Path: utzoo!attcan!uunet!ncrlnk!ncrwic!wsuiar!mnjafar
From: mnjafar@wsuiar.uucp
Newsgroups: comp.lang.lisp
Subject: about EVAL
Message-ID: <200.266aae50@wsuiar.uucp>
Date: 4 Jun 90 23:54:08 GMT
Organization: Wichita State Univ., Wichita KS
Lines: 85


  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 ?


The first test
---------------------------------------------------
;Test1.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)
 )
)
(strange)
---------------------------------------------------


The second test
---------------------------------------------------
;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)
----------------------------------------------------



 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



Machine: PC/AT compatible
         Golden Common Lisp Version 1.01

Test       |     Error Message or value of j
---------------------------------------------
test1      |     12
test2      |     12



Mohammad Jafar
mnjafar@wsuiar.wsu.ukans.edu