Emacs Lisp

This is the GNU Emacs Lisp Reference Manual corresponding to Emacs version 24.3.

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Copyright © 1990–1996, 1998–2013 Free Software Foundation, Inc.

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(a) The FSF's Back-Cover Text is: “You have the freedom to copy and modify this GNU manual. Buying copies from the FSF supports it in developing GNU and promoting software freedom.”

Introduction	Introduction and conventions used.
Lisp Data Types	Data types of objects in Emacs Lisp.
Numbers	Numbers and arithmetic functions.
Strings and Characters	Strings, and functions that work on them.
Lists	Lists, cons cells, and related functions.
Sequences Arrays Vectors	Lists, strings and vectors are called sequences. Certain functions act on any kind of sequence. The description of vectors is here as well.
Hash Tables	Very fast lookup-tables.
Symbols	Symbols represent names, uniquely.
Evaluation	How Lisp expressions are evaluated.
Control Structures	Conditionals, loops, nonlocal exits.
Variables	Using symbols in programs to stand for values.
Functions	A function is a Lisp program that can be invoked from other functions.
Macros	Macros are a way to extend the Lisp language.
Customization	Making variables and faces customizable.
Loading	Reading files of Lisp code into Lisp.
Byte Compilation	Compilation makes programs run faster.
Advising Functions	Adding to the definition of a function.
Debugging	Tools and tips for debugging Lisp programs.
Read and Print	Converting Lisp objects to text and back.
Minibuffers	Using the minibuffer to read input.
Command Loop	How the editor command loop works, and how you can call its subroutines.
Keymaps	Defining the bindings from keys to commands.
Modes	Defining major and minor modes.
Documentation	Writing and using documentation strings.
Files	Accessing files.
Backups and Auto-Saving	Controlling how backups and auto-save files are made.
Buffers	Creating and using buffer objects.
Windows	Manipulating windows and displaying buffers.
Frames	Making multiple system-level windows.
Positions	Buffer positions and motion functions.
Markers	Markers represent positions and update automatically when the text is changed.
Text	Examining and changing text in buffers.
Non-ASCII Characters	Non-ASCII text in buffers and strings.
Searching and Matching	Searching buffers for strings or regexps.
Syntax Tables	The syntax table controls word and list parsing.
Abbrevs	How Abbrev mode works, and its data structures.
Processes	Running and communicating with subprocesses.
Display	Features for controlling the screen display.
System Interface	Getting the user id, system type, environment variables, and other such things.
Packaging	Preparing Lisp code for distribution.
Appendices
Antinews	Info for users downgrading to Emacs 23.
GNU Free Documentation License	The license for this documentation.
GPL	Conditions for copying and changing GNU Emacs.
Tips	Advice and coding conventions for Emacs Lisp.
GNU Emacs Internals	Building and dumping Emacs; internal data structures.
Standard Errors	List of some standard error symbols.
Standard Keymaps	List of some standard keymaps.
Standard Hooks	List of some standard hook variables.
Index	Index including concepts, functions, variables, and other terms.

Detailed Node Listing

Here are other nodes that are subnodes of those already listed, mentioned here so you can get to them in one step:

Introduction
Caveats	Flaws and a request for help.
Lisp History	Emacs Lisp is descended from Maclisp.
Conventions	How the manual is formatted.
Version Info	Which Emacs version is running?
Acknowledgments	The authors, editors, and sponsors of this manual.
Conventions
Some Terms	Explanation of terms we use in this manual.
nil and t	How the symbols nil and t are used.
Evaluation Notation	The format we use for examples of evaluation.
Printing Notation	The format we use when examples print text.
Error Messages	The format we use for examples of errors.
Buffer Text Notation	The format we use for buffer contents in examples.
Format of Descriptions	Notation for describing functions, variables, etc.
Format of Descriptions
A Sample Function Description	A description of an imaginary function, foo.
A Sample Variable Description	A description of an imaginary variable, electric-future-map.
Lisp Data Types
Printed Representation	How Lisp objects are represented as text.
Comments	Comments and their formatting conventions.
Programming Types	Types found in all Lisp systems.
Editing Types	Types specific to Emacs.
Circular Objects	Read syntax for circular structure.
Type Predicates	Tests related to types.
Equality Predicates	Tests of equality between any two objects.
Programming Types
Integer Type	Numbers without fractional parts.
Floating Point Type	Numbers with fractional parts and with a large range.
Character Type	The representation of letters, numbers and control characters.
Symbol Type	A multi-use object that refers to a function, variable, or property list, and has a unique identity.
Sequence Type	Both lists and arrays are classified as sequences.
Cons Cell Type	Cons cells, and lists (which are made from cons cells).
Array Type	Arrays include strings and vectors.
String Type	An (efficient) array of characters.
Vector Type	One-dimensional arrays.
Char-Table Type	One-dimensional sparse arrays indexed by characters.
Bool-Vector Type	One-dimensional arrays of t or nil.
Hash Table Type	Super-fast lookup tables.
Function Type	A piece of executable code you can call from elsewhere.
Macro Type	A method of expanding an expression into another expression, more fundamental but less pretty.
Primitive Function Type	A function written in C, callable from Lisp.
Byte-Code Type	A function written in Lisp, then compiled.
Autoload Type	A type used for automatically loading seldom-used functions.
Character Type
Basic Char Syntax	Syntax for regular characters.
General Escape Syntax	How to specify characters by their codes.
Ctl-Char Syntax	Syntax for control characters.
Meta-Char Syntax	Syntax for meta-characters.
Other Char Bits	Syntax for hyper-, super-, and alt-characters.
Cons Cell and List Types
Box Diagrams	Drawing pictures of lists.
Dotted Pair Notation	A general syntax for cons cells.
Association List Type	A specially constructed list.
String Type
Syntax for Strings	How to specify Lisp strings.
Non-ASCII in Strings	International characters in strings.
Nonprinting Characters	Literal unprintable characters in strings.
Text Props and Strings	Strings with text properties.
Editing Types
Buffer Type	The basic object of editing.
Marker Type	A position in a buffer.
Window Type	Buffers are displayed in windows.
Frame Type	Windows subdivide frames.
Terminal Type	A terminal device displays frames.
Window Configuration Type	Recording the way a frame is subdivided.
Frame Configuration Type	Recording the status of all frames.
Process Type	A subprocess of Emacs running on the underlying OS.
Stream Type	Receive or send characters.
Keymap Type	What function a keystroke invokes.
Overlay Type	How an overlay is represented.
Font Type	Fonts for displaying text.
Numbers
Integer Basics	Representation and range of integers.
Float Basics	Representation and range of floating point.
Predicates on Numbers	Testing for numbers.
Comparison of Numbers	Equality and inequality predicates.
Numeric Conversions	Converting float to integer and vice versa.
Arithmetic Operations	How to add, subtract, multiply and divide.
Rounding Operations	Explicitly rounding floating point numbers.
Bitwise Operations	Logical and, or, not, shifting.
Math Functions	Trig, exponential and logarithmic functions.
Random Numbers	Obtaining random integers, predictable or not.
Strings and Characters
String Basics	Basic properties of strings and characters.
Predicates for Strings	Testing whether an object is a string or char.
Creating Strings	Functions to allocate new strings.
Modifying Strings	Altering the contents of an existing string.
Text Comparison	Comparing characters or strings.
String Conversion	Converting to and from characters and strings.
Formatting Strings	format: Emacs's analogue of printf.
Case Conversion	Case conversion functions.
Case Tables	Customizing case conversion.
Lists
Cons Cells	How lists are made out of cons cells.
List-related Predicates	Is this object a list? Comparing two lists.
List Elements	Extracting the pieces of a list.
Building Lists	Creating list structure.
List Variables	Modifying lists stored in variables.
Modifying Lists	Storing new pieces into an existing list.
Sets And Lists	A list can represent a finite mathematical set.
Association Lists	A list can represent a finite relation or mapping.
Property Lists	A list of paired elements.
Modifying Existing List Structure
Setcar	Replacing an element in a list.
Setcdr	Replacing part of the list backbone. This can be used to remove or add elements.
Rearrangement	Reordering the elements in a list; combining lists.
Property Lists
Plists and Alists	Comparison of the advantages of property lists and association lists.
Plist Access	Accessing property lists stored elsewhere.
Sequences, Arrays, and Vectors
Sequence Functions	Functions that accept any kind of sequence.
Arrays	Characteristics of arrays in Emacs Lisp.
Array Functions	Functions specifically for arrays.
Vectors	Special characteristics of Emacs Lisp vectors.
Vector Functions	Functions specifically for vectors.
Char-Tables	How to work with char-tables.
Bool-Vectors	How to work with bool-vectors.
Rings	Managing a fixed-size ring of objects.
Hash Tables
Creating Hash	Functions to create hash tables.
Hash Access	Reading and writing the hash table contents.
Defining Hash	Defining new comparison methods.
Other Hash	Miscellaneous.
Symbols
Symbol Components	Symbols have names, values, function definitions and property lists.
Definitions	A definition says how a symbol will be used.
Creating Symbols	How symbols are kept unique.
Symbol Properties	Each symbol has a property list for recording miscellaneous information.
Symbol Properties
Symbol Plists	Accessing symbol properties.
Standard Properties	Standard meanings of symbol properties.
Evaluation
Intro Eval	Evaluation in the scheme of things.
Forms	How various sorts of objects are evaluated.
Quoting	Avoiding evaluation (to put constants in the program).
Backquote	Easier construction of list structure.
Eval	How to invoke the Lisp interpreter explicitly.
Kinds of Forms
Self-Evaluating Forms	Forms that evaluate to themselves.
Symbol Forms	Symbols evaluate as variables.
Classifying Lists	How to distinguish various sorts of list forms.
Function Indirection	When a symbol appears as the car of a list, we find the real function via the symbol.
Function Forms	Forms that call functions.
Macro Forms	Forms that call macros.
Special Forms	"Special forms" are idiosyncratic primitives, most of them extremely important.
Autoloading	Functions set up to load files containing their real definitions.
Control Structures
Sequencing	Evaluation in textual order.
Conditionals	if, cond, when, unless.
Combining Conditions	and, or, not.
Iteration	while loops.
Nonlocal Exits	Jumping out of a sequence.
Nonlocal Exits
Catch and Throw	Nonlocal exits for the program's own purposes.
Examples of Catch	Showing how such nonlocal exits can be written.
Errors	How errors are signaled and handled.
Cleanups	Arranging to run a cleanup form if an error happens.
Errors
Signaling Errors	How to report an error.
Processing of Errors	What Emacs does when you report an error.
Handling Errors	How you can trap errors and continue execution.
Error Symbols	How errors are classified for trapping them.
Variables
Global Variables	Variable values that exist permanently, everywhere.
Constant Variables	Certain "variables" have values that never change.
Local Variables	Variable values that exist only temporarily.
Void Variables	Symbols that lack values.
Defining Variables	A definition says a symbol is used as a variable.
Tips for Defining	Things you should think about when you define a variable.
Accessing Variables	Examining values of variables whose names are known only at run time.
Setting Variables	Storing new values in variables.
Variable Scoping	How Lisp chooses among local and global values.
Buffer-Local Variables	Variable values in effect only in one buffer.
File Local Variables	Handling local variable lists in files.
Directory Local Variables	Local variables common to all files in a directory.
Variable Aliases	Variables that are aliases for other variables.
Variables with Restricted Values	Non-constant variables whose value can not be an arbitrary Lisp object.
Generalized Variables	Extending the concept of variables.
Scoping Rules for Variable Bindings
Dynamic Binding	The default for binding local variables in Emacs.
Dynamic Binding Tips	Avoiding problems with dynamic binding.
Lexical Binding	A different type of local variable binding.
Using Lexical Binding	How to enable lexical binding.
Buffer-Local Variables
Intro to Buffer-Local	Introduction and concepts.
Creating Buffer-Local	Creating and destroying buffer-local bindings.
Default Value	The default value is seen in buffers that don't have their own buffer-local values.
Generalized Variables
Setting Generalized Variables	The setf macro.
Adding Generalized Variables	Defining new setf forms.
Functions
What Is a Function	Lisp functions vs. primitives; terminology.
Lambda Expressions	How functions are expressed as Lisp objects.
Function Names	A symbol can serve as the name of a function.
Defining Functions	Lisp expressions for defining functions.
Calling Functions	How to use an existing function.
Mapping Functions	Applying a function to each element of a list, etc.
Anonymous Functions	Lambda expressions are functions with no names.
Function Cells	Accessing or setting the function definition of a symbol.
Closures	Functions that enclose a lexical environment.
Obsolete Functions	Declaring functions obsolete.
Inline Functions	Defining functions that the compiler will expand inline.
Declare Form	Adding additional information about a function.
Declaring Functions	Telling the compiler that a function is defined.
Function Safety	Determining whether a function is safe to call.
Related Topics	Cross-references to specific Lisp primitives that have a special bearing on how functions work.
Lambda Expressions
Lambda Components	The parts of a lambda expression.
Simple Lambda	A simple example.
Argument List	Details and special features of argument lists.
Function Documentation	How to put documentation in a function.
Macros
Simple Macro	A basic example.
Expansion	How, when and why macros are expanded.
Compiling Macros	How macros are expanded by the compiler.
Defining Macros	How to write a macro definition.
Problems with Macros	Don't evaluate the macro arguments too many times. Don't hide the user's variables.
Indenting Macros	Specifying how to indent macro calls.
Common Problems Using Macros
Wrong Time	Do the work in the expansion, not in the macro.
Argument Evaluation	The expansion should evaluate each macro arg once.
Surprising Local Vars	Local variable bindings in the expansion require special care.
Eval During Expansion	Don't evaluate them; put them in the expansion.
Repeated Expansion	Avoid depending on how many times expansion is done.
Customization Settings
Common Keywords	Common keyword arguments for all kinds of customization declarations.
Group Definitions	Writing customization group definitions.
Variable Definitions	Declaring user options.
Customization Types	Specifying the type of a user option.
Applying Customizations	Functions to apply customization settings.
Custom Themes	Writing Custom themes.
Customization Types
Simple Types	Simple customization types: sexp, integer, etc.
Composite Types	Build new types from other types or data.
Splicing into Lists	Splice elements into list with :inline.
Type Keywords	Keyword-argument pairs in a customization type.
Defining New Types	Give your type a name.
Loading
How Programs Do Loading	The load function and others.
Load Suffixes	Details about the suffixes that load tries.
Library Search	Finding a library to load.
Loading Non-ASCII	Non-ASCII characters in Emacs Lisp files.
Autoload	Setting up a function to autoload.
Repeated Loading	Precautions about loading a file twice.
Named Features	Loading a library if it isn't already loaded.
Where Defined	Finding which file defined a certain symbol.
Unloading	How to "unload" a library that was loaded.
Hooks for Loading	Providing code to be run when particular libraries are loaded.
Byte Compilation
Speed of Byte-Code	An example of speedup from byte compilation.
Compilation Functions	Byte compilation functions.
Docs and Compilation	Dynamic loading of documentation strings.
Dynamic Loading	Dynamic loading of individual functions.
Eval During Compile	Code to be evaluated when you compile.
Compiler Errors	Handling compiler error messages.
Byte-Code Objects	The data type used for byte-compiled functions.
Disassembly	Disassembling byte-code; how to read byte-code.
Advising Emacs Lisp Functions
Simple Advice	A simple example to explain the basics of advice.
Defining Advice	Detailed description of defadvice.
Around-Advice	Wrapping advice around a function's definition.
Computed Advice	...is to defadvice as fset is to defun.
Activation of Advice	Advice doesn't do anything until you activate it.
Enabling Advice	You can enable or disable each piece of advice.
Preactivation	Preactivation is a way of speeding up the loading of compiled advice.
Argument Access in Advice	How advice can access the function's arguments.
Combined Definition	How advice is implemented.
Debugging Lisp Programs
Debugger	A debugger for the Emacs Lisp evaluator.
Edebug	A source-level Emacs Lisp debugger.
Syntax Errors	How to find syntax errors.
Test Coverage	Ensuring you have tested all branches in your code.
Profiling	Measuring the resources that your code uses.
The Lisp Debugger
Error Debugging	Entering the debugger when an error happens.
Infinite Loops	Stopping and debugging a program that doesn't exit.
Function Debugging	Entering it when a certain function is called.
Explicit Debug	Entering it at a certain point in the program.
Using Debugger	What the debugger does; what you see while in it.
Debugger Commands	Commands used while in the debugger.
Invoking the Debugger	How to call the function debug.
Internals of Debugger	Subroutines of the debugger, and global variables.
Edebug
Using Edebug	Introduction to use of Edebug.
Instrumenting	You must instrument your code in order to debug it with Edebug.
Edebug Execution Modes	Execution modes, stopping more or less often.
Jumping	Commands to jump to a specified place.
Edebug Misc	Miscellaneous commands.
Breaks	Setting breakpoints to make the program stop.
Trapping Errors	Trapping errors with Edebug.
Edebug Views	Views inside and outside of Edebug.
Edebug Eval	Evaluating expressions within Edebug.
Eval List	Expressions whose values are displayed each time you enter Edebug.
Printing in Edebug	Customization of printing.
Trace Buffer	How to produce trace output in a buffer.
Coverage Testing	How to test evaluation coverage.
The Outside Context	Data that Edebug saves and restores.
Edebug and Macros	Specifying how to handle macro calls.
Edebug Options	Option variables for customizing Edebug.
Breaks
Breakpoints	Breakpoints at stop points.
Global Break Condition	Breaking on an event.
Source Breakpoints	Embedding breakpoints in source code.
The Outside Context
Checking Whether to Stop	When Edebug decides what to do.
Edebug Display Update	When Edebug updates the display.
Edebug Recursive Edit	When Edebug stops execution.
Edebug and Macros
Instrumenting Macro Calls	The basic problem.
Specification List	How to specify complex patterns of evaluation.
Backtracking	What Edebug does when matching fails.
Specification Examples	To help understand specifications.
Debugging Invalid Lisp Syntax
Excess Open	How to find a spurious open paren or missing close.
Excess Close	How to find a spurious close paren or missing open.
Reading and Printing Lisp Objects
Streams Intro	Overview of streams, reading and printing.
Input Streams	Various data types that can be used as input streams.
Input Functions	Functions to read Lisp objects from text.
Output Streams	Various data types that can be used as output streams.
Output Functions	Functions to print Lisp objects as text.
Output Variables	Variables that control what the printing functions do.
Minibuffers
Intro to Minibuffers	Basic information about minibuffers.
Text from Minibuffer	How to read a straight text string.
Object from Minibuffer	How to read a Lisp object or expression.
Minibuffer History	Recording previous minibuffer inputs so the user can reuse them.
Initial Input	Specifying initial contents for the minibuffer.
Completion	How to invoke and customize completion.
Yes-or-No Queries	Asking a question with a simple answer.
Multiple Queries	Asking a series of similar questions.
Reading a Password	Reading a password from the terminal.
Minibuffer Commands	Commands used as key bindings in minibuffers.
Minibuffer Windows	Operating on the special minibuffer windows.
Minibuffer Contents	How such commands access the minibuffer text.
Recursive Mini	Whether recursive entry to minibuffer is allowed.
Minibuffer Misc	Various customization hooks and variables.
Completion
Basic Completion	Low-level functions for completing strings.
Minibuffer Completion	Invoking the minibuffer with completion.
Completion Commands	Minibuffer commands that do completion.
High-Level Completion	Convenient special cases of completion (reading buffer names, variable names, etc.).
Reading File Names	Using completion to read file names and shell commands.
Completion Variables	Variables controlling completion behavior.
Programmed Completion	Writing your own completion function.
Completion in Buffers	Completing text in ordinary buffers.
Command Loop
Command Overview	How the command loop reads commands.
Defining Commands	Specifying how a function should read arguments.
Interactive Call	Calling a command, so that it will read arguments.
Distinguish Interactive	Making a command distinguish interactive calls.
Command Loop Info	Variables set by the command loop for you to examine.
Adjusting Point	Adjustment of point after a command.
Input Events	What input looks like when you read it.
Reading Input	How to read input events from the keyboard or mouse.
Special Events	Events processed immediately and individually.
Waiting	Waiting for user input or elapsed time.
Quitting	How C-g works. How to catch or defer quitting.
Prefix Command Arguments	How the commands to set prefix args work.
Recursive Editing	Entering a recursive edit, and why you usually shouldn't.
Disabling Commands	How the command loop handles disabled commands.
Command History	How the command history is set up, and how accessed.
Keyboard Macros	How keyboard macros are implemented.
Defining Commands
Using Interactive	General rules for interactive.
Interactive Codes	The standard letter-codes for reading arguments in various ways.
Interactive Examples	Examples of how to read interactive arguments.
Input Events
Keyboard Events	Ordinary characters--keys with symbols on them.
Function Keys	Function keys--keys with names, not symbols.
Mouse Events	Overview of mouse events.
Click Events	Pushing and releasing a mouse button.
Drag Events	Moving the mouse before releasing the button.
Button-Down Events	A button was pushed and not yet released.
Repeat Events	Double and triple click (or drag, or down).
Motion Events	Just moving the mouse, not pushing a button.
Focus Events	Moving the mouse between frames.
Misc Events	Other events the system can generate.
Event Examples	Examples of the lists for mouse events.
Classifying Events	Finding the modifier keys in an event symbol. Event types.
Accessing Mouse	Functions to extract info from mouse events.
Accessing Scroll	Functions to get info from scroll bar events.
Strings of Events	Special considerations for putting keyboard character events in a string.
Reading Input
Key Sequence Input	How to read one key sequence.
Reading One Event	How to read just one event.
Event Mod	How Emacs modifies events as they are read.
Invoking the Input Method	How reading an event uses the input method.
Quoted Character Input	Asking the user to specify a character.
Event Input Misc	How to reread or throw away input events.
Keymaps
Key Sequences	Key sequences as Lisp objects.
Keymap Basics	Basic concepts of keymaps.
Format of Keymaps	What a keymap looks like as a Lisp object.
Creating Keymaps	Functions to create and copy keymaps.
Inheritance and Keymaps	How one keymap can inherit the bindings of another keymap.
Prefix Keys	Defining a key with a keymap as its definition.
Active Keymaps	How Emacs searches the active keymaps for a key binding.
Searching Keymaps	A pseudo-Lisp summary of searching active maps.
Controlling Active Maps	Each buffer has a local keymap to override the standard (global) bindings. A minor mode can also override them.
Key Lookup	Finding a key's binding in one keymap.
Functions for Key Lookup	How to request key lookup.
Changing Key Bindings	Redefining a key in a keymap.
Remapping Commands	A keymap can translate one command to another.
Translation Keymaps	Keymaps for translating sequences of events.
Key Binding Commands	Interactive interfaces for redefining keys.
Scanning Keymaps	Looking through all keymaps, for printing help.
Menu Keymaps	Defining a menu as a keymap.
Menu Keymaps
Defining Menus	How to make a keymap that defines a menu.
Mouse Menus	How users actuate the menu with the mouse.
Keyboard Menus	How users actuate the menu with the keyboard.
Menu Example	Making a simple menu.
Menu Bar	How to customize the menu bar.
Tool Bar	A tool bar is a row of images.
Modifying Menus	How to add new items to a menu.
Easy Menu	A convenience macro for defining menus.
Defining Menus
Simple Menu Items	A simple kind of menu key binding.
Extended Menu Items	More complex menu item definitions.
Menu Separators	Drawing a horizontal line through a menu.
Alias Menu Items	Using command aliases in menu items.
Major and Minor Modes
Hooks	How to use hooks; how to write code that provides hooks.
Major Modes	Defining major modes.
Minor Modes	Defining minor modes.
Mode Line Format	Customizing the text that appears in the mode line.
Imenu	Providing a menu of definitions made in a buffer.
Font Lock Mode	How modes can highlight text according to syntax.
Auto-Indentation	How to teach Emacs to indent for a major mode.
Desktop Save Mode	How modes can have buffer state saved between Emacs sessions.
Hooks
Running Hooks	How to run a hook.
Setting Hooks	How to put functions on a hook, or remove them.
Major Modes
Major Mode Conventions	Coding conventions for keymaps, etc.
Auto Major Mode	How Emacs chooses the major mode automatically.
Mode Help	Finding out how to use a mode.
Derived Modes	Defining a new major mode based on another major mode.
Basic Major Modes	Modes that other modes are often derived from.
Mode Hooks	Hooks run at the end of major mode functions.
Tabulated List Mode	Parent mode for buffers containing tabulated data.
Generic Modes	Defining a simple major mode that supports comment syntax and Font Lock mode.
Example Major Modes	Text mode and Lisp modes.
Minor Modes
Minor Mode Conventions	Tips for writing a minor mode.
Keymaps and Minor Modes	How a minor mode can have its own keymap.
Defining Minor Modes	A convenient facility for defining minor modes.
Mode Line Format
Mode Line Basics	Basic ideas of mode line control.
Mode Line Data	The data structure that controls the mode line.
Mode Line Top	The top level variable, mode-line-format.
Mode Line Variables	Variables used in that data structure.
%-Constructs	Putting information into a mode line.
Properties in Mode	Using text properties in the mode line.
Header Lines	Like a mode line, but at the top.
Emulating Mode Line	Formatting text as the mode line would.
Font Lock Mode
Font Lock Basics	Overview of customizing Font Lock.
Search-based Fontification	Fontification based on regexps.
Customizing Keywords	Customizing search-based fontification.
Other Font Lock Variables	Additional customization facilities.
Levels of Font Lock	Each mode can define alternative levels so that the user can select more or less.
Precalculated Fontification	How Lisp programs that produce the buffer contents can also specify how to fontify it.
Faces for Font Lock	Special faces specifically for Font Lock.
Syntactic Font Lock	Fontification based on syntax tables.
Multiline Font Lock	How to coerce Font Lock into properly highlighting multiline constructs.
Multiline Font Lock Constructs
Font Lock Multiline	Marking multiline chunks with a text property.
Region to Refontify	Controlling which region gets refontified after a buffer change.
Automatic Indentation of code
SMIE	A simple minded indentation engine.
Simple Minded Indentation Engine
SMIE setup	SMIE setup and features.
Operator Precedence Grammars	A very simple parsing technique.
SMIE Grammar	Defining the grammar of a language.
SMIE Lexer	Defining tokens.
SMIE Tricks	Working around the parser's limitations.
SMIE Indentation	Specifying indentation rules.
SMIE Indentation Helpers	Helper functions for indentation rules.
SMIE Indentation Example	Sample indentation rules.
Documentation
Documentation Basics	Where doc strings are defined and stored.
Accessing Documentation	How Lisp programs can access doc strings.
Keys in Documentation	Substituting current key bindings.
Describing Characters	Making printable descriptions of non-printing characters and key sequences.
Help Functions	Subroutines used by Emacs help facilities.
Files
Visiting Files	Reading files into Emacs buffers for editing.
Saving Buffers	Writing changed buffers back into files.
Reading from Files	Reading files into buffers without visiting.
Writing to Files	Writing new files from parts of buffers.
File Locks	Locking and unlocking files, to prevent simultaneous editing by two people.
Information about Files	Testing existence, accessibility, size of files.
Changing Files	Renaming files, changing permissions, etc.
File Names	Decomposing and expanding file names.
Contents of Directories	Getting a list of the files in a directory.
Create/Delete Dirs	Creating and Deleting Directories.
Magic File Names	Special handling for certain file names.
Format Conversion	Conversion to and from various file formats.
Visiting Files
Visiting Functions	The usual interface functions for visiting.
Subroutines of Visiting	Lower-level subroutines that they use.
Information about Files
Testing Accessibility	Is a given file readable? Writable?
Kinds of Files	Is it a directory? A symbolic link?
Truenames	Eliminating symbolic links from a file name.
File Attributes	How large is it? Any other names? Etc.
Locating Files	How to find a file in standard places.
File Names
File Name Components	The directory part of a file name, and the rest.
Relative File Names	Some file names are relative to a current directory.
Directory Names	A directory's name as a directory is different from its name as a file.
File Name Expansion	Converting relative file names to absolute ones.
Unique File Names	Generating names for temporary files.
File Name Completion	Finding the completions for a given file name.
Standard File Names	If your package uses a fixed file name, how to handle various operating systems simply.
File Format Conversion
Format Conversion Overview	insert-file-contents and write-region.
Format Conversion Round-Trip	Using format-alist.
Format Conversion Piecemeal	Specifying non-paired conversion.
Backups and Auto-Saving
Backup Files	How backup files are made; how their names are chosen.
Auto-Saving	How auto-save files are made; how their names are chosen.
Reverting	revert-buffer, and how to customize what it does.
Backup Files
Making Backups	How Emacs makes backup files, and when.
Rename or Copy	Two alternatives: renaming the old file or copying it.
Numbered Backups	Keeping multiple backups for each source file.
Backup Names	How backup file names are computed; customization.
Buffers
Buffer Basics	What is a buffer?
Current Buffer	Designating a buffer as current so that primitives will access its contents.
Buffer Names	Accessing and changing buffer names.
Buffer File Name	The buffer file name indicates which file is visited.
Buffer Modification	A buffer is modified if it needs to be saved.
Modification Time	Determining whether the visited file was changed "behind Emacs's back".
Read Only Buffers	Modifying text is not allowed in a read-only buffer.
The Buffer List	How to look at all the existing buffers.
Creating Buffers	Functions that create buffers.
Killing Buffers	Buffers exist until explicitly killed.
Indirect Buffers	An indirect buffer shares text with some other buffer.
Swapping Text	Swapping text between two buffers.
Buffer Gap	The gap in the buffer.
Windows
Basic Windows	Basic information on using windows.
Windows and Frames	Relating windows to the frame they appear on.
Window Sizes	Accessing a window's size.
Resizing Windows	Changing the sizes of windows.
Splitting Windows	Splitting one window into two windows.
Deleting Windows	Deleting a window gives its space to other windows.
Recombining Windows	Preserving the frame layout when splitting and deleting windows.
Selecting Windows	The selected window is the one that you edit in.
Cyclic Window Ordering	Moving around the existing windows.
Buffers and Windows	Each window displays the contents of a buffer.
Switching Buffers	Higher-level functions for switching to a buffer.
Choosing Window	How to choose a window for displaying a buffer.
Display Action Functions	Subroutines for display-buffer.
Choosing Window Options	Extra options affecting how buffers are displayed.
Window History	Each window remembers the buffers displayed in it.
Dedicated Windows	How to avoid displaying another buffer in a specific window.
Quitting Windows	How to restore the state prior to displaying a buffer.
Window Point	Each window has its own location of point.
Window Start and End	Buffer positions indicating which text is on-screen in a window.
Textual Scrolling	Moving text up and down through the window.
Vertical Scrolling	Moving the contents up and down on the window.
Horizontal Scrolling	Moving the contents sideways on the window.
Coordinates and Windows	Converting coordinates to windows.
Window Configurations	Saving and restoring the state of the screen.
Window Parameters	Associating additional information with windows.
Window Hooks	Hooks for scrolling, window size changes, redisplay going past a certain point, or window configuration changes.
Frames
Creating Frames	Creating additional frames.
Multiple Terminals	Displaying on several different devices.
Frame Parameters	Controlling frame size, position, font, etc.
Terminal Parameters	Parameters common for all frames on terminal.
Frame Titles	Automatic updating of frame titles.
Deleting Frames	Frames last until explicitly deleted.
Finding All Frames	How to examine all existing frames.
Minibuffers and Frames	How a frame finds the minibuffer to use.
Input Focus	Specifying the selected frame.
Visibility of Frames	Frames may be visible or invisible, or icons.
Raising and Lowering	Raising a frame makes it hide other windows; lowering it makes the others hide it.
Frame Configurations	Saving the state of all frames.
Mouse Tracking	Getting events that say when the mouse moves.
Mouse Position	Asking where the mouse is, or moving it.
Pop-Up Menus	Displaying a menu for the user to select from.
Dialog Boxes	Displaying a box to ask yes or no.
Pointer Shape	Specifying the shape of the mouse pointer.
Window System Selections	Transferring text to and from other X clients.
Drag and Drop	Internals of Drag-and-Drop implementation.
Color Names	Getting the definitions of color names.
Text Terminal Colors	Defining colors for text terminals.
Resources	Getting resource values from the server.
Display Feature Testing	Determining the features of a terminal.
Frame Parameters
Parameter Access	How to change a frame's parameters.
Initial Parameters	Specifying frame parameters when you make a frame.
Window Frame Parameters	List of frame parameters for window systems.
Size and Position	Changing the size and position of a frame.
Geometry	Parsing geometry specifications.
Window Frame Parameters
Basic Parameters	Parameters that are fundamental.
Position Parameters	The position of the frame on the screen.
Size Parameters	Frame's size.
Layout Parameters	Size of parts of the frame, and enabling or disabling some parts.
Buffer Parameters	Which buffers have been or should be shown.
Management Parameters	Communicating with the window manager.
Cursor Parameters	Controlling the cursor appearance.
Font and Color Parameters	Fonts and colors for the frame text.
Positions
Point	The special position where editing takes place.
Motion	Changing point.
Excursions	Temporary motion and buffer changes.
Narrowing	Restricting editing to a portion of the buffer.
Motion
Character Motion	Moving in terms of characters.
Word Motion	Moving in terms of words.
Buffer End Motion	Moving to the beginning or end of the buffer.
Text Lines	Moving in terms of lines of text.
Screen Lines	Moving in terms of lines as displayed.
List Motion	Moving by parsing lists and sexps.
Skipping Characters	Skipping characters belonging to a certain set.
Markers
Overview of Markers	The components of a marker, and how it relocates.
Predicates on Markers	Testing whether an object is a marker.
Creating Markers	Making empty markers or markers at certain places.
Information from Markers	Finding the marker's buffer or character position.
Marker Insertion Types	Two ways a marker can relocate when you insert where it points.
Moving Markers	Moving the marker to a new buffer or position.
The Mark	How "the mark" is implemented with a marker.
The Region	How to access "the region".
Text
Near Point	Examining text in the vicinity of point.
Buffer Contents	Examining text in a general fashion.
Comparing Text	Comparing substrings of buffers.
Insertion	Adding new text to a buffer.
Commands for Insertion	User-level commands to insert text.
Deletion	Removing text from a buffer.
User-Level Deletion	User-level commands to delete text.
The Kill Ring	Where removed text sometimes is saved for later use.
Undo	Undoing changes to the text of a buffer.
Maintaining Undo	How to enable and disable undo information. How to control how much information is kept.
Filling	Functions for explicit filling.
Margins	How to specify margins for filling commands.
Adaptive Fill	Adaptive Fill mode chooses a fill prefix from context.
Auto Filling	How auto-fill mode is implemented to break lines.
Sorting	Functions for sorting parts of the buffer.
Columns	Computing horizontal positions, and using them.
Indentation	Functions to insert or adjust indentation.
Case Changes	Case conversion of parts of the buffer.
Text Properties	Assigning Lisp property lists to text characters.
Substitution	Replacing a given character wherever it appears.
Registers	How registers are implemented. Accessing the text or position stored in a register.
Transposition	Swapping two portions of a buffer.
Base 64	Conversion to or from base 64 encoding.
Checksum/Hash	Computing cryptographic hashes.
Parsing HTML/XML	Parsing HTML and XML.
Atomic Changes	Installing several buffer changes "atomically".
Change Hooks	Supplying functions to be run when text is changed.
The Kill Ring
Kill Ring Concepts	What text looks like in the kill ring.
Kill Functions	Functions that kill text.
Yanking	How yanking is done.
Yank Commands	Commands that access the kill ring.
Low-Level Kill Ring	Functions and variables for kill ring access.
Internals of Kill Ring	Variables that hold kill ring data.
Indentation
Primitive Indent	Functions used to count and insert indentation.
Mode-Specific Indent	Customize indentation for different modes.
Region Indent	Indent all the lines in a region.
Relative Indent	Indent the current line based on previous lines.
Indent Tabs	Adjustable, typewriter-like tab stops.
Motion by Indent	Move to first non-blank character.
Text Properties
Examining Properties	Looking at the properties of one character.
Changing Properties	Setting the properties of a range of text.
Property Search	Searching for where a property changes value.
Special Properties	Particular properties with special meanings.
Format Properties	Properties for representing formatting of text.
Sticky Properties	How inserted text gets properties from neighboring text.
Lazy Properties	Computing text properties in a lazy fashion only when text is examined.
Clickable Text	Using text properties to make regions of text do something when you click on them.
Fields	The field property defines fields within the buffer.
Not Intervals	Why text properties do not use Lisp-visible text intervals.
Non-ASCII Characters
Text Representations	How Emacs represents text.
Converting Representations	Converting unibyte to multibyte and vice versa.
Selecting a Representation	Treating a byte sequence as unibyte or multi.
Character Codes	How unibyte and multibyte relate to codes of individual characters.
Character Properties	Character attributes that define their behavior and handling.
Character Sets	The space of possible character codes is divided into various character sets.
Scanning Charsets	Which character sets are used in a buffer?
Translation of Characters	Translation tables are used for conversion.
Coding Systems	Coding systems are conversions for saving files.
Input Methods	Input methods allow users to enter various non-ASCII characters without special keyboards.
Locales	Interacting with the POSIX locale.
Coding Systems
Coding System Basics	Basic concepts.
Encoding and I/O	How file I/O functions handle coding systems.
Lisp and Coding Systems	Functions to operate on coding system names.
User-Chosen Coding Systems	Asking the user to choose a coding system.
Default Coding Systems	Controlling the default choices.
Specifying Coding Systems	Requesting a particular coding system for a single file operation.
Explicit Encoding	Encoding or decoding text without doing I/O.
Terminal I/O Encoding	Use of encoding for terminal I/O.
MS-DOS File Types	How DOS "text" and "binary" files relate to coding systems.
Searching and Matching
String Search	Search for an exact match.
Searching and Case	Case-independent or case-significant searching.
Regular Expressions	Describing classes of strings.
Regexp Search	Searching for a match for a regexp.
POSIX Regexps	Searching POSIX-style for the longest match.
Match Data	Finding out which part of the text matched, after a string or regexp search.
Search and Replace	Commands that loop, searching and replacing.
Standard Regexps	Useful regexps for finding sentences, pages,...
Regular Expressions
Syntax of Regexps	Rules for writing regular expressions.
Regexp Example	Illustrates regular expression syntax.
Regexp Functions	Functions for operating on regular expressions.
Syntax of Regular Expressions
Regexp Special	Special characters in regular expressions.
Char Classes	Character classes used in regular expressions.
Regexp Backslash	Backslash-sequences in regular expressions.
The Match Data
Replacing Match	Replacing a substring that was matched.
Simple Match Data	Accessing single items of match data, such as where a particular subexpression started.
Entire Match Data	Accessing the entire match data at once, as a list.
Saving Match Data	Saving and restoring the match data.
Syntax Tables
Syntax Basics	Basic concepts of syntax tables.
Syntax Descriptors	How characters are classified.
Syntax Table Functions	How to create, examine and alter syntax tables.
Syntax Properties	Overriding syntax with text properties.
Motion and Syntax	Moving over characters with certain syntaxes.
Parsing Expressions	Parsing balanced expressions using the syntax table.
Syntax Table Internals	How syntax table information is stored.
Categories	Another way of classifying character syntax.
Syntax Descriptors
Syntax Class Table	Table of syntax classes.
Syntax Flags	Additional flags each character can have.
Parsing Expressions
Motion via Parsing	Motion functions that work by parsing.
Position Parse	Determining the syntactic state of a position.
Parser State	How Emacs represents a syntactic state.
Low-Level Parsing	Parsing across a specified region.
Control Parsing	Parameters that affect parsing.
Abbrevs and Abbrev Expansion
Abbrev Tables	Creating and working with abbrev tables.
Defining Abbrevs	Specifying abbreviations and their expansions.
Abbrev Files	Saving abbrevs in files.
Abbrev Expansion	Controlling expansion; expansion subroutines.
Standard Abbrev Tables	Abbrev tables used by various major modes.
Abbrev Properties	How to read and set abbrev properties. Which properties have which effect.
Abbrev Table Properties	How to read and set abbrev table properties. Which properties have which effect.
Processes
Subprocess Creation	Functions that start subprocesses.
Shell Arguments	Quoting an argument to pass it to a shell.
Synchronous Processes	Details of using synchronous subprocesses.
Asynchronous Processes	Starting up an asynchronous subprocess.
Deleting Processes	Eliminating an asynchronous subprocess.
Process Information	Accessing run-status and other attributes.
Input to Processes	Sending input to an asynchronous subprocess.
Signals to Processes	Stopping, continuing or interrupting an asynchronous subprocess.
Output from Processes	Collecting output from an asynchronous subprocess.
Sentinels	Sentinels run when process run-status changes.
Query Before Exit	Whether to query if exiting will kill a process.
System Processes	Accessing other processes running on your system.
Transaction Queues	Transaction-based communication with subprocesses.
Network	Opening network connections.
Network Servers	Network servers let Emacs accept net connections.
Datagrams	UDP network connections.
Low-Level Network	Lower-level but more general function to create connections and servers.
Misc Network	Additional relevant functions for net connections.
Serial Ports	Communicating with serial ports.
Byte Packing	Using bindat to pack and unpack binary data.
Receiving Output from Processes
Process Buffers	If no filter, output is put in a buffer.
Filter Functions	Filter functions accept output from the process.
Decoding Output	Filters can get unibyte or multibyte strings.
Accepting Output	How to wait until process output arrives.
Low-Level Network Access
Network Processes	Using make-network-process.
Network Options	Further control over network connections.
Network Feature Testing	Determining which network features work on the machine you are using.
Packing and Unpacking Byte Arrays
Bindat Spec	Describing data layout.
Bindat Functions	Doing the unpacking and packing.
Bindat Examples	Samples of what bindat.el can do for you!
Emacs Display
Refresh Screen	Clearing the screen and redrawing everything on it.
Forcing Redisplay	Forcing redisplay.
Truncation	Folding or wrapping long text lines.
The Echo Area	Displaying messages at the bottom of the screen.
Warnings	Displaying warning messages for the user.
Invisible Text	Hiding part of the buffer text.
Selective Display	Hiding part of the buffer text (the old way).
Temporary Displays	Displays that go away automatically.
Overlays	Use overlays to highlight parts of the buffer.
Width	How wide a character or string is on the screen.
Line Height	Controlling the height of lines.
Faces	A face defines a graphics style for text characters: font, colors, etc.
Fringes	Controlling window fringes.
Scroll Bars	Controlling vertical scroll bars.
Display Property	Enabling special display features.
Images	Displaying images in Emacs buffers.
Buttons	Adding clickable buttons to Emacs buffers.
Abstract Display	Emacs's Widget for Object Collections.
Blinking	How Emacs shows the matching open parenthesis.
Character Display	How Emacs displays individual characters.
Beeping	Audible signal to the user.
Window Systems	Which window system is being used.
Bidirectional Display	Display of bidirectional scripts, such as Arabic and Farsi.
The Echo Area
Displaying Messages	Explicitly displaying text in the echo area.
Progress	Informing user about progress of a long operation.
Logging Messages	Echo area messages are logged for the user.
Echo Area Customization	Controlling the echo area.
Reporting Warnings
Warning Basics	Warnings concepts and functions to report them.
Warning Variables	Variables programs bind to customize their warnings.
Warning Options	Variables users set to control display of warnings.
Delayed Warnings	Deferring a warning until the end of a command.
Overlays
Managing Overlays	Creating and moving overlays.
Overlay Properties	How to read and set properties. What properties do to the screen display.
Finding Overlays	Searching for overlays.
Faces
Face Attributes	What is in a face?
Defining Faces	How to define a face.
Attribute Functions	Functions to examine and set face attributes.
Displaying Faces	How Emacs combines the faces specified for a character.
Face Remapping	Remapping faces to alternative definitions.
Face Functions	How to define and examine faces.
Auto Faces	Hook for automatic face assignment.
Basic Faces	Faces that are defined by default.
Font Selection	Finding the best available font for a face.
Font Lookup	Looking up the names of available fonts and information about them.
Fontsets	A fontset is a collection of fonts that handle a range of character sets.
Low-Level Font	Lisp representation for character display fonts.
Fringes
Fringe Size/Pos	Specifying where to put the window fringes.
Fringe Indicators	Displaying indicator icons in the window fringes.
Fringe Cursors	Displaying cursors in the right fringe.
Fringe Bitmaps	Specifying bitmaps for fringe indicators.
Customizing Bitmaps	Specifying your own bitmaps to use in the fringes.
Overlay Arrow	Display of an arrow to indicate position.
The display Property
Replacing Specs	Display specs that replace the text.
Specified Space	Displaying one space with a specified width.
Pixel Specification	Specifying space width or height in pixels.
Other Display Specs	Displaying an image; adjusting the height, spacing, and other properties of text.
Display Margins	Displaying text or images to the side of the main text.
Images
Image Formats	Supported image formats.
Image Descriptors	How to specify an image for use in :display.
XBM Images	Special features for XBM format.
XPM Images	Special features for XPM format.
GIF Images	Special features for GIF format.
TIFF Images	Special features for TIFF format.
PostScript Images	Special features for PostScript format.
ImageMagick Images	Special features available through ImageMagick.
Other Image Types	Various other formats are supported.
Defining Images	Convenient ways to define an image for later use.
Showing Images	Convenient ways to display an image once it is defined.
Animated Images	Some image formats can be animated.
Image Cache	Internal mechanisms of image display.
Buttons
Button Properties	Button properties with special meanings.
Button Types	Defining common properties for classes of buttons.
Making Buttons	Adding buttons to Emacs buffers.
Manipulating Buttons	Getting and setting properties of buttons.
Button Buffer Commands	Buffer-wide commands and bindings for buttons.
Abstract Display
Abstract Display Functions	Functions in the Ewoc package.
Abstract Display Example	Example of using Ewoc.
Character Display
Usual Display	The usual conventions for displaying characters.
Display Tables	What a display table consists of.
Active Display Table	How Emacs selects a display table to use.
Glyphs	How to define a glyph, and what glyphs mean.
Glyphless Chars	How glyphless characters are drawn.
Operating System Interface
Starting Up	Customizing Emacs startup processing.
Getting Out	How exiting works (permanent or temporary).
System Environment	Distinguish the name and kind of system.
User Identification	Finding the name and user id of the user.
Time of Day	Getting the current time.
Time Conversion	Converting a time from numeric form to calendrical data and vice versa.
Time Parsing	Converting a time from numeric form to text and vice versa.
Processor Run Time	Getting the run time used by Emacs.
Time Calculations	Adding, subtracting, comparing times, etc.
Timers	Setting a timer to call a function at a certain time.
Idle Timers	Setting a timer to call a function when Emacs has been idle for a certain length of time.
Terminal Input	Accessing and recording terminal input.
Terminal Output	Controlling and recording terminal output.
Sound Output	Playing sounds on the computer's speaker.
X11 Keysyms	Operating on key symbols for X Windows.
Batch Mode	Running Emacs without terminal interaction.
Session Management	Saving and restoring state with X Session Management.
Notifications	Desktop notifications.
Dynamic Libraries	On-demand loading of support libraries.
Starting Up Emacs
Startup Summary	Sequence of actions Emacs performs at startup.
Init File	Details on reading the init file.
Terminal-Specific	How the terminal-specific Lisp file is read.
Command-Line Arguments	How command-line arguments are processed, and how you can customize them.
Getting Out of Emacs
Killing Emacs	Exiting Emacs irreversibly.
Suspending Emacs	Exiting Emacs reversibly.
Terminal Input
Input Modes	Options for how input is processed.
Recording Input	Saving histories of recent or all input events.
Preparing Lisp code for distribution
Packaging Basics	The basic concepts of Emacs Lisp packages.
Simple Packages	How to package a single .el file.
Multi-file Packages	How to package multiple files.
Package Archives	Maintaining package archives.
Tips and Conventions
Coding Conventions	Conventions for clean and robust programs.
Key Binding Conventions	Which keys should be bound by which programs.
Programming Tips	Making Emacs code fit smoothly in Emacs.
Compilation Tips	Making compiled code run fast.
Warning Tips	Turning off compiler warnings.
Documentation Tips	Writing readable documentation strings.
Comment Tips	Conventions for writing comments.
Library Headers	Standard headers for library packages.
GNU Emacs Internals
Building Emacs	How the dumped Emacs is made.
Pure Storage	Kludge to make preloaded Lisp functions shareable.
Garbage Collection	Reclaiming space for Lisp objects no longer used.
Memory Usage	Info about total size of Lisp objects made so far.
Writing Emacs Primitives	Writing C code for Emacs.
Object Internals	Data formats of buffers, windows, processes.
Object Internals
Buffer Internals	Components of a buffer structure.
Window Internals	Components of a window structure.
Process Internals	Components of a process structure.

