Emacs Lisp's Library System: What's require, load, load-file, autoload, feature?

Xah Lee, 2010-07-13, …, 2012-01-04

This page explains emacs library system. For example, what's the difference between library, package, features? And what's the difference between {load-file, load, require, autoload}?

What's Library, Package, Feature?

No Namespace

Emacs lisp the language does not have namespaces. Everything is global, with dynamic scope, with some shadowing mechanism. So, don't expect library or module to be language defined namespace constructs like Perl or Python's module system or Java's Package system.

What's the difference between a Package and Library?

The terms “package” and “library”, are used losely in emacs/elisp manual to refer to any useful elisp file. They are not technical definitions in elisp.

A “library” usually refers to elisp file containing a collection of lisp functions, to be called by other lisp source code. For example, the command comment-dwim is defined in 〔newcomment.el〕, which is a library of functions.

A “package” usually refers to something useful for emacs users. e.g. major mode or minor mode.

The term “module” is not used by emacs.

Emacs's Concept of “Feature” and the Functions {provide, require}

The term “feature” has some meaning in elisp, but is not mechanical. A “feature” is elisp symbol that is intended to represent the functionality provided by a emacs package.

For example, type 【Alt+x describe-variable Enter features Enter】, and here's its value:

ibuffer etags ring cc-mode cc-fonts cc-menus cc-cmds cc-styles
cc-align cc-engine cc-vars cc-defs xlsl-mode encoded-kb speck
sgml-mode dired info newcomment desktop recentf tree-widget wid-edit
advice help-fns …

A elisp file can call (provide '‹symbol name›) near the end. Then emacs will add that symbol to the “features” list.

The purpose of features is for emacs to know if a package is already loaded. The “features” variable and the functions {provide, require}, is the mechanism. The require function checks the “features” to see if a symbol is already there before loading the file.

In summary:

“features” is a global variable, holding a list of elisp symbols that are features.
A “feature” is a elisp symbol that represent a lisp package.
A package can call (provide '‹symbol name›). When this code is evaluated, emacs will add the symbol name to the “features” list.
When the code (require '‹symbol name›) is called, emacs check if that symbol name is already in the “features” list. If not, load it. (emacs will guess a file name based on the symbol name. Or, the “require” function may specify a file name in the 2nd argument.)

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 in many bundled emacs packages. 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, but is a useful mode anyway. 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 as well as library.

File/Package Loading Mechanisms

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

Function Name	Purpose	Tech Detail	Comment
`load-file`	Load a file.	Load one specific file.	Use this if you have one very SPECIFIC file at one particular file path.
`load`	Load a file.	Load a file by searching thru var “load-path”. Also, tries to load a compiled version (.elc) if exists.	Use this if the path for the file is not known in advance, and you are using a file name without full path, such as “undo” or “undo.el”, and you want to load the compiled version if it exists (undo.elc).
`require`	Load a package if it has not already been loaded.	Checks the var “features”, if symbol is not there, then call `load` to load it.	Best used in elisp source code, similar to other lang's “require” or “import”.
`autoload`	Load a file only when a function is called.	Associate 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, it's good to have your package installation go by `autoload` (if possible). It saves startup time.

Isn't all this “no namespace”, “not enforced”, “not managed” module system very bad?

Yes it is.

Though, it's just the state of software. Many most popular langs, such as C, C++, PHP, do worse. They don't have a module system neither, but worse, they load a file by “include”. Note that even Scheme Lisp didn't have module system, until R6RS, released in 2007, and the new module system defined in it is widely criticized, and R6RS caused Scheme community to split.

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