CLHS: Function UNREAD-CHAR

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Function UNREAD-CHAR

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*.

Exceptional Situations: None.

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.

The following X3J13 cleanup issues, not part of the specification, apply to this section:

- UNREAD-CHAR-AFTER-PEEK-CHAR:DONT-ALLOW
- PEEK-CHAR-READ-CHAR-ECHO:FIRST-READ-CHAR

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