Elisp: load, load-file, autoload

Emacs lisp's library system is a primitive system, centered on loading file by load, with some slightly high level things such as its autoload, and features, require. However, nothing is strict or enforced by elisp.

load

(load FILE &optional NOERROR NOMESSAGE NOSUFFIX MUST-SUFFIX)
The general function for loading a file.
If no optional argument is given, it'll try to load the FILE by searching for it in a list of directory stored in variable load-path, by first appending .elc to the argument FILE, and if none found, it'll try appending .el, and others, and finally try just FILE as given.
For example, if you (load "x"), it'll try

1. x.elc
2. x.el
3. x

load-file

Load one specific file. Use this when you have a specific full path of a file in mind. The file name argument should contain file name extension, such as .el or .elc.
(load-file file_name) just calls (load (expand-file-name file_name) nil nil t).

autoload

(autoload FUNCTION FILE &optional DOCSTRING INTERACTIVE TYPE)
Load a file only when a function is called. autoload associates a function name with a file path. When the function is called, load the file, and execute the function. If you are writing a major mode, have your package installation go by autoload if possible. It saves startup time.

require

(require FEATURE &optional FILENAME NOERROR)
Load a package if it has not already been loaded by checking the features variable.
require checks if the symbol FEATURE is in variable features. If not, then it calls load to load it. File name is guessed from the feature name FEATURE, or specified in the optional argument. require is best used in elisp libraries or scripts, similar to other language's “require” or “import”. [see Elisp: provide, require, features]

• (info "(elisp) How Programs Do Loading")
• (info "(emacs) Lisp Libraries")
• (info "(elisp) Loading")

See also: Elisp: How to Name Your Major Mode.

What is Library, Package, Feature?

No Namespace

Emacs lisp the language does not have namespace. Everything is global. So, don't expect library or module to be structured namespace like python module system or Java's Package system.

[see Python: How to Write a Module]

[see Java: Package]

What is the difference between a Package and Library?

These words are used losely, and do NOT have TECHNICAL definitions in elisp.

library

Elisp file(s) containing a collection of useful stuff. For example, the command comment-dwim is defined in newcomment.el, which is a library of functions.

package

Any useful elisp library for emacs users. For example: major mode or minor mode.

The term “module” is not used by emacs.

Emacs's Concept of Feature

Elisp: provide, require, features

Package/Library/Feature are not Managed

There is no absolute relation between any concept of package/library/feature/autoload facilities and the file name.

By convention, if a elisp file name is xyz-mode.el, it OFTEN provides a lisp symbol “xyz-mode” as its feature name (if it does at all), and the command to invoke the mode is OFTEN named “xyz-mode”. Sometimes the “-mode” part is omitted in any of {file name, feature symbol name, command name}.

This is only a lose convention. There are a lot exceptions. For example:

• The file lisp-mode.el provides the symbol lisp-mode as feature, and is invoked by a command named emacs-lisp-mode.
• The cua-base.el file provides symbols cua-base and cua as features, and is invoked by a command named cua-mode.
• The text-mode.el file does not provide any symbol for feature. It is invoked by a command named text-mode.
• The file desktop.el provides the symbol desktop as feature, and the command name to invoke it is desktop-save-mode.

All the above means, you could have a file named Joe-xyz-mode_v2.1.el, which provides a feature named “abc”, while the command name to activate it may be “opq”, and it might be displayed in mode line as “OPQ helper”. And, this file can be considered as a package or library.