[FIXED] How to change the use of Common Lisp `with-open-file` so that elements are appended inside a list in a file?

Issue

I am using Common Lisp (SBCL). Currently, I can write a file appending lists. In order to illustrate, let's first define some variables to make it easier to understand:

(defvar example-1 '((d-1 u-1) (i-1) (i-2)))
(defvar example-2 '((d-2 u-2) (i-1) (i-2)))

Now, in the REPL:

CL-USER> (with-open-file (str "/home/pedro/miscellaneous/misc/tests-output/stack-overflow.lisp"
                         :direction :output
                         :if-exists :append
                         :if-does-not-exist :create)
           (format str " ~S ~%" example-1))
    
CL-USER> (with-open-file (str "/home/pedro/miscellaneous/misc/tests-output/stack-overflow.lisp"
                         :direction :output
                         :if-exists :append
                         :if-does-not-exist :create)
           (format str " ~S ~%" example-2))

If I go to checkout the file "stack-overflow.lisp", I can see:

 ((D-1 U-1) (I-1) (I-2)) 
 ((D-2 U-2) (I-1) (I-2)) 

Ok. This is close to what I want.

However, I would like to have everything wrapped in a list:

(

 ((D-1 U-1) (I-1) (I-2)) 
 ((D-2 U-2) (I-1) (I-2)) 

)

Thus, every time something is "appended" to the file, it should be inside this list. I am changing this because it will make this file easier to read and work on. I will need to filter the elements added.

What do I need to change in with-open-file function to have this output?

Solution

Use a single call to WITH-OPEN-FILE and combine everything into a single list.

(with-open-file (str "/home/pedro/miscellaneous/misc/tests-output/stack-overflow.lisp"
                         :direction :output
                         :if-exists :overwrite
                         :if-does-not-exist :create)
  (pprint (list example-1 example-2) str))

If you want to append to the list every time you write, you need to read the list, append to the list, then overwrite the file with the updated list.

(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)))

Answered By - Barmar

Answer Checked By - Gilberto Lyons (PHPFixing Admin)