Emacs Lisp
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This is the GNU Emacs Lisp Reference Manual corresponding to Emacs version 29.1.
Copyright © 1990–1996, 1998–2023 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.”
Table of Contents
- 1 Introduction
- 2 Lisp Data Types
- 2.1 Printed Representation and Read Syntax
- 2.2 Special Read Syntax
- 2.3 Comments
- 2.4 Programming Types
- 2.4.1 Integer Type
- 2.4.2 Floating-Point Type
- 2.4.3 Character Type
- 2.4.4 Symbol Type
- 2.4.5 Sequence Types
- 2.4.6 Cons Cell and List Types
- 2.4.7 Array Type
- 2.4.8 String Type
- 2.4.9 Vector Type
- 2.4.10 Char-Table Type
- 2.4.11 Bool-Vector Type
- 2.4.12 Hash Table Type
- 2.4.13 Function Type
- 2.4.14 Macro Type
- 2.4.15 Primitive Function Type
- 2.4.16 Byte-Code Function Type
- 2.4.17 Record Type
- 2.4.18 Type Descriptors
- 2.4.19 Autoload Type
- 2.4.20 Finalizer Type
- 2.5 Editing Types
- 2.5.1 Buffer Type
- 2.5.2 Marker Type
- 2.5.3 Window Type
- 2.5.4 Frame Type
- 2.5.5 Terminal Type
- 2.5.6 Window Configuration Type
- 2.5.7 Frame Configuration Type
- 2.5.8 Process Type
- 2.5.9 Thread Type
- 2.5.10 Mutex Type
- 2.5.11 Condition Variable Type
- 2.5.12 Stream Type
- 2.5.13 Keymap Type
- 2.5.14 Overlay Type
- 2.5.15 Font Type
- 2.5.16 Xwidget Type
- 2.6 Read Syntax for Circular Objects
- 2.7 Type Predicates
- 2.8 Equality Predicates
- 2.9 Mutability
- 3 Numbers
- 4 Strings and Characters
- 5 Lists
- 6 Sequences, Arrays, and Vectors
- 7 Records
- 8 Hash Tables
- 9 Symbols
- 10 Evaluation
- 11 Control Structures
- 12 Variables
- 12.1 Global Variables
- 12.2 Variables that Never Change
- 12.3 Local Variables
- 12.4 When a Variable is Void
- 12.5 Defining Global Variables
- 12.6 Tips for Defining Variables Robustly
- 12.7 Accessing Variable Values
- 12.8 Setting Variable Values
- 12.9 Running a function when a variable is changed.
- 12.10 Scoping Rules for Variable Bindings
- 12.11 Buffer-Local Variables
- 12.12 File Local Variables
- 12.13 Directory Local Variables
- 12.14 Connection Local Variables
- 12.15 Variable Aliases
- 12.16 Variables with Restricted Values
- 12.17 Generalized Variables
- 12.18 Multisession Variables
- 13 Functions
- 13.1 What Is a Function?
- 13.2 Lambda Expressions
- 13.3 Naming a Function
- 13.4 Defining Functions
- 13.5 Calling Functions
- 13.6 Mapping Functions
- 13.7 Anonymous Functions
- 13.8 Generic Functions
- 13.9 Accessing Function Cell Contents
- 13.10 Closures
- 13.11 Open Closures
- 13.12 Advising Emacs Lisp Functions
- 13.13 Declaring Functions Obsolete
- 13.14 Inline Functions
- 13.15 The
declare
Form - 13.16 Telling the Compiler that a Function is Defined
- 13.17 Determining whether a Function is Safe to Call
- 13.18 Other Topics Related to Functions
- 14 Macros
- 15 Customization Settings
- 16 Loading
- 17 Byte Compilation
- 18 Compilation of Lisp to Native Code
- 19 Debugging Lisp Programs
- 19.1 The Lisp Debugger
- 19.1.1 Entering the Debugger on an Error
- 19.1.2 Debugging Redisplay Errors
- 19.1.3 Debugging Infinite Loops
- 19.1.4 Entering the Debugger on a Function Call
- 19.1.5 Entering the debugger when a variable is modified
- 19.1.6 Explicit Entry to the Debugger
- 19.1.7 Using the Debugger
- 19.1.8 Backtraces
- 19.1.9 Debugger Commands
- 19.1.10 Invoking the Debugger
- 19.1.11 Internals of the Debugger
- 19.2 Edebug
- 19.2.1 Using Edebug
- 19.2.2 Instrumenting for Edebug
- 19.2.3 Edebug Execution Modes
- 19.2.4 Jumping
- 19.2.5 Miscellaneous Edebug Commands
- 19.2.6 Breaks
- 19.2.7 Trapping Errors
- 19.2.8 Edebug Views
- 19.2.9 Evaluation
- 19.2.10 Evaluation List Buffer
- 19.2.11 Printing in Edebug
- 19.2.12 Trace Buffer
- 19.2.13 Coverage Testing
- 19.2.14 The Outside Context
- 19.2.15 Edebug and Macros
- 19.2.16 Edebug Options
- 19.3 Debugging Invalid Lisp Syntax
- 19.4 Test Coverage
- 19.5 Profiling
- 19.1 The Lisp Debugger
- 20 Reading and Printing Lisp Objects
- 21 Minibuffers
- 21.1 Introduction to Minibuffers
- 21.2 Reading Text Strings with the Minibuffer
- 21.3 Reading Lisp Objects with the Minibuffer
- 21.4 Minibuffer History
- 21.5 Initial Input
- 21.6 Completion
- 21.7 Yes-or-No Queries
- 21.8 Asking Multiple-Choice Questions
- 21.9 Reading a Password
- 21.10 Minibuffer Commands
- 21.11 Minibuffer Windows
- 21.12 Minibuffer Contents
- 21.13 Recursive Minibuffers
- 21.14 Inhibiting Interaction
- 21.15 Minibuffer Miscellany
- 22 Command Loop
- 22.1 Command Loop Overview
- 22.2 Defining Commands
- 22.3 Interactive Call
- 22.4 Distinguish Interactive Calls
- 22.5 Information from the Command Loop
- 22.6 Adjusting Point After Commands
- 22.7 Input Events
- 22.7.1 Keyboard Events
- 22.7.2 Function Keys
- 22.7.3 Mouse Events
- 22.7.4 Click Events
- 22.7.5 Drag Events
- 22.7.6 Button-Down Events
- 22.7.7 Repeat Events
- 22.7.8 Motion Events
- 22.7.9 Touchscreen Events
- 22.7.10 Focus Events
- 22.7.11 Xwidget events
- 22.7.12 Miscellaneous System Events
- 22.7.13 Event Examples
- 22.7.14 Classifying Events
- 22.7.15 Accessing Mouse Events
- 22.7.16 Accessing Scroll Bar Events
- 22.7.17 Putting Keyboard Events in Strings
- 22.8 Reading Input
- 22.9 Special Events
- 22.10 Waiting for Elapsed Time or Input
- 22.11 Quitting
- 22.12 Prefix Command Arguments
- 22.13 Recursive Editing
- 22.14 Disabling Commands
- 22.15 Command History
- 22.16 Keyboard Macros
- 23 Keymaps
- 23.1 Key Sequences
- 23.2 Keymap Basics
- 23.3 Format of Keymaps
- 23.4 Creating Keymaps
- 23.5 Inheritance and Keymaps
- 23.6 Prefix Keys
- 23.7 Active Keymaps
- 23.8 Searching the Active Keymaps
- 23.9 Controlling the Active Keymaps
- 23.10 Key Lookup
- 23.11 Functions for Key Lookup
- 23.12 Changing Key Bindings
- 23.13 Low-Level Key Binding
- 23.14 Remapping Commands
- 23.15 Keymaps for Translating Sequences of Events
- 23.16 Commands for Binding Keys
- 23.17 Scanning Keymaps
- 23.18 Menu Keymaps
- 24 Major and Minor Modes
- 24.1 Hooks
- 24.2 Major Modes
- 24.3 Minor Modes
- 24.4 Mode Line Format
- 24.5 Imenu
- 24.6 Font Lock Mode
