Emacs Lisp

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This is the GNU Emacs Lisp Reference Manual corresponding to Emacs version 26.3.

Copyright © 1990–1996, 1998–2019 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with the Invariant Sections being “GNU General Public License,” with the Front-Cover Texts being “A GNU Manual,” and with the Back-Cover Texts as in (a) below. A copy of the license is included in the section entitled “GNU Free Documentation License.”

(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.
Records Compound objects with programmer-defined types.
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.
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.
Threads Concurrency in Emacs Lisp.
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.
Antinews Info for users downgrading to Emacs 25.
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:


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.
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.
Record Type Compound objects with programmer-defined types.
Type Descriptors Objects holding information about types.
Autoload Type A type used for automatically loading seldom-used functions.
Finalizer Type Runs code when no longer reachable.
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.
Thread Type A thread of Emacs Lisp execution.
Mutex Type An exclusive lock for thread synchronization.
Condition Variable Type Condition variable for thread synchronization.
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.
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.
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.
Record Functions Functions for records.
Backward Compatibility Compatibility for cl-defstruct.
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.
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.
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.
Pattern-Matching Conditional How to use pcase and friends.
Iteration while loops.
Generators Generic sequences and coroutines.
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.
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.
Global Variables Variable values that exist permanently, everywhere.
Constant Variables Variables 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.
Watching Variables Running a function when a variable is changed.
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.
Connection Local Variables Local variables common for remote connections.
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.
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.
Generic Functions Polymorphism, Emacs-style.
Function Cells Accessing or setting the function definition of a symbol.
Closures Functions that enclose a lexical environment.
Advising Functions Adding to the definition of a function.
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.
Advising Emacs Lisp Functions
Core Advising Primitives Primitives to manipulate advice.
Advising Named Functions Advising named functions.
Advice combinators Ways to compose advice.
Porting old advice Adapting code using the old defadvice.
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.
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.
Dynamic Modules Modules provide additional Lisp primitives.
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.
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.
Variable Debugging Entering it when a variable is modified.
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.
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.
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.
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.
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.
Generic Commands Select among command alternatives.
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.
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.
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.
SMIE Customization Customizing indentation.
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.
Text Quoting Style Quotation marks in doc strings and messages.
Describing Characters Making printable descriptions of non-printing characters and key sequences.
Help Functions Subroutines used by Emacs help facilities.
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 File sizes, modification times, etc.
Extended Attributes Extended file attributes for access control.
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.
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.
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.
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.
Preserving Window Sizes Preserving the size 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.
Displaying Buffers Displaying a buffer in a suitable window.
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.
Side Windows Special windows on a frame's sides.
Atomic Windows Preserving parts of the window layout.
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.
Mouse Window Auto-selection Automatically selecting windows with the mouse.
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.
Displaying Buffers
Choosing Window How to choose a window for displaying a buffer.
Buffer Display Action Functions Support functions for buffer display.
Buffer Display Action Alists Alists for fine-tuning buffer display action functions.
Choosing Window Options Extra options affecting how buffers are displayed.
Precedence of Action Functions A tutorial explaining the precedence of buffer display action functions.
The Zen of Buffer Display How to avoid that buffers get lost in between windows.
Side Windows
Displaying Buffers in Side Windows An action function for displaying buffers in side windows.
Side Window Options and Functions Further tuning of side windows.
Frame Layouts with Side Windows Setting up frame layouts with side windows.
Creating Frames Creating additional frames.
Multiple Terminals Displaying on several different devices.
Frame Geometry Geometric properties of frames.
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, Lowering and Restacking Frames.
Frame Configurations Saving the state of all frames.
Child Frames Making a frame the child of another.
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 Geometry
Frame Layout Basic layout of frames.
Frame Font The default font of a frame and how to set it.
Frame Position The position of a frame on its display.
Frame Size Specifying and retrieving a frame's size.
Implied Frame Resizing Implied resizing of frames and how to prevent it.
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.
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.
Frame Interaction Parameters Parameters for interacting with other frames.
Mouse Dragging Parameters Parameters for resizing and moving frames with the mouse.
Management Parameters Communicating with the window manager.
Cursor Parameters Controlling the cursor appearance.
Font and Color Parameters Fonts and colors for the frame text.
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.
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.
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.
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.
Decompression Dealing with compressed data.
Base 64 Conversion to or from base 64 encoding.
Checksum/Hash Computing cryptographic hashes.
GnuTLS Cryptography Cryptographic algorithms imported from GnuTLS.
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.
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.
Parsing HTML and XML
Document Object Model Access, manipulate and search the DOM.
Non-ASCII Characters
Text Representations How Emacs represents text.
Disabling Multibyte Controlling whether to use multibyte characters.
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.
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.
Basic Thread Functions Basic thread functions.
Mutexes Mutexes allow exclusive access to data.
Condition Variables Inter-thread events.
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 By default, 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.
Size of Displayed Text How large displayed text is.
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 scroll bars.
Window Dividers Separating windows visually.
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.
Tooltips Tooltip display in Emacs.
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.
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.
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.
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.
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.
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.
Multi-Frame Images Some images contain more than one frame.
Image Cache Internal mechanisms of image display.
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.
Desktop Notifications Desktop notifications.
File Notifications File notifications.
Dynamic Libraries On-demand loading of support libraries.
Security Considerations Running Emacs in an unfriendly environment.
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.
Stack-allocated Objects Temporary conses and strings on C stack.
Memory Usage Info about total size of Lisp objects made so far.
C Dialect What C variant Emacs is written in.
Writing Emacs Primitives Writing C code for Emacs.
Writing Dynamic Modules Writing loadable modules for Emacs.
Object Internals Data formats of buffers, windows, processes.
C Integer Types How C integer types are used inside Emacs.
Writing Dynamic Modules
Module Initialization
Module Functions
Module Values
Module Misc
Module Nonlocal
Object Internals
Buffer Internals Components of a buffer structure.
Window Internals Components of a window structure.
Process Internals Components of a process structure.

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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 26.3.

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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.

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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.

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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.

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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.

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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 that 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.

— Function: booleanp object

Return non-nil if object is one of the two canonical boolean values: t or nil.

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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)

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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

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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

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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 ----------

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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.

Next: , Up: Format of Descriptions 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:

— Function: foo integer1 &optional integer2 &rest integers

The function foo subtracts integer1 from integer2, then adds all the rest of the arguments to the result. If integer2 is not supplied, then the number 19 is used by default.

          (foo 1 5 3 9)
               ⇒ 16
          (foo 5)
               ⇒ 14

More generally,

          (foo w x y...)
          (+ (- x w) y...)

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:

— Special Form: count-loop (var [from to [inc]]) body...

This imaginary special form implements a loop that executes the body forms and then increments the variable var on each iteration. On the first iteration, the variable has the value from; on subsequent iterations, it is incremented by one (or by inc if that is given). The loop exits before executing body if var equals to. Here is an example:

          (count-loop (i 0 10)
            (prin1 i) (princ " ")
            (prin1 (aref vector i))

If from and to are omitted, var is bound to nil before the loop begins, and the loop exits if var is non-nil at the beginning of an iteration. Here is an example:

          (count-loop (done)
            (if (pending)
              (setq done t)))

In this special form, the arguments from and to are optional, but must both be present or both absent. If they are present, inc may optionally be specified as well. These arguments are grouped with the argument var into a list, to distinguish them from body, which includes all remaining elements of the form.

Previous: A Sample Function Description, Up: Format of Descriptions 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.

— Variable: electric-future-map

The value of this variable is a full keymap used by Electric Command Future mode. The functions in this map allow you to edit commands you have not yet thought about executing.

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

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1.4 Version Information

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

— Command: emacs-version &optional here

This function returns a string describing the version of Emacs that is running. It is useful to include this string in bug reports.

            ⇒ "GNU Emacs 26.1 (build 1, x86_64-unknown-linux-gnu,
                       GTK+ Version 3.16) of 2017-06-01"

If here is non-nil, it inserts the text in the buffer before point, and returns nil. When this function is called interactively, it prints the same information in the echo area, but giving a prefix argument makes here non-nil.

— Variable: emacs-build-time

The value of this variable indicates the time at which Emacs was built. It is a list of four integers, like the value of current-time (see Time of Day), or is nil if the information is not available.

               ⇒ (20614 63694 515336 438000)
— Variable: emacs-version

The value of this variable is the version of Emacs being run. It is a string such as "26.1". A value with three numeric components, such as "26.0.91", indicates an unreleased test version. (Prior to Emacs 26.1, the string includes an extra final component with the integer that is now stored in emacs-build-number; e.g., "25.1.1".)

— Variable: emacs-major-version

The major version number of Emacs, as an integer. For Emacs version 23.1, the value is 23.

— Variable: emacs-minor-version

The minor version number of Emacs, as an integer. For Emacs version 23.1, the value is 1.

— Variable: emacs-build-number

An integer that increments each time Emacs is built in the same directory (without cleaning). This is only of relevance when developing Emacs.

Previous: Version Info, Up: Introduction

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 Potortì, 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 change log entries in the Emacs source repository.

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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, byte-code function, and record, 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.

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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:

          ⇒ #<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.

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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, an unescaped 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.

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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.

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2.3.1 Integer Type

The range of values for an integer depends on the machine. The minimum range is −536,870,912 to 536,870,911 (30 bits; i.e., −2**29 to 2**29 − 1) but many machines provide a wider range. Emacs Lisp arithmetic functions do not check for integer overflow. Thus (1+ 536870911) is −536,870,912 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.

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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’, ‘15.0e+2’, ‘+1500000e-3’, and ‘.15e4’ are five ways of writing a floating-point number whose value is 1500. They are all equivalent.

See Numbers, for more information.

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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.

Next: , Up: Character Type 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. However, if the punctuation character has a special syntactic meaning in Lisp, you must quote it with a ‘\’. For example, ‘?\(’ is the way to write the open-paren character. Likewise, 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 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 must add a backslash before any of the characters ‘()[]\;"’, and 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.)

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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.

  1. You can specify characters by their Unicode names, if any. ?\N{NAME} represents the Unicode character named NAME. Thus, ‘?\N{LATIN SMALL LETTER A WITH GRAVE}’ is equivalent to and denotes the Unicode character U+00E0. To simplify entering multi-line strings, you can replace spaces in the names by non-empty sequences of whitespace (e.g., newlines).
  2. You can specify characters by their Unicode values. ?\N{U+X} represents a character with Unicode code point X, where X is a hexadecimal number. Also, ?\uxxxx and ?\Uxxxxxxxx represent code points xxxx and xxxxxxxx, respectively, where each x is a single hexadecimal digit. For example, ?\N{U+E0}, ?\u00e0 and ?\U000000E0 are all equivalent to and to ‘?\N{LATIN SMALL LETTER A WITH GRAVE}’. The Unicode Standard defines code points only up to ‘U+10ffff’, so if you specify a code point higher than that, Emacs signals an error.
  3. 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 à (a with grave accent). You can use any number of hex digits, so you can represent any character code in this way.
  4. 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.

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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.

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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’.

Previous: Meta-Char Syntax, Up: Character Type 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.

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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.)

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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.

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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, the address 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.

Next: , Up: Cons Cell Type 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 |
                 |       |      |     |         |      |
                  --------------       ----------------

Next: , Previous: Box Diagrams, Up: Cons Cell Type 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

Previous: Dotted Pair Notation, Up: Cons Cell Type 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.

Next: , Previous: Cons Cell Type, Up: Programming Types

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.

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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.

Next: , Up: String Type 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 one 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."

Next: , Previous: Syntax for Strings, Up: String Type 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.

Next: , Previous: Non-ASCII in Strings, Up: String Type 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.

Previous: Nonprinting Characters, Up: String Type 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.)

Next: , Previous: String Type, Up: Programming Types

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.

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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:

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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

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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:

          ⇒ #s(hash-table size 65 test eql rehash-size 1.5
                                  rehash-threshold 0.8125 data ())

See Hash Tables, for more information about hash tables.

Next: , Previous: Hash Table Type, Up: Programming Types

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.

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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 macro, 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.

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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.

Next: , Previous: Primitive Function Type, Up: Programming Types

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 ‘[’.

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2.3.17 Record Type

A record is much like a vector. However, the first element is used to hold its type as returned by type-of. The purpose of records is to allow programmers to create objects with new types that are not built into Emacs.

See Records, for functions that work with records.

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2.3.18 Type Descriptors

A type descriptor is a record which holds information about a type. Slot 1 in the record must be a symbol naming the type, and type-of relies on this to return the type of record objects. No other type descriptor slot is used by Emacs; they are free for use by Lisp extensions.

An example of a type descriptor is any instance of cl-structure-class.

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2.3.19 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.

Previous: Autoload Type, Up: Programming Types

2.3.20 Finalizer Type

A finalizer object helps Lisp code clean up after objects that are no longer needed. A finalizer holds a Lisp function object. When a finalizer object becomes unreachable after a garbage collection pass, Emacs calls the finalizer's associated function object. When deciding whether a finalizer is reachable, Emacs does not count references from finalizer objects themselves, allowing you to use finalizers without having to worry about accidentally capturing references to finalized objects themselves.

Errors in finalizers are printed to *Messages*. Emacs runs a given finalizer object's associated function exactly once, even if that function fails.

— Function: make-finalizer function

Make a finalizer that will run function. function will be called after garbage collection when the returned finalizer object becomes unreachable. If the finalizer object is reachable only through references from finalizer objects, it does not count as reachable for the purpose of deciding whether to run function. function will be run once per finalizer object.

Next: , Previous: Programming Types, Up: Lisp Data Types

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.

Next: , Up: Editing Types

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:

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.

          ⇒ #<buffer objects.texi>

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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.

          ⇒ #<marker at 10779 in objects.texi>

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

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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 (see 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.

          ⇒ #<window 1 on objects.texi>

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

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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).

          ⇒ #<frame emacs@psilocin.gnu.org 0xdac80>

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

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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>

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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.

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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.

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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 shell>)

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

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2.4.9 Thread Type

A thread in Emacs represents a separate thread of Emacs Lisp execution. It runs its own Lisp program, has its own current buffer, and can have subprocesses locked to it, i.e. subprocesses whose output only this thread can accept. See Threads.

Thread objects have no read syntax. They print in hash notation, giving the name of the thread (if it has been given a name) or its address in core:

         ⇒ (#<thread 0176fc40>)

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2.4.10 Mutex Type

A mutex is an exclusive lock that threads can own and disown, in order to synchronize between them. See Mutexes.

Mutex objects have no read syntax. They print in hash notation, giving the name of the mutex (if it has been given a name) or its address in core:

     (make-mutex "my-mutex")
         ⇒ #<mutex my-mutex>
         ⇒ #<mutex 01c7e4e0>

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2.4.11 Condition Variable Type

A condition variable is a device for a more complex thread synchronization than the one supported by a mutex. A thread can wait on a condition variable, to be woken up when some other thread notifies the condition.

Condition variable objects have no read syntax. They print in hash notation, giving the name of the condition variable (if it has been given a name) or its address in core:

     (make-condition-variable (make-mutex))
         ⇒ #<condvar 01c45ae8>

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2.4.12 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.

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2.4.13 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.

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2.4.14 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.

Previous: Overlay Type, Up: Editing Types

2.4.15 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.

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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.

Next: , Previous: Circular Objects, Up: Lisp Data Types

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)))
              ;; 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.

See atom.
See arrayp.
See bool-vector-p.
See booleanp.
See bufferp.
See byte-code-function-p.
See case-table-p.
See char-or-string-p.
See char-table-p.
See commandp.
See condition-variable-p.
See consp.
See custom-variable-p.
See floatp.
See Low-Level Font.
See frame-configuration-p.
See frame-live-p.
See framep.
See functionp.
See hash-table-p.
See integer-or-marker-p.
See integerp.
See keymapp.
See Constant Variables.
See listp.
See markerp.
See mutexp.
See nlistp.
See number-or-marker-p.
See numberp.
See overlayp.
See processp.
See recordp.
See sequencep.
See string-or-null-p.
See stringp.
See subrp.
See symbolp.
See syntax-table-p.
See threadp.
See vectorp.
See wholenump.
See window-configuration-p.
See window-live-p.
See windowp.

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.

— Function: type-of object

This function returns a symbol naming the primitive type of object. The value is one of the symbols bool-vector, buffer, char-table, compiled-function, condition-variable, cons, finalizer, float, font-entity, font-object, font-spec, frame, hash-table, integer, marker, mutex, overlay, process, string, subr, symbol, thread, vector, window, or window-configuration. However, if object is a record, the type specified by its first slot is returned; Records.

          (type-of 1)
               ⇒ integer
          (type-of 'nil)
               ⇒ symbol
          (type-of '())    ; () is nil.
               ⇒ symbol
          (type-of '(x))
               ⇒ cons
          (type-of (record 'foo))
               ⇒ foo

Previous: Type Predicates, Up: Lisp Data Types

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.

— Function: eq object1 object2

This function returns t if object1 and object2 are the same object, and nil otherwise.

If object1 and object2 are integers with the same value, they are considered to be the same object (i.e., eq returns t). If object1 and object2 are symbols with the same name, they are normally the same object—but see Creating Symbols for exceptions. For other types (e.g., lists, vectors, strings), two arguments with the same contents or elements are not necessarily eq to each other: they are eq only if they are the same object, meaning that a change in the contents of one will be reflected by the same change in the contents of the other.

          (eq 'foo 'foo)
               ⇒ t
          (eq 456 456)
               ⇒ t
          (eq "asdf" "asdf")
               ⇒ nil
          (eq "" "")
               ⇒ t
          ;; This exception occurs because Emacs Lisp
          ;; makes just one multibyte empty string, to save space.
          (eq '(1 (2 (3))) '(1 (2 (3))))
               ⇒ nil
          (setq foo '(1 (2 (3))))
               ⇒ (1 (2 (3)))
          (eq foo foo)
               ⇒ t
          (eq foo '(1 (2 (3))))
               ⇒ nil
          (eq [(1 2) 3] [(1 2) 3])
               ⇒ nil
          (eq (point-marker) (point-marker))
               ⇒ nil

The make-symbol function returns an uninterned symbol, distinct from the symbol that is used if you write the name in a Lisp expression. Distinct symbols with the same name are not eq. See Creating Symbols.

          (eq (make-symbol "foo") 'foo)
               ⇒ nil

The Emacs Lisp byte compiler may collapse identical literal objects, such as literal strings, into references to the same object, with the effect that the byte-compiled code will compare such objects as eq, while the interpreted version of the same code will not. Therefore, your code should never rely on objects with the same literal contents being either eq or not eq, it should instead use functions that compare object contents such as equal, described below. Similarly, your code should not modify literal objects (e.g., put text properties on literal strings), since doing that might affect other literal objects of the same contents, if the byte compiler collapses them.

— Function: equal object1 object2

This function returns t if object1 and object2 have equal components, and nil otherwise. Whereas eq tests if its arguments are the same object, equal looks inside nonidentical arguments to see if their elements or contents are the same. So, if two objects are eq, they are equal, but the converse is not always true.

          (equal 'foo 'foo)
               ⇒ t
          (equal 456 456)
               ⇒ t
          (equal "asdf" "asdf")
               ⇒ t
          (eq "asdf" "asdf")
               ⇒ nil
          (equal '(1 (2 (3))) '(1 (2 (3))))
               ⇒ t
          (eq '(1 (2 (3))) '(1 (2 (3))))
               ⇒ nil
          (equal [(1 2) 3] [(1 2) 3])
               ⇒ t
          (eq [(1 2) 3] [(1 2) 3])
               ⇒ nil
          (equal (point-marker) (point-marker))
               ⇒ t
          (eq (point-marker) (point-marker))
               ⇒ nil

Comparison of strings is case-sensitive, but does not take account of text properties—it compares only the characters in the strings. See Text Properties. Use equal-including-properties to also compare text properties. For technical reasons, a unibyte string and a multibyte string are equal if and only if they contain the same sequence of character codes and all these codes are in the range 0 through 127 (ASCII).

          (equal "asdf" "ASDF")
               ⇒ nil

However, two distinct buffers are never considered equal, even if their textual contents are the same.

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).

— Function: equal-including-properties object1 object2

This function behaves like equal in all cases but also requires that for two strings to be equal, they have the same text properties.

          (equal "asdf" (propertize "asdf" 'asdf t))
               ⇒ t
          (equal-including-properties "asdf"
                                      (propertize "asdf" 'asdf t))
               ⇒ nil

Next: , Previous: Lisp Data Types, Up: Top

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. Floating-point numbers are numbers with fractional parts, such as −4.5, 0.0, and 2.71828. They can also be expressed in exponential notation: ‘1.5e2’ is the same as ‘150.0’; here, ‘e2’ stands for ten to the second power, and that is multiplied by 1.5. Integer computations are exact, though they may overflow. Floating-point computations often involve rounding errors, as the numbers have a fixed amount of precision.

Next: , Up: Numbers

3.1 Integer Basics

The range of values for an integer depends on the machine. The minimum range is −536,870,912 to 536,870,911 (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.
                      ; The floating-point number 9e18.
      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.)

Subtracting 4 from −1 returns the negative integer −5. 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 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.

— Variable: most-positive-fixnum

The value of this variable is the largest integer that Emacs Lisp can handle. Typical values are 2**29 − 1 on 32-bit and 2**61 − 1 on 64-bit platforms.

— Variable: most-negative-fixnum

The value of this variable is the smallest integer that Emacs Lisp can handle. It is negative. Typical values are −2**29 on 32-bit and −2**61 on 64-bit platforms.

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 Character Codes.

Next: , Previous: Integer Basics, Up: Numbers

3.2 Floating-Point Basics

Floating-point numbers are useful for representing numbers that are not integral. The range of floating-point numbers is the same as the range of the C data type double on the machine you are using. On all computers currently supported by Emacs, this is double-precision IEEE floating point.

The read syntax for floating-point numbers requires either a decimal point, an exponent, or both. Optional signs (‘+’ or ‘-’) precede the number and its exponent. For example, ‘1500.0’, ‘+15e2’, ‘15.0e+2’, ‘+1500000e-3’, and ‘.15e4’ are five ways of writing a floating-point number whose value is 1500. They are all equivalent. Like Common Lisp, Emacs Lisp requires at least one digit after any decimal point in a floating-point number; ‘1500.’ is an integer, not a floating-point number.

Emacs Lisp treats -0.0 as numerically equal to ordinary zero with respect to equal and =. This follows the IEEE floating-point standard, which says -0.0 and 0.0 are numerically equal even though other operations can distinguish them.

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. Although NaN values carry a sign, for practical purposes there is no other significant difference between different NaN values in Emacs Lisp.

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

1.0e+INF’ and ‘-1.0e+INF
0.0e+NaN’ and ‘-0.0e+NaN

The following functions are specialized for handling floating-point numbers:

— Function: isnan x

This predicate returns t if its floating-point argument is a NaN, nil otherwise.

— Function: frexp x

This function returns a cons cell (s . e), where s and e are respectively the significand and exponent of the floating-point number x.

If x is finite, then s is a floating-point number between 0.5 (inclusive) and 1.0 (exclusive), e is an integer, and x = s * 2**e. If x is zero or infinity, then s is the same as x. If x is a NaN, then s is also a NaN. If x is zero, then e is 0.

— Function: ldexp s e

Given a numeric significand s and an integer exponent e, this function returns the floating point number s * 2**e.

— Function: copysign x1 x2

This function copies the sign of x2 to the value of x1, and returns the result. x1 and x2 must be floating point.

— Function: logb x

This function returns the binary exponent of x. More precisely, the value is the logarithm base 2 of |x|, rounded down to an integer.

          (logb 10)
               ⇒ 3
          (logb 10.0e20)
               ⇒ 69

Next: , Previous: Float Basics, Up: 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.

— Function: floatp object

This predicate tests whether its argument is floating point and returns t if so, nil otherwise.

— Function: integerp object

This predicate tests whether its argument is an integer, and returns t if so, nil otherwise.

— Function: numberp object

This predicate tests whether its argument is a number (either integer or floating point), and returns t if so, nil otherwise.

— Function: natnump object

This predicate (whose name comes from the phrase “natural number”) tests to see whether its argument is a nonnegative integer, and returns t if so, nil otherwise. 0 is considered non-negative.

wholenump is a synonym for natnump.

— Function: zerop number

This predicate tests whether its argument is zero, and returns t if so, nil otherwise. The argument must be a number.

(zerop x) is equivalent to (= x 0).

Next: , Previous: Predicates on Numbers, Up: Numbers

3.4 Comparison of Numbers

To test numbers for numerical equality, you should normally use =, not eq. There can be many distinct floating-point 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 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 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 (= x y)
           (< (/ (abs (- x y))
                 (max (abs x) (abs y)))

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 integers.
— Function: = number-or-marker &rest number-or-markers

This function tests whether all its arguments are numerically equal, and returns t if so, nil otherwise.

— Function: eql value1 value2

This function acts like eq except when both arguments are numbers. It compares numbers by type and numeric value, so that (eql 1.0 1) returns nil, but (eql 1.0 1.0) and (eql 1 1) both return t.

— Function: /= number-or-marker1 number-or-marker2

This function tests whether its arguments are numerically equal, and returns t if they are not, and nil if they are.

— Function: < number-or-marker &rest number-or-markers

This function tests whether each argument is strictly less than the following argument. It returns t if so, nil otherwise.

— Function: <= number-or-marker &rest number-or-markers

This function tests whether each argument is less than or equal to the following argument. It returns t if so, nil otherwise.

— Function: > number-or-marker &rest number-or-markers

This function tests whether each argument is strictly greater than the following argument. It returns t if so, nil otherwise.

— Function: >= number-or-marker &rest number-or-markers

This function tests whether each argument is greater than or equal to the following argument. It returns t if so, nil otherwise.

— Function: max number-or-marker &rest numbers-or-markers

This function returns the largest of its arguments.

          (max 20)
               ⇒ 20
          (max 1 2.5)
               ⇒ 2.5
          (max 1 3 2.5)
               ⇒ 3
— Function: min number-or-marker &rest numbers-or-markers

This function returns the smallest of its arguments.

          (min -4 1)
               ⇒ -4
— Function: abs number

This function returns the absolute value of number.

Next: , Previous: Comparison of Numbers, Up: Numbers

3.5 Numeric Conversions

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

— Function: float number

This returns number converted to floating point. If number is already floating point, float returns it unchanged.

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. If divisor is zero (whether integer or floating point), Emacs signals an arith-error error.

— Function: truncate number &optional divisor

This returns number, converted to an integer by rounding towards zero.

          (truncate 1.2)
               ⇒ 1
          (truncate 1.7)
               ⇒ 1
          (truncate -1.2)
               ⇒ -1
          (truncate -1.7)
               ⇒ -1
— Function: floor number &optional divisor

This returns number, converted to an integer by rounding downward (towards negative infinity).

If divisor is specified, this uses the kind of division operation that corresponds to mod, rounding downward.

          (floor 1.2)
               ⇒ 1
          (floor 1.7)
               ⇒ 1
          (floor -1.2)
               ⇒ -2
          (floor -1.7)
               ⇒ -2
          (floor 5.99 3)
               ⇒ 1
— Function: ceiling number &optional divisor

This returns number, converted to an integer by rounding upward (towards positive infinity).

          (ceiling 1.2)
               ⇒ 2
          (ceiling 1.7)
               ⇒ 2
          (ceiling -1.2)
               ⇒ -1
          (ceiling -1.7)
               ⇒ -1
— Function: round number &optional divisor

This returns number, converted to an integer by rounding towards the nearest integer. Rounding a value equidistant between two integers returns the even integer.

          (round 1.2)
               ⇒ 1
          (round 1.7)
               ⇒ 2
          (round -1.2)
               ⇒ -1
          (round -1.7)
               ⇒ -2

Next: , Previous: Numeric Conversions, Up: Numbers

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 floating point.

Emacs Lisp arithmetic functions do not check for integer overflow. Thus (1+ 536870911) may evaluate to −536870912, depending on your hardware.

— Function: 1+ number-or-marker

This function returns number-or-marker plus 1. For example,

          (setq foo 4)
               ⇒ 4
          (1+ foo)
               ⇒ 5

This function is not analogous to the C operator ++—it does not increment a variable. It just computes a sum. Thus, if we continue,

               ⇒ 4

If you want to increment the variable, you must use setq, like this:

          (setq foo (1+ foo))
               ⇒ 5
— Function: 1- number-or-marker

This function returns number-or-marker minus 1.

— Function: + &rest numbers-or-markers

This function adds its arguments together. When given no arguments, + returns 0.

               ⇒ 0
          (+ 1)
               ⇒ 1
          (+ 1 2 3 4)
               ⇒ 10
— Function: - &optional number-or-marker &rest more-numbers-or-markers

The - function serves two purposes: negation and subtraction. When - has a single argument, the value is the negative of the argument. When there are multiple arguments, - subtracts each of the more-numbers-or-markers from number-or-marker, cumulatively. If there are no arguments, the result is 0.

          (- 10 1 2 3 4)
               ⇒ 0
          (- 10)
               ⇒ -10
               ⇒ 0
— Function: * &rest numbers-or-markers

This function multiplies its arguments together, and returns the product. When given no arguments, * returns 1.

               ⇒ 1
          (* 1)
               ⇒ 1
          (* 1 2 3 4)
               ⇒ 24
— Function: / number &rest divisors

With one or more divisors, this function divides number by each divisor in divisors in turn, and returns the quotient. With no divisors, this function returns 1/number, i.e., the multiplicative inverse of number. Each argument may be a number or a marker.

If all the arguments are integers, the result is an integer, obtained by rounding the quotient towards zero after each division.

          (/ 6 2)
               ⇒ 3
          (/ 5 2)
               ⇒ 2
          (/ 5.0 2)
               ⇒ 2.5
          (/ 5 2.0)
               ⇒ 2.5
          (/ 5.0 2.0)
               ⇒ 2.5
          (/ 4.0)
               ⇒ 0.25
          (/ 4)
               ⇒ 0
          (/ 25 3 2)
               ⇒ 4
          (/ -17 6)
               ⇒ -2

If you divide an integer by the integer 0, Emacs signals an arith-error error (see Errors). Floating-point division of a nonzero number by zero yields either positive or negative infinity (see Float Basics).

— Function: % dividend divisor

This function returns the integer remainder after division of dividend by divisor. The arguments must be integers or markers.

For any two integers dividend and divisor,

          (+ (% dividend divisor)
             (* (/ dividend divisor) divisor))

always equals dividend if divisor is nonzero.

          (% 9 4)
               ⇒ 1
          (% -9 4)
               ⇒ -1
          (% 9 -4)
               ⇒ 1
          (% -9 -4)
               ⇒ -1
— Function: mod dividend divisor

This function returns the value of dividend modulo divisor; in other words, the remainder after division of dividend by divisor, but with the same sign as divisor. The arguments must be numbers or markers.

Unlike %, mod permits floating-point arguments; it rounds the quotient downward (towards minus infinity) to an integer, and uses that quotient to compute the remainder.

If divisor is zero, mod signals an arith-error error if both arguments are integers, and returns a NaN otherwise.

          (mod 9 4)
               ⇒ 1
          (mod -9 4)
               ⇒ 3
          (mod 9 -4)
               ⇒ -3
          (mod -9 -4)
               ⇒ -1
          (mod 5.5 2.5)
               ⇒ .5

For any two numbers dividend and divisor,

          (+ (mod dividend divisor)
             (* (floor dividend divisor) divisor))

always equals dividend, subject to rounding error if either argument is floating point and to an arith-error if dividend is an integer and divisor is 0. For floor, see Numeric Conversions.

Next: , Previous: Arithmetic Operations, Up: Numbers

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.

— Function: ffloor float

This function rounds float to the next lower integral value, and returns that value as a floating-point number.

— Function: fceiling float

This function rounds float to the next higher integral value, and returns that value as a floating-point number.

— Function: ftruncate float

This function rounds float towards zero to an integral value, and returns that value as a floating-point number.

— Function: fround float

This function rounds float to the nearest integral value, and returns that value as a floating-point number. Rounding a value equidistant between two integers returns the even integer.

Next: , Previous: Rounding Operations, Up: Numbers

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.

