From 7b0cc54a306486e2ca09b7863f7330f85bf6531c Mon Sep 17 00:00:00 2001
From: Kaz Kylheku <kaz@kylheku.com>
Date: Tue, 29 Jul 2014 07:27:38 -0700
Subject: * eval.c (eval_init): Register nconc as intrinsic.

* lib.c (nconcv): New function.

* lib.h (nconcv): Declared.

* txr.1: Documented nconc.
---
 ChangeLog | 10 ++++++++++
 eval.c    |  1 +
 lib.c     | 12 ++++++++++++
 lib.h     |  1 +
 txr.1     | 11 ++++++++---
 5 files changed, 32 insertions(+), 3 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 2e3a0e50..ef3197ba 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,13 @@
+2014-07-29  Kaz Kylheku  <kaz@kylheku.com>
+
+	* eval.c (eval_init): Register nconc as intrinsic.
+
+	* lib.c (nconcv): New function.
+
+	* lib.h (nconcv): Declared.
+
+	* txr.1: Documented nconc.
+
 2014-07-29  Kaz Kylheku  <kaz@kylheku.com>
 
 	* lib.c (reduce_left, reduce_right): Nullify incoming sequence
diff --git a/eval.c b/eval.c
index 1c9724cd..dabb40bf 100644
--- a/eval.c
+++ b/eval.c
@@ -3598,6 +3598,7 @@ void eval_init(void)
   reg_fun(intern(lit("replace-list"), user_package), func_n4o(replace_list, 2));
   reg_fun(append_s, func_n0v(appendv));
   reg_fun(intern(lit("append*"), user_package), func_n0v(lazy_appendv));
+  reg_fun(intern(lit("nconc"), user_package), func_n0v(nconcv));
   reg_fun(list_s, list_f);
   reg_fun(intern(lit("list*"), user_package), func_n0v(list_star_intrinsic));
   reg_fun(identity_s, identity_f);
diff --git a/lib.c b/lib.c
index 2b209b77..afb134b8 100644
--- a/lib.c
+++ b/lib.c
@@ -727,6 +727,18 @@ val nappend2(val list1, val list2)
   return out;
 }
 
+val nconcv(val lists)
+{
+  list_collect_decl (out, ptail);
+
+  for (; lists; lists = cdr(lists)) {
+    val item = car(lists);
+    ptail = list_collect_nconc(ptail, item);
+  }
+
+  return out;
+}
+
 val sub_list(val list, val from, val to)
 {
   val len = nil;
diff --git a/lib.h b/lib.h
index 157a7994..b6051100 100644
--- a/lib.h
+++ b/lib.h
@@ -436,6 +436,7 @@ val reverse(val in);
 val append2(val list1, val list2);
 val nappend2(val list1, val list2);
 val appendv(val lists);
+val nconcv(val lists);
 val sub_list(val list, val from, val to);
 val replace_list(val list, val items, val from, val to);
 val lazy_appendv(val lists);
diff --git a/txr.1 b/txr.1
index e8da4242..7fda0ce7 100644
--- a/txr.1
+++ b/txr.1
@@ -7331,13 +7331,14 @@ Examples:
   (second '(1 2)) -> 2
   (third '(1 2 . 3)) -> **error**
 
-.SS Functions append and append*
+.SS Functions append, nconc and append*
 
 .TP
 Syntax:
 
-(append [<list>* <last-arg>])
-(append* [<list>* <last-arg>])
+  (append [<list>* <last-arg>])
+  (nconc [<list>* <last-arg>])
+  (append* [<list>* <last-arg>])
 
 .TP
 Description:
@@ -7357,6 +7358,10 @@ Thus, if argument N is also a list, it is catenated onto the resulting list,
 but without being copied. Argument N may be an atom other than nil; in that
 case append produces an improper list.
 
+The nconc function works like append, but avoids consing. It destructively
+manipulates (that is to say, mutates) incoming lists to catenate them, and so
+must be used with care.
+
 The append* function works like append, but returns a lazy list which produces
 the catenation of the lists on demand.  If some of the arguments are
 themselves lazy lists which are infinite, then append* can return immediately,
-- 
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