- 24.6.1 Font Lock Basics
- 24.6.2 Search-based Fontification
- 24.6.3 Customizing Search-Based Fontification
- 24.6.4 Other Font Lock Variables
- 24.6.5 Levels of Font Lock
- 24.6.6 Precalculated Fontification
- 24.6.7 Faces for Font Lock
- 24.6.8 Syntactic Font Lock
- 24.6.9 Multiline Font Lock Constructs
- 24.6.10 Parser-based Font Lock
- 24.7 Automatic Indentation of code
- 24.7.1 Simple Minded Indentation Engine
- 24.7.1.1 SMIE Setup and Features
- 24.7.1.2 Operator Precedence Grammars
- 24.7.1.3 Defining the Grammar of a Language
- 24.7.1.4 Defining Tokens
- 24.7.1.5 Living With a Weak Parser
- 24.7.1.6 Specifying Indentation Rules
- 24.7.1.7 Helper Functions for Indentation Rules
- 24.7.1.8 Sample Indentation Rules
- 24.7.1.9 Customizing Indentation
- 24.7.2 Parser-based Indentation
- 24.7.1 Simple Minded Indentation Engine
- 24.8 Desktop Save Mode
- 25 Documentation
- 26 Files
- 26.1 Visiting Files
- 26.2 Saving Buffers
- 26.3 Reading from Files
- 26.4 Writing to Files
- 26.5 File Locks
- 26.6 Information about Files
- 26.7 Changing File Names and Attributes
- 26.8 Files and Secondary Storage
- 26.9 File Names
- 26.10 Contents of Directories
- 26.11 Creating, Copying and Deleting Directories
- 26.12 Making Certain File Names “Magic”
- 26.13 File Format Conversion
- 27 Backups and Auto-Saving
- 28 Buffers
- 28.1 Buffer Basics
- 28.2 The Current Buffer
- 28.3 Buffer Names
- 28.4 Buffer File Name
- 28.5 Buffer Modification
- 28.6 Buffer Modification Time
- 28.7 Read-Only Buffers
- 28.8 The Buffer List
- 28.9 Creating Buffers
- 28.10 Killing Buffers
- 28.11 Indirect Buffers
- 28.12 Swapping Text Between Two Buffers
- 28.13 The Buffer Gap
- 29 Windows
- 29.1 Basic Concepts of Emacs Windows
- 29.2 Windows and Frames
- 29.3 Selecting Windows
- 29.4 Window Sizes
- 29.5 Resizing Windows
- 29.6 Preserving Window Sizes
- 29.7 Splitting Windows
- 29.8 Deleting Windows
- 29.9 Recombining Windows
- 29.10 Cyclic Ordering of Windows
- 29.11 Buffers and Windows
- 29.12 Switching to a Buffer in a Window
- 29.13 Displaying a Buffer in a Suitable Window
- 29.14 Window History
- 29.15 Dedicated Windows
- 29.16 Quitting Windows
- 29.17 Side Windows
- 29.18 Atomic Windows
- 29.19 Windows and Point
- 29.20 The Window Start and End Positions
- 29.21 Textual Scrolling
- 29.22 Vertical Fractional Scrolling
- 29.23 Horizontal Scrolling
- 29.24 Coordinates and Windows
- 29.25 Mouse Window Auto-selection
- 29.26 Window Configurations
- 29.27 Window Parameters
- 29.28 Hooks for Window Scrolling and Changes
- 30 Frames
- 30.1 Creating Frames
- 30.2 Multiple Terminals
- 30.3 Frame Geometry
- 30.4 Frame Parameters
- 30.4.1 Access to Frame Parameters
- 30.4.2 Initial Frame Parameters
- 30.4.3 Window Frame Parameters
- 30.4.3.1 Basic Parameters
- 30.4.3.2 Position Parameters
- 30.4.3.3 Size Parameters
- 30.4.3.4 Layout Parameters
- 30.4.3.5 Buffer Parameters
- 30.4.3.6 Frame Interaction Parameters
- 30.4.3.7 Mouse Dragging Parameters
- 30.4.3.8 Window Management Parameters
- 30.4.3.9 Cursor Parameters
- 30.4.3.10 Font and Color Parameters
- 30.4.4 Geometry
- 30.5 Terminal Parameters
- 30.6 Frame Titles
- 30.7 Deleting Frames
- 30.8 Finding All Frames
- 30.9 Minibuffers and Frames
- 30.10 Input Focus
- 30.11 Visibility of Frames
- 30.12 Raising, Lowering and Restacking Frames
- 30.13 Frame Configurations
- 30.14 Child Frames
- 30.15 Mouse Tracking
- 30.16 Mouse Position
- 30.17 Pop-Up Menus
- 30.18 Dialog Boxes
- 30.19 Pointer Shape
- 30.20 Window System Selections
- 30.21 Yanking Media
- 30.22 Drag and Drop
- 30.23 Color Names
- 30.24 Text Terminal Colors
- 30.25 X Resources
- 30.26 Display Feature Testing
- 31 Positions
- 32 Markers
- 33 Text
- 33.1 Examining Text Near Point
- 33.2 Examining Buffer Contents
- 33.3 Comparing Text
- 33.4 Inserting Text
- 33.5 User-Level Insertion Commands
- 33.6 Deleting Text
- 33.7 User-Level Deletion Commands
- 33.8 The Kill Ring
- 33.9 Undo
- 33.10 Maintaining Undo Lists
- 33.11 Filling
- 33.12 Margins for Filling
- 33.13 Adaptive Fill Mode
- 33.14 Auto Filling
- 33.15 Sorting Text
- 33.16 Counting Columns
- 33.17 Indentation
- 33.18 Case Changes
- 33.19 Text Properties
- 33.19.1 Examining Text Properties
- 33.19.2 Changing Text Properties
- 33.19.3 Text Property Search Functions
- 33.19.4 Properties with Special Meanings
- 33.19.5 Formatted Text Properties
- 33.19.6 Stickiness of Text Properties
- 33.19.7 Lazy Computation of Text Properties
- 33.19.8 Defining Clickable Text
- 33.19.9 Defining and Using Fields
- 33.19.10 Why Text Properties are not Intervals
- 33.20 Substituting for a Character Code
- 33.21 Registers
- 33.22 Transposition of Text
- 33.23 Replacing Buffer Text
- 33.24 Dealing With Compressed Data
- 33.25 Base 64 Encoding
- 33.26 Checksum/Hash
- 33.27 Suspicious Text
- 33.28 GnuTLS Cryptography
- 33.29 Database
- 33.30 Parsing HTML and XML
- 33.31 Parsing and generating JSON values
- 33.32 JSONRPC communication
- 33.33 Atomic Change Groups
- 33.34 Change Hooks
- 34 Non-ASCII Characters
- 34.1 Text Representations
- 34.2 Disabling Multibyte Characters
- 34.3 Converting Text Representations
- 34.4 Selecting a Representation
- 34.5 Character Codes
- 34.6 Character Properties
- 34.7 Character Sets
- 34.8 Scanning for Character Sets
- 34.9 Translation of Characters
- 34.10 Coding Systems
- 34.11 Input Methods
- 34.12 Locales
- 35 Searching and Matching
- 35.1 Searching for Strings
- 35.2 Searching and Case
- 35.3 Regular Expressions
- 35.4 Regular Expression Searching
- 35.5 POSIX Regular Expression Searching
- 35.6 The Match Data
- 35.7 Search and Replace
- 35.8 Standard Regular Expressions Used in Editing
- 36 Syntax Tables
- 37 Parsing Program Source
- 38 Abbrevs and Abbrev Expansion
- 39 Threads
- 40 Processes
- 40.1 Functions that Create Subprocesses
- 40.2 Shell Arguments
- 40.3 Creating a Synchronous Process
- 40.4 Creating an Asynchronous Process
- 40.5 Deleting Processes
- 40.6 Process Information
- 40.7 Sending Input to Processes
