with-function-input-stream

Arguments: (sym function &rest args) &body body

with-function-input-stream evalutes body as an implicit progn with sym bound to the stream returned by make-function-input-stream. When control leaves the body, either normally or abnormally (such as by the use of throw), the stream is automatically closed.

See make-function-input-stream for details.

Example 1

(defun filler (writeto count)
  (dotimes (n count)
     (write-byte n writeto))
  (close writeto))

(with-function-input-stream (readfrom #'filler 5)
  (let ((count 0)
        (sum 0)
        got)
    (while (setf got (read-byte readfrom nil nil))
       (incf sum got)
       (incf count))
    (format t "Read ~d bytes, totalling ~a~%" count sum)))

Example 2

(defun head-filter (outstream instream &key (lines 10))
  (dotimes (n lines)
    (let ((line (read-line instream nil nil)))
      (if (null line)
	  (return))
      (write-line line outstream))))

(defun tail-filter (outstream instream &key (lines 10))
  (let ((buffer (make-array lines))
	(pos 0)
	(wrapped nil)
	line)
    (while (setf line (read-line instream nil nil))
      (setf (aref buffer pos) line)
      (incf pos)
      (if (= pos lines)
	  (progn 
	    (setf pos 0)
	    (setf wrapped t))))

    (if wrapped
	(dotimes (n (- lines pos))
	  (write-line (aref buffer (+ n pos)) outstream)))

    (dotimes (n pos)
      (write-line (aref buffer n) outstream))))

(defun uniq-filter (outstream instream)
  (let (lastline 
	line)
    (while (setf line (read-line instream nil nil))
      (if (or (not lastline) (string/= line lastline))
	  (progn
	    (write-line line outstream)
	    (setf lastline line))))))

;; uniq the last 5 of the first 10 lines of the sourcefile.
(defun test-filters (sourcefile)
  (with-open-file (f sourcefile)
    (with-function-input-stream (headstream #'head-filter f)
      (with-function-input-stream (tailstream #'tail-filter headstream
					      :lines 5)
	(uniq-filter *terminal-io* tailstream)))))

Copyright (c) 1998-2016, Franz Inc. Oakland, CA., USA. All rights reserved.
This page was not revised from the 8.1 page.
Created 2010.1.21.