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Issue: UNREAD-CHAR-AFTER-PEEK-CHAR (Version 1)
- To: firstname.lastname@example.org
- Subject: Issue: UNREAD-CHAR-AFTER-PEEK-CHAR (Version 1)
- From: masinter.pa@Xerox.COM
- Date: 1 Aug 88 10:26 PDT
- Cc: cutting.pa@Xerox.COM
References: pp 379, 380 of CLtL
Edit history: Version 1 by Doug Cutting <Cutting.PA@Xerox.COM> on July 29, 1988
PEEK-CHAR and UNREAD-CHAR are very similar mechanisms. The description of
PEEK-CHAR in CLtL even states that "it is as if one had called READ-CHAR and
then UNREAD-CHAR in succession." But while CLtL prohibits calling UNREAD-CHAR
twice in succession it does not prohibit calling UNREAD-CHAR after PEEK-CHAR.
The obvious implementation of PEEK-CHAR and UNREAD-CHAR (a one-character buffer)
will not work unless this prohibition is present.
Proposal (UNREAD-CHAR-AFTER-PEEK-CHAR:DONT-ALLOW): UNREAD-CHAR may not be called
after PEEK-CHAR without an intervening call to READ-CHAR.
;;; Following is an example of code which should not be valid CL.
(defun test (stream)
(let ((char (read-char stream)))
(peek-char nil stream)
(unread-char char stream)
(assert (eql char (read-char stream)))))
PEEK-CHAR and UNREAD-CHAR provide equivalent functionality and it is thus
reasonable for an implementation to implement them in terms of the same
In Xerox Common Lisp, different (non-random-access) stream types behave
differently. One, (TCP/FTP) handled this correctly, while another (keyboard with
line-buffering turned off) did not.
Cost to Implementors:
Zero. Implementations which allow this are still correct.
Cost to Users:
Small. I suspect there is very little code which depends upon this working
correctly, as most code uses either PEEK-CHAR or UNREAD-CHAR, but not both.
Cost of non-adoption:
Implementations of sequential streams are forced to be unnecessarily complex in
order to be correct.
Allows simple yet adequately powerful implementation of sequential streams.
Requires that users have shared, one-char buffer model of how UNREAD-CHAR and
PEEK-CHAR work, rather than two separate one-char buffers.