- 40.8 Sending Signals to Processes
- 40.9 Receiving Output from Processes
- 40.10 Sentinels: Detecting Process Status Changes
- 40.11 Querying Before Exit
- 40.12 Accessing Other Processes
- 40.13 Transaction Queues
- 40.14 Network Connections
- 40.15 Network Servers
- 40.16 Datagrams
- 40.17 Low-Level Network Access
- 40.18 Misc Network Facilities
- 40.19 Communicating with Serial Ports
- 40.20 Packing and Unpacking Byte Arrays
- 41 Emacs Display
- 41.1 Refreshing the Screen
- 41.2 Forcing Redisplay
- 41.3 Truncation
- 41.4 The Echo Area
- 41.5 Reporting Warnings
- 41.6 Invisible Text
- 41.7 Selective Display
- 41.8 Temporary Displays
- 41.9 Overlays
- 41.10 Size of Displayed Text
- 41.11 Line Height
- 41.12 Faces
- 41.12.1 Face Attributes
- 41.12.2 Defining Faces
- 41.12.3 Face Attribute Functions
- 41.12.4 Displaying Faces
- 41.12.5 Face Remapping
- 41.12.6 Functions for Working with Faces
- 41.12.7 Automatic Face Assignment
- 41.12.8 Basic Faces
- 41.12.9 Font Selection
- 41.12.10 Looking Up Fonts
- 41.12.11 Fontsets
- 41.12.12 Low-Level Font Representation
- 41.13 Fringes
- 41.14 Scroll Bars
- 41.15 Window Dividers
- 41.16 The
display
Property - 41.17 Images
- 41.18 Icons
- 41.19 Embedded Native Widgets
- 41.20 Buttons
- 41.21 Abstract Display
- 41.22 Blinking Parentheses
- 41.23 Character Display
- 41.24 Beeping
- 41.25 Window Systems
- 41.26 Tooltips
- 41.27 Bidirectional Display
- 42 Operating System Interface
- 42.1 Starting Up Emacs
- 42.2 Getting Out of Emacs
- 42.3 Operating System Environment
- 42.4 User Identification
- 42.5 Time of Day
- 42.6 Time Zone Rules
- 42.7 Time Conversion
- 42.8 Parsing and Formatting Times
- 42.9 Processor Run time
- 42.10 Time Calculations
- 42.11 Timers for Delayed Execution
- 42.12 Idle Timers
- 42.13 Terminal Input
- 42.14 Terminal Output
- 42.15 Sound Output
- 42.16 Operating on X11 Keysyms
- 42.17 Batch Mode
- 42.18 Session Management
- 42.19 Desktop Notifications
- 42.20 Notifications on File Changes
- 42.21 Dynamically Loaded Libraries
- 42.22 Security Considerations
- 43 Preparing Lisp code for distribution
- Appendix A Emacs 28 Antinews
- Appendix B GNU Free Documentation License
- Appendix C GNU General Public License
- Appendix D Tips and Conventions
- Appendix E GNU Emacs Internals
- Appendix F Standard Errors
- Appendix G Standard Keymaps
- Appendix H Standard Hooks
- Index
Short Table of Contents
- 1 Introduction
- 2 Lisp Data Types
- 3 Numbers
- 4 Strings and Characters
- 5 Lists
- 6 Sequences, Arrays, and Vectors
- 7 Records
- 8 Hash Tables
- 9 Symbols
- 10 Evaluation
- 11 Control Structures
- 12 Variables
- 13 Functions
- 14 Macros
- 15 Customization Settings
- 16 Loading
- 17 Byte Compilation
- 18 Compilation of Lisp to Native Code
- 19 Debugging Lisp Programs
- 20 Reading and Printing Lisp Objects
- 21 Minibuffers
- 22 Command Loop
- 23 Keymaps
- 24 Major and Minor Modes
- 25 Documentation
- 26 Files
- 27 Backups and Auto-Saving
- 28 Buffers
- 29 Windows
- 30 Frames
- 31 Positions
- 32 Markers
- 33 Text
- 34 Non-ASCII Characters
- 35 Searching and Matching
- 36 Syntax Tables
- 37 Parsing Program Source
- 38 Abbrevs and Abbrev Expansion
- 39 Threads
- 40 Processes
- 41 Emacs Display
- 42 Operating System Interface
- 43 Preparing Lisp code for distribution
- Appendix A Emacs 28 Antinews
- Appendix B GNU Free Documentation License
- Appendix C GNU General Public License
- Appendix D Tips and Conventions
- Appendix E GNU Emacs Internals
- Appendix F Standard Errors
- Appendix G Standard Keymaps
- Appendix H Standard Hooks
- Index
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 29.1.
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. If you wish to contribute new code (or send a patch to fix a problem), use M-x submit-emacs-patch.
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 in Common Lisp Extensions.
Emacs Lisp is not at all influenced by Scheme; but the GNU project has an implementation of Scheme, called Guile. We use it in all new GNU software that calls for extensibility.
1.3 Conventions
This section explains the notational conventions that are used in this manual. You may want to skip this section and refer back to it later.
- Some Terms
nil
andt
- Evaluation Notation
- Printing Notation
- Error Messages
- Buffer Text Notation
- Format of Descriptions
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 and Read Syntax, for more details. You, the person reading this manual, are thought of as the programmer and are addressed as “you”. The user is the person who uses Lisp programs, including those you write.
Examples of Lisp code are formatted like this: (list 1 2 3)
.
Names that represent metasyntactic variables, or arguments to a function
being described, are formatted like this: first-number.
1.3.2 nil
and t
In Emacs Lisp, the symbol nil
has three separate meanings: it
is a symbol with the name ‘nil’; it is the logical truth value
false; and it is the empty list—the list of zero elements.
When used as a variable, nil
always has the value nil
.
As far as the Lisp reader is concerned, ‘()’ and ‘nil’ are
identical: they stand for the same object, the symbol nil
. The
different ways of writing the symbol are intended entirely for human
readers. After the Lisp reader has read either ‘()’ or ‘nil’,
there is no way to determine which representation was actually written
by the programmer.
In this manual, we write ()
when we wish to emphasize that it
means the empty list, and we write nil
when we wish to emphasize
that it means the truth value false. That is a good convention to use
in Lisp programs also.
(cons 'foo ()) ; Emphasize the empty list (setq foo-flag nil) ; Emphasize the truth value false
In contexts where a truth value is expected, any non-nil
value
is considered to be true. However, t
is the preferred way
to represent the truth value true. When you need to choose a
value 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 Variables that Never Change.
