From c66aa75d955df8eae0b5553a5a57601245ad65af Mon Sep 17 00:00:00 2001
From: Kaz Kylheku <kaz@kylheku.com>
Date: Tue, 3 Jun 2014 06:21:36 -0700
Subject: Documenting the return value of rplaca and rplacd, and changing it to
 be the cell, rather than the new value.

* eval.c (transform_op): Fix use of rplacd that uses return value.
(force): Likewise.

* lib.c (rplaca, rplacd): Return cons.
(rem_lazy_rec, obj_init): Fix use of rplacd that uses return value.

* txr.1: Documented.
---
 ChangeLog | 13 +++++++++++++
 eval.c    |  7 ++++---
 lib.c     | 18 +++++++++++-------
 txr.1     |  5 +++++
 4 files changed, 33 insertions(+), 10 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index c2fc1205..3ff7b5bf 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,16 @@
+2014-06-03  Kaz Kylheku  <kaz@kylheku.com>
+
+	Documenting the return value of rplaca and rplacd, and changing
+	it to be the cell, rather than the new value.
+
+	* eval.c (transform_op): Fix use of rplacd that uses return value.
+	(force): Likewise.
+
+	* lib.c (rplaca, rplacd): Return cons.
+	(rem_lazy_rec, obj_init): Fix use of rplacd that uses return value.
+
+	* txr.1: Documented.
+
 2014-05-20  Kaz Kylheku  <kaz@kylheku.com>
 
 	The call operator should be a function!
diff --git a/eval.c b/eval.c
index de1180ed..bbd76c2d 100644
--- a/eval.c
+++ b/eval.c
@@ -2284,8 +2284,9 @@ static val transform_op(val forms, val syms, val rg)
         val newsyms = syms;
         val new_p;
         val cell = acons_new_c(vararg, mkcloc(new_p), mkcloc(newsyms));
-        val sym = if3(new_p, rplacd(cell, gensym(format_op_arg(vararg))),
-                             cdr(cell));
+        val sym = cdr(if3(new_p,
+                          rplacd(cell, gensym(format_op_arg(vararg))),
+                          cell));
         cons_bind (outsyms, outforms, transform_op(re, newsyms, rg)); 
         return cons(outsyms, rlcp(cons(sym, outforms), outforms));
       } else if (vararg == rest_s) {
@@ -2933,7 +2934,7 @@ static val force(val promise)
     return cdr(promise);
 
   rplaca(promise, nil);
-  return rplacd(promise, funcall(cdr(promise)));
+  return cdr(rplacd(promise, funcall(cdr(promise))));
 }
 
 static val errno_wrap(val newval)
diff --git a/lib.c b/lib.c
index fed06664..5c51ed7f 100644
--- a/lib.c
+++ b/lib.c
@@ -252,9 +252,11 @@ val rplaca(val cons, val new_car)
 {
   switch (type(cons)) {
   case CONS:
-    return set(mkloc(cons->c.car, cons), new_car);
+    set(mkloc(cons->c.car, cons), new_car);
+    return cons;
   case LCONS:
-    return set(mkloc(cons->lc.car, cons), new_car);
+    set(mkloc(cons->lc.car, cons), new_car);
+    return cons;
   default:
     type_mismatch(lit("~s is not a cons"), cons, nao);
   }
@@ -265,9 +267,11 @@ val rplacd(val cons, val new_cdr)
 {
   switch (type(cons)) {
   case CONS:
-    return set(mkloc(cons->c.cdr, cons), new_cdr);
+    set(mkloc(cons->c.cdr, cons), new_cdr);
+    return cons;
   case LCONS:
-    return set(mkloc(cons->lc.cdr, cons), new_cdr);
+    set(mkloc(cons->lc.cdr, cons), new_cdr);
+    return cons;
   default:
     type_mismatch(lit("~s is not a cons"), cons, nao);
   }
@@ -950,7 +954,7 @@ static val rem_lazy_rec(val obj, val list, val env, val func);
 static val rem_lazy_func(val env, val lcons)
 {
   cons_bind (pred, list, env);
-  return rplacd(lcons, rem_lazy_rec(pred, list, env, lcons_fun(lcons)));
+  return cdr(rplacd(lcons, rem_lazy_rec(pred, list, env, lcons_fun(lcons))));
 }
 
 static val rem_lazy_rec(val pred, val list, val env, val func)
@@ -5165,8 +5169,8 @@ static void obj_init(void)
   rplacd(gethash_c(user_package->pk.symhash, nil_string, nulloc), nil);
 
   /* t can't be interned, because intern needs t in order to do its job. */
-  t = rplacd(gethash_c(user_package->pk.symhash,
-                       lit("t"), nulloc), make_sym(lit("t")));
+  t = cdr(rplacd(gethash_c(user_package->pk.symhash,
+                           lit("t"), nulloc), make_sym(lit("t"))));
   set(mkloc(t->s.package, t), user_package);
 
   null_s = intern(lit("null"), user_package);
diff --git a/txr.1 b/txr.1
index b54ce957..2a62737d 100644
--- a/txr.1
+++ b/txr.1
@@ -7102,6 +7102,11 @@ is the same as the more generic (set (car x) y), and likewise
 It is an error if <cons> is not a cons or lazy cons. In particular,
 whereas (car nil) is correct, (rplaca nil ...) is erroneous.
 
+The rplaca and rplacd functions return <cons>.
+
+Note: TXR versions 89 and earlier, these functions returned the new value.
+The behavior was undocumented.
+
 .SS Functions second, third, fourth, fifth and sixth
 
 .TP
-- 
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