Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!linus!philabs!cmcl2!harvard!husc6!panda!genrad!decvax!decwrl!glacier!kestrel!king
From: king@kestrel.UUCP
Newsgroups: net.ai,net.lang.lisp
Subject: Re: Common LISP style standards.
Message-ID: <8760@kestrel.ARPA>
Date: Tue, 3-Jun-86 15:41:30 EDT
Article-I.D.: kestrel.8760
Posted: Tue Jun  3 15:41:30 1986
Date-Received: Thu, 5-Jun-86 19:25:09 EDT
Organization: Kestrel Institute, Palo Alto, CA
Lines: 42
Xref: linus net.ai:3275 net.lang.lisp:771


   From: michaelm@bcsaic.UUCP (michael maxwell)
   Newsgroups: net.ai,net.lang.lisp
   Date: 15 May 86 17:42:18 GMT
   Reply-To: michaelm@bcsaic.UUCP (michael maxwell)
   Distribution: net

	.	.	.

   A common situation we find ourselves in is the following.  We have
	a long list, 
   and we wish to apply some test to each member of the list.  However, at some
   point in the list, if the test returns a certain value, there is no need to
   look further: we can jump out of processing the list right there, and thus
   save time.  Now you can jump out of a do loop with "(return
	<value>)", but you 
   can't jump out of a mapc (mapcar etc.) with "return."  So we wind up using
   "do" a lot of places where it would otherwise be natural to use "mapcar".  I
   suppose I could use "catch" and "throw", but that looks so much like "goto"
   that I feel sinful if I use that solution...

   Any style suggestions?

I'm way behind in this group, so I apologize in advance if you have
seen this solution or a better one before.

You might try

(prog () 
   (mapcar #'(lambda (y) (when (you-like y) (return (result-for y))))
           x)))

I tried it, and it works.  It doesn't seem dirty to me, and it should
be efficient.  Even if the return point of a prog is such that it
forces the lexical closure to be non-vacuous, this shouldn't be a
problem when compiled.

   -- 
   Mike Maxwell
   Boeing Artificial Intelligence Center
	   ...uw-beaver!uw-june!bcsaic!michaelm

-dick