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.

1 files changed, 2 insertions, 2 deletions

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))