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Make the "Dia Primitives" section of the manual more clear.

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* doc/ref/libguile-program.texi (Dia Primitives): Use
  scm_assert_smob_type, and update the text.
This commit is contained in:
Andy Wingo 2011-12-22 09:32:09 -05:00
parent 61fe8eafc2
commit e0a8221dcf
1 changed files with 6 additions and 7 deletions
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13
doc/ref/libguile-program.texi
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@ -279,13 +279,12 @@ As an example, here is a possible implementation of the @code{square?}
primitive:
@lisp
#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);
scm_assert_smob_type (shape_tag, shape);
/* Access Scheme-specific shape structure. */
guile_shape = SCM_SMOB_DATA (shape);
@ -295,7 +294,6 @@ static SCM square_p (SCM shape)
return scm_from_bool (guile_shape->c_shape &&
(guile_shape->c_shape->type == DIA_SQUARE));
@}
#undef FUNC_NAME
@end lisp
Notice how easy it is to chain through from the @code{SCM shape}
@ -303,10 +301,11 @@ parameter that @code{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, @code{SCM_SMOB_DATA} and @code{scm_from_bool} are from
the standard Guile API. @code{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
In this code, @code{scm_assert_smob_type}, @code{SCM_SMOB_DATA}, and
@code{scm_from_bool} are from the standard Guile API. We assume that
@code{shape_tag} was given to us when we made the shape SMOB type, using
@code{scm_make_smob_type}. The call to @code{scm_assert_smob_type}
ensures that @var{shape} is indeed a shape. This is needed to guard
against Scheme code using the @code{square?} procedure incorrectly, as
in @code{(square? "hello")}; Scheme's latent typing means that usage
errors like this must be caught at run time.