Path: utzoo!utgpu!watmath!clyde!ima!think!barmar
From: barmar@think.COM (Barry Margolin)
Newsgroups: comp.lang.lisp
Subject: Re: unread'ing
Message-ID: <35330@think.UUCP>
Date: 11 Jan 89 09:58:43 GMT
References: <1302@arctic.nprdc.arpa>
Sender: news@think.UUCP
Reply-To: barmar@kulla.think.com.UUCP (Barry Margolin)
Organization: Thinking Machines Corporation, Cambridge MA, USA
Lines: 19

Common Lisp does not require an implementation to complain if you try
to do more than one unread in a row (CLtL doesn't say that doing so
"signals an error").  It merely specifies that no implementation is
required to allow more than one.  In other words, every implementation
must allow at last one unread-char in a row.  So, all the
implementations you described conform to the spec.

In some implementations, the actual number of characters that may be
unread may depend on the stream.  For instance, a file stream may
allow unreading all the way back to the beginning of the file, because
it simply reposition's the file offset, but a terminal stream might
require the Lisp implementation to do its own buffering and it could
chose to maintain only a single character buffer.

Barry Margolin
Thinking Machines Corp.

barmar@think.com
{uunet,harvard}!think!barmar