CLHS: Function LISTEN

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Function LISTEN

Syntax:

listen `&optional` input-stream => generalized-boolean

Arguments and Values:

input-stream---an input stream designator. The default is standard input.

generalized-boolean---a generalized boolean.

Description:

Returns true if there is a character immediately available from input-stream; otherwise, returns false. On a non-interactive input-stream, listen returns true except when at end of file[1]. If an end of file is encountered, listen returns false. listen is intended to be used when input-stream obtains characters from an interactive device such as a keyboard.

Examples:

     (progn (unread-char (read-char)) (list (listen) (read-char)))
    >>  1
    =>  (T #\1)
     (progn (clear-input) (listen))
    =>  NIL ;Unless you're a very fast typist!

Side Effects: None.

Affected By:

*standard-input*

Exceptional Situations: None.

See Also:

interactive-stream-p, read-char-no-hang

Notes: None.

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