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(elisp)Dynamic Loading

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Dynamic Loading of Individual Functions
=======================================

   When you compile a file, you can optionally enable the "dynamic
function loading" feature (also known as "lazy loading").  With dynamic
function loading, loading the file doesn't fully read the function
definitions in the file.  Instead, each function definition contains a
place-holder which refers to the file.  The first time each function is
called, it reads the full definition from the file, to replace the
place-holder.

   The advantage of dynamic function loading is that loading the file
becomes much faster.  This is a good thing for a file which contains
many separate user-callable functions, if using one of them does not
imply you will probably also use the rest.  A specialized mode which
provides many keyboard commands often has that usage pattern: a user may
invoke the mode, but use only a few of the commands it provides.

   The dynamic loading feature has certain disadvantages:

   * If you delete or move the compiled file after loading it, Emacs
     can no longer load the remaining function definitions not already
     loaded.

   * If you alter the compiled file (such as by compiling a new
     version), then trying to load any function not already loaded will
     yield nonsense results.

   These problems will never happen in normal circumstances with
installed Emacs files.  But they are quite likely to happen with Lisp
files that you are changing.  The easiest way to prevent these problems
is to reload the new compiled file immediately after each recompilation.

   The byte compiler uses the dynamic function loading feature if the
variable `byte-compile-dynamic' is non-`nil' at compilation time.  Do
not set this variable globally, since dynamic loading is desirable only
for certain files.  Instead, enable the feature for specific source
files with file-local variable bindings.  For example, you could do it
by writing this text in the source file's first line:

     -*-byte-compile-dynamic: t;-*-

 - Variable: byte-compile-dynamic
     If this is non-`nil', the byte compiler generates compiled files
     that are set up for dynamic function loading.

 - Function: fetch-bytecode function
     This immediately finishes loading the definition of FUNCTION from
     its byte-compiled file, if it is not fully loaded already.  The
     argument FUNCTION may be a byte-code function object or a function
     name.