From 28bd257494d8a0a6337d6fdd06b820412a2f1c55 Mon Sep 17 00:00:00 2001
From: Kaz Kylheku <kaz@kylheku.com>
Date: Tue, 14 Jan 2014 06:57:43 -0800
Subject: * eval.c (bind_args): Implemented default values for optional
 arguments. (op_defun): Permit the syntax.

* txr.1: Documented.
---
 ChangeLog |  8 ++++++++
 eval.c    | 18 +++++++++++++++---
 txr.1     |  6 ++++++
 3 files changed, 29 insertions(+), 3 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index f7a0f4ac..46ff1909 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,11 @@
+2014-01-14  Kaz Kylheku  <kaz@kylheku.com>
+
+	* eval.c (bind_args): Implemented default values for optional
+	arguments.
+	(op_defun): Permit the syntax.
+
+	* txr.1: Documented.
+
 2014-01-13  Kaz Kylheku  <kaz@kylheku.com>
 
 	Support for pushing back bytes and characters into streams.
diff --git a/eval.c b/eval.c
index f9a68f95..ce610dd1 100644
--- a/eval.c
+++ b/eval.c
@@ -235,8 +235,11 @@ static val bind_args(val env, val params, val args, val ctx_form)
       param = car(params);
     }
 
+    if (optargs && consp(param))
+      param = car(param);
+
     if (!bindable(param))
-      eval_error(ctx_form, lit("~a: ~s is not a bindable sybol"),
+      eval_error(ctx_form, lit("~a: ~s is not a bindable symbol"),
                  car(ctx_form), param, nao);
 
     new_bindings = acons(param, car(args), new_bindings);
@@ -254,9 +257,16 @@ static val bind_args(val env, val params, val args, val ctx_form)
     if (!optargs)
       eval_error(ctx_form, lit("~s: too few arguments"), car(ctx_form), nao);
     while (consp(params)) {
-      if (car(params) == colon_k)
+      val param = car(params);
+      val val = nil;
+      if (param == colon_k)
         goto twocol;
-      new_bindings = acons(car(params), nil, new_bindings);
+      if (consp(param)) {
+        val = car(cdr(param));
+        param = car(param);
+      }
+      new_bindings = acons(param, eval(val, env, ctx_form),
+                           new_bindings);
       params = cdr(params);
     }
     if (bindable(params))
@@ -758,6 +768,8 @@ static val op_defun(val form, val env)
           colon = t;
       continue;
     }
+    if (colon && consp(param))
+      continue;
     if (!bindable(param))
       eval_error(form, lit("defun: parameter ~s is not a bindable symbol"), param, nao);
   }
diff --git a/txr.1 b/txr.1
index a6760076..888d4cbc 100644
--- a/txr.1
+++ b/txr.1
@@ -5821,6 +5821,12 @@ but a separator between required parameters and optional parameters.
 When the function is called, optional parameter for which arguments
 are not supplied take on the value nil.
 
+An optional parameter can also be written in the form (<name> <expr>).
+In this situation, if the call does not specify a value for the parameter,
+then the parameter takes on the value of the expression <expr>. 
+The expression is evaluated in the environment in which the lambda
+was constructed.
+
 Functions created by lambda capture the surrounding variable bindings.
 
 
-- 
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