Warning: This is the manual of the legacy Guile 2.0 series. You may want to read the manual of the current stable series instead.
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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:
static SCM square_p (SCM shape) { struct dia_guile_shape * guile_shape; /* Check that arg is really a shape SMOB. */ scm_assert_smob_type (shape_tag, 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_from_bool (guile_shape->c_shape && (guile_shape->c_shape->type == DIA_SQUARE)); }
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_assert_smob_type
, SCM_SMOB_DATA
, and
scm_from_bool
are from the standard Guile API. We assume that
shape_tag
was given to us when we made the shape SMOB type, using
scm_make_smob_type
. The call to scm_assert_smob_type
ensures that shape is indeed a shape. 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
(see Primitive Procedures) 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 (see Dia Structure).
Next: Dia Hook, Previous: Dia Smobs, Up: Extending Dia [Contents][Index]