From: Alex Vong <alexvong1995@gmail.com>
To: Mark H Weaver <mhw@netris.org>
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: Thu, 14 Dec 2017 21:50:59 +0800 [thread overview]
Message-ID: <87374dwh8c.fsf@gmail.com> (raw)
In-Reply-To: <876099cx0l.fsf@netris.org> (Mark H. Weaver's message of "Thu, 14 Dec 2017 07:30:18 -0500")
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Hello Mark,
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?
Mark H Weaver <mhw@netris.org> writes:
> Hi Alex,
>
> Alex Vong <alexvong1995@gmail.com> writes:
>
>> From the Guile manual ``6.1 Overview of the Guile API'',
>>
>>> For some Scheme functions, some last arguments are optional; the
>>> corresponding C function must always be invoked with all optional
>>> arguments specified. To get the effect as if an argument has not been
>>> specified, pass ‘SCM_UNDEFINED’ as its value. You can not do this for
>>> an argument in the middle; when one argument is ‘SCM_UNDEFINED’ all the
>>> ones following it must be ‘SCM_UNDEFINED’ as well.
>>
>> Therefore, we can check if opt_arg2 has the value SCM_UNDEFINED, to
>> decide if we have received an optional argument.
>
> Yes indeed, and your attached code is *almost* correct, except for one
> serious problem:
>
>> #include <libguile.h>
>>
>> static SCM myfun(SCM arg1,SCM opt_arg2)
>> {
>> if (opt_arg2 == SCM_UNDEFINED)
>
> You must not use '==' to compare values of type SCM. Instead, use
> 'scm_is_eq', like this:
>
> if (scm_is_eq (opt_arg2, SCM_UNDEFINED))
>
> Guile explicitly makes no promises about the SCM type, so there's no
> guarantee that '==' will do the right thing. SCM is to be treated as an
> abstract type. Look up 'SCM' in the Guile manual's index. Section 6.3
> (The SCM Type) of the manual states:
>
> -- C Type: SCM
> ‘SCM’ is the user level abstract C type that is used to represent
> all of Guile’s Scheme objects, no matter what the Scheme object
> type is. No C operation except assignment is guaranteed to work
> with variables of type ‘SCM’, so you should only use macros and
> functions to work with ‘SCM’ values. [...]
>
> Here's the Guile manual entry for 'scm_is_eq', which also mentions that
> you must not use '==':
>
> -- C Function: int scm_is_eq (SCM x, SCM y)
> Return ‘1’ when X and Y are equal in the sense of ‘eq?’, otherwise
> return ‘0’.
>
> The ‘==’ operator should not be used on ‘SCM’ values, an ‘SCM’ is a
> C type which cannot necessarily be compared using ‘==’ (*note The
> SCM Type::).
>
> Other than that, the attached code looks good to me.
>
> Mark
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next prev parent reply other threads:[~2017-12-14 13:50 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 [this message]
2017-12-15 8:06 ` Mark H Weaver
2017-12-15 16:59 ` Alex Vong
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