— Function: lsh integer1 count

lsh, which is an abbreviation for logical shift, shifts the bits in integer1 to the left count places, or to the right if count is negative, bringing zeros into the vacated bits. If count is negative, lsh shifts zeros into the leftmost (most-significant) bit, producing a positive result even if integer1 is negative. Contrast this with ash, below.

Here are two examples of lsh, shifting a pattern of bits one place to the left. We show only the low-order eight bits of the binary pattern; the rest are all zero.

          (lsh 5 1)
               ⇒ 10
          ;; Decimal 5 becomes decimal 10.
          00000101 ⇒ 00001010
          (lsh 7 1)
               ⇒ 14
          ;; Decimal 7 becomes decimal 14.
          00000111 ⇒ 00001110

As the examples illustrate, shifting the pattern of bits one place to the left produces a number that is twice the value of the previous number.

Shifting a pattern of bits two places to the left produces results like this (with 8-bit binary numbers):

          (lsh 3 2)
               ⇒ 12
          ;; Decimal 3 becomes decimal 12.
          00000011 ⇒ 00001100

On the other hand, shifting one place to the right looks like this:

          (lsh 6 -1)
               ⇒ 3
          ;; Decimal 6 becomes decimal 3.
          00000110 ⇒ 00000011
          (lsh 5 -1)
               ⇒ 2
          ;; Decimal 5 becomes decimal 2.
          00000101 ⇒ 00000010

As the example illustrates, shifting one place to the right divides the value of a positive integer by two, rounding downward.

The function lsh, like all Emacs Lisp arithmetic functions, does not check for overflow, so shifting left can discard significant bits and change the sign of the number. For example, left shifting 536,870,911 produces −2 in the 30-bit implementation:

          (lsh 536870911 1)          ; left shift
               ⇒ -2

In binary, the argument looks like this:

          ;; Decimal 536,870,911
          0111...111111 (30 bits total)

which becomes the following when left shifted:

          ;; Decimal −2
          1111...111110 (30 bits total)
— Function: ash integer1 count

ash (arithmetic shift) shifts the bits in integer1 to the left count places, or to the right if count is negative.

ash gives the same results as lsh except when integer1 and count are both negative. In that case, ash puts ones in the empty bit positions on the left, while lsh puts zeros in those bit positions.

Thus, with ash, shifting the pattern of bits one place to the right looks like this:

          (ash -6 -1) ⇒ -3
          ;; Decimal −6 becomes decimal −3.
          1111...111010 (30 bits total)
          1111...111101 (30 bits total)

In contrast, shifting the pattern of bits one place to the right with lsh looks like this:

          (lsh -6 -1) ⇒ 536870909
          ;; Decimal −6 becomes decimal 536,870,909.
          1111...111010 (30 bits total)
          0111...111101 (30 bits total)

Here are other examples:

                             ;         30-bit binary values
          (lsh 5 2)          ;   5  =  0000...000101
               ⇒ 20         ;      =  0000...010100
          (ash 5 2)
               ⇒ 20
          (lsh -5 2)         ;  -5  =  1111...111011
               ⇒ -20        ;      =  1111...101100
          (ash -5 2)
               ⇒ -20
          (lsh 5 -2)         ;   5  =  0000...000101
               ⇒ 1          ;      =  0000...000001
          (ash 5 -2)
               ⇒ 1
          (lsh -5 -2)        ;  -5  =  1111...111011
               ⇒ 268435454
                             ;      =  0011...111110
          (ash -5 -2)        ;  -5  =  1111...111011
               ⇒ -2         ;      =  1111...111110
— Function: logand &rest ints-or-markers

This function returns the bitwise AND of the arguments: the nth bit is 1 in the result if, and only if, the nth bit is 1 in all the arguments.

For example, using 4-bit binary numbers, the bitwise AND of 13 and 12 is 12: 1101 combined with 1100 produces 1100. In both the binary numbers, the leftmost two bits are both 1 so the leftmost two bits of the returned value are both 1. However, for the rightmost two bits, each is 0 in at least one of the arguments, so the rightmost two bits of the returned value are both 0.


          (logand 13 12)
               ⇒ 12

If logand is not passed any argument, it returns a value of −1. This number is an identity element for logand because its binary representation consists entirely of ones. If logand is passed just one argument, it returns that argument.

                             ;        30-bit binary values
          (logand 14 13)     ; 14  =  0000...001110
                             ; 13  =  0000...001101
               ⇒ 12         ; 12  =  0000...001100
          (logand 14 13 4)   ; 14  =  0000...001110
                             ; 13  =  0000...001101
                             ;  4  =  0000...000100
               ⇒ 4          ;  4  =  0000...000100
               ⇒ -1         ; -1  =  1111...111111
— Function: logior &rest ints-or-markers

This function returns the bitwise inclusive OR of its arguments: the nth bit is 1 in the result if, and only if, the nth bit is 1 in at least one of the arguments. If there are no arguments, the result is 0, which is an identity element for this operation. If logior is passed just one argument, it returns that argument.

                             ;        30-bit binary values
          (logior 12 5)      ; 12  =  0000...001100
                             ;  5  =  0000...000101
               ⇒ 13         ; 13  =  0000...001101
          (logior 12 5 7)    ; 12  =  0000...001100
                             ;  5  =  0000...000101
                             ;  7  =  0000...000111
               ⇒ 15         ; 15  =  0000...001111
— Function: logxor &rest ints-or-markers

This function returns the bitwise exclusive OR of its arguments: the nth bit is 1 in the result if, and only if, the nth bit is 1 in an odd number of the arguments. If there are no arguments, the result is 0, which is an identity element for this operation. If logxor is passed just one argument, it returns that argument.

                             ;        30-bit binary values
          (logxor 12 5)      ; 12  =  0000...001100
                             ;  5  =  0000...000101
               ⇒ 9          ;  9  =  0000...001001
          (logxor 12 5 7)    ; 12  =  0000...001100
                             ;  5  =  0000...000101
                             ;  7  =  0000...000111
               ⇒ 14         ; 14  =  0000...001110
— Function: lognot integer

This function returns the bitwise complement of its argument: the nth bit is one in the result if, and only if, the nth bit is zero in integer, and vice-versa.

          (lognot 5)
               ⇒ -6
          ;;  5  =  0000...000101 (30 bits total)
          ;; becomes
          ;; -6  =  1111...111010 (30 bits total)

Next: , Previous: Bitwise Operations, Up: Numbers

3.9 Standard Mathematical Functions

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

— Function: sin arg
— Function: cos arg
— Function: tan arg

These are the basic trigonometric functions, with argument arg measured in radians.

— Function: asin arg

The value of (asin arg) is a number between −pi/2 and pi/2 (inclusive) whose sine is arg. If arg is out of range (outside [−1, 1]), asin returns a NaN.

— Function: acos arg

The value of (acos arg) is a number between 0 and pi (inclusive) whose cosine is arg. If arg is out of range (outside [−1, 1]), acos returns a NaN.

— Function: atan y &optional x

The value of (atan y) is a number between −pi/2 and pi/2 (exclusive) whose tangent is y. If the optional second argument x is given, the value of (atan y x) is the angle in radians between the vector [x, y] and the X axis.

— Function: exp arg

This is the exponential function; it returns e to the power arg.

— Function: log arg &optional base

This function returns the logarithm of arg, with base base. If you don't specify base, the natural base e is used. If arg or base is negative, log returns a NaN.

— Function: expt x y

This function returns x raised to power y. If both arguments are integers and y is positive, the result is an integer; in this case, overflow causes truncation, so watch out. If x is a finite negative number and y is a finite non-integer, expt returns a NaN.

— Function: sqrt arg

This returns the square root of arg. If arg is finite and less than zero, sqrt returns a NaN.

In addition, Emacs defines the following common mathematical constants:

— Variable: float-e

The mathematical constant e (2.71828...).

— Variable: float-pi

The mathematical constant pi (3.14159...).

Previous: Math Functions, Up: Numbers

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 value. 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.

— Function: random &optional limit

This function returns a pseudo-random integer. Repeated calls return a series of pseudo-random integers.

If limit is a positive integer, the value is chosen to be nonnegative and less than limit. Otherwise, the value might be any integer representable in Lisp, i.e., an integer between most-negative-fixnum and most-positive-fixnum (see Integer Basics).

If limit is t, it means to choose a new seed as if Emacs were restarting, typically from the system entropy. On systems lacking entropy pools, choose the seed from less-random volatile data such as the current time.

If limit is a string, it means to choose a new seed based on the string's contents.

Next: , Previous: Numbers, Up: Top

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.

Next: , Up: Strings and Characters

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 Size of Displayed Text) 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.

Next: , Previous: String Basics, Up: Strings and Characters

4.2 Predicates for Strings

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

— Function: stringp object

This function returns t if object is a string, nil otherwise.

— Function: string-or-null-p object

This function returns t if object is a string or nil. It returns nil otherwise.

— Function: char-or-string-p object

This function returns t if object is a string or a character (i.e., an integer), nil otherwise.

Next: , Previous: Predicates for Strings, Up: Strings and Characters

4.3 Creating Strings

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

— Function: make-string count character

This function returns a string made up of count repetitions of character. If count is negative, an error is signaled.

          (make-string 5 ?x)
               ⇒ "xxxxx"
          (make-string 0 ?x)
               ⇒ ""

Other functions to compare with this one include make-vector (see Vectors) and make-list (see Building Lists).

— Function: string &rest characters

This returns a string containing the characters characters.

          (string ?a ?b ?c)
               ⇒ "abc"
— Function: substring string &optional start end

This function returns a new string which consists of those characters from string in the range from (and including) the character at the index start up to (but excluding) the character at the index end. The first character is at index zero. With one argument, this function just copies string.

          (substring "abcdefg" 0 3)
               ⇒ "abc"

In the above example, the index for ‘a’ is 0, the index for ‘b’ is 1, and the index for ‘c’ is 2. The index 3—which is the fourth character in the string—marks the character position up to which the substring is copied. Thus, ‘abc’ is copied from the string "abcdefg".

A negative number counts from the end of the string, so that −1 signifies the index of the last character of the string. For example:

          (substring "abcdefg" -3 -1)
               ⇒ "ef"

In this example, the index for ‘e’ is −3, the index for ‘f’ is −2, and the index for ‘g’ is −1. Therefore, ‘e’ and ‘f’ are included, and ‘g’ is excluded.

When nil is used for end, it stands for the length of the string. Thus,

          (substring "abcdefg" -3 nil)
               ⇒ "efg"

Omitting the argument end is equivalent to specifying nil. It follows that (substring string 0) returns a copy of all of string.

          (substring "abcdefg" 0)
               ⇒ "abcdefg"

But we recommend copy-sequence for this purpose (see Sequence Functions).

If the characters copied from string have text properties, the properties are copied into the new string also. See Text Properties.

substring also accepts a vector for the first argument. For example:

          (substring [a b (c) "d"] 1 3)
               ⇒ [b (c)]

A wrong-type-argument error is signaled if start is not an integer or if end is neither an integer nor nil. An args-out-of-range error is signaled if start indicates a character following end, or if either integer is out of range for string.

Contrast this function with buffer-substring (see Buffer Contents), which returns a string containing a portion of the text in the current buffer. The beginning of a string is at index 0, but the beginning of a buffer is at index 1.

— Function: substring-no-properties string &optional start end

This works like substring but discards all text properties from the value. Also, start may be omitted or nil, which is equivalent to 0. Thus, (substring-no-properties string) returns a copy of string, with all text properties removed.

— Function: concat &rest sequences

This function returns a new string consisting of the characters in the arguments passed to it (along with their text properties, if any). The arguments may be strings, lists of numbers, or vectors of numbers; they are not themselves changed. If concat receives no arguments, it returns an empty string.

          (concat "abc" "-def")
               ⇒ "abc-def"
          (concat "abc" (list 120 121) [122])
               ⇒ "abcxyz"
          ;; nil is an empty sequence.
          (concat "abc" nil "-def")
               ⇒ "abc-def"
          (concat "The " "quick brown " "fox.")
               ⇒ "The quick brown fox."
               ⇒ ""

This function always constructs a new string that is not eq to any existing string, except when the result is the empty string (to save space, Emacs makes only one empty multibyte string).

For information about other concatenation functions, see the description of mapconcat in Mapping Functions, vconcat in Vector Functions, and append in Building Lists. For concatenating individual command-line arguments into a string to be used as a shell command, see combine-and-quote-strings.

— Function: split-string string &optional separators omit-nulls trim

This function splits string into substrings based on the regular expression separators (see Regular Expressions). Each match for separators defines a splitting point; the substrings between splitting points are made into a list, which is returned.

If separators is nil (or omitted), the default is the value of split-string-default-separators and the function behaves as if omit-nulls were t.

If omit-nulls is nil (or omitted), the result contains null strings whenever there are two consecutive matches for separators, or a match is adjacent to the beginning or end of string. If omit-nulls is t, these null strings are omitted from the result.

If the optional argument trim is non-nil, it should be a regular expression to match text to trim from the beginning and end of each substring. If trimming makes the substring empty, it is treated as null.

If you need to split a string into a list of individual command-line arguments suitable for call-process or start-process, see split-string-and-unquote.


          (split-string "  two words ")
               ⇒ ("two" "words")

The result is not ("" "two" "words" ""), which would rarely be useful. If you need such a result, use an explicit value for separators:

          (split-string "  two words "
               ⇒ ("" "two" "words" "")
          (split-string "Soup is good food" "o")
               ⇒ ("S" "up is g" "" "d f" "" "d")
          (split-string "Soup is good food" "o" t)
               ⇒ ("S" "up is g" "d f" "d")
          (split-string "Soup is good food" "o+")
               ⇒ ("S" "up is g" "d f" "d")

Empty matches do count, except that split-string will not look for a final empty match when it already reached the end of the string using a non-empty match or when string is empty:

          (split-string "aooob" "o*")
               ⇒ ("" "a" "" "b" "")
          (split-string "ooaboo" "o*")
               ⇒ ("" "" "a" "b" "")
          (split-string "" "")
               ⇒ ("")

However, when separators can match the empty string, omit-nulls is usually t, so that the subtleties in the three previous examples are rarely relevant:

          (split-string "Soup is good food" "o*" t)
               ⇒ ("S" "u" "p" " " "i" "s" " " "g" "d" " " "f" "d")
          (split-string "Nice doggy!" "" t)
               ⇒ ("N" "i" "c" "e" " " "d" "o" "g" "g" "y" "!")
          (split-string "" "" t)
               ⇒ nil

Somewhat odd, but predictable, behavior can occur for certain “non-greedy” values of separators that can prefer empty matches over non-empty matches. Again, such values rarely occur in practice:

          (split-string "ooo" "o*" t)
               ⇒ nil
          (split-string "ooo" "\\|o+" t)
               ⇒ ("o" "o" "o")
— Variable: split-string-default-separators

The default value of separators for split-string. Its usual value is "[ \f\t\n\r\v]+".

Next: , Previous: Creating Strings, Up: Strings and Characters

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:

— Function: store-substring string idx obj

This function alters part of the contents of the string string, by storing obj starting at index idx. The argument obj may be either a character or a (smaller) string.

Since it is impossible to change the length of an existing string, it is an error if obj doesn't fit within string's actual length, or if any new character requires a different number of bytes from the character currently present at that point in string.

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

— Function: clear-string string

This makes string a unibyte string and clears its contents to zeros. It may also change string's length.

Next: , Previous: Modifying Strings, Up: Strings and Characters

4.5 Comparison of Characters and Strings

— Function: char-equal character1 character2

This function returns t if the arguments represent the same character, nil otherwise. This function ignores differences in case if case-fold-search is non-nil.

          (char-equal ?x ?x)
               ⇒ t
          (let ((case-fold-search nil))
            (char-equal ?x ?X))
               ⇒ nil
— Function: string= string1 string2

This function returns t if the characters of the two strings match exactly. Symbols are also allowed as arguments, in which case the symbol names are used. Case is always significant, regardless of case-fold-search.

This function is equivalent to equal for comparing two strings (see Equality Predicates). In particular, the text properties of the two strings are ignored; use equal-including-properties if you need to distinguish between strings that differ only in their text properties. However, unlike equal, if either argument is not a string or symbol, string= signals an error.

          (string= "abc" "abc")
               ⇒ t
          (string= "abc" "ABC")
               ⇒ nil
          (string= "ab" "ABC")
               ⇒ nil

For technical reasons, a unibyte and a multibyte string are equal if and only if they contain the same sequence of character codes and all these codes are either in the range 0 through 127 (ASCII) or 160 through 255 (eight-bit-graphic). However, when a unibyte string is converted to a multibyte string, all characters with codes in the range 160 through 255 are converted to characters with higher codes, whereas ASCII characters remain unchanged. Thus, a unibyte string and its conversion to multibyte are only equal if the string is all ASCII. Character codes 160 through 255 are not entirely proper in multibyte text, even though they can occur. As a consequence, the situation where a unibyte and a multibyte string are equal without both being all ASCII is a technical oddity that very few Emacs Lisp programmers ever get confronted with. See Text Representations.

— Function: string-equal string1 string2

string-equal is another name for string=.

— Function: string-collate-equalp string1 string2 &optional locale ignore-case

This function returns t if string1 and string2 are equal with respect to collation rules. A collation rule is not only determined by the lexicographic order of the characters contained in string1 and string2, but also further rules about relations between these characters. Usually, it is defined by the locale environment Emacs is running with.

For example, characters with different coding points but the same meaning might be considered as equal, like different grave accent Unicode characters:

          (string-collate-equalp (string ?\uFF40) (string ?\u1FEF))
               ⇒ t

The optional argument locale, a string, overrides the setting of your current locale identifier for collation. The value is system dependent; a locale "en_US.UTF-8" is applicable on POSIX systems, while it would be, e.g., "enu_USA.1252" on MS-Windows systems.

If ignore-case is non-nil, characters are converted to lower-case before comparing them.

To emulate Unicode-compliant collation on MS-Windows systems, bind w32-collate-ignore-punctuation to a non-nil value, since the codeset part of the locale cannot be "UTF-8" on MS-Windows.

If your system does not support a locale environment, this function behaves like string-equal.

Do not use this function to compare file names for equality, as filesystems generally don't honor linguistic equivalence of strings that collation implements.

— Function: string< string1 string2

This function compares two strings a character at a time. It scans both the strings at the same time to find the first pair of corresponding characters that do not match. If the lesser character of these two is the character from string1, then string1 is less, and this function returns t. If the lesser character is the one from string2, then string1 is greater, and this function returns nil. If the two strings match entirely, the value is nil.

Pairs of characters are compared according to their character codes. Keep in mind that lower case letters have higher numeric values in the ASCII character set than their upper case counterparts; digits and many punctuation characters have a lower numeric value than upper case letters. An ASCII character is less than any non-ASCII character; a unibyte non-ASCII character is always less than any multibyte non-ASCII character (see Text Representations).

          (string< "abc" "abd")
               ⇒ t
          (string< "abd" "abc")
               ⇒ nil
          (string< "123" "abc")
               ⇒ t

When the strings have different lengths, and they match up to the length of string1, then the result is t. If they match up to the length of string2, the result is nil. A string of no characters is less than any other string.

          (string< "" "abc")
               ⇒ t
          (string< "ab" "abc")
               ⇒ t
          (string< "abc" "")
               ⇒ nil
          (string< "abc" "ab")
               ⇒ nil
          (string< "" "")
               ⇒ nil

Symbols are also allowed as arguments, in which case their print names are compared.

— Function: string-lessp string1 string2

string-lessp is another name for string<.

— Function: string-greaterp string1 string2

This function returns the result of comparing string1 and string2 in the opposite order, i.e., it is equivalent to calling (string-lessp string2 string1).

— Function: string-collate-lessp string1 string2 &optional locale ignore-case

This function returns t if string1 is less than string2 in collation order. A collation order is not only determined by the lexicographic order of the characters contained in string1 and string2, but also further rules about relations between these characters. Usually, it is defined by the locale environment Emacs is running with.

For example, punctuation and whitespace characters might be ignored for sorting (see Sequence Functions):

          (sort '("11" "12" "1 1" "1 2" "1.1" "1.2") 'string-collate-lessp)
               ⇒ ("11" "1 1" "1.1" "12" "1 2" "1.2")

This behavior is system-dependent; e.g., punctuation and whitespace are never ignored on Cygwin, regardless of locale.

The optional argument locale, a string, overrides the setting of your current locale identifier for collation. The value is system dependent; a locale "en_US.UTF-8" is applicable on POSIX systems, while it would be, e.g., "enu_USA.1252" on MS-Windows systems. The locale value of "POSIX" or "C" lets string-collate-lessp behave like string-lessp:

          (sort '("11" "12" "1 1" "1 2" "1.1" "1.2")
                (lambda (s1 s2) (string-collate-lessp s1 s2 "POSIX")))
               ⇒ ("1 1" "1 2" "1.1" "1.2" "11" "12")

If ignore-case is non-nil, characters are converted to lower-case before comparing them.

To emulate Unicode-compliant collation on MS-Windows systems, bind w32-collate-ignore-punctuation to a non-nil value, since the codeset part of the locale cannot be "UTF-8" on MS-Windows.

If your system does not support a locale environment, this function behaves like string-lessp.

— Function: string-version-lessp string1 string2

This function compares strings lexicographically, except it treats sequences of numerical characters as if they comprised a base-ten number, and then compares the numbers. So ‘foo2.png’ is “smaller” than ‘foo12.png’ according to this predicate, even if ‘12’ is lexicographically “smaller” than ‘2’.

— Function: string-prefix-p string1 string2 &optional ignore-case

This function returns non-nil if string1 is a prefix of string2; i.e., if string2 starts with string1. If the optional argument ignore-case is non-nil, the comparison ignores case differences.

— Function: string-suffix-p suffix string &optional ignore-case

This function returns non-nil if suffix is a suffix of string; i.e., if string ends with suffix. If the optional argument ignore-case is non-nil, the comparison ignores case differences.

— Function: compare-strings string1 start1 end1 string2 start2 end2 &optional ignore-case

This function compares a specified part of string1 with a specified part of string2. The specified part of string1 runs from index start1 (inclusive) up to index end1 (exclusive); nil for start1 means the start of the string, while nil for end1 means the length of the string. Likewise, the specified part of string2 runs from index start2 up to index end2.

The strings are compared by the numeric values of their characters. For instance, str1 is considered less than str2 if its first differing character has a smaller numeric value. If ignore-case is non-nil, characters are converted to upper-case before comparing them. Unibyte strings are converted to multibyte for comparison (see Text Representations), so that a unibyte string and its conversion to multibyte are always regarded as equal.

If the specified portions of the two strings match, the value is t. Otherwise, the value is an integer which indicates how many leading characters agree, and which string is less. Its absolute value is one plus the number of characters that agree at the beginning of the two strings. The sign is negative if string1 (or its specified portion) is less.

— Function: assoc-string key alist &optional case-fold

This function works like assoc, except that key must be a string or symbol, and comparison is done using compare-strings. Symbols are converted to strings before testing. If case-fold is non-nil, key and the elements of alist are converted to upper-case before comparison. Unlike assoc, this function can also match elements of the alist that are strings or symbols rather than conses. In particular, alist can be a list of strings or symbols rather than an actual alist. See Association Lists.

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.

Next: , Previous: Text Comparison, Up: Strings and Characters

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.

— Function: number-to-string number

This function returns a string consisting of the printed base-ten representation of number. The returned value starts with a minus sign if the argument is negative.

          (number-to-string 256)
               ⇒ "256"
          (number-to-string -23)
               ⇒ "-23"
          (number-to-string -23.5)
               ⇒ "-23.5"

int-to-string is a semi-obsolete alias for this function.

See also the function format in Formatting Strings.

— Function: string-to-number string &optional base

This function returns the numeric value of the characters in string. If base is non-nil, it must be an integer between 2 and 16 (inclusive), and integers are converted in that base. If base is nil, then base ten is used. Floating-point conversion only works in base ten; we have not implemented other radices for floating-point numbers, because that would be much more work and does not seem useful. If string looks like an integer but its value is too large to fit into a Lisp integer, string-to-number returns a floating-point result.

The parsing skips spaces and tabs at the beginning of string, then reads as much of string as it can interpret as a number in the given base. (On some systems it ignores other whitespace at the beginning, not just spaces and tabs.) If string cannot be interpreted as a number, this function returns 0.

          (string-to-number "256")
               ⇒ 256
          (string-to-number "25 is a perfect square.")
               ⇒ 25
          (string-to-number "X256")
               ⇒ 0
          (string-to-number "-4.5")
               ⇒ -4.5
          (string-to-number "1e5")
               ⇒ 100000.0

string-to-int is an obsolete alias for this function.

— Function: char-to-string character

This function returns a new string containing one character, character. This function is semi-obsolete because the function string is more general. See Creating Strings.

— Function: string-to-char string

This function returns the first character in string. This mostly identical to (aref string 0), except that it returns 0 if the string is empty. (The value is also 0 when the first character of string is the null character, ASCII code 0.) This function may be eliminated in the future if it does not seem useful enough to retain.

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

This function converts a vector or a list into a string. See Creating Strings.
This function converts a string into a vector. See Vector Functions.
This function converts a string into a list. See Building Lists.
This function converts a byte of character data into a unibyte string. See Converting Representations.

Next: , Previous: String Conversion, Up: Strings and Characters

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-message only in how they use the result of formatting.

— Function: format string &rest objects

This function returns a string equal to string, replacing any format specifications with encodings of the corresponding objects. The arguments objects are the computed values to be formatted.

The characters in string, other than the format specifications, are copied directly into the output, including their text properties, if any. Any text properties of the format specifications are copied to the produced string representations of the argument objects.

The output string need not be newly-allocated. For example, if x is the string "foo", the expressions (eq x (format x)) and (eq x (format "%s" x)) might both yield t.

— Function: format-message string &rest objects

This function acts like format, except it also converts any grave accents (`) and apostrophes (') in string as per the value of text-quoting-style.

Typically grave accent and apostrophe in the format translate to matching curved quotes, e.g., "Missing `%s'" might result in "Missing ‘foo’". See Text Quoting Style, for how to influence or inhibit this translation.

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)

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:

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.

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.
Replace the specification with the base-eight representation of an unsigned integer.
Replace the specification with the base-ten representation of a signed integer.
Replace the specification with the base-sixteen representation of an unsigned integer. ‘%x’ uses lower case and ‘%X’ uses upper case.
Replace the specification with the character which is the value given.
Replace the specification with the exponential notation for a floating-point number.
Replace the specification with the decimal-point notation for a floating-point number.
Replace the specification with notation for a floating-point number, using either exponential notation or decimal-point notation. The exponential notation is used if the exponent would be less than −4 or greater than or equal to the precision (default: 6). By default, trailing zeros are removed from the fractional portion of the result and a decimal-point character appears only if it is followed by a digit.
Replace the specification with a single ‘%’. This format specification is unusual in that its only form is plain ‘%%’ and 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, which assume the typical text-quoting-style settings:

     (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."
      "The name of this buffer is ‘%s’." (buffer-name))
          ⇒ "The name of this buffer is ‘strings.texi’."
      "The buffer object prints as `%s'." (current-buffer))
          ⇒ "The buffer object prints as ‘strings.texi’."

By default, 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.

A format specification can have a field number, which is a decimal number immediately after the initial ‘%’, followed by a literal dollar sign ‘$’. It causes the format specification to convert the argument with the given number instead of the next argument. Field numbers start at 1. A format can contain either numbered or unnumbered format specifications but not both, except that ‘%%’ can be mixed with numbered specifications.

     (format "%2$s, %3$s, %%, %1$s" "x" "y" "z")
          ⇒ "y, z, %, x"

After the ‘%’ and any field number, you can 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’ and ‘%f’, the ‘#’ flag means include a decimal point even if the precision is zero. For ‘%g’, it always includes a decimal point, and also forces any trailing zeros after the decimal point to be left in place where they would otherwise be removed.

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 any padding inserted by the width, if specified, 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."

A specification can have a width, which is a decimal number that appears after any field number and flags. If the printed representation of the object contains fewer characters than this width, format extends it with padding. Any padding introduced by the width 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 two 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."

All the specification characters allow an optional precision after the field number, flags and width, if present. The precision is a decimal-point ‘.’ followed by a digit-string. For the floating-point specifications (‘%e’ and ‘%f’), the precision specifies how many digits following the decimal point to show; if zero, the decimal-point itself is also omitted. For ‘%g’, the precision specifies how many significant digits to show (significant digits are the first digit before the decimal point and all the digits after it). If the precision of %g is zero or unspecified, it is treated as 1. 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. For other specification characters, the effect of precision is what the local library functions of the printf family produce.

Next: , Previous: Formatting Strings, Up: Strings and 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.

— Function: downcase string-or-char

This function converts string-or-char, which should be either a character or a string, to lower case.

When string-or-char is a string, this function returns a new string in which each letter in the argument that is upper case is converted to lower case. When string-or-char is a character, this function returns the corresponding lower case character (an integer); if the original character is lower case, or is not a letter, the return value is equal to the original character.

          (downcase "The cat in the hat")
               ⇒ "the cat in the hat"
          (downcase ?X)
               ⇒ 120
— Function: upcase string-or-char

This function converts string-or-char, which should be either a character or a string, to upper case.

When string-or-char is a string, this function returns a new string in which each letter in the argument that is lower case is converted to upper case. When string-or-char is a character, this function returns the corresponding upper case character (an integer); if the original character is upper case, or is not a letter, the return value is equal to the original character.

          (upcase "The cat in the hat")
               ⇒ "THE CAT IN THE HAT"
          (upcase ?x)
               ⇒ 88
— Function: capitalize string-or-char

This function capitalizes strings or characters. If string-or-char is a string, the function returns a new string whose contents are a copy of string-or-char in which each word has been capitalized. This means that the first character of each word is converted to upper case, and the rest are converted to lower case.

The definition of a word is any sequence of consecutive characters that are assigned to the word constituent syntax class in the current syntax table (see Syntax Class Table).

When string-or-char is a character, this function does the same thing as upcase.

          (capitalize "The cat in the hat")
               ⇒ "The Cat In The Hat"
          (capitalize "THE 77TH-HATTED CAT")
               ⇒ "The 77th-Hatted Cat"
          (capitalize ?x)
               ⇒ 88
— Function: upcase-initials string-or-char

If string-or-char is a string, this function capitalizes the initials of the words in string-or-char, without altering any letters other than the initials. It returns a new string whose contents are a copy of string-or-char, in which each word has had its initial letter converted to upper case.

The definition of a word is any sequence of consecutive characters that are assigned to the word constituent syntax class in the current syntax table (see Syntax Class Table).

When the argument to upcase-initials is a character, upcase-initials has the same result as upcase.

          (upcase-initials "The CAT in the hAt")
               ⇒ "The CAT In The HAt"

Note that case conversion is not a one-to-one mapping of codepoints and length of the result may differ from length of the argument. Furthermore, because passing a character forces return type to be a character, functions are unable to perform proper substitution and result may differ compared to treating a one-character string. For example:

     (upcase "fi")  ; note: single character, ligature "fi"
          ⇒ "FI"
     (upcase ?fi)
          ⇒ 64257  ; i.e. ?fi

To avoid this, a character must first be converted into a string, using string function, before being passed to one of the casing functions. Of course, no assumptions on the length of the result may be made.

Mapping for such special cases are taken from special-uppercase, special-lowercase and special-titlecase See Character Properties.

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

Previous: Case Conversion, Up: Strings and Characters

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:

The upcase table maps each character into the corresponding upper case character.
The canonicalize table maps all of a set of case-related characters into a particular member of that set.
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:

— Function: case-table-p object

This predicate returns non-nil if object is a valid case table.

— Function: set-standard-case-table table

This function makes table the standard case table, so that it will be used in any buffers created subsequently.

— Function: standard-case-table

This returns the standard case table.

— Function: current-case-table

This function returns the current buffer's case table.

— Function: set-case-table table

This sets the current buffer's case table to table.

— Macro: with-case-table table body...

The with-case-table macro saves the current case table, makes table the current case table, evaluates the body forms, and finally restores the case table. The return value is the value of the last form in body. The case table is restored even in case of an abnormal exit via throw or error (see Nonlocal Exits).

Some language environments modify the case conversions of ASCII characters; for example, in the Turkish language environment, the ASCII capital I is downcased into a Turkish dotless i (‘ı’). 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.

