input-stream—an input stream designator. The default is standard input.
generalized-boolean—a generalized boolean.
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 file1.
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.
(progn (unread-char (read-char)) (list (listen) (read-char))) ▷ 1 → (T #\1) (progn (clear-input) (listen)) → NIL ;Unless you're a very fast typist!