From e91d406a7ff82b8dfd57b3d84065597b640328db Mon Sep 17 00:00:00 2001
From: Kaz Kylheku <kaz@kylheku.com>
Date: Wed, 16 Oct 2019 07:57:15 -0700
Subject: copy-cons: more efficient; copies lconses.

* lib.c (copy_cons): Rewrite to copy the object in a more
low-level way, rather than going through the accessors
and constructors. Now copies LCONS as LCONS, including its
update function.

* txr.1: copy-cons re-documented.
---
 lib.c | 14 ++++++++++++--
 txr.1 | 28 ++++++++++++++++++++++------
 2 files changed, 34 insertions(+), 8 deletions(-)

diff --git a/lib.c b/lib.c
index b6f22971..aa5fcde8 100644
--- a/lib.c
+++ b/lib.c
@@ -8160,9 +8160,19 @@ val alist_nremove1(val list, val key)
   return list;
 }
 
-val copy_cons(val c)
+val copy_cons(val cell)
 {
-  return cons(car(c), cdr(c));
+  switch (type(cell)) {
+  case CONS:
+  case LCONS:
+    {
+      val obj = make_obj();
+      *obj = *cell;
+      return obj;
+    }
+  default:
+    type_mismatch(lit("copy-cons: ~s is not a cons"), cell, nao);
+  }
 }
 
 val copy_alist(val list)
diff --git a/txr.1 b/txr.1
index 5a536e50..0d357c56 100644
--- a/txr.1
+++ b/txr.1
@@ -19377,12 +19377,28 @@ for the empty list
 .desc
 The
 .code copy-cons
-function creates a fresh cons cell, whose
-.code car
-and
-.code cdr
-fields are copied from
-.codn cons .
+function creates and returns a new object that is a replica of
+.metn cons .
+
+The
+.meta cons
+argument must be either a
+.code cons
+cell, or else a lazy cons: an object of type
+.codn lcons .
+
+A new cell of the same type as
+.meta cons
+is created, and all of its fields are initialized by
+copying the corresponding fields from
+.metn cons .
+
+If
+.meta cons
+is lazy, the newly created object is in the same
+state as the original. If the original has not yet been updated
+and thus has an update function, the copy also has not yet been
+updated and has the same update function.
 
 .coNP Functions @ reverse and @ nreverse
 .synb
-- 
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