- Function: booleanp object ¶
Return non-
nil
if object is one of the two canonical boolean values:t
ornil
.
1.3.3 Evaluation Notation
A Lisp expression that you can evaluate is called a form. Evaluating a form always produces a result, which is a Lisp object. In the examples in this manual, this is indicated with ‘⇒’:
(car '(1 2)) ⇒ 1
You can read this as “(car '(1 2))
evaluates to 1”.
When a form is a macro call, it expands into a new form for Lisp to evaluate. We show the result of the expansion with ‘→’. We may or may not show the result of the evaluation of the expanded form.
(third '(a b c)) → (car (cdr (cdr '(a b c)))) ⇒ c
To help describe one form, we sometimes show another form that produces identical results. The exact equivalence of two forms is indicated with ‘≡’.
(make-sparse-keymap) ≡ (list 'keymap)
1.3.4 Printing Notation
Many of the examples in this manual print text when they are
evaluated. If you execute example code in a Lisp Interaction buffer
(such as the buffer *scratch*) by typing C-j after the
closing parenthesis of the example, the printed text is inserted into
the buffer. If you execute the example by other means (such as by
evaluating the function eval-region
), the printed text is
displayed in the echo area.
Examples in this manual indicate printed text with ‘-|’, irrespective of where that text goes. The value returned by evaluating the form follows on a separate line with ‘⇒’.
(progn (prin1 'foo) (princ "\n") (prin1 'bar)) -| foo -| bar ⇒ bar
1.3.5 Error Messages
Some examples signal errors. This normally displays an error message in the echo area. We show the error message on a line starting with ‘error→’. Note that ‘error→’ itself does not appear in the echo area.
(+ 23 'x) error→ Wrong type argument: number-or-marker-p, x
1.3.6 Buffer Text Notation
Some examples describe modifications to the contents of a buffer, by showing the before and after versions of the text. These examples show the contents of the buffer in question between two lines of dashes containing the buffer name. In addition, ‘∗’ indicates the location of point. (The symbol for point, of course, is not part of the text in the buffer; it indicates the place between two characters where point is currently located.)
---------- Buffer: foo ---------- This is the ∗contents of foo. ---------- Buffer: foo ---------- (insert "changed ") ⇒ nil ---------- Buffer: foo ---------- This is the changed ∗contents of foo. ---------- Buffer: foo ----------
1.3.7 Format of Descriptions
Functions, variables, macros, commands, user options, and special forms are described in this manual in a uniform format. The first line of a description contains the name of the item followed by its arguments, if any. The category—function, variable, or whatever—appears at the beginning of the line. The description follows on succeeding lines, sometimes with examples.
1.3.7.1 A Sample Function Description
In a function description, the name of the function being described appears first. It is followed on the same line by a list of argument names. These names are also used in the body of the description, to stand for the values of the arguments.
The appearance of the keyword &optional
in the argument list
indicates that the subsequent arguments may be omitted (omitted
arguments default to nil
). Do not write &optional
when
you call the function.
The keyword &rest
(which must be followed by a single
argument name) indicates that any number of arguments can follow. The
single argument name following &rest
receives, as its
value, a list of all the remaining arguments passed to the function.
Do not write &rest
when you call the function.
Here is a description of an imaginary function foo
:
- 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)) (terpri))
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) (fixit) (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.
1.3.7.2 A Sample Variable Description
A variable is a name that can be bound (or set) to an object. The object to which a variable is bound is called a value; we say also that variable holds that value. Although nearly all variables can be set by the user, certain variables exist specifically so that users can change them; these are called user options. Ordinary variables and user options are described using a format like that for functions, except that there are no arguments.
Here is a description of the imaginary electric-future-map
variable.
- 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’.
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.
(emacs-version) ⇒ "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 returnsnil
. 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 uses the style of
current-time
(see Time of Day), or isnil
if the information is not available.emacs-build-time ⇒ (25194 55894 8547 617000)
(This timestamp is
(1651169878008547617 . 1000000000)
ifcurrent-time-list
wasnil
when Emacs was built.)
- 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 inemacs-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.
- Variable: emacs-repository-version ¶
A string that gives the repository revision from which Emacs was built. If Emacs was built outside revision control, the value is
nil
.
- Variable: emacs-repository-branch ¶
A string that gives the repository branch from which Emacs was built. In the most cases this is
"master"
. If Emacs was built outside revision control, the value isnil
.
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.
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.
- Printed Representation and Read Syntax
- Special Read Syntax
- Comments
- Programming Types
- Editing Types
- Read Syntax for Circular Objects
- Type Predicates
- Equality Predicates
- Mutability
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 Reading and Printing Lisp Objects.
In most cases, an object’s printed representation is also a read syntax for the object. However, some types have no read syntax, since it does not make sense to enter objects of these types as constants in a Lisp program. These objects are printed in hash notation, which consists of the characters ‘#<’, a descriptive string (typically the type name followed by the name of the object), and a closing ‘>’. For example:
(current-buffer) ⇒ #<buffer objects.texi>
Hash notation cannot be read at all, so the Lisp reader signals the
error invalid-read-syntax
whenever it encounters ‘#<’.
In other languages, an expression is text; it has no other form. In Lisp, an expression is primarily a Lisp object and only secondarily the text that is the object’s read syntax. Often there is no need to emphasize this distinction, but you must keep it in the back of your mind, or you will occasionally be very confused.
When you evaluate an expression interactively, the Lisp interpreter
first reads the textual representation of it, producing a Lisp object,
and then evaluates that object (see Evaluation). However,
evaluation and reading are separate activities. Reading returns the
Lisp object represented by the text that is read; the object may or may
not be evaluated later. See Input Functions, for a description of
read
, the basic function for reading objects.
2.2 Special Read Syntax
Emacs Lisp represents many special objects and constructs via special hash notations.
- ‘#<…>’
Objects that have no read syntax are presented like this (see Printed Representation and Read Syntax).
- ‘##’
The printed representation of an interned symbol whose name is an empty string (see Symbol Type).
- ‘#'’
This is a shortcut for
function
, see Anonymous Functions.- ‘#:’
The printed representation of an uninterned symbol whose name is foo is ‘#:foo’ (see Symbol Type).
- ‘#N’
When printing circular structures, this construct is used to represent where the structure loops back onto itself, and ‘N’ is the starting list count:
(let ((a (list 1))) (setcdr a a)) => (1 . #0)
- ‘#N=’
- ‘#N#’
‘#N=’ gives the name to an object, and ‘#N#’ represents that object, so when reading back the object, they will be the same object instead of copies (see Read Syntax for Circular Objects).
- ‘#xN’
‘N’ represented as a hexadecimal number (‘#x2a’).
- ‘#oN’
‘N’ represented as an octal number (‘#o52’).
- ‘#bN’
‘N’ represented as a binary number (‘#b101010’).
- ‘#(…)’
String text properties (see Text Properties in Strings).
- ‘#^’
A char table (see Char-Table Type).
- ‘#s(hash-table …)’
A hash table (see Hash Table Type).
- ‘?C’
A character (see Basic Char Syntax).
- ‘#$’
The current file name in byte-compiled files (see Documentation Strings and Compilation). This is not meant to be used in Emacs Lisp source files.
- ‘#@N’
Skip the next ‘N’ characters (see Comments). This is used in byte-compiled files, and is not meant to be used in Emacs Lisp source files.
- ‘#f’
Indicates that the following form isn’t readable by the Emacs Lisp reader. This is only in text for display purposes (when that would look prettier than alternative ways of indicating an unreadable form) and will never appear in any Lisp file.
