#emacs #protip: buffer-file-name is nil when not viewing a file

protip.el

;; I wanted to setup a hook which would compile the user init file upon exit or save.
;; So I wrote the code below to do so:

(defun my-emacs-lisp-mode-hook ()
  (when (files-equal-p buffer-file-name user-init-file)
    (add-hook 'after-save-hook 'byte-compile-user-init-file t t)))

(add-hook 'emacs-lisp-mode-hook 'my-emacs-lisp-mode-hook)
(add-hook 'kill-emacs-hook 'byte-compile-user-init-file t t)

;; When evaluating this directly, it worked fine.
;; However, when opening emacs it would fail and I got the following message:

;; files-equal-p: Wrong type argument: stringp, nil

;; It turns out that buffer-file-name is nil when not visiting a file, e.g. opening emacs.
;; So here is my code change to fix the issue:

(defun my-emacs-lisp-mode-hook ()
  (when (files-equal-p (or buffer-file-name "") user-init-file)
    (add-hook 'after-save-hook 'byte-compile-user-init-file t t)))