Tighten up environment visibility semantics of default argument

init forms, while streamlining the implementation.

* eval.c (bind_args): Get rid of opt_init_parm array hack;
turns out we already have a useful function for extending an
environment object, namely env_vbind: hitherto unused!  We now make a
blank environment upfront and extend it destructively with env_vbind.
Except that after evaluating an init form, we create a new environment:
this is in case a lexical closure was made by the init form, in which
event we don't want to be destructively manipulating the environment
any more.

* txr.1: Clarified.

3 files changed, 26 insertions, 23 deletions

diff --git a/ChangeLog b/ChangeLog
index b5056ac5..912e2854 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,21 @@
 2014-02-03  Kaz Kylheku  <kaz@kylheku.com>
 
+	Tighten up environment visibility semantics of default argument
+	init forms, while streamlining the implementation.
+
+	* eval.c (bind_args): Get rid of opt_init_parm array hack;
+	turns out we already have a useful function for extending an
+	environment object, namely env_vbind: hitherto unused!  We now make a
+	blank environment upfront and extend it destructively with env_vbind.
+	Except that after evaluating an init form, we create a new environment:
+	this is in case a lexical closure was made by the init form, in which
+	event we don't want to be destructively manipulating the environment
+	any more.
+
+	* txr.1: Clarified.
+
+2014-02-03  Kaz Kylheku  <kaz@kylheku.com>
+
 	* eval.c (eval_init): Fix incorrect registration of
 	list_star_intrinsic, diagnosed by compiler.
 
diff --git a/eval.c b/eval.c
index d1f27cfd..b4644bce 100644
--- a/eval.c
+++ b/eval.c
@@ -220,11 +220,8 @@ static val lookup_sym_lisp1(val env, val sym)
 
 static val bind_args(val env, val params, val args, val ctx_form)
 {
-  val opt_init_parm[32];
-  val *oi_parm = &opt_init_parm[0], *oi_end = &opt_init_parm[32], *oi_iter;
-  val new_bindings = nil;
+  val new_env = make_env(nil, nil, env);
   val optargs = nil;
-  val new_env;
 
   for (; args && consp(params); args = cdr(args), params = cdr(params)) {
     val param = car(params);
@@ -246,11 +243,11 @@ static val bind_args(val env, val params, val args, val ctx_form)
       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);
+    env_vbind(new_env, param, car(args));
   }
 
   if (bindable(params)) {
-    new_bindings = acons(params, args, new_bindings);
+    env_vbind(new_env, params, args);
   } else if (consp(params)) {
     if (car(params) == colon_k) {
       if (optargs)
@@ -265,19 +262,16 @@ static val bind_args(val env, val params, val args, val ctx_form)
       if (param == colon_k)
         goto twocol;
       if (consp(param)) {
-        if (oi_parm == oi_end)
-          eval_error(ctx_form,
-                     lit("~s: too many optional args with initializers"),
-                     car(ctx_form), nao);
-        new_bindings = acons(car(param), car(cdr(param)), new_bindings);
-        *oi_parm++ = car(new_bindings);
+        val initval = eval(car(cdr(param)), new_env, ctx_form);
+        new_env = make_env(nil, nil, new_env);
+        env_vbind(new_env, car(param), initval);
       } else {
-        new_bindings = acons(param, nil, new_bindings);
+        env_vbind(new_env, param, nil);
       }
       params = cdr(params);
     }
     if (bindable(params))
-      new_bindings = acons(params, args, new_bindings);
+      env_vbind(new_env, params, nil);
   } else if (params) {
     eval_error(ctx_form, lit("~a: ~s is not a bindable sybol"),
                car(ctx_form), params, nao);
@@ -285,13 +279,6 @@ static val bind_args(val env, val params, val args, val ctx_form)
     eval_error(ctx_form, lit("~s: too many arguments"), car(ctx_form), nao);
   }
 
-  new_env = make_env(new_bindings, nil, env);
-
-  for (oi_iter = &opt_init_parm[0]; oi_iter < oi_parm; oi_iter++) {
-    val initval = eval(cdr(*oi_iter), new_env, ctx_form);
-    rplacd(*oi_iter, initval);
-  }
-
   return new_env;
 twocol:
   eval_error(ctx_form, lit("~a: multiple colons in parameter list"),
diff --git a/txr.1 b/txr.1
index d6fc7fdd..bb2325a5 100644
--- a/txr.1
+++ b/txr.1
@@ -5977,8 +5977,8 @@ 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 initializer expressions are evaluated an environment in which
-all of the parameters are visible, which extends the environment in
-which the lambda was constructed. For instance:
+all of the previous parameters are visible, in addition to the surrounding
+environment of the lambda. For instance:
 
   (let ((default 0))
     (lambda (str : (end (length str)) (counter default))