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/lib.c b/lib.c
index 99284d9f..225ea92d 100644
--- a/lib.c
+++ b/lib.c
@@ -232,6 +232,13 @@ val subtypep(val sub, val sup)
   } else if (sup == list_s) {
     return tnil(sub == null_s || sub == cons_s || sub == lcons_s);
   } else if (sup == sequence_s) {
+    val sub_struct = find_struct_type(sub);
+    if (sub_struct) {
+      if (get_special_slot_by_type(sub_struct, length_m) ||
+          get_special_slot_by_type(sub_struct, car_m))
+        return t;
+      return nil;
+    }
     return tnil(sub == str_s || sub == lit_s || sub == lstr_s ||
                 sub == vec_s || sub == null_s || sub == cons_s ||
                 sub == lcons_s || sub == list_s || sub == string_s);