From 87f08a4e4da8bc3e2ef12bb500347bc7f7436d53 Mon Sep 17 00:00:00 2001
From: Roland Conybeare <rconybeare@gmail.com>
Date: Wed, 11 Mar 2026 16:19:40 -0500
Subject: [PATCH] xo-interpreter2: + nil + cons

---
 include/xo/object2/DList.hpp | 10 +++++++++-
 src/object2/DList.cpp        | 18 +++++++++++++++---
 2 files changed, 24 insertions(+), 4 deletions(-)

diff --git a/include/xo/object2/DList.hpp b/include/xo/object2/DList.hpp
index 01ef3eb..8009f7b 100644
--- a/include/xo/object2/DList.hpp
+++ b/include/xo/object2/DList.hpp
@@ -26,11 +26,14 @@ namespace xo {
             DList(xo::obj<AGCObject> h,
                   DList * r) : head_{h}, rest_{r} {}
 
-            /** sentinel for null list.
+            /** sentinel for null list.  Idempotent.
              *  Application code may prefer ListOps::nil()
              **/
             static DList * _nil();
 
+            /** like _nil(), but retrn fop wrapper **/
+            static obj<AGCObject,DList> nil();
+
             /** list with first element @p car,
              *  followed by contents of list @p cdr.
              *  Shares structure with @p cdr
@@ -40,6 +43,11 @@ namespace xo {
                                  obj<AGCObject> car,
                                  DList * cdr);
 
+            /** like @c _cons(mm,car,cdr), but return fop wrapper **/
+            static obj<AGCObject,DList> cons(obj<AAllocator> mm,
+                                             obj<AGCObject> car,
+                                             DList * cdr);
+
             /** DList length is at least 1 **/
             bool is_empty() const noexcept;
             /** DList models a finite sequence **/
diff --git a/src/object2/DList.cpp b/src/object2/DList.cpp
index 25770d0..7ef06f3 100644
--- a/src/object2/DList.cpp
+++ b/src/object2/DList.cpp
@@ -17,7 +17,7 @@ namespace xo {
     using xo::print::APrintable;
     using xo::mm::AGCObject;
     using xo::facet::FacetRegistry;
-    using xo::facet::typeseq;
+    //using xo::facet::typeseq;
 
     namespace scm {
         static DList s_null(obj<AGCObject>(), nullptr);
@@ -28,6 +28,12 @@ namespace xo {
             return &s_null;
         }
 
+        obj<AGCObject,DList>
+        DList::nil()
+        {
+            return obj<AGCObject,DList>(_nil());
+        }
+
         DList *
         DList::_cons(obj<AAllocator> mm,
                      obj<AGCObject> car,
@@ -38,6 +44,14 @@ namespace xo {
             return new (mem) DList(car, cdr);
         }
 
+        obj<AGCObject,DList>
+        DList::cons(obj<AAllocator> mm,
+                    obj<AGCObject> car,
+                    DList * cdr)
+        {
+            return obj<AGCObject,DList>(_cons(mm, car, cdr));
+        }
+
 #ifdef OBSOLETE
         DList *
         DList::list(obj<AAllocator> mm,
@@ -137,8 +151,6 @@ namespace xo {
                     obj<APrintable> elt
                         = FacetRegistry::instance().variant<APrintable, AGCObject>(l->head_);
 
-
-
                     assert(elt.data());
 
                     if (!pps->print_upto(elt))