quote doesn't produce a fresh object

quote doesn't produce a fresh object

[Don Bashford]

(defun myfun (x)
  (let ((xs '(nil)))
    (print xs)
    (setcar xs "foo"))
  t)

On first invocation, (myfun 'arg) prints (nil) and returns t, as
expected.  But on second invocation, it prints ("foo").  It seems in the
second invocation, the let variable xs gets the old object from the
previous invocation.  If '(nil) is changed to (list nil) in myfun, the
problem does not occur, so the problem is with quote, not with let.

GNU Emacs 21.2.1 (i386-redhat-linux-gnu, X toolkit, Xaw3d scroll bars)
of 2002-04-08 on porky.devel.redhat.com

(Also occurs with 21.3 from redhat)

-Don

Re: quote doesn't produce a fresh object

[Kevin Rodgers]

Don Bashford wrote:
 > (defun myfun (x)
 >   (let ((xs '(nil)))
 >     (print xs)
 >     (setcar xs "foo"))
 >   t)
 >
 > On first invocation, (myfun 'arg) prints (nil) and returns t, as
 > expected.  But on second invocation, it prints ("foo").  It seems in the
 > second invocation, the let variable xs gets the old object from the
 > previous invocation.  If '(nil) is changed to (list nil) in myfun, the
 > problem does not occur, so the problem is with quote, not with let.

That is not a bug. 'FORM is equivalent to (quote FORM) and is used to
denote a constant Lisp object, so the xs symbol should reference the
original object (which is cons'ed when the defun form is read, not when
it is evaluated).

-- 
Kevin Rodgers