New equality substitution.

If the equal method is defined for structs, its return
value is used in their place for hashing and comparison.

* eval.h (eq_s, eql_s, equal_s): Declared.

* hash.c (equal_hash): If a COBJ defines an equalsub
function, we call it. If it returns non-nil, we
take the object in its place and recurse.

* lib.c (equal): Refactored to support equality substitution.
(less): Support equality substitution.

* lib.h (cobj_ops): New function pointer member, equalsub.
Only struct instances define this, currently.
(cobj_ops_init): Add null entry to initializer for equalsub.
(cobj_ops_init_ex): New initialiation macro for
situations when the equalsub member must be provided.

* struct.c (struct struct_type): new member eqmslot.
(make_struct_type): Initialize emslot to zero.
(static_slot_set, static_slot_ensure): If eqmslot is -1,
indicating positive knowledge that there is no equal method
static slot, we must invalidate that with a zero: it is no
longer known whether there is or isn't such a slot.
(get_equal_method, struct_inst_equalsub): New static functions.
(struct_inst_ops): Initialize the equalsub member using
new cobj_ops_init_ex macro.

* txr.1: Document equality substitution.

1 files changed, 5 insertions, 0 deletions

diff --git a/hash.c b/hash.c
index 77de4dea..e844d5f3 100644
--- a/hash.c
+++ b/hash.c
@@ -173,6 +173,11 @@ static cnum equal_hash(val obj)
   case FLNUM:
     return hash_double(obj->fl.n);
   case COBJ:
+    if (obj->co.ops->equalsub) {
+      val sub = obj->co.ops->equalsub(obj);
+      if (sub)
+        return equal_hash(sub);
+    }
     return obj->co.ops->hash(obj) & NUM_MAX;
   case RNG:
     return (equal_hash(obj->rn.from)