Path: utzoo!utgpu!water!watmath!clyde!bellcore!rutgers!ucsd!ames!ai.etl.army.mil!hoey
From: hoey@ai.etl.army.mil (Dan Hoey)
Newsgroups: comp.lang.lisp
Subject: Re: Amusing Code
Message-ID: <179@ai.etl.army.mil>
Date: 24 Aug 88 15:01:34 GMT
References: <389@soi.UUCP>
Reply-To: hoey@ai.etl.army.mil (Dan Hoey)
Organization: Naval Research Lab, Washington, DC
Lines: 19

In article <389@soi.UUCP> alex@soi.UUCP (Alex Zatsman) writes about modifying
quoted structure, as in

>(defun Init-Stack () (setf *Stack* '((:Bottom-Frame))))

>(defun Push-Object (Object) (push Object (car *Stack*)))

where calling PUSH-OBJECT can modify the quoted constant in INIT-STACK.

The current feeling of a lot of the Common Lisp developers is that such
behavior is an error.  In particular, if an implementation has the capability
of supporting read-only storage, then it may use that storage for quoted
structure, and calling PUSH-OBJECT may signal an error.

A better way of writing INIT-STACK is

	(defun Init-Stack () (setf *Stack* (list '(:Bottom-Frame))))

Dan