— Variable: ascii-case-table

The case table for the ASCII character set. This should not be modified by any language environment settings.

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.

— Function: set-case-syntax-pair uc lc case-table

This function specifies a pair of corresponding letters, one upper case and one lower case.

— Function: set-case-syntax-delims l r case-table

This function makes characters l and r a matching pair of case-invariant delimiters.

— Function: set-case-syntax char syntax case-table

This function makes char case-invariant, with syntax syntax.

— Command: describe-buffer-case-table

This command displays a description of the contents of the current buffer's case table.

Next: , Previous: Strings and Characters, Up: Top

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.

Next: , Up: Lists

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 proper list2. 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 proper list is always a proper list. The cdr of a nonempty proper list is a proper 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 proper, 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 proper 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.

Next: , Previous: Cons Cells, Up: Lists

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.)

— Function: consp object

This function returns t if object is a cons cell, nil otherwise. nil is not a cons cell, although it is a list.

— Function: atom object

This function returns t if object is an atom, nil otherwise. All objects except cons cells are atoms. The symbol nil is an atom and is also a list; it is the only Lisp object that is both.

          (atom object) == (not (consp object))
— Function: listp object

This function returns t if object is a cons cell or nil. Otherwise, it returns nil.

          (listp '(1))
               ⇒ t
          (listp '())
               ⇒ t
— Function: nlistp object

This function is the opposite of listp: it returns t if object is not a list. Otherwise, it returns nil.

          (listp object) == (not (nlistp object))
— Function: null object

This function returns t if object is nil, and returns nil otherwise. This function is identical to not, but as a matter of clarity we use null when object is considered a list and not when it is considered a truth value (see not in Combining Conditions).

          (null '(1))
               ⇒ nil
          (null '())
               ⇒ t

Next: , Previous: List-related Predicates, Up: Lists

5.3 Accessing Elements of Lists

— Function: car cons-cell

This function returns the value referred to by the first slot of the cons cell cons-cell. In other words, it returns the car of cons-cell.

As a special case, if cons-cell is nil, this function returns nil. Therefore, any list is a valid argument. An error is signaled if the argument is not a cons cell or nil.

          (car '(a b c))
               ⇒ a
          (car '())
               ⇒ nil
— Function: cdr cons-cell

This function returns the value referred to by the second slot of the cons cell cons-cell. In other words, it returns the cdr of cons-cell.

As a special case, if cons-cell is nil, this function returns nil; therefore, any list is a valid argument. An error is signaled if the argument is not a cons cell or nil.

          (cdr '(a b c))
               ⇒ (b c)
          (cdr '())
               ⇒ nil
— Function: car-safe object

This function lets you take the car of a cons cell while avoiding errors for other data types. It returns the car of object if object is a cons cell, nil otherwise. This is in contrast to car, which signals an error if object is not a list.

          (car-safe object)
          (let ((x object))
            (if (consp x)
                (car x)
— Function: cdr-safe object

This function lets you take the cdr of a cons cell while avoiding errors for other data types. It returns the cdr of object if object is a cons cell, nil otherwise. This is in contrast to cdr, which signals an error if object is not a list.

          (cdr-safe object)
          (let ((x object))
            (if (consp x)
                (cdr x)
— Macro: pop listname

This macro provides a convenient way to examine the car of a list, and take it off the list, all at once. It operates on the list stored in listname. It removes the first element from the list, saves the cdr into listname, then returns the removed element.

In the simplest case, listname is an unquoted symbol naming a list; in that case, this macro is equivalent to (prog1 (car listname) (setq listname (cdr listname))).

               ⇒ (a b c)
          (pop x)
               ⇒ a
               ⇒ (b c)

More generally, listname can be a generalized variable. In that case, this macro saves into listname using setf. See Generalized Variables.

For the push macro, which adds an element to a list, See List Variables.

— Function: nth n list

This function returns the nth element of list. Elements are numbered starting with zero, so the car of list is element number zero. If the length of list is n or less, the value is nil.

          (nth 2 '(1 2 3 4))
               ⇒ 3
          (nth 10 '(1 2 3 4))
               ⇒ nil
          (nth n x) == (car (nthcdr n x))

The function elt is similar, but applies to any kind of sequence. For historical reasons, it takes its arguments in the opposite order. See Sequence Functions.

— Function: nthcdr n list

This function returns the nth cdr of list. In other words, it skips past the first n links of list and returns what follows.

If n is zero, nthcdr returns all of list. If the length of list is n or less, nthcdr returns nil.

          (nthcdr 1 '(1 2 3 4))
               ⇒ (2 3 4)
          (nthcdr 10 '(1 2 3 4))
               ⇒ nil
          (nthcdr 0 '(1 2 3 4))
               ⇒ (1 2 3 4)
— Function: last list &optional n

This function returns the last link of list. The car of this link is the list's last element. If list is null, nil is returned. If n is non-nil, the nth-to-last link is returned instead, or the whole of list if n is bigger than list's length.

— Function: safe-length list

This function returns the length of list, with no risk of either an error or an infinite loop. It generally returns the number of distinct cons cells in the list. However, for circular lists, the value is just an upper bound; it is often too large.

If list is not nil or a cons cell, safe-length returns 0.

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.

— Function: caar cons-cell

This is the same as (car (car cons-cell)).

— Function: cadr cons-cell

This is the same as (car (cdr cons-cell)) or (nth 1 cons-cell).

— Function: cdar cons-cell

This is the same as (cdr (car cons-cell)).

— Function: cddr cons-cell

This is the same as (cdr (cdr cons-cell)) or (nthcdr 2 cons-cell).

In addition to the above, 24 additional compositions of car and cdr are defined as cxxxr and cxxxxr, where each x is either a or d. cadr, caddr, and cadddr pick out the second, third or fourth elements of a list, respectively. cl-lib provides the same under the names cl-second, cl-third, and cl-fourth. See List Functions.

— Function: butlast x &optional n

This function returns the list x with the last element, or the last n elements, removed. If n is greater than zero it makes a copy of the list so as not to damage the original list. In general, (append (butlast x n) (last x n)) will return a list equal to x.

— Function: nbutlast x &optional n

This is a version of butlast that works by destructively modifying the cdr of the appropriate element, rather than making a copy of the list.

Next: , Previous: List Elements, Up: Lists

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.

— Function: cons object1 object2

This function is the most basic function for building new list structure. It creates a new cons cell, making object1 the car, and object2 the cdr. It then returns the new cons cell. The arguments object1 and object2 may be any Lisp objects, but most often object2 is a list.

          (cons 1 '(2))
               ⇒ (1 2)
          (cons 1 '())
               ⇒ (1)
          (cons 1 2)
               ⇒ (1 . 2)

cons is often used to add a single element to the front of a list. This is called consing the element onto the list. 3 For example:

          (setq list (cons newelt list))

Note that there is no conflict between the variable named list used in this example and the function named list described below; any symbol can serve both purposes.

— Function: list &rest objects

This function creates a list with objects as its elements. The resulting list is always nil-terminated. If no objects are given, the empty list is returned.

          (list 1 2 3 4 5)
               ⇒ (1 2 3 4 5)
          (list 1 2 '(3 4 5) 'foo)
               ⇒ (1 2 (3 4 5) foo)
               ⇒ nil
— Function: make-list length object

This function creates a list of length elements, in which each element is object. Compare make-list with make-string (see Creating Strings).

          (make-list 3 'pigs)
               ⇒ (pigs pigs pigs)
          (make-list 0 'pigs)
               ⇒ nil
          (setq l (make-list 3 '(a b)))
               ⇒ ((a b) (a b) (a b))
          (eq (car l) (cadr l))
               ⇒ t
— Function: append &rest sequences

This function returns a list containing all the elements of sequences. The sequences may be lists, vectors, bool-vectors, or strings, but the last one should usually be a list. All arguments except the last one are copied, so none of the arguments is altered. (See nconc in Rearrangement, for a way to join lists with no copying.)

More generally, the final argument to append may be any Lisp object. The final argument is not copied or converted; it becomes the cdr of the last cons cell in the new list. If the final argument is itself a list, then its elements become in effect elements of the result list. If the final element is not a list, the result is a dotted list since its final cdr is not nil as required in a proper list (see Cons Cells).

Here is an example of using append:

     (setq trees '(pine oak))
          ⇒ (pine oak)
     (setq more-trees (append '(maple birch) trees))
          ⇒ (maple birch pine oak)
          ⇒ (pine oak)
          ⇒ (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:

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

          ⇒ 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.

— Function: copy-tree tree &optional vecp

This function returns a copy of the tree tree. If tree is a cons cell, this makes a new cons cell with the same car and cdr, then recursively copies the car and cdr in the same way.

Normally, when tree is anything other than a cons cell, copy-tree simply returns tree. However, if vecp is non-nil, it copies vectors too (and operates recursively on their elements).

— Function: number-sequence from &optional to separation

This returns a list of numbers starting with from and incrementing by separation, and ending at or just before to. separation can be positive or negative and defaults to 1. If to is nil or numerically equal to from, the value is the one-element list (from). If to is less than from with a positive separation, or greater than from with a negative separation, the value is nil because those arguments specify an empty sequence.

If separation is 0 and to is neither nil nor numerically equal to from, number-sequence signals an error, since those arguments specify an infinite sequence.

All arguments are numbers. Floating-point arguments can be tricky, because floating-point arithmetic is inexact. For instance, depending on the machine, it may quite well happen that (number-sequence 0.4 0.6 0.2) returns the one element list (0.4), whereas (number-sequence 0.4 0.8 0.2) returns a list with three elements. The nth element of the list is computed by the exact formula (+ from (* n separation)). Thus, if one wants to make sure that to is included in the list, one can pass an expression of this exact type for to. Alternatively, one can replace to with a slightly larger value (or a slightly more negative value if separation is negative).

Some examples:

          (number-sequence 4 9)
               ⇒ (4 5 6 7 8 9)
          (number-sequence 9 4 -1)
               ⇒ (9 8 7 6 5 4)
          (number-sequence 9 4 -2)
               ⇒ (9 7 5)
          (number-sequence 8)
               ⇒ (8)
          (number-sequence 8 5)
               ⇒ nil
          (number-sequence 5 8 -1)
               ⇒ nil
          (number-sequence 1.5 6 2)
               ⇒ (1.5 3.5 5.5)

Next: , Previous: Building Lists, Up: Lists

5.5 Modifying List Variables

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

— Macro: push element listname

This macro creates a new list whose car is element and whose cdr is the list specified by listname, and saves that list in listname. In the simplest case, listname is an unquoted symbol naming a list, and this macro is equivalent to (setq listname (cons element listname)).

          (setq l '(a b))
               ⇒ (a b)
          (push 'c l)
               ⇒ (c a b)
               ⇒ (c a b)

More generally, listname can be a generalized variable. In that case, this macro does the equivalent of (setf listname (cons element listname)). See Generalized Variables.

For the pop macro, which removes the first element from a list, See List Elements.

Two functions modify lists that are the values of variables.

— Function: add-to-list symbol element &optional append compare-fn

This function sets the variable symbol by consing element onto the old value, if element is not already a member of that value. It returns the resulting list, whether updated or not. The value of symbol had better be a list already before the call. add-to-list uses compare-fn to compare element against existing list members; if compare-fn is nil, it uses equal.

Normally, if element is added, it is added to the front of symbol, but if the optional argument append is non-nil, it is added at the end.

The argument symbol is not implicitly quoted; add-to-list is an ordinary function, like set and unlike setq. Quote the argument yourself if that is what you want.

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)))
— Function: add-to-ordered-list symbol element &optional order

This function sets the variable symbol by inserting element into the old value, which must be a list, at the position specified by order. If element is already a member of the list, its position in the list is adjusted according to order. Membership is tested using eq. This function returns the resulting list, whether updated or not.

The order is typically a number (integer or float), and the elements of the list are sorted in non-decreasing numerical order.

order may also be omitted or nil. Then the numeric order of element stays unchanged if it already has one; otherwise, element has no numeric order. Elements without a numeric list order are placed at the end of the list, in no particular order.

Any other value for order removes the numeric order of element if it already has one; otherwise, it is equivalent to nil.

The argument symbol is not implicitly quoted; add-to-ordered-list is an ordinary function, like set and unlike setq. Quote the argument yourself if necessary.

The ordering information is stored in a hash table on symbol's list-order property.

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)

Next: , Previous: List Variables, Up: Lists

5.6 Modifying Existing List Structure

You can modify the car and cdr contents of a cons cell with the primitives setcar and setcdr. These are 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.

Next: , Up: Modifying 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.

— Function: setcar cons object

This function stores object as the new car of cons, replacing its previous car. In other words, it changes the car slot of cons to refer to object. It returns the value object. For example:

          (setq x '(1 2))
               ⇒ (1 2)
          (setcar x 4)
               ⇒ 4
               ⇒ (4 2)

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)
          ⇒ (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)
          ⇒ (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:

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

Next: , Previous: Setcar, Up: Modifying Lists

5.6.2 Altering the CDR of a List

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

— Function: setcdr cons object

This function stores object as the new cdr of cons, replacing its previous cdr. In other words, it changes the cdr slot of cons to refer to object. It returns the value object.

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)
          ⇒ (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)
          ⇒ (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)
          ⇒ (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------
             |       |       |

Previous: Setcdr, Up: Modifying Lists

5.6.3 Functions that Rearrange Lists

Here are some functions that rearrange lists destructively by modifying the cdrs of their component cons cells. These functions are 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.

— Function: nconc &rest lists

This function returns a list containing all the elements of lists. Unlike append (see Building Lists), the lists are not copied. Instead, the last cdr of each of the lists is changed to refer to the following list. The last of the lists is not altered. For example:

          (setq x '(1 2 3))
               ⇒ (1 2 3)
          (nconc x '(4 5))
               ⇒ (1 2 3 4 5)
               ⇒ (1 2 3 4 5)

Since the last argument of nconc is not itself modified, it is reasonable to use a constant list, such as '(4 5), as in the above example. For the same reason, the last argument need not be a list:

          (setq x '(1 2 3))
               ⇒ (1 2 3)
          (nconc x 'z)
               ⇒ (1 2 3 . z)
               ⇒ (1 2 3 . z)

However, the other arguments (all but the last) must be lists.

A common pitfall is to use a quoted constant list as a non-last argument to nconc. If you do this, your program will change each time you run it! Here is what happens:

          (defun add-foo (x)            ; We want this function to add
            (nconc '(foo) x))           ;   foo to the front of its arg.
          (symbol-function 'add-foo)
               ⇒ (lambda (x) (nconc (quote (foo)) x))
          (setq xx (add-foo '(1 2)))    ; It seems to work.
               ⇒ (foo 1 2)
          (setq xy (add-foo '(3 4)))    ; What happened?
               ⇒ (foo 1 2 3 4)
          (eq xx xy)
               ⇒ t
          (symbol-function 'add-foo)
               ⇒ (lambda (x) (nconc (quote (foo 1 2 3 4) x)))

Next: , Previous: Modifying Lists, Up: Lists

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.
— Function: memq object list

This function tests to see whether object is a member of list. If it is, memq returns a list starting with the first occurrence of object. Otherwise, it returns nil. The letter ‘q’ in memq says that it uses eq to compare object against the elements of the list. For example:

          (memq 'b '(a b c b a))
               ⇒ (b c b a)
          (memq '(2) '((1) (2)))    ; (2) and (2) are not eq.
               ⇒ nil
— Function: delq object list

This function destructively removes all elements eq to object from list, and returns the resulting list. The letter ‘q’ in delq says that it uses eq to compare object against the elements of the list, like memq and remq.

Typically, when you invoke delq, you should use the return value by assigning it to the variable which held the original list. The reason for this is explained below.

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

— Function: remq object list

This function returns a copy of list, with all elements removed which are eq to object. The letter ‘q’ in remq says that it uses eq to compare object against the elements of list.

          (setq sample-list '(a b c a b c))
               ⇒ (a b c a b c)
          (remq 'a sample-list)
               ⇒ (b c b c)
               ⇒ (a b c a b c)
— Function: memql object list

The function memql tests to see whether object is a member of list, comparing members with object using eql, so floating-point elements are compared by value. If object is a member, memql returns a list starting with its first occurrence in list. Otherwise, it returns nil.

Compare this with memq:

          (memql 1.2 '(1.1 1.2 1.3))  ; 1.2 and 1.2 are eql.
               ⇒ (1.2 1.3)
          (memq 1.2 '(1.1 1.2 1.3))  ; 1.2 and 1.2 are not eq.
               ⇒ nil

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

— Function: member object list

The function member tests to see whether object is a member of list, comparing members with object using equal. If object is a member, member returns a list starting with its first occurrence in list. Otherwise, it returns nil.

Compare this with memq:

          (member '(2) '((1) (2)))  ; (2) and (2) are equal.
               ⇒ ((2))
          (memq '(2) '((1) (2)))    ; (2) and (2) are not eq.
               ⇒ nil
          ;; Two strings with the same contents are equal.
          (member "foo" '("foo" "bar"))
               ⇒ ("foo" "bar")
— Function: delete object sequence

This function removes all elements equal to object from sequence, and returns the resulting sequence.

If sequence is a list, delete is to delq as member is to memq: it uses equal to compare elements with object, like member; when it finds an element that matches, it cuts the element out just as delq would. As with delq, you should typically use the return value by assigning it to the variable which held the original list.

If sequence is a vector or string, delete returns a copy of sequence with all elements equal to object removed.

For example:

          (setq l '((2) (1) (2)))
          (delete '(2) l)
               ⇒ ((1))
               ⇒ ((2) (1))
          ;; If you want to change l reliably,
          ;; write (setq l (delete '(2) l)).
          (setq l '((2) (1) (2)))
          (delete '(1) l)
               ⇒ ((2) (2))
               ⇒ ((2) (2))
          ;; In this case, it makes no difference whether you set l,
          ;; but you should do so for the sake of the other case.
          (delete '(2) [(2) (1) (2)])
               ⇒ [(1)]
— Function: remove object sequence

This function is the non-destructive counterpart of delete. It returns a copy of sequence, a list, vector, or string, with elements equal to object removed. For example:

          (remove '(2) '((2) (1) (2)))
               ⇒ ((1))
          (remove '(2) [(2) (1) (2)])
               ⇒ [(1)]
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.
— Function: member-ignore-case object list

This function is like member, except that object should be a string and that it ignores differences in letter-case and text representation: upper-case and lower-case letters are treated as equal, and unibyte strings are converted to multibyte prior to comparison.

— Function: delete-dups list

This function destructively removes all equal duplicates from list, stores the result in list and returns it. Of several equal occurrences of an element in list, delete-dups keeps the first one.

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.

Next: , Previous: Sets And Lists, Up: Lists

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.4

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.

— Function: assoc key alist &optional testfn

This function returns the first association for key in alist, comparing key against the alist elements using testfn if it is non-nil and equal otherwise (see Equality Predicates). It returns nil if no association in alist has a car equal to key. For example:

          (setq trees '((pine . cones) (oak . acorns) (maple . seeds)))
               ⇒ ((pine . cones) (oak . acorns) (maple . seeds))
          (assoc 'oak trees)
               ⇒ (oak . acorns)
          (cdr (assoc 'oak trees))
               ⇒ acorns
          (assoc 'birch trees)
               ⇒ nil

Here is another example, in which the keys and values are not symbols:

          (setq needles-per-cluster
                '((2 "Austrian Pine" "Red Pine")
                  (3 "Pitch Pine")
                  (5 "White Pine")))
          (cdr (assoc 3 needles-per-cluster))
               ⇒ ("Pitch Pine")
          (cdr (assoc 2 needles-per-cluster))
               ⇒ ("Austrian Pine" "Red Pine")

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

— Function: rassoc value alist

This function returns the first association with value value in alist. It returns nil if no association in alist has a cdr equal to value.

rassoc is like assoc except that it compares the cdr of each alist association instead of the car. You can think of this as reverse assoc, finding the key for a given value.

— Function: assq key alist

This function is like assoc in that it returns the first association for key in alist, but it makes the comparison using eq. assq returns nil if no association in alist has a car eq to key. This function is used more often than assoc, since eq is faster than equal and most alists use symbols as keys. See Equality Predicates.

          (setq trees '((pine . cones) (oak . acorns) (maple . seeds)))
               ⇒ ((pine . cones) (oak . acorns) (maple . seeds))
          (assq 'pine trees)
               ⇒ (pine . cones)

On the other hand, assq is not usually useful in alists where the keys may not be symbols:

          (setq leaves
                '(("simple leaves" . oak)
                  ("compound leaves" . horsechestnut)))
          (assq "simple leaves" leaves)
               ⇒ nil
          (assoc "simple leaves" leaves)
               ⇒ ("simple leaves" . oak)
— Function: alist-get key alist &optional default remove testfn

This function is similar to assq. It finds the first association (key . value) by comparing key with alist elements, and, if found, returns the value of that association. If no association is found, the function returns default. Comparison of key against alist elements uses the function specified by testfn, defaulting to eq.

This is a generalized variable (see Generalized Variables) that can be used to change a value with setf. When using it to set a value, optional argument remove non-nil means to remove key's association from alist if the new value is eql to default.

— Function: rassq value alist

This function returns the first association with value value in alist. It returns nil if no association in alist has a cdr eq to value.

rassq is like assq except that it compares the cdr of each alist association instead of the car. You can think of this as reverse assq, finding the key for a given value.

For example:

          (setq trees '((pine . cones) (oak . acorns) (maple . seeds)))
          (rassq 'acorns trees)
               ⇒ (oak . acorns)
          (rassq 'spores trees)
               ⇒ nil

rassq cannot search for a value stored in the car of the cdr of an element:

          (setq colors '((rose red) (lily white) (buttercup yellow)))
          (rassq 'white colors)
               ⇒ nil

In this case, the cdr of the association (lily white) is not the symbol white, but rather the list (white). This becomes clearer if the association is written in dotted pair notation:

          (lily white) == (lily . (white))
— Function: assoc-default key alist &optional test default

This function searches alist for a match for key. For each element of alist, it compares the element (if it is an atom) or the element's car (if it is a cons) against key, by calling test with two arguments: the element or its car, and key. The arguments are passed in that order so that you can get useful results using string-match with an alist that contains regular expressions (see Regexp Search). If test is omitted or nil, equal is used for comparison.

If an alist element matches key by this criterion, then assoc-default returns a value based on this element. If the element is a cons, then the value is the element's cdr. Otherwise, the return value is default.

If no alist element matches key, assoc-default returns nil.

— Function: copy-alist alist

This function returns a two-level deep copy of alist: it creates a new copy of each association, so that you can alter the associations of the new alist without changing the old one.

          (setq needles-per-cluster
                '((2 . ("Austrian Pine" "Red Pine"))
                  (3 . ("Pitch Pine"))
                  (5 . ("White Pine"))))
          ((2 "Austrian Pine" "Red Pine")
           (3 "Pitch Pine")
           (5 "White Pine"))
          (setq copy (copy-alist needles-per-cluster))
          ((2 "Austrian Pine" "Red Pine")
           (3 "Pitch Pine")
           (5 "White Pine"))
          (eq needles-per-cluster copy)
               ⇒ nil
          (equal needles-per-cluster copy)
               ⇒ t
          (eq (car needles-per-cluster) (car copy))
               ⇒ nil
          (cdr (car (cdr needles-per-cluster)))
               ⇒ ("Pitch Pine")
          (eq (cdr (car (cdr needles-per-cluster)))
              (cdr (car (cdr copy))))
               ⇒ t

This example shows how copy-alist makes it possible to change the associations of one copy without affecting the other:

          (setcdr (assq 3 copy) '("Martian Vacuum Pine"))
          (cdr (assq 3 needles-per-cluster))
               ⇒ ("Pitch Pine")
— Function: assq-delete-all key alist

This function deletes from alist all the elements whose car is eq to key, much as if you used delq to delete each such element one by one. It returns the shortened alist, and often modifies the original list structure of alist. For correct results, use the return value of assq-delete-all rather than looking at the saved value of alist.

          (setq alist '((foo 1) (bar 2) (foo 3) (lose 4)))
               ⇒ ((foo 1) (bar 2) (foo 3) (lose 4))
          (assq-delete-all 'foo alist)
               ⇒ ((bar 2) (lose 4))
               ⇒ ((foo 1) (bar 2) (lose 4))
— Function: assoc-delete-all key alist

This function deletes from alist all the elements whose car is equal to key. It works like assq-delete-all, except for the predicate used for comparing alist elements with key.

— Function: rassq-delete-all value alist

This function deletes from alist all the elements whose cdr is eq to value. It returns the shortened alist, and often modifies the original list structure of alist. rassq-delete-all is like assq-delete-all except that it compares the cdr of each alist association instead of the car.

Previous: Association Lists, Up: Lists

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.

Next: , Up: Property Lists

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.

Previous: Plists and Alists, Up: Property Lists

5.9.2 Property Lists Outside Symbols

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

— Function: plist-get plist property

This returns the value of the property property stored in the property list plist. It accepts a malformed plist argument. If property is not found in the plist, it returns nil. For example,

          (plist-get '(foo 4) 'foo)
               ⇒ 4
          (plist-get '(foo 4 bad) 'foo)
               ⇒ 4
          (plist-get '(foo 4 bad) 'bad)
               ⇒ nil
          (plist-get '(foo 4 bad) 'bar)
               ⇒ nil
— Function: plist-put plist property value

This stores value as the value of the property property in the property list plist. It may modify plist destructively, or it may construct a new list structure without altering the old. The function returns the modified property list, so you can store that back in the place where you got plist. For example,

          (setq my-plist '(bar t foo 4))
               ⇒ (bar t foo 4)
          (setq my-plist (plist-put my-plist 'foo 69))
               ⇒ (bar t foo 69)
          (setq my-plist (plist-put my-plist 'quux '(a)))
               ⇒ (bar t foo 69 quux (a))
— Function: lax-plist-get plist property

Like plist-get except that it compares properties using equal instead of eq.

— Function: lax-plist-put plist property value

Like plist-put except that it compares properties using equal instead of eq.

— Function: plist-member plist property

This returns non-nil if plist contains the given property. Unlike plist-get, this allows you to distinguish between a missing property and a property with the value nil. The value is actually the tail of plist whose car is property.

Next: , Previous: Lists, Up: Top

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 | | |
              |          | |____________| |_____________| | |
              |          |________________________________| |

Next: , Up: Sequences Arrays Vectors

6.1 Sequences

This section describes functions that accept any kind of sequence.

— Function: sequencep object

This function returns t if object is a list, vector, string, bool-vector, or char-table, nil otherwise. See also seqp below.

— Function: length sequence

This function returns the number of elements in sequence. The function signals the wrong-type-argument error if the argument is not a sequence or is a dotted list; it signals the circular-list error if the argument is a circular list. For a char-table, the value returned is always one more than the maximum Emacs character code.

See Definition of safe-length, for the related function safe-length.

          (length '(1 2 3))
              ⇒ 3
          (length ())
              ⇒ 0
          (length "foobar")
              ⇒ 6
          (length [1 2 3])
              ⇒ 3
          (length (make-bool-vector 5 nil))
              ⇒ 5

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 Size of Displayed Text), not length, since length only counts the number of characters, but does not account for the display width of each character.

— Function: elt sequence index

This function returns the element of sequence indexed by index. Legitimate values of index are integers ranging from 0 up to one less than the length of sequence. If sequence is a list, out-of-range values behave as for nth. See Definition of nth. Otherwise, out-of-range values trigger an args-out-of-range error.

          (elt [1 2 3 4] 2)
               ⇒ 3
          (elt '(1 2 3 4) 2)
               ⇒ 3
          ;; We use string to show clearly which character elt returns.
          (string (elt "1234" 2))
               ⇒ "3"
          (elt [1 2 3 4] 4)
               error--> Args out of range: [1 2 3 4], 4
          (elt [1 2 3 4] -1)
               error--> Args out of range: [1 2 3 4], -1

This function generalizes aref (see Array Functions) and nth (see Definition of nth).

— Function: copy-sequence seqr

This function returns a copy of seqr, which should be either a sequence or a record. The copy is the same type of object as the original, and it has the same elements in the same order. However, if seqr is empty, like a string or a vector of zero length, the value returned by this function might not be a copy, but an empty object of the same type and identical to seqr.

Storing a new element into the copy does not affect the original seqr, and vice versa. However, the elements of the copy are not copies; they are identical (eq) to the elements of the original. Therefore, changes made within these elements, as found via the copy, are also visible in the original.

If the argument is a string with text properties, the property list in the copy is itself a copy, not shared with the original's property list. However, the actual values of the properties are shared. See Text Properties.

This function does not work for dotted lists. Trying to copy a circular list may cause an infinite loop.

See also append in Building Lists, concat in Creating Strings, and vconcat in Vector Functions, for other ways to copy sequences.

          (setq bar '(1 2))
               ⇒ (1 2)
          (setq x (vector 'foo bar))
               ⇒ [foo (1 2)]
          (setq y (copy-sequence x))
               ⇒ [foo (1 2)]
          (eq x y)
               ⇒ nil
          (equal x y)
               ⇒ t
          (eq (elt x 1) (elt y 1))
               ⇒ t
          ;; Replacing an element of one sequence.
          (aset x 0 'quux)
          x ⇒ [quux (1 2)]
          y ⇒ [foo (1 2)]
          ;; Modifying the inside of a shared element.
          (setcar (aref x 1) 69)
          x ⇒ [quux (69 2)]
          y ⇒ [foo (69 2)]
— Function: reverse sequence

This function creates a new sequence whose elements are the elements of sequence, but in reverse order. The original argument sequence is not altered. Note that char-tables cannot be reversed.

          (setq x '(1 2 3 4))
               ⇒ (1 2 3 4)
          (reverse x)
               ⇒ (4 3 2 1)
               ⇒ (1 2 3 4)
          (setq x [1 2 3 4])
               ⇒ [1 2 3 4]
          (reverse x)
               ⇒ [4 3 2 1]
               ⇒ [1 2 3 4]
          (setq x "xyzzy")
               ⇒ "xyzzy"
          (reverse x)
               ⇒ "yzzyx"
               ⇒ "xyzzy"
— Function: nreverse sequence

This function reverses the order of the elements of sequence. Unlike reverse the original sequence may be modified.

For example:

          (setq x '(a b c))
               ⇒ (a b c)
               ⇒ (a b c)
          (nreverse x)
               ⇒ (c b a)
          ;; The cons cell that was first is now last.
               ⇒ (a)

To avoid confusion, we usually store the result of nreverse back in the same variable which held the original list:

          (setq x (nreverse x))

Here is the nreverse of our favorite example, (a b c), presented graphically:

          Original list head:                       Reversed list:
           -------------        -------------        ------------
          | car  | cdr  |      | car  | cdr  |      | car | cdr  |
          |   a  |  nil |<--   |   b  |   o  |<--   |   c |   o  |
          |      |      |   |  |      |   |  |   |  |     |   |  |
           -------------    |   --------- | -    |   -------- | -
                            |             |      |            |
                             -------------        ------------

For the vector, it is even simpler because you don't need setq:

          (setq x [1 2 3 4])
               ⇒ [1 2 3 4]
          (nreverse x)
               ⇒ [4 3 2 1]
               ⇒ [4 3 2 1]

Note that unlike reverse, this function doesn't work with strings. Although you can alter string data by using aset, it is strongly encouraged to treat strings as immutable.

— Function: sort sequence predicate

This function sorts sequence stably. Note that this function doesn't work for all sequences; it may be used only for lists and vectors. If sequence is a list, it is modified destructively. This functions returns the sorted sequence and compares elements using predicate. A stable sort is one in which elements with equal sort keys maintain their relative order before and after the sort. Stability is important when successive sorts are used to order elements according to different criteria.

The argument predicate must be a function that accepts two arguments. It is called with two elements of sequence. To get an increasing order sort, the predicate should return non-nil if the first element is “less” than the second, or nil if not.

The comparison function predicate must give reliable results for any given pair of arguments, at least within a single call to sort. It must be antisymmetric; that is, if a is less than b, b must not be less than a. It must be transitive—that is, if a is less than b, and b is less than c, then a must be less than c. If you use a comparison function which does not meet these requirements, the result of sort is unpredictable.

