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  ANSI Common Lisp   21 Streams   21.2 Dictionary of Streams

21.2.20 unread-char Function

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.

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

Affected By:
*standard-input*, *terminal-io*.

See Also:
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.

Allegro CL Implementation Details:

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