CODEWITHSUNDEEP
fileiocommon-lisp
Pedro Delfino
Pedro Delfino

Reputation: 2681

How can I succesfully re-write a file appending an element to a list using Common Lisp?

I want to write a Common Lisp list in a .lisp file. If the file does not exist, it will be created and add the element.

If the file already exists, it will re-write the file appending new content to the list.

This implementation partially works:

(defun append-to-list-in-file (filename new-item &aux contents) ;;;;
  (setq contents (list)) ;; default in case reading fails
  (ignore-errors
    (with-open-file (str filename :direction :input)
      (setq contents (read str))))
  (setq contents (nconc contents (list new-item)))
  (with-open-file (str filename :direction :output :if-exists :overwrite)
    (write contents :stream str)))

If I do:

(append-to-list-in-file  "/home/pedro/miscellaneous/misc/tests-output/CL.lisp" 4)

It works. The code creates the file AND puts 4 inside of it as '(4). However, if I run the code again with a new element using the file that was just created:

(append-to-list-in-file  "/home/pedro/miscellaneous/misc/tests-output/CL.lisp" 5)

It throws an error:

Error opening #P"/home/pedro/miscellaneous/misc/tests-output/CL.lisp" [Condition of type SB-EXT:FILE-EXISTS]

I was expecting: '(4 5)

What do I need to change?

Upvotes: 0

Views: 121

Answers (1)

Rainer Joswig
Rainer Joswig

Reputation: 139251

Probably a good idea to create the file. Otherwise you can't overwrite it.

... :if-does-not-exist :create :if-exists :overwrite ...

Upvotes: 2

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