From 6cd6af985619569e7e12df7b28c6aa1dc10726c1 Mon Sep 17 00:00:00 2001
From: Kaz Kylheku <kaz@kylheku.com>
Date: Thu, 6 Nov 2014 06:35:34 -0800
Subject: Allow then-func to be omitted in iff. Cleanup.

* eval.c (eval_init): Register iff as requiring only one arg.

* lib.c (do_iff): Do not check thenfun for nil. Just call it.
(iff): Default thenfun to identity.

* txr.1: Documentation updated.
---
 ChangeLog | 11 +++++++++++
 eval.c    |  2 +-
 lib.c     |  3 ++-
 txr.1     | 53 +++++++++++++++++++++++++++++++++++++----------------
 4 files changed, 51 insertions(+), 18 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 97693d32..523d9f74 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,14 @@
+2014-11-06  Kaz Kylheku  <kaz@kylheku.com>
+
+	Allow then-func to be omitted in iff. Cleanup.
+
+	* eval.c (eval_init): Register iff as requiring only one arg.
+
+	* lib.c (do_iff): Do not check thenfun for nil. Just call it.
+	(iff): Default thenfun to identity.
+
+	* txr.1: Documentation updated.
+
 2014-11-05  Kaz Kylheku  <kaz@kylheku.com>
 
 	* lib.c (iff): Bugfix: forgotten handling of default
diff --git a/eval.c b/eval.c
index aab5a683..402d19a2 100644
--- a/eval.c
+++ b/eval.c
@@ -3749,7 +3749,7 @@ void eval_init(void)
   reg_fun(intern(lit("andf"), user_package), func_n0v(andv));
   reg_fun(intern(lit("orf"), user_package), func_n0v(orv));
   reg_fun(intern(lit("notf"), user_package), func_n1(notf));
-  reg_fun(intern(lit("iff"), user_package), func_n3o(iff, 2));
+  reg_fun(intern(lit("iff"), user_package), func_n3o(iff, 1));
   reg_fun(intern(lit("iffi"), user_package), func_n3o(iffi, 2));
   reg_fun(intern(lit("if"), user_package), func_n3o(if_fun, 2));
   reg_fun(intern(lit("or"), user_package), func_n0v(or_fun));
diff --git a/lib.c b/lib.c
index 4674ff26..6abbf1f1 100644
--- a/lib.c
+++ b/lib.c
@@ -4606,12 +4606,13 @@ static val do_iff(val env, val args)
   cons_bind (thenfun, elsefun, choices);
 
   return if3(apply(condfun, args, nil),
-             if2(thenfun, apply(thenfun, args, nil)),
+             apply(thenfun, args, nil),
              if2(elsefun, apply(elsefun, args, nil)));
 }
 
 val iff(val condfun, val thenfun, val elsefun)
 {
+  thenfun = default_arg(thenfun, identity_f);
   elsefun = default_bool_arg(elsefun);
   return func_f0v(cons(condfun, cons(thenfun, elsefun)), do_iff);
 }
diff --git a/txr.1 b/txr.1
index 165848b1..2ca3a0a1 100644
--- a/txr.1
+++ b/txr.1
@@ -20628,7 +20628,7 @@ and then inverts the result as if by application of the
 
 .coNP Functions @ iff and @ iffi
 .synb
-.mets (iff < cond-func < then-func <> [ else-func ])
+.mets (iff < cond-func >> [ then-func <> [ else-func ]])
 .mets (iffi < cond-func < then-func <> [ else-func ])
 .syne
 .desc
@@ -20639,7 +20639,7 @@ function is the functional equivalent of the
 operator. It accepts
 functional arguments and returns a function.
 
-The resulting function takes its arguments and applies them to
+The resulting function takes its arguments, if any, and applies them to
 .metn cond-func .
 If
 .meta cond-func
@@ -20650,6 +20650,15 @@ resulting value is returned. Otherwise the arguments are passed to
 .meta else-func
 and the resulting value is returned.
 
+If
+.meta then-func
+is omitted then
+.code identity
+is used as default. This omission is not permitted by
+.codn iffi ,
+only
+.codn iff .
+
 If
 .meta else-func
 needs to be called, but is omitted, then
@@ -20663,26 +20672,38 @@ function differs from
 only in the defaulting behavior with respect
 to the
 .meta else-func
-argument. The following equivalences hold:
+argument. If
+.meta else-func
+is omitted in a call to
+.code iffi
+then the deafult function is
+.codn identity .
+This is useful in situations when one value is to be
+replaced with another one when the condition is true, otherwise
+preserved.
+
+The following equivalences hold between
+.code iffi
+and
+.codn iff :
 
 .cblk
-  (iffi a b c)   <--> (iff a b c)
+  (iffi a b c)            <--> (iff a b c)
 
-  (iffi a b)     <--> (iff a b identity)
+  (iffi a b)              <--> (iff a b identity)
 
-  (iffi a b nil) <--> (iff a b identity)
+  (iffi a b false)        <--> (iff a b)
+
+  (iffi a identity false) <--> (iff a)
 .cble
 
-The
-.code iffi
-function defaults to the identity function when
-.meta else-func
-is
-omitted or
-.codn nil ,
-and therefore is useful in situations when one value is to be
-replaced with another one when the condition is true, otherwise
-preserved.
+The following equivalence illustrates
+.code iff
+with both optional arguments omitted:
+
+.cblk
+  (iff a)  <--->  (iff a identity false)
+.cble
 
 .coNP Functions @ tf and @ nilf
 .synb
-- 
cgit v1.2.3