Is it possible to append to a list via .dir-locals.el?

Question

So, I know that I can assign a variable via .dir-locals.el...

("thingy" (1 2 3 4))

but is there any way that I can append to an array, if at all?

("that" . "is correct")
(add-to-list 'thingy 5)

Answer 1

From the GNU Emacs Manual:

You can specify the variables mode, eval, and unibyte in your .dir-locals.el, and they have the same meanings as they would have in file local variables. coding cannot be specified as a directory local variable. See Local Variables in Files.

Variables in .dir-locals.el are mode specific. You have to explicitly specify the mode that you are using, it is not a simple key/value store.

The eval keyword means you can execute Lisp code, this means you may call add-to-list and modify variables as needed.

((org-mode . ((eval . (add-to-list 'org-publish-project-alist
 ("org-project" (:output-dir "~/org-out")))))