How can backup-directory-alist be set to save in the files directory?

Question

I want the backup files to be created in sub-directories of the files being edited, and I have created a function returns that directory.

(defun tempfilepath ()
  (interactive)
  (concat (file-name-directory buffer-file-name) "./editorbackups")
)

So I need backup-directory-alist to be something like

(setq backup-directory-alist `("." . tempfilepath))

where tempfilepath will be evaluated when the file at runtime so all the backup files are saved under the files folders.

I am not sure how to get the syntax right and the role make-backup-file-name-function if it is necessary in this case.

Answer 1

Just use relative directory name

(setq backup-directory-alist '(("." . "editorbackups")))

Function make-backup-file-name-1 will make directory name relative to file's directory and create it:

;; If backup-directory is relative, it should be relative to the
;; file's directory.  By expanding explicitly here, we avoid
;; depending on default-directory.
(if backup-directory
    (setq abs-backup-directory
          (expand-file-name backup-directory
                            (file-name-directory file))))
(if (and abs-backup-directory (not (file-exists-p abs-backup-directory)))
    (condition-case nil
        (make-directory abs-backup-directory 'parents)
      (file-error (setq backup-directory nil
                        abs-backup-directory nil))))

Also make sure that variable make-backup-file-name-function is set to make-backup-file-name--default-function.