Re: complex numbers in libguile

[Jim Blandy <jimb@red-bean.com>]

> The gsl contains many functions that involve complex numbers. While they
> are a custom datatype in C libraries, they're a built-in type in scheme.
> How does one pass real- and imgainary part of a SCM obj representing a
> complex number through libguile, so that the gsl can map it from and to
> an instance of the corresponding struct ? I don't feel like constructing 
> a smob for something that already has a built-in equivalent.

I take it the custom datatype used by gsl is something like

	struct complex { double x, y };

What's wrong with converting between that and Guile's normal complex
number representation?  That is, to convert a gsl complex number to a
Scheme value, call scm_makdbl; to do the reverse, do something like
this:

struct complex scm2complex (SCM z);
{
  struct complex out;

  out.x = scm_num2dbl (z);
  if (SCM_CPLXP (z))
    out.y = SCM_IMAG(z);
  else
    out.y = 0.0;

  return out;
}