Re: How to create a new procedure type?

[John Tobey <jtobey@channel1.com>]

Thanks, this is what I wanted to know.

Cheers
-John

Russ McManus <russell.mcmanus@gs.com> wrote:
> 
> (nobody) writes:
> 
> > This question has gone unanswered for two days, and I, too, would like
> > to know the answer.  From what I grasped of Guile's pre-1.3 dynamic
> > loading support, callbacks could only take strings as args.  This
> > seemed to me rather silly, since the actual arguments are of course
> > going to be Scheme objects.  I guess you could sprintf(buf,"%ld",scm)
> > but you'd probably outsmart the gc by doing it.
> > 
> > Is there any way to make a C function of type SCM (*)(int nargs, SCM*
> > args) in a dynamically loaded module callable from Scheme?
> 
> First let me say that I'm not sure that I understand your question.
> 
> But I'll attempt an answer.  Sure there is!  I'll try to answer your
> question in the context of one of the examples that appear in the
> guile FAQ (http://idt.net/~mcmanus/guile-faq/guile-faq.html).  Here is
> the example:
> 
>      /* $Id: hack.c,v 1.1 1999/04/14 21:18:02 mcmanr Exp $ */
>      
>      /* this code creates a new dynamically linkable module called (silly
>         hack).  there is precisely one primitive defined in this module,
>         '2+', which adds 2 to it's argument */
>      
>      #include <guile/gh.h>
>      
>      /* add new primitives here */
>      SCM_PROC(s_2_plus, "2+", 1, 0, 0, scm_2_plus);
>      static SCM
>      scm_2_plus(SCM x)
>      {
>          SCM_ASSERT(gh_number_p(x), x, SCM_ARG1, s_2_plus);
>          return(scm_sum(x, gh_int2scm(2)));
>      }
>      
>      /* the init function */
>      void
>      scm_init_hack(void)
>      {
>      #   include "hack.x"
>          return;
>      }
>      
>      /* the pre-init function */
>      void
>      scm_init_silly_hack_module()
>      {
>          scm_register_module_xxx("silly hack", (void*)scm_init_hack);
>      }
> 
> 
> This is just about the smallest dynamically loadable C module
> possible.  (You'll note that I'm not using gh_ functions, which is
> probably a bug in the FAQ, but for now, I'm just creating example the
> way I know how).
> 
> Take the SCM_PROC line.  It says that there is a new primitive
> function named "2+" from scheme, whose C implementation is the
> function scm_2_plus.  You can identify the c function in error
> messages with the C constant string s_2_plus.
> 
> The numeric argument in position three represents the number of
> required parameters.  The signature of scm_2_plus should start with
> this many parameters of type SCM.  These parameters will always have
> valid Scheme data when the runtime calls your function.
> 
> The numeric argument in position four represents the number of
> optional parameters.  The signature of scm_2_plus should contain, in
> addition to and following the parameters described previously, this
> many parameters.  Each of these parameters may have the special value
> SCM_UNDEFINED, depending on whether the user supplied the parameter.
> 
> The numeric argument in position five represents a C boolean
> expression that specifies whether the function accepts a 'rest' list,
> which corresponds to the dot notation in a lambda list.  I will try to
> add an example of this functionality.  If the parameter is true (!= 0
> in C parlance), then the signature for scm_2_plus should contain an
> additional parameter, appearing at the end of the parameter list.
> This final parameter will contain the user supplied list of additional
> arguments to the C primitive, or SCM_EOL if no list was supplied.  So
> this case is the one that I think will enable you to do what you want.
> 
> I think that there is a gh_ analog to all of this, which I need to
> research and add to the FAQ.  If I'm totally off base about your
> question, my regrets.
> 
> -russ
> 
> 
> --
> Death to all fanatics!
> 
>