The destructive aspect of sort for lists is that it rearranges the cons cells forming sequence by changing cdrs. A nondestructive sort function would create new cons cells to store the elements in their sorted order. If you wish to make a sorted copy without destroying the original, copy it first with copy-sequence and then sort.

Sorting does not change the cars of the cons cells in sequence; the cons cell that originally contained the element a in sequence still has a in its car after sorting, but it now appears in a different position in the list due to the change of cdrs. For example:

          (setq nums '(1 3 2 6 5 4 0))
               ⇒ (1 3 2 6 5 4 0)
          (sort nums '<)
               ⇒ (0 1 2 3 4 5 6)
               ⇒ (1 2 3 4 5 6)

Warning: Note that the list in nums no longer contains 0; this is the same cons cell that it was before, but it is no longer the first one in the list. Don't assume a variable that formerly held the argument now holds the entire sorted list! Instead, save the result of sort and use that. Most often we store the result back into the variable that held the original list:

          (setq nums (sort nums '<))

For the better understanding of what stable sort is, consider the following vector example. After sorting, all items whose car is 8 are grouped at the beginning of vector, but their relative order is preserved. All items whose car is 9 are grouped at the end of vector, but their relative order is also preserved:

            (vector '(8 . "xxx") '(9 . "aaa") '(8 . "bbb") '(9 . "zzz")
                    '(9 . "ppp") '(8 . "ttt") '(8 . "eee") '(9 . "fff")))
               ⇒ [(8 . "xxx") (9 . "aaa") (8 . "bbb") (9 . "zzz")
                   (9 . "ppp") (8 . "ttt") (8 . "eee") (9 . "fff")]
          (sort vector (lambda (x y) (< (car x) (car y))))
               ⇒ [(8 . "xxx") (8 . "bbb") (8 . "ttt") (8 . "eee")
                   (9 . "aaa") (9 . "zzz") (9 . "ppp") (9 . "fff")]

See Sorting, for more functions that perform sorting. See documentation in Accessing Documentation, for a useful example of sort.

The seq.el library provides the following additional sequence manipulation macros and functions, prefixed with seq-. To use them, you must first load the seq library.

All functions defined in this library are free of side-effects; i.e., they do not modify any sequence (list, vector, or string) that you pass as an argument. Unless otherwise stated, the result is a sequence of the same type as the input. For those functions that take a predicate, this should be a function of one argument.

The seq.el library can be extended to work with additional types of sequential data-structures. For that purpose, all functions are defined using cl-defgeneric. See Generic Functions, for more details about using cl-defgeneric for adding extensions.

— Function: seq-elt sequence index

This function returns the element of sequence at the specified index, which is an integer whose valid value range is zero to one less than the length of sequence. For out-of-range values on built-in sequence types, seq-elt behaves like elt. For the details, see Definition of elt.

          (seq-elt [1 2 3 4] 2)
          ⇒ 3

seq-elt returns places settable using setf (see Setting Generalized Variables).

          (setq vec [1 2 3 4])
          (setf (seq-elt vec 2) 5)
          ⇒ [1 2 5 4]
— Function: seq-length sequence

This function returns the number of elements in sequence. For built-in sequence types, seq-length behaves like length. See Definition of length.

— Function: seqp object

This function returns non-nil if object is a sequence (a list or array), or any additional type of sequence defined via seq.el generic functions. This is an extensible variant of sequencep.

          (seqp [1 2])
          ⇒ t
          (seqp 2)
          ⇒ nil
— Function: seq-drop sequence n

This function returns all but the first n (an integer) elements of sequence. If n is negative or zero, the result is sequence.

          (seq-drop [1 2 3 4 5 6] 3)
          ⇒ [4 5 6]
          (seq-drop "hello world" -4)
          ⇒ "hello world"
— Function: seq-take sequence n

This function returns the first n (an integer) elements of sequence. If n is negative or zero, the result is nil.

          (seq-take '(1 2 3 4) 3)
          ⇒ (1 2 3)
          (seq-take [1 2 3 4] 0)
          ⇒ []
— Function: seq-take-while predicate sequence

This function returns the members of sequence in order, stopping before the first one for which predicate returns nil.

          (seq-take-while (lambda (elt) (> elt 0)) '(1 2 3 -1 -2))
          ⇒ (1 2 3)
          (seq-take-while (lambda (elt) (> elt 0)) [-1 4 6])
          ⇒ []
— Function: seq-drop-while predicate sequence

This function returns the members of sequence in order, starting from the first one for which predicate returns nil.

          (seq-drop-while (lambda (elt) (> elt 0)) '(1 2 3 -1 -2))
          ⇒ (-1 -2)
          (seq-drop-while (lambda (elt) (< elt 0)) [1 4 6])
          ⇒ [1 4 6]
— Function: seq-do function sequence

This function applies function to each element of sequence in turn (presumably for side effects), and returns sequence.

— Function: seq-map function sequence

This function returns the result of applying function to each element of sequence. The returned value is a list.

          (seq-map #'1+ '(2 4 6))
          ⇒ (3 5 7)
          (seq-map #'symbol-name [foo bar])
          ⇒ ("foo" "bar")
— Function: seq-map-indexed function sequence

This function returns the result of applying function to each element of sequence and its index within seq. The returned value is a list.

          (seq-map-indexed (lambda (elt idx)
                             (list idx elt))
                           '(a b c))
          ⇒ ((0 a) (b 1) (c 2))
— Function: seq-mapn function &rest sequences

This function returns the result of applying function to each element of sequences. The arity (see subr-arity) of function must match the number of sequences. Mapping stops at the end of the shortest sequence, and the returned value is a list.

          (seq-mapn #'+ '(2 4 6) '(20 40 60))
          ⇒ (22 44 66)
          (seq-mapn #'concat '("moskito" "bite") ["bee" "sting"])
          ⇒ ("moskitobee" "bitesting")
— Function: seq-filter predicate sequence

This function returns a list of all the elements in sequence for which predicate returns non-nil.

          (seq-filter (lambda (elt) (> elt 0)) [1 -1 3 -3 5])
          ⇒ (1 3 5)
          (seq-filter (lambda (elt) (> elt 0)) '(-1 -3 -5))
          ⇒ nil
— Function: seq-remove predicate sequence

This function returns a list of all the elements in sequence for which predicate returns nil.

          (seq-remove (lambda (elt) (> elt 0)) [1 -1 3 -3 5])
          ⇒ (-1 -3)
          (seq-remove (lambda (elt) (< elt 0)) '(-1 -3 -5))
          ⇒ nil
— Function: seq-reduce function sequence initial-value

This function returns the result of calling function with initial-value and the first element of sequence, then calling function with that result and the second element of sequence, then with that result and the third element of sequence, etc. function should be a function of two arguments. If sequence is empty, this returns initial-value without calling function.

          (seq-reduce #'+ [1 2 3 4] 0)
          ⇒ 10
          (seq-reduce #'+ '(1 2 3 4) 5)
          ⇒ 15
          (seq-reduce #'+ '() 3)
          ⇒ 3
— Function: seq-some predicate sequence

This function returns the first non-nil value returned by applying predicate to each element of sequence in turn.

          (seq-some #'numberp ["abc" 1 nil])
          ⇒ t
          (seq-some #'numberp ["abc" "def"])
          ⇒ nil
          (seq-some #'null ["abc" 1 nil])
          ⇒ t
          (seq-some #'1+ [2 4 6])
          ⇒ 3
— Function: seq-find predicate sequence &optional default

This function returns the first element in sequence for which predicate returns non-nil. If no element matches predicate, the function returns default.

Note that this function has an ambiguity if the found element is identical to default, as in that case it cannot be known whether an element was found or not.

          (seq-find #'numberp ["abc" 1 nil])
          ⇒ 1
          (seq-find #'numberp ["abc" "def"])
          ⇒ nil
— Function: seq-every-p predicate sequence

This function returns non-nil if applying predicate to every element of sequence returns non-nil.

          (seq-every-p #'numberp [2 4 6])
          ⇒ t
          (seq-every-p #'numberp [2 4 "6"])
          ⇒ nil
— Function: seq-empty-p sequence

This function returns non-nil if sequence is empty.

          (seq-empty-p "not empty")
          ⇒ nil
          (seq-empty-p "")
          ⇒ t
— Function: seq-count predicate sequence

This function returns the number of elements in sequence for which predicate returns non-nil.

          (seq-count (lambda (elt) (> elt 0)) [-1 2 0 3 -2])
          ⇒ 2

— Function: seq-sort function sequence

This function returns a copy of sequence that is sorted according to function, a function of two arguments that returns non-nil if the first argument should sort before the second.

— Function: seq-sort-by function predicate sequence

This function is similar to seq-sort, but the elements of sequence are transformed by applying function on them before being sorted. function is a function of one argument.

          (seq-sort-by #'seq-length #'> ["a" "ab" "abc"])
          ⇒ ["abc" "ab" "a"]
— Function: seq-contains sequence elt &optional function

This function returns the first element in sequence that is equal to elt. If the optional argument function is non-nil, it is a function of two arguments to use instead of the default equal.

          (seq-contains '(symbol1 symbol2) 'symbol1)
          ⇒ symbol1
          (seq-contains '(symbol1 symbol2) 'symbol3)
          ⇒ nil
— Function: seq-set-equal-p sequence1 sequence2 &optional testfn

This function checks whether sequence1 and sequence2 contain the same elements, regardless of the order. If the optional argument testfn is non-nil, it is a function of two arguments to use instead of the default equal.

          (seq-set-equal-p '(a b c) '(c b a))
          ⇒ t
          (seq-set-equal-p '(a b c) '(c b))
          ⇒ nil
          (seq-set-equal-p '("a" "b" "c") '("c" "b" "a"))
          ⇒ t
          (seq-set-equal-p '("a" "b" "c") '("c" "b" "a") #'eq)
          ⇒ nil
— Function: seq-position sequence elt &optional function

This function returns the index of the first element in sequence that is equal to elt. If the optional argument function is non-nil, it is a function of two arguments to use instead of the default equal.

          (seq-position '(a b c) 'b)
          ⇒ 1
          (seq-position '(a b c) 'd)
          ⇒ nil
— Function: seq-uniq sequence &optional function

This function returns a list of the elements of sequence with duplicates removed. If the optional argument function is non-nil, it is a function of two arguments to use instead of the default equal.

          (seq-uniq '(1 2 2 1 3))
          ⇒ (1 2 3)
          (seq-uniq '(1 2 2.0 1.0) #'=)
          ⇒ (1 2)
— Function: seq-subseq sequence start &optional end

This function returns a subset of sequence from start to end, both integers (end defaults to the last element). If start or end is negative, it counts from the end of sequence.

          (seq-subseq '(1 2 3 4 5) 1)
          ⇒ (2 3 4 5)
          (seq-subseq '[1 2 3 4 5] 1 3)
          ⇒ [2 3]
          (seq-subseq '[1 2 3 4 5] -3 -1)
          ⇒ [3 4]
— Function: seq-concatenate type &rest sequences

This function returns a sequence of type type made of the concatenation of sequences. type may be: vector, list or string.

          (seq-concatenate 'list '(1 2) '(3 4) [5 6])
          ⇒ (1 2 3 4 5 6)
          (seq-concatenate 'string "Hello " "world")
          ⇒ "Hello world"
— Function: seq-mapcat function sequence &optional type

This function returns the result of applying seq-concatenate to the result of applying function to each element of sequence. The result is a sequence of type type, or a list if type is nil.

          (seq-mapcat #'seq-reverse '((3 2 1) (6 5 4)))
          ⇒ (1 2 3 4 5 6)
— Function: seq-partition sequence n

This function returns a list of the elements of sequence grouped into sub-sequences of length n. The last sequence may contain less elements than n. n must be an integer. If n is a negative integer or 0, the return value is nil.

          (seq-partition '(0 1 2 3 4 5 6 7) 3)
          ⇒ ((0 1 2) (3 4 5) (6 7))
— Function: seq-intersection sequence1 sequence2 &optional function

This function returns a list of the elements that appear both in sequence1 and sequence2. If the optional argument function is non-nil, it is a function of two arguments to use to compare elements instead of the default equal.

          (seq-intersection [2 3 4 5] [1 3 5 6 7])
          ⇒ (3 5)
— Function: seq-difference sequence1 sequence2 &optional function

This function returns a list of the elements that appear in sequence1 but not in sequence2. If the optional argument function is non-nil, it is a function of two arguments to use to compare elements instead of the default equal.

          (seq-difference '(2 3 4 5) [1 3 5 6 7])
          ⇒ (2 4)
— Function: seq-group-by function sequence

This function separates the elements of sequence into an alist whose keys are the result of applying function to each element of sequence. Keys are compared using equal.

          (seq-group-by #'integerp '(1 2.1 3 2 3.2))
          ⇒ ((t 1 3 2) (nil 2.1 3.2))
          (seq-group-by #'car '((a 1) (b 2) (a 3) (c 4)))
          ⇒ ((b (b 2)) (a (a 1) (a 3)) (c (c 4)))
— Function: seq-into sequence type

This function converts the sequence sequence into a sequence of type type. type can be one of the following symbols: vector, string or list.

          (seq-into [1 2 3] 'list)
          ⇒ (1 2 3)
          (seq-into nil 'vector)
          ⇒ []
          (seq-into "hello" 'vector)
          ⇒ [104 101 108 108 111]
— Function: seq-min sequence

This function returns the smallest element of sequence. The elements of sequence must be numbers or markers (see Markers).

          (seq-min [3 1 2])
          ⇒ 1
          (seq-min "Hello")
          ⇒ 72
— Function: seq-max sequence

This function returns the largest element of sequence. The elements of sequence must be numbers or markers.

          (seq-max [1 3 2])
          ⇒ 3
          (seq-max "Hello")
          ⇒ 111
— Macro: seq-doseq (var sequence) body...

This macro is like dolist (see dolist), except that sequence can be a list, vector or string. This is primarily useful for side-effects.

— Macro: seq-let var-sequence val-sequence body...

This macro binds the variables defined in var-sequence to the values that are the corresponding elements of val-sequence. This is known as destructuring binding. The elements of var-sequence can themselves include sequences, allowing for nested destructuring.

The var-sequence sequence can also include the &rest marker followed by a variable name to be bound to the rest of val-sequence.

          (seq-let [first second] [1 2 3 4]
            (list first second))
          ⇒ (1 2)
          (seq-let (_ a _ b) '(1 2 3 4)
            (list a b))
          ⇒ (2 4)
          (seq-let [a [b [c]]] [1 [2 [3]]]
            (list a b c))
          ⇒ (1 2 3)
          (seq-let [a b &rest others] [1 2 3 4]
          ⇒ [3 4]

The pcase patterns provide an alternative facility for destructuring binding, see Destructuring with pcase Patterns.

— Function: seq-random-elt sequence

This function returns an element of sequence taken at random.

          (seq-random-elt [1 2 3 4])
          ⇒ 3
          (seq-random-elt [1 2 3 4])
          ⇒ 2
          (seq-random-elt [1 2 3 4])
          ⇒ 4
          (seq-random-elt [1 2 3 4])
          ⇒ 2
          (seq-random-elt [1 2 3 4])
          ⇒ 1

If sequence is empty, this function signals an error.

Next: , Previous: Sequence Functions, Up: Sequences Arrays Vectors

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:

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:

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.

Next: , Previous: Arrays, Up: Sequences Arrays Vectors

6.3 Functions that Operate on Arrays

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

— Function: arrayp object

This function returns t if object is an array (i.e., a vector, a string, a bool-vector or a char-table).

          (arrayp [a])
               ⇒ t
          (arrayp "asdf")
               ⇒ t
          (arrayp (syntax-table))    ;; A char-table.
               ⇒ t
— Function: aref arr index

This function returns the indexth element of the array or record arr. The first element is at index zero.

          (setq primes [2 3 5 7 11 13])
               ⇒ [2 3 5 7 11 13]
          (aref primes 4)
               ⇒ 11
          (aref "abcdefg" 1)
               ⇒ 98           ; b’ is ASCII code 98.

See also the function elt, in Sequence Functions.

— Function: aset array index object

This function sets the indexth element of array to be object. It returns object.

          (setq w [foo bar baz])
               ⇒ [foo bar baz]
          (aset w 0 'fu)
               ⇒ fu
               ⇒ [fu bar baz]
          (setq x "asdfasfd")
               ⇒ "asdfasfd"
          (aset x 3 ?Z)
               ⇒ 90
               ⇒ "asdZasfd"

If array is a string and object is not a character, a wrong-type-argument error results. The function converts a unibyte string to multibyte if necessary to insert a character.

— Function: fillarray array object

This function fills the array array with object, so that each element of array is object. It returns array.

          (setq a [a b c d e f g])
               ⇒ [a b c d e f g]
          (fillarray a 0)
               ⇒ [0 0 0 0 0 0 0]
               ⇒ [0 0 0 0 0 0 0]
          (setq s "When in the course")
               ⇒ "When in the course"
          (fillarray s ?-)
               ⇒ "------------------"

If array is a string and object is not a character, a wrong-type-argument error results.

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

Next: , Previous: Array Functions, Up: Sequences Arrays Vectors

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

Next: , Previous: Vectors, Up: Sequences Arrays Vectors

6.5 Functions for Vectors

Here are some functions that relate to vectors:

— Function: vectorp object

This function returns t if object is a vector.

          (vectorp [a])
               ⇒ t
          (vectorp "asdf")
               ⇒ nil
— Function: vector &rest objects

This function creates and returns a vector whose elements are the arguments, objects.

          (vector 'foo 23 [bar baz] "rats")
               ⇒ [foo 23 [bar baz] "rats"]
               ⇒ []
— Function: make-vector length object

This function returns a new vector consisting of length elements, each initialized to object.

          (setq sleepy (make-vector 9 'Z))
               ⇒ [Z Z Z Z Z Z Z Z Z]
— Function: vconcat &rest sequences

This function returns a new vector containing all the elements of sequences. The arguments sequences may be proper lists, vectors, strings or bool-vectors. If no sequences are given, the empty vector is returned.

The value is either the empty vector, or is a newly constructed nonempty vector that is not eq to any existing vector.

          (setq a (vconcat '(A B C) '(D E F)))
               ⇒ [A B C D E F]
          (eq a (vconcat a))
               ⇒ nil
               ⇒ []
          (vconcat [A B C] "aa" '(foo (6 7)))
               ⇒ [A B C 97 97 foo (6 7)]

The vconcat function also allows byte-code function objects as arguments. This is a special feature to make it easy to access the entire contents of a byte-code function object. See Byte-Code Objects.

For other concatenation functions, see mapconcat in Mapping Functions, concat in Creating Strings, and append in Building Lists.

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])

Next: , Previous: Vector Functions, Up: Sequences Arrays Vectors

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:

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.

— Function: make-char-table subtype &optional init

Return a newly-created char-table, with subtype subtype (a symbol). Each element is initialized to init, which defaults to nil. You cannot alter the subtype of a char-table after the char-table is created.

There is no argument to specify the length of the char-table, because all char-tables have room for any valid character code as an index.

If subtype has the char-table-extra-slots symbol property, that specifies the number of extra slots in the char-table. This should be an integer between 0 and 10; otherwise, make-char-table raises an error. If subtype has no char-table-extra-slots symbol property (see Property Lists), the char-table has no extra slots.

— Function: char-table-p object

This function returns t if object is a char-table, and nil otherwise.

— Function: char-table-subtype char-table

This function returns the subtype symbol of char-table.

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

— Function: char-table-parent char-table

This function returns the parent of char-table. The parent is always either nil or another char-table.

— Function: set-char-table-parent char-table new-parent

This function sets the parent of char-table to new-parent.

— Function: char-table-extra-slot char-table n

This function returns the contents of extra slot n (zero based) of char-table. The number of extra slots in a char-table is determined by its subtype.

— Function: set-char-table-extra-slot char-table n value

This function stores value in extra slot n (zero based) of char-table.

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

— Function: char-table-range char-table range

This returns the value specified in char-table for a range of characters range. Here are the possibilities for range:

Refers to the default value.
Refers to the element for character char (supposing char is a valid character code).
(from . to)
A cons cell refers to all the characters in the inclusive range ‘[from..to]’.

— Function: set-char-table-range char-table range value

This function sets the value in char-table for a range of characters range. Here are the possibilities for range:

Refers to the default value.
Refers to the whole range of character codes.
Refers to the element for character char (supposing char is a valid character code).
(from . to)
A cons cell refers to all the characters in the inclusive range ‘[from..to]’.

— Function: map-char-table function char-table

This function calls its argument function for each element of char-table that has a non-nil value. The call to function is with two arguments, a key and a value. The key is a possible range argument for char-table-range—either a valid character or a cons cell (from . to), specifying a range of characters that share the same value. The value is what (char-table-range char-table key) returns.

Overall, the key-value pairs passed to function describe all the values stored in char-table.

The return value is always nil; to make calls to map-char-table useful, function should have side effects. For example, here is how to examine the elements of the syntax table:

          (let (accumulator)
              #'(lambda (key value)
                  (setq accumulator
                        (cons (list
                               (if (consp key)
                                   (list (car key) (cdr key))
          (((2597602 4194303) (2)) ((2597523 2597601) (3))
           ... (65379 (5 . 65378)) (65378 (4 . 65379)) (65377 (1))
           ... (12 (0)) (11 (3)) (10 (12)) (9 (0)) ((0 8) (3)))

Next: , Previous: Char-Tables, Up: Sequences Arrays Vectors

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.

Several functions work specifically with bool-vectors; aside from that, you manipulate them with same functions used for other kinds of arrays.

— Function: make-bool-vector length initial

Return a new bool-vector of length elements, each one initialized to initial.

— Function: bool-vector &rest objects

This function creates and returns a bool-vector whose elements are the arguments, objects.

— Function: bool-vector-p object

This returns t if object is a bool-vector, and nil otherwise.

There are also some bool-vector set operation functions, described below:

— Function: bool-vector-exclusive-or a b &optional c

Return bitwise exclusive or of bool vectors a and b. If optional argument c is given, the result of this operation is stored into c. All arguments should be bool vectors of the same length.

— Function: bool-vector-union a b &optional c

Return bitwise or of bool vectors a and b. If optional argument c is given, the result of this operation is stored into c. All arguments should be bool vectors of the same length.

— Function: bool-vector-intersection a b &optional c

Return bitwise and of bool vectors a and b. If optional argument c is given, the result of this operation is stored into c. All arguments should be bool vectors of the same length.

— Function: bool-vector-set-difference a b &optional c

Return set difference of bool vectors a and b. If optional argument c is given, the result of this operation is stored into c. All arguments should be bool vectors of the same length.

— Function: bool-vector-not a &optional b

Return set complement of bool vector a. If optional argument b is given, the result of this operation is stored into b. All arguments should be bool vectors of the same length.

— Function: bool-vector-subsetp a b

Return t if every t value in a is also t in b, nil otherwise. All arguments should be bool vectors of the same length.

— Function: bool-vector-count-consecutive a b i

Return the number of consecutive elements in a equal b starting at i. a is a bool vector, b is t or nil, and i is an index into a.

— Function: bool-vector-count-population a

Return the number of elements that are t in bool vector a.

The printed form represents up to 8 boolean values as a single character:

     (bool-vector t nil t nil)
          ⇒ #&4"^E"
          ⇒ #&0""

You can use vconcat to print a bool-vector like other vectors:

     (vconcat (bool-vector nil t nil t))
          ⇒ [nil t nil t]

Here is another example of creating, examining, and updating a bool-vector:

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

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

Previous: Bool-Vectors, Up: Sequences Arrays Vectors

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.

— Function: make-ring size

This returns a new ring capable of holding size objects. size should be an integer.

— Function: ring-p object

This returns t if object is a ring, nil otherwise.

— Function: ring-size ring

This returns the maximum capacity of the ring.

— Function: ring-length ring

This returns the number of objects that ring currently contains. The value will never exceed that returned by ring-size.

— Function: ring-elements ring

This returns a list of the objects in ring, in order, newest first.

— Function: ring-copy ring

This returns a new ring which is a copy of ring. The new ring contains the same (eq) objects as ring.

— Function: ring-empty-p ring

This returns t if ring is empty, nil otherwise.

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.

— Function: ring-ref ring index

This returns the object in ring found at index index. index may be negative or greater than the ring length. If ring is empty, ring-ref signals an error.

— Function: ring-insert ring object

This inserts object into ring, making it the newest element, and returns object.

If the ring is full, insertion removes the oldest element to make room for the new element.

— Function: ring-remove ring &optional index

Remove an object from ring, and return that object. The argument index specifies which item to remove; if it is nil, that means to remove the oldest item. If ring is empty, ring-remove signals an error.

— Function: ring-insert-at-beginning ring object

This inserts object into ring, treating it as the oldest element. The return value is not significant.

If the ring is full, this function removes the newest element to make room for the inserted element.

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")

Next: , Previous: Sequences Arrays Vectors, Up: Top

7 Records

The purpose of records is to allow programmers to create objects with new types that are not built into Emacs. They are used as the underlying representation of cl-defstruct and defclass instances.

Internally, a record object is much like a vector; its slots can be accessed using aref and it can be copied using copy-sequence. However, the first slot is used to hold its type as returned by type-of. Also, in the current implementation records can have at most 4096 slots, whereas vectors can be much larger. Like arrays, records use zero-origin indexing: the first slot has index 0.

The type slot should be a symbol or a type descriptor. If it's a type descriptor, the symbol naming its type will be returned; Type Descriptors. Any other kind of object is returned as-is.

The printed representation of records is ‘#s’ followed by a list specifying the contents. The first list element must be the record type. The following elements are the record slots.

To avoid conflicts with other type names, Lisp programs that define new types of records should normally use the naming conventions of the package where these record types are introduced for the names of the types. Note that the names of the types which could possibly conflict might not be known at the time the package defining a record type is loaded; they could be loaded at some future point in time.

A record is considered a constant for evaluation: the result of evaluating it is the same record. This does not evaluate or even examine the slots. See Self-Evaluating Forms.

Next: , Up: Records

7.1 Record Functions

— Function: recordp object

This function returns t if object is a record.

          (recordp #s(a))
               ⇒ t
— Function: record type &rest objects

This function creates and returns a record whose type is type and remaining slots are the rest of the arguments, objects.

          (record 'foo 23 [bar baz] "rats")
               ⇒ #s(foo 23 [bar baz] "rats")
— Function: make-record type length object

This function returns a new record with type type and length more slots, each initialized to object.

          (setq sleepy (make-record 'foo 9 'Z))
               ⇒ #s(foo Z Z Z Z Z Z Z Z Z)

Previous: Record Functions, Up: Records

7.2 Backward Compatibility

Code compiled with older versions of cl-defstruct that doesn't use records may run into problems when used in a new Emacs. To alleviate this, Emacs detects when an old cl-defstruct is used, and enables a mode in which type-of handles old struct objects as if they were records.

— Function: cl-old-struct-compat-mode arg

If arg is positive, enable backward compatibility with old-style structs.

Next: , Previous: Records, Up: Top

8 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:

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. (Hash notation, the initial ‘#’ character used in the printed representations of objects with no read representation, has nothing to do with hash tables. 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).

Next: , Up: Hash Tables

8.1 Creating Hash Tables

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

— Function: make-hash-table &rest keyword-args

This function creates a new hash table according to the specified arguments. The arguments should consist of alternating keywords (particular symbols recognized specially) and values corresponding to them.

Several keywords make sense in make-hash-table, but the only two that you really need to know about are :test and :weakness.

:test test
This specifies the method of key lookup for this hash table. The default is eql; eq and equal are other alternatives:
Keys which are numbers are the same if they are equal, that is, if they are equal in value and either both are integers or both are floating point; otherwise, two distinct objects are never the same.
Any two distinct Lisp objects are different as keys.
Two Lisp objects are the same, as keys, if they are equal according to equal.

You can use define-hash-table-test (see Defining Hash) to define additional possibilities for test.

:weakness weak
The weakness of a hash table specifies whether the presence of a key or value in the hash table preserves it from garbage collection.

The value, weak, must be one of nil, key, value, key-or-value, key-and-value, or t which is an alias for key-and-value. If weak is key then the hash table does not prevent its keys from being collected as garbage (if they are not referenced anywhere else); if a particular key does get collected, the corresponding association is removed from the hash table.

If weak is value, then the hash table does not prevent values from being collected as garbage (if they are not referenced anywhere else); if a particular value does get collected, the corresponding association is removed from the hash table.

If weak is key-and-value or t, both the key and the value must be live in order to preserve the association. Thus, the hash table does not protect either keys or values from garbage collection; if either one is collected as garbage, that removes the association.

If weak is key-or-value, either the key or the value can preserve the association. Thus, associations are removed from the hash table when both their key and value would be collected as garbage (if not for references from weak hash tables).

The default for weak is nil, so that all keys and values referenced in the hash table are preserved from garbage collection.

:size size
This specifies a hint for how many associations you plan to store in the hash table. If you know the approximate number, you can make things a little more efficient by specifying it this way. If you specify too small a size, the hash table will grow automatically when necessary, but doing that takes some extra time.

The default size is 65.

:rehash-size rehash-size
When you add an association to a hash table and the table is full, it grows automatically. This value specifies how to make the hash table larger, at that time.

If rehash-size is an integer, it should be positive, and the hash table grows by adding approximately that much to the nominal size. If rehash-size is floating point, it had better be greater than 1, and the hash table grows by multiplying the old size by approximately that number.

The default value is 1.5.

:rehash-threshold threshold
This specifies the criterion for when the hash table is full (so it should be made larger). The value, threshold, should be a positive floating-point number, no greater than 1. The hash table is full whenever the actual number of entries exceeds the nominal size multiplied by an approximation to this value. The default for threshold is 0.8125.

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.

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8.2 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.

— Function: gethash key table &optional default

This function looks up key in table, and returns its associated value—or default, if key has no association in table.

— Function: puthash key value table

This function enters an association for key in table, with value value. If key already has an association in table, value replaces the old associated value.

— Function: remhash key table

This function removes the association for key from table, if there is one. If key has no association, remhash does nothing.

Common Lisp note: In Common Lisp, remhash returns non-nil if it actually removed an association and nil otherwise. In Emacs Lisp, remhash always returns nil.

— Function: clrhash table

This function removes all the associations from hash table table, so that it becomes empty. This is also called clearing the hash table.

Common Lisp note: In Common Lisp, clrhash returns the empty table. In Emacs Lisp, it returns nil.

— Function: maphash function table

This function calls function once for each of the associations in table. The function function should accept two arguments—a key listed in table, and its associated value. maphash returns nil.

Next: , Previous: Hash Access, Up: Hash Tables

8.3 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.

— Function: define-hash-table-test name test-fn hash-fn

This function defines a new hash table test, named name.

After defining name in this way, you can use it as the test argument in make-hash-table. When you do that, the hash table will use test-fn to compare key values, and hash-fn to compute a hash code from a key value.

The function test-fn should accept two arguments, two keys, and return non-nil if they are considered the same.

The function hash-fn should accept one argument, a key, and return an integer that is the hash code of that key. For good results, the function should use the whole range of integers for hash codes, including negative integers.

The specified functions are stored in the property list of name under the property hash-table-test; the property value's form is (test-fn hash-fn).

— Function: sxhash-equal obj

This function returns a hash code for Lisp object obj. This is an integer which reflects the contents of obj and the other Lisp objects it points to.

If two objects obj1 and obj2 are equal, then (sxhash-equal obj1) and (sxhash-equal obj2) are the same integer.

If the two objects are not equal, the values returned by sxhash-equal are usually different, but not always; once in a rare while, by luck, you will encounter two distinct-looking objects that give the same result from sxhash-equal.

Common Lisp note: In Common Lisp a similar function is called sxhash. Emacs provides this name as a compatibility alias for sxhash-equal.

— Function: sxhash-eq obj

This function returns a hash code for Lisp object obj. Its result reflects identity of obj, but not its contents.

If two objects obj1 and obj2 are eq, then (sxhash-eq obj1) and (sxhash-eq obj2) are the same integer.

— Function: sxhash-eql obj

This function returns a hash code for Lisp object obj suitable for eql comparison. I.e. it reflects identity of obj except for the case where the object is a float number, in which case hash code is generated for the value.

