Emacs Lisp


Emacs Lisp is a dialect of the Lisp programming language used as a scripting language by 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. Emacs Lisp is also termed Elisp, although there is also an older, unrelated Lisp dialect with that name.
Users of Emacs commonly write Emacs Lisp code to customize and extend Emacs. Other options include the Customize feature that's been in GNU Emacs since version 20. Itself written in Emacs Lisp, Customize provides a set of preferences pages allowing the user to set options and preview their effect in the running Emacs session. When the user saves their changes, Customize simply writes the necessary Emacs Lisp code to the user's config file, which can be set to a special file that only Customize uses, to avoid the possibility of altering the user's own file.
Emacs Lisp can also function as a scripting language, much like the Unix Bourne shell or Perl, by calling Emacs in batch mode. In this way it may be called from the command line or via an executable file, and its editing functions, such as buffers and movement commands are available to the program just as in the normal mode. No user interface is presented when Emacs is started in batch mode; it simply executes the passed-in script and exits, displaying any output from the script.

Compared to other Lisp dialects

Emacs Lisp is most closely related to Maclisp, 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 because of its powerful features, including the ability to treat functions as data. Although the Common Lisp standard had yet to be formulated, Scheme existed at the time Stallman was rewriting Gosling Emacs into GNU Emacs. He chose not to use it because of its comparatively poor performance on workstations, and he wanted to develop a dialect which he thought would be more easily optimized.
The Lisp dialect used in Emacs differs substantially from the more modern Common Lisp and Scheme dialects used for applications programming. A prominent characteristic of Emacs Lisp is in its use of dynamic rather than lexical scope by default. That is, a function may reference local variables in the scope it is called from, but not in the scope where it was defined.

Example

To understand the logic behind Emacs Lisp, it is important to remember that there is an emphasis on providing data structures and features specific to making a versatile text editor over implementing a general-purpose programming language. For example, Emacs Lisp cannot easily read a file a line at a time—the entire file must be read into an Emacs buffer. However, Emacs Lisp provides many features for navigating and modifying buffer text at a sentence, paragraph, or higher syntactic level as defined by modes.
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 which can be saved into a text document.
Users can press the default C-x 2 key binding to open a new window. This runs the Emacs Lisp function split-window-below. 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:



))

The first statement, , defines a new function, my-split-window-func, which calls split-window-below, then tells the new window to display another buffer. The second statement, re-binds the key sequence "C-x 2" to the new function.
This can also be written using the feature called advice, which allows the user to create wrappers around existing functions instead of defining their own. This has the advantage of not requiring keybindings to be changed and working wherever the original function is called, as well as being simpler to write but the disadvantage of making debugging more complicated. For this reason, advice is not allowed in the source code of GNU Emacs, but if a user wishes, the advice feature can be used in their code to reimplement the above code as follows:

))

This instructs split-window-below to execute the user-supplied code whenever it is called, before executing the rest of the function. Advice can also be specified to execute after the original function, around it—literally wrapping the original, or to conditionally execute the original function based on the results of the advice.
Emacs 24.4 replaces this defadvice mechanism with advice-add, which is claimed to be more flexible and simpler. The advice above could be reimplemented using the new system as:

))

These changes take effect as soon as the code is evaluated. It is not necessary to recompile, restart Emacs, or even rehash a configuration file. If the code is saved into an Emacs init file, then Emacs will load the extension the next time it starts. Otherwise, the changes must be reevaluated manually when Emacs is restarted.

Byte code

Byte-compiling can make Emacs Lisp code execute faster. Emacs contains a compiler which can translate Emacs Lisp source files into a special representation termed 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.

Language features

Notably, the "cl" package implements a fairly large subset of Common Lisp.
Emacs Lisp does not do tail-call optimization. Without this, tail recursions can eventually lead to stack overflow.
The apel library aids in writing portable Emacs Lisp code, with the help of the polysylabi platform bridge.
Emacs Lisp is a Lisp-2 meaning that it has a function namespace which is separate from the namespace it uses for other variables.

From dynamic to lexical scoping

Like MacLisp, Emacs Lisp uses dynamic scope, offering static as an option starting from version 24. It can be activated by setting the file local variable lexical-binding.
In dynamic scoping, 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. "I asked RMS when he was implementing emacs lisp why it was dynamically scoped and his exact reply was that lexical scope was too inefficient." 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.
Also, 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.