unread-char
unread-char Function
Syntax:
unread-char character &optional input-stream → nil
Arguments and Values:
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.
Description:
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.
Examples:
(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
Affected By:
*standard-input*, *terminal-io*.
See Also:
peek-char, read-char, Section 21.1 (Stream Concepts)
Notes:
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.
Expanded Reference: unread-char
TODO: Please contribute to this page by adding explanations and examples
(unread-char )