1 Introduction

Most of the GNU Emacs text editor is written in the programming language called Emacs Lisp. You can write new code in Emacs Lisp and install it as an extension to the editor. However, Emacs Lisp is more than a mere “extension language”; it is a full computer programming language in its own right. You can use it as you would any other programming language.

Because Emacs Lisp is designed for use in an editor, it has special features for scanning and parsing text as well as features for handling files, buffers, displays, subprocesses, and so on. Emacs Lisp is closely integrated with the editing facilities; thus, editing commands are functions that can also conveniently be called from Lisp programs, and parameters for customization are ordinary Lisp variables.

This manual attempts to be a full description of Emacs Lisp. For a beginner's introduction to Emacs Lisp, see An Introduction to Emacs Lisp Programming, by Bob Chassell, also published by the Free Software Foundation. This manual presumes considerable familiarity with the use of Emacs for editing; see The GNU Emacs Manual for this basic information.

Generally speaking, the earlier chapters describe features of Emacs Lisp that have counterparts in many programming languages, and later chapters describe features that are peculiar to Emacs Lisp or relate specifically to editing.

This is the GNU Emacs Lisp Reference Manual, corresponding to Emacs version 24.3.

• Caveats: Flaws and a request for help.
• Lisp History: Emacs Lisp is descended from Maclisp.
• Conventions: How the manual is formatted.
• Version Info: Which Emacs version is running?
• Acknowledgments: The authors, editors, and sponsors of this manual.

1.1 Caveats

This manual has gone through numerous drafts. It is nearly complete but not flawless. There are a few topics that are not covered, either because we consider them secondary (such as most of the individual modes) or because they are yet to be written. Because we are not able to deal with them completely, we have left out several parts intentionally.

The manual should be fully correct in what it does cover, and it is therefore open to criticism on anything it says—from specific examples and descriptive text, to the ordering of chapters and sections. If something is confusing, or you find that you have to look at the sources or experiment to learn something not covered in the manual, then perhaps the manual should be fixed. Please let us know.

As you use this manual, we ask that you send corrections as soon as you find them. If you think of a simple, real life example for a function or group of functions, please make an effort to write it up and send it in. Please reference any comments to the node name and function or variable name, as appropriate. Also state the number of the edition you are criticizing.

Please send comments and corrections using M-x report-emacs-bug.

1.2 Lisp History

Lisp (LISt Processing language) was first developed in the late 1950s at the Massachusetts Institute of Technology for research in artificial intelligence. The great power of the Lisp language makes it ideal for other purposes as well, such as writing editing commands.

Dozens of Lisp implementations have been built over the years, each with its own idiosyncrasies. Many of them were inspired by Maclisp, which was written in the 1960s at MIT's Project MAC. Eventually the implementers of the descendants of Maclisp came together and developed a standard for Lisp systems, called Common Lisp. In the meantime, Gerry Sussman and Guy Steele at MIT developed a simplified but very powerful dialect of Lisp, called Scheme.

GNU Emacs Lisp is largely inspired by Maclisp, and a little by Common Lisp. If you know Common Lisp, you will notice many similarities. However, many features of Common Lisp have been omitted or simplified in order to reduce the memory requirements of GNU Emacs. Sometimes the simplifications are so drastic that a Common Lisp user might be very confused. We will occasionally point out how GNU Emacs Lisp differs from Common Lisp. If you don't know Common Lisp, don't worry about it; this manual is self-contained.

A certain amount of Common Lisp emulation is available via the cl-lib library. See Overview.

Emacs Lisp is not at all influenced by Scheme; but the GNU project has an implementation of Scheme, called Guile. We use it in all new GNU software that calls for extensibility.

1.3 Conventions

This section explains the notational conventions that are used in this manual. You may want to skip this section and refer back to it later.

• Some Terms: Explanation of terms we use in this manual.
• nil and t: How the symbols nil and t are used.
• Evaluation Notation: The format we use for examples of evaluation.
• Printing Notation: The format we use when examples print text.
• Error Messages: The format we use for examples of errors.
• Buffer Text Notation: The format we use for buffer contents in examples.
• Format of Descriptions: Notation for describing functions, variables, etc.

1.3.1 Some Terms

Throughout this manual, the phrases “the Lisp reader” and “the Lisp printer” refer to those routines in Lisp that convert textual representations of Lisp objects into actual Lisp objects, and vice versa. See Printed Representation, for more details. You, the person reading this manual, are thought of as “the programmer” and are addressed as “you”. “The user” is the person who uses Lisp programs, including those you write.

Examples of Lisp code are formatted like this: (list 1 2 3). Names that represent metasyntactic variables, or arguments to a function being described, are formatted like this: first-number.

1.3.2 nil and t

In Emacs Lisp, the symbol nil has three separate meanings: it is a symbol with the name ‘nil’; it is the logical truth value false; and it is the empty list—the list of zero elements. When used as a variable, nil always has the value nil.

As far as the Lisp reader is concerned, ‘()’ and ‘nil’ are identical: they stand for the same object, the symbol nil. The different ways of writing the symbol are intended entirely for human readers. After the Lisp reader has read either ‘()’ or ‘nil’, there is no way to determine which representation was actually written by the programmer.

In this manual, we write () when we wish to emphasize that it means the empty list, and we write nil when we wish to emphasize that it means the truth value false. That is a good convention to use in Lisp programs also.

     (cons 'foo ())                ; Emphasize the empty list
     (setq foo-flag nil)           ; Emphasize the truth value false

In contexts where a truth value is expected, any non-nil value is considered to be true. However, t is the preferred way to represent the truth value true. When you need to choose a value which represents true, and there is no other basis for choosing, use t. The symbol t always has the value t.

In Emacs Lisp, nil and t are special symbols that always evaluate to themselves. This is so that you do not need to quote them to use them as constants in a program. An attempt to change their values results in a setting-constant error. See Constant Variables.

1.3.3 Evaluation Notation

A Lisp expression that you can evaluate is called a form. Evaluating a form always produces a result, which is a Lisp object. In the examples in this manual, this is indicated with ‘⇒’:

     (car '(1 2))
          ⇒ 1

You can read this as “(car '(1 2)) evaluates to 1”.

When a form is a macro call, it expands into a new form for Lisp to evaluate. We show the result of the expansion with ‘==>’. We may or may not show the result of the evaluation of the expanded form.

     (third '(a b c))
          ==> (car (cdr (cdr '(a b c))))
          ⇒ c

To help describe one form, we sometimes show another form that produces identical results. The exact equivalence of two forms is indicated with ‘==’.

     (make-sparse-keymap) == (list 'keymap)

1.3.4 Printing Notation

Many of the examples in this manual print text when they are evaluated. If you execute example code in a Lisp Interaction buffer (such as the buffer *scratch*), the printed text is inserted into the buffer. If you execute the example by other means (such as by evaluating the function eval-region), the printed text is displayed in the echo area.

Examples in this manual indicate printed text with ‘-|’, irrespective of where that text goes. The value returned by evaluating the form follows on a separate line with ‘⇒’.

     (progn (prin1 'foo) (princ "\n") (prin1 'bar))
          -| foo
          -| bar
          ⇒ bar

1.3.5 Error Messages

Some examples signal errors. This normally displays an error message in the echo area. We show the error message on a line starting with ‘error-->’. Note that ‘error-->’ itself does not appear in the echo area.

     (+ 23 'x)
     error--> Wrong type argument: number-or-marker-p, x

1.3.6 Buffer Text Notation

Some examples describe modifications to the contents of a buffer, by showing the “before” and “after” versions of the text. These examples show the contents of the buffer in question between two lines of dashes containing the buffer name. In addition, ‘-!-’ indicates the location of point. (The symbol for point, of course, is not part of the text in the buffer; it indicates the place between two characters where point is currently located.)

     ---------- Buffer: foo ----------
     This is the -!-contents of foo.
     ---------- Buffer: foo ----------
     
     (insert "changed ")
          ⇒ nil
     ---------- Buffer: foo ----------
     This is the changed -!-contents of foo.
     ---------- Buffer: foo ----------

1.3.7 Format of Descriptions

Functions, variables, macros, commands, user options, and special forms are described in this manual in a uniform format. The first line of a description contains the name of the item followed by its arguments, if any. The category—function, variable, or whatever—appears at the beginning of the line. The description follows on succeeding lines, sometimes with examples.

• A Sample Function Description: A description of an imaginary function, foo.
• A Sample Variable Description: A description of an imaginary variable, electric-future-map.

1.3.7.1 A Sample Function Description

In a function description, the name of the function being described appears first. It is followed on the same line by a list of argument names. These names are also used in the body of the description, to stand for the values of the arguments.

The appearance of the keyword &optional in the argument list indicates that the subsequent arguments may be omitted (omitted arguments default to nil). Do not write &optional when you call the function.

The keyword &rest (which must be followed by a single argument name) indicates that any number of arguments can follow. The single argument name following &rest receives, as its value, a list of all the remaining arguments passed to the function. Do not write &rest when you call the function.

Here is a description of an imaginary function foo:

By convention, any argument whose name contains the name of a type (e.g., integer, integer1 or buffer) is expected to be of that type. A plural of a type (such as buffers) often means a list of objects of that type. An argument named object may be of any type. (For a list of Emacs object types, see Lisp Data Types.) An argument with any other sort of name (e.g., new-file) is specific to the function; if the function has a documentation string, the type of the argument should be described there (see Documentation).

See Lambda Expressions, for a more complete description of arguments modified by &optional and &rest.

Command, macro, and special form descriptions have the same format, but the word ‘Function’ is replaced by ‘Command’, ‘Macro’, or ‘Special Form’, respectively. Commands are simply functions that may be called interactively; macros process their arguments differently from functions (the arguments are not evaluated), but are presented the same way.

The descriptions of macros and special forms use a more complex notation to specify optional and repeated arguments, because they can break the argument list down into separate arguments in more complicated ways. ‘[optional-arg]’ means that optional-arg is optional and ‘repeated-args...’ stands for zero or more arguments. Parentheses are used when several arguments are grouped into additional levels of list structure. Here is an example:

1.3.7.2 A Sample Variable Description

A variable is a name that can be bound (or set) to an object. The object to which a variable is bound is called a value; we say also that variable holds that value. Although nearly all variables can be set by the user, certain variables exist specifically so that users can change them; these are called user options. Ordinary variables and user options are described using a format like that for functions, except that there are no arguments.

Here is a description of the imaginary electric-future-map variable.

User option descriptions have the same format, but ‘Variable’ is replaced by ‘User Option’.

1.4 Version Information

These facilities provide information about which version of Emacs is in use.

1.5 Acknowledgments

This manual was originally written by Robert Krawitz, Bil Lewis, Dan LaLiberte, Richard M. Stallman and Chris Welty, the volunteers of the GNU manual group, in an effort extending over several years. Robert J. Chassell helped to review and edit the manual, with the support of the Defense Advanced Research Projects Agency, ARPA Order 6082, arranged by Warren A. Hunt, Jr. of Computational Logic, Inc. Additional sections have since been written by Miles Bader, Lars Brinkhoff, Chong Yidong, Kenichi Handa, Lute Kamstra, Juri Linkov, Glenn Morris, Thien-Thi Nguyen, Dan Nicolaescu, Martin Rudalics, Kim F. Storm, Luc Teirlinck, and Eli Zaretskii, and others.

Corrections were supplied by Drew Adams, Juanma Barranquero, Karl Berry, Jim Blandy, Bard Bloom, Stephane Boucher, David Boyes, Alan Carroll, Richard Davis, Lawrence R. Dodd, Peter Doornbosch, David A. Duff, Chris Eich, Beverly Erlebacher, David Eckelkamp, Ralf Fassel, Eirik Fuller, Stephen Gildea, Bob Glickstein, Eric Hanchrow, Jesper Harder, George Hartzell, Nathan Hess, Masayuki Ida, Dan Jacobson, Jak Kirman, Bob Knighten, Frederick M. Korz, Joe Lammens, Glenn M. Lewis, K. Richard Magill, Brian Marick, Roland McGrath, Stefan Monnier, Skip Montanaro, John Gardiner Myers, Thomas A. Peterson, Francesco Potorti, Friedrich Pukelsheim, Arnold D. Robbins, Raul Rockwell, Jason Rumney, Per Starbäck, Shinichirou Sugou, Kimmo Suominen, Edward Tharp, Bill Trost, Rickard Westman, Jean White, Eduard Wiebe, Matthew Wilding, Carl Witty, Dale Worley, Rusty Wright, and David D. Zuhn.

For a more complete list of contributors, please see the relevant ChangeLog file in the Emacs sources.

2 Lisp Data Types

A Lisp object is a piece of data used and manipulated by Lisp programs. For our purposes, a type or data type is a set of possible objects.

Every object belongs to at least one type. Objects of the same type have similar structures and may usually be used in the same contexts. Types can overlap, and objects can belong to two or more types. Consequently, we can ask whether an object belongs to a particular type, but not for “the” type of an object.

A few fundamental object types are built into Emacs. These, from which all other types are constructed, are called primitive types. Each object belongs to one and only one primitive type. These types include integer, float, cons, symbol, string, vector, hash-table, subr, and byte-code function, plus several special types, such as buffer, that are related to editing. (See Editing Types.)

Each primitive type has a corresponding Lisp function that checks whether an object is a member of that type.

Lisp is unlike many other languages in that its objects are self-typing: the primitive type of each object is implicit in the object itself. For example, if an object is a vector, nothing can treat it as a number; Lisp knows it is a vector, not a number.

In most languages, the programmer must declare the data type of each variable, and the type is known by the compiler but not represented in the data. Such type declarations do not exist in Emacs Lisp. A Lisp variable can have any type of value, and it remembers whatever value you store in it, type and all. (Actually, a small number of Emacs Lisp variables can only take on values of a certain type. See Variables with Restricted Values.)

This chapter describes the purpose, printed representation, and read syntax of each of the standard types in GNU Emacs Lisp. Details on how to use these types can be found in later chapters.

• Printed Representation: How Lisp objects are represented as text.
• Comments: Comments and their formatting conventions.
• Programming Types: Types found in all Lisp systems.
• Editing Types: Types specific to Emacs.
• Circular Objects: Read syntax for circular structure.
• Type Predicates: Tests related to types.
• Equality Predicates: Tests of equality between any two objects.

2.1 Printed Representation and Read Syntax

The printed representation of an object is the format of the output generated by the Lisp printer (the function prin1) for that object. Every data type has a unique printed representation. The read syntax of an object is the format of the input accepted by the Lisp reader (the function read) for that object. This is not necessarily unique; many kinds of object have more than one syntax. See Read and Print.

In most cases, an object's printed representation is also a read syntax for the object. However, some types have no read syntax, since it does not make sense to enter objects of these types as constants in a Lisp program. These objects are printed in hash notation, which consists of the characters ‘#<’, a descriptive string (typically the type name followed by the name of the object), and a closing ‘>’. For example:

     (current-buffer)
          ⇒ #<buffer objects.texi>

Hash notation cannot be read at all, so the Lisp reader signals the error invalid-read-syntax whenever it encounters ‘#<’. In other languages, an expression is text; it has no other form. In Lisp, an expression is primarily a Lisp object and only secondarily the text that is the object's read syntax. Often there is no need to emphasize this distinction, but you must keep it in the back of your mind, or you will occasionally be very confused.

When you evaluate an expression interactively, the Lisp interpreter first reads the textual representation of it, producing a Lisp object, and then evaluates that object (see Evaluation). However, evaluation and reading are separate activities. Reading returns the Lisp object represented by the text that is read; the object may or may not be evaluated later. See Input Functions, for a description of read, the basic function for reading objects.

2.2 Comments

A comment is text that is written in a program only for the sake of humans that read the program, and that has no effect on the meaning of the program. In Lisp, a semicolon (‘;’) starts a comment if it is not within a string or character constant. The comment continues to the end of line. The Lisp reader discards comments; they do not become part of the Lisp objects which represent the program within the Lisp system.

The ‘#@count’ construct, which skips the next count characters, is useful for program-generated comments containing binary data. The Emacs Lisp byte compiler uses this in its output files (see Byte Compilation). It isn't meant for source files, however.

See Comment Tips, for conventions for formatting comments.

2.3 Programming Types

There are two general categories of types in Emacs Lisp: those having to do with Lisp programming, and those having to do with editing. The former exist in many Lisp implementations, in one form or another. The latter are unique to Emacs Lisp.

• Integer Type: Numbers without fractional parts.
• Floating Point Type: Numbers with fractional parts and with a large range.
• Character Type: The representation of letters, numbers and control characters.
• Symbol Type: A multi-use object that refers to a function, variable, or property list, and has a unique identity.
• Sequence Type: Both lists and arrays are classified as sequences.
• Cons Cell Type: Cons cells, and lists (which are made from cons cells).
• Array Type: Arrays include strings and vectors.
• String Type: An (efficient) array of characters.
• Vector Type: One-dimensional arrays.
• Char-Table Type: One-dimensional sparse arrays indexed by characters.
• Bool-Vector Type: One-dimensional arrays of t or nil.
• Hash Table Type: Super-fast lookup tables.
• Function Type: A piece of executable code you can call from elsewhere.
• Macro Type: A method of expanding an expression into another expression, more fundamental but less pretty.
• Primitive Function Type: A function written in C, callable from Lisp.
• Byte-Code Type: A function written in Lisp, then compiled.
• Autoload Type: A type used for automatically loading seldom-used functions.

2.3.1 Integer Type

The range of values for integers in Emacs Lisp is −536870912 to 536870911 (30 bits; i.e., -2**29 to 2**29 − 1) on typical 32-bit machines. (Some machines provide a wider range.) Emacs Lisp arithmetic functions do not check for overflow. Thus (1+ 536870911) is −536870912 if Emacs integers are 30 bits.

The read syntax for integers is a sequence of (base ten) digits with an optional sign at the beginning and an optional period at the end. The printed representation produced by the Lisp interpreter never has a leading ‘+’ or a final ‘.’.

     -1               ; The integer -1.
     1                ; The integer 1.
     1.               ; Also the integer 1.
     +1               ; Also the integer 1.

As a special exception, if a sequence of digits specifies an integer too large or too small to be a valid integer object, the Lisp reader reads it as a floating-point number (see Floating Point Type). For instance, if Emacs integers are 30 bits, 536870912 is read as the floating-point number 536870912.0.

See Numbers, for more information.

2.3.2 Floating Point Type

Floating point numbers are the computer equivalent of scientific notation; you can think of a floating point number as a fraction together with a power of ten. The precise number of significant figures and the range of possible exponents is machine-specific; Emacs uses the C data type double to store the value, and internally this records a power of 2 rather than a power of 10.

The printed representation for floating point numbers requires either a decimal point (with at least one digit following), an exponent, or both. For example, ‘1500.0’, ‘15e2’, ‘15.0e2’, ‘1.5e3’, and ‘.15e4’ are five ways of writing a floating point number whose value is 1500. They are all equivalent.

See Numbers, for more information.

2.3.3 Character Type

A character in Emacs Lisp is nothing more than an integer. In other words, characters are represented by their character codes. For example, the character A is represented as the integer 65.

Individual characters are used occasionally in programs, but it is more common to work with strings, which are sequences composed of characters. See String Type.

Characters in strings and buffers are currently limited to the range of 0 to 4194303—twenty two bits (see Character Codes). Codes 0 through 127 are ASCII codes; the rest are non-ASCII (see Non-ASCII Characters). Characters that represent keyboard input have a much wider range, to encode modifier keys such as Control, Meta and Shift.

There are special functions for producing a human-readable textual description of a character for the sake of messages. See Describing Characters.

• Basic Char Syntax: Syntax for regular characters.
• General Escape Syntax: How to specify characters by their codes.
• Ctl-Char Syntax: Syntax for control characters.
• Meta-Char Syntax: Syntax for meta-characters.
• Other Char Bits: Syntax for hyper-, super-, and alt-characters.

2.3.3.1 Basic Char Syntax

Since characters are really integers, the printed representation of a character is a decimal number. This is also a possible read syntax for a character, but writing characters that way in Lisp programs is not clear programming. You should always use the special read syntax formats that Emacs Lisp provides for characters. These syntax formats start with a question mark.

The usual read syntax for alphanumeric characters is a question mark followed by the character; thus, ‘?A’ for the character A, ‘?B’ for the character B, and ‘?a’ for the character a.

For example:

     ?Q ⇒ 81     ?q ⇒ 113

You can use the same syntax for punctuation characters, but it is often a good idea to add a ‘\’ so that the Emacs commands for editing Lisp code don't get confused. For example, ‘?\(’ is the way to write the open-paren character. If the character is ‘\’, you must use a second ‘\’ to quote it: ‘?\\’.

You can express the characters control-g, backspace, tab, newline, vertical tab, formfeed, space, return, del, and escape as ‘?\a’, ‘?\b’, ‘?\t’, ‘?\n’, ‘?\v’, ‘?\f’, ‘?\s’, ‘?\r’, ‘?\d’, and ‘?\e’, respectively. (‘?\s’ followed by a dash has a different meaning—it applies the “super” modifier to the following character.) Thus,

     ?\a ⇒ 7                 ; control-g, C-g
     ?\b ⇒ 8                 ; backspace, <BS>, C-h
     ?\t ⇒ 9                 ; tab, <TAB>, C-i
     ?\n ⇒ 10                ; newline, C-j
     ?\v ⇒ 11                ; vertical tab, C-k
     ?\f ⇒ 12                ; formfeed character, C-l
     ?\r ⇒ 13                ; carriage return, <RET>, C-m
     ?\e ⇒ 27                ; escape character, <ESC>, C-[
     ?\s ⇒ 32                ; space character, <SPC>
     ?\\ ⇒ 92                ; backslash character, \
     ?\d ⇒ 127               ; delete character, <DEL>

These sequences which start with backslash are also known as escape sequences, because backslash plays the role of an “escape character”; this terminology has nothing to do with the character <ESC>. ‘\s’ is meant for use in character constants; in string constants, just write the space.

A backslash is allowed, and harmless, preceding any character without a special escape meaning; thus, ‘?\+’ is equivalent to ‘?+’. There is no reason to add a backslash before most characters. However, you should add a backslash before any of the characters ‘()\|;'`"#.,’ to avoid confusing the Emacs commands for editing Lisp code. You can also add a backslash before whitespace characters such as space, tab, newline and formfeed. However, it is cleaner to use one of the easily readable escape sequences, such as ‘\t’ or ‘\s’, instead of an actual whitespace character such as a tab or a space. (If you do write backslash followed by a space, you should write an extra space after the character constant to separate it from the following text.)

2.3.3.2 General Escape Syntax

In addition to the specific escape sequences for special important control characters, Emacs provides several types of escape syntax that you can use to specify non-ASCII text characters.

Firstly, you can specify characters by their Unicode values. ?\unnnn represents a character with Unicode code point ‘U+nnnn’, where nnnn is (by convention) a hexadecimal number with exactly four digits. The backslash indicates that the subsequent characters form an escape sequence, and the ‘u’ specifies a Unicode escape sequence.

There is a slightly different syntax for specifying Unicode characters with code points higher than U+ffff: ?\U00nnnnnn represents the character with code point ‘U+nnnnnn’, where nnnnnn is a six-digit hexadecimal number. The Unicode Standard only defines code points up to ‘U+10ffff’, so if you specify a code point higher than that, Emacs signals an error.

Secondly, you can specify characters by their hexadecimal character codes. A hexadecimal escape sequence consists of a backslash, ‘x’, and the hexadecimal character code. Thus, ‘?\x41’ is the character A, ‘?\x1’ is the character C-a, and ?\xe0 is the character ‘a’ with grave accent. You can use any number of hex digits, so you can represent any character code in this way.

Thirdly, you can specify characters by their character code in octal. An octal escape sequence consists of a backslash followed by up to three octal digits; thus, ‘?\101’ for the character A, ‘?\001’ for the character C-a, and ?\002 for the character C-b. Only characters up to octal code 777 can be specified this way.

These escape sequences may also be used in strings. See Non-ASCII in Strings.

2.3.3.3 Control-Character Syntax

Control characters can be represented using yet another read syntax. This consists of a question mark followed by a backslash, caret, and the corresponding non-control character, in either upper or lower case. For example, both ‘?\^I’ and ‘?\^i’ are valid read syntax for the character C-i, the character whose value is 9.

Instead of the ‘^’, you can use ‘C-’; thus, ‘?\C-i’ is equivalent to ‘?\^I’ and to ‘?\^i’:

     ?\^I ⇒ 9     ?\C-I ⇒ 9

In strings and buffers, the only control characters allowed are those that exist in ASCII; but for keyboard input purposes, you can turn any character into a control character with ‘C-’. The character codes for these non-ASCII control characters include the 2**26 bit as well as the code for the corresponding non-control character. Ordinary text terminals have no way of generating non-ASCII control characters, but you can generate them straightforwardly using X and other window systems.

For historical reasons, Emacs treats the <DEL> character as the control equivalent of ?:

     ?\^? ⇒ 127     ?\C-? ⇒ 127

As a result, it is currently not possible to represent the character Control-?, which is a meaningful input character under X, using ‘\C-’. It is not easy to change this, as various Lisp files refer to <DEL> in this way.

For representing control characters to be found in files or strings, we recommend the ‘^’ syntax; for control characters in keyboard input, we prefer the ‘C-’ syntax. Which one you use does not affect the meaning of the program, but may guide the understanding of people who read it.

2.3.3.4 Meta-Character Syntax

A meta character is a character typed with the <META> modifier key. The integer that represents such a character has the 2**27 bit set. We use high bits for this and other modifiers to make possible a wide range of basic character codes.

In a string, the 2**7 bit attached to an ASCII character indicates a meta character; thus, the meta characters that can fit in a string have codes in the range from 128 to 255, and are the meta versions of the ordinary ASCII characters. See Strings of Events, for details about <META>-handling in strings.

The read syntax for meta characters uses ‘\M-’. For example, ‘?\M-A’ stands for M-A. You can use ‘\M-’ together with octal character codes (see below), with ‘\C-’, or with any other syntax for a character. Thus, you can write M-A as ‘?\M-A’, or as ‘?\M-\101’. Likewise, you can write C-M-b as ‘?\M-\C-b’, ‘?\C-\M-b’, or ‘?\M-\002’.

2.3.3.5 Other Character Modifier Bits

The case of a graphic character is indicated by its character code; for example, ASCII distinguishes between the characters ‘a’ and ‘A’. But ASCII has no way to represent whether a control character is upper case or lower case. Emacs uses the 2**25 bit to indicate that the shift key was used in typing a control character. This distinction is possible only when you use X terminals or other special terminals; ordinary text terminals do not report the distinction. The Lisp syntax for the shift bit is ‘\S-’; thus, ‘?\C-\S-o’ or ‘?\C-\S-O’ represents the shifted-control-o character.

The X Window System defines three other modifier bits that can be set in a character: hyper, super and alt. The syntaxes for these bits are ‘\H-’, ‘\s-’ and ‘\A-’. (Case is significant in these prefixes.) Thus, ‘?\H-\M-\A-x’ represents Alt-Hyper-Meta-x. (Note that ‘\s’ with no following ‘-’ represents the space character.) Numerically, the bit values are 2**22 for alt, 2**23 for super and 2**24 for hyper.

2.3.4 Symbol Type

A symbol in GNU Emacs Lisp is an object with a name. The symbol name serves as the printed representation of the symbol. In ordinary Lisp use, with one single obarray (see Creating Symbols), a symbol's name is unique—no two symbols have the same name.

A symbol can serve as a variable, as a function name, or to hold a property list. Or it may serve only to be distinct from all other Lisp objects, so that its presence in a data structure may be recognized reliably. In a given context, usually only one of these uses is intended. But you can use one symbol in all of these ways, independently.

A symbol whose name starts with a colon (‘:’) is called a keyword symbol. These symbols automatically act as constants, and are normally used only by comparing an unknown symbol with a few specific alternatives. See Constant Variables.

A symbol name can contain any characters whatever. Most symbol names are written with letters, digits, and the punctuation characters ‘-+=*/’. Such names require no special punctuation; the characters of the name suffice as long as the name does not look like a number. (If it does, write a ‘\’ at the beginning of the name to force interpretation as a symbol.) The characters ‘_~!@$%^&:<>{}?’ are less often used but also require no special punctuation. Any other characters may be included in a symbol's name by escaping them with a backslash. In contrast to its use in strings, however, a backslash in the name of a symbol simply quotes the single character that follows the backslash. For example, in a string, ‘\t’ represents a tab character; in the name of a symbol, however, ‘\t’ merely quotes the letter ‘t’. To have a symbol with a tab character in its name, you must actually use a tab (preceded with a backslash). But it's rare to do such a thing.

Common Lisp note: In Common Lisp, lower case letters are always “folded” to upper case, unless they are explicitly escaped. In Emacs Lisp, upper case and lower case letters are distinct.

Here are several examples of symbol names. Note that the ‘+’ in the fourth example is escaped to prevent it from being read as a number. This is not necessary in the sixth example because the rest of the name makes it invalid as a number.

     foo                 ; A symbol named ‘foo’.
     FOO                 ; A symbol named ‘FOO’, different from ‘foo’.
     1+                  ; A symbol named ‘1+’
                         ;   (not ‘+1’, which is an integer).
     \+1                 ; A symbol named ‘+1’
                         ;   (not a very readable name).
     \(*\ 1\ 2\)         ; A symbol named ‘(* 1 2)’ (a worse name).
     
     
     +-*/_~!@$%^&=:<>{}  ; A symbol named ‘+-*/_~!@$%^&=:<>{}’.
                         ;   These characters need not be escaped.

As an exception to the rule that a symbol's name serves as its printed representation, ‘##’ is the printed representation for an interned symbol whose name is an empty string. Furthermore, ‘#:foo’ is the printed representation for an uninterned symbol whose name is foo. (Normally, the Lisp reader interns all symbols; see Creating Symbols.)

2.3.5 Sequence Types

A sequence is a Lisp object that represents an ordered set of elements. There are two kinds of sequence in Emacs Lisp: lists and arrays.

Lists are the most commonly-used sequences. A list can hold elements of any type, and its length can be easily changed by adding or removing elements. See the next subsection for more about lists.

Arrays are fixed-length sequences. They are further subdivided into strings, vectors, char-tables and bool-vectors. Vectors can hold elements of any type, whereas string elements must be characters, and bool-vector elements must be t or nil. Char-tables are like vectors except that they are indexed by any valid character code. The characters in a string can have text properties like characters in a buffer (see Text Properties), but vectors do not support text properties, even when their elements happen to be characters.

Lists, strings and the other array types also share important similarities. For example, all have a length l, and all have elements which can be indexed from zero to l minus one. Several functions, called sequence functions, accept any kind of sequence. For example, the function length reports the length of any kind of sequence. See Sequences Arrays Vectors.

