Re: while break and continue

[Kevin Ryde <user42@zip.com.au>]

Marius Vollmer <mvo@zagadka.de> writes:
>
> Do you want to put it into boot-9.scm?

Yes, I think it'd be good.  Actually what I'd like to put in is
something like

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

All the backquoting being designed to protect against local bindings
at expansion time.  I know what you said about compiler friendliness,
but I think for the moment protection against local defines would be
better.

(I imagine a compiler might want to try to cooperate with application
macros like this using only basic list or procedure stuff, through
some sort of eval in a restricted environment or whatever.  But no
doubt plenty of people smarter than me have thought about such things
before.)


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