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| ANSI Common Lisp 21 Streams 21.2 Dictionary of Streams
21.2.35 clear-input | Function
  Syntax: clear-input `&optional` input-stream nil Arguments and Values: input-stream - an input stream designator. The default is standard input. Description: Clears any available input from input-stream. If clear-input does not make sense for input-stream, then clear-input does nothing. Examples:
      ;; The exact I/O behavior of this example might vary from implementation
      ;; to implementation depending on the kind of interactive buffering that
      ;; occurs.  (The call to SLEEP here is intended to help even out the
      ;; differences in implementations which do not do line-at-a-time buffering.)

      (defun read-sleepily (&optional (clear-p nil) (zzz 0))
        (list (progn (print '>) (read))
              ;; Note that input typed within the first ZZZ seconds
              ;; will be discarded.
              (progn (print '>)
                     (if zzz (sleep zzz))
                     (print '>>)
                     (if clear-p (clear-input))
                     (read))))

      (read-sleepily)
      > 10
      >
      >> 20
       (10 20)

      (read-sleepily t)
      > 10
      >
      >> 20
       (10 20)

      (read-sleepily t 10)
      > 10
      > 20  ; Some implementations won't echo typeahead here.
      >> 30
       (10 30)
Side Effects: The input-stream is modified. Affected By: *standard-input* Exceptional Situations: Should signal an error of type type-error if input-stream is not a stream designator. See Also: clear-output Allegro CL Implementation Details: None.

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