It is generally impossible to read the same sequence twice, since sequences are always created anew upon reading. If you read the read syntax for a sequence twice, you get two sequences with equal contents. There is one exception: the empty list () always stands for the same object, nil.

2.3.6 Cons Cell and List Types

A cons cell is an object that consists of two slots, called the car slot and the cdr slot. Each slot can hold any Lisp object. We also say that “the car of this cons cell is” whatever object its car slot currently holds, and likewise for the cdr.

A list is a series of cons cells, linked together so that the cdr slot of each cons cell holds either the next cons cell or the empty list. The empty list is actually the symbol nil. See Lists, for details. Because most cons cells are used as part of lists, we refer to any structure made out of cons cells as a list structure.

A note to C programmers: a Lisp list thus works as a linked list built up of cons cells. Because pointers in Lisp are implicit, we do not distinguish between a cons cell slot “holding” a value versus “pointing to” the value.

Because cons cells are so central to Lisp, we also have a word for “an object which is not a cons cell”. These objects are called atoms.

The read syntax and printed representation for lists are identical, and consist of a left parenthesis, an arbitrary number of elements, and a right parenthesis. Here are examples of lists:

     (A 2 "A")            ; A list of three elements.
     ()                   ; A list of no elements (the empty list).
     nil                  ; A list of no elements (the empty list).
     ("A ()")             ; A list of one element: the string "A ()".
     (A ())               ; A list of two elements: A and the empty list.
     (A nil)              ; Equivalent to the previous.
     ((A B C))            ; A list of one element
                          ;   (which is a list of three elements).

Upon reading, each object inside the parentheses becomes an element of the list. That is, a cons cell is made for each element. The car slot of the cons cell holds the element, and its cdr slot refers to the next cons cell of the list, which holds the next element in the list. The cdr slot of the last cons cell is set to hold nil.

The names car and cdr derive from the history of Lisp. The original Lisp implementation ran on an IBM 704 computer which divided words into two parts, called the “address” part and the “decrement”; car was an instruction to extract the contents of the address part of a register, and cdr an instruction to extract the contents of the decrement. By contrast, “cons cells” are named for the function cons that creates them, which in turn was named for its purpose, the construction of cells.

• Box Diagrams: Drawing pictures of lists.
• Dotted Pair Notation: A general syntax for cons cells.
• Association List Type: A specially constructed list.

2.3.6.1 Drawing Lists as Box Diagrams

A list can be illustrated by a diagram in which the cons cells are shown as pairs of boxes, like dominoes. (The Lisp reader cannot read such an illustration; unlike the textual notation, which can be understood by both humans and computers, the box illustrations can be understood only by humans.) This picture represents the three-element list (rose violet buttercup):

         --- ---      --- ---      --- ---
        |   |   |--> |   |   |--> |   |   |--> nil
         --- ---      --- ---      --- ---
          |            |            |
          |            |            |
           --> rose     --> violet   --> buttercup

In this diagram, each box represents a slot that can hold or refer to any Lisp object. Each pair of boxes represents a cons cell. Each arrow represents a reference to a Lisp object, either an atom or another cons cell.

In this example, the first box, which holds the car of the first cons cell, refers to or “holds” rose (a symbol). The second box, holding the cdr of the first cons cell, refers to the next pair of boxes, the second cons cell. The car of the second cons cell is violet, and its cdr is the third cons cell. The cdr of the third (and last) cons cell is nil.

Here is another diagram of the same list, (rose violet buttercup), sketched in a different manner:

      ---------------       ----------------       -------------------
     | car   | cdr   |     | car    | cdr   |     | car       | cdr   |
     | rose  |   o-------->| violet |   o-------->| buttercup |  nil  |
     |       |       |     |        |       |     |           |       |
      ---------------       ----------------       -------------------

A list with no elements in it is the empty list; it is identical to the symbol nil. In other words, nil is both a symbol and a list.

Here is the list (A ()), or equivalently (A nil), depicted with boxes and arrows:

         --- ---      --- ---
        |   |   |--> |   |   |--> nil
         --- ---      --- ---
          |            |
          |            |
           --> A        --> nil

Here is a more complex illustration, showing the three-element list, ((pine needles) oak maple), the first element of which is a two-element list:

         --- ---      --- ---      --- ---
        |   |   |--> |   |   |--> |   |   |--> nil
         --- ---      --- ---      --- ---
          |            |            |
          |            |            |
          |             --> oak      --> maple
          |
          |     --- ---      --- ---
           --> |   |   |--> |   |   |--> nil
                --- ---      --- ---
                 |            |
                 |            |
                  --> pine     --> needles

The same list represented in the second box notation looks like this:

      --------------       --------------       --------------
     | car   | cdr  |     | car   | cdr  |     | car   | cdr  |
     |   o   |   o------->| oak   |   o------->| maple |  nil |
     |   |   |      |     |       |      |     |       |      |
      -- | ---------       --------------       --------------
         |
         |
         |        --------------       ----------------
         |       | car   | cdr  |     | car     | cdr  |
          ------>| pine  |   o------->| needles |  nil |
                 |       |      |     |         |      |
                  --------------       ----------------

2.3.6.2 Dotted Pair Notation

Dotted pair notation is a general syntax for cons cells that represents the car and cdr explicitly. In this syntax, (a . b) stands for a cons cell whose car is the object a and whose cdr is the object b. Dotted pair notation is more general than list syntax because the cdr does not have to be a list. However, it is more cumbersome in cases where list syntax would work. In dotted pair notation, the list ‘(1 2 3)’ is written as ‘(1 . (2 . (3 . nil)))’. For nil-terminated lists, you can use either notation, but list notation is usually clearer and more convenient. When printing a list, the dotted pair notation is only used if the cdr of a cons cell is not a list.

Here's an example using boxes to illustrate dotted pair notation. This example shows the pair (rose . violet):

         --- ---
        |   |   |--> violet
         --- ---
          |
          |
           --> rose

You can combine dotted pair notation with list notation to represent conveniently a chain of cons cells with a non-nil final cdr. You write a dot after the last element of the list, followed by the cdr of the final cons cell. For example, (rose violet . buttercup) is equivalent to (rose . (violet . buttercup)). The object looks like this:

         --- ---      --- ---
        |   |   |--> |   |   |--> buttercup
         --- ---      --- ---
          |            |
          |            |
           --> rose     --> violet

The syntax (rose . violet . buttercup) is invalid because there is nothing that it could mean. If anything, it would say to put buttercup in the cdr of a cons cell whose cdr is already used for violet.

The list (rose violet) is equivalent to (rose . (violet)), and looks like this:

         --- ---      --- ---
        |   |   |--> |   |   |--> nil
         --- ---      --- ---
          |            |
          |            |
           --> rose     --> violet

Similarly, the three-element list (rose violet buttercup) is equivalent to (rose . (violet . (buttercup))). It looks like this:

         --- ---      --- ---      --- ---
        |   |   |--> |   |   |--> |   |   |--> nil
         --- ---      --- ---      --- ---
          |            |            |
          |            |            |
           --> rose     --> violet   --> buttercup

2.3.6.3 Association List Type

An association list or alist is a specially-constructed list whose elements are cons cells. In each element, the car is considered a key, and the cdr is considered an associated value. (In some cases, the associated value is stored in the car of the cdr.) Association lists are often used as stacks, since it is easy to add or remove associations at the front of the list.

For example,

     (setq alist-of-colors
           '((rose . red) (lily . white) (buttercup . yellow)))

sets the variable alist-of-colors to an alist of three elements. In the first element, rose is the key and red is the value.

See Association Lists, for a further explanation of alists and for functions that work on alists. See Hash Tables, for another kind of lookup table, which is much faster for handling a large number of keys.

2.3.7 Array Type

An array is composed of an arbitrary number of slots for holding or referring to other Lisp objects, arranged in a contiguous block of memory. Accessing any element of an array takes approximately the same amount of time. In contrast, accessing an element of a list requires time proportional to the position of the element in the list. (Elements at the end of a list take longer to access than elements at the beginning of a list.)

Emacs defines four types of array: strings, vectors, bool-vectors, and char-tables.

A string is an array of characters and a vector is an array of arbitrary objects. A bool-vector can hold only t or nil. These kinds of array may have any length up to the largest integer. Char-tables are sparse arrays indexed by any valid character code; they can hold arbitrary objects.

The first element of an array has index zero, the second element has index 1, and so on. This is called zero-origin indexing. For example, an array of four elements has indices 0, 1, 2, and 3. The largest possible index value is one less than the length of the array. Once an array is created, its length is fixed.

All Emacs Lisp arrays are one-dimensional. (Most other programming languages support multidimensional arrays, but they are not essential; you can get the same effect with nested one-dimensional arrays.) Each type of array has its own read syntax; see the following sections for details.

The array type is a subset of the sequence type, and contains the string type, the vector type, the bool-vector type, and the char-table type.

2.3.8 String Type

A string is an array of characters. Strings are used for many purposes in Emacs, as can be expected in a text editor; for example, as the names of Lisp symbols, as messages for the user, and to represent text extracted from buffers. Strings in Lisp are constants: evaluation of a string returns the same string.

See Strings and Characters, for functions that operate on strings.

• Syntax for Strings: How to specify Lisp strings.
• Non-ASCII in Strings: International characters in strings.
• Nonprinting Characters: Literal unprintable characters in strings.
• Text Props and Strings: Strings with text properties.

2.3.8.1 Syntax for Strings

The read syntax for a string is a double-quote, an arbitrary number of characters, and another double-quote, "like this". To include a double-quote in a string, precede it with a backslash; thus, "\"" is a string containing just a single double-quote character. Likewise, you can include a backslash by preceding it with another backslash, like this: "this \\ is a single embedded backslash".

The newline character is not special in the read syntax for strings; if you write a new line between the double-quotes, it becomes a character in the string. But an escaped newline—one that is preceded by ‘\’—does not become part of the string; i.e., the Lisp reader ignores an escaped newline while reading a string. An escaped space ‘\ ’ is likewise ignored.

     "It is useful to include newlines
     in documentation strings,
     but the newline is \
     ignored if escaped."
          ⇒ "It is useful to include newlines
     in documentation strings,
     but the newline is ignored if escaped."

2.3.8.2 Non-ASCII Characters in Strings

There are two text representations for non-ASCII characters in Emacs strings: multibyte and unibyte (see Text Representations). Roughly speaking, unibyte strings store raw bytes, while multibyte strings store human-readable text. Each character in a unibyte string is a byte, i.e., its value is between 0 and 255. By contrast, each character in a multibyte string may have a value between 0 to 4194303 (see Character Type). In both cases, characters above 127 are non-ASCII.

You can include a non-ASCII character in a string constant by writing it literally. If the string constant is read from a multibyte source, such as a multibyte buffer or string, or a file that would be visited as multibyte, then Emacs reads each non-ASCII character as a multibyte character and automatically makes the string a multibyte string. If the string constant is read from a unibyte source, then Emacs reads the non-ASCII character as unibyte, and makes the string unibyte.

Instead of writing a character literally into a multibyte string, you can write it as its character code using an escape sequence. See General Escape Syntax, for details about escape sequences.

If you use any Unicode-style escape sequence ‘\uNNNN’ or ‘\U00NNNNNN’ in a string constant (even for an ASCII character), Emacs automatically assumes that it is multibyte.

You can also use hexadecimal escape sequences (‘\xn’) and octal escape sequences (‘\n’) in string constants. But beware: If a string constant contains hexadecimal or octal escape sequences, and these escape sequences all specify unibyte characters (i.e., less than 256), and there are no other literal non-ASCII characters or Unicode-style escape sequences in the string, then Emacs automatically assumes that it is a unibyte string. That is to say, it assumes that all non-ASCII characters occurring in the string are 8-bit raw bytes.

In hexadecimal and octal escape sequences, the escaped character code may contain a variable number of digits, so the first subsequent character which is not a valid hexadecimal or octal digit terminates the escape sequence. If the next character in a string could be interpreted as a hexadecimal or octal digit, write ‘\ ’ (backslash and space) to terminate the escape sequence. For example, ‘\xe0\ ’ represents one character, ‘a’ with grave accent. ‘\ ’ in a string constant is just like backslash-newline; it does not contribute any character to the string, but it does terminate any preceding hex escape.

2.3.8.3 Nonprinting Characters in Strings

You can use the same backslash escape-sequences in a string constant as in character literals (but do not use the question mark that begins a character constant). For example, you can write a string containing the nonprinting characters tab and C-a, with commas and spaces between them, like this: "\t, \C-a". See Character Type, for a description of the read syntax for characters.

However, not all of the characters you can write with backslash escape-sequences are valid in strings. The only control characters that a string can hold are the ASCII control characters. Strings do not distinguish case in ASCII control characters.

Properly speaking, strings cannot hold meta characters; but when a string is to be used as a key sequence, there is a special convention that provides a way to represent meta versions of ASCII characters in a string. If you use the ‘\M-’ syntax to indicate a meta character in a string constant, this sets the 2**7 bit of the character in the string. If the string is used in define-key or lookup-key, this numeric code is translated into the equivalent meta character. See Character Type.

Strings cannot hold characters that have the hyper, super, or alt modifiers.

2.3.8.4 Text Properties in Strings

A string can hold properties for the characters it contains, in addition to the characters themselves. This enables programs that copy text between strings and buffers to copy the text's properties with no special effort. See Text Properties, for an explanation of what text properties mean. Strings with text properties use a special read and print syntax:

     #("characters" property-data...)

where property-data consists of zero or more elements, in groups of three as follows:

     beg end plist

The elements beg and end are integers, and together specify a range of indices in the string; plist is the property list for that range. For example,

     #("foo bar" 0 3 (face bold) 3 4 nil 4 7 (face italic))

represents a string whose textual contents are ‘foo bar’, in which the first three characters have a face property with value bold, and the last three have a face property with value italic. (The fourth character has no text properties, so its property list is nil. It is not actually necessary to mention ranges with nil as the property list, since any characters not mentioned in any range will default to having no properties.)

2.3.9 Vector Type

A vector is a one-dimensional array of elements of any type. It takes a constant amount of time to access any element of a vector. (In a list, the access time of an element is proportional to the distance of the element from the beginning of the list.)

The printed representation of a vector consists of a left square bracket, the elements, and a right square bracket. This is also the read syntax. Like numbers and strings, vectors are considered constants for evaluation.

     [1 "two" (three)]      ; A vector of three elements.
          ⇒ [1 "two" (three)]

See Vectors, for functions that work with vectors.

2.3.10 Char-Table Type

A char-table is a one-dimensional array of elements of any type, indexed by character codes. Char-tables have certain extra features to make them more useful for many jobs that involve assigning information to character codes—for example, a char-table can have a parent to inherit from, a default value, and a small number of extra slots to use for special purposes. A char-table can also specify a single value for a whole character set.

The printed representation of a char-table is like a vector except that there is an extra ‘#^’ at the beginning.¹

See Char-Tables, for special functions to operate on char-tables. Uses of char-tables include:

• Case tables (see Case Tables).
• Character category tables (see Categories).
• Display tables (see Display Tables).
• Syntax tables (see Syntax Tables).

2.3.11 Bool-Vector Type

A bool-vector is a one-dimensional array whose elements must be t or nil.

The printed representation of a bool-vector is like a string, except that it begins with ‘#&’ followed by the length. The string constant that follows actually specifies the contents of the bool-vector as a bitmap—each “character” in the string contains 8 bits, which specify the next 8 elements of the bool-vector (1 stands for t, and 0 for nil). The least significant bits of the character correspond to the lowest indices in the bool-vector.

     (make-bool-vector 3 t)
          ⇒ #&3"^G"
     (make-bool-vector 3 nil)
          ⇒ #&3"^@"

These results make sense, because the binary code for ‘C-g’ is 111 and ‘C-@’ is the character with code 0.

If the length is not a multiple of 8, the printed representation shows extra elements, but these extras really make no difference. For instance, in the next example, the two bool-vectors are equal, because only the first 3 bits are used:

     (equal #&3"\377" #&3"\007")
          ⇒ t

2.3.12 Hash Table Type

A hash table is a very fast kind of lookup table, somewhat like an alist in that it maps keys to corresponding values, but much faster. The printed representation of a hash table specifies its properties and contents, like this:

     (make-hash-table)
          ⇒ #s(hash-table size 65 test eql rehash-size 1.5
                                  rehash-threshold 0.8 data ())

See Hash Tables, for more information about hash tables.

2.3.13 Function Type

Lisp functions are executable code, just like functions in other programming languages. In Lisp, unlike most languages, functions are also Lisp objects. A non-compiled function in Lisp is a lambda expression: that is, a list whose first element is the symbol lambda (see Lambda Expressions).

In most programming languages, it is impossible to have a function without a name. In Lisp, a function has no intrinsic name. A lambda expression can be called as a function even though it has no name; to emphasize this, we also call it an anonymous function (see Anonymous Functions). A named function in Lisp is just a symbol with a valid function in its function cell (see Defining Functions).

Most of the time, functions are called when their names are written in Lisp expressions in Lisp programs. However, you can construct or obtain a function object at run time and then call it with the primitive functions funcall and apply. See Calling Functions.

2.3.14 Macro Type

A Lisp macro is a user-defined construct that extends the Lisp language. It is represented as an object much like a function, but with different argument-passing semantics. A Lisp macro has the form of a list whose first element is the symbol macro and whose cdr is a Lisp function object, including the lambda symbol.

Lisp macro objects are usually defined with the built-in defmacro function, but any list that begins with macro is a macro as far as Emacs is concerned. See Macros, for an explanation of how to write a macro.

Warning: Lisp macros and keyboard macros (see Keyboard Macros) are entirely different things. When we use the word “macro” without qualification, we mean a Lisp macro, not a keyboard macro.

2.3.15 Primitive Function Type

A primitive function is a function callable from Lisp but written in the C programming language. Primitive functions are also called subrs or built-in functions. (The word “subr” is derived from “subroutine”.) Most primitive functions evaluate all their arguments when they are called. A primitive function that does not evaluate all its arguments is called a special form (see Special Forms).

It does not matter to the caller of a function whether the function is primitive. However, this does matter if you try to redefine a primitive with a function written in Lisp. The reason is that the primitive function may be called directly from C code. Calls to the redefined function from Lisp will use the new definition, but calls from C code may still use the built-in definition. Therefore, we discourage redefinition of primitive functions.

The term function refers to all Emacs functions, whether written in Lisp or C. See Function Type, for information about the functions written in Lisp.

Primitive functions have no read syntax and print in hash notation with the name of the subroutine.

     (symbol-function 'car)          ; Access the function cell
                                     ;   of the symbol.
          ⇒ #<subr car>
     (subrp (symbol-function 'car))  ; Is this a primitive function?
          ⇒ t                       ; Yes.

2.3.16 Byte-Code Function Type

Byte-code function objects are produced by byte-compiling Lisp code (see Byte Compilation). Internally, a byte-code function object is much like a vector; however, the evaluator handles this data type specially when it appears in a function call. See Byte-Code Objects.

The printed representation and read syntax for a byte-code function object is like that for a vector, with an additional ‘#’ before the opening ‘[’.

2.3.17 Autoload Type

An autoload object is a list whose first element is the symbol autoload. It is stored as the function definition of a symbol, where it serves as a placeholder for the real definition. The autoload object says that the real definition is found in a file of Lisp code that should be loaded when necessary. It contains the name of the file, plus some other information about the real definition.

After the file has been loaded, the symbol should have a new function definition that is not an autoload object. The new definition is then called as if it had been there to begin with. From the user's point of view, the function call works as expected, using the function definition in the loaded file.

An autoload object is usually created with the function autoload, which stores the object in the function cell of a symbol. See Autoload, for more details.

2.4 Editing Types

The types in the previous section are used for general programming purposes, and most of them are common to most Lisp dialects. Emacs Lisp provides several additional data types for purposes connected with editing.

• Buffer Type: The basic object of editing.
• Marker Type: A position in a buffer.
• Window Type: Buffers are displayed in windows.
• Frame Type: Windows subdivide frames.
• Terminal Type: A terminal device displays frames.
• Window Configuration Type: Recording the way a frame is subdivided.
• Frame Configuration Type: Recording the status of all frames.
• Process Type: A subprocess of Emacs running on the underlying OS.
• Stream Type: Receive or send characters.
• Keymap Type: What function a keystroke invokes.
• Overlay Type: How an overlay is represented.
• Font Type: Fonts for displaying text.

2.4.1 Buffer Type

A buffer is an object that holds text that can be edited (see Buffers). Most buffers hold the contents of a disk file (see Files) so they can be edited, but some are used for other purposes. Most buffers are also meant to be seen by the user, and therefore displayed, at some time, in a window (see Windows). But a buffer need not be displayed in any window. Each buffer has a designated position called point (see Positions); most editing commands act on the contents of the current buffer in the neighborhood of point. At any time, one buffer is the current buffer.

The contents of a buffer are much like a string, but buffers are not used like strings in Emacs Lisp, and the available operations are different. For example, you can insert text efficiently into an existing buffer, altering the buffer's contents, whereas “inserting” text into a string requires concatenating substrings, and the result is an entirely new string object.

Many of the standard Emacs functions manipulate or test the characters in the current buffer; a whole chapter in this manual is devoted to describing these functions (see Text).

Several other data structures are associated with each buffer:

• a local syntax table (see Syntax Tables);
• a local keymap (see Keymaps); and,
• a list of buffer-local variable bindings (see Buffer-Local Variables).
• overlays (see Overlays).
• text properties for the text in the buffer (see Text Properties).

The local keymap and variable list contain entries that individually override global bindings or values. These are used to customize the behavior of programs in different buffers, without actually changing the programs.

A buffer may be indirect, which means it shares the text of another buffer, but presents it differently. See Indirect Buffers.

Buffers have no read syntax. They print in hash notation, showing the buffer name.

     (current-buffer)
          ⇒ #<buffer objects.texi>

2.4.2 Marker Type

A marker denotes a position in a specific buffer. Markers therefore have two components: one for the buffer, and one for the position. Changes in the buffer's text automatically relocate the position value as necessary to ensure that the marker always points between the same two characters in the buffer.

Markers have no read syntax. They print in hash notation, giving the current character position and the name of the buffer.

     (point-marker)
          ⇒ #<marker at 10779 in objects.texi>

See Markers, for information on how to test, create, copy, and move markers.

2.4.3 Window Type

A window describes the portion of the terminal screen that Emacs uses to display a buffer. Every window has one associated buffer, whose contents appear in the window. By contrast, a given buffer may appear in one window, no window, or several windows.

Though many windows may exist simultaneously, at any time one window is designated the selected window. This is the window where the cursor is (usually) displayed when Emacs is ready for a command. The selected window usually displays the current buffer, but this is not necessarily the case.

Windows are grouped on the screen into frames; each window belongs to one and only one frame. See Frame Type.

Windows have no read syntax. They print in hash notation, giving the window number and the name of the buffer being displayed. The window numbers exist to identify windows uniquely, since the buffer displayed in any given window can change frequently.

     (selected-window)
          ⇒ #<window 1 on objects.texi>

See Windows, for a description of the functions that work on windows.

2.4.4 Frame Type

A frame is a screen area that contains one or more Emacs windows; we also use the term “frame” to refer to the Lisp object that Emacs uses to refer to the screen area.

Frames have no read syntax. They print in hash notation, giving the frame's title, plus its address in core (useful to identify the frame uniquely).

     (selected-frame)
          ⇒ #<frame emacs@psilocin.gnu.org 0xdac80>

See Frames, for a description of the functions that work on frames.

2.4.5 Terminal Type

A terminal is a device capable of displaying one or more Emacs frames (see Frame Type).

Terminals have no read syntax. They print in hash notation giving the terminal's ordinal number and its TTY device file name.

     (get-device-terminal nil)
          ⇒ #<terminal 1 on /dev/tty>

2.4.6 Window Configuration Type

A window configuration stores information about the positions, sizes, and contents of the windows in a frame, so you can recreate the same arrangement of windows later.

Window configurations do not have a read syntax; their print syntax looks like ‘#<window-configuration>’. See Window Configurations, for a description of several functions related to window configurations.

2.4.7 Frame Configuration Type

A frame configuration stores information about the positions, sizes, and contents of the windows in all frames. It is not a primitive type—it is actually a list whose car is frame-configuration and whose cdr is an alist. Each alist element describes one frame, which appears as the car of that element.

See Frame Configurations, for a description of several functions related to frame configurations.

2.4.8 Process Type

The word process usually means a running program. Emacs itself runs in a process of this sort. However, in Emacs Lisp, a process is a Lisp object that designates a subprocess created by the Emacs process. Programs such as shells, GDB, ftp, and compilers, running in subprocesses of Emacs, extend the capabilities of Emacs. An Emacs subprocess takes textual input from Emacs and returns textual output to Emacs for further manipulation. Emacs can also send signals to the subprocess.

Process objects have no read syntax. They print in hash notation, giving the name of the process:

     (process-list)
          ⇒ (#<process shell>)

See Processes, for information about functions that create, delete, return information about, send input or signals to, and receive output from processes.

2.4.9 Stream Type

A stream is an object that can be used as a source or sink for characters—either to supply characters for input or to accept them as output. Many different types can be used this way: markers, buffers, strings, and functions. Most often, input streams (character sources) obtain characters from the keyboard, a buffer, or a file, and output streams (character sinks) send characters to a buffer, such as a *Help* buffer, or to the echo area.

The object nil, in addition to its other meanings, may be used as a stream. It stands for the value of the variable standard-input or standard-output. Also, the object t as a stream specifies input using the minibuffer (see Minibuffers) or output in the echo area (see The Echo Area).

Streams have no special printed representation or read syntax, and print as whatever primitive type they are.

See Read and Print, for a description of functions related to streams, including parsing and printing functions.

2.4.10 Keymap Type

A keymap maps keys typed by the user to commands. This mapping controls how the user's command input is executed. A keymap is actually a list whose car is the symbol keymap.

See Keymaps, for information about creating keymaps, handling prefix keys, local as well as global keymaps, and changing key bindings.

2.4.11 Overlay Type

An overlay specifies properties that apply to a part of a buffer. Each overlay applies to a specified range of the buffer, and contains a property list (a list whose elements are alternating property names and values). Overlay properties are used to present parts of the buffer temporarily in a different display style. Overlays have no read syntax, and print in hash notation, giving the buffer name and range of positions.

See Overlays, for information on how you can create and use overlays.

2.4.12 Font Type

A font specifies how to display text on a graphical terminal. There are actually three separate font types—font objects, font specs, and font entities—each of which has slightly different properties. None of them have a read syntax; their print syntax looks like ‘#<font-object>’, ‘#<font-spec>’, and ‘#<font-entity>’ respectively. See Low-Level Font, for a description of these Lisp objects.

2.5 Read Syntax for Circular Objects

To represent shared or circular structures within a complex of Lisp objects, you can use the reader constructs ‘#n=’ and ‘#n#’.

Use #n= before an object to label it for later reference; subsequently, you can use #n# to refer the same object in another place. Here, n is some integer. For example, here is how to make a list in which the first element recurs as the third element:

     (#1=(a) b #1#)

This differs from ordinary syntax such as this

     ((a) b (a))

which would result in a list whose first and third elements look alike but are not the same Lisp object. This shows the difference:

     (prog1 nil
       (setq x '(#1=(a) b #1#)))
     (eq (nth 0 x) (nth 2 x))
          ⇒ t
     (setq x '((a) b (a)))
     (eq (nth 0 x) (nth 2 x))
          ⇒ nil

You can also use the same syntax to make a circular structure, which appears as an “element” within itself. Here is an example:

     #1=(a #1#)

This makes a list whose second element is the list itself. Here's how you can see that it really works:

     (prog1 nil
       (setq x '#1=(a #1#)))
     (eq x (cadr x))
          ⇒ t

The Lisp printer can produce this syntax to record circular and shared structure in a Lisp object, if you bind the variable print-circle to a non-nil value. See Output Variables.

2.6 Type Predicates

The Emacs Lisp interpreter itself does not perform type checking on the actual arguments passed to functions when they are called. It could not do so, since function arguments in Lisp do not have declared data types, as they do in other programming languages. It is therefore up to the individual function to test whether each actual argument belongs to a type that the function can use.

All built-in functions do check the types of their actual arguments when appropriate, and signal a wrong-type-argument error if an argument is of the wrong type. For example, here is what happens if you pass an argument to + that it cannot handle:

     (+ 2 'a)
          error--> Wrong type argument: number-or-marker-p, a

If you want your program to handle different types differently, you must do explicit type checking. The most common way to check the type of an object is to call a type predicate function. Emacs has a type predicate for each type, as well as some predicates for combinations of types.

A type predicate function takes one argument; it returns t if the argument belongs to the appropriate type, and nil otherwise. Following a general Lisp convention for predicate functions, most type predicates' names end with ‘p’.

Here is an example which uses the predicates listp to check for a list and symbolp to check for a symbol.

     (defun add-on (x)
       (cond ((symbolp x)
              ;; If X is a symbol, put it on LIST.
              (setq list (cons x list)))
             ((listp x)
              ;; If X is a list, add its elements to LIST.
              (setq list (append x list)))
             (t
              ;; We handle only symbols and lists.
              (error "Invalid argument %s in add-on" x))))

Here is a table of predefined type predicates, in alphabetical order, with references to further information.

atom

See atom.

arrayp

See arrayp.

bool-vector-p

See bool-vector-p.

bufferp

See bufferp.

byte-code-function-p

See byte-code-function-p.

case-table-p

See case-table-p.

char-or-string-p

See char-or-string-p.

char-table-p

See char-table-p.

commandp

See commandp.

consp

See consp.

custom-variable-p

See custom-variable-p.

display-table-p

See display-table-p.

floatp

See floatp.

fontp

See Low-Level Font.

frame-configuration-p

See frame-configuration-p.

frame-live-p

See frame-live-p.

framep

See framep.

functionp

See functionp.

hash-table-p

See hash-table-p.

integer-or-marker-p

See integer-or-marker-p.

integerp

See integerp.

keymapp

See keymapp.

keywordp

See Constant Variables.

listp

See listp.

markerp

See markerp.

wholenump

See wholenump.

nlistp

See nlistp.

numberp

See numberp.

number-or-marker-p

See number-or-marker-p.

overlayp

See overlayp.

processp

See processp.

sequencep

See sequencep.

stringp

See stringp.

subrp

See subrp.

symbolp

See symbolp.

syntax-table-p

See syntax-table-p.

vectorp

See vectorp.

window-configuration-p

See window-configuration-p.

window-live-p

See window-live-p.

windowp

See windowp.

booleanp

See booleanp.

string-or-null-p

See string-or-null-p.

The most general way to check the type of an object is to call the function type-of. Recall that each object belongs to one and only one primitive type; type-of tells you which one (see Lisp Data Types). But type-of knows nothing about non-primitive types. In most cases, it is more convenient to use type predicates than type-of.

2.7 Equality Predicates

Here we describe functions that test for equality between two objects. Other functions test equality of contents between objects of specific types, e.g., strings. For these predicates, see the appropriate chapter describing the data type.

The test for equality is implemented recursively; for example, given two cons cells x and y, (equal x y) returns t if and only if both the expressions below return t:

     (equal (car x) (car y))
     (equal (cdr x) (cdr y))

Because of this recursive method, circular lists may therefore cause infinite recursion (leading to an error).

3 Numbers

GNU Emacs supports two numeric data types: integers and floating point numbers. Integers are whole numbers such as −3, 0, 7, 13, and 511. Their values are exact. Floating point numbers are numbers with fractional parts, such as −4.5, 0.0, or 2.71828. They can also be expressed in exponential notation: 1.5e2 equals 150; in this example, ‘e2’ stands for ten to the second power, and that is multiplied by 1.5. Floating point values are not exact; they have a fixed, limited amount of precision.

• Integer Basics: Representation and range of integers.
• Float Basics: Representation and range of floating point.
• Predicates on Numbers: Testing for numbers.
• Comparison of Numbers: Equality and inequality predicates.
• Numeric Conversions: Converting float to integer and vice versa.
• Arithmetic Operations: How to add, subtract, multiply and divide.
• Rounding Operations: Explicitly rounding floating point numbers.
• Bitwise Operations: Logical and, or, not, shifting.
• Math Functions: Trig, exponential and logarithmic functions.
• Random Numbers: Obtaining random integers, predictable or not.

3.1 Integer Basics

The range of values for an integer depends on the machine. The minimum range is −536870912 to 536870911 (30 bits; i.e., -2**29 to 2**29 − 1), but many machines provide a wider range. Many examples in this chapter assume the minimum integer width of 30 bits. The Lisp reader reads an integer as a sequence of digits with optional initial sign and optional final period. An integer that is out of the Emacs range is treated as a floating-point number.

      1               ; The integer 1.
      1.              ; The integer 1.
     +1               ; Also the integer 1.
     -1               ; The integer −1.
      1073741825      ; The floating point number 1073741825.0.
      0               ; The integer 0.
     -0               ; The integer 0.

The syntax for integers in bases other than 10 uses ‘#’ followed by a letter that specifies the radix: ‘b’ for binary, ‘o’ for octal, ‘x’ for hex, or ‘radixr’ to specify radix radix. Case is not significant for the letter that specifies the radix. Thus, ‘#binteger’ reads integer in binary, and ‘#radixrinteger’ reads integer in radix radix. Allowed values of radix run from 2 to 36. For example:

     #b101100 ⇒ 44
     #o54 ⇒ 44
     #x2c ⇒ 44
     #24r1k ⇒ 44

To understand how various functions work on integers, especially the bitwise operators (see Bitwise Operations), it is often helpful to view the numbers in their binary form.

In 30-bit binary, the decimal integer 5 looks like this:

     0000...000101 (30 bits total)

(The ‘...’ stands for enough bits to fill out a 30-bit word; in this case, ‘...’ stands for twenty 0 bits. Later examples also use the ‘...’ notation to make binary integers easier to read.)

The integer −1 looks like this:

     1111...111111 (30 bits total)

−1 is represented as 30 ones. (This is called two's complement notation.)

The negative integer, −5, is creating by subtracting 4 from −1. In binary, the decimal integer 4 is 100. Consequently, −5 looks like this:

     1111...111011 (30 bits total)

In this implementation, the largest 30-bit binary integer value is 536,870,911 in decimal. In binary, it looks like this:

     0111...111111 (30 bits total)

Since the arithmetic functions do not check whether integers go outside their range, when you add 1 to 536,870,911, the value is the negative integer −536,870,912:

     (+ 1 536870911)
          ⇒ -536870912
          ⇒ 1000...000000 (30 bits total)

Many of the functions described in this chapter accept markers for arguments in place of numbers. (See Markers.) Since the actual arguments to such functions may be either numbers or markers, we often give these arguments the name number-or-marker. When the argument value is a marker, its position value is used and its buffer is ignored.

In Emacs Lisp, text characters are represented by integers. Any integer between zero and the value of max-char, inclusive, is considered to be valid as a character. See String Basics.

3.2 Floating Point Basics

Floating point numbers are useful for representing numbers that are not integral. The precise range of floating point numbers is machine-specific; it is the same as the range of the C data type double on the machine you are using. Emacs uses the IEEE floating point standard, which is supported by all modern computers.

The read syntax for floating point numbers requires either a decimal point (with at least one digit following), an exponent, or both. For example, ‘1500.0’, ‘15e2’, ‘15.0e2’, ‘1.5e3’, and ‘.15e4’ are five ways of writing a floating point number whose value is 1500. They are all equivalent. You can also use a minus sign to write negative floating point numbers, as in ‘-1.0’.

Emacs Lisp treats -0.0 as equal to ordinary zero (with respect to equal and =), even though the two are distinguishable in the IEEE floating point standard.

The IEEE floating point standard supports positive infinity and negative infinity as floating point values. It also provides for a class of values called NaN or “not-a-number”; numerical functions return such values in cases where there is no correct answer. For example, (/ 0.0 0.0) returns a NaN. (NaN values can also carry a sign, but for practical purposes there's no significant difference between different NaN values in Emacs Lisp.)

When a function is documented to return a NaN, it returns an implementation-defined value when Emacs is running on one of the now-rare platforms that do not use IEEE floating point. For example, (log -1.0) typically returns a NaN, but on non-IEEE platforms it returns an implementation-defined value.

Here are the read syntaxes for these special floating point values:

positive infinity

‘1.0e+INF’

negative infinity

‘-1.0e+INF’

Not-a-number

‘0.0e+NaN’ or ‘-0.0e+NaN’.

The following functions are specialized for handling floating point numbers:

3.3 Type Predicates for Numbers

The functions in this section test for numbers, or for a specific type of number. The functions integerp and floatp can take any type of Lisp object as argument (they would not be of much use otherwise), but the zerop predicate requires a number as its argument. See also integer-or-marker-p and number-or-marker-p, in Predicates on Markers.

3.4 Comparison of Numbers

To test numbers for numerical equality, you should normally use =, not eq. There can be many distinct floating point number objects with the same numeric value. If you use eq to compare them, then you test whether two values are the same object. By contrast, = compares only the numeric values of the objects.

In Emacs Lisp, each integer value is a unique Lisp object. Therefore, eq is equivalent to = where integers are concerned. It is sometimes convenient to use eq for comparing an unknown value with an integer, because eq does not report an error if the unknown value is not a number—it accepts arguments of any type. By contrast, = signals an error if the arguments are not numbers or markers. However, it is better programming practice to use = if you can, even for comparing integers.

Sometimes it is useful to compare numbers with equal, which treats two numbers as equal if they have the same data type (both integers, or both floating point) and the same value. By contrast, = can treat an integer and a floating point number as equal. See Equality Predicates.

There is another wrinkle: because floating point arithmetic is not exact, it is often a bad idea to check for equality of two floating point values. Usually it is better to test for approximate equality. Here's a function to do this:

     (defvar fuzz-factor 1.0e-6)
     (defun approx-equal (x y)
       (or (and (= x 0) (= y 0))
           (< (/ (abs (- x y))
                 (max (abs x) (abs y)))
              fuzz-factor)))

Common Lisp note: Comparing numbers in Common Lisp always requires = because Common Lisp implements multi-word integers, and two distinct integer objects can have the same numeric value. Emacs Lisp can have just one integer object for any given value because it has a limited range of integer values.

3.5 Numeric Conversions

To convert an integer to floating point, use the function float.

There are four functions to convert floating point numbers to integers; they differ in how they round. All accept an argument number and an optional argument divisor. Both arguments may be integers or floating point numbers. divisor may also be nil. If divisor is nil or omitted, these functions convert number to an integer, or return it unchanged if it already is an integer. If divisor is non-nil, they divide number by divisor and convert the result to an integer. integer. If divisor is zero (whether integer or floating-point), Emacs signals an arith-error error.

3.6 Arithmetic Operations

Emacs Lisp provides the traditional four arithmetic operations (addition, subtraction, multiplication, and division), as well as remainder and modulus functions, and functions to add or subtract 1. Except for %, each of these functions accepts both integer and floating point arguments, and returns a floating point number if any argument is a floating point number.

It is important to note that in Emacs Lisp, arithmetic functions do not check for overflow. Thus (1+ 536870911) may evaluate to −536870912, depending on your hardware.

3.7 Rounding Operations

The functions ffloor, fceiling, fround, and ftruncate take a floating point argument and return a floating point result whose value is a nearby integer. ffloor returns the nearest integer below; fceiling, the nearest integer above; ftruncate, the nearest integer in the direction towards zero; fround, the nearest integer.

3.8 Bitwise Operations on Integers

In a computer, an integer is represented as a binary number, a sequence of bits (digits which are either zero or one). A bitwise operation acts on the individual bits of such a sequence. For example, shifting moves the whole sequence left or right one or more places, reproducing the same pattern “moved over”.

The bitwise operations in Emacs Lisp apply only to integers.

