NetNews Usenet Archive 1992 #16/NN_1992_16.iso/spool/comp/lang/lisp/2127

Text File | 1992-07-28 | 2.1 KB | 52 lines

Newsgroups: comp.lang.lisp
Path: sparky!uunet!stanford.edu!CSD-NewsHost.Stanford.EDU!Xenon.Stanford.EDU!phil
From: [e-mail] (Phil Stubblefield)
Subject: Re: Help with EVAL-WHEN
Message-ID: <[e-mail]>
Sender: [e-mail]
Organization: Computer Science Department, Stanford University.
References: <[e-mail]> <[e-mail]>
Date: Tue, 28 Jul 1992 18:32:37 GMT
Lines: 40

In article <[e-mail]> [e-mail]
 (Sandra Loosemore) writes:
>The problem is that the code nested inside the UNLESS is not at
>top-level.  EVAL-WHEN can cause compile-time evaluation only when
>it appears at top-level, so the inner EVAL-WHEN in your code doesn't
>do anything useful.  Defining forms such as DEFCONSTANT also have
>no compile-time effect unless they appear at top-level.

Sandra,

I think my confusion stems from a misunderstanding of X3J13 action #50, or
DEFINING-MACROS-NON-TOP-LEVEL:ALLOW, if you don't remember off the top of
your head.  :)  I had interpreted the various CLtL/2 references to this vote
to mean that these defining forms would work exactly the same in non-
top-level contexts.  Rereading the top paragraph on p. 84 helped to clarify
that the real issue behind this vote is that the various defining forms
must capture the current lexical environment correctly, although it is
still the case that "certain compile-time side effects of these defining
forms are performed only when the defining forms occur at top level."

>A better way to do this kind of conditionalization would be something
>like this:
>
>(eval-when (:compile-toplevel :load-toplevel)
>  (if (fboundp 'foo) (pushnew :foo-feature *features*)))
>
>#-:foo-feature
>(progn
>  ; code to define your own foo goes here.
>  )
>
>Notice that PROGN does pass "top-level-ness" on to its subforms.
>
>-Sandra

Thanks much!
--

Phil Stubblefield                                        [phone]
Rockwell Palo Alto Laboratory                    [e-mail]