unread-char character {&optional input-stream} => nil
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~
|> 0 #\0
|> 2 #\1
|> 4 #\2
=> NIL
*standard-input*, *terminal-io*.
section peek-char [Function] , section read-char [Function] , section 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.
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