3.9 Standard Mathematical Functions

These mathematical functions allow integers as well as floating point numbers as arguments.

In addition, Emacs defines the following common mathematical constants:

3.10 Random Numbers

A deterministic computer program cannot generate true random numbers. For most purposes, pseudo-random numbers suffice. A series of pseudo-random numbers is generated in a deterministic fashion. The numbers are not truly random, but they have certain properties that mimic a random series. For example, all possible values occur equally often in a pseudo-random series.

Pseudo-random numbers are generated from a “seed”. Starting from any given seed, the random function always generates the same sequence of numbers. By default, Emacs initializes the random seed at startup, in such a way that the sequence of values of random (with overwhelming likelihood) differs in each Emacs run.

Sometimes you want the random number sequence to be repeatable. For example, when debugging a program whose behavior depends on the random number sequence, it is helpful to get the same behavior in each program run. To make the sequence repeat, execute (random ""). This sets the seed to a constant value for your particular Emacs executable (though it may differ for other Emacs builds). You can use other strings to choose various seed values.

4 Strings and Characters

A string in Emacs Lisp is an array that contains an ordered sequence of characters. Strings are used as names of symbols, buffers, and files; to send messages to users; to hold text being copied between buffers; and for many other purposes. Because strings are so important, Emacs Lisp has many functions expressly for manipulating them. Emacs Lisp programs use strings more often than individual characters.

See Strings of Events, for special considerations for strings of keyboard character events.

• Basics: Basic properties of strings and characters.
• Predicates for Strings: Testing whether an object is a string or char.
• Creating Strings: Functions to allocate new strings.
• Modifying Strings: Altering the contents of an existing string.
• Text Comparison: Comparing characters or strings.
• String Conversion: Converting to and from characters and strings.
• Formatting Strings: format: Emacs's analogue of printf.
• Case Conversion: Case conversion functions.
• Case Tables: Customizing case conversion.

4.1 String and Character Basics

A character is a Lisp object which represents a single character of text. In Emacs Lisp, characters are simply integers; whether an integer is a character or not is determined only by how it is used. See Character Codes, for details about character representation in Emacs.

A string is a fixed sequence of characters. It is a type of sequence called a array, meaning that its length is fixed and cannot be altered once it is created (see Sequences Arrays Vectors). Unlike in C, Emacs Lisp strings are not terminated by a distinguished character code.

Since strings are arrays, and therefore sequences as well, you can operate on them with the general array and sequence functions documented in Sequences Arrays Vectors. For example, you can access or change individual characters in a string using the functions aref and aset (see Array Functions). However, note that length should not be used for computing the width of a string on display; use string-width (see Width) instead.

There are two text representations for non-ASCII characters in Emacs strings (and in buffers): unibyte and multibyte. For most Lisp programming, you don't need to be concerned with these two representations. See Text Representations, for details.

Sometimes key sequences are represented as unibyte strings. When a unibyte string is a key sequence, string elements in the range 128 to 255 represent meta characters (which are large integers) rather than character codes in the range 128 to 255. Strings cannot hold characters that have the hyper, super or alt modifiers; they can hold ASCII control characters, but no other control characters. They do not distinguish case in ASCII control characters. If you want to store such characters in a sequence, such as a key sequence, you must use a vector instead of a string. See Character Type, for more information about keyboard input characters.

Strings are useful for holding regular expressions. You can also match regular expressions against strings with string-match (see Regexp Search). The functions match-string (see Simple Match Data) and replace-match (see Replacing Match) are useful for decomposing and modifying strings after matching regular expressions against them.

Like a buffer, a string can contain text properties for the characters in it, as well as the characters themselves. See Text Properties. All the Lisp primitives that copy text from strings to buffers or other strings also copy the properties of the characters being copied.

See Text, for information about functions that display strings or copy them into buffers. See Character Type, and String Type, for information about the syntax of characters and strings. See Non-ASCII Characters, for functions to convert between text representations and to encode and decode character codes.

4.2 Predicates for Strings

For more information about general sequence and array predicates, see Sequences Arrays Vectors, and Arrays.

4.3 Creating Strings

The following functions create strings, either from scratch, or by putting strings together, or by taking them apart.

4.4 Modifying Strings

The most basic way to alter the contents of an existing string is with aset (see Array Functions). (aset string idx char) stores char into string at index idx. Each character occupies one or more bytes, and if char needs a different number of bytes from the character already present at that index, aset signals an error.

A more powerful function is store-substring:

To clear out a string that contained a password, use clear-string:

4.5 Comparison of Characters and Strings

See also the function compare-buffer-substrings in Comparing Text, for a way to compare text in buffers. The function string-match, which matches a regular expression against a string, can be used for a kind of string comparison; see Regexp Search.

4.6 Conversion of Characters and Strings

This section describes functions for converting between characters, strings and integers. format (see Formatting Strings) and prin1-to-string (see Output Functions) can also convert Lisp objects into strings. read-from-string (see Input Functions) can “convert” a string representation of a Lisp object into an object. The functions string-to-multibyte and string-to-unibyte convert the text representation of a string (see Converting Representations).

See Documentation, for functions that produce textual descriptions of text characters and general input events (single-key-description and text-char-description). These are used primarily for making help messages.

Here are some other functions that can convert to or from a string:

concat

This function converts a vector or a list into a string. See Creating Strings.

vconcat

This function converts a string into a vector. See Vector Functions.

append

This function converts a string into a list. See Building Lists.

byte-to-string

This function converts a byte of character data into a unibyte string. See Converting Representations.

4.7 Formatting Strings

Formatting means constructing a string by substituting computed values at various places in a constant string. This constant string controls how the other values are printed, as well as where they appear; it is called a format string.

Formatting is often useful for computing messages to be displayed. In fact, the functions message and error provide the same formatting feature described here; they differ from format only in how they use the result of formatting.

A format specification is a sequence of characters beginning with a ‘%’. Thus, if there is a ‘%d’ in string, the format function replaces it with the printed representation of one of the values to be formatted (one of the arguments objects). For example:

     (format "The value of fill-column is %d." fill-column)
          ⇒ "The value of fill-column is 72."

Since format interprets ‘%’ characters as format specifications, you should never pass an arbitrary string as the first argument. This is particularly true when the string is generated by some Lisp code. Unless the string is known to never include any ‘%’ characters, pass "%s", described below, as the first argument, and the string as the second, like this:

       (format "%s" arbitrary-string)

If string contains more than one format specification, the format specifications correspond to successive values from objects. Thus, the first format specification in string uses the first such value, the second format specification uses the second such value, and so on. Any extra format specifications (those for which there are no corresponding values) cause an error. Any extra values to be formatted are ignored.

Certain format specifications require values of particular types. If you supply a value that doesn't fit the requirements, an error is signaled.

Here is a table of valid format specifications:

‘%s’

Replace the specification with the printed representation of the object, made without quoting (that is, using princ, not prin1—see Output Functions). Thus, strings are represented by their contents alone, with no ‘"’ characters, and symbols appear without ‘\’ characters.

If the object is a string, its text properties are copied into the output. The text properties of the ‘%s’ itself are also copied, but those of the object take priority.

‘%S’

Replace the specification with the printed representation of the object, made with quoting (that is, using prin1—see Output Functions). Thus, strings are enclosed in ‘"’ characters, and ‘\’ characters appear where necessary before special characters.

‘%o’

Replace the specification with the base-eight representation of an integer.

‘%d’

Replace the specification with the base-ten representation of an integer.

‘%x’

‘%X’

Replace the specification with the base-sixteen representation of an integer. ‘%x’ uses lower case and ‘%X’ uses upper case.

‘%c’

Replace the specification with the character which is the value given.

‘%e’

Replace the specification with the exponential notation for a floating point number.

‘%f’

Replace the specification with the decimal-point notation for a floating point number.

‘%g’

Replace the specification with notation for a floating point number, using either exponential notation or decimal-point notation, whichever is shorter.

‘%%’

Replace the specification with a single ‘%’. This format specification is unusual in that it does not use a value. For example, (format "%% %d" 30) returns "% 30".

Any other format character results in an ‘Invalid format operation’ error.

Here are several examples:

     (format "The name of this buffer is %s." (buffer-name))
          ⇒ "The name of this buffer is strings.texi."
     
     (format "The buffer object prints as %s." (current-buffer))
          ⇒ "The buffer object prints as strings.texi."
     
     (format "The octal value of %d is %o,
              and the hex value is %x." 18 18 18)
          ⇒ "The octal value of 18 is 22,
              and the hex value is 12."

A specification can have a width, which is a decimal number between the ‘%’ and the specification character. If the printed representation of the object contains fewer characters than this width, format extends it with padding. The width specifier is ignored for the ‘%%’ specification. Any padding introduced by the width specifier normally consists of spaces inserted on the left:

     (format "%5d is padded on the left with spaces" 123)
          ⇒ "  123 is padded on the left with spaces"

If the width is too small, format does not truncate the object's printed representation. Thus, you can use a width to specify a minimum spacing between columns with no risk of losing information. In the following three examples, ‘%7s’ specifies a minimum width of 7. In the first case, the string inserted in place of ‘%7s’ has only 3 letters, and needs 4 blank spaces as padding. In the second case, the string "specification" is 13 letters wide but is not truncated.

     (format "The word `%7s' has %d letters in it."
             "foo" (length "foo"))
          ⇒ "The word `    foo' has 3 letters in it."
     (format "The word `%7s' has %d letters in it."
             "specification" (length "specification"))
          ⇒ "The word `specification' has 13 letters in it."

Immediately after the ‘%’ and before the optional width specifier, you can also put certain flag characters.

The flag ‘+’ inserts a plus sign before a positive number, so that it always has a sign. A space character as flag inserts a space before a positive number. (Otherwise, positive numbers start with the first digit.) These flags are useful for ensuring that positive numbers and negative numbers use the same number of columns. They are ignored except for ‘%d’, ‘%e’, ‘%f’, ‘%g’, and if both flags are used, ‘+’ takes precedence.

The flag ‘#’ specifies an “alternate form” which depends on the format in use. For ‘%o’, it ensures that the result begins with a ‘0’. For ‘%x’ and ‘%X’, it prefixes the result with ‘0x’ or ‘0X’. For ‘%e’, ‘%f’, and ‘%g’, the ‘#’ flag means include a decimal point even if the precision is zero.

The flag ‘0’ ensures that the padding consists of ‘0’ characters instead of spaces. This flag is ignored for non-numerical specification characters like ‘%s’, ‘%S’ and ‘%c’. These specification characters accept the ‘0’ flag, but still pad with spaces.

The flag ‘-’ causes the padding inserted by the width specifier, if any, to be inserted on the right rather than the left. If both ‘-’ and ‘0’ are present, the ‘0’ flag is ignored.

     (format "%06d is padded on the left with zeros" 123)
          ⇒ "000123 is padded on the left with zeros"
     
     (format "%-6d is padded on the right" 123)
          ⇒ "123    is padded on the right"
     
     (format "The word `%-7s' actually has %d letters in it."
             "foo" (length "foo"))
          ⇒ "The word `foo    ' actually has 3 letters in it."

All the specification characters allow an optional precision before the character (after the width, if present). The precision is a decimal-point ‘.’ followed by a digit-string. For the floating-point specifications (‘%e’, ‘%f’, ‘%g’), the precision specifies how many decimal places to show; if zero, the decimal-point itself is also omitted. For ‘%s’ and ‘%S’, the precision truncates the string to the given width, so ‘%.3s’ shows only the first three characters of the representation for object. Precision has no effect for other specification characters.

4.8 Case Conversion in Lisp

The character case functions change the case of single characters or of the contents of strings. The functions normally convert only alphabetic characters (the letters ‘A’ through ‘Z’ and ‘a’ through ‘z’, as well as non-ASCII letters); other characters are not altered. You can specify a different case conversion mapping by specifying a case table (see Case Tables).

These functions do not modify the strings that are passed to them as arguments.

The examples below use the characters ‘X’ and ‘x’ which have ASCII codes 88 and 120 respectively.

See Text Comparison, for functions that compare strings; some of them ignore case differences, or can optionally ignore case differences.

4.9 The Case Table

You can customize case conversion by installing a special case table. A case table specifies the mapping between upper case and lower case letters. It affects both the case conversion functions for Lisp objects (see the previous section) and those that apply to text in the buffer (see Case Changes). Each buffer has a case table; there is also a standard case table which is used to initialize the case table of new buffers.

A case table is a char-table (see Char-Tables) whose subtype is case-table. This char-table maps each character into the corresponding lower case character. It has three extra slots, which hold related tables:

upcase

The upcase table maps each character into the corresponding upper case character.

canonicalize

The canonicalize table maps all of a set of case-related characters into a particular member of that set.

equivalences

The equivalences table maps each one of a set of case-related characters into the next character in that set.

In simple cases, all you need to specify is the mapping to lower-case; the three related tables will be calculated automatically from that one.

For some languages, upper and lower case letters are not in one-to-one correspondence. There may be two different lower case letters with the same upper case equivalent. In these cases, you need to specify the maps for both lower case and upper case.

The extra table canonicalize maps each character to a canonical equivalent; any two characters that are related by case-conversion have the same canonical equivalent character. For example, since ‘a’ and ‘A’ are related by case-conversion, they should have the same canonical equivalent character (which should be either ‘a’ for both of them, or ‘A’ for both of them).

The extra table equivalences is a map that cyclically permutes each equivalence class (of characters with the same canonical equivalent). (For ordinary ASCII, this would map ‘a’ into ‘A’ and ‘A’ into ‘a’, and likewise for each set of equivalent characters.)

When constructing a case table, you can provide nil for canonicalize; then Emacs fills in this slot from the lower case and upper case mappings. You can also provide nil for equivalences; then Emacs fills in this slot from canonicalize. In a case table that is actually in use, those components are non-nil. Do not try to specify equivalences without also specifying canonicalize.

Here are the functions for working with case tables:

Some language environments modify the case conversions of ASCII characters; for example, in the Turkish language environment, the ASCII character ‘I’ is downcased into a Turkish “dotless i”. This can interfere with code that requires ordinary ASCII case conversion, such as implementations of ASCII-based network protocols. In that case, use the with-case-table macro with the variable ascii-case-table, which stores the unmodified case table for the ASCII character set.

The following three functions are convenient subroutines for packages that define non-ASCII character sets. They modify the specified case table case-table; they also modify the standard syntax table. See Syntax Tables. Normally you would use these functions to change the standard case table.

5 Lists

A list represents a sequence of zero or more elements (which may be any Lisp objects). The important difference between lists and vectors is that two or more lists can share part of their structure; in addition, you can insert or delete elements in a list without copying the whole list.

• Cons Cells: How lists are made out of cons cells.
• List-related Predicates: Is this object a list? Comparing two lists.
• List Elements: Extracting the pieces of a list.
• Building Lists: Creating list structure.
• List Variables: Modifying lists stored in variables.
• Modifying Lists: Storing new pieces into an existing list.
• Sets And Lists: A list can represent a finite mathematical set.
• Association Lists: A list can represent a finite relation or mapping.
• Property Lists: A list of paired elements.

5.1 Lists and Cons Cells

Lists in Lisp are not a primitive data type; they are built up from cons cells (see Cons Cell Type). A cons cell is a data object that represents an ordered pair. That is, it has two slots, and each slot holds, or refers to, some Lisp object. One slot is known as the car, and the other is known as the cdr. (These names are traditional; see Cons Cell Type.) cdr is pronounced “could-er”.

We say that “the car of this cons cell is” whatever object its car slot currently holds, and likewise for the cdr.

A list is a series of cons cells “chained together”, so that each cell refers to the next one. There is one cons cell for each element of the list. By convention, the cars of the cons cells hold the elements of the list, and the cdrs are used to chain the list (this asymmetry between car and cdr is entirely a matter of convention; at the level of cons cells, the car and cdr slots have similar properties). Hence, the cdr slot of each cons cell in a list refers to the following cons cell.

Also by convention, the cdr of the last cons cell in a list is nil. We call such a nil-terminated structure a true list. In Emacs Lisp, the symbol nil is both a symbol and a list with no elements. For convenience, the symbol nil is considered to have nil as its cdr (and also as its car).

Hence, the cdr of a true list is always a true list. The cdr of a nonempty true list is a true list containing all the elements except the first.

If the cdr of a list's last cons cell is some value other than nil, we call the structure a dotted list, since its printed representation would use dotted pair notation (see Dotted Pair Notation). There is one other possibility: some cons cell's cdr could point to one of the previous cons cells in the list. We call that structure a circular list.

For some purposes, it does not matter whether a list is true, circular or dotted. If a program doesn't look far enough down the list to see the cdr of the final cons cell, it won't care. However, some functions that operate on lists demand true lists and signal errors if given a dotted list. Most functions that try to find the end of a list enter infinite loops if given a circular list.

Because most cons cells are used as part of lists, we refer to any structure made out of cons cells as a list structure.

5.2 Predicates on Lists

The following predicates test whether a Lisp object is an atom, whether it is a cons cell or is a list, or whether it is the distinguished object nil. (Many of these predicates can be defined in terms of the others, but they are used so often that it is worth having them.)

5.3 Accessing Elements of Lists

The most common way to compute the length of a list, when you are not worried that it may be circular, is with length. See Sequence Functions.

5.4 Building Cons Cells and Lists

Many functions build lists, as lists reside at the very heart of Lisp. cons is the fundamental list-building function; however, it is interesting to note that list is used more times in the source code for Emacs than cons.

Here is an example of using append:

     (setq trees '(pine oak))
          ⇒ (pine oak)
     (setq more-trees (append '(maple birch) trees))
          ⇒ (maple birch pine oak)
     
     trees
          ⇒ (pine oak)
     more-trees
          ⇒ (maple birch pine oak)
     (eq trees (cdr (cdr more-trees)))
          ⇒ t

You can see how append works by looking at a box diagram. The variable trees is set to the list (pine oak) and then the variable more-trees is set to the list (maple birch pine oak). However, the variable trees continues to refer to the original list:

     more-trees                trees
     |                           |
     |     --- ---      --- ---   -> --- ---      --- ---
      --> |   |   |--> |   |   |--> |   |   |--> |   |   |--> nil
           --- ---      --- ---      --- ---      --- ---
            |            |            |            |
            |            |            |            |
             --> maple    -->birch     --> pine     --> oak

An empty sequence contributes nothing to the value returned by append. As a consequence of this, a final nil argument forces a copy of the previous argument:

     trees
          ⇒ (pine oak)
     (setq wood (append trees nil))
          ⇒ (pine oak)
     wood
          ⇒ (pine oak)
     (eq wood trees)
          ⇒ nil

This once was the usual way to copy a list, before the function copy-sequence was invented. See Sequences Arrays Vectors.

Here we show the use of vectors and strings as arguments to append:

     (append [a b] "cd" nil)
          ⇒ (a b 99 100)

With the help of apply (see Calling Functions), we can append all the lists in a list of lists:

     (apply 'append '((a b c) nil (x y z) nil))
          ⇒ (a b c x y z)

If no sequences are given, nil is returned:

     (append)
          ⇒ nil

Here are some examples where the final argument is not a list:

     (append '(x y) 'z)
          ⇒ (x y . z)
     (append '(x y) [z])
          ⇒ (x y . [z])

The second example shows that when the final argument is a sequence but not a list, the sequence's elements do not become elements of the resulting list. Instead, the sequence becomes the final cdr, like any other non-list final argument.

5.5 Modifying List Variables

These functions, and one macro, provide convenient ways to modify a list which is stored in a variable.

Two functions modify lists that are the values of variables.

Here's a scenario showing how to use add-to-list:

     (setq foo '(a b))
          ⇒ (a b)
     
     (add-to-list 'foo 'c)     ;; Add c.
          ⇒ (c a b)
     
     (add-to-list 'foo 'b)     ;; No effect.
          ⇒ (c a b)
     
     foo                       ;; foo was changed.
          ⇒ (c a b)

An equivalent expression for (add-to-list 'var value) is this:

     (or (member value var)
         (setq var (cons value var)))

Here's a scenario showing how to use add-to-ordered-list:

     (setq foo '())
          ⇒ nil
     
     (add-to-ordered-list 'foo 'a 1)     ;; Add a.
          ⇒ (a)
     
     (add-to-ordered-list 'foo 'c 3)     ;; Add c.
          ⇒ (a c)
     
     (add-to-ordered-list 'foo 'b 2)     ;; Add b.
          ⇒ (a b c)
     
     (add-to-ordered-list 'foo 'b 4)     ;; Move b.
          ⇒ (a c b)
     
     (add-to-ordered-list 'foo 'd)       ;; Append d.
          ⇒ (a c b d)
     
     (add-to-ordered-list 'foo 'e)       ;; Add e.
          ⇒ (a c b e d)
     
     foo                       ;; foo was changed.
          ⇒ (a c b e d)

5.6 Modifying Existing List Structure

You can modify the car and cdr contents of a cons cell with the primitives setcar and setcdr. We call these “destructive” operations because they change existing list structure.

Common Lisp note: Common Lisp uses functions rplaca and rplacd to alter list structure; they change structure the same way as setcar and setcdr, but the Common Lisp functions return the cons cell while setcar and setcdr return the new car or cdr.

• Setcar: Replacing an element in a list.
• Setcdr: Replacing part of the list backbone. This can be used to remove or add elements.
• Rearrangement: Reordering the elements in a list; combining lists.

5.6.1 Altering List Elements with setcar

Changing the car of a cons cell is done with setcar. When used on a list, setcar replaces one element of a list with a different element.

When a cons cell is part of the shared structure of several lists, storing a new car into the cons changes one element of each of these lists. Here is an example:

     ;; Create two lists that are partly shared.
     (setq x1 '(a b c))
          ⇒ (a b c)
     (setq x2 (cons 'z (cdr x1)))
          ⇒ (z b c)
     
     ;; Replace the car of a shared link.
     (setcar (cdr x1) 'foo)
          ⇒ foo
     x1                           ; Both lists are changed.
          ⇒ (a foo c)
     x2
          ⇒ (z foo c)
     
     ;; Replace the car of a link that is not shared.
     (setcar x1 'baz)
          ⇒ baz
     x1                           ; Only one list is changed.
          ⇒ (baz foo c)
     x2
          ⇒ (z foo c)

Here is a graphical depiction of the shared structure of the two lists in the variables x1 and x2, showing why replacing b changes them both:

             --- ---        --- ---      --- ---
     x1---> |   |   |----> |   |   |--> |   |   |--> nil
             --- ---        --- ---      --- ---
              |        -->   |            |
              |       |      |            |
               --> a  |       --> b        --> c
                      |
            --- ---   |
     x2--> |   |   |--
            --- ---
             |
             |
              --> z

Here is an alternative form of box diagram, showing the same relationship:

     x1:
      --------------       --------------       --------------
     | car   | cdr  |     | car   | cdr  |     | car   | cdr  |
     |   a   |   o------->|   b   |   o------->|   c   |  nil |
     |       |      |  -->|       |      |     |       |      |
      --------------  |    --------------       --------------
                      |
     x2:              |
      --------------  |
     | car   | cdr  | |
     |   z   |   o----
     |       |      |
      --------------

5.6.2 Altering the CDR of a List

The lowest-level primitive for modifying a cdr is setcdr:

Here is an example of replacing the cdr of a list with a different list. All but the first element of the list are removed in favor of a different sequence of elements. The first element is unchanged, because it resides in the car of the list, and is not reached via the cdr.

     (setq x '(1 2 3))
          ⇒ (1 2 3)
     (setcdr x '(4))
          ⇒ (4)
     x
          ⇒ (1 4)

You can delete elements from the middle of a list by altering the cdrs of the cons cells in the list. For example, here we delete the second element, b, from the list (a b c), by changing the cdr of the first cons cell:

     (setq x1 '(a b c))
          ⇒ (a b c)
     (setcdr x1 (cdr (cdr x1)))
          ⇒ (c)
     x1
          ⇒ (a c)

Here is the result in box notation:

                        --------------------
                       |                    |
      --------------   |   --------------   |    --------------
     | car   | cdr  |  |  | car   | cdr  |   -->| car   | cdr  |
     |   a   |   o-----   |   b   |   o-------->|   c   |  nil |
     |       |      |     |       |      |      |       |      |
      --------------       --------------        --------------

The second cons cell, which previously held the element b, still exists and its car is still b, but it no longer forms part of this list.

It is equally easy to insert a new element by changing cdrs:

     (setq x1 '(a b c))
          ⇒ (a b c)
     (setcdr x1 (cons 'd (cdr x1)))
          ⇒ (d b c)
     x1
          ⇒ (a d b c)

Here is this result in box notation:

      --------------        -------------       -------------
     | car  | cdr   |      | car  | cdr  |     | car  | cdr  |
     |   a  |   o   |   -->|   b  |   o------->|   c  |  nil |
     |      |   |   |  |   |      |      |     |      |      |
      --------- | --   |    -------------       -------------
                |      |
          -----         --------
         |                      |
         |    ---------------   |
         |   | car   | cdr   |  |
          -->|   d   |   o------
             |       |       |
              ---------------

5.6.3 Functions that Rearrange Lists

Here are some functions that rearrange lists “destructively” by modifying the cdrs of their component cons cells. We call these functions “destructive” because they chew up the original lists passed to them as arguments, relinking their cons cells to form a new list that is the returned value.

See delq, in Sets And Lists, for another function that modifies cons cells.

5.7 Using Lists as Sets

A list can represent an unordered mathematical set—simply consider a value an element of a set if it appears in the list, and ignore the order of the list. To form the union of two sets, use append (as long as you don't mind having duplicate elements). You can remove equal duplicates using delete-dups. Other useful functions for sets include memq and delq, and their equal versions, member and delete.

Common Lisp note: Common Lisp has functions union (which avoids duplicate elements) and intersection for set operations. Although standard GNU Emacs Lisp does not have them, the cl-lib library provides versions. See Lists as Sets.

The delq function deletes elements from the front of the list by simply advancing down the list, and returning a sublist that starts after those elements. For example:

     (delq 'a '(a b c)) == (cdr '(a b c))

When an element to be deleted appears in the middle of the list, removing it involves changing the cdrs (see Setcdr).

     (setq sample-list '(a b c (4)))
          ⇒ (a b c (4))
     (delq 'a sample-list)
          ⇒ (b c (4))
     sample-list
          ⇒ (a b c (4))
     (delq 'c sample-list)
          ⇒ (a b (4))
     sample-list
          ⇒ (a b (4))

Note that (delq 'c sample-list) modifies sample-list to splice out the third element, but (delq 'a sample-list) does not splice anything—it just returns a shorter list. Don't assume that a variable which formerly held the argument list now has fewer elements, or that it still holds the original list! Instead, save the result of delq and use that. Most often we store the result back into the variable that held the original list:

     (setq flowers (delq 'rose flowers))

In the following example, the (4) that delq attempts to match and the (4) in the sample-list are not eq:

     (delq '(4) sample-list)
          ⇒ (a c (4))

If you want to delete elements that are equal to a given value, use delete (see below).

The following three functions are like memq, delq and remq, but use equal rather than eq to compare elements. See Equality Predicates.

Common Lisp note: The functions member, delete and remove in GNU Emacs Lisp are derived from Maclisp, not Common Lisp. The Common Lisp versions do not use equal to compare elements.

See also the function add-to-list, in List Variables, for a way to add an element to a list stored in a variable and used as a set.

5.8 Association Lists

An association list, or alist for short, records a mapping from keys to values. It is a list of cons cells called associations: the car of each cons cell is the key, and the cdr is the associated value.³

Here is an example of an alist. The key pine is associated with the value cones; the key oak is associated with acorns; and the key maple is associated with seeds.

     ((pine . cones)
      (oak . acorns)
      (maple . seeds))

Both the values and the keys in an alist may be any Lisp objects. For example, in the following alist, the symbol a is associated with the number 1, and the string "b" is associated with the list (2 3), which is the cdr of the alist element:

     ((a . 1) ("b" 2 3))

Sometimes it is better to design an alist to store the associated value in the car of the cdr of the element. Here is an example of such an alist:

     ((rose red) (lily white) (buttercup yellow))

Here we regard red as the value associated with rose. One advantage of this kind of alist is that you can store other related information—even a list of other items—in the cdr of the cdr. One disadvantage is that you cannot use rassq (see below) to find the element containing a given value. When neither of these considerations is important, the choice is a matter of taste, as long as you are consistent about it for any given alist.

The same alist shown above could be regarded as having the associated value in the cdr of the element; the value associated with rose would be the list (red).

Association lists are often used to record information that you might otherwise keep on a stack, since new associations may be added easily to the front of the list. When searching an association list for an association with a given key, the first one found is returned, if there is more than one.

In Emacs Lisp, it is not an error if an element of an association list is not a cons cell. The alist search functions simply ignore such elements. Many other versions of Lisp signal errors in such cases.

Note that property lists are similar to association lists in several respects. A property list behaves like an association list in which each key can occur only once. See Property Lists, for a comparison of property lists and association lists.

The function assoc-string is much like assoc except that it ignores certain differences between strings. See Text Comparison.

5.9 Property Lists

A property list (plist for short) is a list of paired elements. Each of the pairs associates a property name (usually a symbol) with a property or value. Here is an example of a property list:

     (pine cones numbers (1 2 3) color "blue")

This property list associates pine with cones, numbers with (1 2 3), and color with "blue". The property names and values can be any Lisp objects, but the names are usually symbols (as they are in this example).

Property lists are used in several contexts. For instance, the function put-text-property takes an argument which is a property list, specifying text properties and associated values which are to be applied to text in a string or buffer. See Text Properties.

Another prominent use of property lists is for storing symbol properties. Every symbol possesses a list of properties, used to record miscellaneous information about the symbol; these properties are stored in the form of a property list. See Symbol Properties.

• Plists and Alists: Comparison of the advantages of property lists and association lists.
• Plist Access: Accessing property lists stored elsewhere.

5.9.1 Property Lists and Association Lists

Association lists (see Association Lists) are very similar to property lists. In contrast to association lists, the order of the pairs in the property list is not significant, since the property names must be distinct.

Property lists are better than association lists for attaching information to various Lisp function names or variables. If your program keeps all such information in one association list, it will typically need to search that entire list each time it checks for an association for a particular Lisp function name or variable, which could be slow. By contrast, if you keep the same information in the property lists of the function names or variables themselves, each search will scan only the length of one property list, which is usually short. This is why the documentation for a variable is recorded in a property named variable-documentation. The byte compiler likewise uses properties to record those functions needing special treatment.

However, association lists have their own advantages. Depending on your application, it may be faster to add an association to the front of an association list than to update a property. All properties for a symbol are stored in the same property list, so there is a possibility of a conflict between different uses of a property name. (For this reason, it is a good idea to choose property names that are probably unique, such as by beginning the property name with the program's usual name-prefix for variables and functions.) An association list may be used like a stack where associations are pushed on the front of the list and later discarded; this is not possible with a property list.

5.9.2 Property Lists Outside Symbols

The following functions can be used to manipulate property lists. They all compare property names using eq.

6 Sequences, Arrays, and Vectors

The sequence type is the union of two other Lisp types: lists and arrays. In other words, any list is a sequence, and any array is a sequence. The common property that all sequences have is that each is an ordered collection of elements.

An array is a fixed-length object with a slot for each of its elements. All the elements are accessible in constant time. The four types of arrays are strings, vectors, char-tables and bool-vectors.

A list is a sequence of elements, but it is not a single primitive object; it is made of cons cells, one cell per element. Finding the nth element requires looking through n cons cells, so elements farther from the beginning of the list take longer to access. But it is possible to add elements to the list, or remove elements.

The following diagram shows the relationship between these types:

               _____________________________________________
              |                                             |
              |          Sequence                           |
              |  ______   ________________________________  |
              | |      | |                                | |
              | | List | |             Array              | |
              | |      | |    ________       ________     | |
              | |______| |   |        |     |        |    | |
              |          |   | Vector |     | String |    | |
              |          |   |________|     |________|    | |
              |          |  ____________   _____________  | |
              |          | |            | |             | | |
              |          | | Char-table | | Bool-vector | | |
              |          | |____________| |_____________| | |
              |          |________________________________| |
              |_____________________________________________|

• Sequence Functions: Functions that accept any kind of sequence.
• Arrays: Characteristics of arrays in Emacs Lisp.
• Array Functions: Functions specifically for arrays.
• Vectors: Special characteristics of Emacs Lisp vectors.
• Vector Functions: Functions specifically for vectors.
• Char-Tables: How to work with char-tables.
• Bool-Vectors: How to work with bool-vectors.
• Rings: Managing a fixed-size ring of objects.

6.1 Sequences

This section describes functions that accept any kind of sequence.

See also string-bytes, in Text Representations.

If you need to compute the width of a string on display, you should use string-width (see Width), not length, since length only counts the number of characters, but does not account for the display width of each character.

6.2 Arrays

An array object has slots that hold a number of other Lisp objects, called the elements of the array. Any element of an array may be accessed in constant time. In contrast, the time to access an element of a list is proportional to the position of that element in the list.

Emacs defines four types of array, all one-dimensional: strings (see String Type), vectors (see Vector Type), bool-vectors (see Bool-Vector Type), and char-tables (see Char-Table Type). Vectors and char-tables can hold elements of any type, but strings can only hold characters, and bool-vectors can only hold t and nil.

All four kinds of array share these characteristics:

• The first element of an array has index zero, the second element has index 1, and so on. This is called zero-origin indexing. For example, an array of four elements has indices 0, 1, 2, and 3.
• The length of the array is fixed once you create it; you cannot change the length of an existing array.
• For purposes of evaluation, the array is a constant—i.e., it evaluates to itself.
• The elements of an array may be referenced or changed with the functions aref and aset, respectively (see Array Functions).

When you create an array, other than a char-table, you must specify its length. You cannot specify the length of a char-table, because that is determined by the range of character codes.

In principle, if you want an array of text characters, you could use either a string or a vector. In practice, we always choose strings for such applications, for four reasons:

• They occupy one-fourth the space of a vector of the same elements.
• Strings are printed in a way that shows the contents more clearly as text.
• Strings can hold text properties. See Text Properties.
• Many of the specialized editing and I/O facilities of Emacs accept only strings. For example, you cannot insert a vector of characters into a buffer the way you can insert a string. See Strings and Characters.

By contrast, for an array of keyboard input characters (such as a key sequence), a vector may be necessary, because many keyboard input characters are outside the range that will fit in a string. See Key Sequence Input.

6.3 Functions that Operate on Arrays

In this section, we describe the functions that accept all types of arrays.

The general sequence functions copy-sequence and length are often useful for objects known to be arrays. See Sequence Functions.

6.4 Vectors

A vector is a general-purpose array whose elements can be any Lisp objects. (By contrast, the elements of a string can only be characters. See Strings and Characters.) Vectors are used in Emacs for many purposes: as key sequences (see Key Sequences), as symbol-lookup tables (see Creating Symbols), as part of the representation of a byte-compiled function (see Byte Compilation), and more.

Like other arrays, vectors use zero-origin indexing: the first element has index 0.

Vectors are printed with square brackets surrounding the elements. Thus, a vector whose elements are the symbols a, b and a is printed as [a b a]. You can write vectors in the same way in Lisp input.

A vector, like a string or a number, is considered a constant for evaluation: the result of evaluating it is the same vector. This does not evaluate or even examine the elements of the vector. See Self-Evaluating Forms.

Here are examples illustrating these principles:

     (setq avector [1 two '(three) "four" [five]])
          ⇒ [1 two (quote (three)) "four" [five]]
     (eval avector)
          ⇒ [1 two (quote (three)) "four" [five]]
     (eq avector (eval avector))
          ⇒ t

6.5 Functions for Vectors

Here are some functions that relate to vectors:

The append function also provides a way to convert a vector into a list with the same elements:

     (setq avector [1 two (quote (three)) "four" [five]])
          ⇒ [1 two (quote (three)) "four" [five]]
     (append avector nil)
          ⇒ (1 two (quote (three)) "four" [five])

6.6 Char-Tables

A char-table is much like a vector, except that it is indexed by character codes. Any valid character code, without modifiers, can be used as an index in a char-table. You can access a char-table's elements with aref and aset, as with any array. In addition, a char-table can have extra slots to hold additional data not associated with particular character codes. Like vectors, char-tables are constants when evaluated, and can hold elements of any type.

Each char-table has a subtype, a symbol, which serves two purposes:

• The subtype provides an easy way to tell what the char-table is for. For instance, display tables are char-tables with display-table as the subtype, and syntax tables are char-tables with syntax-table as the subtype. The subtype can be queried using the function char-table-subtype, described below.
• The subtype controls the number of extra slots in the char-table. This number is specified by the subtype's char-table-extra-slots symbol property (see Symbol Properties), whose value should be an integer between 0 and 10. If the subtype has no such symbol property, the char-table has no extra slots.

A char-table can have a parent, which is another char-table. If it does, then whenever the char-table specifies nil for a particular character c, it inherits the value specified in the parent. In other words, (aref char-table c) returns the value from the parent of char-table if char-table itself specifies nil.

A char-table can also have a default value. If so, then (aref char-table c) returns the default value whenever the char-table does not specify any other non-nil value.

There is no special function to access default values in a char-table. To do that, use char-table-range (see below).

A char-table can specify an element value for a single character code; it can also specify a value for an entire character set.

6.7 Bool-vectors

A bool-vector is much like a vector, except that it stores only the values t and nil. If you try to store any non-nil value into an element of the bool-vector, the effect is to store t there. As with all arrays, bool-vector indices start from 0, and the length cannot be changed once the bool-vector is created. Bool-vectors are constants when evaluated.

There are two special functions for working with bool-vectors; aside from that, you manipulate them with same functions used for other kinds of arrays.

Here is an example of creating, examining, and updating a bool-vector. Note that the printed form represents up to 8 boolean values as a single character.

     (setq bv (make-bool-vector 5 t))
          ⇒ #&5"^_"
     (aref bv 1)
          ⇒ t
     (aset bv 3 nil)
          ⇒ nil
     bv
          ⇒ #&5"^W"

These results make sense because the binary codes for control-_ and control-W are 11111 and 10111, respectively.

6.8 Managing a Fixed-Size Ring of Objects

A ring is a fixed-size data structure that supports insertion, deletion, rotation, and modulo-indexed reference and traversal. An efficient ring data structure is implemented by the ring package. It provides the functions listed in this section.

Note that several “rings” in Emacs, like the kill ring and the mark ring, are actually implemented as simple lists, not using the ring package; thus the following functions won't work on them.

The newest element in the ring always has index 0. Higher indices correspond to older elements. Indices are computed modulo the ring length. Index −1 corresponds to the oldest element, −2 to the next-oldest, and so forth.

If you are careful not to exceed the ring size, you can use the ring as a first-in-first-out queue. For example:

     (let ((fifo (make-ring 5)))
       (mapc (lambda (obj) (ring-insert fifo obj))
             '(0 one "two"))
       (list (ring-remove fifo) t
             (ring-remove fifo) t
             (ring-remove fifo)))
          ⇒ (0 t one t "two")

7 Hash Tables

A hash table is a very fast kind of lookup table, somewhat like an alist (see Association Lists) in that it maps keys to corresponding values. It differs from an alist in these ways:

• Lookup in a hash table is extremely fast for large tables—in fact, the time required is essentially independent of how many elements are stored in the table. For smaller tables (a few tens of elements) alists may still be faster because hash tables have a more-or-less constant overhead.
• The correspondences in a hash table are in no particular order.
• There is no way to share structure between two hash tables, the way two alists can share a common tail.

Emacs Lisp provides a general-purpose hash table data type, along with a series of functions for operating on them. Hash tables have a special printed representation, which consists of ‘#s’ followed by a list specifying the hash table properties and contents. See Creating Hash. (Note that the term “hash notation”, which refers to the initial ‘#’ character used in the printed representations of objects with no read representation, has nothing to do with the term “hash table”. See Printed Representation.)

