From 129b1323a6d30e219dbd2f59413da084924e73c7 Mon Sep 17 00:00:00 2001
From: Kaz Kylheku <kaz@kylheku.com>
Date: Fri, 6 Jan 2017 14:30:18 -0800
Subject: Allow last var to be omitted in whilet.

* eval.c (me_whilet): insert gensym if last var is missing.
Warn if init-form looks like a variable.

* txr.1: Documented by copy and paste from iflet.
---
 eval.c | 16 +++++++++++++---
 txr.1  | 17 +++++++++++++++++
 2 files changed, 30 insertions(+), 3 deletions(-)

diff --git a/eval.c b/eval.c
index c5151391..319faa5b 100644
--- a/eval.c
+++ b/eval.c
@@ -3466,16 +3466,26 @@ static val me_whilet(val form, val env)
   val body = form;
   val sym = pop(&body);
   val lets = pop(&body);
-  val lastlet = last(lets, nil);
+  val lastlet_cons = last(lets, nil);
+  val lastlet = car(lastlet_cons);
   val not_done = gensym(lit("not-done"));
 
-  if (nilp(lastlet))
+  if (nilp(lastlet_cons))
     eval_error(form, lit("~s: empty binding list"), sym, nao);
 
+  if (!cdr(lastlet)) {
+    val var = car(lastlet);
+    if (symbolp(var) && bindable(var))
+      eval_warn(form, lit("~s: ~s is init-form here, not new variable"),
+                sym, var, nao);
+    push(gensym(nil), &lastlet);
+    lets = append2(butlast(lets, nil), cons(lastlet, nil));
+  }
+
   return list(let_s, cons(list(not_done, t, nao), nil),
               list(while_s, not_done,
                    list(let_star_s, lets,
-                        list(if_s, car(car(lastlet)),
+                        list(if_s, car(lastlet),
                              cons(progn_s, body),
                              list(set_s, not_done, nil, nao), nao), nao), nao), nao);
 }
diff --git a/txr.1 b/txr.1
index e1ef3748..0ef550de 100644
--- a/txr.1
+++ b/txr.1
@@ -13757,6 +13757,23 @@ loop is thus terminated by an explicit
 a return value can be specified. Under normal termination, the return value is
 .codn nil .
 
+In the syntax, a small convenience is permitted. Instead of the last
+.cblk
+.meti >> ( sym << init-form )
+.cble
+it is permissible for the syntax
+.cblk
+.meti <> ( init-form )
+.cble
+to appear, the
+.meta sym
+being omitted. A machine-generated variable is substituted
+in place of the missing
+.meta sym
+and that variable is then initialized from
+.meta init-form
+and used as the basis of the test.
+
 .TP* Examples:
 .cblk
   ;; read lines of text from *std-input* and print them,
-- 
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