Re: C extensions

[Tim Meehan <btmeehan@gmail.com>]

Hello Olivier, thanks!
Those functions helped, and now I know where in the source of Guile to look.

Is "inexact" the same thing as "floating point number" in Guile-speak?

Here is one of the functions in my extension - it returns a lognormal
variate. There are a bunch of things like this that could be added to
"random.c" - but I'm imagining that the only user of stuff like that might
be ... me ... anyhow:

Right now, it doesn't give really helpful errors when you give it a
negative standard deviation:
scheme@(guile-user)> (random:lognormal 0 -1)
$1 = #f

SCM_DEFINE (scm_random_lognormal, "random:lognormal", 2, 1, 0,
            (SCM _mu, SCM _sigma, SCM state),
            "Return an inexact real in a Log Normal distribution.\n"
            "\n"
            "@var{mu} - mean of the underlying normal distribution.\n"
            "@var{sigma} - standard deviation of the same.\n"
            "Optionally, a random @var{state} can be provided.\n"
            "See @code{seed->random-state}.")
#define FUNC_NAME s_scm_random_lognormal
{
  if (SCM_UNBNDP (state))
    state = SCM_VARIABLE_REF (scm_var_random_state);
  SCM_VALIDATE_RSTATE (1, state);

  if (scm_is_false (scm_positive_p (_sigma)))
    return SCM_BOOL_F; // I'd like to return some meaningful error here ...

  double mu = scm_to_double (_mu);
  double sigma = scm_to_double (_sigma);
  return scm_from_double (exp (mu + sigma*scm_c_normal01
(SCM_RSTATE(state))));
}
#undef FUNC_NAME

On Sun, Feb 21, 2021 at 9:59 AM Olivier Dion <olivier.dion@polymtl.ca>
wrote:

> On Sat, 20 Feb 2021, Tim Meehan <btmeehan@gmail.com> wrote:
> > I'm trying my hand a writing C extensions.
> > I've done this for stuff like Matlab before, and was wondering how you do
> > the usual checking of the arguments that are passed in from Guile.
> >
> > In the manual, 6.13.13.1 "C Support" has a few functions.
> > libguile/numbers.h has a bunch more ...
> >
> > What I have is an extension function, sort of like the bessel function in
> > the tutorial:
> >
> https://www.gnu.org/software/guile/manual/html_node/A-Sample-Guile-Extension.html
>
> Usually I do this in 2 or 3 steps.
>
>   1. Define the primitive in C with raw arguments.
>
>   2. Make a wrapper of this primitive for Guile to use. Optionnaly do
>   the arguments checking here or go to step 3.
>
>   3. Make a second wrapper in Scheme and check the arguments there.
>   This wrapper will call wrapper in made in 2.
>
> Here's a full example:
> ----------------------------------------------------------------------
> float c_sin(double x)
> {
>    return sin(x);
> }
>
> SCM_DEFINE(c_sin_wrapper, "sin", 1, 0, 0,
>            (SCM x),
>            "My sine")
> {
>         if (scm_is_number(x)) {
>                 return c_sin(scm_to_double(x));
>         }
>
>         return SCM_BOOL_F;
> }
> ----------------------------------------------------------------------
>
> > What I would like to do is verify that the first argument is an inexact
> > number, larger than 0. How would I go about that? Perhaps some of it
> > could
>
> For your example, use the following predicates:
> ----------------------------------------------------------------------
> scm_is_inexact(x) && scm_is_true(scm_positive_p(x))
> ----------------------------------------------------------------------
>
> > be:
> > SCM_REALP
> > scm_misc_error
> >
> > Any tips?
> --
> Olivier Dion
> PolyMtl
>