You can load a given file more than once in an Emacs session. For example, after you have rewritten and reinstalled a function definition by editing it in a buffer, you may wish to return to the original version; you can do this by reloading the file it came from.
When you load or reload files, bear in mind that the load and
load-library functions automatically load a byte-compiled file
rather than a non-compiled file of similar name. If you rewrite a file
that you intend to save and reinstall, you need to byte-compile the new
version; otherwise Emacs will load the older, byte-compiled file instead
of your newer, non-compiled file! If that happens, the message
displayed when loading the file includes, ‘(compiled; note, source is
newer)’, to remind you to recompile it.
When writing the forms in a Lisp library file, keep in mind that the
file might be loaded more than once. For example, think about whether
each variable should be reinitialized when you reload the library;
defvar does not change the value if the variable is already
initialized. (See Defining Global Variables.)
The simplest way to add an element to an alist is like this:
(push '(leif-mode " Leif") minor-mode-alist)
But this would add multiple elements if the library is reloaded. To
avoid the problem, use add-to-list (see Modifying List Variables):
(add-to-list 'minor-mode-alist '(leif-mode " Leif"))
Occasionally you will want to test explicitly whether a library has
already been loaded. If the library uses provide to provide a
named feature, you can use featurep earlier in the file to test
whether the provide call has been executed before (see Features). Alternatively, you could use something like this:
(defvar foo-was-loaded nil) (unless foo-was-loaded execute-first-time-only (setq foo-was-loaded t))