If two objects obj1 and obj2 are eql, then (sxhash-eql obj1) and (sxhash-eql obj2) are the same integer.

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

Previous: Defining Hash, Up: Hash Tables

8.4 Other Hash Table Functions

Here are some other functions for working with hash tables.

— Function: hash-table-p table

This returns non-nil if table is a hash table object.

— Function: copy-hash-table table

This function creates and returns a copy of table. Only the table itself is copied—the keys and values are shared.

— Function: hash-table-count table

This function returns the actual number of entries in table.

— Function: hash-table-test table

This returns the test value that was given when table was created, to specify how to hash and compare keys. See make-hash-table (see Creating Hash).

— Function: hash-table-weakness table

This function returns the weak value that was specified for hash table table.

— Function: hash-table-rehash-size table

This returns the rehash size of table.

— Function: hash-table-rehash-threshold table

This returns the rehash threshold of table.

— Function: hash-table-size table

This returns the current nominal size of table.

Next: , Previous: Hash Tables, Up: Top

9 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:

— Function: symbolp object

This function returns t if object is a symbol, nil otherwise.

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9.1 Symbol Components

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

Print name
The symbol's name.
The symbol's current value as a variable.
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):

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

Next: , Previous: Symbol Components, Up: Symbols

9.2 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 definitions 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.

Next: , Previous: Definitions, Up: Symbols

9.3 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.

— Function: symbol-name symbol

This function returns the string that is symbol's name. For example:

          (symbol-name 'foo)
               ⇒ "foo"

Warning: Changing the string by substituting characters does change the name of the symbol, but fails to update the obarray, so don't do it!

— Function: make-symbol name

This function returns a newly-allocated, uninterned symbol whose name is name (which must be a string). Its value and function definition are void, and its property list is nil. In the example below, the value of sym is not eq to foo because it is a distinct uninterned symbol whose name is also ‘foo’.

          (setq sym (make-symbol "foo"))
               ⇒ foo
          (eq sym 'foo)
               ⇒ nil
— Function: gensym &optional prefix

This function returns a symbol using make-symbol, whose name is made by appending gensym-counter to prefix. The prefix defaults to "g".

— Function: intern name &optional obarray

This function returns the interned symbol whose name is name. If there is no such symbol in the obarray obarray, intern creates a new one, adds it to the obarray, and returns it. If obarray is omitted, the value of the global variable obarray is used.

          (setq sym (intern "foo"))
               ⇒ foo
          (eq sym 'foo)
               ⇒ t
          (setq sym1 (intern "foo" other-obarray))
               ⇒ foo
          (eq sym1 'foo)
               ⇒ nil

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.
— Function: intern-soft name &optional obarray

This function returns the symbol in obarray whose name is name, or nil if obarray has no symbol with that name. Therefore, you can use intern-soft to test whether a symbol with a given name is already interned. If obarray is omitted, the value of the global variable obarray is used.

The argument name may also be a symbol; in that case, the function returns name if name is interned in the specified obarray, and otherwise nil.

          (intern-soft "frazzle")        ; No such symbol exists.
               ⇒ nil
          (make-symbol "frazzle")        ; Create an uninterned one.
               ⇒ frazzle
          (intern-soft "frazzle")        ; That one cannot be found.
               ⇒ nil
          (setq sym (intern "frazzle"))  ; Create an interned one.
               ⇒ frazzle
          (intern-soft "frazzle")        ; That one can be found!
               ⇒ frazzle
          (eq sym 'frazzle)              ; And it is the same one.
               ⇒ t
— Variable: obarray

This variable is the standard obarray for use by intern and read.

— Function: mapatoms function &optional obarray

This function calls function once with each symbol in the obarray obarray. Then it returns nil. If obarray is omitted, it defaults to the value of obarray, the standard obarray for ordinary symbols.

          (setq count 0)
               ⇒ 0
          (defun count-syms (s)
            (setq count (1+ count)))
               ⇒ count-syms
          (mapatoms 'count-syms)
               ⇒ nil
               ⇒ 1871

See documentation in Accessing Documentation, for another example using mapatoms.

— Function: unintern symbol obarray

This function deletes symbol from the obarray obarray. If symbol is not actually in the obarray, unintern does nothing. If obarray is nil, the current obarray is used.

If you provide a string instead of a symbol as symbol, it stands for a symbol name. Then unintern deletes the symbol (if any) in the obarray which has that name. If there is no such symbol, unintern does nothing.

If unintern does delete a symbol, it returns t. Otherwise it returns nil.

Previous: Creating Symbols, Up: Symbols

9.4 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).

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9.4.1 Accessing Symbol Properties

The following functions can be used to access symbol properties.

— Function: get symbol property

This function returns the value of the property named property in symbol's property list. If there is no such property, it returns nil. Thus, there is no distinction between a value of nil and the absence of the property.

The name property is compared with the existing property names using eq, so any object is a legitimate property.

See put for an example.

— Function: put symbol property value

This function puts value onto symbol's property list under the property name property, replacing any previous property value. The put function returns value.

          (put 'fly 'verb 'transitive)
          (put 'fly 'noun '(a buzzing little bug))
               ⇒ (a buzzing little bug)
          (get 'fly 'verb)
               ⇒ transitive
          (symbol-plist 'fly)
               ⇒ (verb transitive noun (a buzzing little bug))
— Function: symbol-plist symbol

This function returns the property list of symbol.

— Function: setplist symbol plist

This function sets symbol's property list to plist. Normally, plist should be a well-formed property list, but this is not enforced. The return value is plist.

          (setplist 'foo '(a 1 b (2 3) c nil))
               ⇒ (a 1 b (2 3) c nil)
          (symbol-plist 'foo)
               ⇒ (a 1 b (2 3) c nil)

For symbols in special obarrays, which are not used for ordinary purposes, it may make sense to use the property list cell in a nonstandard fashion; in fact, the abbrev mechanism does so (see Abbrevs).

You could define put in terms of setplist and plist-put, as follows:

          (defun put (symbol prop value)
            (setplist symbol
                      (plist-put (symbol-plist symbol) prop value)))
— Function: function-get symbol property &optional autoload

This function is identical to get, except that if symbol is the name of a function alias, it looks in the property list of the symbol naming the actual function. See Defining Functions. If the optional argument autoload is non-nil, and symbol is auto-loaded, this function will try to autoload it, since autoloading might set property of symbol. If autoload is the symbol macro, only try autoloading if symbol is an auto-loaded macro.

— Function: function-put function property value

This function sets property of function to value. function should be a symbol. This function is preferred to calling put for setting properties of a function, because it will allow us some day to implement remapping of old properties to new ones.

Previous: Symbol Plists, Up: Symbol Properties

9.4.2 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.

This property value specifies the preferred key binding, when showing documentation, for the named function. See Keys in Documentation.
The value, if non-nil, specifies the number of extra slots in the named char-table type. See Char-Tables.
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.
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.
If the value is non-nil, the named function is disabled as a command. See Disabling Commands.
The value stores the documentation string of the named face. This is set automatically by defface. See Defining Faces.
The value, if non-nil, specifies the maximum minibuffer history length for the named history list variable. See Minibuffer History.
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.
The value is an expression for determining whether the named menu item should be enabled in menus. See Simple Menu Items.
If the value is special, the named major mode is special. See Major Mode Conventions.
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.
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.
If the value is non-nil, the named function is considered to be pure (see What Is a Function). Calls with constant arguments can be evaluated at compile time. This may shift run time errors to compile time. Not to be confused with pure storage (see Pure Storage).
If the value is non-nil, the named variable is considered risky as a file-local variable. See File Local Variables.
If the value is non-nil, the named function is considered generally safe for evaluation. See Function Safety.
If the value is non-nil, the named function is safe to call in file-local evaluation forms. See File Local Variables.
The value specifies a function for determining safe file-local values for the named variable. See File Local Variables.
A non-nil value indicates that the named function is free of side effects (see What Is a Function), so the byte compiler may ignore a call whose value is unused. If the property's value is error-free, the byte compiler may even delete such unused calls. In addition to byte compiler optimizations, this property is also used for determining function safety (see Function Safety).
If non-nil, this specifies the named variable's documentation string. This is set automatically by defvar and related functions. See Defining Faces.

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10 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.

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10.1 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 expression5. 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).6 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.

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10.2 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 other types, which are self-evaluating forms.

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10.2.1 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>

Next: , Previous: Self-Evaluating Forms, Up: Forms

10.2.2 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
          ⇒ 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.

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10.2.3 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.

Next: , Previous: Classifying Lists, Up: Forms

10.2.4 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. Otherwise, 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.

— Function: indirect-function function &optional noerror

This function returns the meaning of function as a function. If function is a symbol, then it finds function's function definition and starts over with that value. If function is not a symbol, then it returns function itself.

This function returns nil if the final symbol is unbound. It signals a cyclic-function-indirection error if there is a loop in the chain of symbols.

The optional argument noerror is obsolete, kept for backward compatibility, and has no effect.

Here is how you could define indirect-function in Lisp:

          (defun indirect-function (function)
            (if (symbolp function)
                (indirect-function (symbol-function function))

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10.2.5 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.

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10.2.6 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.

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10.2.7 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.

If an expression's first symbol is that of a special form, the expression should follow the rules of that special form; otherwise, Emacs's behavior is not well-defined (though it will not crash). For example, ((lambda (x) x . 3) 4) contains a subexpression that begins with lambda but is not a well-formed lambda expression, so Emacs may signal an error, or may return 3 or 4 or nil, or may behave in other ways.

— Function: special-form-p object

This predicate tests whether its argument is a special form, and returns t if so, nil otherwise.

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

see Combining Conditions
see Catch and Throw
see Conditionals
see Handling Errors
see Defining Variables
see Defining Variables
see Anonymous Functions
see Conditionals
see Interactive Call
see Lambda Expressions
see Local Variables
see Combining Conditions
see Sequencing
see Quoting
see Current Buffer
see Excursions
see Narrowing
see Setting Variables
see Creating Buffer-Local
see Nonlocal Exits
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).

Previous: Special Forms, Up: Forms

10.2.8 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.

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10.3 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.)

— Special Form: quote object

This special form returns object, without evaluating it.

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
          ⇒ (quote foo)
     '(quote foo)
          ⇒ (quote 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.

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10.4 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)

Previous: Backquote, Up: Evaluation

10.5 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.

— Function: eval form &optional lexical

This is the basic function for evaluating an expression. It evaluates form in the current environment, and returns the result. The type of the form object determines how it is evaluated. See Forms.

The argument lexical specifies the scoping rule for local variables (see Variable Scoping). If it is omitted or nil, that means to evaluate form using the default dynamic scoping rule. If it is t, that means to use the lexical scoping rule. The value of lexical can also be a non-empty alist specifying a particular lexical environment for lexical bindings; however, this feature is only useful for specialized purposes, such as in Emacs Lisp debuggers. See Lexical Binding.

Since eval is a function, the argument expression that appears in a call to eval is evaluated twice: once as preparation before eval is called, and again by the eval function itself. Here is an example:

          (setq foo 'bar)
               ⇒ bar
          (setq bar 'baz)
               ⇒ baz
          ;; Here eval receives argument foo
          (eval 'foo)
               ⇒ bar
          ;; Here eval receives argument bar, which is the value of foo
          (eval foo)
               ⇒ baz

The number of currently active calls to eval is limited to max-lisp-eval-depth (see below).

— Command: eval-region start end &optional stream read-function

This function evaluates the forms in the current buffer in the region defined by the positions start and end. It reads forms from the region and calls eval on them until the end of the region is reached, or until an error is signaled and not handled.

By default, eval-region does not produce any output. However, if stream is non-nil, any output produced by output functions (see Output Functions), as well as the values that result from evaluating the expressions in the region are printed using stream. See Output Streams.

If read-function is non-nil, it should be a function, which is used instead of read to read expressions one by one. This function is called with one argument, the stream for reading input. You can also use the variable load-read-function (see How Programs Do Loading) to specify this function, but it is more robust to use the read-function argument.

eval-region does not move point. It always returns nil.

— Command: eval-buffer &optional buffer-or-name stream filename unibyte print

This is similar to eval-region, but the arguments provide different optional features. eval-buffer operates on the entire accessible portion of buffer buffer-or-name (see Narrowing). buffer-or-name can be a buffer, a buffer name (a string), or nil (or omitted), which means to use the current buffer. stream is used as in eval-region, unless stream is nil and print non-nil. In that case, values that result from evaluating the expressions are still discarded, but the output of the output functions is printed in the echo area. filename is the file name to use for load-history (see Unloading), and defaults to buffer-file-name (see Buffer File Name). If unibyte is non-nil, read converts strings to unibyte whenever possible.

eval-current-buffer is an alias for this command.

— User Option: max-lisp-eval-depth

This variable defines the maximum depth allowed in calls to eval, apply, and funcall before an error is signaled (with error message "Lisp nesting exceeds max-lisp-eval-depth").

This limit, with the associated error when it is exceeded, is one way Emacs Lisp avoids infinite recursion on an ill-defined function. If you increase the value of max-lisp-eval-depth too much, such code can cause stack overflow instead. On some systems, this overflow can be handled. In that case, normal Lisp evaluation is interrupted and control is transferred back to the top level command loop (top-level). Note that there is no way to enter Emacs Lisp debugger in this situation. See Error Debugging.

The depth limit counts internal uses of eval, apply, and funcall, such as for calling the functions mentioned in Lisp expressions, and recursive evaluation of function call arguments and function body forms, as well as explicit calls in Lisp code.

The default value of this variable is 800. If you set it to a value less than 100, Lisp will reset it to 100 if the given value is reached. Entry to the Lisp debugger increases the value, if there is little room left, to make sure the debugger itself has room to execute.

max-specpdl-size provides another limit on nesting. See Local Variables.

— Variable: values

The value of this variable is a list of the values returned by all the expressions that were read, evaluated, and printed from buffers (including the minibuffer) by the standard Emacs commands which do this. (Note that this does not include evaluation in *ielm* buffers, nor evaluation using C-j, C-x C-e, and similar evaluation commands in lisp-interaction-mode.) The elements are ordered most recent first.

          (setq x 1)
               ⇒ 1
          (list 'A (1+ 2) auto-save-default)
               ⇒ (A 3 t)
               ⇒ ((A 3 t) 1 ...)

This variable is useful for referring back to values of forms recently evaluated. It is generally a bad idea to print the value of values itself, since this may be very long. Instead, examine particular elements, like this:

          ;; Refer to the most recent evaluation result.
          (nth 0 values)
               ⇒ (A 3 t)
          ;; That put a new element on,
          ;;   so all elements move back one.
          (nth 1 values)
               ⇒ (A 3 t)
          ;; This gets the element that was next-to-most-recent
          ;;   before this example.
          (nth 3 values)
               ⇒ 1

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11 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).

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11.1 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.

— Special Form: progn forms...

This special form evaluates all of the forms, in textual order, returning the result of the final form.

          (progn (print "The first form")
                 (print "The second form")
                 (print "The third form"))
               -| "The first form"
               -| "The second form"
               -| "The third form"
          ⇒ "The third form"

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

— Special Form: prog1 form1 forms...

This special form evaluates form1 and all of the forms, in textual order, returning the result of form1.

          (prog1 (print "The first form")
                 (print "The second form")
                 (print "The third form"))
               -| "The first form"
               -| "The second form"
               -| "The third form"
          ⇒ "The first form"

Here is a way to remove the first element from a list in the variable x, then return the value of that former element:

          (prog1 (car x) (setq x (cdr x)))
— Special Form: prog2 form1 form2 forms...

This special form evaluates form1, form2, and all of the following forms, in textual order, returning the result of form2.

          (prog2 (print "The first form")
                 (print "The second form")
                 (print "The third form"))
               -| "The first form"
               -| "The second form"
               -| "The third form"
          ⇒ "The second form"

Next: , Previous: Sequencing, Up: Control Structures

11.2 Conditionals

Conditional control structures choose among alternatives. Emacs Lisp has five conditional forms: if, which is much the same as in other languages; when and unless, which are variants of if; cond, which is a generalized case statement; and pcase, which is a generalization of cond (see Pattern-Matching Conditional).

— Special Form: if condition then-form else-forms...

if chooses between the then-form and the else-forms based on the value of condition. If the evaluated condition is non-nil, then-form is evaluated and the result returned. Otherwise, the else-forms are evaluated in textual order, and the value of the last one is returned. (The else part of if is an example of an implicit progn. See Sequencing.)

If condition has the value nil, and no else-forms are given, if returns nil.

if is a special form because the branch that is not selected is never evaluated—it is ignored. Thus, in this example, true is not printed because print is never called:

          (if nil
              (print 'true)
          ⇒ very-false
— Macro: when condition then-forms...

This is a variant of if where there are no else-forms, and possibly several then-forms. In particular,

          (when condition a b c)

is entirely equivalent to

          (if condition (progn a b c) nil)
— Macro: unless condition forms...

This is a variant of if where there is no then-form:

          (unless condition a b c)

is entirely equivalent to

          (if condition nil
             a b c)
— Special Form: cond clause...

cond chooses among an arbitrary number of alternatives. Each clause in the cond must be a list. The car of this list is the condition; the remaining elements, if any, the body-forms. Thus, a clause looks like this:

          (condition body-forms...)

cond tries the clauses in textual order, by evaluating the condition of each clause. If the value of condition is non-nil, the clause succeeds; then cond evaluates its body-forms, and returns the value of the last of body-forms. Any remaining clauses are ignored.

If the value of condition is nil, the clause fails, so the cond moves on to the following clause, trying its condition.

A clause may also look like this:


Then, if condition is non-nil when tested, the cond form returns the value of condition.

If every condition evaluates to nil, so that every clause fails, cond returns nil.

The following example has four clauses, which test for the cases where the value of x is a number, string, buffer and symbol, respectively:

          (cond ((numberp x) x)
                ((stringp x) x)
                ((bufferp x)
                 (setq temporary-hack x) ; multiple body-forms
                 (buffer-name x))        ; in one clause
                ((symbolp x) (symbol-value x)))

Often we want to execute the last clause whenever none of the previous clauses was successful. To do this, we use t as the condition of the last clause, like this: (t body-forms). The form t evaluates to t, which is never nil, so this clause never fails, provided the cond gets to it at all. For example:

          (setq a 5)
          (cond ((eq a 'hack) 'foo)
                (t "default"))
          ⇒ "default"

This cond expression returns foo if the value of a is hack, and returns the string "default" otherwise.

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))

Next: , Previous: Conditionals, Up: Control Structures

11.3 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.

— Function: not condition

This function tests for the falsehood of condition. It returns t if condition is nil, and nil otherwise. The function not is identical to null, and we recommend using the name null if you are testing for an empty list.

— Special Form: and conditions...

The and special form tests whether all the conditions are true. It works by evaluating the conditions one by one in the order written.

If any of the conditions evaluates to nil, then the result of the and must be nil regardless of the remaining conditions; so and returns nil right away, ignoring the remaining conditions.

If all the conditions turn out non-nil, then the value of the last of them becomes the value of the and form. Just (and), with no conditions, returns t, appropriate because all the conditions turned out non-nil. (Think about it; which one did not?)

Here is an example. The first condition returns the integer 1, which is not nil. Similarly, the second condition returns the integer 2, which is not nil. The third condition is nil, so the remaining condition is never evaluated.

          (and (print 1) (print 2) nil (print 3))
               -| 1
               -| 2
          ⇒ nil

Here is a more realistic example of using and:

          (if (and (consp foo) (eq (car foo) 'x))
              (message "foo is a list starting with x"))

Note that (car foo) is not executed if (consp foo) returns nil, thus avoiding an error.

and expressions can also be written using either if or cond. Here's how:

          (and arg1 arg2 arg3)
          (if arg1 (if arg2 arg3))
          (cond (arg1 (cond (arg2 arg3))))
— Special Form: or conditions...

The or special form tests whether at least one of the conditions is true. It works by evaluating all the conditions one by one in the order written.

If any of the conditions evaluates to a non-nil value, then the result of the or must be non-nil; so or returns right away, ignoring the remaining conditions. The value it returns is the non-nil value of the condition just evaluated.

If all the conditions turn out nil, then the or expression returns nil. Just (or), with no conditions, returns nil, appropriate because all the conditions turned out nil. (Think about it; which one did not?)

For example, this expression tests whether x is either nil or the integer zero:

          (or (eq x nil) (eq x 0))

Like the and construct, or can be written in terms of cond. For example:

          (or arg1 arg2 arg3)
          (cond (arg1)

You could almost write or in terms of if, but not quite:

          (if arg1 arg1
            (if arg2 arg2

This is not completely equivalent because it can evaluate arg1 or arg2 twice. By contrast, (or arg1 arg2 arg3) never evaluates any argument more than once.

Next: , Previous: Combining Conditions, Up: Control Structures

11.4 Pattern-Matching Conditional

Aside from the four basic conditional forms, Emacs Lisp also has a pattern-matching conditional form, the pcase macro, a hybrid of cond and cl-case (see Conditionals) that overcomes their limitations and introduces the pattern matching programming style. The limitations that pcase overcomes are:

Conceptually, the pcase macro borrows the first-arg focus of cl-case and the clause-processing flow of cond, replacing condition with a generalization of the equality test which is a variant of pattern matching, and adding facilities so that you can concisely express a clause's predicate, and arrange to share let-bindings between a clause's predicate and body-forms.

The concise expression of a predicate is known as a pattern. When the predicate, called on the value of the first arg, returns non-nil, we say that “the pattern matches the value” (or sometimes “the value matches the pattern”).

Next: , Up: Pattern-Matching Conditional

11.4.1 The pcase macro

For background, See Pattern-Matching Conditional.

— Macro: pcase expression &rest clauses

Each clause in clauses has the form: (pattern body-forms...).

Evaluate expression to determine its value, expval. Find the first clause in clauses whose pattern matches expval and pass control to that clause's body-forms.

If there is a match, the value of pcase is the value of the last of body-forms in the successful clause. Otherwise, pcase evaluates to nil.

Each pattern has to be a pcase pattern, which can use either one of the core patterns defined below, or one of the patterns defined via pcase-defmacro (see Extending pcase).

The rest of this subsection describes different forms of core patterns, presents some examples, and concludes with important caveats on using the let-binding facility provided by some pattern forms. A core pattern can have the following forms:

Matches any expval. This is also known as don't care or wildcard.
Matches if expval equals val. The comparison is done as if by equal (see Equality Predicates).
Matches if expval equals the literal object. This is a special case of 'val, above, possible because literal objects of these types are self-quoting.
Matches any expval, and additionally let-binds symbol to expval, such that this binding is available to body-forms (see Dynamic Binding).

If symbol is part of a sequencing pattern seqpat (e.g., by using and, below), the binding is also available to the portion of seqpat following the appearance of symbol. This usage has some caveats, see caveats.

Two symbols to avoid are t, which behaves like _ (above) and is deprecated, and nil, which signals an error. Likewise, it makes no sense to bind keyword symbols (see Constant Variables).

(pred function)
Matches if the predicate function returns non-nil when called on expval. the predicate function can have one of the following forms:
function name (a symbol)
Call the named function with one argument, expval.

Example: integerp

lambda expression
Call the anonymous function with one argument, expval (see Lambda Expressions).

Example: (lambda (n) (= 42 n))

function call with n args
Call the function (the first element of the function call) with n arguments (the other elements) and an additional n+1-th argument that is expval.

Example: (= 42)
In this example, the function is =, n is one, and the actual function call becomes: (= 42 expval).

(app function pattern)
Matches if function called on expval returns a value that matches pattern. function can take one of the forms described for pred, above. Unlike pred, however, app tests the result against pattern, rather than against a boolean truth value.
(guard boolean-expression)
Matches if boolean-expression evaluates to non-nil.
(let pattern expr)
Evaluates expr to get exprval and matches if exprval matches pattern. (It is called let because pattern can bind symbols to values using symbol.)

A sequencing pattern (also known as seqpat) is a pattern that processes its sub-pattern arguments in sequence. There are two for pcase: and and or. They behave in a similar manner to the special forms that share their name (see Combining Conditions), but instead of processing values, they process sub-patterns.

(and pattern1...)
Attempts to match pattern1..., in order, until one of them fails to match. In that case, and likewise fails to match, and the rest of the sub-patterns are not tested. If all sub-patterns match, and matches.
(or pattern1 pattern2...)
Attempts to match pattern1, pattern2, ..., in order, until one of them succeeds. In that case, or likewise matches, and the rest of the sub-patterns are not tested. (Note that there must be at least two sub-patterns. Simply (or pattern1) signals error.)

To present a consistent environment (see Intro Eval) to body-forms (thus avoiding an evaluation error on match), if any of the sub-patterns let-binds a set of symbols, they must all bind the same set of symbols.

Example: Advantage Over cl-case

Here's an example that highlights some advantages pcase has over cl-case (see Conditionals).

     (pcase (get-return-code x)
       ;; string
       ((and (pred stringp) msg)
        (message "%s" msg))
       ;; symbol
       ('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"))
       ;; default
       (code           (message "Unknown return code %S" code)))

With cl-case, you would need to explicitly declare a local variable code to hold the return value of get-return-code. Also cl-case is difficult to use with strings because it uses eql for comparison.

Example: Using and

A common idiom is to write a pattern starting with and, with one or more symbol sub-patterns providing bindings to the sub-patterns that follow (as well as to the body forms). For example, the following pattern matches single-digit integers.

       (pred integerp)
       n                     ; bind n to expval
       (guard (<= -9 n 9)))

First, pred matches if (integerp expval) evaluates to non-nil. Next, n is a symbol pattern that matches anything and binds n to expval. Lastly, guard matches if the boolean expression (<= -9 n 9) (note the reference to n) evaluates to non-nil. If all these sub-patterns match, and matches.

Example: Reformulation with pcase

Here is another example that shows how to reformulate a simple matching task from its traditional implementation (function grok/traditional) to one using pcase (function grok/pcase). The docstring for both these functions is: “If OBJ is a string of the form "key:NUMBER", return NUMBER (a string). Otherwise, return the list ("149" default).” First, the traditional implementation (see Regular Expressions):

     (defun grok/traditional (obj)
       (if (and (stringp obj)
                (string-match "^key:\\([[:digit:]]+\\)$" obj))
           (match-string 1 obj)
         (list "149" 'default)))
     (grok/traditional "key:0")   ⇒ "0"
     (grok/traditional "key:149") ⇒ "149"
     (grok/traditional 'monolith) ⇒ ("149" default)

The reformulation demonstrates symbol binding as well as or, and, pred, app and let.

     (defun grok/pcase (obj)
       (pcase obj
         ((or                                     ; line 1
           (and                                   ; line 2
            (pred stringp)                        ; line 3
            (pred (string-match                   ; line 4
                   "^key:\\([[:digit:]]+\\)$"))   ; line 5
            (app (match-string 1)                 ; line 6
                 val))                            ; line 7
           (let val (list "149" 'default)))       ; line 8
          val)))                                  ; line 9
     (grok/pcase "key:0")   ⇒ "0"
     (grok/pcase "key:149") ⇒ "149"
     (grok/pcase 'monolith) ⇒ ("149" default)

The bulk of grok/pcase is a single clause of a pcase form, the pattern on lines 1-8, the (single) body form on line 9. The pattern is or, which tries to match in turn its argument sub-patterns, first and (lines 2-7), then let (line 8), until one of them succeeds.

As in the previous example (see Example 1), and begins with a pred sub-pattern to ensure the following sub-patterns work with an object of the correct type (string, in this case). If (stringp expval) returns nil, pred fails, and thus and fails, too.

The next pred (lines 4-5) evaluates (string-match RX expval) and matches if the result is non-nil, which means that expval has the desired form: key:NUMBER. Again, failing this, pred fails and and, too.

Lastly (in this series of and sub-patterns), app evaluates (match-string 1 expval) (line 6) to get a temporary value tmp (i.e., the “NUMBER” substring) and tries to match tmp against pattern val (line 7). Since that is a symbol pattern, it matches unconditionally and additionally binds val to tmp.

Now that app has matched, all and sub-patterns have matched, and so and matches. Likewise, once and has matched, or matches and does not proceed to try sub-pattern let (line 8).

Let's consider the situation where obj is not a string, or it is a string but has the wrong form. In this case, one of the pred (lines 3-5) fails to match, thus and (line 2) fails to match, thus or (line 1) proceeds to try sub-pattern let (line 8).

First, let evaluates (list "149" 'default) to get ("149" default), the exprval, and then tries to match exprval against pattern val. Since that is a symbol pattern, it matches unconditionally and additionally binds val to exprval. Now that let has matched, or matches.

Note how both and and let sub-patterns finish in the same way: by trying (always successfully) to match against the symbol pattern val, in the process binding val. Thus, or always matches and control always passes to the body form (line 9). Because that is the last body form in a successfully matched pcase clause, it is the value of pcase and likewise the return value of grok/pcase (see What Is a Function).

Caveats for symbol in Sequencing Patterns

The preceding examples all use sequencing patterns which include the symbol sub-pattern in some way. Here are some important details about that usage.

  1. When symbol occurs more than once in seqpat, the second and subsequent occurrences do not expand to re-binding, but instead expand to an equality test using eq.

    The following example features a pcase form with two clauses and two seqpat, A and B. Both A and B first check that expval is a pair (using pred), and then bind symbols to the car and cdr of expval (using one app each).

    For A, because symbol st is mentioned twice, the second mention becomes an equality test using eq. On the other hand, B uses two separate symbols, s1 and s2, both of which become independent bindings.

              (defun grok (object)
                (pcase object
                  ((and (pred consp)        ; seqpat A
                        (app car st)        ; first mention: st
                        (app cdr st))       ; second mention: st
                   (list 'eq st))
                  ((and (pred consp)        ; seqpat B
                        (app car s1)        ; first mention: s1
                        (app cdr s2))       ; first mention: s2
                   (list 'not-eq s1 s2))))
              (let ((s "yow!"))
                (grok (cons s s)))      ⇒ (eq "yow!")
              (grok (cons "yo!" "yo!")) ⇒ (not-eq "yo!" "yo!")
              (grok '(4 2))             ⇒ (not-eq 4 (2))
  2. Side-effecting code referencing symbol is undefined. Avoid. For example, here are two similar functions. Both use and, symbol and guard:
              (defun square-double-digit-p/CLEAN (integer)
                (pcase (* integer integer)
                  ((and n (guard (< 9 n 100))) (list 'yes n))
                  (sorry (list 'no sorry))))
              (square-double-digit-p/CLEAN 9) ⇒ (yes 81)
              (square-double-digit-p/CLEAN 3) ⇒ (no 9)
              (defun square-double-digit-p/MAYBE (integer)
                (pcase (* integer integer)
                  ((and n (guard (< 9 (incf n) 100))) (list 'yes n))
                  (sorry (list 'no sorry))))
              (square-double-digit-p/MAYBE 9) ⇒ (yes 81)
              (square-double-digit-p/MAYBE 3) ⇒ (yes 9)  ; WRONG!

    The difference is in boolean-expression in guard: CLEAN references n simply and directly, while MAYBE references n with a side-effect, in the expression (incf n). When integer is 3, here's what happens:

    • The first n binds it to expval, i.e., the result of evaluating (* 3 3), or 9.
    • boolean-expression is evaluated:
                     start:   (< 9 (incf n)        100)
                     becomes: (< 9 (setq n (1+ n)) 100)
                     becomes: (< 9 (setq n (1+ 9)) 100)
                     becomes: (< 9 (setq n 10)     100)
                                                        ; side-effect here!
                     becomes: (< 9       n         100) ; n now bound to 10
                     becomes: (< 9      10         100)
                     becomes: t
    • Because the result of the evaluation is non-nil, guard matches, and matches, and control passes to that clause's body forms.

    Aside from the mathematical incorrectness of asserting that 9 is a double-digit integer, there is another problem with MAYBE. The body form references n once more, yet we do not see the updated value—10—at all. What happened to it?

    To sum up, it's best to avoid side-effecting references to symbol patterns entirely, not only in boolean-expression (in guard), but also in expr (in let) and function (in pred and app).

  3. On match, the clause's body forms can reference the set of symbols the pattern let-binds. When seqpat is and, this set is the union of all the symbols each of its sub-patterns let-binds. This makes sense because, for and to match, all the sub-patterns must match.