2.3 Comments
A comment is text that is written in a program only for the sake of humans that read the program, and that has no effect on the meaning of the program. In Lisp, 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 Tips on Writing Comments, for conventions for formatting comments.
2.4 Programming Types
There are two general categories of types in Emacs Lisp: those having to do with Lisp programming, and those having to do with editing. The former exist in many Lisp implementations, in one form or another. The latter are unique to Emacs Lisp.
- Integer Type
- Floating-Point Type
- Character Type
- Symbol Type
- Sequence Types
- Cons Cell and List Types
- Array Type
- String Type
- Vector Type
- Char-Table Type
- Bool-Vector Type
- Hash Table Type
- Function Type
- Macro Type
- Primitive Function Type
- Byte-Code Function Type
- Record Type
- Type Descriptors
- Autoload Type
- Finalizer Type
2.4.1 Integer Type
Under the hood, there are two kinds of integers—small integers, called fixnums, and large integers, called bignums.
The range of values for a fixnum 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.
Bignums can have arbitrary precision. Operations that overflow a fixnum will return a bignum instead.
All numbers can be compared with eql
or =
; fixnums can
also be compared with eq
. To test whether an integer is a fixnum or a
bignum, you can compare it to most-negative-fixnum
and
most-positive-fixnum
, or you can use the convenience predicates
fixnump
and bignump
on any object.
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.
See Numbers, for more information.
2.4.2 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.
2.4.3 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 for Help Messages.
- Basic Char Syntax
- General Escape Syntax
- Control-Character Syntax
- Meta-Character Syntax
- Other Character Modifier Bits
2.4.3.1 Basic Char Syntax
Since characters are really integers, the printed representation of a character is a decimal number. This is also a possible read syntax for a character, but writing characters that way in Lisp programs is not clear programming. You should always use the special read syntax formats that Emacs Lisp provides for characters. These syntax formats start with a question mark.
The usual read syntax for alphanumeric characters is a question mark followed by the character; thus, ‘?A’ for the character A, ‘?B’ for the character B, and ‘?a’ for the character a.
For example:
?Q ⇒ 81 ?q ⇒ 113
You can use the same syntax for punctuation characters. 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 should also add a backslash before Unicode characters which resemble the previously mentioned ASCII ones, to avoid confusing people reading your code. Emacs will highlight some non-escaped commonly confused characters such as ‘‘’ to encourage this. You can also add a backslash before whitespace characters such as space, tab, newline and formfeed. However, it is cleaner to use one of the easily readable escape sequences, such as ‘\t’ or ‘\s’, instead of an actual whitespace character such as a tab or a space. (If you do write backslash followed by a space, you should write an extra space after the character constant to separate it from the following text.)
2.4.3.2 General Escape Syntax
In addition to the specific escape sequences for special important control characters, Emacs provides several types of escape syntax that you can use to specify non-ASCII text characters.
-
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?à
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). - 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?à
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. - 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 one or more hex digits after ‘x’, so you can represent any character code in this way. -
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 Characters in Strings.
2.4.3.3 Control-Character Syntax
Control characters can be represented using yet another read syntax. This consists of a question mark followed by a backslash, caret, and the corresponding non-control character, in either upper or lower case. For example, both ‘?\^I’ and ‘?\^i’ are valid read syntax for the character C-i, the character whose value is 9.
Instead of the ‘^’, you can use ‘C-’; thus, ‘?\C-i’ is equivalent to ‘?\^I’ and to ‘?\^i’:
?\^I ⇒ 9 ?\C-I ⇒ 9
In strings and buffers, the only control characters allowed are those that exist in ASCII; but for keyboard input purposes, you can turn any character into a control character with ‘C-’. The character codes for these non-ASCII control characters include the 2**26 bit as well as the code for the corresponding non-control character. Not all text terminals can generate non-ASCII control characters, but it is straightforward to generate them using X and other window systems.
For historical reasons, Emacs treats the DEL character as the control equivalent of ?:
?\^? ⇒ 127 ?\C-? ⇒ 127
As a result, it is currently not possible to represent the character Control-?, which is a meaningful input character under X, using ‘\C-’. It is not easy to change this, as various Lisp files refer to DEL in this way.
For representing control characters to be found in files or strings, we recommend the ‘^’ syntax; for control characters in keyboard input, we prefer the ‘C-’ syntax. Which one you use does not affect the meaning of the program, but may guide the understanding of people who read it.
2.4.3.4 Meta-Character Syntax
A meta character is a character typed with the META modifier key. The integer that represents such a character has the 2**27 bit set. We use high bits for this and other modifiers to make possible a wide range of basic character codes.
In a string, the 2**7 bit attached to an ASCII character indicates a meta character; thus, the meta characters that can fit in a string have codes in the range from 128 to 255, and are the meta versions of the ordinary ASCII characters. See Putting Keyboard Events in Strings, for details about META-handling in strings.
The read syntax for meta characters uses ‘\M-’. For example, ‘?\M-A’ stands for M-A. You can use ‘\M-’ together with octal character codes (see below), with ‘\C-’, or with any other syntax for a character. Thus, you can write M-A as ‘?\M-A’, or as ‘?\M-\101’. Likewise, you can write C-M-b as ‘?\M-\C-b’, ‘?\C-\M-b’, or ‘?\M-\002’.
2.4.3.5 Other Character Modifier Bits
The case of a graphic character is indicated by its character code; for example, ASCII distinguishes between the characters ‘a’ and ‘A’. But ASCII has no way to represent whether a control character is upper case or lower case. Emacs uses the 2**25 bit to indicate that the shift key was used in typing a control character. This distinction is possible only on a graphical display such as a GUI display on X; text terminals do not report the distinction. The Lisp syntax for the shift bit is ‘\S-’; thus, ‘?\C-\S-o’ or ‘?\C-\S-O’ represents the shifted-control-o character.
The X Window System defines three other modifier bits that can be set in a character: hyper, super and alt. The syntaxes for these bits are ‘\H-’, ‘\s-’ and ‘\A-’. (Case is significant in these prefixes.) Thus, ‘?\H-\M-\A-x’ represents Alt-Hyper-Meta-x. (Note that ‘\s’ with no following ‘-’ represents the space character.) Numerically, the bit values are 2**22 for alt, 2**23 for super and 2**24 for hyper.
2.4.4 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 and Interning 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 Variables that Never Change.
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 and Interning Symbols.)
2.4.5 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, and Vectors.
It is generally impossible to read the same sequence twice, since
sequences are always created anew upon reading. If you read the read
syntax for a sequence twice, you get two sequences with equal contents.
There is one exception: the empty list ()
always stands for the
same object, nil
.
2.4.6 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.
2.4.6.1 Drawing Lists as Box Diagrams
A list can be illustrated by a diagram in which the cons cells are
shown as pairs of boxes, like dominoes. (The Lisp reader cannot read
such an illustration; unlike the textual notation, which can be
understood by both humans and computers, the box illustrations can be
understood only by humans.) This picture represents the three-element
list (rose violet buttercup)
:
--- --- --- --- --- --- | | |--> | | |--> | | |--> nil --- --- --- --- --- --- | | | | | | --> rose --> violet --> buttercup
In this diagram, each box represents a slot that can hold or refer to any Lisp object. Each pair of boxes represents a cons cell. Each arrow represents a reference to a Lisp object, either an atom or another cons cell.
In this example, the first box, which holds the CAR of the first
cons cell, refers to or holds rose
(a symbol). The second
box, holding the CDR of the first cons cell, refers to the next
pair of boxes, the second cons cell. The CAR of the second cons
cell is violet
, and its CDR is the third cons cell. The
CDR of the third (and last) cons cell is nil
.
