Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!wiley!scott
From: scott@wiley.uucp (Scott Simpson)
Newsgroups: comp.lang.lisp
Subject: Re: about EVAL
Summary: Eval executes in a different environment
Keywords: eval, lisp, environment
Message-ID: <10090@wiley.UUCP>
Date: 8 Jun 90 18:46:34 GMT
References: <200.266aae50@wsuiar.uucp>
Sender: news@wiley.UUCP
Reply-To: scott@wiley.UUCP (Scott Simpson)
Organization: TRW Inc., Redondo Beach, CA
Lines: 130

In article <200.266aae50@wsuiar.uucp> mnjafar@wsuiar.uucp writes:
>  I am a newcomer to lisp, so forgive my ignorance. I ran the
>test programs shown below on three different machines with
>Common Lisp. The results of the tests are shown below. Does
>anyone out there know the answer to why Eval behaves in the way
>it does. I looked in Guy Steele's book, but did not find an answer,
>maybe I don't know where to look . What exactly is supposed  to
>happen when I issue an (eval '(setq i j)) inside a block ?
>;Test1.lsp
>(defun strange()
> (let (j i xpr)		;j, i and xpr are nil
>  (setq i 12)
>  (setq xpr '(setq j i))
>  (format t " ~% The value of i inside the function is : ~a~%" i)
>  (format t " ~% The value of xpr inside the function is : ~a~%" xpr)
>  (eval xpr)
>  (format t " ~% The value of j inside the function is :~a~%" j)
> )
>)
>(strange)

Eval is executed in its own local environment.  The stack of execution
looks like

	|---------------------|
	|eval + environment   |___
	|---------------------|  |
	|strange + environment|  |
	-----------------------  |  Can't find i!
				 V

Since i is not in eval's environment and it is not in the global
environment you get an error.  If you change the line

(let (j i xpr) 

to

(let (j i xpr) (declare (special i))

then i is dynamically scoped and it will be found in strange's
environment.

	|---------------------|
	|eval + environment   |___
	|---------------------|  |   finds i dynamically, sets global j
	|strange + environment|<--
	-----------------------

The function strange will now completes correctly and returns

 The value of i inside the function is : 12
 The value of xpr inside the function is : (SETQ J I)
 The value of j inside the function is :NIL

Notice that j is nil.  The setq in the eval creates and sets a
global variable named j (try evaluating j after running the new
strange function with the declare). The j inside the function strange
is never set.
	If you wish to set the j inside the function, declare it
dynamically scoped also.

(let (j i xpr) (declare (special i) (special j))

The result is now

 The value of i inside the function is : 12
 The value of xpr inside the function is : (SETQ J I)
 The value of j inside the function is :12

No global j is created.

>;Test2.lsp
>(defun strange()
> (let (j i xpr)
>  (setq i 12)
>  (setq xpr '(setq j i))
>  (format t " ~% The value of i inside the function is : ~a~%" i)
>  (format t " ~% The value of xpr inside the function is : ~a~%" xpr)
>  (eval xpr)
>  (format t " ~% The value of j inside the function is :~a~%" j)
>    j
> )
>)
>(setq i 13)
>(format t   " ~% The value of i outside the function is : ~a~%" i)
>(strange)

This yielded

 The value of i inside the function is : 12
 The value of xpr inside the function is : (SETQ J I)
 The value of j inside the function is :NIL

J is still nil.  The j set by the eval was a global j.  A global i
exists now though so you only need to make j special.  Try making j
special and you get

 The value of i inside the function is : 12
 The value of xpr inside the function is : (SETQ J I)
 The value of j inside the function is :12


> The results obtained
>Machine: VAX 8650 running VMS 5.2
>         Vax Lisp  V 2.2
>Test       |     Error Message or value of j
>---------------------------------------------
>test1      |     symbol has no value : I
>test2      |     nil
>
>Machine: SUN 
>         Sun Common Lisp, development environment 3.0.2
>Test       |     Error Message or value of j
>---------------------------------------------
>test1      |     the symbol I has no global value
>test2      |     nil

These are correct.

>Machine: PC/AT compatible
>         Golden Common Lisp Version 1.01
>
>Test       |     Error Message or value of j
>---------------------------------------------
>test1      |     12
>test2      |     12

This is incorrect.
Scott Simpson    		TRW 		scott@wiley.coyote.trw.com