Xref: utzoo comp.lang.lisp:624 comp.emacs:2442
Path: utzoo!mnetor!uunet!husc6!uwvax!ai!neves
From: neves@ai.WISC.EDU (David M. Neves)
Newsgroups: comp.lang.lisp,comp.emacs
Subject: Re: A CL iteration macro, "while".
Message-ID: <4935@spool.cs.wisc.edu>
Date: 22 Dec 87 22:40:41 GMT
References: <13639@beta.UUCP>
Sender: news@spool.cs.wisc.edu
Reply-To: neves@ai.WISC.EDU (David M. Neves)
Organization: U of Wisconsin CS Dept
Lines: 58
Keywords: Let's have a little non-flamable fun...
Summary: misunderstanding macros

[[When I see articles that have obvious errors I usually wait a few
days because I know that others will most likely post responses.  In
this case I happened to have defined a while macro myself.  So...]]

In article <13639@beta.UUCP> dzzr@beta.UUCP (Douglas J Roberts) writes:
>
...
>
>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.
>
...
>
>(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)
>	       ))
>  ))
There are a couple of problems with this code.
1.  You use mapcar to iterate through the forms.  Mapcar is going to
create a list as a result.  Since you are not using the result of
mapcar you have done unnecessary CONSing.  This means your program will
run slower because it will do more garbage collection.  Use mapc
instead. 
2.  Even more serious is the way you defined the macro.  You are
almost using it as we used to use fexprs or nlambdas[1].  A macro
translates its input into another form, which then is evaluated.  Your
code will work if it is interpreted but not when it is compiled.  The
compiler will substitute the translation (in this case it will be nil,
the value of the prog) for each call to "while". 

[1] (A reason for using macros rather than fexprs is to get the
correct scoping when the forms are evaluated.  I won't go more into
this here.)

Here is a correct definition of while in Common Lisp.

(defmacro while (test &rest body)
  `(do nil (,test) ,@body))

"`" is the backquote character.  "," evaluates the s-expression after
it (within a backquote).  ",@" is similar to "," in that it evaluates
the s-expression that follows it.  It is different in that the value
of the s-expression is spliced into the existing list.



David Neves, Computer Sciences Department, University of Wisconsin-Madison
Usenet:  {rutgers,ucbvax,ihnp4}!uwvax!neves
Arpanet: neves@cs.wisc.edu