From e6e2346e5cae8dd1a0ac6ad4676fd2ae1ad4ce98 Mon Sep 17 00:00:00 2001
From: Kaz Kylheku <kaz@kylheku.com>
Date: Mon, 28 Mar 2016 06:15:18 -0700
Subject: New macro: lset.

* lisplib.c (place_set_entries): Added "lset" to autoload
list.

* share/txr/stdlib/place.tl (lset): New macro.

* txr.1: Documented lset.
---
 lisplib.c                 |  2 +-
 share/txr/stdlib/place.tl | 13 +++++++++++++
 txr.1                     | 31 +++++++++++++++++++++++++++++++
 3 files changed, 45 insertions(+), 1 deletion(-)

diff --git a/lisplib.c b/lisplib.c
index 9ac7e3a7..bf41ab82 100644
--- a/lisplib.c
+++ b/lisplib.c
@@ -69,7 +69,7 @@ static val place_set_entries(val dlt, val fun)
     lit("with-delete-expander"),
     lit("set"), lit("pset"), lit("zap"), lit("flip"), lit("inc"), lit("dec"),
     lit("push"), lit("pop"), lit("swap"), lit("shift"), lit("rotate"),
-    lit("pushnew"), lit("del"),
+    lit("pushnew"), lit("del"), lit("lset"),
     lit("defplace"), lit("define-place-macro"), lit("define-modify-macro"),
     lit("placelet"), lit("placelet*"), lit("define-acessor"),
     lit("with-slots"),
diff --git a/share/txr/stdlib/place.tl b/share/txr/stdlib/place.tl
index 7ef7f44d..5d17acb7 100644
--- a/share/txr/stdlib/place.tl
+++ b/share/txr/stdlib/place.tl
@@ -295,6 +295,19 @@
   (with-delete-expander (deleter) place env
     ^(,deleter)))
 
+(defmacro lset (. places-source)
+  (let ((places (butlast places-source))
+        (source (last places-source))
+        (orig (gensym))
+        (iter (gensym)))
+    (unless places
+      (sys:eval-err "lset: require one or more places followed by expression"))
+    ^(let* ((,orig ,(car source))
+            (,iter ,orig))
+       ,*(butlast (mappend (ret ^((set ,@1 (car ,iter)) (set ,iter (cdr ,iter))))
+                           places))
+       ,orig)))
+
 (defmacro defplace (place-destructuring-args body-sym
                     (getter-sym setter-sym update-body) :
                     ((ssetter-sym clobber-body))
diff --git a/txr.1 b/txr.1
index 2a068815..26be2887 100644
--- a/txr.1
+++ b/txr.1
@@ -25452,6 +25452,37 @@ place denotes a value stored in a dynamic data set such as a hash table,
 then deletion of that place implies deletion of the entry which holds
 that value. If the entry is identified by a key, that key is also removed.
 
+.coNP Macro @ lset
+.synb
+.mets (lset <> { place }+ << list-expr )
+.syne
+.desc
+The
+.code lset
+operator's parameter list consists of one or more places followed
+by an expression
+.metn list-expr .
+
+The macro evaluates
+.codn list-expr ,
+which is expected to produce a proper list.
+
+Successive elements of the resulting list are then assigned to each
+successive
+.codn place .
+
+If there are fewer elements in the list than places, the
+unmatched places receive the value
+.codn nil .
+
+Excess elements in the list are ignored.
+
+A
+.code lset
+form produces the value of
+.meta list-expr
+as its result value.
+
 .SS* User-Defined Places and Place Operators
 \*(TL provides a number of place-modifying operators such as
 .codn set ,
-- 
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