Here is another diagram of the same list, (rose violet
buttercup)
, sketched in a different manner:
--------------- ---------------- ------------------- | car | cdr | | car | cdr | | car | cdr | | rose | o-------->| violet | o-------->| buttercup | nil | | | | | | | | | | --------------- ---------------- -------------------
A list with no elements in it is the empty list; it is identical
to the symbol nil
. In other words, nil
is both a symbol
and a list.
Here is the list (A ())
, or equivalently (A nil)
,
depicted with boxes and arrows:
--- --- --- --- | | |--> | | |--> nil --- --- --- --- | | | | --> A --> nil
Here is a more complex illustration, showing the three-element list,
((pine needles) oak maple)
, the first element of which is a
two-element list:
--- --- --- --- --- --- | | |--> | | |--> | | |--> nil --- --- --- --- --- --- | | | | | | | --> oak --> maple | | --- --- --- --- --> | | |--> | | |--> nil --- --- --- --- | | | | --> pine --> needles
The same list represented in the second box notation looks like this:
-------------- -------------- -------------- | car | cdr | | car | cdr | | car | cdr | | o | o------->| oak | o------->| maple | nil | | | | | | | | | | | -- | --------- -------------- -------------- | | | -------------- ---------------- | | car | cdr | | car | cdr | ------>| pine | o------->| needles | nil | | | | | | | -------------- ----------------
2.4.6.2 Dotted Pair Notation
Dotted pair notation is a general syntax for cons cells that
represents the CAR and CDR explicitly. In this syntax,
(a . b)
stands for a cons cell whose CAR is
the object a and whose CDR is the object b. Dotted
pair notation is more general than list syntax because the CDR
does not have to be a list. However, it is more cumbersome in cases
where list syntax would work. In dotted pair notation, the list
‘(1 2 3)’ is written as ‘(1 . (2 . (3 . nil)))’. For
nil
-terminated lists, you can use either notation, but list
notation is usually clearer and more convenient. When printing a
list, the dotted pair notation is only used if the CDR of a cons
cell is not a list.
Here’s an example using boxes to illustrate dotted pair notation.
This example shows the pair (rose . violet)
:
--- --- | | |--> violet --- --- | | --> rose
You can combine dotted pair notation with list notation to represent
conveniently a chain of cons cells with a non-nil
final CDR.
You write a dot after the last element of the list, followed by the
CDR of the final cons cell. For example, (rose violet
. buttercup)
is equivalent to (rose . (violet . buttercup))
.
The object looks like this:
--- --- --- --- | | |--> | | |--> buttercup --- --- --- --- | | | | --> rose --> violet
The syntax (rose . violet . buttercup)
is invalid because
there is nothing that it could mean. If anything, it would say to put
buttercup
in the CDR of a cons cell whose CDR is already
used for violet
.
The list (rose violet)
is equivalent to (rose . (violet))
,
and looks like this:
--- --- --- --- | | |--> | | |--> nil --- --- --- --- | | | | --> rose --> violet
Similarly, the three-element list (rose violet buttercup)
is equivalent to (rose . (violet . (buttercup)))
.
It looks like this:
--- --- --- --- --- --- | | |--> | | |--> | | |--> nil --- --- --- --- --- --- | | | | | | --> rose --> violet --> buttercup
2.4.6.3 Association List Type
An association list or alist is a specially-constructed list whose elements are cons cells. In each element, the CAR is considered a key, and the CDR is considered an associated value. (In some cases, the associated value is stored in the CAR of the CDR.) Association lists are often used as stacks, since it is easy to add or remove associations at the front of the list.
For example,
(setq alist-of-colors '((rose . red) (lily . white) (buttercup . yellow)))
sets the variable alist-of-colors
to an alist of three elements. In the
first element, rose
is the key and red
is the value.
See Association Lists, for a further explanation of alists and for functions that work on alists. See Hash Tables, for another kind of lookup table, which is much faster for handling a large number of keys.
2.4.7 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 fixnum,
subject to system architecture limits and available memory.
Char-tables are sparse arrays indexed by any valid character code; they
can hold arbitrary objects.
The first element of an array has index zero, the second element has index 1, and so on. This is called zero-origin indexing. For example, an array of four elements has indices 0, 1, 2, and 3. The largest possible index value is one less than the length of the array. Once an array is created, its length is fixed.
All Emacs Lisp arrays are one-dimensional. (Most other programming languages support multidimensional arrays, but they are not essential; you can get the same effect with nested one-dimensional arrays.) Each type of array has its own read syntax; see the following sections for details.
The array type is a subset of the sequence type, and contains the string type, the vector type, the bool-vector type, and the char-table type.
2.4.8 String Type
A string is an array of characters. Strings are used for many purposes in Emacs, as can be expected in a text editor; for example, as the names of Lisp symbols, as messages for the user, and to represent text extracted from buffers. Strings in Lisp are constants: evaluation of a string returns the same string.
See Strings and Characters, for functions that operate on strings.
- Syntax for Strings
- Non-ASCII Characters in Strings
- Nonprinting Characters in Strings
- Text Properties in Strings
2.4.8.1 Syntax for Strings
The read syntax for a string is a double-quote, an arbitrary number
of characters, and another double-quote, "like this"
. To
include a double-quote in a string, precede it with a backslash; thus,
"\""
is a string containing just 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."
2.4.8.2 Non-ASCII Characters in Strings
There are two text representations for non-ASCII characters in Emacs strings: multibyte and unibyte (see Text Representations). Roughly speaking, unibyte strings store raw bytes, while multibyte strings store human-readable text. Each character in a unibyte string is a byte, i.e., its value is between 0 and 255. By contrast, each character in a multibyte string may have a value between 0 to 4194303 (see Character Type). In both cases, characters above 127 are non-ASCII.
You can include a non-ASCII character in a string constant by writing it literally. If the string constant is read from a multibyte source, such as a multibyte buffer or string, or a file that would be visited as multibyte, then Emacs reads each non-ASCII character as a multibyte character and automatically makes the string a multibyte string. If the string constant is read from a unibyte source, then Emacs reads the non-ASCII character as unibyte, and makes the string unibyte.
Instead of writing a character literally into a multibyte string, you can write it as its character code using an escape sequence. See General Escape Syntax, for details about escape sequences.
If you use any Unicode-style escape sequence ‘\uNNNN’ or ‘\U00NNNNNN’ in a string constant (even for an ASCII character), Emacs automatically assumes that it is multibyte.
You can also use hexadecimal escape sequences (‘\xn’) and octal escape sequences (‘\n’) in string constants. But beware: If a string constant contains hexadecimal or octal escape sequences, and these escape sequences all specify unibyte characters (i.e., less than 256), and there are no other literal non-ASCII characters or Unicode-style escape sequences in the string, then Emacs automatically assumes that it is a unibyte string. That is to say, it assumes that all non-ASCII characters occurring in the string are 8-bit raw bytes.
In hexadecimal and octal escape sequences, the escaped character code may contain a variable number of digits, so the first subsequent character which is not a valid hexadecimal or octal digit terminates the escape sequence. If the next character in a string could be interpreted as a hexadecimal or octal digit, write ‘\ ’ (backslash and space) to terminate the escape sequence. For example, ‘\xe0\ ’ represents one character, ‘a’ with grave accent. ‘\ ’ in a string constant is just like backslash-newline; it does not contribute any character to the string, but it does terminate any preceding hex escape.
