Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!rutgers!clyde!burl!codas!mtune!whuts!davel
From: davel@whuts.UUCP (David Loewenstern)
Newsgroups: comp.lang.lisp
Subject: Re: A CL iteration macro, "while".
Message-ID: <3503@whuts.UUCP>
Date: 30 Dec 87 21:31:34 GMT
References: <13639@beta.UUCP>
Organization: AT&T Bell Laboratories
Lines: 72
Keywords: Let's have a little non-flamable fun...

In article <13639@beta.UUCP>, dzzr@beta.UUCP (Douglas J Roberts) writes:
> 
> I was poking around in the source for Stallman's Gnu Emacs the other day
> and stumbled across a "while" function that I kind of liked. It's not
> Common nor Zeta LISP, and so I'd never seen it before. We have
> Symbolic's big loop macro installed on all of our Common LISP machines
> (Suns, TI Explorers, & Symbolics), but even with it there is no clean
> way to do simple iteration control that I really like. (I don't like the
> syntax of do, dotimes, dolist, etc.)
I certainly don't blame you.  DOTIMES and DOLIST don't bother me, but DO
is REALLY ugly -- not much better than using PROG.
> 
> I thought it would be fun to write a CL while macro and then post it
> for comment, etc. I'd be curious for anybody interested enough to
> suggest other ways of writing it.
> 
> Here 'tis.
> 
> 
> (defmacro while (test-form &rest forms)
>   "This macro evaluates test-form, and if the result is non-nil
> 	all subsequent forms will be iteratively evaluated until
> 	test-form evaluates to nil."
>   (prog ()
>      again 
> 	(cond (
> 	       (eval test-form) 
> 	       (mapcar #'eval forms)
> 	       (go again)
> 	       ))
>   ))
This macro performs the loop on expansion.  It cannot be usefully compiled, 
since it always expands to NIL.  Try calling the following function *twice*:
(DEFUN X (n)
   (WHILE (PLUSP n)
      (WHEN (ODDP n) (PRINT n))
      (DECF n)))

I predict: typing (X 5) will print
5
3
1
NIL
and typing (X 5) again will print
NIL
certainly this is the standard behavior on the Symbolics and Explorers.

What is wrong with
  (LOOP while (< *number* 5)
        do (print *number*)
           ...
           (setq *number* (1+ *number*)))

?  In any event, what you probably meant was

(DEFMACRO While (test &BODY body)
  `(PROG ()
     AGAIN
          (WHEN ,test ,@body (GO AGAIN))))

except that this doesn't do anything more than the LOOP macro which you
already have.

> 			Doug Roberts
> 			dzzr@lanl.gov



-- 
David Loewenstern                     
Bangpath: {rutgers!moss,ihnp4}!whuts!davel
          or -- in extremis -- loewenst@paul.rutgers.edu
Disclaimer: My opinions are controlled by the Bavarian Illuminati, not by AT&T.