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, 6 insertions, 1 deletions

diff --git a/lib.h b/lib.h
index 3b10ce6d..7b33c784 100644
--- a/lib.h
+++ b/lib.h
@@ -219,10 +219,15 @@ struct cobj_ops {
   void (*destroy)(val self);
   void (*mark)(val self);
   cnum (*hash)(val self);
+  val (*equalsub)(val self);
 };
 
 #define cobj_ops_init(equal, print, destroy, mark, hash) \
-  { equal, print, destroy, mark, hash }
+  { equal, print, destroy, mark, hash, 0 }
+
+#define cobj_ops_init_ex(equal, print, destroy, mark, hash, \
+                         equalsub) \
+  { equal, print, destroy, mark, hash, equalsub }
 
 /* Default operations for above structure.
  * Default equal is eq