    When seqpat is or, things are different: or matches at the first sub-pattern that matches; the rest of the sub-patterns are ignored. It makes no sense for each sub-pattern to let-bind a different set of symbols because the body forms have no way to distinguish which sub-pattern matched and choose among the different sets. For example, the following is invalid:

              (pcase (read-number "Enter an integer: ")
                ((or (and (pred evenp)
                          e-num)      ; bind e-num to expval
                     o-num)           ; bind o-num to expval
                 (list e-num o-num)))
              Enter an integer: 42
              error--> Symbol’s value as variable is void: o-num
              Enter an integer: 149
              error--> Symbol’s value as variable is void: e-num

    Evaluating body form (list e-num o-num) signals error. To distinguish between sub-patterns, you can use another symbol, identical in name in all sub-patterns but differing in value. Reworking the above example:

              (pcase (read-number "Enter an integer: ")
                ((and num                                ; line 1
                      (or (and (pred evenp)              ; line 2
                               (let spin 'even))         ; line 3
                          (let spin 'odd)))              ; line 4
                 (list spin num)))                       ; line 5
              Enter an integer: 42
              ⇒ (even 42)
              Enter an integer: 149
              ⇒ (odd 149)

    Line 1 “factors out” the expval binding with and and symbol (in this case, num). On line 2, or begins in the same way as before, but instead of binding different symbols, uses let twice (lines 3-4) to bind the same symbol spin in both sub-patterns. The value of spin distinguishes the sub-patterns. The body form references both symbols (line 5).

Next: , Previous: pcase Macro, Up: Pattern-Matching Conditional

11.4.2 Extending pcase

The pcase macro supports several kinds of patterns (see Pattern-Matching Conditional). You can add support for other kinds of patterns using the pcase-defmacro macro.

— Macro: pcase-defmacro name args [doc] &rest body

Define a new kind of pattern for pcase, to be invoked as (name actual-args). The pcase macro expands this into a function call that evaluates body, whose job it is to rewrite the invoked pattern into some other pattern, in an environment where args are bound to actual-args.

Additionally, arrange to display doc along with the docstring of pcase. By convention, doc should use EXPVAL to stand for the result of evaluating expression (first arg to pcase).

Typically, body rewrites the invoked pattern to use more basic patterns. Although all patterns eventually reduce to core patterns, body need not use core patterns straight away. The following example defines two patterns, named less-than and integer-less-than.

     (pcase-defmacro less-than (n)
       "Matches if EXPVAL is a number less than N."
       `(pred (> ,n)))
     (pcase-defmacro integer-less-than (n)
       "Matches if EXPVAL is an integer less than N."
       `(and (pred integerp)
             (less-than ,n)))

Note that the docstrings mention args (in this case, only one: n) in the usual way, and also mention EXPVAL by convention. The first rewrite (i.e., body for less-than) uses one core pattern: pred. The second uses two core patterns: and and pred, as well as the newly-defined pattern less-than. Both use a single backquote construct (see Backquote).

Next: , Previous: Extending pcase, Up: Pattern-Matching Conditional

11.4.3 Backquote-Style Patterns

This subsection describes backquote-style patterns, a set of builtin patterns that eases structural matching. For background, see Pattern-Matching Conditional.

Backquote-style patterns are a powerful set of pcase pattern extensions (created using pcase-defmacro) that make it easy to match expval against specifications of its structure.

For example, to match expval that must be a list of two elements whose first element is a specific string and the second element is any value, you can write a core pattern:

     (and (pred listp)
          (guard (= 2 (length ls)))
          (guard (string= "first" (car ls)))
          (let second-elem (cadr ls)))

or you can write the equivalent backquote-style pattern:

     `("first" ,second-elem)

The backquote-style pattern is more concise, resembles the structure of expval, and avoids binding ls.

A backquote-style pattern has the form `qpat where qpat can have the following forms:

(qpat1 . qpat2)
Matches if expval is a cons cell whose car matches qpat1 and whose cdr matches qpat2. This readily generalizes to lists as in (qpat1 qpat2 ...).
[qpat1 qpat2 ... qpatm]
Matches if expval is a vector of length m whose 0..(m-1)th elements match qpat1, qpat2 ... qpatm, respectively.
Matches if the corresponding element of expval is equal to the specified literal object. Note that, aside from symbol, this is the same set of self-quoting literal objects that are acceptable as a core pattern.
Matches if the corresponding element of expval matches pattern. Note that pattern is any kind that pcase supports. (In the example above, second-elem is a symbol core pattern; it therefore matches anything, and let-binds second-elem.)

The corresponding element is the portion of expval that is in the same structural position as the structural position of qpat in the backquote-style pattern. (In the example above, the corresponding element of second-elem is the second element of expval.)

Here is an example of using pcase to implement a simple interpreter for a little expression language (note that this requires lexical binding for the lambda expression in the fn clause to properly capture body and arg (see Lexical Binding):

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

The first three clauses use backquote-style patterns. `(add ,x ,y) is a pattern that checks that form is a three-element list starting with the literal symbol add, then extracts the second and third elements and binds them to symbols x and y, respectively. The clause body evaluates x and y and adds the results. Similarly, the call clause implements a function call, and the fn clause implements an anonymous function definition.

The remaining clauses use core patterns. (pred numberp) matches if form is a number. On match, the body evaluates it. (pred symbolp) matches if form is a symbol. On match, the body looks up the symbol in env and returns its association. Finally, _ is the catch-all pattern that matches anything, so it's suitable for reporting syntax errors.

Here are some sample programs in this small language, including their evaluation results:

     (evaluate '(add 1 2) nil)                 ⇒ 3
     (evaluate '(add x y) '((x . 1) (y . 2)))  ⇒ 3
     (evaluate '(call (fn x (add 1 x)) 2) nil) ⇒ 3
     (evaluate '(sub 1 2) nil)                 ⇒ error

Previous: Backquote Patterns, Up: Pattern-Matching Conditional

11.4.4 Destructuring with pcase Patterns

Pcase patterns not only express a condition on the form of the objects they can match, but they can also extract sub-fields of those objects. For example we can extract 2 elements from a list that is the value of the variable my-list with the following code:

       (pcase my-list
         (`(add ,x ,y)  (message "Contains %S and %S" x y)))

This will not only extract x and y but will additionally test that my-list is a list containing exactly 3 elements and whose first element is the symbol add. If any of those tests fail, pcase will immediately return nil without calling message.

Extraction of multiple values stored in an object is known as destructuring. Using pcase patterns allows to perform destructuring binding, which is similar to a local binding (see Local Variables), but gives values to multiple elements of a variable by extracting those values from an object of compatible structure.

The macros described in this section use pcase patterns to perform destructuring binding. The condition of the object to be of compatible structure means that the object must match the pattern, because only then the object's subfields can be extracted. For example:

       (pcase-let ((`(add ,x ,y) my-list))
         (message "Contains %S and %S" x y))

does the same as the previous example, except that it directly tries to extract x and y from my-list without first verifying if my-list is a list which has the right number of elements and has add as its first element. The precise behavior when the object does not actually match the pattern is undefined, although the body will not be silently skipped: either an error is signaled or the body is run with some of the variables potentially bound to arbitrary values like nil.

The pcase patterns that are useful for destructuring bindings are generally those described in Backquote Patterns, since they express a specification of the structure of objects that will match.

For an alternative facility for destructuring binding, see seq-let.

— Macro: pcase-let bindings body...

Perform destructuring binding of variables according to bindings, and then evaluate body.

bindings is a list of bindings of the form (pattern exp), where exp is an expression to evaluate and pattern is a pcase pattern.

All exps are evaluated first, after which they are matched against their respective pattern, introducing new variable bindings that can then be used inside body. The variable bindings are produced by destructuring binding of elements of pattern to the values of the corresponding elements of the evaluated exp.

— Macro: pcase-let* bindings body...

Perform destructuring binding of variables according to bindings, and then evaluate body.

bindings is a list of bindings of the form (pattern exp), where exp is an expression to evaluate and pattern is a pcase pattern. The variable bindings are produced by destructuring binding of elements of pattern to the values of the corresponding elements of the evaluated exp.

Unlike pcase-let, but similarly to let*, each exp is matched against its corresponding pattern before processing the next element of bindings, so the variable bindings introduced in each one of the bindings are available in the exps of the bindings that follow it, additionally to being available in body.

— Macro: pcase-dolist (pattern list) body...

Execute body once for each element of list, on each iteration performing a destructuring binding of variables in pattern to the values of the corresponding subfields of the element of list. The bindings are performed as if by pcase-let. When pattern is a simple variable, this ends up being equivalent to dolist (see Iteration).

Next: , Previous: Pattern-Matching Conditional, Up: Control Structures

11.5 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:

— Special Form: while condition forms...

while first evaluates condition. If the result is non-nil, it evaluates forms in textual order. Then it reevaluates condition, and if the result is non-nil, it evaluates forms again. This process repeats until condition evaluates to nil.

There is no limit on the number of iterations that may occur. The loop will continue until either condition evaluates to nil or until an error or throw jumps out of it (see Nonlocal Exits).

The value of a while form is always nil.

          (setq num 0)
               ⇒ 0
          (while (< num 4)
            (princ (format "Iteration %d." num))
            (setq num (1+ num)))
               -| Iteration 0.
               -| Iteration 1.
               -| Iteration 2.
               -| Iteration 3.
               ⇒ nil

To write a repeat-until loop, which will execute something on each iteration and then do the end-test, put the body followed by the end-test in a progn as the first argument of while, as shown here:

          (while (progn
                   (forward-line 1)
                   (not (looking-at "^$"))))

This moves forward one line and continues moving by lines until it reaches an empty line. It is peculiar in that the while has no body, just the end test (which also does the real work of moving point).

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

— Macro: dolist (var list [result]) body...

This construct executes body once for each element of list, binding the variable var locally to hold the current element. Then it returns the value of evaluating result, or nil if result is omitted. For example, here is how you could use dolist to define the reverse function:

          (defun reverse (list)
            (let (value)
              (dolist (elt list value)
                (setq value (cons elt value)))))
— Macro: dotimes (var count [result]) body...

This construct executes body once for each integer from 0 (inclusive) to count (exclusive), binding the variable var to the integer for the current iteration. Then it returns the value of evaluating result, or nil if result is omitted. Here is an example of using dotimes to do something 100 times:

          (dotimes (i 100)
            (insert "I will not obey absurd orders\n"))

Next: , Previous: Iteration, Up: Control Structures

11.6 Generators

A generator is a function that produces a potentially-infinite stream of values. Each time the function produces a value, it suspends itself and waits for a caller to request the next value.

— Macro: iter-defun name args [doc] [declare] [interactive] body...

iter-defun defines a generator function. A generator function has the same signature as a normal function, but works differently. Instead of executing body when called, a generator function returns an iterator object. That iterator runs body to generate values, emitting a value and pausing where iter-yield or iter-yield-from appears. When body returns normally, iter-next signals iter-end-of-sequence with body's result as its condition data.

Any kind of Lisp code is valid inside body, but iter-yield and iter-yield-from cannot appear inside unwind-protect forms.

— Macro: iter-lambda args [doc] [interactive] body...

iter-lambda produces an unnamed generator function that works just like a generator function produced with iter-defun.

— Macro: iter-yield value

When it appears inside a generator function, iter-yield indicates that the current iterator should pause and return value from iter-next. iter-yield evaluates to the value parameter of next call to iter-next.

— Macro: iter-yield-from iterator

iter-yield-from yields all the values that iterator produces and evaluates to the value that iterator's generator function returns normally. While it has control, iterator receives values sent to the iterator using iter-next.

To use a generator function, first call it normally, producing a iterator object. An iterator is a specific instance of a generator. Then use iter-next to retrieve values from this iterator. When there are no more values to pull from an iterator, iter-next raises an iter-end-of-sequence condition with the iterator's final value.

It's important to note that generator function bodies only execute inside calls to iter-next. A call to a function defined with iter-defun produces an iterator; you must drive this iterator with iter-next for anything interesting to happen. Each call to a generator function produces a different iterator, each with its own state.

— Function: iter-next iterator value

Retrieve the next value from iterator. If there are no more values to be generated (because iterator's generator function returned), iter-next signals the iter-end-of-sequence condition; the data value associated with this condition is the value with which iterator's generator function returned.

value is sent into the iterator and becomes the value to which iter-yield evaluates. value is ignored for the first iter-next call to a given iterator, since at the start of iterator's generator function, the generator function is not evaluating any iter-yield form.

— Function: iter-close iterator

If iterator is suspended inside an unwind-protect's bodyform and becomes unreachable, Emacs will eventually run unwind handlers after a garbage collection pass. (Note that iter-yield is illegal inside an unwind-protect's unwindforms.) To ensure that these handlers are run before then, use iter-close.

Some convenience functions are provided to make working with iterators easier:

— Macro: iter-do (var iterator) body ...

Run body with var bound to each value that iterator produces.

The Common Lisp loop facility also contains features for working with iterators. See Loop Facility.

The following piece of code demonstrates some important principles of working with iterators.

     (require 'generator)
     (iter-defun my-iter (x)
       (iter-yield (1+ (iter-yield (1+ x))))
        ;; Return normally
     (let* ((iter (my-iter 5))
            (iter2 (my-iter 0)))
       ;; Prints 6
       (print (iter-next iter))
       ;; Prints 9
       (print (iter-next iter 8))
       ;; Prints 1; iter and iter2 have distinct states
       (print (iter-next iter2 nil))
       ;; We expect the iter sequence to end now
       (condition-case x
           (iter-next iter)
           ;; Prints -1, which my-iter returned normally
           (print (cdr x)))))

Previous: Generators, Up: Control Structures

11.7 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.

Next: , Up: Nonlocal Exits

11.7.1 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
     (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.
— Special Form: catch tag body...

catch establishes a return point for the throw function. The return point is distinguished from other such return points by tag, which may be any Lisp object except nil. The argument tag is evaluated normally before the return point is established.

With the return point in effect, catch evaluates the forms of the body in textual order. If the forms execute normally (without error or nonlocal exit) the value of the last body form is returned from the catch.

If a throw is executed during the execution of body, specifying the same value tag, the catch form exits immediately; the value it returns is whatever was specified as the second argument of throw.

— Function: throw tag value

The purpose of throw is to return from a return point previously established with catch. The argument tag is used to choose among the various existing return points; it must be eq to the value specified in the catch. If multiple return points match tag, the innermost one is used.

The argument value is used as the value to return from that catch.

If no return point is in effect with tag tag, then a no-catch error is signaled with data (tag value).

Next: , Previous: Catch and Throw, Up: Nonlocal Exits

11.7.2 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))
     -| 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))
     ⇒ 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.

Next: , Previous: Examples of Catch, Up: Nonlocal Exits

11.7.3 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.

Next: , Up: Errors 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.

— Function: error format-string &rest args

This function signals an error with an error message constructed by applying format-message (see Formatting Strings) to format-string and args.

These examples show typical uses of error:

          (error "That is an error -- try something else")
               error--> That is an error -- try something else
          (error "Invalid name `%s'" "A%%B")
               error--> Invalid name ‘A%%B’

error works by calling signal with two arguments: the error symbol error, and a list containing the string returned by format-message.

Typically grave accent and apostrophe in the format translate to matching curved quotes, e.g., "Missing `%s'" might result in "Missing ‘foo’". See Text Quoting Style, for how to influence or inhibit this translation.

Warning: If you want to use your own string as an error message verbatim, don't just write (error string). If string string contains ‘%’, ‘`’, or ‘'’ it may be reformatted, with undesirable results. Instead, use (error "%s" string).

— Function: signal error-symbol data

This function signals an error named by error-symbol. The argument data is a list of additional Lisp objects relevant to the circumstances of the error.

The argument error-symbol must be an error symbol—a symbol defined with define-error. This is how Emacs Lisp classifies different sorts of errors. See Error Symbols, for a description of error symbols, error conditions and condition names.

If the error is not handled, the two arguments are used in printing the error message. Normally, this error message is provided by the error-message property of error-symbol. If data is non-nil, this is followed by a colon and a comma separated list of the unevaluated elements of data. For error, the error message is the car of data (that must be a string). Subcategories of file-error are handled specially.

The number and significance of the objects in data depends on error-symbol. For example, with a wrong-type-argument error, there should be two objects in the list: a predicate that describes the type that was expected, and the object that failed to fit that type.

Both error-symbol and data are available to any error handlers that handle the error: condition-case binds a local variable to a list of the form (error-symbol . data) (see Handling Errors).

The function signal never returns.

          (signal 'wrong-number-of-arguments '(x y))
               error--> Wrong number of arguments: x, y
          (signal 'no-such-error '("My unknown error condition"))
               error--> peculiar error: "My unknown error condition"

— Function: user-error format-string &rest args

This function behaves exactly like error, except that it uses the error symbol user-error rather than error. As the name suggests, this is intended to report errors on the part of the user, rather than errors in the code itself. For example, if you try to use the command Info-history-back (l) to move back beyond the start of your Info browsing history, Emacs signals a user-error. Such errors do not cause entry to the debugger, even when debug-on-error is non-nil. See Error Debugging.

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

Next: , Previous: Signaling Errors, Up: Errors 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:

— Variable: command-error-function

This variable, if non-nil, specifies a function to use to handle errors that return control to the Emacs command loop. The function should take three arguments: data, a list of the same form that condition-case would bind to its variable; context, a string describing the situation in which the error occurred, or (more often) nil; and caller, the Lisp function which called the primitive that signaled the error.

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.

Next: , Previous: Processing of Errors, Up: Errors 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.

— Macro: condition-case-unless-debug var protected-form handlers...

The macro condition-case-unless-debug provides another way to handle debugging of such forms. It behaves exactly like condition-case, unless the variable debug-on-error is non-nil, in which case it does not handle any errors at all.

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).

— Special Form: condition-case var protected-form handlers...

This special form establishes the error handlers handlers around the execution of protected-form. If protected-form executes without error, the value it returns becomes the value of the condition-case form; in this case, the condition-case has no effect. The condition-case form makes a difference when an error occurs during protected-form.

Each of the handlers is a list of the form (conditions body...). Here conditions is an error condition name to be handled, or a list of condition names (which can include debug to allow the debugger to run before the handler); body is one or more Lisp expressions to be executed when this handler handles an error. Here are examples of handlers:

          (error nil)
          (arith-error (message "Division by zero"))
          ((arith-error file-error)
            "Either division by zero or failure to open a file"))

Each error that occurs has an error symbol that describes what kind of error it is, and which describes also a list of condition names (see Error Symbols). Emacs searches all the active condition-case forms for a handler that specifies one or more of these condition names; the innermost matching condition-case handles the error. Within this condition-case, the first applicable handler handles the error.

After executing the body of the handler, the condition-case returns normally, using the value of the last form in the handler body as the overall value.

The argument var is a variable. condition-case does not bind this variable when executing the protected-form, only when it handles an error. At that time, it binds var locally to an error description, which is a list giving the particulars of the error. The error description has the form (error-symbol . data). The handler can refer to this list to decide what to do. For example, if the error is for failure opening a file, the file name is the second element of data—the third element of the error description.

If var is nil, that means no variable is bound. Then the error symbol and associated data are not available to the handler.

Sometimes it is necessary to re-throw a signal caught by condition-case, for some outer-level handler to catch. Here's how to do that:

            (signal (car err) (cdr err))

where err is the error description variable, the first argument to condition-case whose error condition you want to re-throw. See Definition of signal.

— Function: error-message-string error-descriptor

This function returns the error message string for a given error descriptor. It is useful if you want to handle an error by printing the usual error message for that error. See Definition of signal.

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))
     ⇒ 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)
           ;; 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))
     -| The error was: (error "Rats!  The variable baz was 34, not 35")
     ⇒ 2
— Macro: ignore-errors body...

This construct executes body, ignoring any errors that occur during its execution. If the execution is without error, ignore-errors returns the value of the last form in body; otherwise, it returns nil.

Here's the example at the beginning of this subsection rewritten using ignore-errors:

             (delete-file filename))
— Macro: with-demoted-errors format body...

This macro is like a milder version of ignore-errors. Rather than suppressing errors altogether, it converts them into messages. It uses the string format to format the message. format should contain a single ‘%’-sequence; e.g., "Error: %S". Use with-demoted-errors around code that is not expected to signal errors, but should be robust if one does occur. Note that this macro uses condition-case-unless-debug rather than condition-case.

Previous: Handling Errors, Up: Errors 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.

— Function: define-error name message &optional parent

In order for a symbol to be an error symbol, it must be defined with define-error which takes a parent condition (defaults to error). This parent defines the conditions that this kind of error belongs to. The transitive set of parents always includes the error symbol itself, and the symbol error. Because quitting is not considered an error, the set of parents of quit is just (quit).

In addition to its parents, the error symbol has a message which is a string to be printed when that error is signaled but not handled. If that message is not valid, the error message ‘peculiar error’ is used. See Definition of signal.

Internally, the set of parents is stored in the error-conditions property of the error symbol and the message is stored in the error-message property of the error symbol.

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

     (define-error 'new-error "A new error" 'my-own-errors)

This error has several condition names: new-error, the narrowest classification; my-own-errors, which we imagine is a wider classification; and all the conditions of my-own-errors which should include 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 its 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.

Previous: Errors, Up: Nonlocal Exits

11.7.4 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.)

— Special Form: unwind-protect body-form cleanup-forms...

unwind-protect executes body-form with a guarantee that the cleanup-forms will be evaluated if control leaves body-form, no matter how that happens. body-form may complete normally, or execute a throw out of the unwind-protect, or cause an error; in all cases, the cleanup-forms will be evaluated.

If body-form finishes normally, unwind-protect returns the value of body-form, after it evaluates the cleanup-forms. If body-form does not finish, unwind-protect does not return any value in the normal sense.

Only body-form is protected by the unwind-protect. If any of the cleanup-forms themselves exits nonlocally (via a throw or an error), unwind-protect is not guaranteed to evaluate the rest of them. If the failure of one of the cleanup-forms has the potential to cause trouble, then protect it with another unwind-protect around that form.

The number of currently active unwind-protect forms counts, together with the number of local variable bindings, against the limit max-specpdl-size (see Local Variables).

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
           (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))
             (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.

Next: , Previous: Control Structures, Up: Top

12 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 cell7. 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.

Next: , Up: Variables

12.1 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:

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

Next: , Previous: Global Variables, Up: Variables

12.2 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
— Function: keywordp object

function returns t if object is a symbol whose name starts with ‘:’, interned in the standard obarray, and returns nil otherwise.

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.

A small number of additional symbols are made read-only for various practical reasons. These include enable-multibyte-characters, most-positive-fixnum, most-negative-fixnum, and a few others. Any attempt to set or bind these also signals a setting-constant error.

Next: , Previous: Constant Variables, Up: Variables

12.3 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 only 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:

— Special Form: let (bindings...) forms...

This special form sets up local bindings for a certain set of variables, as specified by bindings, and then evaluates all of the forms in textual order. Its return value is the value of the last form in forms. The local bindings set up by let will be in effect only within the body of forms.

Each of the bindings is either (i) a symbol, in which case that symbol is locally bound to nil; or (ii) a list of the form (symbol value-form), in which case symbol is locally bound to the result of evaluating value-form. If value-form is omitted, nil is used.

All of the value-forms in bindings are evaluated in the order they appear and before binding any of the symbols to them. Here is an example of this: z is bound to the old value of y, which is 2, not the new value of y, which is 1.

          (setq y 2)
               ⇒ 2
          (let ((y 1)
                (z y))
            (list y z))
               ⇒ (1 2)

On the other hand, the order of bindings is unspecified: in the following example, either 1 or 2 might be printed.

          (let ((x 1)
                (x 2))
            (print x))

Therefore, avoid binding a variable more than once in a single let form.

— Special Form: let* (bindings...) forms...

This special form is like let, but it binds each variable right after computing its local value, before computing the local value for the next variable. Therefore, an expression in bindings can refer to the preceding symbols bound in this let* form. Compare the following example with the example above for let.

          (setq y 2)
               ⇒ 2
          (let* ((y 1)
                 (z y))    ; Use the just-established value of y.
            (list y z))
               ⇒ (1 1)

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

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.

— User Option: max-specpdl-size

This variable defines the limit on the total number of local variable bindings and unwind-protect cleanups (see Cleaning Up from Nonlocal Exits) that are allowed before Emacs signals an error (with data "Variable binding depth exceeds max-specpdl-size").

This limit, with the associated error when it is exceeded, is one way that Lisp avoids infinite recursion on an ill-defined function. max-lisp-eval-depth provides another limit on depth of nesting. See Eval.

The default value is 1300. Entry to the Lisp debugger increases the value, if there is little room left, to make sure the debugger itself has room to execute.

Next: , Previous: Local Variables, Up: Variables

12.4 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 scoping rule (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, instead of returning a value.

Under the optional lexical scoping rule, the value cell only holds the variable's global value—the value outside of any lexical binding construct. When a variable is lexically bound, the local value is determined by the lexical environment; hence, variables can have local values even if their symbols' value cells are unassigned.

— Function: makunbound symbol

This function empties out the value cell of symbol, making the variable void. It returns symbol.

If symbol has a dynamic local binding, makunbound voids the current binding, and this voidness lasts only as long as the local binding is in effect. Afterwards, the previously shadowed local or global binding is reexposed; then the variable will no longer be void, unless the reexposed binding is void too.

Here are some examples (assuming dynamic binding is in effect):

          (setq x 1)               ; Put a value in the global binding.
               ⇒ 1
          (let ((x 2))             ; Locally bind it.
            (makunbound 'x)        ; Void the local binding.
          error--> Symbol's value as variable is void: x
          x                        ; The global binding is unchanged.
               ⇒ 1
          (let ((x 2))             ; Locally bind it.
            (let ((x 3))           ; And again.
              (makunbound 'x)      ; Void the innermost-local binding.
              x))                  ; And refer: it's void.
          error--> Symbol's value as variable is void: x
          (let ((x 2))
            (let ((x 3))
              (makunbound 'x))     ; Void inner binding, then remove it.
            x)                     ; Now outer let binding is visible.
               ⇒ 2
— Function: boundp variable

This function returns t if variable (a symbol) is not void, and nil if it is void.

Here are some examples (assuming dynamic binding is in effect):

          (boundp 'abracadabra)          ; Starts out void.
               ⇒ nil
          (let ((abracadabra 5))         ; Locally bind it.
            (boundp 'abracadabra))
               ⇒ t
          (boundp 'abracadabra)          ; Still globally void.
               ⇒ nil
          (setq abracadabra 5)           ; Make it globally nonvoid.
               ⇒ 5
          (boundp 'abracadabra)
               ⇒ t

Next: , Previous: Void Variables, Up: Variables

12.5 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.

— Special Form: defvar symbol [value [doc-string]]

This special form defines symbol as a variable. Note that symbol is not evaluated; the symbol to be defined should appear explicitly in the defvar form. The variable is marked as special, meaning that it should always be dynamically bound (see Variable Scoping).

If value is specified, and symbol is void (i.e., it has no dynamically bound value; see Void Variables), then value is evaluated and symbol is set to the result. But if symbol is not void, value is not evaluated, and symbol's value is left unchanged. If value is omitted, the value of symbol is not changed in any case.

Note that specifying a value, even nil, marks the variable as special permanently. Whereas if value is omitted then the variable is only marked special locally (i.e. within the current lexical scope, or file if at the top-level). This can be useful for suppressing byte compilation warnings, see Compiler Errors.

If symbol has a buffer-local binding in the current buffer, defvar acts on the default value, which is buffer-independent, rather than the buffer-local binding. It sets the default value if the default value is void. See Buffer-Local Variables.

If symbol is already lexically bound (e.g., if the defvar form occurs in a let form with lexical binding enabled), then defvar sets the dynamic value. The lexical binding remains in effect until its binding construct exits. See Variable Scoping.

When you evaluate a top-level defvar form with C-M-x in Emacs Lisp mode (eval-defun), a special feature of eval-defun arranges to set the variable unconditionally, without testing whether its value is void.

If the doc-string argument is supplied, it specifies the documentation string for the variable (stored in the symbol's variable-documentation property). See Documentation.

Here are some examples. This form defines foo but does not initialize it:

          (defvar foo)
               ⇒ foo

This example initializes the value of bar to 23, and gives it a documentation string:

          (defvar bar 23
            "The normal weight of a bar.")
               ⇒ bar

The defvar form returns symbol, but it is normally used at top level in a file where its value does not matter.

For a more elaborate example of using defvar without a value, see Local defvar example.

— Special Form: defconst symbol value [doc-string]

This special form defines symbol as a value and initializes it. It informs a person reading your code that symbol has a standard global value, established here, that should not be changed by the user or by other programs. Note that symbol is not evaluated; the symbol to be defined must appear explicitly in the defconst.

The defconst form, like defvar, marks the variable as special, meaning that it should always be dynamically bound (see Variable Scoping). In addition, it marks the variable as risky (see File Local Variables).

defconst always evaluates value, and sets the value of symbol to the result. If symbol does have a buffer-local binding in the current buffer, defconst sets the default value, not the buffer-local value. (But you should not be making buffer-local bindings for a symbol that is defined with defconst.)

An example of the use of defconst is Emacs's definition of float-pi—the mathematical constant pi, which ought not to be changed by anyone (attempts by the Indiana State Legislature notwithstanding). As the second form illustrates, however, defconst is only advisory.

          (defconst float-pi 3.141592653589793 "The value of Pi.")
               ⇒ float-pi
          (setq float-pi 3)
               ⇒ float-pi
               ⇒ 3

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.

Next: , Previous: Defining Variables, Up: Variables

12.6 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:

The variable is a normal hook (see Hooks).
The value is a function.
The value is a list of functions.
The value is a form (an expression).
The value is a list of forms (expressions).
The value is a predicate—a function of one argument that returns non-nil for success and nil for failure.
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.
The value is a program name.
The value is a whole shell command.
The value specifies options for a command.
The variable is intended for internal use and is defined in the file prefix.el. (Emacs code contributed before 2018 may follow other conventions, which are being phased out.)
The variable is intended for internal use and is defined in C code. (Emacs code contributed before 2018 may follow other conventions, which are being phased out.)

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)

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
     (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.

Next: , Previous: Tips for Defining, Up: Variables

12.7 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.

— Function: symbol-value symbol

This function returns the value stored in symbol's value cell. This is where the variable's current (dynamic) value is stored. If the variable has no local binding, this is simply its global value. If the variable is void, a void-variable error is signaled.

If the variable is lexically bound, the value reported by symbol-value is not necessarily the same as the variable's lexical value, which is determined by the lexical environment rather than the symbol's value cell. See Variable Scoping.

          (setq abracadabra 5)
               ⇒ 5
          (setq foo 9)
               ⇒ 9
          ;; Here the symbol abracadabra
          ;;   is the symbol whose value is examined.
          (let ((abracadabra 'foo))
            (symbol-value 'abracadabra))
               ⇒ foo
          ;; Here, the value of abracadabra,
          ;;   which is foo,
          ;;   is the symbol whose value is examined.
          (let ((abracadabra 'foo))
            (symbol-value abracadabra))
               ⇒ 9
          (symbol-value 'abracadabra)
               ⇒ 5

Next: , Previous: Accessing Variables, Up: Variables

12.8 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.

— Special Form: setq [symbol form]...

This special form is the most common method of changing a variable's value. Each symbol is given a new value, which is the result of evaluating the corresponding form. The current binding of the symbol is changed.

setq does not evaluate symbol; it sets the symbol that you write. We say that this argument is automatically quoted. The ‘q’ in setq stands for “quoted”.

The value of the setq form is the value of the last form.

          (setq x (1+ 2))
               ⇒ 3
          x                   ; x now has a global value.
               ⇒ 3
          (let ((x 5))
            (setq x 6)        ; The local binding of x is set.
               ⇒ 6
          x                   ; The global value is unchanged.
               ⇒ 3

Note that the first form is evaluated, then the first symbol is set, then the second form is evaluated, then the second symbol is set, and so on:

          (setq x 10          ; Notice that x is set before
                y (1+ x))     ;   the value of y is computed.
               ⇒ 11
— Function: set symbol value

This function puts value in the value cell of symbol. Since it is a function rather than a special form, the expression written for symbol is evaluated to obtain the symbol to set. The return value is value.

