Different behaviors of evaluator and compiler

[barmar@Think.COM (Barry Margolin)]

    Date: Wed, 21 Aug 1991 08:25 EDT
    From: CLanning@pdesds1.atg.trc.scra.org (Craig Lanning)

	Date: Wed, 21 Aug 91 12:25+0100
	From: Vincent Keunen <keunen@milou.nrb.be>

	...

	This is a simple function to demonstrate the strange behaviour:
 
	(defun test ()
	  (let ((result '(nil)))
	    (loop for item in '(a b c)
		  do (nconc result (list item)))
	    result))
 
	(test) ;when the function definition is evaluated returns
 
	(NIL A B C) ;...which is what I want
 
Try calling it again, and you'll see that it doesn't always do what you
want.  The second time it will return (NIL A B C A B C), the next time
it will return (NIL A B C A B C A B C), etc.

	(test) ;when the function definition is compiled returns
 
	Error: Attempt to RPLACD a list that is embedded in a structure and
	       therefore cannot be RPLACD'ed.  The list is (NIL)
	While in the function SYS:RPLACD-ESCAPE  SYS:*NCONC  TEST
 
	...

	Can someone help me on this one?
 
	Thank you
 
	Vincent Keunen
	keunen@nrb.be
 
    The error reported after the function is compiled has to do with the way
    constants are stored in the COMPILED-FUNCTION object.  No doubt, someone
    else will send you a more in-depth explanation of why that happens.

Someone else just asked this same question less than two weeks ago.
Yes, the problem is that constants are stored in the function object,
and lists in structure regions can't be RPLACD'ed.

In general, Common Lisp doesn't permit constants to be modified.  See
p.70 of CLtL 2nd Edition, the second paragraph of section "Self
Evaluating Forms".  This restriction exists to permit implementations to
share structure between multiple instances of equivalent constants
(referred to as "coalescing constants"), to store constants in read-only
memory, and to obviate copying constants when evaluating or compiling
code.

                                                barmar