Re: C extensions

[Olivier Dion via General Guile related discussions <guile-user@gnu.org>]

On Sun, 21 Feb 2021, Tim Meehan <btmeehan@gmail.com> wrote:
> 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?

I don't think so.  If I remember correctly, exact numbers are numbers
that can be represented by a quotient of two integers.  In other words,
any rational number is an exact number.  I might be wrong here.  I
highly suggest that you read the numerical tower from the Guile's
texinfo manual for more informations on that.

>
> 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

In my example, I returned false because this is how I would approach the
problem from a C perspective.  i.e., returning an error code.  I'm still
learning on how to work with Scheme.  I guess that if you want something
more Pythonic, you would throw an exception.

> 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 ...

You can use `scm_throw()` to throw an exception.
----------------------------------------------------------------------
/*
 * This create a symbol that has static linkage in C.  You might want
 * to declare it globally instead.
 */
SCM_SYMBOL(lognormal_error_key, "invalid-argument");

if (scm_is_false (scm_positive_p (_sigma))) {
   scm_throw(lognormal_error_key, scm_list_n(SCM_UNDEFINED));
   /* Unreachable path! */
   assert(0);
}
----------------------------------------------------------------------

Note that throwing an error might be easier to do in Scheme itself:
----------------------------------------------------------------------
(throw 'invalid-argument)
----------------------------------------------------------------------

>
>   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
>>
-- 
Olivier Dion
PolyMtl