Obarrays are also a kind of hash table, but they are a different type of object and are used only for recording interned symbols (see Creating Symbols).

• Creating Hash: Functions to create hash tables.
• Hash Access: Reading and writing the hash table contents.
• Defining Hash: Defining new comparison methods.
• Other Hash: Miscellaneous.

7.1 Creating Hash Tables

The principal function for creating a hash table is make-hash-table.

You can also create a new hash table using the printed representation for hash tables. The Lisp reader can read this printed representation, provided each element in the specified hash table has a valid read syntax (see Printed Representation). For instance, the following specifies a new hash table containing the keys key1 and key2 (both symbols) associated with val1 (a symbol) and 300 (a number) respectively.

     #s(hash-table size 30 data (key1 val1 key2 300))

The printed representation for a hash table consists of ‘#s’ followed by a list beginning with ‘hash-table’. The rest of the list should consist of zero or more property-value pairs specifying the hash table's properties and initial contents. The properties and values are read literally. Valid property names are size, test, weakness, rehash-size, rehash-threshold, and data. The data property should be a list of key-value pairs for the initial contents; the other properties have the same meanings as the matching make-hash-table keywords (:size, :test, etc.), described above.

Note that you cannot specify a hash table whose initial contents include objects that have no read syntax, such as buffers and frames. Such objects may be added to the hash table after it is created.

7.2 Hash Table Access

This section describes the functions for accessing and storing associations in a hash table. In general, any Lisp object can be used as a hash key, unless the comparison method imposes limits. Any Lisp object can also be used as the value.

7.3 Defining Hash Comparisons

You can define new methods of key lookup by means of define-hash-table-test. In order to use this feature, you need to understand how hash tables work, and what a hash code means.

You can think of a hash table conceptually as a large array of many slots, each capable of holding one association. To look up a key, gethash first computes an integer, the hash code, from the key. It reduces this integer modulo the length of the array, to produce an index in the array. Then it looks in that slot, and if necessary in other nearby slots, to see if it has found the key being sought.

Thus, to define a new method of key lookup, you need to specify both a function to compute the hash code from a key, and a function to compare two keys directly.

This example creates a hash table whose keys are strings that are compared case-insensitively.

     (defun case-fold-string= (a b)
       (eq t (compare-strings a nil nil b nil nil t)))
     (defun case-fold-string-hash (a)
       (sxhash (upcase a)))
     
     (define-hash-table-test 'case-fold
       'case-fold-string= 'case-fold-string-hash)
     
     (make-hash-table :test 'case-fold)

Here is how you could define a hash table test equivalent to the predefined test value equal. The keys can be any Lisp object, and equal-looking objects are considered the same key.

     (define-hash-table-test 'contents-hash 'equal 'sxhash)
     
     (make-hash-table :test 'contents-hash)

7.4 Other Hash Table Functions

Here are some other functions for working with hash tables.

8 Symbols

A symbol is an object with a unique name. This chapter describes symbols, their components, their property lists, and how they are created and interned. Separate chapters describe the use of symbols as variables and as function names; see Variables, and Functions. For the precise read syntax for symbols, see Symbol Type.

You can test whether an arbitrary Lisp object is a symbol with symbolp:

• Symbol Components: Symbols have names, values, function definitions and property lists.
• Definitions: A definition says how a symbol will be used.
• Creating Symbols: How symbols are kept unique.
• Symbol Properties: Each symbol has a property list for recording miscellaneous information.

8.1 Symbol Components

Each symbol has four components (or “cells”), each of which references another object:

Print name

The symbol's name.

Value

The symbol's current value as a variable.

Function

The symbol's function definition. It can also hold a symbol, a keymap, or a keyboard macro.

Property list

The symbol's property list.

The print name cell always holds a string, and cannot be changed. Each of the other three cells can be set to any Lisp object.

The print name cell holds the string that is the name of a symbol. Since symbols are represented textually by their names, it is important not to have two symbols with the same name. The Lisp reader ensures this: every time it reads a symbol, it looks for an existing symbol with the specified name before it creates a new one. To get a symbol's name, use the function symbol-name (see Creating Symbols).

The value cell holds a symbol's value as a variable, which is what you get if the symbol itself is evaluated as a Lisp expression. See Variables, for details about how values are set and retrieved, including complications such as local bindings and scoping rules. Most symbols can have any Lisp object as a value, but certain special symbols have values that cannot be changed; these include nil and t, and any symbol whose name starts with ‘:’ (those are called keywords). See Constant Variables.

The function cell holds a symbol's function definition. Often, we refer to “the function foo” when we really mean the function stored in the function cell of foo; we make the distinction explicit only when necessary. Typically, the function cell is used to hold a function (see Functions) or a macro (see Macros). However, it can also be used to hold a symbol (see Function Indirection), keyboard macro (see Keyboard Macros), keymap (see Keymaps), or autoload object (see Autoloading). To get the contents of a symbol's function cell, use the function symbol-function (see Function Cells).

The property list cell normally should hold a correctly formatted property list. To get a symbol's property list, use the function symbol-plist. See Symbol Properties.

The function cell or the value cell may be void, which means that the cell does not reference any object. (This is not the same thing as holding the symbol void, nor the same as holding the symbol nil.) Examining a function or value cell that is void results in an error, such as ‘Symbol's value as variable is void’.

Because each symbol has separate value and function cells, variables names and function names do not conflict. For example, the symbol buffer-file-name has a value (the name of the file being visited in the current buffer) as well as a function definition (a primitive function that returns the name of the file):

     buffer-file-name
          ⇒ "/gnu/elisp/symbols.texi"
     (symbol-function 'buffer-file-name)
          ⇒ #<subr buffer-file-name>

8.2 Defining Symbols

A definition is a special kind of Lisp expression that announces your intention to use a symbol in a particular way. It typically specifies a value or meaning for the symbol for one kind of use, plus documentation for its meaning when used in this way. Thus, when you define a symbol as a variable, you can supply an initial value for the variable, plus documentation for the variable.

defvar and defconst are special forms that define a symbol as a global variable—a variable that can be accessed at any point in a Lisp program. See Variables, for details about variables. To define a customizable variable, use the defcustom macro, which also calls defvar as a subroutine (see Customization).

In principle, you can assign a variable value to any symbol with setq, whether not it has first been defined as a variable. However, you ought to write a variable definition for each global variable that you want to use; otherwise, your Lisp program may not act correctly if it is evaluated with lexical scoping enabled (see Variable Scoping).

defun defines a symbol as a function, creating a lambda expression and storing it in the function cell of the symbol. This lambda expression thus becomes the function definition of the symbol. (The term “function definition”, meaning the contents of the function cell, is derived from the idea that defun gives the symbol its definition as a function.) defsubst and defalias are two other ways of defining a function. See Functions.

defmacro defines a symbol as a macro. It creates a macro object and stores it in the function cell of the symbol. Note that a given symbol can be a macro or a function, but not both at once, because both macro and function definitions are kept in the function cell, and that cell can hold only one Lisp object at any given time. See Macros.

As previously noted, Emacs Lisp allows the same symbol to be defined both as a variable (e.g., with defvar) and as a function or macro (e.g., with defun). Such definitions do not conflict.

These definition also act as guides for programming tools. For example, the C-h f and C-h v commands create help buffers containing links to the relevant variable, function, or macro definitions. See Name Help.

8.3 Creating and Interning Symbols

To understand how symbols are created in GNU Emacs Lisp, you must know how Lisp reads them. Lisp must ensure that it finds the same symbol every time it reads the same set of characters. Failure to do so would cause complete confusion.

When the Lisp reader encounters a symbol, it reads all the characters of the name. Then it “hashes” those characters to find an index in a table called an obarray. Hashing is an efficient method of looking something up. For example, instead of searching a telephone book cover to cover when looking up Jan Jones, you start with the J's and go from there. That is a simple version of hashing. Each element of the obarray is a bucket which holds all the symbols with a given hash code; to look for a given name, it is sufficient to look through all the symbols in the bucket for that name's hash code. (The same idea is used for general Emacs hash tables, but they are a different data type; see Hash Tables.)

If a symbol with the desired name is found, the reader uses that symbol. If the obarray does not contain a symbol with that name, the reader makes a new symbol and adds it to the obarray. Finding or adding a symbol with a certain name is called interning it, and the symbol is then called an interned symbol.

Interning ensures that each obarray has just one symbol with any particular name. Other like-named symbols may exist, but not in the same obarray. Thus, the reader gets the same symbols for the same names, as long as you keep reading with the same obarray.

Interning usually happens automatically in the reader, but sometimes other programs need to do it. For example, after the M-x command obtains the command name as a string using the minibuffer, it then interns the string, to get the interned symbol with that name.

No obarray contains all symbols; in fact, some symbols are not in any obarray. They are called uninterned symbols. An uninterned symbol has the same four cells as other symbols; however, the only way to gain access to it is by finding it in some other object or as the value of a variable.

Creating an uninterned symbol is useful in generating Lisp code, because an uninterned symbol used as a variable in the code you generate cannot clash with any variables used in other Lisp programs.

In Emacs Lisp, an obarray is actually a vector. Each element of the vector is a bucket; its value is either an interned symbol whose name hashes to that bucket, or 0 if the bucket is empty. Each interned symbol has an internal link (invisible to the user) to the next symbol in the bucket. Because these links are invisible, there is no way to find all the symbols in an obarray except using mapatoms (below). The order of symbols in a bucket is not significant.

In an empty obarray, every element is 0, so you can create an obarray with (make-vector length 0). This is the only valid way to create an obarray. Prime numbers as lengths tend to result in good hashing; lengths one less than a power of two are also good.

Do not try to put symbols in an obarray yourself. This does not work—only intern can enter a symbol in an obarray properly.

Common Lisp note: Unlike Common Lisp, Emacs Lisp does not provide for interning a single symbol in several obarrays.

Most of the functions below take a name and sometimes an obarray as arguments. A wrong-type-argument error is signaled if the name is not a string, or if the obarray is not a vector.

Common Lisp note: In Common Lisp, you can intern an existing symbol in an obarray. In Emacs Lisp, you cannot do this, because the argument to intern must be a string, not a symbol.

8.4 Symbol Properties

A symbol may possess any number of symbol properties, which can be used to record miscellaneous information about the symbol. For example, when a symbol has a risky-local-variable property with a non-nil value, that means the variable which the symbol names is a risky file-local variable (see File Local Variables).

Each symbol's properties and property values are stored in the symbol's property list cell (see Symbol Components), in the form of a property list (see Property Lists).

• Symbol Plists: Accessing symbol properties.
• Standard Properties: Standard meanings of symbol properties.

8.4.1 Accessing Symbol Properties

The following functions can be used to access symbol properties.

8.4.2 Standard Symbol Properties

Here, we list the symbol properties which are used for special purposes in Emacs. In the following table, whenever we say “the named function”, that means the function whose name is the relevant symbol; similarly for “the named variable” etc.

:advertised-binding

This property value specifies the preferred key binding, when showing documentation, for the named function. See Keys in Documentation.

char-table-extra-slots

The value, if non-nil, specifies the number of extra slots in the named char-table type. See Char-Tables.

customized-face

face-defface-spec

saved-face

theme-face

These properties are used to record a face's standard, saved, customized, and themed face specs. Do not set them directly; they are managed by defface and related functions. See Defining Faces.

customized-value

saved-value

standard-value

theme-value

These properties are used to record a customizable variable's standard value, saved value, customized-but-unsaved value, and themed values. Do not set them directly; they are managed by defcustom and related functions. See Variable Definitions.

disabled

If the value is non-nil, the named function is disabled as a command. See Disabling Commands.

face-documentation

The value stores the documentation string of the named face. This is set automatically by defface. See Defining Faces.

history-length

The value, if non-nil, specifies the maximum minibuffer history length for the named history list variable. See Minibuffer History.

interactive-form

The value is an interactive form for the named function. Normally, you should not set this directly; use the interactive special form instead. See Interactive Call.

menu-enable

The value is an expression for determining whether the named menu item should be enabled in menus. See Simple Menu Items.

mode-class

If the value is special, the named major mode is “special”. See Major Mode Conventions.

permanent-local

If the value is non-nil, the named variable is a buffer-local variable whose value should not be reset when changing major modes. See Creating Buffer-Local.

permanent-local-hook

If the value is non-nil, the named function should not be deleted from the local value of a hook variable when changing major modes. See Setting Hooks.

pure

This property is used internally to mark certain named functions for byte compiler optimization. Do not set it.

risky-local-variable

If the value is non-nil, the named variable is considered risky as a file-local variable. See File Local Variables.

safe-function

If the value is non-nil, the named function is considered generally safe for evaluation. See Function Safety.

safe-local-eval-function

If the value is non-nil, the named function is safe to call in file-local evaluation forms. See File Local Variables.

safe-local-variable

The value specifies a function for determining safe file-local values for the named variable. See File Local Variables.

side-effect-free

A non-nil value indicates that the named function is free of side-effects, for determining function safety (see Function Safety) as well as for byte compiler optimizations. Do not set it.

variable-documentation

If non-nil, this specifies the named vaariable's documentation string. This is set automatically by defvar and related functions. See Defining Faces.

9 Evaluation

The evaluation of expressions in Emacs Lisp is performed by the Lisp interpreter—a program that receives a Lisp object as input and computes its value as an expression. How it does this depends on the data type of the object, according to rules described in this chapter. The interpreter runs automatically to evaluate portions of your program, but can also be called explicitly via the Lisp primitive function eval.

• Intro Eval: Evaluation in the scheme of things.
• Forms: How various sorts of objects are evaluated.
• Quoting: Avoiding evaluation (to put constants in the program).
• Backquote: Easier construction of list structure.
• Eval: How to invoke the Lisp interpreter explicitly.

9.1 Introduction to Evaluation

The Lisp interpreter, or evaluator, is the part of Emacs that computes the value of an expression that is given to it. When a function written in Lisp is called, the evaluator computes the value of the function by evaluating the expressions in the function body. Thus, running any Lisp program really means running the Lisp interpreter.

A Lisp object that is intended for evaluation is called a form or expression⁴. The fact that forms are data objects and not merely text is one of the fundamental differences between Lisp-like languages and typical programming languages. Any object can be evaluated, but in practice only numbers, symbols, lists and strings are evaluated very often.

In subsequent sections, we will describe the details of what evaluation means for each kind of form.

It is very common to read a Lisp form and then evaluate the form, but reading and evaluation are separate activities, and either can be performed alone. Reading per se does not evaluate anything; it converts the printed representation of a Lisp object to the object itself. It is up to the caller of read to specify whether this object is a form to be evaluated, or serves some entirely different purpose. See Input Functions.

Evaluation is a recursive process, and evaluating a form often involves evaluating parts within that form. For instance, when you evaluate a function call form such as (car x), Emacs first evaluates the argument (the subform x). After evaluating the argument, Emacs executes the function (car), and if the function is written in Lisp, execution works by evaluating the body of the function (in this example, however, car is not a Lisp function; it is a primitive function implemented in C). See Functions, for more information about functions and function calls.

Evaluation takes place in a context called the environment, which consists of the current values and bindings of all Lisp variables (see Variables).⁵ Whenever a form refers to a variable without creating a new binding for it, the variable evaluates to the value given by the current environment. Evaluating a form may also temporarily alter the environment by binding variables (see Local Variables).

Evaluating a form may also make changes that persist; these changes are called side effects. An example of a form that produces a side effect is (setq foo 1).

Do not confuse evaluation with command key interpretation. The editor command loop translates keyboard input into a command (an interactively callable function) using the active keymaps, and then uses call-interactively to execute that command. Executing the command usually involves evaluation, if the command is written in Lisp; however, this step is not considered a part of command key interpretation. See Command Loop.

9.2 Kinds of Forms

A Lisp object that is intended to be evaluated is called a form (or an expression). How Emacs evaluates a form depends on its data type. Emacs has three different kinds of form that are evaluated differently: symbols, lists, and “all other types”. This section describes all three kinds, one by one, starting with the “all other types” which are self-evaluating forms.

• Self-Evaluating Forms: Forms that evaluate to themselves.
• Symbol Forms: Symbols evaluate as variables.
• Classifying Lists: How to distinguish various sorts of list forms.
• Function Indirection: When a symbol appears as the car of a list, we find the real function via the symbol.
• Function Forms: Forms that call functions.
• Macro Forms: Forms that call macros.
• Special Forms: "Special forms" are idiosyncratic primitives, most of them extremely important.
• Autoloading: Functions set up to load files containing their real definitions.

9.2.1 Self-Evaluating Forms

A self-evaluating form is any form that is not a list or symbol. Self-evaluating forms evaluate to themselves: the result of evaluation is the same object that was evaluated. Thus, the number 25 evaluates to 25, and the string "foo" evaluates to the string "foo". Likewise, evaluating a vector does not cause evaluation of the elements of the vector—it returns the same vector with its contents unchanged.

     '123               ; A number, shown without evaluation.
          ⇒ 123
     123                ; Evaluated as usual---result is the same.
          ⇒ 123
     (eval '123)        ; Evaluated ``by hand''---result is the same.
          ⇒ 123
     (eval (eval '123)) ; Evaluating twice changes nothing.
          ⇒ 123

It is common to write numbers, characters, strings, and even vectors in Lisp code, taking advantage of the fact that they self-evaluate. However, it is quite unusual to do this for types that lack a read syntax, because there's no way to write them textually. It is possible to construct Lisp expressions containing these types by means of a Lisp program. Here is an example:

     ;; Build an expression containing a buffer object.
     (setq print-exp (list 'print (current-buffer)))
          ⇒ (print #<buffer eval.texi>)
     ;; Evaluate it.
     (eval print-exp)
          -| #<buffer eval.texi>
          ⇒ #<buffer eval.texi>

9.2.2 Symbol Forms

When a symbol is evaluated, it is treated as a variable. The result is the variable's value, if it has one. If the symbol has no value as a variable, the Lisp interpreter signals an error. For more information on the use of variables, see Variables.

In the following example, we set the value of a symbol with setq. Then we evaluate the symbol, and get back the value that setq stored.

     (setq a 123)
          ⇒ 123
     (eval 'a)
          ⇒ 123
     a
          ⇒ 123

The symbols nil and t are treated specially, so that the value of nil is always nil, and the value of t is always t; you cannot set or bind them to any other values. Thus, these two symbols act like self-evaluating forms, even though eval treats them like any other symbol. A symbol whose name starts with ‘:’ also self-evaluates in the same way; likewise, its value ordinarily cannot be changed. See Constant Variables.

9.2.3 Classification of List Forms

A form that is a nonempty list is either a function call, a macro call, or a special form, according to its first element. These three kinds of forms are evaluated in different ways, described below. The remaining list elements constitute the arguments for the function, macro, or special form.

The first step in evaluating a nonempty list is to examine its first element. This element alone determines what kind of form the list is and how the rest of the list is to be processed. The first element is not evaluated, as it would be in some Lisp dialects such as Scheme.

9.2.4 Symbol Function Indirection

If the first element of the list is a symbol then evaluation examines the symbol's function cell, and uses its contents instead of the original symbol. If the contents are another symbol, this process, called symbol function indirection, is repeated until it obtains a non-symbol. See Function Names, for more information about symbol function indirection.

One possible consequence of this process is an infinite loop, in the event that a symbol's function cell refers to the same symbol. Or a symbol may have a void function cell, in which case the subroutine symbol-function signals a void-function error. But if neither of these things happens, we eventually obtain a non-symbol, which ought to be a function or other suitable object.

More precisely, we should now have a Lisp function (a lambda expression), a byte-code function, a primitive function, a Lisp macro, a special form, or an autoload object. Each of these types is a case described in one of the following sections. If the object is not one of these types, Emacs signals an invalid-function error.

The following example illustrates the symbol indirection process. We use fset to set the function cell of a symbol and symbol-function to get the function cell contents (see Function Cells). Specifically, we store the symbol car into the function cell of first, and the symbol first into the function cell of erste.

     ;; Build this function cell linkage:
     ;;   -------------       -----        -------        -------
     ;;  | #<subr car> | <-- | car |  <-- | first |  <-- | erste |
     ;;   -------------       -----        -------        -------
     (symbol-function 'car)
          ⇒ #<subr car>
     (fset 'first 'car)
          ⇒ car
     (fset 'erste 'first)
          ⇒ first
     (erste '(1 2 3))   ; Call the function referenced by erste.
          ⇒ 1

By contrast, the following example calls a function without any symbol function indirection, because the first element is an anonymous Lisp function, not a symbol.

     ((lambda (arg) (erste arg))
      '(1 2 3))
          ⇒ 1

Executing the function itself evaluates its body; this does involve symbol function indirection when calling erste.

This form is rarely used and is now deprecated. Instead, you should write it as:

     (funcall (lambda (arg) (erste arg))
              '(1 2 3))

or just

     (let ((arg '(1 2 3))) (erste arg))

The built-in function indirect-function provides an easy way to perform symbol function indirection explicitly.

9.2.5 Evaluation of Function Forms

If the first element of a list being evaluated is a Lisp function object, byte-code object or primitive function object, then that list is a function call. For example, here is a call to the function +:

     (+ 1 x)

The first step in evaluating a function call is to evaluate the remaining elements of the list from left to right. The results are the actual argument values, one value for each list element. The next step is to call the function with this list of arguments, effectively using the function apply (see Calling Functions). If the function is written in Lisp, the arguments are used to bind the argument variables of the function (see Lambda Expressions); then the forms in the function body are evaluated in order, and the value of the last body form becomes the value of the function call.

9.2.6 Lisp Macro Evaluation

If the first element of a list being evaluated is a macro object, then the list is a macro call. When a macro call is evaluated, the elements of the rest of the list are not initially evaluated. Instead, these elements themselves are used as the arguments of the macro. The macro definition computes a replacement form, called the expansion of the macro, to be evaluated in place of the original form. The expansion may be any sort of form: a self-evaluating constant, a symbol, or a list. If the expansion is itself a macro call, this process of expansion repeats until some other sort of form results.

Ordinary evaluation of a macro call finishes by evaluating the expansion. However, the macro expansion is not necessarily evaluated right away, or at all, because other programs also expand macro calls, and they may or may not evaluate the expansions.

Normally, the argument expressions are not evaluated as part of computing the macro expansion, but instead appear as part of the expansion, so they are computed when the expansion is evaluated.

For example, given a macro defined as follows:

     (defmacro cadr (x)
       (list 'car (list 'cdr x)))

an expression such as (cadr (assq 'handler list)) is a macro call, and its expansion is:

     (car (cdr (assq 'handler list)))

Note that the argument (assq 'handler list) appears in the expansion.

See Macros, for a complete description of Emacs Lisp macros.

9.2.7 Special Forms

A special form is a primitive function specially marked so that its arguments are not all evaluated. Most special forms define control structures or perform variable bindings—things which functions cannot do.

Each special form has its own rules for which arguments are evaluated and which are used without evaluation. Whether a particular argument is evaluated may depend on the results of evaluating other arguments.

Here is a list, in alphabetical order, of all of the special forms in Emacs Lisp with a reference to where each is described.

and

see Combining Conditions

catch

see Catch and Throw

cond

see Conditionals

condition-case

see Handling Errors

defconst

see Defining Variables

defvar

see Defining Variables

function

see Anonymous Functions

if

see Conditionals

interactive

see Interactive Call

let

let*

see Local Variables

or

see Combining Conditions

prog1

prog2

progn

see Sequencing

quote

see Quoting

save-current-buffer

see Current Buffer

save-excursion

see Excursions

save-restriction

see Narrowing

setq

see Setting Variables

setq-default

see Creating Buffer-Local

track-mouse

see Mouse Tracking

unwind-protect

see Nonlocal Exits

while

see Iteration

Common Lisp note: Here are some comparisons of special forms in GNU Emacs Lisp and Common Lisp. setq, if, and catch are special forms in both Emacs Lisp and Common Lisp. save-excursion is a special form in Emacs Lisp, but doesn't exist in Common Lisp. throw is a special form in Common Lisp (because it must be able to throw multiple values), but it is a function in Emacs Lisp (which doesn't have multiple values).

9.2.8 Autoloading

The autoload feature allows you to call a function or macro whose function definition has not yet been loaded into Emacs. It specifies which file contains the definition. When an autoload object appears as a symbol's function definition, calling that symbol as a function automatically loads the specified file; then it calls the real definition loaded from that file. The way to arrange for an autoload object to appear as a symbol's function definition is described in Autoload.

9.3 Quoting

The special form quote returns its single argument, as written, without evaluating it. This provides a way to include constant symbols and lists, which are not self-evaluating objects, in a program. (It is not necessary to quote self-evaluating objects such as numbers, strings, and vectors.)

Because quote is used so often in programs, Lisp provides a convenient read syntax for it. An apostrophe character (‘'’) followed by a Lisp object (in read syntax) expands to a list whose first element is quote, and whose second element is the object. Thus, the read syntax 'x is an abbreviation for (quote x).

Here are some examples of expressions that use quote:

     (quote (+ 1 2))
          ⇒ (+ 1 2)
     (quote foo)
          ⇒ foo
     'foo
          ⇒ foo
     ''foo
          ⇒ (quote foo)
     '(quote foo)
          ⇒ (quote foo)
     ['foo]
          ⇒ [(quote foo)]

Other quoting constructs include function (see Anonymous Functions), which causes an anonymous lambda expression written in Lisp to be compiled, and ‘`’ (see Backquote), which is used to quote only part of a list, while computing and substituting other parts.

9.4 Backquote

Backquote constructs allow you to quote a list, but selectively evaluate elements of that list. In the simplest case, it is identical to the special form quote (described in the previous section; see Quoting). For example, these two forms yield identical results:

     `(a list of (+ 2 3) elements)
          ⇒ (a list of (+ 2 3) elements)
     '(a list of (+ 2 3) elements)
          ⇒ (a list of (+ 2 3) elements)

The special marker ‘,’ inside of the argument to backquote indicates a value that isn't constant. The Emacs Lisp evaluator evaluates the argument of ‘,’, and puts the value in the list structure:

     `(a list of ,(+ 2 3) elements)
          ⇒ (a list of 5 elements)

Substitution with ‘,’ is allowed at deeper levels of the list structure also. For example:

     `(1 2 (3 ,(+ 4 5)))
          ⇒ (1 2 (3 9))

You can also splice an evaluated value into the resulting list, using the special marker ‘,@’. The elements of the spliced list become elements at the same level as the other elements of the resulting list. The equivalent code without using ‘`’ is often unreadable. Here are some examples:

     (setq some-list '(2 3))
          ⇒ (2 3)
     (cons 1 (append some-list '(4) some-list))
          ⇒ (1 2 3 4 2 3)
     `(1 ,@some-list 4 ,@some-list)
          ⇒ (1 2 3 4 2 3)
     
     (setq list '(hack foo bar))
          ⇒ (hack foo bar)
     (cons 'use
       (cons 'the
         (cons 'words (append (cdr list) '(as elements)))))
          ⇒ (use the words foo bar as elements)
     `(use the words ,@(cdr list) as elements)
          ⇒ (use the words foo bar as elements)

9.5 Eval

Most often, forms are evaluated automatically, by virtue of their occurrence in a program being run. On rare occasions, you may need to write code that evaluates a form that is computed at run time, such as after reading a form from text being edited or getting one from a property list. On these occasions, use the eval function. Often eval is not needed and something else should be used instead. For example, to get the value of a variable, while eval works, symbol-value is preferable; or rather than store expressions in a property list that then need to go through eval, it is better to store functions instead that are then passed to funcall.

The functions and variables described in this section evaluate forms, specify limits to the evaluation process, or record recently returned values. Loading a file also does evaluation (see Loading).

It is generally cleaner and more flexible to store a function in a data structure, and call it with funcall or apply, than to store an expression in the data structure and evaluate it. Using functions provides the ability to pass information to them as arguments.

10 Control Structures

A Lisp program consists of a set of expressions, or forms (see Forms). We control the order of execution of these forms by enclosing them in control structures. Control structures are special forms which control when, whether, or how many times to execute the forms they contain.

The simplest order of execution is sequential execution: first form a, then form b, and so on. This is what happens when you write several forms in succession in the body of a function, or at top level in a file of Lisp code—the forms are executed in the order written. We call this textual order. For example, if a function body consists of two forms a and b, evaluation of the function evaluates first a and then b. The result of evaluating b becomes the value of the function.

Explicit control structures make possible an order of execution other than sequential.

Emacs Lisp provides several kinds of control structure, including other varieties of sequencing, conditionals, iteration, and (controlled) jumps—all discussed below. The built-in control structures are special forms since their subforms are not necessarily evaluated or not evaluated sequentially. You can use macros to define your own control structure constructs (see Macros).

• Sequencing: Evaluation in textual order.
• Conditionals: if, cond, when, unless.
• Combining Conditions: and, or, not.
• Iteration: while loops.
• Nonlocal Exits: Jumping out of a sequence.

10.1 Sequencing

Evaluating forms in the order they appear is the most common way control passes from one form to another. In some contexts, such as in a function body, this happens automatically. Elsewhere you must use a control structure construct to do this: progn, the simplest control construct of Lisp.

A progn special form looks like this:

     (progn a b c ...)

and it says to execute the forms a, b, c, and so on, in that order. These forms are called the body of the progn form. The value of the last form in the body becomes the value of the entire progn. (progn) returns nil.

In the early days of Lisp, progn was the only way to execute two or more forms in succession and use the value of the last of them. But programmers found they often needed to use a progn in the body of a function, where (at that time) only one form was allowed. So the body of a function was made into an “implicit progn”: several forms are allowed just as in the body of an actual progn. Many other control structures likewise contain an implicit progn. As a result, progn is not used as much as it was many years ago. It is needed now most often inside an unwind-protect, and, or, or in the then-part of an if.

Two other constructs likewise evaluate a series of forms but return different values:

10.2 Conditionals

Conditional control structures choose among alternatives. Emacs Lisp has four conditional forms: if, which is much the same as in other languages; when and unless, which are variants of if; and cond, which is a generalized case statement.

Any conditional construct can be expressed with cond or with if. Therefore, the choice between them is a matter of style. For example:

     (if a b c)
     ==
     (cond (a b) (t c))

• Pattern matching case statement

10.2.1 Pattern matching case statement

To compare a particular value against various possible cases, the macro pcase can come handy. It takes the following form:

     (pcase exp branch1 branch2 branch3 ...)

where each branch takes the form (upattern body-forms...).

It will first evaluate exp and then compare the value against each upattern to see which branch to use, after which it will run the corresponding body-forms. A common use case is to distinguish between a few different constant values:

     (pcase (get-return-code x)
       (`success       (message "Done!"))
       (`would-block   (message "Sorry, can't do it now"))
       (`read-only     (message "The shmliblick is read-only"))
       (`access-denied (message "You do not have the needed rights"))
       (code           (message "Unknown return code %S" code)))

In the last clause, code is a variable that gets bound to the value that was returned by (get-return-code x).

To give a more complex example, a simple interpreter for a little expression language could look like:

     (defun evaluate (exp env)
       (pcase exp
         (`(add ,x ,y)         (+ (evaluate x env) (evaluate y env)))
         (`(call ,fun ,arg)    (funcall (evaluate fun) (evaluate arg env)))
         (`(fn ,arg ,body)     (lambda (val)
                                 (evaluate body (cons (cons arg val) env))))
         ((pred numberp)       exp)
         ((pred symbolp)       (cdr (assq exp env)))
         (_                    (error "Unknown expression %S" exp))))

Where `(add ,x ,y) is a pattern that checks that exp is a three element list starting with the symbol add, then extracts the second and third elements and binds them to the variables x and y. (pred numberp) is a pattern that simply checks that exp is a number, and _ is the catch-all pattern that matches anything.

There are two kinds of patterns involved in pcase, called U-patterns and Q-patterns. The upattern mentioned above are U-patterns and can take the following forms:

`qpattern

This is one of the most common form of patterns. The intention is to mimic the backquote macro: this pattern matches those values that could have been built by such a backquote expression. Since we're pattern matching rather than building a value, the unquote does not indicate where to plug an expression, but instead it lets one specify a U-pattern that should match the value at that location.

More specifically, a Q-pattern can take the following forms:

(qpattern1 . qpattern2)

This pattern matches any cons cell whose car matches QPATTERN1 and whose cdr matches PATTERN2.

atom

This pattern matches any atom equal to atom.

,upattern

This pattern matches any object that matches the upattern.

symbol

A mere symbol in a U-pattern matches anything, and additionally let-binds this symbol to the value that it matched, so that you can later refer to it, either in the body-forms or also later in the pattern.

_

This so-called don't care pattern matches anything, like the previous one, but unless symbol patterns it does not bind any variable.

(pred pred)

This pattern matches if the function pred returns non-nil when called with the object being matched.

(or upattern1 upattern2...)

This pattern matches as soon as one of the argument patterns succeeds. All argument patterns should let-bind the same variables.

(and upattern1 upattern2...)

This pattern matches only if all the argument patterns succeed.

(guard exp)

This pattern ignores the object being examined and simply succeeds if exp evaluates to non-nil and fails otherwise. It is typically used inside an and pattern. For example, (and x (guard (< x 10))) is a pattern which matches any number smaller than 10 and let-binds it to the variable x.

10.3 Constructs for Combining Conditions

This section describes three constructs that are often used together with if and cond to express complicated conditions. The constructs and and or can also be used individually as kinds of multiple conditional constructs.

10.4 Iteration

Iteration means executing part of a program repetitively. For example, you might want to repeat some computation once for each element of a list, or once for each integer from 0 to n. You can do this in Emacs Lisp with the special form while:

The dolist and dotimes macros provide convenient ways to write two common kinds of loops.

10.5 Nonlocal Exits

A nonlocal exit is a transfer of control from one point in a program to another remote point. Nonlocal exits can occur in Emacs Lisp as a result of errors; you can also use them under explicit control. Nonlocal exits unbind all variable bindings made by the constructs being exited.

• Catch and Throw: Nonlocal exits for the program's own purposes.
• Examples of Catch: Showing how such nonlocal exits can be written.
• Errors: How errors are signaled and handled.
• Cleanups: Arranging to run a cleanup form if an error happens.

10.5.1 Explicit Nonlocal Exits: catch and throw

Most control constructs affect only the flow of control within the construct itself. The function throw is the exception to this rule of normal program execution: it performs a nonlocal exit on request. (There are other exceptions, but they are for error handling only.) throw is used inside a catch, and jumps back to that catch. For example:

     (defun foo-outer ()
       (catch 'foo
         (foo-inner)))
     
     (defun foo-inner ()
       ...
       (if x
           (throw 'foo t))
       ...)

The throw form, if executed, transfers control straight back to the corresponding catch, which returns immediately. The code following the throw is not executed. The second argument of throw is used as the return value of the catch.

The function throw finds the matching catch based on the first argument: it searches for a catch whose first argument is eq to the one specified in the throw. If there is more than one applicable catch, the innermost one takes precedence. Thus, in the above example, the throw specifies foo, and the catch in foo-outer specifies the same symbol, so that catch is the applicable one (assuming there is no other matching catch in between).

Executing throw exits all Lisp constructs up to the matching catch, including function calls. When binding constructs such as let or function calls are exited in this way, the bindings are unbound, just as they are when these constructs exit normally (see Local Variables). Likewise, throw restores the buffer and position saved by save-excursion (see Excursions), and the narrowing status saved by save-restriction. It also runs any cleanups established with the unwind-protect special form when it exits that form (see Cleanups).

The throw need not appear lexically within the catch that it jumps to. It can equally well be called from another function called within the catch. As long as the throw takes place chronologically after entry to the catch, and chronologically before exit from it, it has access to that catch. This is why throw can be used in commands such as exit-recursive-edit that throw back to the editor command loop (see Recursive Editing).

Common Lisp note: Most other versions of Lisp, including Common Lisp, have several ways of transferring control nonsequentially: return, return-from, and go, for example. Emacs Lisp has only throw. The cl-lib library provides versions of some of these. See Blocks and Exits.

10.5.2 Examples of catch and throw

One way to use catch and throw is to exit from a doubly nested loop. (In most languages, this would be done with a “goto”.) Here we compute (foo i j) for i and j varying from 0 to 9:

     (defun search-foo ()
       (catch 'loop
         (let ((i 0))
           (while (< i 10)
             (let ((j 0))
               (while (< j 10)
                 (if (foo i j)
                     (throw 'loop (list i j)))
                 (setq j (1+ j))))
             (setq i (1+ i))))))

If foo ever returns non-nil, we stop immediately and return a list of i and j. If foo always returns nil, the catch returns normally, and the value is nil, since that is the result of the while.

Here are two tricky examples, slightly different, showing two return points at once. First, two return points with the same tag, hack:

     (defun catch2 (tag)
       (catch tag
         (throw 'hack 'yes)))
     ⇒ catch2
     
     (catch 'hack
       (print (catch2 'hack))
       'no)
     -| yes
     ⇒ no

Since both return points have tags that match the throw, it goes to the inner one, the one established in catch2. Therefore, catch2 returns normally with value yes, and this value is printed. Finally the second body form in the outer catch, which is 'no, is evaluated and returned from the outer catch.

Now let's change the argument given to catch2:

     (catch 'hack
       (print (catch2 'quux))
       'no)
     ⇒ yes

We still have two return points, but this time only the outer one has the tag hack; the inner one has the tag quux instead. Therefore, throw makes the outer catch return the value yes. The function print is never called, and the body-form 'no is never evaluated.

10.5.3 Errors

When Emacs Lisp attempts to evaluate a form that, for some reason, cannot be evaluated, it signals an error.

When an error is signaled, Emacs's default reaction is to print an error message and terminate execution of the current command. This is the right thing to do in most cases, such as if you type C-f at the end of the buffer.

In complicated programs, simple termination may not be what you want. For example, the program may have made temporary changes in data structures, or created temporary buffers that should be deleted before the program is finished. In such cases, you would use unwind-protect to establish cleanup expressions to be evaluated in case of error. (See Cleanups.) Occasionally, you may wish the program to continue execution despite an error in a subroutine. In these cases, you would use condition-case to establish error handlers to recover control in case of error.

Resist the temptation to use error handling to transfer control from one part of the program to another; use catch and throw instead. See Catch and Throw.

• Signaling Errors: How to report an error.
• Processing of Errors: What Emacs does when you report an error.
• Handling Errors: How you can trap errors and continue execution.
• Error Symbols: How errors are classified for trapping them.

10.5.3.1 How to Signal an Error

Signaling an error means beginning error processing. Error processing normally aborts all or part of the running program and returns to a point that is set up to handle the error (see Processing of Errors). Here we describe how to signal an error.

Most errors are signaled “automatically” within Lisp primitives which you call for other purposes, such as if you try to take the car of an integer or move forward a character at the end of the buffer. You can also signal errors explicitly with the functions error and signal.

Quitting, which happens when the user types C-g, is not considered an error, but it is handled almost like an error. See Quitting.

Every error specifies an error message, one way or another. The message should state what is wrong (“File does not exist”), not how things ought to be (“File must exist”). The convention in Emacs Lisp is that error messages should start with a capital letter, but should not end with any sort of punctuation.

Common Lisp note: Emacs Lisp has nothing like the Common Lisp concept of continuable errors.

10.5.3.2 How Emacs Processes Errors

When an error is signaled, signal searches for an active handler for the error. A handler is a sequence of Lisp expressions designated to be executed if an error happens in part of the Lisp program. If the error has an applicable handler, the handler is executed, and control resumes following the handler. The handler executes in the environment of the condition-case that established it; all functions called within that condition-case have already been exited, and the handler cannot return to them.

