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| ANSI Common Lisp 21 Streams 21.2 Dictionary of Streams
21.2.20 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. Allegro CL Implementation Details: None.

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