1 @node Gray Streams examples
2 @subsection Gray Streams examples
8 Below are two classes of stream that can be conveniently defined as
9 wrappers for Common Lisp streams. These are meant to serve as
10 examples of minimal implementations of the protocols that must be
11 followed when defining Gray streams. Realistic uses of the Gray
12 Streams API would implement the various methods that can do I/O in
13 batches, such as @codew{stream-read-line}, @codew{stream-write-string},
14 @codew{stream-read-sequence}, and @codew{stream-write-sequence}.
18 * Character counting input stream::
19 * Output prefixing character stream::
22 @node Character counting input stream
23 @subsubsection Character counting input stream
25 It is occasionally handy for programs that process input files to
26 count the number of characters and lines seen so far, and the number
27 of characters seen on the current line, so that useful messages may be
28 reported in case of parsing errors, etc. Here is a character input
29 stream class that keeps track of these counts. Note that all
30 character input streams must implement @codew{stream-read-char} and
31 @codew{stream-unread-char}.
35 (defclass wrapped-stream (fundamental-stream)
36 ((stream :initarg :stream :reader stream-of)))
39 (defmethod stream-element-type ((stream wrapped-stream))
40 (stream-element-type (stream-of stream)))
43 (defmethod close ((stream wrapped-stream) &key abort)
44 (close (stream-of stream) :abort abort))
47 (defclass wrapped-character-input-stream
48 (wrapped-stream fundamental-character-input-stream)
52 (defmethod stream-read-char ((stream wrapped-character-input-stream))
53 (read-char (stream-of stream) nil :eof))
56 (defmethod stream-unread-char ((stream wrapped-character-input-stream)
58 (unread-char char (stream-of stream)))
61 (defclass counting-character-input-stream
62 (wrapped-character-input-stream)
63 ((char-count :initform 1 :accessor char-count-of)
64 (line-count :initform 1 :accessor line-count-of)
65 (col-count :initform 1 :accessor col-count-of)
66 (prev-col-count :initform 1 :accessor prev-col-count-of)))
69 (defmethod stream-read-char ((stream counting-character-input-stream))
70 (with-accessors ((inner-stream stream-of) (chars char-count-of)
71 (lines line-count-of) (cols col-count-of)
72 (prev prev-col-count-of)) stream
73 (let ((char (call-next-method)))
74 (cond ((eql char :eof)
76 ((char= char #\Newline)
88 (defmethod stream-unread-char ((stream counting-character-input-stream)
90 (with-accessors ((inner-stream stream-of) (chars char-count-of)
91 (lines line-count-of) (cols col-count-of)
92 (prev prev-col-count-of)) stream
93 (cond ((char= char #\Newline)
105 The default methods for @codew{stream-read-char-no-hang},
106 @codew{stream-peek-char}, @codew{stream-listen},
107 @codew{stream-clear-input}, @codew{stream-read-line}, and
108 @codew{stream-read-sequence} should be sufficient (though the last two
109 will probably be slower than methods that forwarded directly).
111 Here's a sample use of this class:
115 (with-input-from-string (input "1 2
117 (let ((counted-stream (make-instance 'counting-character-input-stream
119 (loop for thing = (read counted-stream) while thing
120 unless (numberp thing) do
121 (error "Non-number ~S (line ~D, column ~D)" thing
122 (line-count-of counted-stream)
123 (- (col-count-of counted-stream)
124 (length (format nil "~S" thing))))
132 Non-number :FOO (line 2, column 5)
133 [Condition of type SIMPLE-ERROR]
137 @node Output prefixing character stream
138 @subsubsection Output prefixing character stream
140 One use for a wrapped output stream might be to prefix each line of
141 text with a timestamp, e.g., for a logging stream. Here's a simple
142 stream that does this, though without any fancy line-wrapping. Note
143 that all character output stream classes must implement
144 @codew{stream-write-char} and @codew{stream-line-column}.
148 (defclass wrapped-stream (fundamental-stream)
149 ((stream :initarg :stream :reader stream-of)))
152 (defmethod stream-element-type ((stream wrapped-stream))
153 (stream-element-type (stream-of stream)))
156 (defmethod close ((stream wrapped-stream) &key abort)
157 (close (stream-of stream) :abort abort))
160 (defclass wrapped-character-output-stream
161 (wrapped-stream fundamental-character-output-stream)
162 ((col-index :initform 0 :accessor col-index-of)))
165 (defmethod stream-line-column ((stream wrapped-character-output-stream))
166 (col-index-of stream))
169 (defmethod stream-write-char ((stream wrapped-character-output-stream)
171 (with-accessors ((inner-stream stream-of) (cols col-index-of)) stream
172 (write-char char inner-stream)
173 (if (char= char #\Newline)
178 (defclass prefixed-character-output-stream
179 (wrapped-character-output-stream)
180 ((prefix :initarg :prefix :reader prefix-of)))
183 (defgeneric write-prefix (prefix stream)
184 (:method ((prefix string) stream) (write-string prefix stream))
185 (:method ((prefix function) stream) (funcall prefix stream)))
188 (defmethod stream-write-char ((stream prefixed-character-output-stream)
190 (with-accessors ((inner-stream stream-of) (cols col-index-of)
191 (prefix prefix-of)) stream
193 (write-prefix prefix inner-stream))
198 As with the example input stream, this implements only the minimal
199 protocol. A production implementation should also provide methods for
200 at least @codew{stream-write-line}, @codew{stream-write-sequence}.
202 And here's a sample use of this class:
206 (flet ((format-timestamp (stream)
207 (apply #'format stream "[~2@@*~2,' D:~1@@*~2,'0D:~0@@*~2,'0D] "
208 (multiple-value-list (get-decoded-time)))))
209 (let ((output (make-instance 'prefixed-character-output-stream
210 :stream *standard-output*
211 :prefix #'format-timestamp)))
212 (loop for string in '("abc" "def" "ghi") do
213 (write-line string output)