If there is no applicable handler for the error, it terminates the current command and returns control to the editor command loop. (The command loop has an implicit handler for all kinds of errors.) The command loop's handler uses the error symbol and associated data to print an error message. You can use the variable command-error-function to control how this is done:

An error that has no explicit handler may call the Lisp debugger. The debugger is enabled if the variable debug-on-error (see Error Debugging) is non-nil. Unlike error handlers, the debugger runs in the environment of the error, so that you can examine values of variables precisely as they were at the time of the error.

10.5.3.3 Writing Code to Handle Errors

The usual effect of signaling an error is to terminate the command that is running and return immediately to the Emacs editor command loop. You can arrange to trap errors occurring in a part of your program by establishing an error handler, with the special form condition-case. A simple example looks like this:

     (condition-case nil
         (delete-file filename)
       (error nil))

This deletes the file named filename, catching any error and returning nil if an error occurs. (You can use the macro ignore-errors for a simple case like this; see below.)

The condition-case construct is often used to trap errors that are predictable, such as failure to open a file in a call to insert-file-contents. It is also used to trap errors that are totally unpredictable, such as when the program evaluates an expression read from the user.

The second argument of condition-case is called the protected form. (In the example above, the protected form is a call to delete-file.) The error handlers go into effect when this form begins execution and are deactivated when this form returns. They remain in effect for all the intervening time. In particular, they are in effect during the execution of functions called by this form, in their subroutines, and so on. This is a good thing, since, strictly speaking, errors can be signaled only by Lisp primitives (including signal and error) called by the protected form, not by the protected form itself.

The arguments after the protected form are handlers. Each handler lists one or more condition names (which are symbols) to specify which errors it will handle. The error symbol specified when an error is signaled also defines a list of condition names. A handler applies to an error if they have any condition names in common. In the example above, there is one handler, and it specifies one condition name, error, which covers all errors.

The search for an applicable handler checks all the established handlers starting with the most recently established one. Thus, if two nested condition-case forms offer to handle the same error, the inner of the two gets to handle it.

If an error is handled by some condition-case form, this ordinarily prevents the debugger from being run, even if debug-on-error says this error should invoke the debugger.

If you want to be able to debug errors that are caught by a condition-case, set the variable debug-on-signal to a non-nil value. You can also specify that a particular handler should let the debugger run first, by writing debug among the conditions, like this:

     (condition-case nil
         (delete-file filename)
       ((debug error) nil))

The effect of debug here is only to prevent condition-case from suppressing the call to the debugger. Any given error will invoke the debugger only if debug-on-error and the other usual filtering mechanisms say it should. See Error Debugging.

Once Emacs decides that a certain handler handles the error, it returns control to that handler. To do so, Emacs unbinds all variable bindings made by binding constructs that are being exited, and executes the cleanups of all unwind-protect forms that are being exited. Once control arrives at the handler, the body of the handler executes normally.

After execution of the handler body, execution returns from the condition-case form. Because the protected form is exited completely before execution of the handler, the handler cannot resume execution at the point of the error, nor can it examine variable bindings that were made within the protected form. All it can do is clean up and proceed.

Error signaling and handling have some resemblance to throw and catch (see Catch and Throw), but they are entirely separate facilities. An error cannot be caught by a catch, and a throw cannot be handled by an error handler (though using throw when there is no suitable catch signals an error that can be handled).

Here is an example of using condition-case to handle the error that results from dividing by zero. The handler displays the error message (but without a beep), then returns a very large number.

     (defun safe-divide (dividend divisor)
       (condition-case err
           ;; Protected form.
           (/ dividend divisor)
         ;; The handler.
         (arith-error                        ; Condition.
          ;; Display the usual message for this error.
          (message "%s" (error-message-string err))
          1000000)))
     ⇒ safe-divide
     
     (safe-divide 5 0)
          -| Arithmetic error: (arith-error)
     ⇒ 1000000

The handler specifies condition name arith-error so that it will handle only division-by-zero errors. Other kinds of errors will not be handled (by this condition-case). Thus:

     (safe-divide nil 3)
          error--> Wrong type argument: number-or-marker-p, nil

Here is a condition-case that catches all kinds of errors, including those from error:

     (setq baz 34)
          ⇒ 34
     
     (condition-case err
         (if (eq baz 35)
             t
           ;; This is a call to the function error.
           (error "Rats!  The variable %s was %s, not 35" 'baz baz))
       ;; This is the handler; it is not a form.
       (error (princ (format "The error was: %s" err))
              2))
     -| The error was: (error "Rats!  The variable baz was 34, not 35")
     ⇒ 2

10.5.3.4 Error Symbols and Condition Names

When you signal an error, you specify an error symbol to specify the kind of error you have in mind. Each error has one and only one error symbol to categorize it. This is the finest classification of errors defined by the Emacs Lisp language.

These narrow classifications are grouped into a hierarchy of wider classes called error conditions, identified by condition names. The narrowest such classes belong to the error symbols themselves: each error symbol is also a condition name. There are also condition names for more extensive classes, up to the condition name error which takes in all kinds of errors (but not quit). Thus, each error has one or more condition names: error, the error symbol if that is distinct from error, and perhaps some intermediate classifications.

In order for a symbol to be an error symbol, it must have an error-conditions property which gives a list of condition names. This list defines the conditions that this kind of error belongs to. (The error symbol itself, and the symbol error, should always be members of this list.) Thus, the hierarchy of condition names is defined by the error-conditions properties of the error symbols. Because quitting is not considered an error, the value of the error-conditions property of quit is just (quit).

In addition to the error-conditions list, the error symbol should have an error-message property whose value is a string to be printed when that error is signaled but not handled. If the error symbol has no error-message property or if the error-message property exists, but is not a string, the error message ‘peculiar error’ is used. See Definition of signal.

Here is how we define a new error symbol, new-error:

     (put 'new-error
          'error-conditions
          '(error my-own-errors new-error))
     ⇒ (error my-own-errors new-error)
     (put 'new-error 'error-message "A new error")
     ⇒ "A new error"

This error has three condition names: new-error, the narrowest classification; my-own-errors, which we imagine is a wider classification; and error, which is the widest of all.

The error string should start with a capital letter but it should not end with a period. This is for consistency with the rest of Emacs.

Naturally, Emacs will never signal new-error on its own; only an explicit call to signal (see Definition of signal) in your code can do this:

     (signal 'new-error '(x y))
          error--> A new error: x, y

This error can be handled through any of the three condition names. This example handles new-error and any other errors in the class my-own-errors:

     (condition-case foo
         (bar nil t)
       (my-own-errors nil))

The significant way that errors are classified is by their condition names—the names used to match errors with handlers. An error symbol serves only as a convenient way to specify the intended error message and list of condition names. It would be cumbersome to give signal a list of condition names rather than one error symbol.

By contrast, using only error symbols without condition names would seriously decrease the power of condition-case. Condition names make it possible to categorize errors at various levels of generality when you write an error handler. Using error symbols alone would eliminate all but the narrowest level of classification.

See Standard Errors, for a list of the main error symbols and their conditions.

10.5.4 Cleaning Up from Nonlocal Exits

The unwind-protect construct is essential whenever you temporarily put a data structure in an inconsistent state; it permits you to make the data consistent again in the event of an error or throw. (Another more specific cleanup construct that is used only for changes in buffer contents is the atomic change group; Atomic Changes.)

For example, here we make an invisible buffer for temporary use, and make sure to kill it before finishing:

     (let ((buffer (get-buffer-create " *temp*")))
       (with-current-buffer buffer
         (unwind-protect
             body-form
           (kill-buffer buffer))))

You might think that we could just as well write (kill-buffer (current-buffer)) and dispense with the variable buffer. However, the way shown above is safer, if body-form happens to get an error after switching to a different buffer! (Alternatively, you could write a save-current-buffer around body-form, to ensure that the temporary buffer becomes current again in time to kill it.)

Emacs includes a standard macro called with-temp-buffer which expands into more or less the code shown above (see Current Buffer). Several of the macros defined in this manual use unwind-protect in this way.

Here is an actual example derived from an FTP package. It creates a process (see Processes) to try to establish a connection to a remote machine. As the function ftp-login is highly susceptible to numerous problems that the writer of the function cannot anticipate, it is protected with a form that guarantees deletion of the process in the event of failure. Otherwise, Emacs might fill up with useless subprocesses.

     (let ((win nil))
       (unwind-protect
           (progn
             (setq process (ftp-setup-buffer host file))
             (if (setq win (ftp-login process host user password))
                 (message "Logged in")
               (error "Ftp login failed")))
         (or win (and process (delete-process process)))))

This example has a small bug: if the user types C-g to quit, and the quit happens immediately after the function ftp-setup-buffer returns but before the variable process is set, the process will not be killed. There is no easy way to fix this bug, but at least it is very unlikely.

11 Variables

A variable is a name used in a program to stand for a value. In Lisp, each variable is represented by a Lisp symbol (see Symbols). The variable name is simply the symbol's name, and the variable's value is stored in the symbol's value cell⁶. See Symbol Components. In Emacs Lisp, the use of a symbol as a variable is independent of its use as a function name.

As previously noted in this manual, a Lisp program is represented primarily by Lisp objects, and only secondarily as text. The textual form of a Lisp program is given by the read syntax of the Lisp objects that constitute the program. Hence, the textual form of a variable in a Lisp program is written using the read syntax for the symbol representing the variable.

• Global Variables: Variable values that exist permanently, everywhere.
• Constant Variables: Certain "variables" have values that never change.
• Local Variables: Variable values that exist only temporarily.
• Void Variables: Symbols that lack values.
• Defining Variables: A definition says a symbol is used as a variable.
• Tips for Defining: Things you should think about when you define a variable.
• Accessing Variables: Examining values of variables whose names are known only at run time.
• Setting Variables: Storing new values in variables.
• Variable Scoping: How Lisp chooses among local and global values.
• Buffer-Local Variables: Variable values in effect only in one buffer.
• File Local Variables: Handling local variable lists in files.
• Directory Local Variables: Local variables common to all files in a directory.
• Variable Aliases: Variables that are aliases for other variables.
• Variables with Restricted Values: Non-constant variables whose value can not be an arbitrary Lisp object.
• Generalized Variables: Extending the concept of variables.

11.1 Global Variables

The simplest way to use a variable is globally. This means that the variable has just one value at a time, and this value is in effect (at least for the moment) throughout the Lisp system. The value remains in effect until you specify a new one. When a new value replaces the old one, no trace of the old value remains in the variable.

You specify a value for a symbol with setq. For example,

     (setq x '(a b))

gives the variable x the value (a b). Note that setq is a special form (see Special Forms); it does not evaluate its first argument, the name of the variable, but it does evaluate the second argument, the new value.

Once the variable has a value, you can refer to it by using the symbol itself as an expression. Thus,

     x ⇒ (a b)

assuming the setq form shown above has already been executed.

If you do set the same variable again, the new value replaces the old one:

     x
          ⇒ (a b)
     (setq x 4)
          ⇒ 4
     x
          ⇒ 4

11.2 Variables that Never Change

In Emacs Lisp, certain symbols normally evaluate to themselves. These include nil and t, as well as any symbol whose name starts with ‘:’ (these are called keywords). These symbols cannot be rebound, nor can their values be changed. Any attempt to set or bind nil or t signals a setting-constant error. The same is true for a keyword (a symbol whose name starts with ‘:’), if it is interned in the standard obarray, except that setting such a symbol to itself is not an error.

     nil == 'nil
          ⇒ nil
     (setq nil 500)
     error--> Attempt to set constant symbol: nil

These constants are fundamentally different from the “constants” defined using the defconst special form (see Defining Variables). A defconst form serves to inform human readers that you do not intend to change the value of a variable, but Emacs does not raise an error if you actually change it.

11.3 Local Variables

Global variables have values that last until explicitly superseded with new values. Sometimes it is useful to give a variable a local value—a value that takes effect only within a certain part of a Lisp program. When a variable has a local value, we say that it is locally bound to that value, and that it is a local variable.

For example, when a function is called, its argument variables receive local values, which are the actual arguments supplied to the function call; these local bindings take effect within the body of the function. To take another example, the let special form explicitly establishes local bindings for specific variables, which take effect within the body of the let form.

We also speak of the global binding, which is where (conceptually) the global value is kept.

Establishing a local binding saves away the variable's previous value (or lack of one). We say that the previous value is shadowed. Both global and local values may be shadowed. If a local binding is in effect, using setq on the local variable stores the specified value in the local binding. When that local binding is no longer in effect, the previously shadowed value (or lack of one) comes back.

A variable can have more than one local binding at a time (e.g., if there are nested let forms that bind the variable). The current binding is the local binding that is actually in effect. It determines the value returned by evaluating the variable symbol, and it is the binding acted on by setq.

For most purposes, you can think of the current binding as the “innermost” local binding, or the global binding if there is no local binding. To be more precise, a rule called the scoping rule determines where in a program a local binding takes effect. The default scoping rule in Emacs Lisp is called dynamic scoping, which simply states that the current binding at any given point in the execution of a program is the most recently-created binding for that variable that still exists. For details about dynamic scoping, and an alternative scoping rule called lexical scoping, See Variable Scoping.

The special forms let and let* exist to create local bindings:

Here is a complete list of the other facilities that create local bindings:

• Function calls (see Functions).
• Macro calls (see Macros).
• condition-case (see Errors).

Variables can also have buffer-local bindings (see Buffer-Local Variables); a few variables have terminal-local bindings (see Multiple Terminals). These kinds of bindings work somewhat like ordinary local bindings, but they are localized depending on “where” you are in Emacs.

11.4 When a Variable is “Void”

We say that a variable is void if its symbol has an unassigned value cell (see Symbol Components). Under Emacs Lisp's default dynamic binding rules (see Variable Scoping), the value cell stores the variable's current (local or global) value. Note that an unassigned value cell is not the same as having nil in the value cell. The symbol nil is a Lisp object and can be the value of a variable, just as any other object can be; but it is still a value. If a variable is void, trying to evaluate the variable signals a void-variable error rather than a value.

Under lexical binding rules, the value cell only holds the variable's global value, i.e., the value outside of any lexical binding construct. When a variable is lexically bound, the local value is determined by the lexical environment; the variable may have a local value if its symbol's value cell is unassigned.

11.5 Defining Global Variables

A variable definition is a construct that announces your intention to use a symbol as a global variable. It uses the special forms defvar or defconst, which are documented below.

A variable definition serves three purposes. First, it informs people who read the code that the symbol is intended to be used a certain way (as a variable). Second, it informs the Lisp system of this, optionally supplying an initial value and a documentation string. Third, it provides information to programming tools such as etags, allowing them to find where the variable was defined.

The difference between defconst and defvar is mainly a matter of intent, serving to inform human readers of whether the value should ever change. Emacs Lisp does not actually prevent you from changing the value of a variable defined with defconst. One notable difference between the two forms is that defconst unconditionally initializes the variable, whereas defvar initializes it only if it is originally void.

To define a customizable variable, you should use defcustom (which calls defvar as a subroutine). See Variable Definitions.

Warning: If you use a defconst or defvar special form while the variable has a local binding (made with let, or a function argument), it sets the local binding rather than the global binding. This is not what you usually want. To prevent this, use these special forms at top level in a file, where normally no local binding is in effect, and make sure to load the file before making a local binding for the variable.

11.6 Tips for Defining Variables Robustly

When you define a variable whose value is a function, or a list of functions, use a name that ends in ‘-function’ or ‘-functions’, respectively.

There are several other variable name conventions; here is a complete list:

‘...-hook’

The variable is a normal hook (see Hooks).

‘...-function’

The value is a function.

‘...-functions’

The value is a list of functions.

‘...-form’

The value is a form (an expression).

‘...-forms’

The value is a list of forms (expressions).

‘...-predicate’

The value is a predicate—a function of one argument that returns non-nil for “good” arguments and nil for “bad” arguments.

‘...-flag’

The value is significant only as to whether it is nil or not. Since such variables often end up acquiring more values over time, this convention is not strongly recommended.

‘...-program’

The value is a program name.

‘...-command’

The value is a whole shell command.

‘...-switches’

The value specifies options for a command.

When you define a variable, always consider whether you should mark it as “safe” or “risky”; see File Local Variables.

When defining and initializing a variable that holds a complicated value (such as a keymap with bindings in it), it's best to put the entire computation of the value into the defvar, like this:

     (defvar my-mode-map
       (let ((map (make-sparse-keymap)))
         (define-key map "\C-c\C-a" 'my-command)
         ...
         map)
       docstring)

This method has several benefits. First, if the user quits while loading the file, the variable is either still uninitialized or initialized properly, never in-between. If it is still uninitialized, reloading the file will initialize it properly. Second, reloading the file once the variable is initialized will not alter it; that is important if the user has run hooks to alter part of the contents (such as, to rebind keys). Third, evaluating the defvar form with C-M-x will reinitialize the map completely.

Putting so much code in the defvar form has one disadvantage: it puts the documentation string far away from the line which names the variable. Here's a safe way to avoid that:

     (defvar my-mode-map nil
       docstring)
     (unless my-mode-map
       (let ((map (make-sparse-keymap)))
         (define-key map "\C-c\C-a" 'my-command)
         ...
         (setq my-mode-map map)))

This has all the same advantages as putting the initialization inside the defvar, except that you must type C-M-x twice, once on each form, if you do want to reinitialize the variable.

11.7 Accessing Variable Values

The usual way to reference a variable is to write the symbol which names it. See Symbol Forms.

Occasionally, you may want to reference a variable which is only determined at run time. In that case, you cannot specify the variable name in the text of the program. You can use the symbol-value function to extract the value.

11.8 Setting Variable Values

The usual way to change the value of a variable is with the special form setq. When you need to compute the choice of variable at run time, use the function set.

11.9 Scoping Rules for Variable Bindings

When you create a local binding for a variable, that binding takes effect only within a limited portion of the program (see Local Variables). This section describes exactly what this means.

Each local binding has a certain scope and extent. Scope refers to where in the textual source code the binding can be accessed. Extent refers to when, as the program is executing, the binding exists.

By default, the local bindings that Emacs creates are dynamic bindings. Such a binding has indefinite scope, meaning that any part of the program can potentially access the variable binding. It also has dynamic extent, meaning that the binding lasts only while the binding construct (such as the body of a let form) is being executed.

Emacs can optionally create lexical bindings. A lexical binding has lexical scope, meaning that any reference to the variable must be located textually within the binding construct. It also has indefinite extent, meaning that under some circumstances the binding can live on even after the binding construct has finished executing, by means of special objects called closures.

The following subsections describe dynamic binding and lexical binding in greater detail, and how to enable lexical binding in Emacs Lisp programs.

• Dynamic Binding: The default for binding local variables in Emacs.
• Dynamic Binding Tips: Avoiding problems with dynamic binding.
• Lexical Binding: A different type of local variable binding.
• Using Lexical Binding: How to enable lexical binding.

11.9.1 Dynamic Binding

By default, the local variable bindings made by Emacs are dynamic bindings. When a variable is dynamically bound, its current binding at any point in the execution of the Lisp program is simply the most recently-created dynamic local binding for that symbol, or the global binding if there is no such local binding.

Dynamic bindings have indefinite scope and dynamic extent, as shown by the following example:

     (defvar x -99)  ; x receives an initial value of -99.
     
     (defun getx ()
       x)            ; x is used ``free'' in this function.
     
     (let ((x 1))    ; x is dynamically bound.
       (getx))
          ⇒ 1
     
     ;; After the let form finishes, x reverts to its
     ;; previous value, which is -99.
     
     (getx)
          ⇒ -99

The function getx refers to x. This is a “free” reference, in the sense that there is no binding for x within that defun construct itself. When we call getx from within a let form in which x is (dynamically) bound, it retrieves the local value of x (i.e., 1). But when we call getx outside the let form, it retrieves the global value of x (i.e., -99).

Here is another example, which illustrates setting a dynamically bound variable using setq:

     (defvar x -99)      ; x receives an initial value of -99.
     
     (defun addx ()
       (setq x (1+ x)))  ; Add 1 to x and return its new value.
     
     (let ((x 1))
       (addx)
       (addx))
          ⇒ 3           ; The two addx calls add to x twice.
     
     ;; After the let form finishes, x reverts to its
     ;; previous value, which is -99.
     
     (addx)
          ⇒ -98

Dynamic binding is implemented in Emacs Lisp in a simple way. Each symbol has a value cell, which specifies its current dynamic value (or absence of value). See Symbol Components. When a symbol is given a dynamic local binding, Emacs records the contents of the value cell (or absence thereof) in a stack, and stores the new local value in the value cell. When the binding construct finishes executing, Emacs pops the old value off the stack, and puts it in the value cell.

11.9.2 Proper Use of Dynamic Binding

Dynamic binding is a powerful feature, as it allows programs to refer to variables that are not defined within their local textual scope. However, if used without restraint, this can also make programs hard to understand. There are two clean ways to use this technique:

• If a variable has no global definition, use it as a local variable only within a binding construct, e.g., the body of the let form where the variable was bound, or the body of the function for an argument variable. If this convention is followed consistently throughout a program, the value of the variable will not affect, nor be affected by, any uses of the same variable symbol elsewhere in the program.
• Otherwise, define the variable with defvar, defconst, or defcustom. See Defining Variables. Usually, the definition should be at top-level in an Emacs Lisp file. As far as possible, it should include a documentation string which explains the meaning and purpose of the variable. You should also choose the variable's name to avoid name conflicts (see Coding Conventions).

  Then you can bind the variable anywhere in a program, knowing reliably what the effect will be. Wherever you encounter the variable, it will be easy to refer back to the definition, e.g., via the C-h v command (provided the variable definition has been loaded into Emacs). See Name Help.

  For example, it is common to use local bindings for customizable variables like case-fold-search:

            (defun search-for-abc ()
              "Search for the string \"abc\", ignoring case differences."
              (let ((case-fold-search nil))
                (re-search-forward "abc")))
  

11.9.3 Lexical Binding

Optionally, you can create lexical bindings in Emacs Lisp. A lexically bound variable has lexical scope, meaning that any reference to the variable must be located textually within the binding construct.

Here is an example (see Using Lexical Binding, for how to actually enable lexical binding):

     (let ((x 1))    ; x is lexically bound.
       (+ x 3))
          ⇒ 4
     
     (defun getx ()
       x)            ; x is used ``free'' in this function.
     
     (let ((x 1))    ; x is lexically bound.
       (getx))
     error--> Symbol's value as variable is void: x

Here, the variable x has no global value. When it is lexically bound within a let form, it can be used in the textual confines of that let form. But it can not be used from within a getx function called from the let form, since the function definition of getx occurs outside the let form itself.

Here is how lexical binding works. Each binding construct defines a lexical environment, specifying the symbols that are bound within the construct and their local values. When the Lisp evaluator wants the current value of a variable, it looks first in the lexical environment; if the variable is not specified in there, it looks in the symbol's value cell, where the dynamic value is stored.

Lexical bindings have indefinite extent. Even after a binding construct has finished executing, its lexical environment can be “kept around” in Lisp objects called closures. A closure is created when you define a named or anonymous function with lexical binding enabled. See Closures, for details.

When a closure is called as a function, any lexical variable references within its definition use the retained lexical environment. Here is an example:

     (defvar my-ticker nil)   ; We will use this dynamically bound
                              ; variable to store a closure.
     
     (let ((x 0))             ; x is lexically bound.
       (setq my-ticker (lambda ()
                         (setq x (1+ x)))))
         ⇒ (closure ((x . 0) t) ()
               (1+ x))
     
     (funcall my-ticker)
         ⇒ 1
     
     (funcall my-ticker)
         ⇒ 2
     
     (funcall my-ticker)
         ⇒ 3
     
     x                        ; Note that x has no global value.
     error--> Symbol's value as variable is void: x

The let binding defines a lexical environment in which the variable x is locally bound to 0. Within this binding construct, we define a lambda expression which increments x by one and returns the incremented value. This lambda expression is automatically turned into a closure, in which the lexical environment lives on even after the let binding construct has exited. Each time we evaluate the closure, it increments x, using the binding of x in that lexical environment.

Note that functions like symbol-value, boundp, and set only retrieve or modify a variable's dynamic binding (i.e., the contents of its symbol's value cell). Also, the code in the body of a defun or defmacro cannot refer to surrounding lexical variables.

Currently, lexical binding is not much used within the Emacs sources. However, we expect its importance to increase in the future. Lexical binding opens up a lot more opportunities for optimization, so Emacs Lisp code that makes use of lexical binding is likely to run faster in future Emacs versions. Such code is also much more friendly to concurrency, which we want to add to Emacs in the near future.

11.9.4 Using Lexical Binding

When loading an Emacs Lisp file or evaluating a Lisp buffer, lexical binding is enabled if the buffer-local variable lexical-binding is non-nil:

When evaluating Emacs Lisp code directly using an eval call, lexical binding is enabled if the lexical argument to eval is non-nil. See Eval.

Even when lexical binding is enabled, certain variables will continue to be dynamically bound. These are called special variables. Every variable that has been defined with defvar, defcustom or defconst is a special variable (see Defining Variables). All other variables are subject to lexical binding.

The use of a special variable as a formal argument in a function is discouraged. Doing so gives rise to unspecified behavior when lexical binding mode is enabled (it may use lexical binding sometimes, and dynamic binding other times).

Converting an Emacs Lisp program to lexical binding is pretty easy. First, add a file-local variable setting of lexical-binding to t in the Emacs Lisp source file. Second, check that every variable in the program which needs to be dynamically bound has a variable definition, so that it is not inadvertently bound lexically.

A simple way to find out which variables need a variable definition is to byte-compile the source file. See Byte Compilation. If a non-special variable is used outside of a let form, the byte-compiler will warn about reference or assignment to a “free variable”. If a non-special variable is bound but not used within a let form, the byte-compiler will warn about an “unused lexical variable”. The byte-compiler will also issue a warning if you use a special variable as a function argument.

(To silence byte-compiler warnings about unused variables, just use a variable name that start with an underscore. The byte-compiler interprets this as an indication that this is a variable known not to be used.)

11.10 Buffer-Local Variables

Global and local variable bindings are found in most programming languages in one form or another. Emacs, however, also supports additional, unusual kinds of variable binding, such as buffer-local bindings, which apply only in one buffer. Having different values for a variable in different buffers is an important customization method. (Variables can also have bindings that are local to each terminal. See Multiple Terminals.)

• Intro to Buffer-Local: Introduction and concepts.
• Creating Buffer-Local: Creating and destroying buffer-local bindings.
• Default Value: The default value is seen in buffers that don't have their own buffer-local values.

11.10.1 Introduction to Buffer-Local Variables

A buffer-local variable has a buffer-local binding associated with a particular buffer. The binding is in effect when that buffer is current; otherwise, it is not in effect. If you set the variable while a buffer-local binding is in effect, the new value goes in that binding, so its other bindings are unchanged. This means that the change is visible only in the buffer where you made it.

The variable's ordinary binding, which is not associated with any specific buffer, is called the default binding. In most cases, this is the global binding.

A variable can have buffer-local bindings in some buffers but not in other buffers. The default binding is shared by all the buffers that don't have their own bindings for the variable. (This includes all newly-created buffers.) If you set the variable in a buffer that does not have a buffer-local binding for it, this sets the default binding, so the new value is visible in all the buffers that see the default binding.

The most common use of buffer-local bindings is for major modes to change variables that control the behavior of commands. For example, C mode and Lisp mode both set the variable paragraph-start to specify that only blank lines separate paragraphs. They do this by making the variable buffer-local in the buffer that is being put into C mode or Lisp mode, and then setting it to the new value for that mode. See Major Modes.

The usual way to make a buffer-local binding is with make-local-variable, which is what major mode commands typically use. This affects just the current buffer; all other buffers (including those yet to be created) will continue to share the default value unless they are explicitly given their own buffer-local bindings.

A more powerful operation is to mark the variable as automatically buffer-local by calling make-variable-buffer-local. You can think of this as making the variable local in all buffers, even those yet to be created. More precisely, the effect is that setting the variable automatically makes the variable local to the current buffer if it is not already so. All buffers start out by sharing the default value of the variable as usual, but setting the variable creates a buffer-local binding for the current buffer. The new value is stored in the buffer-local binding, leaving the default binding untouched. This means that the default value cannot be changed with setq in any buffer; the only way to change it is with setq-default.

Warning: When a variable has buffer-local bindings in one or more buffers, let rebinds the binding that's currently in effect. For instance, if the current buffer has a buffer-local value, let temporarily rebinds that. If no buffer-local bindings are in effect, let rebinds the default value. If inside the let you then change to a different current buffer in which a different binding is in effect, you won't see the let binding any more. And if you exit the let while still in the other buffer, you won't see the unbinding occur (though it will occur properly). Here is an example to illustrate:

     (setq foo 'g)
     (set-buffer "a")
     (make-local-variable 'foo)
     (setq foo 'a)
     (let ((foo 'temp))
       ;; foo ⇒ 'temp  ; let binding in buffer ‘a’
       (set-buffer "b")
       ;; foo ⇒ 'g     ; the global value since foo is not local in ‘b’
       body...)
     foo ⇒ 'g        ; exiting restored the local value in buffer ‘a’,
                      ; but we don't see that in buffer ‘b’
     (set-buffer "a") ; verify the local value was restored
     foo ⇒ 'a

Note that references to foo in body access the buffer-local binding of buffer ‘b’.

When a file specifies local variable values, these become buffer-local values when you visit the file. See File Variables.

A buffer-local variable cannot be made terminal-local (see Multiple Terminals).

11.10.2 Creating and Deleting Buffer-Local Bindings

A buffer-local variable is permanent if the variable name (a symbol) has a permanent-local property that is non-nil. Such variables are unaffected by kill-all-local-variables, and their local bindings are therefore not cleared by changing major modes. Permanent locals are appropriate for data pertaining to where the file came from or how to save it, rather than with how to edit the contents.

11.10.3 The Default Value of a Buffer-Local Variable

The global value of a variable with buffer-local bindings is also called the default value, because it is the value that is in effect whenever neither the current buffer nor the selected frame has its own binding for the variable.

The functions default-value and setq-default access and change a variable's default value regardless of whether the current buffer has a buffer-local binding. For example, you could use setq-default to change the default setting of paragraph-start for most buffers; and this would work even when you are in a C or Lisp mode buffer that has a buffer-local value for this variable.

The special forms defvar and defconst also set the default value (if they set the variable at all), rather than any buffer-local value.

11.11 File Local Variables

A file can specify local variable values; Emacs uses these to create buffer-local bindings for those variables in the buffer visiting that file. See Local Variables in Files, for basic information about file-local variables. This section describes the functions and variables that affect how file-local variables are processed.

If a file-local variable could specify an arbitrary function or Lisp expression that would be called later, visiting a file could take over your Emacs. Emacs protects against this by automatically setting only those file-local variables whose specified values are known to be safe. Other file-local variables are set only if the user agrees.

For additional safety, read-circle is temporarily bound to nil when Emacs reads file-local variables (see Input Functions). This prevents the Lisp reader from recognizing circular and shared Lisp structures (see Circular Objects).

You can specify safe values for a variable with a safe-local-variable property. The property has to be a function of one argument; any value is safe if the function returns non-nil given that value. Many commonly-encountered file variables have safe-local-variable properties; these include fill-column, fill-prefix, and indent-tabs-mode. For boolean-valued variables that are safe, use booleanp as the property value. Lambda expressions should be quoted so that describe-variable can display the predicate.

When defining a user option using defcustom, you can set its safe-local-variable property by adding the arguments :safe function to defcustom (see Variable Definitions).

Some variables are considered risky. If a variable is risky, it is never entered automatically into safe-local-variable-values; Emacs always queries before setting a risky variable, unless the user explicitly allows a value by customizing safe-local-variable-values directly.

Any variable whose name has a non-nil risky-local-variable property is considered risky. When you define a user option using defcustom, you can set its risky-local-variable property by adding the arguments :risky value to defcustom (see Variable Definitions). In addition, any variable whose name ends in any of ‘-command’, ‘-frame-alist’, ‘-function’, ‘-functions’, ‘-hook’, ‘-hooks’, ‘-form’, ‘-forms’, ‘-map’, ‘-map-alist’, ‘-mode-alist’, ‘-program’, or ‘-predicate’ is automatically considered risky. The variables ‘font-lock-keywords’, ‘font-lock-keywords’ followed by a digit, and ‘font-lock-syntactic-keywords’ are also considered risky.

The ‘Eval:’ “variable” is also a potential loophole, so Emacs normally asks for confirmation before handling it.

If the expression is a function call and the function has a safe-local-eval-function property, the property value determines whether the expression is safe to evaluate. The property value can be a predicate to call to test the expression, a list of such predicates (it's safe if any predicate succeeds), or t (always safe provided the arguments are constant).

Text properties are also potential loopholes, since their values could include functions to call. So Emacs discards all text properties from string values specified for file-local variables.

11.12 Directory Local Variables

A directory can specify local variable values common to all files in that directory; Emacs uses these to create buffer-local bindings for those variables in buffers visiting any file in that directory. This is useful when the files in the directory belong to some project and therefore share the same local variables.

There are two different methods for specifying directory local variables: by putting them in a special file, or by defining a project class for that directory.

11.13 Variable Aliases

It is sometimes useful to make two variables synonyms, so that both variables always have the same value, and changing either one also changes the other. Whenever you change the name of a variable—either because you realize its old name was not well chosen, or because its meaning has partly changed—it can be useful to keep the old name as an alias of the new one for compatibility. You can do this with defvaralias.

Variable aliases are convenient for replacing an old name for a variable with a new name. make-obsolete-variable declares that the old name is obsolete and therefore that it may be removed at some stage in the future.

You can make two variables synonyms and declare one obsolete at the same time using the macro define-obsolete-variable-alias.

     (defvaralias 'foo 'bar)
     (indirect-variable 'foo)
          ⇒ bar
     (indirect-variable 'bar)
          ⇒ bar
     (setq bar 2)
     bar
          ⇒ 2
     foo
          ⇒ 2
     (setq foo 0)
     bar
          ⇒ 0
     foo
          ⇒ 0

11.14 Variables with Restricted Values

Ordinary Lisp variables can be assigned any value that is a valid Lisp object. However, certain Lisp variables are not defined in Lisp, but in C. Most of these variables are defined in the C code using DEFVAR_LISP. Like variables defined in Lisp, these can take on any value. However, some variables are defined using DEFVAR_INT or DEFVAR_BOOL. See Writing Emacs Primitives, in particular the description of functions of the type syms_of_filename, for a brief discussion of the C implementation.

Variables of type DEFVAR_BOOL can only take on the values nil or t. Attempting to assign them any other value will set them to t:

     (let ((display-hourglass 5))
       display-hourglass)
          ⇒ t

Variables of type DEFVAR_INT can only take on integer values. Attempting to assign them any other value will result in an error:

     (setq undo-limit 1000.0)
     error--> Wrong type argument: integerp, 1000.0

11.15 Generalized Variables

A generalized variable or place form is one of the many places in Lisp memory where values can be stored. The simplest place form is a regular Lisp variable. But the cars and cdrs of lists, elements of arrays, properties of symbols, and many other locations are also places where Lisp values are stored.

Generalized variables are analogous to “lvalues” in the C language, where ‘x = a[i]’ gets an element from an array and ‘a[i] = x’ stores an element using the same notation. Just as certain forms like a[i] can be lvalues in C, there is a set of forms that can be generalized variables in Lisp.

• Setting Generalized Variables: The setf macro.
• Adding Generalized Variables: Defining new setf forms.

11.15.1 The setf Macro

The setf macro is the most basic way to operate on generalized variables. The setf form is like setq, except that it accepts arbitrary place forms on the left side rather than just symbols. For example, (setf (car a) b) sets the car of a to b, doing the same operation as (setcar a b), but without having to remember two separate functions for setting and accessing every type of place.

The following Lisp forms will work as generalized variables, and so may appear in the place argument of setf:

• A symbol naming a variable. In other words, (setf x y) is exactly equivalent to (setq x y), and setq itself is strictly speaking redundant given that setf exists. Many programmers continue to prefer setq for setting simple variables, though, purely for stylistic or historical reasons. The macro (setf x y) actually expands to (setq x y), so there is no performance penalty for using it in compiled code.
• A call to any of the following standard Lisp functions:

            aref      cddr      symbol-function
            car       elt       symbol-plist
            caar      get       symbol-value
            cadr      gethash
            cdr       nth
            cdar      nthcdr
  

• A call to any of the following Emacs-specific functions:

            default-value                 process-get
            frame-parameter               process-sentinel
            terminal-parameter            window-buffer
            keymap-parent                 window-display-table
            match-data                    window-dedicated-p
            overlay-get                   window-hscroll
            overlay-start                 window-parameter
            overlay-end                   window-point
            process-buffer                window-start
            process-filter
  

setf signals an error if you pass a place form that it does not know how to handle.

Note that for nthcdr, the list argument of the function must itself be a valid place form. For example, (setf (nthcdr 0 foo) 7) will set foo itself to 7.

The macros push (see List Variables) and pop (see List Elements) can manipulate generalized variables, not just lists. (pop place) removes and returns the first element of the list stored in place. It is analogous to (prog1 (car place) (setf place (cdr place))), except that it takes care to evaluate all subforms only once. (push x place) inserts x at the front of the list stored in place. It is analogous to (setf place (cons x place)), except for evaluation of the subforms. Note that push and pop on an nthcdr place can be used to insert or delete at any position in a list.

The cl-lib library defines various extensions for generalized variables, including additional setf places. See Generalized Variables.

11.15.2 Defining new setf forms

This section describes how to define new forms that setf can operate on.

For more control over the expansion, see the macro gv-define-expander. The macro gv-letplace can be useful in defining macros that perform similarly to setf; for example, the incf macro of Common Lisp. Consult the source file gv.el for more details.

Common Lisp note: Common Lisp defines another way to specify the setf behavior of a function, namely “setf functions”, whose names are lists (setf name) rather than symbols. For example, (defun (setf foo) ...) defines the function that is used when setf is applied to foo. Emacs does not support this. It is a compile-time error to use setf on a form that has not already had an appropriate expansion defined. In Common Lisp, this is not an error since the function (setf func) might be defined later.

12 Functions

A Lisp program is composed mainly of Lisp functions. This chapter explains what functions are, how they accept arguments, and how to define them.

• What Is a Function: Lisp functions vs. primitives; terminology.
• Lambda Expressions: How functions are expressed as Lisp objects.
• Function Names: A symbol can serve as the name of a function.
• Defining Functions: Lisp expressions for defining functions.
• Calling Functions: How to use an existing function.
• Mapping Functions: Applying a function to each element of a list, etc.
• Anonymous Functions: Lambda expressions are functions with no names.
• Function Cells: Accessing or setting the function definition of a symbol.
• Closures: Functions that enclose a lexical environment.
• Obsolete Functions: Declaring functions obsolete.
• Inline Functions: Functions that the compiler will expand inline.
• Declare Form: Adding additional information about a function.
• Declaring Functions: Telling the compiler that a function is defined.
• Function Safety: Determining whether a function is safe to call.
• Related Topics: Cross-references to specific Lisp primitives that have a special bearing on how functions work.

12.1 What Is a Function?

In a general sense, a function is a rule for carrying out a computation given input values called arguments. The result of the computation is called the value or return value of the function. The computation can also have side effects, such as lasting changes in the values of variables or the contents of data structures.

In most computer languages, every function has a name. But in Lisp, a function in the strictest sense has no name: it is an object which can optionally be associated with a symbol (e.g., car) that serves as the function name. See Function Names. When a function has been given a name, we usually also refer to that symbol as a “function” (e.g., we refer to “the function car”). In this manual, the distinction between a function name and the function object itself is usually unimportant, but we will take note wherever it is relevant.

Certain function-like objects, called special forms and macros, also accept arguments to carry out computations. However, as explained below, these are not considered functions in Emacs Lisp.

