Variable for file name

Question

How can I tell emacs/e-lisp to check if a user defined variable is a full path to a file or just a folder name? Currently, for one particular mode, user can just define variable for a sqlite database as a folder, and as a result, get database error which is not very informative.

If it matters, the variable is org-roam-db-location and default value is defined in org-roam-db.el as:

(defcustom org-roam-db-location (locate-user-emacs-file "org-roam.db")
  "The path to file where the Org-roam database is stored.

It is the user's responsibility to set this correctly, especially
when used with multiple Org-roam instances."
  :type 'string
  :group 'org-roam)

Answer 1

You can check using the functions file-directory-p and/or file-regular-p:

(file-directory-p org-roam-db-location)

will return t if it is a directory (a "folder") and nil otherwise.

Similarly

(file-regular-p org-roam-db-location)

will return t if it is a regular file and nil otherwise.