2.4.8.3 Nonprinting Characters in Strings
You can use the same backslash escape-sequences in a string constant
as in character literals (but do not use the question mark that begins a
character constant). For example, you can write a string containing the
nonprinting characters tab and C-a, with commas and spaces between
them, like this: "\t, \C-a"
. See Character Type, for a
description of the read syntax for characters.
However, not all of the characters you can write with backslash escape-sequences are valid in strings. The only control characters that a string can hold are the ASCII control characters. Strings do not distinguish case in ASCII control characters.
Properly speaking, strings cannot hold meta characters; but when a
string is to be used as a key sequence, there is a special convention
that provides a way to represent meta versions of ASCII
characters in a string. If you use the ‘\M-’ syntax to indicate
a meta character in a string constant, this sets the
2**7
bit of the character in the string. If the string is used in
define-key
or lookup-key
, this numeric code is translated
into the equivalent meta character. See Character Type.
Strings cannot hold characters that have the hyper, super, or alt modifiers.
2.4.8.4 Text Properties in Strings
A string can hold properties for the characters it contains, in addition to the characters themselves. This enables programs that copy text between strings and buffers to copy the text’s properties with no special effort. See Text Properties, for an explanation of what text properties mean. Strings with text properties use a special read and print syntax:
#("characters" property-data...)
where property-data consists of zero or more elements, in groups of three as follows:
beg end plist
The elements beg and end are integers, and together specify a range of indices in the string; plist is the property list for that range. For example,
#("foo bar" 0 3 (face bold) 3 4 nil 4 7 (face italic))
represents a string whose textual contents are ‘foo bar’, in which
the first three characters have a face
property with value
bold
, and the last three have a face
property with value
italic
. (The fourth character has no text properties, so its
property list is nil
. It is not actually necessary to mention
ranges with nil
as the property list, since any characters not
mentioned in any range will default to having no properties.)
2.4.9 Vector Type
A vector is a one-dimensional array of elements of any type. It takes a constant amount of time to access any element of a vector. (In a list, the access time of an element is proportional to the distance of the element from the beginning of the list.)
The printed representation of a vector consists of a left square bracket, the elements, and a right square bracket. This is also the read syntax. Like numbers and strings, vectors are considered constants for evaluation.
[1 "two" (three)] ; A vector of three elements.
⇒ [1 "two" (three)]
See Vectors, for functions that work with vectors.
2.4.10 Char-Table Type
A char-table is a one-dimensional array of elements of any type, indexed by character codes. Char-tables have certain extra features to make them more useful for many jobs that involve assigning information to character codes—for example, a char-table can have a parent to inherit from, a default value, and a small number of extra slots to use for special purposes. A char-table can also specify a single value for a whole character set.
The printed representation of a char-table is like a vector except that there is an extra ‘#^’ at the beginning.1
See Char-Tables, for special functions to operate on char-tables. Uses of char-tables include:
- Case tables (see The Case Table).
- Character category tables (see Categories).
- Display tables (see Display Tables).
- Syntax tables (see Syntax Tables).
2.4.11 Bool-Vector Type
A bool-vector is a one-dimensional array whose elements must
be t
or nil
.
The printed representation of a bool-vector is like a string, except
that it begins with ‘#&’ followed by the length. The string
constant that follows actually specifies the contents of the bool-vector
as a bitmap—each character in the string contains 8 bits, which
specify the next 8 elements of the bool-vector (1 stands for t
,
and 0 for nil
). The least significant bits of the character
correspond to the lowest indices in the bool-vector.
(make-bool-vector 3 t) ⇒ #&3"^G" (make-bool-vector 3 nil) ⇒ #&3"^@"
These results make sense, because the binary code for ‘C-g’ is 111 and ‘C-@’ is the character with code 0.
If the length is not a multiple of 8, the printed representation shows extra elements, but these extras really make no difference. For instance, in the next example, the two bool-vectors are equal, because only the first 3 bits are used:
(equal #&3"\377" #&3"\007") ⇒ t
2.4.12 Hash Table Type
A hash table is a very fast kind of lookup table, somewhat like an alist in that it maps keys to corresponding values, but much faster. The printed representation of a hash table specifies its properties and contents, like this:
(make-hash-table) ⇒ #s(hash-table size 65 test eql rehash-size 1.5 rehash-threshold 0.8125 data ())
See Hash Tables, for more information about hash tables.
2.4.13 Function Type
Lisp functions are executable code, just like functions in other
programming languages. In Lisp, unlike most languages, functions are
also Lisp objects. A non-compiled function in Lisp is a lambda
expression: that is, a list whose first element is the symbol
lambda
(see Lambda Expressions).
In most programming languages, it is impossible to have a function without a name. In Lisp, a function has no intrinsic name. A lambda expression can be called as a function even though it has no name; to emphasize this, we also call it an anonymous function (see Anonymous Functions). A named function in Lisp is just a symbol with a valid function in its function cell (see Defining Functions).
Most of the time, functions are called when their names are written in
Lisp expressions in Lisp programs. However, you can construct or obtain
a function object at run time and then call it with the primitive
functions funcall
and apply
. See Calling Functions.
2.4.14 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.
2.4.15 Primitive Function Type
A primitive function is a function callable from Lisp but written in the C programming language. Primitive functions are also called subrs or built-in functions. (The word “subr” is derived from “subroutine”.) Most primitive functions evaluate all their arguments when they are called. A primitive function that does not evaluate all its arguments is called a special form (see Special Forms).
It does not matter to the caller of a function whether the function is primitive. However, this does matter if you try to redefine a primitive with a function written in Lisp. The reason is that the primitive function may be called directly from C code. Calls to the redefined function from Lisp will use the new definition, but calls from C code may still use the built-in definition. Therefore, we discourage redefinition of primitive functions.
The term function refers to all Emacs functions, whether written in Lisp or C. See Function Type, for information about the functions written in Lisp.
Primitive functions have no read syntax and print in hash notation with the name of the subroutine.
(symbol-function 'car) ; Access the function cell ; of the symbol. ⇒ #<subr car> (subrp (symbol-function 'car)) ; Is this a primitive function? ⇒ t ; Yes.
2.4.16 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 Function Objects.
The printed representation and read syntax for a byte-code function object is like that for a vector, with an additional ‘#’ before the opening ‘[’.
2.4.17 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.
2.4.18 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
.
2.4.19 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.
2.4.20 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.
2.5 Editing Types
The types in the previous section are used for general programming purposes, and most of them are common to most Lisp dialects. Emacs Lisp provides several additional data types for purposes connected with editing.
- Buffer Type
- Marker Type
- Window Type
- Frame Type
- Terminal Type
- Window Configuration Type
- Frame Configuration Type
- Process Type
- Thread Type
- Mutex Type
- Condition Variable Type
- Stream Type
- Keymap Type
- Overlay Type
- Font Type
- Xwidget Type
2.5.1 Buffer Type
A buffer is an object that holds text that can be edited (see Buffers). Most buffers hold the contents of a disk file (see Files) so they can be edited, but some are used for other purposes. Most buffers are also meant to be seen by the user, and therefore displayed, at some time, in a window (see Windows). But a buffer need not be displayed in any window. Each buffer has a designated position called point (see Positions); most editing commands act on the contents of the current buffer in the neighborhood of point. At any time, one buffer is the current buffer.
The contents of a buffer are much like a string, but buffers are not used like strings in Emacs Lisp, and the available operations are different. For example, you can insert text efficiently into an existing buffer, altering the buffer’s contents, whereas inserting text into a string requires concatenating substrings, and the result is an entirely new string object.