Here are important terms for functions and function-like objects:

lambda expression

A function (in the strict sense, i.e., a function object) which is written in Lisp. These are described in the following section. See Lambda Expressions.

primitive

A function which is callable from Lisp but is actually written in C. Primitives are also called built-in functions, or subrs. Examples include functions like car and append. In addition, all special forms (see below) are also considered primitives.

Usually, a function is implemented as a primitive because it is a fundamental part of Lisp (e.g., car), or because it provides a low-level interface to operating system services, or because it needs to run fast. Unlike functions defined in Lisp, primitives can be modified or added only by changing the C sources and recompiling Emacs. See Writing Emacs Primitives.

special form

A primitive that is like a function but does not evaluate all of its arguments in the usual way. It may evaluate only some of the arguments, or may evaluate them in an unusual order, or several times. Examples include if, and, and while. See Special Forms.

macro

A construct defined in Lisp, which differs from a function in that it translates a Lisp expression into another expression which is to be evaluated instead of the original expression. Macros enable Lisp programmers to do the sorts of things that special forms can do. See Macros.

command

An object which can be invoked via the command-execute primitive, usually due to the user typing in a key sequence bound to that command. See Interactive Call. A command is usually a function; if the function is written in Lisp, it is made into a command by an interactive form in the function definition (see Defining Commands). Commands that are functions can also be called from Lisp expressions, just like other functions.

Keyboard macros (strings and vectors) are commands also, even though they are not functions. See Keyboard Macros. We say that a symbol is a command if its function cell contains a command (see Symbol Components); such a named command can be invoked with M-x.

closure

A function object that is much like a lambda expression, except that it also encloses an “environment” of lexical variable bindings. See Closures.

byte-code function

A function that has been compiled by the byte compiler. See Byte-Code Type.

autoload object

A place-holder for a real function. If the autoload object is called, Emacs loads the file containing the definition of the real function, and then calls the real function. See Autoload.

You can use the function functionp to test if an object is a function:

Unlike functionp, the next three functions do not treat a symbol as its function definition.

12.2 Lambda Expressions

A lambda expression is a function object written in Lisp. Here is an example:

     (lambda (x)
       "Return the hyperbolic cosine of X."
       (* 0.5 (+ (exp x) (exp (- x)))))

In Emacs Lisp, such a list is valid as an expression—it evaluates to itself. But its main use is not to be evaluated as an expression, but to be called as a function.

A lambda expression, by itself, has no name; it is an anonymous function. Although lambda expressions can be used this way (see Anonymous Functions), they are more commonly associated with symbols to make named functions (see Function Names). Before going into these details, the following subsections describe the components of a lambda expression and what they do.

• Lambda Components: The parts of a lambda expression.
• Simple Lambda: A simple example.
• Argument List: Details and special features of argument lists.
• Function Documentation: How to put documentation in a function.

12.2.1 Components of a Lambda Expression

A lambda expression is a list that looks like this:

     (lambda (arg-variables...)
       [documentation-string]
       [interactive-declaration]
       body-forms...)

The first element of a lambda expression is always the symbol lambda. This indicates that the list represents a function. The reason functions are defined to start with lambda is so that other lists, intended for other uses, will not accidentally be valid as functions.

The second element is a list of symbols—the argument variable names. This is called the lambda list. When a Lisp function is called, the argument values are matched up against the variables in the lambda list, which are given local bindings with the values provided. See Local Variables.

The documentation string is a Lisp string object placed within the function definition to describe the function for the Emacs help facilities. See Function Documentation.

The interactive declaration is a list of the form (interactive code-string). This declares how to provide arguments if the function is used interactively. Functions with this declaration are called commands; they can be called using M-x or bound to a key. Functions not intended to be called in this way should not have interactive declarations. See Defining Commands, for how to write an interactive declaration.

The rest of the elements are the body of the function: the Lisp code to do the work of the function (or, as a Lisp programmer would say, “a list of Lisp forms to evaluate”). The value returned by the function is the value returned by the last element of the body.

12.2.2 A Simple Lambda Expression Example

Consider the following example:

     (lambda (a b c) (+ a b c))

We can call this function by passing it to funcall, like this:

     (funcall (lambda (a b c) (+ a b c))
              1 2 3)

This call evaluates the body of the lambda expression with the variable a bound to 1, b bound to 2, and c bound to 3. Evaluation of the body adds these three numbers, producing the result 6; therefore, this call to the function returns the value 6.

Note that the arguments can be the results of other function calls, as in this example:

     (funcall (lambda (a b c) (+ a b c))
              1 (* 2 3) (- 5 4))

This evaluates the arguments 1, (* 2 3), and (- 5 4) from left to right. Then it applies the lambda expression to the argument values 1, 6 and 1 to produce the value 8.

As these examples show, you can use a form with a lambda expression as its car to make local variables and give them values. In the old days of Lisp, this technique was the only way to bind and initialize local variables. But nowadays, it is clearer to use the special form let for this purpose (see Local Variables). Lambda expressions are mainly used as anonymous functions for passing as arguments to other functions (see Anonymous Functions), or stored as symbol function definitions to produce named functions (see Function Names).

12.2.3 Other Features of Argument Lists

Our simple sample function, (lambda (a b c) (+ a b c)), specifies three argument variables, so it must be called with three arguments: if you try to call it with only two arguments or four arguments, you get a wrong-number-of-arguments error.

It is often convenient to write a function that allows certain arguments to be omitted. For example, the function substring accepts three arguments—a string, the start index and the end index—but the third argument defaults to the length of the string if you omit it. It is also convenient for certain functions to accept an indefinite number of arguments, as the functions list and + do.

To specify optional arguments that may be omitted when a function is called, simply include the keyword &optional before the optional arguments. To specify a list of zero or more extra arguments, include the keyword &rest before one final argument.

Thus, the complete syntax for an argument list is as follows:

     (required-vars...
      [&optional optional-vars...]
      [&rest rest-var])

The square brackets indicate that the &optional and &rest clauses, and the variables that follow them, are optional.

A call to the function requires one actual argument for each of the required-vars. There may be actual arguments for zero or more of the optional-vars, and there cannot be any actual arguments beyond that unless the lambda list uses &rest. In that case, there may be any number of extra actual arguments.

If actual arguments for the optional and rest variables are omitted, then they always default to nil. There is no way for the function to distinguish between an explicit argument of nil and an omitted argument. However, the body of the function is free to consider nil an abbreviation for some other meaningful value. This is what substring does; nil as the third argument to substring means to use the length of the string supplied.

Common Lisp note: Common Lisp allows the function to specify what default value to use when an optional argument is omitted; Emacs Lisp always uses nil. Emacs Lisp does not support “supplied-p” variables that tell you whether an argument was explicitly passed.

For example, an argument list that looks like this:

     (a b &optional c d &rest e)

binds a and b to the first two actual arguments, which are required. If one or two more arguments are provided, c and d are bound to them respectively; any arguments after the first four are collected into a list and e is bound to that list. If there are only two arguments, c is nil; if two or three arguments, d is nil; if four arguments or fewer, e is nil.

There is no way to have required arguments following optional ones—it would not make sense. To see why this must be so, suppose that c in the example were optional and d were required. Suppose three actual arguments are given; which variable would the third argument be for? Would it be used for the c, or for d? One can argue for both possibilities. Similarly, it makes no sense to have any more arguments (either required or optional) after a &rest argument.

Here are some examples of argument lists and proper calls:

     (funcall (lambda (n) (1+ n))        ; One required:
              1)                         ; requires exactly one argument.
          ⇒ 2
     (funcall (lambda (n &optional n1)   ; One required and one optional:
                (if n1 (+ n n1) (1+ n))) ; 1 or 2 arguments.
              1 2)
          ⇒ 3
     (funcall (lambda (n &rest ns)       ; One required and one rest:
                (+ n (apply '+ ns)))     ; 1 or more arguments.
              1 2 3 4 5)
          ⇒ 15

12.2.4 Documentation Strings of Functions

A lambda expression may optionally have a documentation string just after the lambda list. This string does not affect execution of the function; it is a kind of comment, but a systematized comment which actually appears inside the Lisp world and can be used by the Emacs help facilities. See Documentation, for how the documentation string is accessed.

It is a good idea to provide documentation strings for all the functions in your program, even those that are called only from within your program. Documentation strings are like comments, except that they are easier to access.

The first line of the documentation string should stand on its own, because apropos displays just this first line. It should consist of one or two complete sentences that summarize the function's purpose.

The start of the documentation string is usually indented in the source file, but since these spaces come before the starting double-quote, they are not part of the string. Some people make a practice of indenting any additional lines of the string so that the text lines up in the program source. That is a mistake. The indentation of the following lines is inside the string; what looks nice in the source code will look ugly when displayed by the help commands.

You may wonder how the documentation string could be optional, since there are required components of the function that follow it (the body). Since evaluation of a string returns that string, without any side effects, it has no effect if it is not the last form in the body. Thus, in practice, there is no confusion between the first form of the body and the documentation string; if the only body form is a string then it serves both as the return value and as the documentation.

The last line of the documentation string can specify calling conventions different from the actual function arguments. Write text like this:

     \(fn arglist)

following a blank line, at the beginning of the line, with no newline following it inside the documentation string. (The ‘\’ is used to avoid confusing the Emacs motion commands.) The calling convention specified in this way appears in help messages in place of the one derived from the actual arguments of the function.

This feature is particularly useful for macro definitions, since the arguments written in a macro definition often do not correspond to the way users think of the parts of the macro call.

12.3 Naming a Function

A symbol can serve as the name of a function. This happens when the symbol's function cell (see Symbol Components) contains a function object (e.g., a lambda expression). Then the symbol itself becomes a valid, callable function, equivalent to the function object in its function cell.

The contents of the function cell are also called the symbol's function definition. The procedure of using a symbol's function definition in place of the symbol is called symbol function indirection; see Function Indirection. If you have not given a symbol a function definition, its function cell is said to be void, and it cannot be used as a function.

In practice, nearly all functions have names, and are referred to by their names. You can create a named Lisp function by defining a lambda expression and putting it in a function cell (see Function Cells). However, it is more common to use the defun special form, described in the next section. See Defining Functions.

We give functions names because it is convenient to refer to them by their names in Lisp expressions. Also, a named Lisp function can easily refer to itself—it can be recursive. Furthermore, primitives can only be referred to textually by their names, since primitive function objects (see Primitive Function Type) have no read syntax.

A function need not have a unique name. A given function object usually appears in the function cell of only one symbol, but this is just a convention. It is easy to store it in several symbols using fset; then each of the symbols is a valid name for the same function.

Note that a symbol used as a function name may also be used as a variable; these two uses of a symbol are independent and do not conflict. (This is not the case in some dialects of Lisp, like Scheme.)

12.4 Defining Functions

We usually give a name to a function when it is first created. This is called defining a function, and it is done with the defun macro.

You cannot create a new primitive function with defun or defalias, but you can use them to change the function definition of any symbol, even one such as car or x-popup-menu whose normal definition is a primitive. However, this is risky: for instance, it is next to impossible to redefine car without breaking Lisp completely. Redefining an obscure function such as x-popup-menu is less dangerous, but it still may not work as you expect. If there are calls to the primitive from C code, they call the primitive's C definition directly, so changing the symbol's definition will have no effect on them.

See also defsubst, which defines a function like defun and tells the Lisp compiler to perform inline expansion on it. See Inline Functions.

12.5 Calling Functions

Defining functions is only half the battle. Functions don't do anything until you call them, i.e., tell them to run. Calling a function is also known as invocation.

The most common way of invoking a function is by evaluating a list. For example, evaluating the list (concat "a" "b") calls the function concat with arguments "a" and "b". See Evaluation, for a description of evaluation.

When you write a list as an expression in your program, you specify which function to call, and how many arguments to give it, in the text of the program. Usually that's just what you want. Occasionally you need to compute at run time which function to call. To do that, use the function funcall. When you also need to determine at run time how many arguments to pass, use apply.

Sometimes it is useful to fix some of the function's arguments at certain values, and leave the rest of arguments for when the function is actually called. The act of fixing some of the function's arguments is called partial application of the function⁸. The result is a new function that accepts the rest of arguments and calls the original function with all the arguments combined.

Here's how to do partial application in Emacs Lisp:

It is common for Lisp functions to accept functions as arguments or find them in data structures (especially in hook variables and property lists) and call them using funcall or apply. Functions that accept function arguments are often called functionals.

Sometimes, when you call a functional, it is useful to supply a no-op function as the argument. Here are two different kinds of no-op function:

Some functions are user-visible commands, which can be called interactively (usually by a key sequence). It is possible to invoke such a command exactly as though it was called interactively, by using the call-interactively function. See Interactive Call.

12.6 Mapping Functions

A mapping function applies a given function (not a special form or macro) to each element of a list or other collection. Emacs Lisp has several such functions; this section describes mapcar, mapc, and mapconcat, which map over a list. See Definition of mapatoms, for the function mapatoms which maps over the symbols in an obarray. See Definition of maphash, for the function maphash which maps over key/value associations in a hash table.

These mapping functions do not allow char-tables because a char-table is a sparse array whose nominal range of indices is very large. To map over a char-table in a way that deals properly with its sparse nature, use the function map-char-table (see Char-Tables).

12.7 Anonymous Functions

Although functions are usually defined with defun and given names at the same time, it is sometimes convenient to use an explicit lambda expression—an anonymous function. Anonymous functions are valid wherever function names are. They are often assigned as variable values, or as arguments to functions; for instance, you might pass one as the function argument to mapcar, which applies that function to each element of a list (see Mapping Functions). See describe-symbols example, for a realistic example of this.

When defining a lambda expression that is to be used as an anonymous function, you can in principle use any method to construct the list. But typically you should use the lambda macro, or the function special form, or the #' read syntax:

The read syntax #' is a short-hand for using function. The following forms are all equivalent:

     (lambda (x) (* x x))
     (function (lambda (x) (* x x)))
     #'(lambda (x) (* x x))

In the following example, we define a change-property function that takes a function as its third argument, followed by a double-property function that makes use of change-property by passing it an anonymous function:

     (defun change-property (symbol prop function)
       (let ((value (get symbol prop)))
         (put symbol prop (funcall function value))))
     
     (defun double-property (symbol prop)
       (change-property symbol prop (lambda (x) (* 2 x))))

Note that we do not quote the lambda form.

If you compile the above code, the anonymous function is also compiled. This would not happen if, say, you had constructed the anonymous function by quoting it as a list:

     (defun double-property (symbol prop)
       (change-property symbol prop (lambda (x) (* 2 x))))

In that case, the anonymous function is kept as a lambda expression in the compiled code. The byte-compiler cannot assume this list is a function, even though it looks like one, since it does not know that change-property intends to use it as a function.

12.8 Accessing Function Cell Contents

The function definition of a symbol is the object stored in the function cell of the symbol. The functions described here access, test, and set the function cell of symbols.

See also the function indirect-function. See Definition of indirect-function.

If you have never given a symbol any function definition, we say that that symbol's function cell is void. In other words, the function cell does not have any Lisp object in it. If you try to call such a symbol as a function, it signals a void-function error.

Note that void is not the same as nil or the symbol void. The symbols nil and void are Lisp objects, and can be stored into a function cell just as any other object can be (and they can be valid functions if you define them in turn with defun). A void function cell contains no object whatsoever.

You can test the voidness of a symbol's function definition with fboundp. After you have given a symbol a function definition, you can make it void once more using fmakunbound.

12.9 Closures

As explained in Variable Scoping, Emacs can optionally enable lexical binding of variables. When lexical binding is enabled, any named function that you create (e.g., with defun), as well as any anonymous function that you create using the lambda macro or the function special form or the #' syntax (see Anonymous Functions), is automatically converted into a closure.

A closure is a function that also carries a record of the lexical environment that existed when the function was defined. When it is invoked, any lexical variable references within its definition use the retained lexical environment. In all other respects, closures behave much like ordinary functions; in particular, they can be called in the same way as ordinary functions.

See Lexical Binding, for an example of using a closure.

Currently, an Emacs Lisp closure object is represented by a list with the symbol closure as the first element, a list representing the lexical environment as the second element, and the argument list and body forms as the remaining elements:

     ;; lexical binding is enabled.
     (lambda (x) (* x x))
          ⇒ (closure (t) (x) (* x x))

However, the fact that the internal structure of a closure is “exposed” to the rest of the Lisp world is considered an internal implementation detail. For this reason, we recommend against directly examining or altering the structure of closure objects.

12.10 Declaring Functions Obsolete

You can mark a named function as obsolete, meaning that it may be removed at some point in the future. This causes Emacs to warn that the function is obsolete whenever it byte-compiles code containing that function, and whenever it displays the documentation for that function. In all other respects, an obsolete function behaves like any other function.

The easiest way to mark a function as obsolete is to put a (declare (obsolete ...)) form in the function's defun definition. See Declare Form. Alternatively, you can use the make-obsolete function, described below.

A macro (see Macros) can also be marked obsolete with make-obsolete; this has the same effects as for a function. An alias for a function or macro can also be marked as obsolete; this makes the alias itself obsolete, not the function or macro which it resolves to.

In addition, you can mark a certain a particular calling convention for a function as obsolete:

12.11 Inline Functions

An inline function is a function that works just like an ordinary function, except for one thing: when you byte-compile a call to the function (see Byte Compilation), the function's definition is expanded into the caller. To define an inline function, use defsubst instead of defun.

Making a function inline often makes its function calls run faster. But it also has disadvantages. For one thing, it reduces flexibility; if you change the definition of the function, calls already inlined still use the old definition until you recompile them.

Another disadvantage is that making a large function inline can increase the size of compiled code both in files and in memory. Since the speed advantage of inline functions is greatest for small functions, you generally should not make large functions inline.

Also, inline functions do not behave well with respect to debugging, tracing, and advising (see Advising Functions). Since ease of debugging and the flexibility of redefining functions are important features of Emacs, you should not make a function inline, even if it's small, unless its speed is really crucial, and you've timed the code to verify that using defun actually has performance problems.

It's possible to define a macro to expand into the same code that an inline function would execute (see Macros). But the macro would be limited to direct use in expressions—a macro cannot be called with apply, mapcar and so on. Also, it takes some work to convert an ordinary function into a macro. To convert it into an inline function is easy; just replace defun with defsubst. Since each argument of an inline function is evaluated exactly once, you needn't worry about how many times the body uses the arguments, as you do for macros.

After an inline function is defined, its inline expansion can be performed later on in the same file, just like macros.

12.12 The declare Form

declare is a special macro which can be used to add “meta” properties to a function or macro: for example, marking it as obsolete, or giving its forms a special <TAB> indentation convention in Emacs Lisp mode.

12.13 Telling the Compiler that a Function is Defined

Byte-compiling a file often produces warnings about functions that the compiler doesn't know about (see Compiler Errors). Sometimes this indicates a real problem, but usually the functions in question are defined in other files which would be loaded if that code is run. For example, byte-compiling fortran.el used to warn:

     In end of data:
     fortran.el:2152:1:Warning: the function `gud-find-c-expr' is not
         known to be defined.

In fact, gud-find-c-expr is only used in the function that Fortran mode uses for the local value of gud-find-expr-function, which is a callback from GUD; if it is called, the GUD functions will be loaded. When you know that such a warning does not indicate a real problem, it is good to suppress the warning. That makes new warnings which might mean real problems more visible. You do that with declare-function.

All you need to do is add a declare-function statement before the first use of the function in question:

     (declare-function gud-find-c-expr "gud.el" nil)

This says that gud-find-c-expr is defined in gud.el (the ‘.el’ can be omitted). The compiler takes for granted that that file really defines the function, and does not check.

The optional third argument specifies the argument list of gud-find-c-expr. In this case, it takes no arguments (nil is different from not specifying a value). In other cases, this might be something like (file &optional overwrite). You don't have to specify the argument list, but if you do the byte compiler can check that the calls match the declaration.

To verify that these functions really are declared where declare-function says they are, use check-declare-file to check all declare-function calls in one source file, or use check-declare-directory check all the files in and under a certain directory.

These commands find the file that ought to contain a function's definition using locate-library; if that finds no file, they expand the definition file name relative to the directory of the file that contains the declare-function call.

You can also say that a function is a primitive by specifying a file name ending in ‘.c’ or ‘.m’. This is useful only when you call a primitive that is defined only on certain systems. Most primitives are always defined, so they will never give you a warning.

Sometimes a file will optionally use functions from an external package. If you prefix the filename in the declare-function statement with ‘ext:’, then it will be checked if it is found, otherwise skipped without error.

There are some function definitions that ‘check-declare’ does not understand (e.g., defstruct and some other macros). In such cases, you can pass a non-nil fileonly argument to declare-function, meaning to only check that the file exists, not that it actually defines the function. Note that to do this without having to specify an argument list, you should set the arglist argument to t (because nil means an empty argument list, as opposed to an unspecified one).

12.14 Determining whether a Function is Safe to Call

Some major modes, such as SES, call functions that are stored in user files. (see Top, for more information on SES.) User files sometimes have poor pedigrees—you can get a spreadsheet from someone you've just met, or you can get one through email from someone you've never met. So it is risky to call a function whose source code is stored in a user file until you have determined that it is safe.

Being quick and simple, unsafep does a very light analysis and rejects many Lisp expressions that are actually safe. There are no known cases where unsafep returns nil for an unsafe expression. However, a “safe” Lisp expression can return a string with a display property, containing an associated Lisp expression to be executed after the string is inserted into a buffer. This associated expression can be a virus. In order to be safe, you must delete properties from all strings calculated by user code before inserting them into buffers.

12.15 Other Topics Related to Functions

Here is a table of several functions that do things related to function calling and function definitions. They are documented elsewhere, but we provide cross references here.

apply

See Calling Functions.

autoload

See Autoload.

call-interactively

See Interactive Call.

called-interactively-p

See Distinguish Interactive.

commandp

See Interactive Call.

documentation

See Accessing Documentation.

eval

See Eval.

funcall

See Calling Functions.

function

See Anonymous Functions.

ignore

See Calling Functions.

indirect-function

See Function Indirection.

interactive

See Using Interactive.

interactive-p

See Distinguish Interactive.

mapatoms

See Creating Symbols.

mapcar

See Mapping Functions.

map-char-table

See Char-Tables.

mapconcat

See Mapping Functions.

undefined

See Functions for Key Lookup.

13 Macros

Macros enable you to define new control constructs and other language features. A macro is defined much like a function, but instead of telling how to compute a value, it tells how to compute another Lisp expression which will in turn compute the value. We call this expression the expansion of the macro.

Macros can do this because they operate on the unevaluated expressions for the arguments, not on the argument values as functions do. They can therefore construct an expansion containing these argument expressions or parts of them.

If you are using a macro to do something an ordinary function could do, just for the sake of speed, consider using an inline function instead. See Inline Functions.

• Simple Macro: A basic example.
• Expansion: How, when and why macros are expanded.
• Compiling Macros: How macros are expanded by the compiler.
• Defining Macros: How to write a macro definition.
• Problems with Macros: Don't evaluate the macro arguments too many times. Don't hide the user's variables.
• Indenting Macros: Specifying how to indent macro calls.

13.1 A Simple Example of a Macro

Suppose we would like to define a Lisp construct to increment a variable value, much like the ++ operator in C. We would like to write (inc x) and have the effect of (setq x (1+ x)). Here's a macro definition that does the job:

     (defmacro inc (var)
        (list 'setq var (list '1+ var)))

When this is called with (inc x), the argument var is the symbol x—not the value of x, as it would be in a function. The body of the macro uses this to construct the expansion, which is (setq x (1+ x)). Once the macro definition returns this expansion, Lisp proceeds to evaluate it, thus incrementing x.

13.2 Expansion of a Macro Call

A macro call looks just like a function call in that it is a list which starts with the name of the macro. The rest of the elements of the list are the arguments of the macro.

Evaluation of the macro call begins like evaluation of a function call except for one crucial difference: the macro arguments are the actual expressions appearing in the macro call. They are not evaluated before they are given to the macro definition. By contrast, the arguments of a function are results of evaluating the elements of the function call list.

Having obtained the arguments, Lisp invokes the macro definition just as a function is invoked. The argument variables of the macro are bound to the argument values from the macro call, or to a list of them in the case of a &rest argument. And the macro body executes and returns its value just as a function body does.

The second crucial difference between macros and functions is that the value returned by the macro body is an alternate Lisp expression, also known as the expansion of the macro. The Lisp interpreter proceeds to evaluate the expansion as soon as it comes back from the macro.

Since the expansion is evaluated in the normal manner, it may contain calls to other macros. It may even be a call to the same macro, though this is unusual.

Note that Emacs tries to expand macros when loading an uncompiled Lisp file. This is not always possible, but if it is, it speeds up subsequent execution. See How Programs Do Loading.

You can see the expansion of a given macro call by calling macroexpand.

13.3 Macros and Byte Compilation

You might ask why we take the trouble to compute an expansion for a macro and then evaluate the expansion. Why not have the macro body produce the desired results directly? The reason has to do with compilation.

When a macro call appears in a Lisp program being compiled, the Lisp compiler calls the macro definition just as the interpreter would, and receives an expansion. But instead of evaluating this expansion, it compiles the expansion as if it had appeared directly in the program. As a result, the compiled code produces the value and side effects intended for the macro, but executes at full compiled speed. This would not work if the macro body computed the value and side effects itself—they would be computed at compile time, which is not useful.

In order for compilation of macro calls to work, the macros must already be defined in Lisp when the calls to them are compiled. The compiler has a special feature to help you do this: if a file being compiled contains a defmacro form, the macro is defined temporarily for the rest of the compilation of that file.

Byte-compiling a file also executes any require calls at top-level in the file, so you can ensure that necessary macro definitions are available during compilation by requiring the files that define them (see Named Features). To avoid loading the macro definition files when someone runs the compiled program, write eval-when-compile around the require calls (see Eval During Compile).

13.4 Defining Macros

A Lisp macro object is a list whose car is macro, and whose cdr is a lambda expression. Expansion of the macro works by applying the lambda expression (with apply) to the list of unevaluated arguments from the macro call.

It is possible to use an anonymous Lisp macro just like an anonymous function, but this is never done, because it does not make sense to pass an anonymous macro to functionals such as mapcar. In practice, all Lisp macros have names, and they are almost always defined with the defmacro macro.

Macros often need to construct large list structures from a mixture of constants and nonconstant parts. To make this easier, use the ‘`’ syntax (see Backquote). For example:

     
          (defmacro t-becomes-nil (variable)
            `(if (eq ,variable t)
                 (setq ,variable nil)))
          
          (t-becomes-nil foo)
               == (if (eq foo t) (setq foo nil))

   

The body of a macro definition can include a declare form, which specifies additional properties about the macro. See Declare Form.

13.5 Common Problems Using Macros

Macro expansion can have counterintuitive consequences. This section describes some important consequences that can lead to trouble, and rules to follow to avoid trouble.

• Wrong Time: Do the work in the expansion, not in the macro.
• Argument Evaluation: The expansion should evaluate each macro arg once.
• Surprising Local Vars: Local variable bindings in the expansion require special care.
• Eval During Expansion: Don't evaluate them; put them in the expansion.
• Repeated Expansion: Avoid depending on how many times expansion is done.

13.5.1 Wrong Time

The most common problem in writing macros is doing some of the real work prematurely—while expanding the macro, rather than in the expansion itself. For instance, one real package had this macro definition:

     (defmacro my-set-buffer-multibyte (arg)
       (if (fboundp 'set-buffer-multibyte)
           (set-buffer-multibyte arg)))

With this erroneous macro definition, the program worked fine when interpreted but failed when compiled. This macro definition called set-buffer-multibyte during compilation, which was wrong, and then did nothing when the compiled package was run. The definition that the programmer really wanted was this:

     (defmacro my-set-buffer-multibyte (arg)
       (if (fboundp 'set-buffer-multibyte)
           `(set-buffer-multibyte ,arg)))

This macro expands, if appropriate, into a call to set-buffer-multibyte that will be executed when the compiled program is actually run.

13.5.2 Evaluating Macro Arguments Repeatedly

When defining a macro you must pay attention to the number of times the arguments will be evaluated when the expansion is executed. The following macro (used to facilitate iteration) illustrates the problem. This macro allows us to write a “for” loop construct.

     (defmacro for (var from init to final do &rest body)
       "Execute a simple \"for\" loop.
     For example, (for i from 1 to 10 do (print i))."
       (list 'let (list (list var init))
             (cons 'while
                   (cons (list '<= var final)
                         (append body (list (list 'inc var)))))))
     
     (for i from 1 to 3 do
        (setq square (* i i))
        (princ (format "\n%d %d" i square)))
     ==>
     (let ((i 1))
       (while (<= i 3)
         (setq square (* i i))
         (princ (format "\n%d %d" i square))
         (inc i)))
     
          -|1       1
          -|2       4
          -|3       9
     ⇒ nil

The arguments from, to, and do in this macro are “syntactic sugar”; they are entirely ignored. The idea is that you will write noise words (such as from, to, and do) in those positions in the macro call.

Here's an equivalent definition simplified through use of backquote:

     (defmacro for (var from init to final do &rest body)
       "Execute a simple \"for\" loop.
     For example, (for i from 1 to 10 do (print i))."
       `(let ((,var ,init))
          (while (<= ,var ,final)
            ,@body
            (inc ,var))))

Both forms of this definition (with backquote and without) suffer from the defect that final is evaluated on every iteration. If final is a constant, this is not a problem. If it is a more complex form, say (long-complex-calculation x), this can slow down the execution significantly. If final has side effects, executing it more than once is probably incorrect.

A well-designed macro definition takes steps to avoid this problem by producing an expansion that evaluates the argument expressions exactly once unless repeated evaluation is part of the intended purpose of the macro. Here is a correct expansion for the for macro:

     (let ((i 1)
           (max 3))
       (while (<= i max)
         (setq square (* i i))
         (princ (format "%d      %d" i square))
         (inc i)))

Here is a macro definition that creates this expansion:

     (defmacro for (var from init to final do &rest body)
       "Execute a simple for loop: (for i from 1 to 10 do (print i))."
       `(let ((,var ,init)
              (max ,final))
          (while (<= ,var max)
            ,@body
            (inc ,var))))

Unfortunately, this fix introduces another problem, described in the following section.

13.5.3 Local Variables in Macro Expansions

In the previous section, the definition of for was fixed as follows to make the expansion evaluate the macro arguments the proper number of times:

     (defmacro for (var from init to final do &rest body)
       "Execute a simple for loop: (for i from 1 to 10 do (print i))."
       `(let ((,var ,init)
              (max ,final))
          (while (<= ,var max)
            ,@body
            (inc ,var))))

The new definition of for has a new problem: it introduces a local variable named max which the user does not expect. This causes trouble in examples such as the following:

     (let ((max 0))
       (for x from 0 to 10 do
         (let ((this (frob x)))
           (if (< max this)
               (setq max this)))))

The references to max inside the body of the for, which are supposed to refer to the user's binding of max, really access the binding made by for.

The way to correct this is to use an uninterned symbol instead of max (see Creating Symbols). The uninterned symbol can be bound and referred to just like any other symbol, but since it is created by for, we know that it cannot already appear in the user's program. Since it is not interned, there is no way the user can put it into the program later. It will never appear anywhere except where put by for. Here is a definition of for that works this way:

     (defmacro for (var from init to final do &rest body)
       "Execute a simple for loop: (for i from 1 to 10 do (print i))."
       (let ((tempvar (make-symbol "max")))
         `(let ((,var ,init)
                (,tempvar ,final))
            (while (<= ,var ,tempvar)
              ,@body
              (inc ,var)))))

This creates an uninterned symbol named max and puts it in the expansion instead of the usual interned symbol max that appears in expressions ordinarily.

13.5.4 Evaluating Macro Arguments in Expansion

Another problem can happen if the macro definition itself evaluates any of the macro argument expressions, such as by calling eval (see Eval). If the argument is supposed to refer to the user's variables, you may have trouble if the user happens to use a variable with the same name as one of the macro arguments. Inside the macro body, the macro argument binding is the most local binding of this variable, so any references inside the form being evaluated do refer to it. Here is an example:

     (defmacro foo (a)
       (list 'setq (eval a) t))
     (setq x 'b)
     (foo x) ==> (setq b t)
          ⇒ t                  ; and b has been set.
     ;; but
     (setq a 'c)
     (foo a) ==> (setq a t)
          ⇒ t                  ; but this set a, not c.

It makes a difference whether the user's variable is named a or x, because a conflicts with the macro argument variable a.

Another problem with calling eval in a macro definition is that it probably won't do what you intend in a compiled program. The byte compiler runs macro definitions while compiling the program, when the program's own computations (which you might have wished to access with eval) don't occur and its local variable bindings don't exist.

To avoid these problems, don't evaluate an argument expression while computing the macro expansion. Instead, substitute the expression into the macro expansion, so that its value will be computed as part of executing the expansion. This is how the other examples in this chapter work.

13.5.5 How Many Times is the Macro Expanded?

Occasionally problems result from the fact that a macro call is expanded each time it is evaluated in an interpreted function, but is expanded only once (during compilation) for a compiled function. If the macro definition has side effects, they will work differently depending on how many times the macro is expanded.

Therefore, you should avoid side effects in computation of the macro expansion, unless you really know what you are doing.

One special kind of side effect can't be avoided: constructing Lisp objects. Almost all macro expansions include constructed lists; that is the whole point of most macros. This is usually safe; there is just one case where you must be careful: when the object you construct is part of a quoted constant in the macro expansion.

If the macro is expanded just once, in compilation, then the object is constructed just once, during compilation. But in interpreted execution, the macro is expanded each time the macro call runs, and this means a new object is constructed each time.

In most clean Lisp code, this difference won't matter. It can matter only if you perform side-effects on the objects constructed by the macro definition. Thus, to avoid trouble, avoid side effects on objects constructed by macro definitions. Here is an example of how such side effects can get you into trouble:

     (defmacro empty-object ()
       (list 'quote (cons nil nil)))
     
     (defun initialize (condition)
       (let ((object (empty-object)))
         (if condition
             (setcar object condition))
         object))

If initialize is interpreted, a new list (nil) is constructed each time initialize is called. Thus, no side effect survives between calls. If initialize is compiled, then the macro empty-object is expanded during compilation, producing a single “constant” (nil) that is reused and altered each time initialize is called.

One way to avoid pathological cases like this is to think of empty-object as a funny kind of constant, not as a memory allocation construct. You wouldn't use setcar on a constant such as '(nil), so naturally you won't use it on (empty-object) either.

13.6 Indenting Macros

Within a macro definition, you can use the declare form (see Defining Macros) to specify how <TAB> should indent calls to the macro. An indentation specification is written like this:

     (declare (indent indent-spec))

Here are the possibilities for indent-spec:

nil

This is the same as no property—use the standard indentation pattern.

defun

Handle this function like a ‘def’ construct: treat the second line as the start of a body.

an integer, number

The first number arguments of the function are distinguished arguments; the rest are considered the body of the expression. A line in the expression is indented according to whether the first argument on it is distinguished or not. If the argument is part of the body, the line is indented lisp-body-indent more columns than the open-parenthesis starting the containing expression. If the argument is distinguished and is either the first or second argument, it is indented twice that many extra columns. If the argument is distinguished and not the first or second argument, the line uses the standard pattern.

a symbol, symbol

symbol should be a function name; that function is called to calculate the indentation of a line within this expression. The function receives two arguments:

pos

The position at which the line being indented begins.

state

The value returned by parse-partial-sexp (a Lisp primitive for indentation and nesting computation) when it parses up to the beginning of this line.

It should return either a number, which is the number of columns of indentation for that line, or a list whose car is such a number. The difference between returning a number and returning a list is that a number says that all following lines at the same nesting level should be indented just like this one; a list says that following lines might call for different indentations. This makes a difference when the indentation is being computed by C-M-q; if the value is a number, C-M-q need not recalculate indentation for the following lines until the end of the list.

14 Customization Settings

Users of Emacs can customize variables and faces without writing Lisp code, by using the Customize interface. See Easy Customization. This chapter describes how to define customization items that users can interact with through the Customize interface.

Customization items include customizable variables, which are defined with the defcustom macro; customizable faces, which are defined with defface (described separately in Defining Faces); and customization groups, defined with defgroup, which act as containers for groups of related customization items.

• Common Keywords: Common keyword arguments for all kinds of customization declarations.
• Group Definitions: Writing customization group definitions.
• Variable Definitions: Declaring user options.
• Customization Types: Specifying the type of a user option.
• Applying Customizations: Functions to apply customization settings.
• Custom Themes: Writing Custom themes.

14.1 Common Item Keywords

The customization declarations that we will describe in the next few sections—defcustom, defgroup, etc.—all accept keyword arguments (see Constant Variables) for specifying various information. This section describes keywords that apply to all types of customization declarations.

All of these keywords, except :tag, can be used more than once in a given item. Each use of the keyword has an independent effect. The keyword :tag is an exception because any given item can only display one name.

:tag label

Use label, a string, instead of the item's name, to label the item in customization menus and buffers. Don't use a tag which is substantially different from the item's real name; that would cause confusion.

:group group

Put this customization item in group group. When you use :group in a defgroup, it makes the new group a subgroup of group.

If you use this keyword more than once, you can put a single item into more than one group. Displaying any of those groups will show this item. Please don't overdo this, since the result would be annoying.

:link link-data

Include an external link after the documentation string for this item. This is a sentence containing a button that references some other documentation.

There are several alternatives you can use for link-data:

(custom-manual info-node)

Link to an Info node; info-node is a string which specifies the node name, as in "(emacs)Top". The link appears as ‘[Manual]’ in the customization buffer and enters the built-in Info reader on info-node.

(info-link info-node)

Like custom-manual except that the link appears in the customization buffer with the Info node name.

(url-link url)

Link to a web page; url is a string which specifies the URL. The link appears in the customization buffer as url and invokes the WWW browser specified by browse-url-browser-function.

(emacs-commentary-link library)

Link to the commentary section of a library; library is a string which specifies the library name. See Library Headers.

(emacs-library-link library)

Link to an Emacs Lisp library file; library is a string which specifies the library name.

(file-link file)

Link to a file; file is a string which specifies the name of the file to visit with find-file when the user invokes this link.

(function-link function)

Link to the documentation of a function; function is a string which specifies the name of the function to describe with describe-function when the user invokes this link.

(variable-link variable)

Link to the documentation of a variable; variable is a string which specifies the name of the variable to describe with describe-variable when the user invokes this link.

(custom-group-link group)

Link to another customization group. Invoking it creates a new customization buffer for group.

You can specify the text to use in the customization buffer by adding :tag name after the first element of the link-data; for example, (info-link :tag "foo" "(emacs)Top") makes a link to the Emacs manual which appears in the buffer as ‘foo’.

You can use this keyword more than once, to add multiple links.

:load file

Load file file (a string) before displaying this customization item (see Loading). Loading is done with load, and only if the file is not already loaded.

:require feature

Execute (require 'feature) when your saved customizations set the value of this item. feature should be a symbol.

The most common reason to use :require is when a variable enables a feature such as a minor mode, and just setting the variable won't have any effect unless the code which implements the mode is loaded.

:version version

This keyword specifies that the item was first introduced in Emacs version version, or that its default value was changed in that version. The value version must be a string.

:package-version '(package . version)

This keyword specifies that the item was first introduced in package version version, or that its meaning or default value was changed in that version. This keyword takes priority over :version.

package should be the official name of the package, as a symbol (e.g., MH-E). version should be a string. If the package package is released as part of Emacs, package and version should appear in the value of customize-package-emacs-version-alist.

Packages distributed as part of Emacs that use the :package-version keyword must also update the customize-package-emacs-version-alist variable.

