> writes:

+(define-macro (while cond . body)
+  (let ((key (make-symbol "while-key")))
+    `(,do ((break    ,(lambda () (throw key #t)))
+	   (continue ,(lambda () (throw key #f))))
+	 ((,catch (,quote ,key)
+		  (,lambda ()
+		    (,do ()
+			((,not ,cond))
+		      ,@body)
+		    #t)
+		  ,(lambda (key arg) arg))))))

I think you need a fresh catch tag for every invocation (not only for
every macro expansion site) of WHILE.  Consider this recursive
example:

(define (rec branch breaker)
  (while #t
    (case branch
      ((1)
       (display ".")
       (breaker))    ; should break the (dynamically) outer loop
                     ; but only breaks the inner loop
      ((2)
       (rec 1 break)))))

(rec 2 (lambda () #t))

This should display one "." and return because the dynamically outer
loop (branch = 2) is broken.  But with your implementation, both loops
share the same catch tag, so only the inner loop (branch = 1) is
broken, hence the outer loop continues indefinitely.

-- 
Matthias Koeppe -- http://www.math.uni-magdeburg.de/~mkoeppe


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