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(elisp)Repeated Loading

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Repeated Loading
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   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.  (Note: Defining Variables.)

   The simplest way to add an element to an alist is like this:

     (setq minor-mode-alist
           (cons '(leif-mode " Leif") minor-mode-alist))

But this would add multiple elements if the library is reloaded.  To
avoid the problem, write this:

     (or (assq 'leif-mode minor-mode-alist)
         (setq minor-mode-alist
               (cons '(leif-mode " Leif") minor-mode-alist)))

   To add an element to a list just once, you can also use `add-to-list'
(Note: Setting Variables).

   Occasionally you will want to test explicitly whether a library has
already been loaded.  Here's one way to test, in a library, whether it
has been loaded before:

     (defvar foo-was-loaded nil)
     
     (unless foo-was-loaded
       EXECUTE-FIRST-TIME-ONLY
       (setq foo-was-loaded t))

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.  Note: Named Features.