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Emacs Lisp

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Emacs Lisp is a dialect of the Lisp programming language used by the GNU Emacs and XEmacs editors, which will simply be called "Emacs" in this article. Emacs Lisp is sometimes also called Elisp, at the risk of confusion with an unrelated Lisp dialect with the same name.

The majority of the editing functionality in Emacs comes from code written in Emacs Lisp; the rest is written in C. Emacs Lisp code can be written by users to customize and extend Emacs.

Emacs Lisp is related to the MacLisp and Common Lisp dialects of Lisp. It supports imperative and functional programming methods. Lisp was chosen as the extension language for Emacs because of its powerful features, including the ability to treat functions as data.

Writing Emacs Lisp is not the only method of customizing GNU Emacs. Since version 20, GNU Emacs has included a "Customize" facility which allows users to set common customization variables through a graphical interface. "Customize" works by writing Emacs Lisp code for the user, and is limited to simple customizations. Not every user needs the full degree of extensibility offered by Emacs; those that do must write their own Emacs Lisp code.

An Emacs Lisp Example

Here is a simple example of an Emacs extension written in Emacs Lisp. In Emacs, the editing area can be split into separate areas called "windows", each containing a different buffer. A buffer is, roughly speaking, a region of text loaded into Emacs' memory, which can be saved into a text document.

The user command for opening a new window is "C-x 2". This runs the lisp function split-window-vertically. Normally, when the new window appears, it displays the same buffer as the previous one. Suppose we wish to make it display the next available buffer. In order to do this, the user writes the following Emacs Lisp code, in either an existing Emacs Lisp source file or an empty Emacs buffer: 

(defun my-split-window-function ()
  (interactive)
  (split-window-vertically)
  (set-window-buffer (next-window) (other-buffer)))

(global-set-key "\C-x2" 'my-split-window-function)

The first statement (defun...), defines a new function, my-split-window-function, which calls split-window-vertically (the old window-splitting function), then tells the new window to display another buffer. The second statement, (global-set-key...) re-binds the keys "C-x 2" to the new function.

However, there is an easier way to write this. Emacs Lisp has a powerful feature called advice, which allows the user to create wrappers around existing functions instead of defining their own. Using advice, the above code can be reimplemented as follows: 

(defadvice split-window-vertically
  (after my-window-splitting-advice first () activate)
  (set-window-buffer (next-window) (other-buffer)))

This instructs split-window-vertically to execute the user-supplied code whenever it is called, before executing the rest of the function.

These changes take effect when the code is evaluated, using (for instance) the command "M-x eval-buffer". It is not necessary to recompile or even restart Emacs, which makes customizing Emacs very convenient. If the code is saved into the Emacs "init file" (usually a file named ".emacs" in the user's home directory), then Emacs will load the extension the next time it starts. Otherwise, the changes will be lost when the user exits Emacs.

Source Code and Byte Code

Emacs Lisp code is stored as plain text, with the filename suffix ".el" (an exception being the user's init file, which is named ".emacs".) When the files are loaded, an interpreter component of the Emacs program reads and parses the functions and variables, storing them in memory. They are then available to other editing functions, and to user commands. Functions and variables can be freely modified and re-loaded.

In order to save memory space, much of the functionality of Emacs is not loaded until it is needed. Each set of optional features is implemented by a collection of Emacs code called a "library". For example, there is a library for highlighting keywords in program source code, and a library for playing the game of Tetris. Each library is implemented using one or more Emacs Lisp source files.

Certain functions are written in C. These are "primitives", also known as "built-in functions" or "subrs". Although primitives can be called from the Lisp code, they can only be modified by editing the C source files and recompiling the editor. Primitives are not available as libraries; they are part of the Emacs executable. Functions are written as primitives because C code is faster than Emacs Lisp code. However, only those few functions that need to run quickly and efficiently are written as primitives, because primitives are not as flexible as Emacs Lisp functions.

The performance of Emacs Lisp code can be further increased by "byte-compilation". Emacs contains a compiler which can translate Emacs Lisp source files into a special representation known as bytecode. Emacs Lisp bytecode files have the filename suffix ".elc". Compared to source files, bytecode files load faster, occupy less space on the disk, use less memory when loaded, and run faster.

Bytecode is still slower than primitives, but functions loaded as bytecode can be easily modified and re-loaded. In addition, bytecode files are platform-independent. The standard Emacs Lisp code distributed with Emacs is loaded as bytecode, although the matching source files are usually provided for the user's reference as well. User-supplied extensions are typically not byte-compiled, as they are neither as large nor as computationally intensive.

Language Features

Emacs Lisp uses dynamic scoping instead of lexical scoping. If a variable is declared within the scope of a function, it is available to subroutines called from within that function. Originally, this was meant to provide greater flexibility for user customizations. However, dynamic scoping has several disadvantages. Firstly, it can easily lead to bugs in large programs, due to unintended interactions between variables in different function. Secondly, accessing variables under dynamic scoping is generally slower than under lexical scoping. As a result, plans have been made to convert Emacs Lisp to lexical scoping, though this has not yet been done.

Emacs does not implement Tail recursion, which is found in most other Lisp implementations.

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