(guile.info.gz) Dia Primitives

 
 17.1.4 Writing Guile Primitives for Dia
 ---------------------------------------
 
 Once the details of object representation are decided, writing the
 primitive function code that you need is usually straightforward.
 
    A primitive is simply a C function whose arguments and return value
 are all of type `SCM', and whose body does whatever you want it to do.
 As an example, here is a possible implementation of the `square?'
 primitive:
 
      #define FUNC_NAME "square?"
      static SCM square_p (SCM shape)
      {
        struct dia_guile_shape * guile_shape;
 
        /* Check that arg is really a shape SMOB. */
        SCM_VALIDATE_SHAPE (SCM_ARG1, shape);
 
        /* Access Scheme-specific shape structure. */
        guile_shape = SCM_SMOB_DATA (shape);
 
        /* Find out if underlying shape exists and is a
           square; return answer as a Scheme boolean. */
        return SCM_BOOL (guile_shape->c_shape &&
                         (guile_shape->c_shape->type == DIA_SQUARE));
      }
      #undef FUNC_NAME
 
    Notice how easy it is to chain through from the `SCM shape'
 parameter that `square_p' receives -- which is a SMOB -- to the
 Scheme-specific structure inside the SMOB, and thence to the underlying
 C structure for the shape.
 
    In this code, `SCM_SMOB_DATA' and `SCM_BOOL' are macros from the
 standard Guile API.  `SCM_VALIDATE_SHAPE' is a macro that you should
 define as part of your SMOB definition: it checks that the passed
 parameter is of the expected type.  This is needed to guard against
 Scheme code using the `square?' procedure incorrectly, as in `(square?
 "hello")'; Scheme's latent typing means that usage errors like this
 must be caught at run time.
 
    Having written the C code for your primitives, you need to make them
 available as Scheme procedures by calling the `scm_c_define_gsubr'
 function.  `scm_c_define_gsubr' (REFFIXME) takes arguments that specify
 the Scheme-level name for the primitive and how many required, optional
 and rest arguments it can accept.  The `square?' primitive always
 requires exactly one argument, so the call to make it available in
 Scheme reads like this:
 
      scm_c_define_gsubr ("square?", 1, 0, 0, square_p);
 
    For where to put this call, see the subsection after next on the
 structure of Guile-enabled code ( Dia Structure).
Info Catalog
(guile.info.gz) Dia Smobs
(guile.info.gz) Extending Dia
(guile.info.gz) Dia Hook
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