Path: utzoo!utgpu!water!watmath!clyde!rutgers!mcnc!decvax!decwrl!sun!pitstop!sundc!seismo!uunet!mcvax!ukc!its63b!aiva!jeff
From: jeff@aiva.ed.ac.uk (Jeff Dalton)
Newsgroups: comp.lang.lisp
Subject: Re: CL question - conditional list elements
Message-ID: <244@aiva.ed.ac.uk>
Date: 29 Jan 88 18:05:31 GMT
References: <1350005@otter.HP.COM> <2002@russell.STANFORD.EDU> <591@cresswell.quintus.UUCP>
Reply-To: jeff@uk.ac.ed.aiva (Jeff Dalton)
Organization: Dept. of AI, Univ. of Edinburgh, UK
Lines: 48

In article <591@cresswell.quintus.UUCP> ok@quintus.UUCP (Richard A. O'Keefe) writes:
>The giveaway was the Pop version.
>	[% a, b, if t1 then c endif, d if t2 then e endif, f, g %]
>operates like this:
>	push a magic marker on the stack
>	push a
>	push b
>	if t1 then push c endif
>	push d
>	if t2 then push e endif
>	push f
>	push g
>	make a list of everything from the top of the stack
>		down to the magic marker.

Oh, so it's pushing on the stack we want.  That can be done:

     1> (defmacro include-if (test values)
          `(if ,test ,values (values)))
     INCLUDE-IF

     2> (multiple-value-call #'list
           1 2 3 4 5
           (include-if t (values 6 7))
           8 9 10
           (include-if nil (values 11 12))
           13 14 15)
     (1 2 3 4 5 6 7 8 9 10 13 14 15)

Is that enough?  I can write the elements not a list thereof.  It's
guaranteed to build a new list.  It doesn't officially generate any
garbage.  (Does someone complain that it's not GUARANTEED not to
produce garbage?  Such guarantees are easy for PopLog because there's
only one implementation.)

>The general answer to questions of the form "here's something I can
>do in Pop, how do I do it in CL" is "you learn how to write Lisp and
>then you won't find yourself wanting it."  Of course the same is true
>in the other direction.  The other general answer seems to be "write
>a macro..."

Some people, I think, will always want some things even after they
learn how to write Lisp.  After all, Lisp can't be all things to all
people.

Jeff Dalton,                      JANET: J.Dalton@uk.ac.ed             
AI Applications Institute,        ARPA:  J.Dalton%uk.ac.ed@nss.cs.ucl.ac.uk
Edinburgh University.             UUCP:  ...!ukc!ed.ac.uk!J.Dalton