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Newsgroups: comp.lang.lisp
Subject: Re: Lexical environment access at runtime
References: <87he1hneb2.fsf@thor.piquan.org> <wkr80kfyye.fsf@nhplace.com>
From: Joel Ray Holveck <jo...@piquan.org>
Message-ID: <87brroqyn1.fsf@thor.piquan.org>
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Date: Fri, 07 Nov 2003 10:08:37 GMT

>> Is there a way for running code to find out, based on a symbol (not
>> known at compile time), what the current lexical value for that symbol
>> is?
> No.
> The easy proof of that is this:
>  (defun foo ()
>    (let ((x 3))
>      #'(lambda () x)))

Thanks for the response.  Let me clarify my question.  When I said
"running code", I meant "code that I write, running within the lexical
environment I'm trying to probe".  So it can have variable accesses to
X that prevent it from being folded away, or whatever else.

So while I agree with you for the general case, I don't think that
proof applies for the special case when I have the luxury of running a
code-walker over the code, instrumenting it before it's submitted to
Lisp.

I apologize; my initial paragraph (the one you quoted) was unclear, or
rather less specific than it should have been.  I don't have the
symbol name at compile time, but I can make adjustments to the code at
compile time, so long as the function still does what it's supposed
to.  In the example I listed later in the message, it instruments the
user-supplied function to call my own code.

The good news about that case is that I can put whatever additional
code I need in there, to add variable accesses that would prevent the
X in your example from being folded away.  The bad news is that, in
the only general solution I've figured out so far, it would require a
list of current lexical bindings (at compile-time), which is what led
to my second question (about MAP-ENVIRONMENT).

I was hoping to avoid making the code-walker look for forms that
create new lexical bindings.  (I'll have the complete top-level form,
so I can start from a null lexical environment.)  Needing to do that
makes the code walker more complex, it couldn't handle
implementation-defined special forms that create new bindings, and
would require almost a complete implementation of complex
binding-creating forms like LOOP.

>> How about for a macro to find out what lexical bindings exist in
>> an &environment variable, like the proposed MAP-ENVIRONMENT?
> Not in the standard.

Nuts.  At this point, that's really what's keeping me from writing The
Debugger I Always Wanted.  ANSI CL is *so* close to what I want, but
just not quite there.

Thanks for your help,
joelh

-- 
Joel Ray Holveck - jo...@piquan.org
   Fourth law of programming:
   Anything that can go wrong wi
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