bug#31154: predicate function for foreign-object not obvious

[Matt Wette <matt.wette@gmail.com>]

The reference manual (guile-2.2.3) does not provide instruction on how to generate
a predicate for a foreign object, and it is not obvious.  Maybe it doesn't need to
be in the manual but should then be in an example somewhere, IMO.

The following patch to the reference manual is one option:


--- doc/ref/api-foreign-objects.texi-orig	2018-04-14 11:26:35.779502997 -0700
+++ doc/ref/api-foreign-objects.texi	2018-04-14 11:28:42.823072217 -0700
@@ -96,6 +96,33 @@
  needed.
  @end deftypefn
  
+To generate a predicate for a foreign object use the @code{SCM_IS_A_P}
+macro as follows.
+
+@example
+static SCM foo_type;
+
+static SCM
+scm_foo_p(SCM obj) {
+  return SCM_IS_A_P(obj, foo_type)? SCM_BOOL_T: SCM_BOOL_F;
+}
+
+void
+init_foo()
+{
+  SCM name, slots;
+
+  name = scm_from_utf8_symbol("foo");
+  slots = scm_list_2 (scm_from_utf8_symbol ("data"),
+		      scm_from_utf8_symbol ("deps"));
+  finalizer = NULL;
+  foo_type = scm_make_foreign_object_type(name, slots, finalizer);
+
+  scm_c_define_gsubr("foo?", 1, 0, 0, scm_foo_p);
+  ...
+}
+@end example
+
  One can also access foreign objects from Scheme.  @xref{Foreign Objects
  and Scheme}, for some examples.