Serializing Lisp lists

In Lisp, it is very easy to serialize and deserialize lists into files. You can use the macros with-open-file and print for this. Using macro with-open-file makes sure that the file is closed automatically even in case of failures. The print macro prints lists in a way that they can be read by the Lisp reader. For example, you define the following function for serialization.

(defun serialize (file-name list)
  (with-open-file (stream file-name :direction :output)
    (print list stream)))

You might want to use keyword argument :if-exists :supersede if you want to overwrite existing files. Add another function for deserialization. It uses the read function to read the list back from the file.

(defun deserialize (file-name)
  (with-open-file (stream file-name :direction :input)
    (read stream)))

Now you can use these two functions to save your lists to file and read them back in. I have defined the two functions in a package called mhn-serial.

CL-USER> (mhn-serial:serialize “~/tmp/list.ser” ‘(1 2 3))
(1 2 3)
CL-USER> (mhn-serial:deserialize “/home/neifer/tmp/list.ser”)
(1 3 4)
CL-USER> (car (mhn-serial:deserialize “/home/neifer/tmp/list.ser”))
CL-USER> (cdr (mhn-serial:deserialize “/home/neifer/tmp/list.ser”))
(3 4)
CL-USER> (nth 2 (mhn-serial:deserialize “/home/neifer/tmp/list.ser”))
CL-USER> (mhn-serial:deserialize “/home/neifer/tmp/list.ser”)
((1 2 3) (4 5 6))
CL-USER> (nth 1 (car (cdr (mhn-serial:deserialize “/home/neifer/tmp/list.ser”))))

Please note that I have edited the file between the calls. I have changed the file content from (1 2 3) to (1 3 4) and then to ((1 2 3)(4 5 6)). Just to check if reading works. You could change the content to something more interesting, of course.

CL-USER> (mhn-serial:deserialize “/home/neifer/tmp/list.ser”)
(+ 2 3)
CL-USER> (eval (mhn-serial:deserialize “/home/neifer/tmp/list.ser”))

You see that you have to be very careful when using eval on objects that you have read from another source. The bad guys could easily compromise your system.

Serializing Lisp lists

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