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### 15.6 Repeated Loading

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 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 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 Named 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))

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