From: Mark H Weaver <mhw@netris.org>
To: Alex Vong <alexvong1995@gmail.com>
Cc: guile-user@gnu.org, Pierre LINDENBAUM <Pierre.Lindenbaum@univ-nantes.fr>
Subject: Re: function registered with scm_c_define_gsubr: how can i handle an optional parameter?
Date: Fri, 15 Dec 2017 03:06:46 -0500 [thread overview]
Message-ID: <87y3m4za7d.fsf@netris.org> (raw)
In-Reply-To: <87374dwh8c.fsf@gmail.com> (Alex Vong's message of "Thu, 14 Dec 2017 21:50:59 +0800")
Hi Alex,
Alex Vong <alexvong1995@gmail.com> writes:
> Oh, I didn't really think too much about this. I always thought immediate
> objects like SCM_EOL, SCM_BOOL_T, ..., SCM_UNDEFINED can be compared
> using '=='. Is this an implementation detail that I should not depend
> on?
That's right. You should not rely on SCM being represented in any
particular way. In C, '==' works only on pointers and values of
arithmetic type, not on structs or unions. Therefore, in general, you
cannot use '==' on a value of an abstract type without some knowledge of
the underlying type.
Guile's API is designed to avoid propagating assumptions about the SCM
type. Please use only the functions and macros in Guile's public API to
inspect, compare, or manipulate values of type SCM. This will enable
Guile implementors and other hackers to experiment with other SCM data
representations. We reserve the right to change the SCM type in future
versions of Guile (but not within a stable release series e.g. 2.2.x).
I should also mention that Guile provides a mechanism to check for such
mistakes. If you test compiling your libguile-using code with
-DSCM_DEBUG_TYPING_STRICTNESS=2, it will change SCM to be a struct type,
and you'll get a compile error if you attempt to use '==' on values of
type SCM. (Note that this will produce code that is ABI-incompatible
with libguile compiled using the default SCM_DEBUG_TYPING_STRICTNESS=1
setting, so typically you would set SCM_DEBUG_TYPING_STRICTNESS=2 only
as a compile-time check.)
For details, see the description of SCM_DEBUG_TYPING_STRICTNESS in
__scm.h, and also the top few pages of tags.h, which describes the
fundamental concepts of how Guile Scheme objects are represented in C,
including the definitions of the SCM type and the 'scm_is_eq' macro.
Regards,
Mark
next prev parent reply other threads:[~2017-12-15 8:06 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-12-13 21:50 function registered with scm_c_define_gsubr: how can i handle an optional parameter? Pierre LINDENBAUM
2017-12-14 10:19 ` Alex Vong
2017-12-14 12:30 ` Mark H Weaver
2017-12-14 13:50 ` Alex Vong
2017-12-15 8:06 ` Mark H Weaver [this message]
2017-12-15 16:59 ` Alex Vong
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=87y3m4za7d.fsf@netris.org \
--to=mhw@netris.org \
--cc=Pierre.Lindenbaum@univ-nantes.fr \
--cc=alexvong1995@gmail.com \
--cc=guile-user@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).