20.2 Autoload

The autoload facility allows you to make a function or macro known in Lisp, but put off loading the file that defines it. The first call to the function automatically reads the proper file to install the real definition and other associated code, then runs the real definition as if it had been loaded all along.

There are two ways to set up an autoloaded function: by calling autoload, and by writing a special “magic” comment in the source before the real definition. autoload is the low-level primitive for autoloading; any Lisp program can call autoload at any time. Magic comments do nothing on their own; they serve as a guide for the command update-file-autoloads, which constructs calls to autoload and arranges to execute them when SXEmacs is built. Magic comments are the most convenient way to make a function autoload, but only for packages installed along with SXEmacs.

Function: autoload function filename &optional docstring interactive type

This function defines the function (or macro) named function so as to load automatically from filename. The string filename specifies the file to load to get the real definition of function.

The argument docstring is the documentation string for the function. Normally, this is identical to the documentation string in the function definition itself. Specifying the documentation string in the call to autoload makes it possible to look at the documentation without loading the function’s real definition.

If interactive is non-nil, then the function can be called interactively. This lets completion in M-x work without loading the function’s real definition. The complete interactive specification need not be given here; it’s not needed unless the user actually calls function, and when that happens, it’s time to load the real definition.

You can autoload macros and keymaps as well as ordinary functions. Specify type as macro if function is really a macro. Specify type as keymap if function is really a keymap. Various parts of SXEmacs need to know this information without loading the real definition.

An autoloaded keymap loads automatically during key lookup when a prefix key’s binding is the symbol function. Autoloading does not occur for other kinds of access to the keymap. In particular, it does not happen when a Lisp program gets the keymap from the value of a variable and calls define-key; not even if the variable name is the same symbol function.

If function already has a non-void function definition that is not an autoload object, autoload does nothing and returns nil. If the function cell of function is void, or is already an autoload object, then it is defined as an autoload object like this:

(autoload filename docstring interactive type)

For example,

(symbol-function 'run-prolog)
     ⇒ (autoload "prolog" 169681 t nil)

In this case, "prolog" is the name of the file to load, 169681 refers to the documentation string in the DOC file (see Documentation Basics), t means the function is interactive, and nil that it is not a macro or a keymap.

The autoloaded file usually contains other definitions and may require or provide one or more features. If the file is not completely loaded (due to an error in the evaluation of its contents), any function definitions or provide calls that occurred during the load are undone. This is to ensure that the next attempt to call any function autoloading from this file will try again to load the file. If not for this, then some of the functions in the file might appear defined, but they might fail to work properly for the lack of certain subroutines defined later in the file and not loaded successfully.

SXEmacs as distributed comes with many autoloaded functions. The calls to autoload are in the file loaddefs.el. There is a convenient way of updating them automatically.

If the autoloaded file fails to define the desired Lisp function or macro, then an error is signaled with data "Autoloading failed to define function function-name".

A magic autoload comment looks like ‘;;;###autoload’, on a line by itself, just before the real definition of the function in its autoloadable source file. The command M-x update-file-autoloads writes a corresponding autoload call into loaddefs.el. Building SXEmacs loads loaddefs.el and thus calls autoload. M-x update-directory-autoloads is even more powerful; it updates autoloads for all files in the current directory.

The same magic comment can copy any kind of form into loaddefs.el. If the form following the magic comment is not a function definition, it is copied verbatim. You can also use a magic comment to execute a form at build time without executing it when the file itself is loaded. To do this, write the form on the same line as the magic comment. Since it is in a comment, it does nothing when you load the source file; but update-file-autoloads copies it to loaddefs.el, where it is executed while building SXEmacs.

The following example shows how doctor is prepared for autoloading with a magic comment:

;;;###autoload
(defun doctor ()
  "Switch to *doctor* buffer and start giving psychotherapy."
  (interactive)
  (switch-to-buffer "*doctor*")
  (doctor-mode))

Here’s what that produces in loaddefs.el:

(autoload 'doctor "doctor"
  "\
Switch to *doctor* buffer and start giving psychotherapy."
  t)

The backslash and newline immediately following the double-quote are a convention used only in the preloaded Lisp files such as loaddefs.el; they tell make-docfile to put the documentation string in the DOC file. See Building SXEmacs.