You can ask for code to be executed each time Emacs loads a library,
by using the variable after-load-functions:
This abnormal hook is run after loading a file. Each function in the hook is called with a single argument, the absolute filename of the file that was just loaded.
If you want code to be executed when a particular library is
loaded, use the macro with-eval-after-load:
This macro arranges to evaluate body at the end of loading the file library, each time library is loaded. If library is already loaded, it evaluates body right away.
You don’t need to give a directory or extension in the file name library. Normally, you just give a bare file name, like this:
(with-eval-after-load "js" (keymap-set js-mode-map "C-c C-c" 'js-eval))
To restrict which files can trigger the evaluation, include a
directory or an extension or both in library. Only a file whose
absolute true name (i.e., the name with all symbolic links chased out)
matches all the given name components will match. In the following
example, my_inst.elc or my_inst.elc.gz in some directory
..../foo/bar will trigger the evaluation, but not
my_inst.el:
(with-eval-after-load "foo/bar/my_inst.elc" …)
library can also be a feature (i.e., a symbol), in which case
body is evaluated at the end of any file where
(provide library) is called.
An error in body does not undo the load, but does prevent execution of the rest of body.
Normally, well-designed Lisp programs should not use
with-eval-after-load. If you need to examine and set the
variables defined in another library (those meant for outside use),
you can do it immediately—there is no need to wait until the library
is loaded. If you need to call functions defined by that library, you
should load the library, preferably with require (see Features).