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Function UNREAD-CHAR

Syntax

  • unread-char character &optional input-streamnil

Arguments and Values

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

Exceptional Situations

None.

See Also

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.

\issue{PEEK-CHAR-READ-CHAR-ECHO:FIRST-READ-CHAR} \issue{UNREAD-CHAR-AFTER-PEEK-CHAR:DONT-ALLOW}