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Re: function registered with scm_c_define_gsubr: how can i handle an opt

From: Mark H Weaver
Subject: Re: function registered with scm_c_define_gsubr: how can i handle an optional parameter?
Date: Thu, 14 Dec 2017 07:30:18 -0500
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/25.3 (gnu/linux)

Hi Alex,

Alex Vong <address@hidden> 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.


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