emacs - eval-after-load vs. mode hook

Question

Is there a difference between setting things for a mode using eval-after-load and using the mode hook?

I've seen some code where define-key is used inside a major mode hook, and some other code where define-key is used in eval-after-load form.

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Update:

For better understanding, here is an example of using eval-after-load and mode hooks with org-mode. The code can run before (load "org") or (require 'org) or (package-initialize).

;; The following two lines of code set some org-mode options.
;; Usually, these can be outside (eval-after-load ...) and work.
;; In cases that doesn't work, try using setq-default or set-variable
;; and putting them in (eval-after-load ...), if the
;; doc for the variables don't say what to do.
;; Or use Customize interface.
(setq org-hide-leading-stars t)
(setq org-return-follows-link t)

;; "org" because C-h f org-mode RET says that org-mode is defined in org.el
(eval-after-load "org"
  '(progn
     ;; Establishing your own keybindings for org-mode.
     ;; Variable org-mode-map is available only after org.el or org.elc is loaded.
     (define-key org-mode-map (kbd "<C-M-return>") 'org-insert-heading-respect-content)
     (define-key org-mode-map (kbd "<M-right>") nil) ; erasing a keybinding.
     (define-key org-mode-map (kbd "<M-left>") nil) ; erasing a keybinding.

     (defun my-org-mode-hook ()
       ;; The following two lines of code is run from the mode hook.
       ;; These are for buffer-specific things.
       ;; In this setup, you want to enable flyspell-mode
       ;; and run org-reveal for every org buffer.
       (flyspell-mode 1)
       (org-reveal))
     (add-hook 'org-mode-hook 'my-org-mode-hook)))

Answer 1

Code wrapped in eval-after-load will be executed only once, so it is typically used to perform one-time setup such as setting default global values and behaviour. An example might be setting up a default keymap for a particular mode. In eval-after-load code, there's no notion of the "current buffer".

Mode hooks execute once for every buffer in which the mode is enabled, so they're used for per-buffer configuration. Mode hooks are therefore run later than eval-after-load code; this lets them take actions based upon such information as whether other modes are enabled in the current buffer.