When dynamic variable binding is in effect (the default), set has the same effect as setq, apart from the fact that set evaluates its symbol argument whereas setq does not. But when a variable is lexically bound, set affects its dynamic value, whereas setq affects its current (lexical) value. See Variable Scoping.

          (set one 1)
          error--> Symbol's value as variable is void: one
          (set 'one 1)
               ⇒ 1
          (set 'two 'one)
               ⇒ one
          (set two 2)         ; two evaluates to symbol one.
               ⇒ 2
          one                 ; So it is one that was set.
               ⇒ 2
          (let ((one 1))      ; This binding of one is set,
            (set 'one 3)      ;   not the global value.
               ⇒ 3
               ⇒ 2

If symbol is not actually a symbol, a wrong-type-argument error is signaled.

          (set '(x y) 'z)
          error--> Wrong type argument: symbolp, (x y)

Next: , Previous: Setting Variables, Up: Variables

12.9 Running a function when a variable is changed.

It is sometimes useful to take some action when a variable changes its value. The watchpoint facility provides the means to do so. Some possible uses for this feature include keeping display in sync with variable settings, and invoking the debugger to track down unexpected changes to variables (see Variable Debugging).

The following functions may be used to manipulate and query the watch functions for a variable.

— Function: add-variable-watcher symbol watch-function

This function arranges for watch-function to be called whenever symbol is modified. Modifications through aliases (see Variable Aliases) will have the same effect.

watch-function will be called with 4 arguments: (symbol newval operation where).

symbol is the variable being changed. newval is the value it will be changed to. operation is a symbol representing the kind of change, one of: `set', `let', `unlet', `makunbound', and `defvaralias'. where is a buffer if the buffer-local value of the variable is being changed, nil otherwise.

— Function: remove-variable-watch symbol watch-function

This function removes watch-function from symbol's list of watchers.

— Function: get-variable-watchers symbol

This function returns the list of symbol's active watcher functions.

12.9.1 Limitations

There are a couple of ways in which a variable could be modified (or at least appear to be modified) without triggering a watchpoint.

Since watchpoints are attached to symbols, modification to the objects contained within variables (e.g., by a list modification function see Modifying Lists) is not caught by this mechanism.

Additionally, C code can modify the value of variables directly, bypassing the watchpoint mechanism.

A minor limitation of this feature, again because it targets symbols, is that only variables of dynamic scope may be watched. This poses little difficulty, since modifications to lexical variables can be discovered easily by inspecting the code within the scope of the variable (unlike dynamic variables, which can be modified by any code at all, see Variable Scoping).

Next: , Previous: Watching Variables, Up: Variables

12.10 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 dynamic 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 construct8. 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.

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12.10.1 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 dynamic scope and 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.
          ⇒ 1
     ;; After the let form finishes, x reverts to its
     ;; previous value, which is −99.
          ⇒ -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 (i.e., 1). But when we call getx outside the let form, it retrieves the global value (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))
          ⇒ 3           ; The two addx calls add to x twice.
     ;; After the let form finishes, x reverts to its
     ;; previous value, which is −99.
          ⇒ -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.

Next: , Previous: Dynamic Binding, Up: Variable Scoping

12.10.2 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:

Next: , Previous: Dynamic Binding Tips, Up: Variable Scoping

12.10.3 Lexical Binding

Lexical binding was introduced to Emacs, as an optional feature, in version 24.1. We expect its importance to increase with time. Lexical binding opens up many more opportunities for optimization, so programs using it are likely to run faster in future Emacs versions. Lexical binding is also more compatible with concurrency, which was added to Emacs in version 26.1.

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.
     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 variables 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.

(Internally, the lexical environment is an alist of symbol-value pairs, with the final element in the alist being the symbol t rather than a cons cell. Such an alist can be passed as the second argument to the eval function, in order to specify a lexical environment in which to evaluate a form. See Eval. Most Emacs Lisp programs, however, should not interact directly with lexical environments in this way; only specialized programs like debuggers.)

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) ()
               (setq x (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 unlike dynamic variables which are tied to the symbol object itself, the relationship between lexical variables and symbols is only present in the interpreter (or compiler). Therefore, functions which take a symbol argument (like symbol-value, boundp, and set) can only retrieve or modify a variable's dynamic binding (i.e., the contents of its symbol's value cell).

Previous: Lexical Binding, Up: Variable Scoping

12.10.4 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:

— Variable: lexical-binding

If this buffer-local variable is non-nil, Emacs Lisp files and buffers are evaluated using lexical binding instead of dynamic binding. (However, special variables are still dynamically bound; see below.) If nil, dynamic binding is used for all local variables. This variable is typically set for a whole Emacs Lisp file, as a file local variable (see File Local Variables). Note that unlike other such variables, this one must be set in the first line of a file.

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.

Using defvar without a value, it is possible to bind a variable dynamically just in one file, or in just one part of a file while still binding it lexically elsewhere. For example:

     (let (_)
       (defvar x)      ; Let-bindings of x will be dynamic within this let.
       (let ((x -99))  ; This is a dynamic binding of x.
         (defun get-dynamic-x ()
     (let ((x 'lexical)) ; This is a lexical binding of x.
       (defun get-lexical-x ()
     (let (_)
       (defvar x)
       (let ((x 'dynamic))
         (list (get-lexical-x)
         ⇒ (lexical dynamic)
— Function: special-variable-p symbol

This function returns non-nil if symbol is a special variable (i.e., it has a defvar, defcustom, or defconst variable definition). Otherwise, the return value is nil.

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 easy. First, add a file-local variable setting of lexical-binding to t in the header line of the Emacs Lisp source file (see File Local Variables). 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 starts with an underscore. The byte-compiler interprets this as an indication that this is a variable known not to be used.)

Next: , Previous: Variable Scoping, Up: Variables

12.11 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.)

Next: , Up: Buffer-Local Variables

12.11.1 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
     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).

Next: , Previous: Intro to Buffer-Local, Up: Buffer-Local Variables

12.11.2 Creating and Deleting Buffer-Local Bindings

— Command: make-local-variable variable

This function creates a buffer-local binding in the current buffer for variable (a symbol). Other buffers are not affected. The value returned is variable.

The buffer-local value of variable starts out as the same value variable previously had. If variable was void, it remains void.

          ;; In buffer ‘b1’:
          (setq foo 5)                ; Affects all buffers.
               ⇒ 5
          (make-local-variable 'foo)  ; Now it is local in ‘b1’.
               ⇒ foo
          foo                         ; That did not change
               ⇒ 5                   ;   the value.
          (setq foo 6)                ; Change the value
               ⇒ 6                   ;   in ‘b1’.
               ⇒ 6
          ;; In buffer ‘b2’, the value hasn't changed.
          (with-current-buffer "b2"
               ⇒ 5

Making a variable buffer-local within a let-binding for that variable does not work reliably, unless the buffer in which you do this is not current either on entry to or exit from the let. This is because let does not distinguish between different kinds of bindings; it knows only which variable the binding was made for.

It is an error to make a constant or a read-only variable buffer-local. See Constant Variables.

If the variable is terminal-local (see Multiple Terminals), this function signals an error. Such variables cannot have buffer-local bindings as well.

Warning: do not use make-local-variable for a hook variable. The hook variables are automatically made buffer-local as needed if you use the local argument to add-hook or remove-hook.

— Macro: setq-local variable value

This macro creates a buffer-local binding in the current buffer for variable, and gives it the buffer-local value value. It is equivalent to calling make-local-variable followed by setq. variable should be an unquoted symbol.

— Command: make-variable-buffer-local variable

This function marks variable (a symbol) automatically buffer-local, so that any subsequent attempt to set it will make it local to the current buffer at the time. Unlike make-local-variable, with which it is often confused, this cannot be undone, and affects the behavior of the variable in all buffers.

A peculiar wrinkle of this feature is that binding the variable (with let or other binding constructs) does not create a buffer-local binding for it. Only setting the variable (with set or setq), while the variable does not have a let-style binding that was made in the current buffer, does so.

If variable does not have a default value, then calling this command will give it a default value of nil. If variable already has a default value, that value remains unchanged. Subsequently calling makunbound on variable will result in a void buffer-local value and leave the default value unaffected.

The value returned is variable.

It is an error to make a constant or a read-only variable buffer-local. See Constant Variables.

Warning: Don't assume that you should use make-variable-buffer-local for user-option variables, simply because users might want to customize them differently in different buffers. Users can make any variable local, when they wish to. It is better to leave the choice to them.

The time to use make-variable-buffer-local is when it is crucial that no two buffers ever share the same binding. For example, when a variable is used for internal purposes in a Lisp program which depends on having separate values in separate buffers, then using make-variable-buffer-local can be the best solution.

— Macro: defvar-local variable value &optional docstring

This macro defines variable as a variable with initial value value and docstring, and marks it as automatically buffer-local. It is equivalent to calling defvar followed by make-variable-buffer-local. variable should be an unquoted symbol.

— Function: local-variable-p variable &optional buffer

This returns t if variable is buffer-local in buffer buffer (which defaults to the current buffer); otherwise, nil.

— Function: local-variable-if-set-p variable &optional buffer

This returns t if variable either has a buffer-local value in buffer buffer, or is automatically buffer-local. Otherwise, it returns nil. If omitted or nil, buffer defaults to the current buffer.

— Function: buffer-local-value variable buffer

This function returns the buffer-local binding of variable (a symbol) in buffer buffer. If variable does not have a buffer-local binding in buffer buffer, it returns the default value (see Default Value) of variable instead.

— Function: buffer-local-variables &optional buffer

This function returns a list describing the buffer-local variables in buffer buffer. (If buffer is omitted, the current buffer is used.) Normally, each list element has the form (sym . val), where sym is a buffer-local variable (a symbol) and val is its buffer-local value. But when a variable's buffer-local binding in buffer is void, its list element is just sym.

          (make-local-variable 'foobar)
          (makunbound 'foobar)
          (make-local-variable 'bind-me)
          (setq bind-me 69)
          (setq lcl (buffer-local-variables))
              ;; First, built-in variables local in all buffers:
          ⇒ ((mark-active . nil)
              (buffer-undo-list . nil)
              (mode-name . "Fundamental")
              ;; Next, non-built-in buffer-local variables.
              ;; This one is buffer-local and void:
              ;; This one is buffer-local and nonvoid:
              (bind-me . 69))

Note that storing new values into the cdrs of cons cells in this list does not change the buffer-local values of the variables.

— Command: kill-local-variable variable

This function deletes the buffer-local binding (if any) for variable (a symbol) in the current buffer. As a result, the default binding of variable becomes visible in this buffer. This typically results in a change in the value of variable, since the default value is usually different from the buffer-local value just eliminated.

If you kill the buffer-local binding of a variable that automatically becomes buffer-local when set, this makes the default value visible in the current buffer. However, if you set the variable again, that will once again create a buffer-local binding for it.

kill-local-variable returns variable.

This function is a command because it is sometimes useful to kill one buffer-local variable interactively, just as it is useful to create buffer-local variables interactively.

— Function: kill-all-local-variables

This function eliminates all the buffer-local variable bindings of the current buffer except for variables marked as permanent and local hook functions that have a non-nil permanent-local-hook property (see Setting Hooks). As a result, the buffer will see the default values of most variables.

This function also resets certain other information pertaining to the buffer: it sets the local keymap to nil, the syntax table to the value of (standard-syntax-table), the case table to (standard-case-table), and the abbrev table to the value of fundamental-mode-abbrev-table.

The very first thing this function does is run the normal hook change-major-mode-hook (see below).

Every major mode command begins by calling this function, which has the effect of switching to Fundamental mode and erasing most of the effects of the previous major mode. To ensure that this does its job, the variables that major modes set should not be marked permanent.

kill-all-local-variables returns nil.

— Variable: change-major-mode-hook

The function kill-all-local-variables runs this normal hook before it does anything else. This gives major modes a way to arrange for something special to be done if the user switches to a different major mode. It is also useful for buffer-specific minor modes that should be forgotten if the user changes the major mode.

For best results, make this variable buffer-local, so that it will disappear after doing its job and will not interfere with the subsequent major mode. See Hooks.

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.

Previous: Creating Buffer-Local, Up: Buffer-Local Variables

12.11.3 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.

— Function: default-value symbol

This function returns symbol's default value. This is the value that is seen in buffers and frames that do not have their own values for this variable. If symbol is not buffer-local, this is equivalent to symbol-value (see Accessing Variables).

— Function: default-boundp symbol

The function default-boundp tells you whether symbol's default value is nonvoid. If (default-boundp 'foo) returns nil, then (default-value 'foo) would get an error.

default-boundp is to default-value as boundp is to symbol-value.

— Special Form: setq-default [symbol form]...

This special form gives each symbol a new default value, which is the result of evaluating the corresponding form. It does not evaluate symbol, but does evaluate form. The value of the setq-default form is the value of the last form.

If a symbol is not buffer-local for the current buffer, and is not marked automatically buffer-local, setq-default has the same effect as setq. If symbol is buffer-local for the current buffer, then this changes the value that other buffers will see (as long as they don't have a buffer-local value), but not the value that the current buffer sees.

          ;; In buffer ‘foo’:
          (make-local-variable 'buffer-local)
               ⇒ buffer-local
          (setq buffer-local 'value-in-foo)
               ⇒ value-in-foo
          (setq-default buffer-local 'new-default)
               ⇒ new-default
               ⇒ value-in-foo
          (default-value 'buffer-local)
               ⇒ new-default
          ;; In (the new) buffer ‘bar’:
               ⇒ new-default
          (default-value 'buffer-local)
               ⇒ new-default
          (setq buffer-local 'another-default)
               ⇒ another-default
          (default-value 'buffer-local)
               ⇒ another-default
          ;; Back in buffer ‘foo’:
               ⇒ value-in-foo
          (default-value 'buffer-local)
               ⇒ another-default
— Function: set-default symbol value

This function is like setq-default, except that symbol is an ordinary evaluated argument.

          (set-default (car '(a b c)) 23)
               ⇒ 23
          (default-value 'a)
               ⇒ 23

A variable can be let-bound (see Local Variables) to a value. This makes its global value shadowed by the binding; default-value will then return the value from that binding, not the global value, and set-default will be prevented from setting the global value (it will change the let-bound value instead). The following two functions allow to reference the global value even if it's shadowed by a let-binding.

— Function: default-toplevel-value symbol

This function returns the top-level default value of symbol, which is its value outside of any let-binding.

     (defvar variable 'global-value)
         ⇒ variable
     (let ((variable 'let-binding))
       (default-value 'variable))
         ⇒ let-binding
     (let ((variable 'let-binding))
       (default-toplevel-value 'variable))
         ⇒ global-value
— Function: set-default-toplevel-value symbol value

This function sets the top-level default value of symbol to the specified value. This comes in handy when you want to set the global value of symbol regardless of whether your code runs in the context of symbol's let-binding.

Next: , Previous: Buffer-Local Variables, Up: Variables

12.12 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).

— User Option: enable-local-variables

This variable controls whether to process file-local variables. The possible values are:

t (the default)
Set the safe variables, and query (once) about any unsafe variables.
Set only the safe variables and do not query.
Set all the variables and do not query.
Don't set any variables.
anything else
Query (once) about all the variables.

— Variable: inhibit-local-variables-regexps

This is a list of regular expressions. If a file has a name matching an element of this list, then it is not scanned for any form of file-local variable. For examples of why you might want to use this, see Auto Major Mode.

— Function: hack-local-variables &optional handle-mode

This function parses, and binds or evaluates as appropriate, any local variables specified by the contents of the current buffer. The variable enable-local-variables has its effect here. However, this function does not look for the ‘mode:’ local variable in the ‘-*- line. set-auto-mode does that, also taking enable-local-variables into account (see Auto Major Mode).

This function works by walking the alist stored in file-local-variables-alist and applying each local variable in turn. It calls before-hack-local-variables-hook and hack-local-variables-hook before and after applying the variables, respectively. It only calls the before-hook if the alist is non-nil; it always calls the other hook. This function ignores a ‘mode’ element if it specifies the same major mode as the buffer already has.

If the optional argument handle-mode is t, then all this function does is return a symbol specifying the major mode, if the ‘-*- line or the local variables list specifies one, and nil otherwise. It does not set the mode or any other file-local variable. If handle-mode has any value other than nil or t, any settings of ‘mode’ in the ‘-*- line or the local variables list are ignored, and the other settings are applied. If handle-mode is nil, all the file local variables are set.

— Variable: file-local-variables-alist

This buffer-local variable holds the alist of file-local variable settings. Each element of the alist is of the form (var . value), where var is a symbol of the local variable and value is its value. When Emacs visits a file, it first collects all the file-local variables into this alist, and then the hack-local-variables function applies them one by one.

— Variable: before-hack-local-variables-hook

Emacs calls this hook immediately before applying file-local variables stored in file-local-variables-alist.

— Variable: hack-local-variables-hook

Emacs calls this hook immediately after it finishes applying file-local variables stored in file-local-variables-alist.

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.

If you want to define safe-local-variable properties for variables defined in C source code, add the names and the properties of those variables to the list in the “Safe local variables” section of files.el.

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). However, a safety predicate defined using :safe will only be known once the package that contains the defcustom is loaded, which is often too late. As an alternative, you can use the autoload cookie (see Autoload) to assign the option its safety predicate, like this:

     ;;;###autoload (put 'var 'safe-local-variable 'pred)

The safe value definitions specified with autoload are copied into the package's autoloads file (loaddefs.el for most packages bundled with Emacs), and are known to Emacs since the beginning of a session.

— User Option: safe-local-variable-values

This variable provides another way to mark some variable values as safe. It is a list of cons cells (var . val), where var is a variable name and val is a value which is safe for that variable.

When Emacs asks the user whether or not to obey a set of file-local variable specifications, the user can choose to mark them as safe. Doing so adds those variable/value pairs to safe-local-variable-values, and saves it to the user's custom file.

— Function: safe-local-variable-p sym val

This function returns non-nil if it is safe to give sym the value val, based on the above criteria.

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.

— Function: risky-local-variable-p sym

This function returns non-nil if sym is a risky variable, based on the above criteria.

— Variable: ignored-local-variables

This variable holds a list of variables that should not be given local values by files. Any value specified for one of these variables is completely ignored.

The ‘Eval:’ “variable” is also a potential loophole, so Emacs normally asks for confirmation before handling it.

— User Option: enable-local-eval

This variable controls processing of ‘Eval:’ in ‘-*-’ lines or local variables lists in files being visited. A value of t means process them unconditionally; nil means ignore them; anything else means ask the user what to do for each file. The default value is maybe.

— User Option: safe-local-eval-forms

This variable holds a list of expressions that are safe to evaluate when found in the ‘Eval:’ “variable” in a file local variables list.

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.

Next: , Previous: File Local Variables, Up: Variables

12.13 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.

— Constant: dir-locals-file

This constant is the name of the file where Emacs expects to find the directory-local variables. The name of the file is .dir-locals.el9. A file by that name in a directory causes Emacs to apply its settings to any file in that directory or any of its subdirectories (optionally, you can exclude subdirectories; see below). If some of the subdirectories have their own .dir-locals.el files, Emacs uses the settings from the deepest file it finds starting from the file's directory and moving up the directory tree. This constant is also used to derive the name of a second dir-locals file .dir-locals-2.el. If this second dir-locals file is present, then that is loaded instead of .dir-locals.el. This is useful when .dir-locals.el is under version control in a shared repository and cannot be used for personal customizations. The file specifies local variables as a specially formatted list; see Per-directory Local Variables, for more details.

— Function: hack-dir-local-variables

This function reads the .dir-locals.el file and stores the directory-local variables in file-local-variables-alist that is local to the buffer visiting any file in the directory, without applying them. It also stores the directory-local settings in dir-locals-class-alist, where it defines a special class for the directory in which .dir-locals.el file was found. This function works by calling dir-locals-set-class-variables and dir-locals-set-directory-class, described below.

— Function: hack-dir-local-variables-non-file-buffer

This function looks for directory-local variables, and immediately applies them in the current buffer. It is intended to be called in the mode commands for non-file buffers, such as Dired buffers, to let them obey directory-local variable settings. For non-file buffers, Emacs looks for directory-local variables in default-directory and its parent directories.

— Function: dir-locals-set-class-variables class variables

This function defines a set of variable settings for the named class, which is a symbol. You can later assign the class to one or more directories, and Emacs will apply those variable settings to all files in those directories. The list in variables can be of one of the two forms: (major-mode . alist) or (directory . list). With the first form, if the file's buffer turns on a mode that is derived from major-mode, then all the variables in the associated alist are applied; alist should be of the form (name . value). A special value nil for major-mode means the settings are applicable to any mode. In alist, you can use a special name: subdirs. If the associated value is nil, the alist is only applied to files in the relevant directory, not to those in any subdirectories.

With the second form of variables, if directory is the initial substring of the file's directory, then list is applied recursively by following the above rules; list should be of one of the two forms accepted by this function in variables.

— Function: dir-locals-set-directory-class directory class &optional mtime

This function assigns class to all the files in directory and its subdirectories. Thereafter, all the variable settings specified for class will be applied to any visited file in directory and its children. class must have been already defined by dir-locals-set-class-variables.

Emacs uses this function internally when it loads directory variables from a .dir-locals.el file. In that case, the optional argument mtime holds the file modification time (as returned by file-attributes). Emacs uses this time to check stored local variables are still valid. If you are assigning a class directly, not via a file, this argument should be nil.

— Variable: dir-locals-class-alist

This alist holds the class symbols and the associated variable settings. It is updated by dir-locals-set-class-variables.

— Variable: dir-locals-directory-cache

This alist holds directory names, their assigned class names, and modification times of the associated directory local variables file (if there is one). The function dir-locals-set-directory-class updates this list.

— Variable: enable-dir-local-variables

If nil, directory-local variables are ignored. This variable may be useful for modes that want to ignore directory-locals while still respecting file-local variables (see File Local Variables).

Next: , Previous: Directory Local Variables, Up: Variables

12.14 Connection Local Variables

Connection-local variables provide a general mechanism for different variable settings in buffers with a remote connection. They are bound and set depending on the remote connection a buffer is dedicated to.

— Function: connection-local-set-profile-variables profile variables

This function defines a set of variable settings for the connection profile, which is a symbol. You can later assign the connection profile to one or more remote connections, and Emacs will apply those variable settings to all process buffers for those connections. The list in variables is an alist of the form (name . value). Example:

            '((shell-file-name . "/bin/bash")
              (shell-command-switch . "-c")
              (shell-interactive-switch . "-i")
              (shell-login-switch . "-l")))
            '((shell-file-name . "/bin/ksh")
              (shell-command-switch . "-c")
              (shell-interactive-switch . "-i")
              (shell-login-switch . "-l")))
            '((null-device . "/dev/null")))
— Variable: connection-local-profile-alist

This alist holds the connection profile symbols and the associated variable settings. It is updated by connection-local-set-profile-variables.

— Function: connection-local-set-profiles criteria &rest profiles

This function assigns profiles, which are symbols, to all remote connections identified by criteria. criteria is a plist identifying a connection and the application using this connection. Property names might be :application, :protocol, :user and :machine. The property value of :application is a symbol, all other property values are strings. All properties are optional; if criteria is nil, it always applies. Example:

            '(:application 'tramp :protocol "ssh" :machine "localhost")
            'remote-bash 'remote-null-device)
            '(:application 'tramp :protocol "sudo"
              :user "root" :machine "localhost")
            'remote-ksh 'remote-null-device)

If criteria is nil, it applies for all remote connections. Therefore, the example above would be equivalent to

            '(:application 'tramp :protocol "ssh" :machine "localhost")
            '(:application 'tramp :protocol "sudo"
              :user "root" :machine "localhost")
            nil 'remote-null-device)

Any connection profile of profiles must have been already defined by connection-local-set-profile-variables.

— Variable: connection-local-criteria-alist

This alist contains connection criteria and their assigned profile names. The function connection-local-set-profiles updates this list.

— Function: hack-connection-local-variables criteria

This function collects applicable connection-local variables associated with criteria in connection-local-variables-alist, without applying them. Example:

            '(:application 'tramp :protocol "ssh" :machine "localhost"))
               ⇒ ((null-device . "/dev/null")
                  (shell-login-switch . "-l")
                  (shell-interactive-switch . "-i")
                  (shell-command-switch . "-c")
                  (shell-file-name . "/bin/bash"))
— Function: hack-connection-local-variables-apply criteria

This function looks for connection-local variables according to criteria, and immediately applies them in the current buffer.

— Macro: with-connection-local-profiles profiles &rest body

All connection-local variables, which are specified by a connection profile in profiles, are applied.

After that, body is executed, and the connection-local variables are unwound. Example:

            '((perl-command-name . "/usr/local/bin/perl")
              (perl-command-switch . "-e %s")))
          (with-connection-local-profiles '(remote-perl)
            do something useful)
— Variable: enable-connection-local-variables

If nil, connection-local variables are ignored. This variable shall be changed temporarily only in special modes.

Next: , Previous: Connection Local Variables, Up: Variables

12.15 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.

— Function: defvaralias new-alias base-variable &optional docstring

This function defines the symbol new-alias as a variable alias for symbol base-variable. This means that retrieving the value of new-alias returns the value of base-variable, and changing the value of new-alias changes the value of base-variable. The two aliased variable names always share the same value and the same bindings.

If the docstring argument is non-nil, it specifies the documentation for new-alias; otherwise, the alias gets the same documentation as base-variable has, if any, unless base-variable is itself an alias, in which case new-alias gets the documentation of the variable at the end of the chain of aliases.

This function returns base-variable.

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.

— Function: make-obsolete-variable obsolete-name current-name when &optional access-type

This function makes the byte compiler warn that the variable obsolete-name is obsolete. If current-name is a symbol, it is the variable's new name; then the warning message says to use current-name instead of obsolete-name. If current-name is a string, this is the message and there is no replacement variable. when should be a string indicating when the variable was first made obsolete (usually a version number string).

The optional argument access-type, if non-nil, should specify the kind of access that will trigger obsolescence warnings; it can be either get or set.

You can make two variables synonyms and declare one obsolete at the same time using the macro define-obsolete-variable-alias.

— Macro: define-obsolete-variable-alias obsolete-name current-name &optional when docstring

This macro marks the variable obsolete-name as obsolete and also makes it an alias for the variable current-name. It is equivalent to the following:

          (defvaralias obsolete-name current-name docstring)
          (make-obsolete-variable obsolete-name current-name when)
— Function: indirect-variable variable

This function returns the variable at the end of the chain of aliases of variable. If variable is not a symbol, or if variable is not defined as an alias, the function returns variable.

This function signals a cyclic-variable-indirection error if there is a loop in the chain of symbols.

     (defvaralias 'foo 'bar)
     (indirect-variable 'foo)
          ⇒ bar
     (indirect-variable 'bar)
          ⇒ bar
     (setq bar 2)
          ⇒ 2
          ⇒ 2
     (setq foo 0)
          ⇒ 0
          ⇒ 0

Next: , Previous: Variable Aliases, Up: Variables

12.16 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))
          ⇒ t
— Variable: byte-boolean-vars

This variable holds a list of all variables of type DEFVAR_BOOL.

Variables of type DEFVAR_INT can take on only 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

Previous: Variables with Restricted Values, Up: Variables

12.17 Generalized Variables

A generalized variable or place form is one of the many places in Lisp memory where values can be stored using the setf macro (see Setting Generalized Variables). 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 get 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.

Next: , Up: Generalized Variables

12.17.1 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 you having to use two separate functions for setting and accessing this type of place.

— Macro: setf [place form]...

This macro evaluates form and stores it in place, which must be a valid generalized variable form. If there are several place and form pairs, the assignments are done sequentially just as with setq. setf returns the value of the last form.

The following Lisp forms are the forms in Emacs that will work as generalized variables, and so may appear in the place argument of setf:

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.

Previous: Setting Generalized Variables, Up: Generalized Variables

12.17.2 Defining new setf forms

This section describes how to define new forms that setf can operate on.

— Macro: gv-define-simple-setter name setter &optional fix-return

This macro enables you to easily define setf methods for simple cases. name is the name of a function, macro, or special form. You can use this macro whenever name has a directly corresponding setter function that updates it, e.g., (gv-define-simple-setter car setcar).

This macro translates a call of the form

          (setf (name args...) value)


          (setter args... value)

Such a setf call is documented to return value. This is no problem with, e.g., car and setcar, because setcar returns the value that it set. If your setter function does not return value, use a non-nil value for the fix-return argument of gv-define-simple-setter. This expands into something equivalent to

          (let ((temp value))
            (setter args... temp)

so ensuring that it returns the correct result.

— Macro: gv-define-setter name arglist &rest body

This macro allows for more complex setf expansions than the previous form. You may need to use this form, for example, if there is no simple setter function to call, or if there is one but it requires different arguments to the place form.

This macro expands the form (setf (name args...) value) by first binding the setf argument forms (value args...) according to arglist, and then executing body. body should return a Lisp form that does the assignment, and finally returns the value that was set. An example of using this macro is:

          (gv-define-setter caar (val x) `(setcar (car ,x) ,val))

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.

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13 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.

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13.1 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 (see Definition of side effect). A pure function is a function which, in addition to having no side effects, always returns the same value for the same combination of arguments, regardless of external factors such as machine type or system state.

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.
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.
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.
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.

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:

— Function: functionp object

This function returns t if object is any kind of function, i.e., can be passed to funcall. Note that functionp returns t for symbols that are function names, and returns nil for special forms.

It is also possible to find out how many arguments an arbitrary function expects:

— Function: func-arity function

This function provides information about the argument list of the specified function. The returned value is a cons cell of the form (min . max), where min is the minimum number of arguments, and max is either the maximum number of arguments, or the symbol many for functions with &rest arguments, or the symbol unevalled if function is a special form.

Note that this function might return inaccurate results in some situations, such as the following:

  • Functions defined using apply-partially (see apply-partially).
  • Functions that are advised using advice-add (see Advising Named Functions).
  • Functions that determine the argument list dynamically, as part of their code.

Unlike functionp, the next three functions do not treat a symbol as its function definition.

— Function: subrp object

This function returns t if object is a built-in function (i.e., a Lisp primitive).

          (subrp 'message)            ; message is a symbol,
               ⇒ nil                 ;   not a subr object.
          (subrp (symbol-function 'message))
               ⇒ t
— Function: byte-code-function-p object

This function returns t if object is a byte-code function. For example:

          (byte-code-function-p (symbol-function 'next-line))
               ⇒ t
— Function: subr-arity subr

This works like func-arity, but only for built-in functions and without symbol indirection. It signals an error for non-built-in functions. We recommend to use func-arity instead.

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13.2 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 a valid expression which evaluates to a function object.

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.

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13.2.1 Components of a Lambda Expression

A lambda expression is a list that looks like this:

     (lambda (arg-variables...)

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.

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13.2.2 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).

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13.2.3 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 (see Errors).

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:

      [&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. Thus, if there are only two arguments, c, d and e are nil; if two or three arguments, d and e are nil; if four arguments or fewer, e is nil. Note that exactly five arguments with an explicit nil argument provided for e will cause that nil argument to be passed as a list with one element, (nil), as with any other single value for e.

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

Previous: Argument List, Up: Lambda Expressions

13.2.4 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.

Do not use this feature if you want to deprecate the calling convention and favor the one you advertise by the above specification. Instead, use the advertised-calling-convention declaration (see Declare Form) or set-advertised-calling-convention (see Obsolete Functions), because these two will cause the byte compiler emit a warning message when it compiles Lisp programs which use the deprecated calling convention.

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13.3 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.)

By convention, if a function's symbol consists of two names separated by ‘--’, the function is intended for internal use and the first part names the file defining the function. For example, a function named vc-git--rev-parse is an internal function defined in vc-git.el. Internal-use functions written in C have names ending in ‘-internal’, e.g., bury-buffer-internal. Emacs code contributed before 2018 may follow other internal-use naming conventions, which are being phased out.

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13.4 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.

— Macro: defun name args [doc] [declare] [interactive] body...

defun is the usual way to define new Lisp functions. It defines the symbol name as a function with argument list args and body forms given by body. Neither name nor args should be quoted.

doc, if present, should be a string specifying the function's documentation string (see Function Documentation). declare, if present, should be a declare form specifying function metadata (see Declare Form). interactive, if present, should be an interactive form specifying how the function is to be called interactively (see Interactive Call).

