灰色的溪流

灰色流是允许用户定义的流的非标准扩展。它提供了用户可以扩展的类和方法。你应该检查你的实施手册，看它是否提供灰色流。

举一个简单的例子，返回随机字符的字符输入流可以像这样实现：

(defclass random-character-input-stream (fundamental-character-input-stream)
  ((character-table
    :initarg :character-table
    :initform "abcdefghijklmnopqrstuvwxyz
" ; The newline is necessary.
    :accessor character-table))
  (:documentation "A stream of random characters."))

(defmethod stream-read-char ((stream random-character-input-stream))
  (let ((table (character-table stream)))
    (aref table (random (length table)))))

(let ((stream (make-instance 'random-character-input-stream)))
  (dotimes (i 5)
    (print (read-line stream))))
; "gyaexyfjsqdcpciaaftoytsygdeycrrzwivwcfb" 
; "gctnoxpajovjqjbkiqykdflbhfspmexjaaggonhydhayvknwpdydyiabithpt" 
; "nvfxwzczfalosaqw" 
; "sxeiejcovrtesbpmoppfvvjfvx" 
; "hjplqgstbodbalnmxhsvxdox" 
;=> NIL