14.2 Defining Customization Groups

Each Emacs Lisp package should have one main customization group which contains all the options, faces and other groups in the package. If the package has a small number of options and faces, use just one group and put everything in it. When there are more than twenty or so options and faces, then you should structure them into subgroups, and put the subgroups under the package's main customization group. It is OK to put some of the options and faces in the package's main group alongside the subgroups.

The package's main or only group should be a member of one or more of the standard customization groups. (To display the full list of them, use M-x customize.) Choose one or more of them (but not too many), and add your group to each of them using the :group keyword.

The way to declare new customization groups is with defgroup.

14.3 Defining Customization Variables

Customizable variables, also called user options, are global Lisp variables whose values can be set through the Customize interface. Unlike other global variables, which are defined with defvar (see Defining Variables), customizable variables are defined using the defcustom macro. In addition to calling defvar as a subroutine, defcustom states how the variable should be displayed in the Customize interface, the values it is allowed to take, etc.

In addition to the keywords listed in Common Keywords, this macro accepts the following keywords:

:type type

Use type as the data type for this option. It specifies which values are legitimate, and how to display the value (see Customization Types).

:options value-list

Specify the list of reasonable values for use in this option. The user is not restricted to using only these values, but they are offered as convenient alternatives.

This is meaningful only for certain types, currently including hook, plist and alist. See the definition of the individual types for a description of how to use :options.

:set setfunction

Specify setfunction as the way to change the value of this option when using the Customize interface. The function setfunction should take two arguments, a symbol (the option name) and the new value, and should do whatever is necessary to update the value properly for this option (which may not mean simply setting the option as a Lisp variable). The default for setfunction is set-default.

If you specify this keyword, the variable's documentation string should describe how to do the same job in hand-written Lisp code.

:get getfunction

Specify getfunction as the way to extract the value of this option. The function getfunction should take one argument, a symbol, and should return whatever customize should use as the “current value” for that symbol (which need not be the symbol's Lisp value). The default is default-value.

You have to really understand the workings of Custom to use :get correctly. It is meant for values that are treated in Custom as variables but are not actually stored in Lisp variables. It is almost surely a mistake to specify getfunction for a value that really is stored in a Lisp variable.

:initialize function

function should be a function used to initialize the variable when the defcustom is evaluated. It should take two arguments, the option name (a symbol) and the value. Here are some predefined functions meant for use in this way:

custom-initialize-set

Use the variable's :set function to initialize the variable, but do not reinitialize it if it is already non-void.

custom-initialize-default

Like custom-initialize-set, but use the function set-default to set the variable, instead of the variable's :set function. This is the usual choice for a variable whose :set function enables or disables a minor mode; with this choice, defining the variable will not call the minor mode function, but customizing the variable will do so.

custom-initialize-reset

Always use the :set function to initialize the variable. If the variable is already non-void, reset it by calling the :set function using the current value (returned by the :get method). This is the default :initialize function.

custom-initialize-changed

Use the :set function to initialize the variable, if it is already set or has been customized; otherwise, just use set-default.

custom-initialize-safe-set

custom-initialize-safe-default

These functions behave like custom-initialize-set (custom-initialize-default, respectively), but catch errors. If an error occurs during initialization, they set the variable to nil using set-default, and signal no error.

These functions are meant for options defined in pre-loaded files, where the standard expression may signal an error because some required variable or function is not yet defined. The value normally gets updated in startup.el, ignoring the value computed by defcustom. After startup, if one unsets the value and reevaluates the defcustom, the standard expression can be evaluated without error.

:risky value

Set the variable's risky-local-variable property to value (see File Local Variables).

:safe function

Set the variable's safe-local-variable property to function (see File Local Variables).

:set-after variables

When setting variables according to saved customizations, make sure to set the variables variables before this one; i.e., delay setting this variable until after those others have been handled. Use :set-after if setting this variable won't work properly unless those other variables already have their intended values.

It is useful to specify the :require keyword for an option that “turns on” a certain feature. This causes Emacs to load the feature, if it is not already loaded, whenever the option is set. See Common Keywords. Here is an example, from the library saveplace.el:

     (defcustom save-place nil
       "Non-nil means automatically save place in each file..."
       :type 'boolean
       :require 'saveplace
       :group 'save-place)

If a customization item has a type such as hook or alist, which supports :options, you can add additional values to the list from outside the defcustom declaration by calling custom-add-frequent-value. For example, if you define a function my-lisp-mode-initialization intended to be called from emacs-lisp-mode-hook, you might want to add that to the list of reasonable values for emacs-lisp-mode-hook, but not by editing its definition. You can do it thus:

     (custom-add-frequent-value 'emacs-lisp-mode-hook
        'my-lisp-mode-initialization)

Internally, defcustom uses the symbol property standard-value to record the expression for the standard value, saved-value to record the value saved by the user with the customization buffer, and customized-value to record the value set by the user with the customization buffer, but not saved. See Symbol Properties. These properties are lists, the car of which is an expression that evaluates to the value.

14.4 Customization Types

When you define a user option with defcustom, you must specify its customization type. That is a Lisp object which describes (1) which values are legitimate and (2) how to display the value in the customization buffer for editing.

You specify the customization type in defcustom with the :type keyword. The argument of :type is evaluated, but only once when the defcustom is executed, so it isn't useful for the value to vary. Normally we use a quoted constant. For example:

     (defcustom diff-command "diff"
       "The command to use to run diff."
       :type '(string)
       :group 'diff)

In general, a customization type is a list whose first element is a symbol, one of the customization type names defined in the following sections. After this symbol come a number of arguments, depending on the symbol. Between the type symbol and its arguments, you can optionally write keyword-value pairs (see Type Keywords).

Some type symbols do not use any arguments; those are called simple types. For a simple type, if you do not use any keyword-value pairs, you can omit the parentheses around the type symbol. For example just string as a customization type is equivalent to (string).

All customization types are implemented as widgets; see Introduction, for details.

• Simple Types: Simple customization types: sexp, integer, etc.
• Composite Types: Build new types from other types or data.
• Splicing into Lists: Splice elements into list with :inline.
• Type Keywords: Keyword-argument pairs in a customization type.
• Defining New Types: Give your type a name.

14.4.1 Simple Types

This section describes all the simple customization types. For several of these customization types, the customization widget provides inline completion with C-M-i or M-<TAB>.

sexp

The value may be any Lisp object that can be printed and read back. You can use sexp as a fall-back for any option, if you don't want to take the time to work out a more specific type to use.

integer

The value must be an integer.

number

The value must be a number (floating point or integer).

float

The value must be a floating point number.

string

The value must be a string. The customization buffer shows the string without delimiting ‘"’ characters or ‘\’ quotes.

regexp

Like string except that the string must be a valid regular expression.

character

The value must be a character code. A character code is actually an integer, but this type shows the value by inserting the character in the buffer, rather than by showing the number.

file

The value must be a file name. The widget provides completion.

(file :must-match t)

The value must be a file name for an existing file. The widget provides completion.

directory

The value must be a directory name. The widget provides completion.

hook

The value must be a list of functions. This customization type is used for hook variables. You can use the :options keyword in a hook variable's defcustom to specify a list of functions recommended for use in the hook; See Variable Definitions.

symbol

The value must be a symbol. It appears in the customization buffer as the symbol name. The widget provides completion.

function

The value must be either a lambda expression or a function name. The widget provides completion for function names.

variable

The value must be a variable name. The widget provides completion.

face

The value must be a symbol which is a face name. The widget provides completion.

boolean

The value is boolean—either nil or t. Note that by using choice and const together (see the next section), you can specify that the value must be nil or t, but also specify the text to describe each value in a way that fits the specific meaning of the alternative.

key-sequence

The value is a key sequence. The customization buffer shows the key sequence using the same syntax as the kbd function. See Key Sequences.

coding-system

The value must be a coding-system name, and you can do completion with M-<TAB>.

color

The value must be a valid color name. The widget provides completion for color names, as well as a sample and a button for selecting a color name from a list of color names shown in a *Colors* buffer.

14.4.2 Composite Types

When none of the simple types is appropriate, you can use composite types, which build new types from other types or from specified data. The specified types or data are called the arguments of the composite type. The composite type normally looks like this:

     (constructor arguments...)

but you can also add keyword-value pairs before the arguments, like this:

     (constructor {keyword value}... arguments...)

Here is a table of constructors and how to use them to write composite types:

(cons car-type cdr-type)

The value must be a cons cell, its car must fit car-type, and its cdr must fit cdr-type. For example, (cons string symbol) is a customization type which matches values such as ("foo" . foo).

In the customization buffer, the car and cdr are displayed and edited separately, each according to their specified type.

(list element-types...)

The value must be a list with exactly as many elements as the element-types given; and each element must fit the corresponding element-type.

For example, (list integer string function) describes a list of three elements; the first element must be an integer, the second a string, and the third a function.

In the customization buffer, each element is displayed and edited separately, according to the type specified for it.

(group element-types...)

This works like list except for the formatting of text in the Custom buffer. list labels each element value with its tag; group does not.

(vector element-types...)

Like list except that the value must be a vector instead of a list. The elements work the same as in list.

(alist :key-type key-type :value-type value-type)

The value must be a list of cons-cells, the car of each cell representing a key of customization type key-type, and the cdr of the same cell representing a value of customization type value-type. The user can add and delete key/value pairs, and edit both the key and the value of each pair.

If omitted, key-type and value-type default to sexp.

The user can add any key matching the specified key type, but you can give some keys a preferential treatment by specifying them with the :options (see Variable Definitions). The specified keys will always be shown in the customize buffer (together with a suitable value), with a checkbox to include or exclude or disable the key/value pair from the alist. The user will not be able to edit the keys specified by the :options keyword argument.

The argument to the :options keywords should be a list of specifications for reasonable keys in the alist. Ordinarily, they are simply atoms, which stand for themselves. For example:

          :options '("foo" "bar" "baz")

specifies that there are three “known” keys, namely "foo", "bar" and "baz", which will always be shown first.

You may want to restrict the value type for specific keys, for example, the value associated with the "bar" key can only be an integer. You can specify this by using a list instead of an atom in the list. The first element will specify the key, like before, while the second element will specify the value type. For example:

          :options '("foo" ("bar" integer) "baz")

Finally, you may want to change how the key is presented. By default, the key is simply shown as a const, since the user cannot change the special keys specified with the :options keyword. However, you may want to use a more specialized type for presenting the key, like function-item if you know it is a symbol with a function binding. This is done by using a customization type specification instead of a symbol for the key.

          :options '("foo"
                     ((function-item some-function) integer)
                     "baz")

Many alists use lists with two elements, instead of cons cells. For example,

          (defcustom list-alist
            '(("foo" 1) ("bar" 2) ("baz" 3))
            "Each element is a list of the form (KEY VALUE).")

instead of

          (defcustom cons-alist
            '(("foo" . 1) ("bar" . 2) ("baz" . 3))
            "Each element is a cons-cell (KEY . VALUE).")

Because of the way lists are implemented on top of cons cells, you can treat list-alist in the example above as a cons cell alist, where the value type is a list with a single element containing the real value.

          (defcustom list-alist '(("foo" 1) ("bar" 2) ("baz" 3))
            "Each element is a list of the form (KEY VALUE)."
            :type '(alist :value-type (group integer)))

The group widget is used here instead of list only because the formatting is better suited for the purpose.

Similarly, you can have alists with more values associated with each key, using variations of this trick:

          (defcustom person-data '(("brian"  50 t)
                                   ("dorith" 55 nil)
                                   ("ken"    52 t))
            "Alist of basic info about people.
          Each element has the form (NAME AGE MALE-FLAG)."
            :type '(alist :value-type (group integer boolean)))

(plist :key-type key-type :value-type value-type)

This customization type is similar to alist (see above), except that (i) the information is stored as a property list, (see Property Lists), and (ii) key-type, if omitted, defaults to symbol rather than sexp.

(choice alternative-types...)

The value must fit one of alternative-types. For example, (choice integer string) allows either an integer or a string.

In the customization buffer, the user selects an alternative using a menu, and can then edit the value in the usual way for that alternative.

Normally the strings in this menu are determined automatically from the choices; however, you can specify different strings for the menu by including the :tag keyword in the alternatives. For example, if an integer stands for a number of spaces, while a string is text to use verbatim, you might write the customization type this way,

          (choice (integer :tag "Number of spaces")
                  (string :tag "Literal text"))

so that the menu offers ‘Number of spaces’ and ‘Literal text’.

In any alternative for which nil is not a valid value, other than a const, you should specify a valid default for that alternative using the :value keyword. See Type Keywords.

If some values are covered by more than one of the alternatives, customize will choose the first alternative that the value fits. This means you should always list the most specific types first, and the most general last. Here's an example of proper usage:

          (choice (const :tag "Off" nil)
                  symbol (sexp :tag "Other"))

This way, the special value nil is not treated like other symbols, and symbols are not treated like other Lisp expressions.

(radio element-types...)

This is similar to choice, except that the choices are displayed using `radio buttons' rather than a menu. This has the advantage of displaying documentation for the choices when applicable and so is often a good choice for a choice between constant functions (function-item customization types).

(const value)

The value must be value—nothing else is allowed.

The main use of const is inside of choice. For example, (choice integer (const nil)) allows either an integer or nil.

:tag is often used with const, inside of choice. For example,

          (choice (const :tag "Yes" t)
                  (const :tag "No" nil)
                  (const :tag "Ask" foo))

describes a variable for which t means yes, nil means no, and foo means “ask”.

(other value)

This alternative can match any Lisp value, but if the user chooses this alternative, that selects the value value.

The main use of other is as the last element of choice. For example,

          (choice (const :tag "Yes" t)
                  (const :tag "No" nil)
                  (other :tag "Ask" foo))

describes a variable for which t means yes, nil means no, and anything else means “ask”. If the user chooses ‘Ask’ from the menu of alternatives, that specifies the value foo; but any other value (not t, nil or foo) displays as ‘Ask’, just like foo.

(function-item function)

Like const, but used for values which are functions. This displays the documentation string as well as the function name. The documentation string is either the one you specify with :doc, or function's own documentation string.

(variable-item variable)

Like const, but used for values which are variable names. This displays the documentation string as well as the variable name. The documentation string is either the one you specify with :doc, or variable's own documentation string.

(set types...)

The value must be a list, and each element of the list must match one of the types specified.

This appears in the customization buffer as a checklist, so that each of types may have either one corresponding element or none. It is not possible to specify two different elements that match the same one of types. For example, (set integer symbol) allows one integer and/or one symbol in the list; it does not allow multiple integers or multiple symbols. As a result, it is rare to use nonspecific types such as integer in a set.

Most often, the types in a set are const types, as shown here:

          (set (const :bold) (const :italic))

Sometimes they describe possible elements in an alist:

          (set (cons :tag "Height" (const height) integer)
               (cons :tag "Width" (const width) integer))

That lets the user specify a height value optionally and a width value optionally.

(repeat element-type)

The value must be a list and each element of the list must fit the type element-type. This appears in the customization buffer as a list of elements, with ‘[INS]’ and ‘[DEL]’ buttons for adding more elements or removing elements.

(restricted-sexp :match-alternatives criteria)

This is the most general composite type construct. The value may be any Lisp object that satisfies one of criteria. criteria should be a list, and each element should be one of these possibilities:

• A predicate—that is, a function of one argument that has no side effects, and returns either nil or non-nil according to the argument. Using a predicate in the list says that objects for which the predicate returns non-nil are acceptable.
• A quoted constant—that is, 'object. This sort of element in the list says that object itself is an acceptable value.

For example,

          (restricted-sexp :match-alternatives
                           (integerp 't 'nil))

allows integers, t and nil as legitimate values.

The customization buffer shows all legitimate values using their read syntax, and the user edits them textually.

Here is a table of the keywords you can use in keyword-value pairs in a composite type:

:tag tag

Use tag as the name of this alternative, for user communication purposes. This is useful for a type that appears inside of a choice.

:match-alternatives criteria

Use criteria to match possible values. This is used only in restricted-sexp.

:args argument-list

Use the elements of argument-list as the arguments of the type construct. For instance, (const :args (foo)) is equivalent to (const foo). You rarely need to write :args explicitly, because normally the arguments are recognized automatically as whatever follows the last keyword-value pair.

14.4.3 Splicing into Lists

The :inline feature lets you splice a variable number of elements into the middle of a list or vector customization type. You use it by adding :inline t to a type specification which is contained in a list or vector specification.

Normally, each entry in a list or vector type specification describes a single element type. But when an entry contains :inline t, the value it matches is merged directly into the containing sequence. For example, if the entry matches a list with three elements, those become three elements of the overall sequence. This is analogous to ‘,@’ in a backquote construct (see Backquote).

For example, to specify a list whose first element must be baz and whose remaining arguments should be zero or more of foo and bar, use this customization type:

     (list (const baz) (set :inline t (const foo) (const bar)))

This matches values such as (baz), (baz foo), (baz bar) and (baz foo bar).

When the element-type is a choice, you use :inline not in the choice itself, but in (some of) the alternatives of the choice. For example, to match a list which must start with a file name, followed either by the symbol t or two strings, use this customization type:

     (list file
           (choice (const t)
                   (list :inline t string string)))

If the user chooses the first alternative in the choice, then the overall list has two elements and the second element is t. If the user chooses the second alternative, then the overall list has three elements and the second and third must be strings.

14.4.4 Type Keywords

You can specify keyword-argument pairs in a customization type after the type name symbol. Here are the keywords you can use, and their meanings:

:value default

Provide a default value.

If nil is not a valid value for the alternative, then it is essential to specify a valid default with :value.

If you use this for a type that appears as an alternative inside of choice; it specifies the default value to use, at first, if and when the user selects this alternative with the menu in the customization buffer.

Of course, if the actual value of the option fits this alternative, it will appear showing the actual value, not default.

:format format-string

This string will be inserted in the buffer to represent the value corresponding to the type. The following ‘%’ escapes are available for use in format-string:

‘%[button%]’

Display the text button marked as a button. The :action attribute specifies what the button will do if the user invokes it; its value is a function which takes two arguments—the widget which the button appears in, and the event.

There is no way to specify two different buttons with different actions.

‘%{sample%}’

Show sample in a special face specified by :sample-face.

‘%v’

Substitute the item's value. How the value is represented depends on the kind of item, and (for variables) on the customization type.

‘%d’

Substitute the item's documentation string.

‘%h’

Like ‘%d’, but if the documentation string is more than one line, add a button to control whether to show all of it or just the first line.

‘%t’

Substitute the tag here. You specify the tag with the :tag keyword.

‘%%’

Display a literal ‘%’.

:action action

Perform action if the user clicks on a button.

:button-face face

Use the face face (a face name or a list of face names) for button text displayed with ‘%[...%]’.

:button-prefix prefix

:button-suffix suffix

These specify the text to display before and after a button. Each can be:

nil

No text is inserted.

a string

The string is inserted literally.

a symbol

The symbol's value is used.

:tag tag

Use tag (a string) as the tag for the value (or part of the value) that corresponds to this type.

:doc doc

Use doc as the documentation string for this value (or part of the value) that corresponds to this type. In order for this to work, you must specify a value for :format, and use ‘%d’ or ‘%h’ in that value.

The usual reason to specify a documentation string for a type is to provide more information about the meanings of alternatives inside a :choice type or the parts of some other composite type.

:help-echo motion-doc

When you move to this item with widget-forward or widget-backward, it will display the string motion-doc in the echo area. In addition, motion-doc is used as the mouse help-echo string and may actually be a function or form evaluated to yield a help string. If it is a function, it is called with one argument, the widget.

:match function

Specify how to decide whether a value matches the type. The corresponding value, function, should be a function that accepts two arguments, a widget and a value; it should return non-nil if the value is acceptable.

:validate function

Specify a validation function for input. function takes a widget as an argument, and should return nil if the widget's current value is valid for the widget. Otherwise, it should return the widget containing the invalid data, and set that widget's :error property to a string explaining the error.

14.4.5 Defining New Types

In the previous sections we have described how to construct elaborate type specifications for defcustom. In some cases you may want to give such a type specification a name. The obvious case is when you are using the same type for many user options: rather than repeat the specification for each option, you can give the type specification a name, and use that name each defcustom. The other case is when a user option's value is a recursive data structure. To make it possible for a datatype to refer to itself, it needs to have a name.

Since custom types are implemented as widgets, the way to define a new customize type is to define a new widget. We are not going to describe the widget interface here in details, see Introduction, for that. Instead we are going to demonstrate the minimal functionality needed for defining new customize types by a simple example.

     (define-widget 'binary-tree-of-string 'lazy
       "A binary tree made of cons-cells and strings."
       :offset 4
       :tag "Node"
       :type '(choice (string :tag "Leaf" :value "")
                      (cons :tag "Interior"
                            :value ("" . "")
                            binary-tree-of-string
                            binary-tree-of-string)))
     
     (defcustom foo-bar ""
       "Sample variable holding a binary tree of strings."
       :type 'binary-tree-of-string)

The function to define a new widget is called define-widget. The first argument is the symbol we want to make a new widget type. The second argument is a symbol representing an existing widget, the new widget is going to be defined in terms of difference from the existing widget. For the purpose of defining new customization types, the lazy widget is perfect, because it accepts a :type keyword argument with the same syntax as the keyword argument to defcustom with the same name. The third argument is a documentation string for the new widget. You will be able to see that string with the M-x widget-browse <RET> binary-tree-of-string <RET> command.

After these mandatory arguments follow the keyword arguments. The most important is :type, which describes the data type we want to match with this widget. Here a binary-tree-of-string is described as being either a string, or a cons-cell whose car and cdr are themselves both binary-tree-of-string. Note the reference to the widget type we are currently in the process of defining. The :tag attribute is a string to name the widget in the user interface, and the :offset argument is there to ensure that child nodes are indented four spaces relative to the parent node, making the tree structure apparent in the customization buffer.

The defcustom shows how the new widget can be used as an ordinary customization type.

The reason for the name lazy is that the other composite widgets convert their inferior widgets to internal form when the widget is instantiated in a buffer. This conversion is recursive, so the inferior widgets will convert their inferior widgets. If the data structure is itself recursive, this conversion is an infinite recursion. The lazy widget prevents the recursion: it convert its :type argument only when needed.

14.5 Applying Customizations

The following functions are responsible for installing the user's customization settings for variables and faces, respectively. When the user invokes ‘Save for future sessions’ in the Customize interface, that takes effect by writing a custom-set-variables and/or a custom-set-faces form into the custom file, to be evaluated the next time Emacs starts.

14.6 Custom Themes

Custom themes are collections of settings that can be enabled or disabled as a unit. See Custom Themes. Each Custom theme is defined by an Emacs Lisp source file, which should follow the conventions described in this section. (Instead of writing a Custom theme by hand, you can also create one using a Customize-like interface; see Creating Custom Themes.)

A Custom theme file should be named foo-theme.el, where foo is the theme name. The first Lisp form in the file should be a call to deftheme, and the last form should be a call to provide-theme.

In between deftheme and provide-theme are Lisp forms specifying the theme settings: usually a call to custom-theme-set-variables and/or a call to custom-theme-set-faces.

In theory, a theme file can also contain other Lisp forms, which would be evaluated when loading the theme, but that is “bad form”. To protect against loading themes containing malicious code, Emacs displays the source file and asks for confirmation from the user before loading any non-built-in theme for the first time.

The following functions are useful for programmatically enabling and disabling themes:

15 Loading

Loading a file of Lisp code means bringing its contents into the Lisp environment in the form of Lisp objects. Emacs finds and opens the file, reads the text, evaluates each form, and then closes the file. Such a file is also called a Lisp library.

The load functions evaluate all the expressions in a file just as the eval-buffer function evaluates all the expressions in a buffer. The difference is that the load functions read and evaluate the text in the file as found on disk, not the text in an Emacs buffer.

The loaded file must contain Lisp expressions, either as source code or as byte-compiled code. Each form in the file is called a top-level form. There is no special format for the forms in a loadable file; any form in a file may equally well be typed directly into a buffer and evaluated there. (Indeed, most code is tested this way.) Most often, the forms are function definitions and variable definitions.

• How Programs Do Loading: The load function and others.
• Load Suffixes: Details about the suffixes that load tries.
• Library Search: Finding a library to load.
• Loading Non-ASCII: Non-ASCII characters in Emacs Lisp files.
• Autoload: Setting up a function to autoload.
• Repeated Loading: Precautions about loading a file twice.
• Named Features: Loading a library if it isn't already loaded.
• Where Defined: Finding which file defined a certain symbol.
• Unloading: How to "unload" a library that was loaded.
• Hooks for Loading: Providing code to be run when particular libraries are loaded.

15.1 How Programs Do Loading

Emacs Lisp has several interfaces for loading. For example, autoload creates a placeholder object for a function defined in a file; trying to call the autoloading function loads the file to get the function's real definition (see Autoload). require loads a file if it isn't already loaded (see Named Features). Ultimately, all these facilities call the load function to do the work.

For information about how load is used in building Emacs, see Building Emacs.

15.2 Load Suffixes

We now describe some technical details about the exact suffixes that load tries.

To summarize, load normally first tries the suffixes in the value of (get-load-suffixes) and then those in load-file-rep-suffixes. If nosuffix is non-nil, it skips the former group, and if must-suffix is non-nil, it skips the latter group.

15.3 Library Search

When Emacs loads a Lisp library, it searches for the library in a list of directories specified by the variable load-path.

Each time Emacs starts up, it sets up the value of load-path in several steps. First, it initializes load-path to the directories specified by the environment variable EMACSLOADPATH, if that exists. The syntax of EMACSLOADPATH is the same as used for PATH; directory names are separated by ‘:’ (or ‘;’, on some operating systems), and ‘.’ stands for the current default directory. Here is an example of how to set EMACSLOADPATH variable from sh:

     export EMACSLOADPATH
     EMACSLOADPATH=/home/foo/.emacs.d/lisp:/opt/emacs/lisp

Here is how to set it from csh:

     setenv EMACSLOADPATH /home/foo/.emacs.d/lisp:/opt/emacs/lisp

If EMACSLOADPATH is not set (which is usually the case), Emacs initializes load-path with the following two directories:

     "/usr/local/share/emacs/version/site-lisp"

and

     "/usr/local/share/emacs/site-lisp"

The first one is for locally installed packages for a particular Emacs version; the second is for locally installed packages meant for use with all installed Emacs versions.

If you run Emacs from the directory where it was built—that is, an executable that has not been formally installed—Emacs puts two more directories in load-path. These are the lisp and site-lisp subdirectories of the main build directory. (Both are represented as absolute file names.)

Next, Emacs “expands” the initial list of directories in load-path by adding the subdirectories of those directories. Both immediate subdirectories and subdirectories multiple levels down are added. But it excludes subdirectories whose names do not start with a letter or digit, and subdirectories named RCS or CVS, and subdirectories containing a file named .nosearch.

Next, Emacs adds any extra load directory that you specify using the ‘-L’ command-line option (see Action Arguments). It also adds the directories where optional packages are installed, if any (see Packaging Basics).

It is common to add code to one's init file (see Init File) to add one or more directories to load-path. For example:

     (push "~/.emacs.d/lisp" load-path)

Dumping Emacs uses a special value of load-path. If the value of load-path at the end of dumping is unchanged (that is, still the same special value), the dumped Emacs switches to the ordinary load-path value when it starts up, as described above. But if load-path has any other value at the end of dumping, that value is used for execution of the dumped Emacs also.

15.4 Loading Non-ASCII Characters

When Emacs Lisp programs contain string constants with non-ASCII characters, these can be represented within Emacs either as unibyte strings or as multibyte strings (see Text Representations). Which representation is used depends on how the file is read into Emacs. If it is read with decoding into multibyte representation, the text of the Lisp program will be multibyte text, and its string constants will be multibyte strings. If a file containing Latin-1 characters (for example) is read without decoding, the text of the program will be unibyte text, and its string constants will be unibyte strings. See Coding Systems.

In most Emacs Lisp programs, the fact that non-ASCII strings are multibyte strings should not be noticeable, since inserting them in unibyte buffers converts them to unibyte automatically. However, if this does make a difference, you can force a particular Lisp file to be interpreted as unibyte by writing ‘coding: raw-text’ in a local variables section. With that designator, the file will unconditionally be interpreted as unibyte. This can matter when making keybindings to non-ASCII characters written as ?vliteral.

15.5 Autoload

The autoload facility lets you register the existence of a function or macro, but put off loading the file that defines it. The first call to the function automatically loads the proper library, in order to install the real definition and other associated code, then runs the real definition as if it had been loaded all along. Autoloading can also be triggered by looking up the documentation of the function or macro (see Documentation Basics).

There are two ways to set up an autoloaded function: by calling autoload, and by writing a special “magic” comment in the source before the real definition. autoload is the low-level primitive for autoloading; any Lisp program can call autoload at any time. Magic comments are the most convenient way to make a function autoload, for packages installed along with Emacs. These comments do nothing on their own, but they serve as a guide for the command update-file-autoloads, which constructs calls to autoload and arranges to execute them when Emacs is built.

The autoloaded file usually contains other definitions and may require or provide one or more features. If the file is not completely loaded (due to an error in the evaluation of its contents), any function definitions or provide calls that occurred during the load are undone. This is to ensure that the next attempt to call any function autoloading from this file will try again to load the file. If not for this, then some of the functions in the file might be defined by the aborted load, but fail to work properly for the lack of certain subroutines not loaded successfully because they come later in the file.

If the autoloaded file fails to define the desired Lisp function or macro, then an error is signaled with data "Autoloading failed to define function function-name".

A magic autoload comment (often called an autoload cookie) consists of ‘;;;###autoload’, on a line by itself, just before the real definition of the function in its autoloadable source file. The command M-x update-file-autoloads writes a corresponding autoload call into loaddefs.el. (The string that serves as the autoload cookie and the name of the file generated by update-file-autoloads can be changed from the above defaults, see below.) Building Emacs loads loaddefs.el and thus calls autoload. M-x update-directory-autoloads is even more powerful; it updates autoloads for all files in the current directory.

The same magic comment can copy any kind of form into loaddefs.el. The form following the magic comment is copied verbatim, except if it is one of the forms which the autoload facility handles specially (e.g., by conversion into an autoload call). The forms which are not copied verbatim are the following:

Definitions for function or function-like objects:

defun and defmacro; also cl-defun and cl-defmacro (see Argument Lists), and define-overloadable-function (see the commentary in mode-local.el).

Definitions for major or minor modes:

define-minor-mode, define-globalized-minor-mode, define-generic-mode, define-derived-mode, easy-mmode-define-minor-mode, easy-mmode-define-global-mode, define-compilation-mode, and define-global-minor-mode.

Other definition types:

defcustom, defgroup, defclass (see EIEIO), and define-skeleton (see the commentary in skeleton.el).

You can also use a magic comment to execute a form at build time without executing it when the file itself is loaded. To do this, write the form on the same line as the magic comment. Since it is in a comment, it does nothing when you load the source file; but M-x update-file-autoloads copies it to loaddefs.el, where it is executed while building Emacs.

The following example shows how doctor is prepared for autoloading with a magic comment:

     ;;;###autoload
     (defun doctor ()
       "Switch to *doctor* buffer and start giving psychotherapy."
       (interactive)
       (switch-to-buffer "*doctor*")
       (doctor-mode))

Here's what that produces in loaddefs.el:

     (autoload (quote doctor) "doctor" "\
     Switch to *doctor* buffer and start giving psychotherapy.
     
     \(fn)" t nil)

The backslash and newline immediately following the double-quote are a convention used only in the preloaded uncompiled Lisp files such as loaddefs.el; they tell make-docfile to put the documentation string in the etc/DOC file. See Building Emacs. See also the commentary in lib-src/make-docfile.c. ‘(fn)’ in the usage part of the documentation string is replaced with the function's name when the various help functions (see Help Functions) display it.

If you write a function definition with an unusual macro that is not one of the known and recognized function definition methods, use of an ordinary magic autoload comment would copy the whole definition into loaddefs.el. That is not desirable. You can put the desired autoload call into loaddefs.el instead by writing this:

     ;;;###autoload (autoload 'foo "myfile")
     (mydefunmacro foo
       ...)

You can use a non-default string as the autoload cookie and have the corresponding autoload calls written into a file whose name is different from the default loaddefs.el. Emacs provides two variables to control this:

The following function may be used to explicitly load the library specified by an autoload object:

15.6 Repeated Loading

You can load a given file more than once in an Emacs session. For example, after you have rewritten and reinstalled a function definition by editing it in a buffer, you may wish to return to the original version; you can do this by reloading the file it came from.

When you load or reload files, bear in mind that the load and load-library functions automatically load a byte-compiled file rather than a non-compiled file of similar name. If you rewrite a file that you intend to save and reinstall, you need to byte-compile the new version; otherwise Emacs will load the older, byte-compiled file instead of your newer, non-compiled file! If that happens, the message displayed when loading the file includes, ‘(compiled; note, source is newer)’, to remind you to recompile it.

When writing the forms in a Lisp library file, keep in mind that the file might be loaded more than once. For example, think about whether each variable should be reinitialized when you reload the library; defvar does not change the value if the variable is already initialized. (See Defining Variables.)

The simplest way to add an element to an alist is like this:

     (push '(leif-mode " Leif") minor-mode-alist)

But this would add multiple elements if the library is reloaded. To avoid the problem, use add-to-list (see List Variables):

     (add-to-list 'minor-mode-alist '(leif-mode " Leif"))

Occasionally you will want to test explicitly whether a library has already been loaded. If the library uses provide to provide a named feature, you can use featurep earlier in the file to test whether the provide call has been executed before (see Named Features). Alternatively, you could use something like this:

     (defvar foo-was-loaded nil)
     
     (unless foo-was-loaded
       execute-first-time-only
       (setq foo-was-loaded t))

15.7 Features

provide and require are an alternative to autoload for loading files automatically. They work in terms of named features. Autoloading is triggered by calling a specific function, but a feature is loaded the first time another program asks for it by name.

A feature name is a symbol that stands for a collection of functions, variables, etc. The file that defines them should provide the feature. Another program that uses them may ensure they are defined by requiring the feature. This loads the file of definitions if it hasn't been loaded already.

To require the presence of a feature, call require with the feature name as argument. require looks in the global variable features to see whether the desired feature has been provided already. If not, it loads the feature from the appropriate file. This file should call provide at the top level to add the feature to features; if it fails to do so, require signals an error.

For example, in idlwave.el, the definition for idlwave-complete-filename includes the following code:

     (defun idlwave-complete-filename ()
       "Use the comint stuff to complete a file name."
        (require 'comint)
        (let* ((comint-file-name-chars "~/A-Za-z0-9+_.$#%={}\\-")
               (comint-completion-addsuffix nil)
               ...)
            (comint-dynamic-complete-filename)))

The expression (require 'comint) loads the file comint.el if it has not yet been loaded, ensuring that comint-dynamic-complete-filename is defined. Features are normally named after the files that provide them, so that require need not be given the file name. (Note that it is important that the require statement be outside the body of the let. Loading a library while its variables are let-bound can have unintended consequences, namely the variables becoming unbound after the let exits.)

The comint.el file contains the following top-level expression:

     (provide 'comint)

This adds comint to the global features list, so that (require 'comint) will henceforth know that nothing needs to be done.

When require is used at top level in a file, it takes effect when you byte-compile that file (see Byte Compilation) as well as when you load it. This is in case the required package contains macros that the byte compiler must know about. It also avoids byte compiler warnings for functions and variables defined in the file loaded with require.

Although top-level calls to require are evaluated during byte compilation, provide calls are not. Therefore, you can ensure that a file of definitions is loaded before it is byte-compiled by including a provide followed by a require for the same feature, as in the following example.

     (provide 'my-feature)  ; Ignored by byte compiler,
                            ;   evaluated by load.
     (require 'my-feature)  ; Evaluated by byte compiler.

The compiler ignores the provide, then processes the require by loading the file in question. Loading the file does execute the provide call, so the subsequent require call does nothing when the file is loaded.

15.8 Which File Defined a Certain Symbol

The basis for symbol-file is the data in the variable load-history.

The command eval-region updates load-history, but does so by adding the symbols defined to the element for the file being visited, rather than replacing that element. See Eval.

15.9 Unloading

You can discard the functions and variables loaded by a library to reclaim memory for other Lisp objects. To do this, use the function unload-feature:

The unload-feature function is written in Lisp; its actions are based on the variable load-history.

15.10 Hooks for Loading

You can ask for code to be executed each time Emacs loads a library, by using the variable after-load-functions:

If you want code to be executed when a particular library is loaded, use the function eval-after-load:

Normally, well-designed Lisp programs should not use eval-after-load. If you need to examine and set the variables defined in another library (those meant for outside use), you can do it immediately—there is no need to wait until the library is loaded. If you need to call functions defined by that library, you should load the library, preferably with require (see Named Features).

16 Byte Compilation

Emacs Lisp has a compiler that translates functions written in Lisp into a special representation called byte-code that can be executed more efficiently. The compiler replaces Lisp function definitions with byte-code. When a byte-code function is called, its definition is evaluated by the byte-code interpreter.

Because the byte-compiled code is evaluated by the byte-code interpreter, instead of being executed directly by the machine's hardware (as true compiled code is), byte-code is completely transportable from machine to machine without recompilation. It is not, however, as fast as true compiled code.

In general, any version of Emacs can run byte-compiled code produced by recent earlier versions of Emacs, but the reverse is not true.

If you do not want a Lisp file to be compiled, ever, put a file-local variable binding for no-byte-compile into it, like this:

     ;; -*-no-byte-compile: t; -*-

• Speed of Byte-Code: An example of speedup from byte compilation.
• Compilation Functions: Byte compilation functions.
• Docs and Compilation: Dynamic loading of documentation strings.
• Dynamic Loading: Dynamic loading of individual functions.
• Eval During Compile: Code to be evaluated when you compile.
• Compiler Errors: Handling compiler error messages.
• Byte-Code Objects: The data type used for byte-compiled functions.
• Disassembly: Disassembling byte-code; how to read byte-code.

16.1 Performance of Byte-Compiled Code

A byte-compiled function is not as efficient as a primitive function written in C, but runs much faster than the version written in Lisp. Here is an example:

     (defun silly-loop (n)
       "Return the time, in seconds, to run N iterations of a loop."
       (let ((t1 (float-time)))
         (while (> (setq n (1- n)) 0))
         (- (float-time) t1)))
     ⇒ silly-loop
     
     (silly-loop 50000000)
     ⇒ 10.235304117202759
     
     (byte-compile 'silly-loop)
     ⇒ [Compiled code not shown]
     
     (silly-loop 50000000)
     ⇒ 3.705854892730713

In this example, the interpreted code required 10 seconds to run, whereas the byte-compiled code required less than 4 seconds. These results are representative, but actual results may vary.

16.2 Byte-Compilation Functions

You can byte-compile an individual function or macro definition with the byte-compile function. You can compile a whole file with byte-compile-file, or several files with byte-recompile-directory or batch-byte-compile.

Sometimes, the byte compiler produces warning and/or error messages (see Compiler Errors, for details). These messages are recorded in a buffer called *Compile-Log*, which uses Compilation mode. See Compilation Mode.

Be careful when writing macro calls in files that you intend to byte-compile. Since macro calls are expanded when they are compiled, the macros need to be loaded into Emacs or the byte compiler will not do the right thing. The usual way to handle this is with require forms which specify the files containing the needed macro definitions (see Named Features). Normally, the byte compiler does not evaluate the code that it is compiling, but it handles require forms specially, by loading the specified libraries. To avoid loading the macro definition files when someone runs the compiled program, write eval-when-compile around the require calls (see Eval During Compile). For more details, See Compiling Macros.

Inline (defsubst) functions are less troublesome; if you compile a call to such a function before its definition is known, the call will still work right, it will just run slower.