character—a character; must be the last character that was read from input-stream.
input-stream—an input stream designator. The default is standard input.
unread-char
places character back onto the front of
input-stream so that it will again be the next character
in input-stream.
When input-stream is an echo stream,
no attempt is made to undo any echoing of the character that might already
have been done on input-stream. However, characters placed on
input-stream by unread-char
are marked in such a way
as to inhibit later re-echo by read-char
.
It is an error to invoke unread-char
twice consecutively on the same stream
without an intervening call to read-char
(or some other input operation which implicitly reads characters)
on that stream.
Invoking peek-char
or read-char
commits all previous characters.
The consequences of invoking unread-char
on any character preceding that which is returned by
peek-char
(including those passed over by
peek-char
that has a non-nil peek-type)
are unspecified.
In particular, the consequences of
invoking unread-char
after peek-char
are unspecified.
(with-input-from-string (is "0123")
(dotimes (i 6)
(let ((c (read-char is)))
(if (evenp i) (format t "~&~S ~S~%" i c) (unread-char c is)))))
▷ 0 #\0
▷ 2 #\1
▷ 4 #\2
→ NIL
*standard-input*
,
*terminal-io*
.
peek-char, read-char, Section 21.1 (Stream Concepts)
unread-char
is intended to be an efficient mechanism for allowing
the Lisp reader and other parsers to perform one-character lookahead
in input-stream.