From 95f05cafb8da2dc1a20a41b165c2e03c24d04099 Mon Sep 17 00:00:00 2001
From: Kaz Kylheku <kaz@kylheku.com>
Date: Tue, 17 Jun 2014 06:59:46 -0700
Subject: * eval.c (not_null): New static function. (eval_init): Use null_f in
 existing registration of null and not. Add registration for not_null as
 "true", and for null_f as "false".

* txr.1: Documented true and false.
---
 ChangeLog |  9 +++++++++
 eval.c    | 11 +++++++++--
 txr.1     | 33 ++++++++++++++++++++++++++++++---
 3 files changed, 48 insertions(+), 5 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 0385c09b..20254472 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,12 @@
+2014-06-17  Kaz Kylheku  <kaz@kylheku.com>
+
+	* eval.c (not_null): New static function.
+	(eval_init): Use null_f in existing registration of null
+	and not. Add registration for not_null as "true",
+	and for null_f as "false".
+
+	* txr.1: Documented true and false.
+
 2014-06-17  Kaz Kylheku  <kaz@kylheku.com>
 
 	* eval.c (eval_init): register tuples as intrinsic.
diff --git a/eval.c b/eval.c
index d9acfda4..7339281e 100644
--- a/eval.c
+++ b/eval.c
@@ -3057,6 +3057,11 @@ static val and_fun(val vals)
   return item;
 }
 
+static val not_null(val obj)
+{
+  return if3(nilp(obj), nil, t);
+}
+
 static val prinl(val obj, val stream)
 {
   val ret = obj_print(obj, stream);
@@ -3224,8 +3229,10 @@ void eval_init(void)
   reg_fun(intern(lit("typeof"), user_package), func_n1(typeof));
 
   reg_fun(intern(lit("atom"), user_package), func_n1(atom));
-  reg_fun(intern(lit("null"), user_package), func_n1(null));
-  reg_fun(not_s, func_n1(null));
+  reg_fun(intern(lit("null"), user_package), null_f);
+  reg_fun(intern(lit("false"), user_package), null_f);
+  reg_fun(intern(lit("true"), user_package), func_n1(not_null));
+  reg_fun(not_s, null_f);
   reg_fun(intern(lit("consp"), user_package), func_n1(consp));
   reg_fun(intern(lit("listp"), user_package), func_n1(listp));
   reg_fun(intern(lit("proper-listp"), user_package), func_n1(proper_listp));
diff --git a/txr.1 b/txr.1
index 503b7771..23b55eac 100644
--- a/txr.1
+++ b/txr.1
@@ -6900,19 +6900,20 @@ A bignum integer: arbitrary precision integer that is heap-allocated.
 .PP
 There are additional kinds of objects, such as streams.
 
-.SS Functions null and not
+.SS Functions null, not, false
 
 .TP
 Syntax:
 
   (null <value>)
   (not <value>)
+  (false <value>)
 
 .TP
 Description:
 
-The null and not functions are synonyms.  They tests whether <value> is the
-object nil. They return t if this is the case, nil otherwise.
+The null, not and false functions are synonyms.  They tests whether <value> is
+the object nil. They return t if this is the case, nil otherwise.
 
 .TP
 Examples:
@@ -6920,6 +6921,7 @@ Examples:
   (null '()) -> t
   (null nil) -> t
   (null ()) -> t
+  (false t) -> nil
 
   (if (null x) (format t "x is nil!"))
 
@@ -6927,6 +6929,31 @@ Examples:
     (if (not (memq 'a list))
       (format t "list ~s does not contain the symbol a\en")))
 
+.SS Function true
+
+.TP
+Syntax:
+
+  (true <value>)
+
+.TP
+Description:
+
+The true function is the complement of the null, not and false functions.
+It return t if the <value> is any object other than nil. If <value> is
+nil, it returns nil.
+
+Note: programs should avoid explicitly testing values with true.
+For instance (if x ...) should be favored over (if (true x) ...)
+
+.TP
+Example:
+
+   ;; Compute indices where the list '(1 nil 2 nil 3) has true values:
+   ;; "Where is (1 nil 2 nil 3) true?"
+
+   [where '(1 nil 2 nil 3) true] -> (1 3)
+
 .SS Functions eq, eql and equal
 
 .TP
-- 
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