The return value of defun is undefined.

Here are some examples:

          (defun foo () 5)
               ⇒ 5
          (defun bar (a &optional b &rest c)
              (list a b c))
          (bar 1 2 3 4 5)
               ⇒ (1 2 (3 4 5))
          (bar 1)
               ⇒ (1 nil nil)
          error--> Wrong number of arguments.
          (defun capitalize-backwards ()
            "Upcase the last letter of the word at point."
            (backward-word 1)
            (forward-word 1)
            (backward-char 1)
            (capitalize-word 1))

Be careful not to redefine existing functions unintentionally. defun redefines even primitive functions such as car without any hesitation or notification. Emacs does not prevent you from doing this, because redefining a function is sometimes done deliberately, and there is no way to distinguish deliberate redefinition from unintentional redefinition.

— Function: defalias name definition &optional doc

This function defines the symbol name as a function, with definition definition (which can be any valid Lisp function). Its return value is undefined.

If doc is non-nil, it becomes the function documentation of name. Otherwise, any documentation provided by definition is used.

Internally, defalias normally uses fset to set the definition. If name has a defalias-fset-function property, however, the associated value is used as a function to call in place of fset.

The proper place to use defalias is where a specific function name is being defined—especially where that name appears explicitly in the source file being loaded. This is because defalias records which file defined the function, just like defun (see Unloading).

By contrast, in programs that manipulate function definitions for other purposes, it is better to use fset, which does not keep such records. See Function Cells.

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.

Alternatively, you can define a function by providing the code which will inline it as a compiler macro. The following macros make this possible.

— Macro: define-inline name args [doc] [declare] body...

Define a function name by providing code that does its inlining, as a compiler macro. The function will accept the argument list args and will have the specified body.

If present, doc should be the function's documentation string (see Function Documentation); declare, if present, should be a declare form (see Declare Form) specifying the function's metadata.

Functions defined via define-inline have several advantages with respect to macros defined by defsubst or defmacro:

Like defmacro, a function inlined with define-inline inherits the scoping rules, either dynamic or lexical, from the call site. See Variable Scoping.

The following macros should be used in the body of a function defined by define-inline.

— Macro: inline-quote expression

Quote expression for define-inline. This is similar to the backquote (see Backquote), but quotes code and accepts only ,, not ,@.

— Macro: inline-letevals (bindings...) body...

This is similar to let (see Local Variables): it sets up local variables as specified by bindings, and then evaluates body with those bindings in effect. Each element of bindings should be either a symbol or a list of the form (var expr); the result is to evaluate expr and bind var to the result. The tail of bindings can be either nil or a symbol which should hold a list of arguments, in which case each argument is evaluated, and the symbol is bound to the resulting list.

— Macro: inline-const-p expression

Return non-nil if the value of expression is already known.

— Macro: inline-const-val expression

Return the value of expression.

— Macro: inline-error format &rest args

Signal an error, formatting args according to format.

Here's an example of using define-inline:

     (define-inline myaccessor (obj)
       (inline-letevals (obj)
         (inline-quote (if (foo-p ,obj) (aref (cdr ,obj) 3) (aref ,obj 2)))))

This is equivalent to

     (defsubst myaccessor (obj)
       (if (foo-p obj) (aref (cdr obj) 3) (aref obj 2)))

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13.5 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.

— Function: funcall function &rest arguments

funcall calls function with arguments, and returns whatever function returns.

Since funcall is a function, all of its arguments, including function, are evaluated before funcall is called. This means that you can use any expression to obtain the function to be called. It also means that funcall does not see the expressions you write for the arguments, only their values. These values are not evaluated a second time in the act of calling function; the operation of funcall is like the normal procedure for calling a function, once its arguments have already been evaluated.

The argument function must be either a Lisp function or a primitive function. Special forms and macros are not allowed, because they make sense only when given the unevaluated argument expressions. funcall cannot provide these because, as we saw above, it never knows them in the first place.

If you need to use funcall to call a command and make it behave as if invoked interactively, use funcall-interactively (see Interactive Call).

          (setq f 'list)
               ⇒ list
          (funcall f 'x 'y 'z)
               ⇒ (x y z)
          (funcall f 'x 'y '(z))
               ⇒ (x y (z))
          (funcall 'and t nil)
          error--> Invalid function: #<subr and>

Compare these examples with the examples of apply.

— Function: apply function &rest arguments

apply calls function with arguments, just like funcall but with one difference: the last of arguments is a list of objects, which are passed to function as separate arguments, rather than a single list. We say that apply spreads this list so that each individual element becomes an argument.

apply returns the result of calling function. As with funcall, function must either be a Lisp function or a primitive function; special forms and macros do not make sense in apply.

          (setq f 'list)
               ⇒ list
          (apply f 'x 'y 'z)
          error--> Wrong type argument: listp, z
          (apply '+ 1 2 '(3 4))
               ⇒ 10
          (apply '+ '(1 2 3 4))
               ⇒ 10
          (apply 'append '((a b c) nil (x y z) nil))
               ⇒ (a b c x y z)

For an interesting example of using apply, see Definition of mapcar.

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 function10. 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:

— Function: apply-partially func &rest args

This function returns a new function which, when called, will call func with the list of arguments composed from args and additional arguments specified at the time of the call. If func accepts n arguments, then a call to apply-partially with m < n arguments will produce a new function of n - m arguments.

Here's how we could define the built-in function 1+, if it didn't exist, using apply-partially and +, another built-in function:

          (defalias '1+ (apply-partially '+ 1)
            "Increment argument by one.")
          (1+ 10)
               ⇒ 11

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:

— Function: identity arg

This function returns arg and has no side effects.

— Function: ignore &rest args

This function ignores any arguments and returns nil.

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.

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13.6 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, mapconcat, and mapcan, 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).

— Function: mapcar function sequence

mapcar applies function to each element of sequence in turn, and returns a list of the results.

The argument sequence can be any kind of sequence except a char-table; that is, a list, a vector, a bool-vector, or a string. The result is always a list. The length of the result is the same as the length of sequence. For example:

          (mapcar 'car '((a b) (c d) (e f)))
               ⇒ (a c e)
          (mapcar '1+ [1 2 3])
               ⇒ (2 3 4)
          (mapcar 'string "abc")
               ⇒ ("a" "b" "c")
          ;; Call each function in my-hooks.
          (mapcar 'funcall my-hooks)
          (defun mapcar* (function &rest args)
            "Apply FUNCTION to successive cars of all ARGS.
          Return the list of results."
            ;; If no list is exhausted,
            (if (not (memq nil args))
                ;; apply function to cars.
                (cons (apply function (mapcar 'car args))
                      (apply 'mapcar* function
                             ;; Recurse for rest of elements.
                             (mapcar 'cdr args)))))
          (mapcar* 'cons '(a b c) '(1 2 3 4))
               ⇒ ((a . 1) (b . 2) (c . 3))
— Function: mapcan function sequence

This function applies function to each element of sequence, like mapcar, but instead of collecting the results into a list, it returns a single list with all the elements of the results (which must be lists), by altering the results (using nconc; see Rearrangement). Like with mapcar, sequence can be of any type except a char-table.

          ;; Contrast this:
          (mapcar 'list '(a b c d))
               ⇒ ((a) (b) (c) (d))
          ;; with this:
          (mapcan 'list '(a b c d))
               ⇒ (a b c d)
— Function: mapc function sequence

mapc is like mapcar except that function is used for side-effects only—the values it returns are ignored, not collected into a list. mapc always returns sequence.

— Function: mapconcat function sequence separator

mapconcat applies function to each element of sequence; the results, which must be sequences of characters (strings, vectors, or lists), are concatenated into a single string return value. Between each pair of result sequences, mapconcat inserts the characters from separator, which also must be a string, or a vector or list of characters. See Sequences Arrays Vectors.

The argument function must be a function that can take one argument and returns a sequence of characters: a string, a vector, or a list. The argument sequence can be any kind of sequence except a char-table; that is, a list, a vector, a bool-vector, or a string.

          (mapconcat 'symbol-name
                     '(The cat in the hat)
                     " ")
               ⇒ "The cat in the hat"
          (mapconcat (function (lambda (x) (format "%c" (1+ x))))
               ⇒ "IBM.9111"

Next: , Previous: Mapping Functions, Up: Functions

13.7 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:

— Macro: lambda args [doc] [interactive] body...

This macro returns an anonymous function with argument list args, documentation string doc (if any), interactive spec interactive (if any), and body forms given by body.

Under dynamic binding, this macro effectively makes lambda forms self-quoting: evaluating a form whose car is lambda yields the form itself:

          (lambda (x) (* x x))
               ⇒ (lambda (x) (* x x))

Note that when evaluating under lexical binding the result is a closure object (see Closures).

The lambda form has one other effect: it tells the Emacs evaluator and byte-compiler that its argument is a function, by using function as a subroutine (see below).

— Special Form: function function-object

This special form returns function-object without evaluating it. In this, it is similar to quote (see Quoting). But unlike quote, it also serves as a note to the Emacs evaluator and byte-compiler that function-object is intended to be used as a function. Assuming function-object is a valid lambda expression, this has two effects:

  • When the code is byte-compiled, function-object is compiled into a byte-code function object (see Byte Compilation).
  • When lexical binding is enabled, function-object is converted into a closure. See Closures.

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.

Next: , Previous: Anonymous Functions, Up: Functions

13.8 Generic Functions

Functions defined using defun have a hard-coded set of assumptions about the types and expected values of their arguments. For example, a function that was designed to handle values of its argument that are either numbers or lists of numbers will fail or signal an error if called with a value of any other type, such as a vector or a string. This happens because the implementation of the function is not prepared to deal with types other than those assumed during the design.

By contrast, object-oriented programs use polymorphic functions: a set of specialized functions having the same name, each one of which was written for a certain specific set of argument types. Which of the functions is actually called is decided at run time based on the types of the actual arguments.

Emacs provides support for polymorphism. Like other Lisp environments, notably Common Lisp and its Common Lisp Object System (CLOS), this support is based on generic functions. The Emacs generic functions closely follow CLOS, including use of similar names, so if you have experience with CLOS, the rest of this section will sound very familiar.

A generic function specifies an abstract operation, by defining its name and list of arguments, but (usually) no implementation. The actual implementation for several specific classes of arguments is provided by methods, which should be defined separately. Each method that implements a generic function has the same name as the generic function, but the method's definition indicates what kinds of arguments it can handle by specializing the arguments defined by the generic function. These argument specializers can be more or less specific; for example, a string type is more specific than a more general type, such as sequence.

Note that, unlike in message-based OO languages, such as C++ and Simula, methods that implement generic functions don't belong to a class, they belong to the generic function they implement.

When a generic function is invoked, it selects the applicable methods by comparing the actual arguments passed by the caller with the argument specializers of each method. A method is applicable if the actual arguments of the call are compatible with the method's specializers. If more than one method is applicable, they are combined using certain rules, described below, and the combination then handles the call.

— Macro: cl-defgeneric name arguments [documentation] [options-and-methods...] &rest body

This macro defines a generic function with the specified name and arguments. If body is present, it provides the default implementation. If documentation is present (it should always be), it specifies the documentation string for the generic function, in the form (:documentation docstring). The optional options-and-methods can be one of the following forms:

(declare declarations)
A declare form, as described in Declare Form.
(:argument-precedence-order &rest args)
This form affects the sorting order for combining applicable methods. Normally, when two methods are compared during combination, method arguments are examined left to right, and the first method whose argument specializer is more specific will come before the other one. The order defined by this form overrides that, and the arguments are examined according to their order in this form, and not left to right.
(:method [qualifiers...] args &rest body)
This form defines a method like cl-defmethod does.

— Macro: cl-defmethod name [qualifier] arguments &rest [docstring] body

This macro defines a particular implementation for the generic function called name. The implementation code is given by body. If present, docstring is the documentation string for the method. The arguments list, which must be identical in all the methods that implement a generic function, and must match the argument list of that function, provides argument specializers of the form (arg spec), where arg is the argument name as specified in the cl-defgeneric call, and spec is one of the following specializer forms:

This specializer requires the argument to be of the given type, one of the types from the type hierarchy described below.
(eql object)
This specializer requires the argument be eql to the given object.
(head object)
The argument must be a cons cell whose car is eql to object.
The argument must be an instance of a class named struct-type defined with cl-defstruct (see Structures), or of one of its child classes.

Alternatively, the argument specializer can be of the form &context (expr spec), in which case the value of expr must be compatible with the specializer provided by spec; spec can be any of the forms described above. In other words, this form of specializer uses the value of expr instead of arguments for the decision whether the method is applicable. For example, &context (overwrite-mode (eql t)) will make the method compatible only when overwrite-mode is turned on.

The type specializer, (arg type), can specify one of the system types in the following list. When a parent type is specified, an argument whose type is any of its more specific child types, as well as grand-children, grand-grand-children, etc. will also be compatible.

Parent type: number.
Parent type: symbol
Parent type: array.
Parent type: sequence.
Parent type: list.
Parent type: sequence.
Parent type: number.
Parent type: array.
Parent type: array.
Parent type: array.

The optional qualifier allows combining several applicable methods. If it is not present, the defined method is a primary method, responsible for providing the primary implementation of the generic function for the specialized arguments. You can also define auxiliary methods, by using one of the following values as qualifier:

This auxiliary method will run before the primary method. More accurately, all the :before methods will run before the primary, in the most-specific-first order.
This auxiliary method will run after the primary method. More accurately, all such methods will run after the primary, in the most-specific-last order.
This auxiliary method will run instead of the primary method. The most specific of such methods will be run before any other method. Such methods normally use cl-call-next-method, described below, to invoke the other auxiliary or primary methods.
:extra string
This allows you to add more methods, distinguished by string, for the same specializers and qualifiers.

Functions defined using cl-defmethod cannot be made interactive, i.e. commands (see Defining Commands), by adding the interactive form to them. If you need a polymorphic command, we recommend defining a normal command that calls a polymorphic function defined via cl-defgeneric and cl-defmethod.

Each time a generic function is called, it builds the effective method which will handle this invocation by combining the applicable methods defined for the function. The process of finding the applicable methods and producing the effective method is called dispatch. The applicable methods are those all of whose specializers are compatible with the actual arguments of the call. Since all of the arguments must be compatible with the specializers, they all determine whether a method is applicable. Methods that explicitly specialize more than one argument are called multiple-dispatch methods.

The applicable methods are sorted into the order in which they will be combined. The method whose left-most argument specializer is the most specific one will come first in the order. (Specifying :argument-precedence-order as part of cl-defmethod overrides that, as described above.) If the method body calls cl-call-next-method, the next most-specific method will run. If there are applicable :around methods, the most-specific of them will run first; it should call cl-call-next-method to run any of the less specific :around methods. Next, the :before methods run in the order of their specificity, followed by the primary method, and lastly the :after methods in the reverse order of their specificity.

— Function: cl-call-next-method &rest args

When invoked from within the lexical body of a primary or an :around auxiliary method, call the next applicable method for the same generic function. Normally, it is called with no arguments, which means to call the next applicable method with the same arguments that the calling method was invoked. Otherwise, the specified arguments are used instead.

— Function: cl-next-method-p

This function, when called from within the lexical body of a primary or an :around auxiliary method, returns non-nil if there is a next method to call.

Next: , Previous: Generic Functions, Up: Functions

13.9 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.

— Function: symbol-function symbol

This returns the object in the function cell of symbol. It does not check that the returned object is a legitimate function.

If the function cell is void, the return value is nil. To distinguish between a function cell that is void and one set to nil, use fboundp (see below).

          (defun bar (n) (+ n 2))
          (symbol-function 'bar)
               ⇒ (lambda (n) (+ n 2))
          (fset 'baz 'bar)
               ⇒ bar
          (symbol-function 'baz)
               ⇒ bar

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 the symbol as a function, Emacs 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.

— Function: fboundp symbol

This function returns t if the symbol has an object in its function cell, nil otherwise. It does not check that the object is a legitimate function.

— Function: fmakunbound symbol

This function makes symbol's function cell void, so that a subsequent attempt to access this cell will cause a void-function error. It returns symbol. (See also makunbound, in Void Variables.)

          (defun foo (x) x)
          (foo 1)
          (fmakunbound 'foo)
               ⇒ foo
          (foo 1)
          error--> Symbol's function definition is void: foo
— Function: fset symbol definition

This function stores definition in the function cell of symbol. The result is definition. Normally definition should be a function or the name of a function, but this is not checked. The argument symbol is an ordinary evaluated argument.

The primary use of this function is as a subroutine by constructs that define or alter functions, like defun or advice-add (see Advising Functions). You can also use it to give a symbol a function definition that is not a function, e.g., a keyboard macro (see Keyboard Macros):

          ;; Define a named keyboard macro.
          (fset 'kill-two-lines "\^u2\^k")
               ⇒ "\^u2\^k"

It you wish to use fset to make an alternate name for a function, consider using defalias instead. See Definition of defalias.

Next: , Previous: Function Cells, Up: Functions

13.10 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.

Next: , Previous: Closures, Up: Functions

13.11 Advising Emacs Lisp Functions

When you need to modify a function defined in another library, or when you need to modify a hook like foo-function, a process filter, or basically any variable or object field which holds a function value, you can use the appropriate setter function, such as fset or defun for named functions, setq for hook variables, or set-process-filter for process filters, but those are often too blunt, completely throwing away the previous value.

The advice feature lets you add to the existing definition of a function, by advising the function. This is a cleaner method than redefining the whole function.

Emacs's advice system provides two sets of primitives for that: the core set, for function values held in variables and object fields (with the corresponding primitives being add-function and remove-function) and another set layered on top of it for named functions (with the main primitives being advice-add and advice-remove).

For example, in order to trace the calls to the process filter of a process proc, you could use:

     (defun my-tracing-function (proc string)
       (message "Proc %S received %S" proc string))
     (add-function :before (process-filter proc) #'my-tracing-function)

This will cause the process's output to be passed to my-tracing-function before being passed to the original process filter. my-tracing-function receives the same arguments as the original function. When you're done with it, you can revert to the untraced behavior with:

     (remove-function (process-filter proc) #'my-tracing-function)

Similarly, if you want to trace the execution of the function named display-buffer, you could use:

     (defun his-tracing-function (orig-fun &rest args)
       (message "display-buffer called with args %S" args)
       (let ((res (apply orig-fun args)))
         (message "display-buffer returned %S" res)
     (advice-add 'display-buffer :around #'his-tracing-function)

Here, his-tracing-function is called instead of the original function and receives the original function (additionally to that function's arguments) as argument, so it can call it if and when it needs to. When you're tired of seeing this output, you can revert to the untraced behavior with:

     (advice-remove 'display-buffer #'his-tracing-function)

The arguments :before and :around used in the above examples specify how the two functions are composed, since there are many different ways to do it. The added function is also called a piece of advice.

Next: , Up: Advising Functions

13.11.1 Primitives to manipulate advices

— Macro: add-function where place function &optional props

This macro is the handy way to add the advice function to the function stored in place (see Generalized Variables).

where determines how function is composed with the existing function, e.g., whether function should be called before, or after the original function. See Advice combinators, for the list of available ways to compose the two functions.

When modifying a variable (whose name will usually end with -function), you can choose whether function is used globally or only in the current buffer: if place is just a symbol, then function is added to the global value of place. Whereas if place is of the form (local symbol), where symbol is an expression which returns the variable name, then function will only be added in the current buffer. Finally, if you want to modify a lexical variable, you will have to use (var variable).

Every function added with add-function can be accompanied by an association list of properties props. Currently only two of those properties have a special meaning:

This gives a name to the advice, which remove-function can use to identify which function to remove. Typically used when function is an anonymous function.
This specifies how to order the advice, should several pieces of advice be present. By default, the depth is 0. A depth of 100 indicates that this piece of advice should be kept as deep as possible, whereas a depth of −100 indicates that it should stay as the outermost piece. When two pieces of advice specify the same depth, the most recently added one will be outermost.

For :before advice, being outermost means that this advice will be run first, before any other advice, whereas being innermost means that it will run right before the original function, with no other advice run between itself and the original function. Similarly, for :after advice innermost means that it will run right after the original function, with no other advice run in between, whereas outermost means that it will be run right at the end after all other advice. An innermost :override piece of advice will only override the original function and other pieces of advice will apply to it, whereas an outermost :override piece of advice will override not only the original function but all other advice applied to it as well.

If function is not interactive, then the combined function will inherit the interactive spec, if any, of the original function. Else, the combined function will be interactive and will use the interactive spec of function. One exception: if the interactive spec of function is a function (rather than an expression or a string), then the interactive spec of the combined function will be a call to that function with as sole argument the interactive spec of the original function. To interpret the spec received as argument, use advice-eval-interactive-spec.

Note: The interactive spec of function will apply to the combined function and should hence obey the calling convention of the combined function rather than that of function. In many cases, it makes no difference since they are identical, but it does matter for :around, :filter-args, and :filter-return, where function receives different arguments than the original function stored in place.

— Macro: remove-function place function

This macro removes function from the function stored in place. This only works if function was added to place using add-function.

function is compared with functions added to place using equal, to try and make it work also with lambda expressions. It is additionally compared also with the name property of the functions added to place, which can be more reliable than comparing lambda expressions using equal.

— Function: advice-function-member-p advice function-def

Return non-nil if advice is already in function-def. Like for remove-function above, instead of advice being the actual function, it can also be the name of the piece of advice.

— Function: advice-function-mapc f function-def

Call the function f for every piece of advice that was added to function-def. f is called with two arguments: the advice function and its properties.

— Function: advice-eval-interactive-spec spec

Evaluate the interactive spec just like an interactive call to a function with such a spec would, and then return the corresponding list of arguments that was built. E.g., (advice-eval-interactive-spec "r\nP") will return a list of three elements, containing the boundaries of the region and the current prefix argument.

Next: , Previous: Core Advising Primitives, Up: Advising Functions

13.11.2 Advising Named Functions

A common use of advice is for named functions and macros. You could just use add-function as in:

     (add-function :around (symbol-function 'fun) #'his-tracing-function)

But you should use advice-add and advice-remove for that instead. This separate set of functions to manipulate pieces of advice applied to named functions, offers the following extra features compared to add-function: they know how to deal with macros and autoloaded functions, they let describe-function preserve the original docstring as well as document the added advice, and they let you add and remove advice before a function is even defined.

advice-add can be useful for altering the behavior of existing calls to an existing function without having to redefine the whole function. However, it can be a source of bugs, since existing callers to the function may assume the old behavior, and work incorrectly when the behavior is changed by advice. Advice can also cause confusion in debugging, if the person doing the debugging does not notice or remember that the function has been modified by advice.

For these reasons, advice should be reserved for the cases where you cannot modify a function's behavior in any other way. If it is possible to do the same thing via a hook, that is preferable (see Hooks). If you simply want to change what a particular key does, it may be better to write a new command, and remap the old command's key bindings to the new one (see Remapping Commands). In particular, Emacs's own source files should not put advice on functions in Emacs. (There are currently a few exceptions to this convention, but we aim to correct them.)

Special forms (see Special Forms) cannot be advised, however macros can be advised, in much the same way as functions. Of course, this will not affect code that has already been macro-expanded, so you need to make sure the advice is installed before the macro is expanded.

It is possible to advise a primitive (see What Is a Function), but one should typically not do so, for two reasons. Firstly, some primitives are used by the advice mechanism, and advising them could cause an infinite recursion. Secondly, many primitives are called directly from C, and such calls ignore advice; hence, one ends up in a confusing situation where some calls (occurring from Lisp code) obey the advice and other calls (from C code) do not.

— Macro: define-advice symbol (where lambda-list &optional name depth) &rest body

This macro defines a piece of advice and adds it to the function named symbol. The advice is an anonymous function if name is nil or a function named symbol@name. See advice-add for explanation of other arguments.

— Function: advice-add symbol where function &optional props

Add the advice function to the named function symbol. where and props have the same meaning as for add-function (see Core Advising Primitives).

— Function: advice-remove symbol function

Remove the advice function from the named function symbol. function can also be the name of a piece of advice.

— Function: advice-member-p function symbol

Return non-nil if the advice function is already in the named function symbol. function can also be the name of a piece of advice.

— Function: advice-mapc function symbol

Call function for every piece of advice that was added to the named function symbol. function is called with two arguments: the advice function and its properties.

Next: , Previous: Advising Named Functions, Up: Advising Functions

13.11.3 Ways to compose advice

Here are the different possible values for the where argument of add-function and advice-add, specifying how the advice function and the original function should be composed.

Call function before the old function. Both functions receive the same arguments, and the return value of the composition is the return value of the old function. More specifically, the composition of the two functions behaves like:
          (lambda (&rest r) (apply function r) (apply oldfun r))

(add-function :before funvar function) is comparable for single-function hooks to (add-hook 'hookvar function) for normal hooks.

Call function after the old function. Both functions receive the same arguments, and the return value of the composition is the return value of the old function. More specifically, the composition of the two functions behaves like:
          (lambda (&rest r) (prog1 (apply oldfun r) (apply function r)))

(add-function :after funvar function) is comparable for single-function hooks to (add-hook 'hookvar function 'append) for normal hooks.

This completely replaces the old function with the new one. The old function can of course be recovered if you later call remove-function.
Call function instead of the old function, but provide the old function as an extra argument to function. This is the most flexible composition. For example, it lets you call the old function with different arguments, or many times, or within a let-binding, or you can sometimes delegate the work to the old function and sometimes override it completely. More specifically, the composition of the two functions behaves like:
          (lambda (&rest r) (apply function oldfun r))

Call function before the old function and don't call the old function if function returns nil. Both functions receive the same arguments, and the return value of the composition is the return value of the old function. More specifically, the composition of the two functions behaves like:
          (lambda (&rest r) (and (apply function r) (apply oldfun r)))

(add-function :before-while funvar function) is comparable for single-function hooks to (add-hook 'hookvar function) when hookvar is run via run-hook-with-args-until-failure.

Call function before the old function and only call the old function if function returns nil. More specifically, the composition of the two functions behaves like:
          (lambda (&rest r) (or (apply function r) (apply oldfun r)))

(add-function :before-until funvar function) is comparable for single-function hooks to (add-hook 'hookvar function) when hookvar is run via run-hook-with-args-until-success.

Call function after the old function and only if the old function returned non-nil. Both functions receive the same arguments, and the return value of the composition is the return value of function. More specifically, the composition of the two functions behaves like:
          (lambda (&rest r) (and (apply oldfun r) (apply function r)))

(add-function :after-while funvar function) is comparable for single-function hooks to (add-hook 'hookvar function 'append) when hookvar is run via run-hook-with-args-until-failure.

Call function after the old function and only if the old function returned nil. More specifically, the composition of the two functions behaves like:
          (lambda (&rest r) (or  (apply oldfun r) (apply function r)))

(add-function :after-until funvar function) is comparable for single-function hooks to (add-hook 'hookvar function 'append) when hookvar is run via run-hook-with-args-until-success.

Call function first and use the result (which should be a list) as the new arguments to pass to the old function. More specifically, the composition of the two functions behaves like:
          (lambda (&rest r) (apply oldfun (funcall function r)))

Call the old function first and pass the result to function. More specifically, the composition of the two functions behaves like:
          (lambda (&rest r) (funcall function (apply oldfun r)))

Previous: Advice combinators, Up: Advising Functions

13.11.4 Adapting code using the old defadvice

A lot of code uses the old defadvice mechanism, which is largely made obsolete by the new advice-add, whose implementation and semantics is significantly simpler.

An old piece of advice such as:

     (defadvice previous-line (before next-line-at-end
                                      (&optional arg try-vscroll))
       "Insert an empty line when moving up from the top line."
       (if (and next-line-add-newlines (= arg 1)
                (save-excursion (beginning-of-line) (bobp)))

could be translated in the new advice mechanism into a plain function:

     (defun previous-line--next-line-at-end (&optional arg try-vscroll)
       "Insert an empty line when moving up from the top line."
       (if (and next-line-add-newlines (= arg 1)
                (save-excursion (beginning-of-line) (bobp)))

Obviously, this does not actually modify previous-line. For that the old advice needed:

     (ad-activate 'previous-line)

whereas the new advice mechanism needs:

     (advice-add 'previous-line :before #'previous-line--next-line-at-end)

Note that ad-activate had a global effect: it activated all pieces of advice enabled for that specified function. If you wanted to only activate or deactivate a particular piece, you needed to enable or disable it with ad-enable-advice and ad-disable-advice. The new mechanism does away with this distinction.

Around advice such as:

     (defadvice foo (around foo-around)
       "Ignore case in `foo'."
       (let ((case-fold-search t))
     (ad-activate 'foo)

could translate into:

     (defun foo--foo-around (orig-fun &rest args)
       "Ignore case in `foo'."
       (let ((case-fold-search t))
         (apply orig-fun args)))
     (advice-add 'foo :around #'foo--foo-around)

Regarding the advice's class, note that the new :before is not quite equivalent to the old before, because in the old advice you could modify the function's arguments (e.g., with ad-set-arg), and that would affect the argument values seen by the original function, whereas in the new :before, modifying an argument via setq in the advice has no effect on the arguments seen by the original function. When porting before advice which relied on this behavior, you'll need to turn it into new :around or :filter-args advice instead.

Similarly old after advice could modify the returned value by changing ad-return-value, whereas new :after advice cannot, so when porting such old after advice, you'll need to turn it into new :around or :filter-return advice instead.

Next: , Previous: Advising Functions, Up: Functions

13.12 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.

— Function: make-obsolete obsolete-name current-name &optional when

This function marks obsolete-name as obsolete. obsolete-name should be a symbol naming a function or macro, or an alias for a function or macro.

If current-name is a symbol, the warning message says to use current-name instead of obsolete-name. current-name does not need to be an alias for obsolete-name; it can be a different function with similar functionality. current-name can also be a string, which serves as the warning message. The message should begin in lower case, and end with a period. It can also be nil, in which case the warning message provides no additional details.

If provided, when should be a string indicating when the function was first made obsolete—for example, a date or a release number.

— Macro: define-obsolete-function-alias obsolete-name current-name &optional when doc

This convenience macro marks the function obsolete-name obsolete and also defines it as an alias for the function current-name. It is equivalent to the following:

          (defalias obsolete-name current-name doc)
          (make-obsolete obsolete-name current-name when)

In addition, you can mark a particular calling convention for a function as obsolete:

— Function: set-advertised-calling-convention function signature when

This function specifies the argument list signature as the correct way to call function. This causes the Emacs byte compiler to issue a warning whenever it comes across an Emacs Lisp program that calls function any other way (however, it will still allow the code to be byte compiled). when should be a string indicating when the variable was first made obsolete (usually a version number string).

For instance, in old versions of Emacs the sit-for function accepted three arguments, like this

            (sit-for seconds milliseconds nodisp)

However, calling sit-for this way is considered obsolete (see Waiting). The old calling convention is deprecated like this:

            'sit-for '(seconds &optional nodisp) "22.1")

Next: , Previous: Obsolete Functions, Up: Functions

13.13 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.

— Macro: defsubst name args [doc] [declare] [interactive] body...

This macro defines an inline function. Its syntax is exactly the same as defun (see Defining Functions).

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.

After an inline function is defined, its inline expansion can be performed later on in the same file, just like macros.

It's possible to use defmacro 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.

As an alternative to defsubst, you can use define-inline to define functions via their exhaustive compiler macro. See define-inline.

Next: , Previous: Inline Functions, Up: Functions

13.14 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.

— Macro: declare specs...

This macro ignores its arguments and evaluates to nil; it has no run-time effect. However, when a declare form occurs in the declare argument of a defun or defsubst function definition (see Defining Functions) or a defmacro macro definition (see Defining Macros), it appends the properties specified by specs to the function or macro. This work is specially performed by defun, defsubst, and defmacro.

Each element in specs should have the form (property args...), which should not be quoted. These have the following effects:

(advertised-calling-convention signature when)
This acts like a call to set-advertised-calling-convention (see Obsolete Functions); signature specifies the correct argument list for calling the function or macro, and when should be a string indicating when the old argument list was first made obsolete.