Path: utzoo!attcan!uunet!lll-winken!lll-ncis!helios.ee.lbl.gov!nosc!ucsd!nprdc!stanonik
From: stanonik@nprdc.arpa (Ron Stanonik)
Newsgroups: comp.lang.lisp
Subject: unread'ing
Message-ID: <1302@arctic.nprdc.arpa>
Date: 10 Jan 89 16:24:27 GMT
Sender: news@nprdc.arpa
Reply-To: stanonik@nprdc.arpa (Ron Stanonik)
Organization: Navy Personnel R&D Center, San Diego
Lines: 28

We've just started using Sun Common Lisp 2.1.1 (Lucid really?)
on a Sun 3 running SunOS 3.5, so sorry if this has come up
before.

A program developed under this lisp, failed to work on a
Symbolics 3670 running Genera 7.2.

The problem seems to be that the program does something
like
	(setq c (read-char))
	(if (eql (peek-char) #\space)
	    (unread-char c))
That is, if the peek-char is a space, the program unread's
the previous char.

Genera complains correctly (as I read CLtL, 1984) about trying
to unread two characters in a row; ie, the peek-char is equivalent
to read-char followed by an unread-char.

Sun Common Lisp unread's two (maybe more?) characters without
complaint, which allowed the program to work even though it
violated the Common Lisp spec.

KCL also allows two (maybe more?) unread's in a row.

Ron Stanonik
stanonik@nprdc.arpa
ucsd!nprdc!stanonik