Emacs Lisp is a dialect of the Lisp programming language used by the GNU Emacs and XEmacs text editors (which this article will refer to collectively as "Emacs"). It is used for implementing most of the editing functionality built into Emacs, the remainder being written in C (as is the Lisp interpreter itself). Users of Emacs commonly write Emacs Lisp code to customize and extend Emacs.
Emacs Lisp can also function as a scripting language, much like the Unix Bourne shell, Perl, Python, scsh, or GNU Guile. Just as with those languages, it may be called from the command line or via an executable file. Its editing functions, such as buffers and movement commands, complement the features of Lisp and work in batch mode.
Some people refer to Emacs Lisp as Elisp, at the risk of confusion with an older unrelated Lisp dialect with the same name. In terms of features, it is closely related to the Maclisp dialect, with some later influence from Common Lisp . It supports imperative and functional programming methods. Richard Stallman chose Lisp as the extension language for his rewrite of Emacs (the original used TECO as its extension language) because of its powerful features, including the ability to treat functions as data. Unlike Common Lisp, Scheme existed at the time Stallman was rewriting Gosling Emacs into GNU Emacs, but it was not used because of Scheme's then-poor performance on workstations, and he had to develop a dialect more easily optimized.
The Lisp dialect used in Emacs differs substantially from the more modern Common Lisp and Scheme dialects commonly used for applications programming. For example: Emacs Lisp uses dynamic rather than lexical scope by default. That is, local variables in a calling function can be referenced from a called function without passing pointers or references.
Other methods exist for customizing GNU Emacs apart from writing Emacs Lisp. 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 relatively simple customizations. Not every user needs the full degree of extensibility offered by Emacs; those that do can write their own Emacs Lisp code.
Here follows 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 displaying a different buffer. A buffer is a region of text loaded into Emacs' memory (possibly from a file) which can be saved into a text document.
Users issue the "C-x 2" command to open a new window. This runs the Emacs 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-func () (interactive) (split-window-vertically) (set-window-buffer (next-window) (other-buffer))) (global-set-key "\C-x2" 'my-split-window-func)
The first statement, (defun ...), defines a new function, my-split-window-func, 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 key sequence "C-x 2" to the new function.
However, an easier way exists 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 my-window-splitting-advice (after split-window-vertically 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 at code evaluation time, 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.
In file-systems, Emacs Lisp code exists as plain text files with the filename suffix ".el" (a common exception, the user's init file, often appears or appeared as ".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 loads only when required. 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.
Emacs developers write certain functions in C. These are "primitives", also known as "built-in functions" or "subrs". Although primitives can be called from Lisp code, they can only be modified by editing the C source files and recompiling. In GNU Emacs, primitives are not available as external libraries; they are part of the Emacs executable. In XEmacs, runtime loading of such primitives is possible, using the operating system's support for dynamic linking. Functions may be written as primitives because they need access to external data and libraries not otherwise available from Emacs Lisp, or because they are called often enough that the comparative speed of C versus Emacs Lisp makes a worthwhile difference.
However, because errors in C code can easily lead to segmentation violations or to more subtle bugs, crashing the editor, and writing C code that interacts correctly with the Emacs Lisp garbage collector is error-prone, relatively few functions are implemented as primitives.
"Byte-compilation" can further increase the performance of Emacs Lisp code. 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 still runs more slowly 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.
Notably, the "cl" package implements a fairly large subset of Common Lisp.
Emacs Lisp uses dynamic, not static (or lexical), scope.
If a programmer declares a variable within the scope of a function,
it is available to subroutines called from within that function.
Originally, this was intended as an optimization; lexical scoping was still
uncommon and of uncertain performance.
Dynamic scoping was also 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 functions. 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. The
lexical-let macro in the "cl" package does provide
effective lexical scope to Emacs Lisp programmers, but while `cl'
is widely used,
lexical-let is rarely used. Variables
lexical-let are never released, even if
they are never used.
The apel library aids in writing portable Emacs Lisp code.