subtypep: structs with car or length method are sequences.

* lib.c (subtypep): For the sequence supertype, check
whether the subtype is a structure that has a length or
car method, returning t if so.

* struct.c (get_special_slot_by_type): New function.

* struct.h (get_special_slot_by_type): Declared.

* txr.1: Add <structures with cars or length methods> to the
type hierarchy diagram.

1 files changed, 7 insertions, 0 deletions

diff --git a/struct.c b/struct.c
index 87ea9b96..82ea5ef4 100644
--- a/struct.c
+++ b/struct.c
@@ -1773,6 +1773,13 @@ val get_special_slot(val obj, enum special_slot spidx)
   return get_special_static_slot(si->type, spidx, slot);
 }
 
+val get_special_slot_by_type(val stype, enum special_slot spidx)
+{
+  struct struct_type *st = coerce(struct struct_type *, stype->co.handle);
+  val slot = *special_sym[spidx];
+  return get_special_static_slot(st, spidx, slot);
+}
+
 static_def(struct cobj_ops struct_type_ops =
            cobj_ops_init(eq, struct_type_print, struct_type_destroy,
                          struct_type_mark, cobj_eq_hash_op));