Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!mailrus!ames!vsi1!altnet!uunet!steinmetz!duff@eraserhead
From: duff@eraserhead (David A Duff)
Newsgroups: comp.lang.lisp
Subject: Re: Scope of EVAL (was: Re: Summing a list)
Summary: Eval does NOT use the lexical environment.
Keywords: eval lexical environment
Message-ID: <12473@steinmetz.ge.com>
Date: 28 Oct 88 20:33:09 GMT
References: <10794@srcsip.UUCP> <10813@srcsip.UUCP> <249@pitstop.UUCP> <4154@phoenix.Princeton.EDU>
Sender: news@steinmetz.ge.com
Reply-To: duff@eraserhead (David A Duff)
Organization: GE Corporate Research and Development, Schenectady, NY
Lines: 40
In-reply-to: eliot@phoenix.Princeton.EDU (Eliot Handelman)

In article <4154@phoenix.Princeton.EDU>, eliot@phoenix (Eliot Handelman) writes:
>In article <249@pitstop.UUCP> robv@pitstop.UUCP (Rob Vollum) writes:

[... stuff about eval only being able to perform evaluation in the top-level
environment.] 

>Is that really true?  In the following example, EVAL consults the lexical
>environment, rather that the global.

Yes.

>(setq x nil)
>And now (let* ((x t) (y x)) (eval y)) => t

Remember: eval is a function, and the semantics of lisp are such that when
evaluating a function application (a list whose car is neither a macro nor a
special form), the arguments are evaluated first, then the function is
applied.  So, basically, whatever you put in place of ... in "(eval ...)" ends
up getting evaluated TWICE -- once before eval is called and once after.  In
your example above, the eval function never sees the symbol y, but is instead
passed the value t.  

To better illustrate the difference between evaluation within a lexical
environment and evaluation in the top-level environment (the kind that the
lisp eval function does) you probably meant to try something like this:

(setq x nil)

;; here x is eval'd in lexical environment:
(let ((x t)) x) ==> t 

;; here x is eval'd in top-level environment:
(let ((x t)) (eval 'x)) ==> nil

Dave Duff                                     GE Research and Development Center
duff@eraserhead.steinmetz.ge.com                           Schenectady, New York
uunet!steinmetz!eraserhead!duff                                     518-387-5649
Dave Duff                                     GE Research and Development Center
duff@eraserhead.steinmetz.ge.com                           Schenectady, New York
uunet!steinmetz!eraserhead!duff                                     518-387-5649