Path: utzoo!utgpu!water!watmath!clyde!ima!think!barmar
From: barmar@think.COM (Barry Margolin)
Newsgroups: comp.lang.lisp
Subject: Re: LISP PROBLEM
Message-ID: <24909@think.UUCP>
Date: 4 Aug 88 04:05:53 GMT
References: <5071@zodiac.UUCP>
Sender: usenet@think.UUCP
Reply-To: barmar@kulla.think.com.UUCP (Barry Margolin)
Organization: Thinking Machines Corporation, Cambridge, MA
Lines: 31

In article <5071@zodiac.UUCP> yushen@ads.com (Yu-shen Ng) writes:
>How can I do an "eval" within the lexical scope from which the "eval" is called?

You can't.  EVAL is an ordinary function, so it has no idea what
lexical context it was called in (only special forms and macros can
access the lexical context).

How would it possibly work in a compiled function, when references to
lexical variables have been converted to stack frame offsets, and the
names of the local variables have been discarded?  If the Lisp
implementation saves the local variable information (as it does on
Lisp Machines) then there will likely be an implementation-specific
function for evaluating in a given lexical environment (for use by the
debugger, for instance); a common mechanism is to allow EVAL to take
an optional second argument, which is the lexical environment.
However, Common Lisp doesn't require this capability.

Why do you need this capability?  If the form you want to evaluate is
coming from outside the function, how could it possibly know the names
of your local variables?

Perhaps you should describe what you are trying to accomplish, since I
suspect you may be going about it wrong.  It is almost always wrong to
call EVAL explicitly from a function.  You can usually do what you
want with FUNCALL or APPLY.

Barry Margolin
Thinking Machines Corp.

barmar@think.com
{uunet,harvard}!think!barmar