unofficial mirror of guile-devel@gnu.org 
 help / color / mirror / Atom feed
From: David Kastrup <dak@gnu.org>
To: guile-devel@gnu.org
Subject: Re: when and unless
Date: Tue, 06 Dec 2011 12:17:06 +0100	[thread overview]
Message-ID: <87mxb6kkzx.fsf@fencepost.gnu.org> (raw)
In-Reply-To: CAMMPzYMoKuQSWnBMCoZ_+ggcrmk+SGvbyaKvYLb=eJVRbR6E1Q@mail.gmail.com

Alex Shinn <alexshinn@gmail.com> writes:

> On Tue, Dec 6, 2011 at 4:48 PM, Marijn <hkBst@gentoo.org> wrote:
>>
>> Couldn't help but wonder why they don't return the value of the last
>> body form, so I looked around a bit and both CLHS[1] and my racket
>> REPL seem to agree that they should:
>>
>> $ racket
>> Welcome to Racket v5.2.0.4.
>>> (when #t 'hello)
>> 'hello
>>> (unless #f 'hi)
>> 'hi
>>
>> Is there some other source that suggests that the return value should
>> be unspecified?
>
> Because the result is meaningless when the condition is false.
>
> CLHS returns nil in this case, but that fits with CL idioms and
> not Scheme idioms.  when and unless are for side-effects - it's
> better to write (and #t 'hello) if you want the result.
>
> R7RS also leaves it unspecified.

I've actually wondered if it would not make sense to return
*unspecified* in the case of the plain else-less if even if the
condition is true, namely when you write (if #t #t).

There is probably code relying on this to be #t, but frankly, this
appears like a recipe for breakage.

-- 
David Kastrup




  reply	other threads:[~2011-12-06 11:17 UTC|newest]

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2011-06-30 10:44 when and unless Andy Wingo
2011-06-30 21:46 ` Ludovic Courtès
2011-07-01  7:50   ` Andy Wingo
2011-07-01 12:47     ` Ludovic Courtès
2011-12-05 20:23 ` Andy Wingo
2011-12-06  7:48   ` Marijn
2011-12-06  8:29     ` Alex Shinn
2011-12-06 11:17       ` David Kastrup [this message]
2011-12-06 16:25         ` Andy Wingo
2011-12-06 16:42           ` David Kastrup
2011-12-06 17:35             ` Andy Wingo
2011-12-06 22:08               ` David Kastrup
2011-12-06 23:05                 ` Chris K. Jester-Young
2011-12-07  9:23                   ` David Kastrup
2011-12-07 15:58                     ` Chris K. Jester-Young
2011-12-08  8:42                       ` David Kastrup
2011-12-08 15:34                         ` Chris K. Jester-Young
2011-12-08 16:10                           ` David Kastrup
2011-12-08 18:10                         ` Ian Price
2011-12-07 16:10         ` Chris K. Jester-Young
2011-12-06 19:05     ` Chris K. Jester-Young
2011-12-06 19:33       ` Andy Wingo
2011-12-06 14:39   ` Ludovic Courtès
2011-12-07 14:19     ` Ludovic Courtès
2011-12-07 14:27       ` David Kastrup
2012-01-07  0:16       ` Andy Wingo
2012-01-07 22:36         ` Ludovic Courtès
2012-01-20 20:19           ` Andy Wingo

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

  List information: https://www.gnu.org/software/guile/

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=87mxb6kkzx.fsf@fencepost.gnu.org \
    --to=dak@gnu.org \
    --cc=guile-devel@gnu.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).