Many of the standard Emacs functions manipulate or test the characters in the current buffer; a whole chapter in this manual is devoted to describing these functions (see Text).
Several other data structures are associated with each buffer:
- a local syntax table (see Syntax Tables);
- a local keymap (see Keymaps); and,
- a list of buffer-local variable bindings (see Buffer-Local Variables).
- overlays (see Overlays).
- text properties for the text in the buffer (see Text Properties).
The local keymap and variable list contain entries that individually override global bindings or values. These are used to customize the behavior of programs in different buffers, without actually changing the programs.
A buffer may be indirect, which means it shares the text of another buffer, but presents it differently. See Indirect Buffers.
Buffers have no read syntax. They print in hash notation, showing the buffer name.
(current-buffer) ⇒ #<buffer objects.texi>
2.5.2 Marker Type
A marker denotes a position in a specific buffer. Markers therefore have two components: one for the buffer, and one for the position. Changes in the buffer’s text automatically relocate the position value as necessary to ensure that the marker always points between the same two characters in the buffer.
Markers have no read syntax. They print in hash notation, giving the current character position and the name of the buffer.
(point-marker) ⇒ #<marker at 10779 in objects.texi>
See Markers, for information on how to test, create, copy, and move markers.
2.5.3 Window Type
A window describes the portion of the screen that Emacs uses to display buffers. Every live window (see Basic Concepts of Emacs Windows) has one associated buffer, whose contents appear in that window. By contrast, a given buffer may appear in one window, no window, or several windows. Windows are grouped on the screen into frames; each window belongs to one and only one frame. See Frame Type.
Though many windows may exist simultaneously, at any time one window is designated the selected window (see Selecting Windows). 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 The Current Buffer), but this is not necessarily the case.
Windows have no read syntax. They print in hash notation, giving the window number and the name of the buffer being displayed. The window numbers exist to identify windows uniquely, since the buffer displayed in any given window can change frequently.
(selected-window) ⇒ #<window 1 on objects.texi>
See Windows, for a description of the functions that work on windows.
2.5.4 Frame Type
A frame is a screen area that contains one or more Emacs windows; we also use the term “frame” to refer to the Lisp object that Emacs uses to refer to the screen area.
Frames have no read syntax. They print in hash notation, giving the frame’s title, plus its address in core (useful to identify the frame uniquely).
(selected-frame) ⇒ #<frame emacs@psilocin.gnu.org 0xdac80>
See Frames, for a description of the functions that work on frames.
2.5.5 Terminal Type
A terminal is a device capable of displaying one or more Emacs frames (see Frame Type).
Terminals have no read syntax. They print in hash notation giving the terminal’s ordinal number and its TTY device file name.
(get-device-terminal nil) ⇒ #<terminal 1 on /dev/tty>
2.5.6 Window Configuration Type
A window configuration stores information about the positions, sizes, and contents of the windows in a frame, so you can recreate the same arrangement of windows later.
Window configurations do not have a read syntax; their print syntax looks like ‘#<window-configuration>’. See Window Configurations, for a description of several functions related to window configurations.
2.5.7 Frame Configuration Type
A frame configuration stores information about the positions,
sizes, and contents of the windows in all frames. It is not a
primitive type—it is actually a list whose CAR is
frame-configuration
and whose CDR is an alist. Each alist
element describes one frame, which appears as the CAR of that
element.
See Frame Configurations, for a description of several functions related to frame configurations.
2.5.8 Process Type
The word process usually means a running program. Emacs itself runs in a process of this sort. However, in Emacs Lisp, a process is a Lisp object that designates a subprocess created by the Emacs process. Programs such as shells, GDB, ftp, and compilers, running in subprocesses of Emacs, extend the capabilities of Emacs. An Emacs subprocess takes textual input from Emacs and returns textual output to Emacs for further manipulation. Emacs can also send signals to the subprocess.
Process objects have no read syntax. They print in hash notation, giving the name of the process:
(process-list) ⇒ (#<process shell>)
See Processes, for information about functions that create, delete, return information about, send input or signals to, and receive output from processes.
2.5.9 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:
(all-threads) ⇒ (#<thread 0176fc40>)
2.5.10 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> (make-mutex) ⇒ #<mutex 01c7e4e0>
2.5.11 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>
2.5.12 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 Reading and Printing Lisp Objects, for a description of functions related to streams, including parsing and printing functions.
2.5.13 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.
2.5.14 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.
2.5.15 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 Representation, for a description of these Lisp objects.
2.5.16 Xwidget Type
An xwidget is a special display element, such as a web browser, that can be embedded inside a buffer. Each window that displays an xwidget will also have an xwidget view, which on X-Windows corresponds to a single X window used to display the widget.
Neither of these objects are readable; their print syntaxes look like ‘#<xwidget>’ and ‘#<xwidget-view>’, respectively. See Embedded Native Widgets, for a more detailed description of xwidgets.
2.6 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 Variables Affecting Output.
2.7 Type Predicates
The Emacs Lisp interpreter itself does not perform type checking on the actual arguments passed to functions when they are called. It could not do so, since function arguments in Lisp do not have declared data types, as they do in other programming languages. It is therefore up to the individual function to test whether each actual argument belongs to a type that the function can use.
All built-in functions do check the types of their actual arguments
when appropriate, and signal a wrong-type-argument
error if an
argument is of the wrong type. For example, here is what happens if you
pass an argument to +
that it cannot handle:
(+ 2 'a) error→ Wrong type argument: number-or-marker-p, a
If you want your program to handle different types differently, you must do explicit type checking. The most common way to check the type of an object is to call a type predicate function. Emacs has a type predicate for each type, as well as some predicates for combinations of types.
A type predicate function takes one argument; it returns t
if
the argument belongs to the appropriate type, and nil
otherwise.
Following a general Lisp convention for predicate functions, most type
predicates’ names end with ‘p’.
Here is an example which uses the predicates listp
to check for
a list and symbolp
to check for a symbol.
(defun add-on (x) (cond ((symbolp x) ;; If X is a symbol, put it on LIST. (setq list (cons x list))) ((listp x) ;; If X is a list, add its elements to LIST. (setq list (append x list))) (t ;; We handle only symbols and lists. (error "Invalid argument %s in add-on" x))))
Here is a table of predefined type predicates, in alphabetical order, with references to further information.
atom
See atom.
arrayp
See arrayp.
bignump
See floatp.
bool-vector-p
See bool-vector-p.
booleanp
See booleanp.
bufferp
See bufferp.
byte-code-function-p
See byte-code-function-p.
compiled-function-p
See compiled-function-p.
case-table-p
See case-table-p.
char-or-string-p
See char-or-string-p.
char-table-p
See char-table-p.
commandp
See commandp.
condition-variable-p
See condition-variable-p.
consp
See consp.
custom-variable-p
See custom-variable-p.
fixnump
See floatp.
floatp
See floatp.
fontp
frame-configuration-p
frame-live-p
See frame-live-p.
framep
See framep.
functionp
See functionp.
hash-table-p
See hash-table-p.
integer-or-marker-p
See integer-or-marker-p.
integerp
See integerp.
keymapp
See keymapp.
keywordp
listp
See listp.
markerp
See markerp.
mutexp
See mutexp.
nlistp
See nlistp.
number-or-marker-p
See number-or-marker-p.
numberp
See numberp.
overlayp
See overlayp.
processp
See processp.
recordp
See recordp.
sequencep
See sequencep.
string-or-null-p
See string-or-null-p.
stringp
See stringp.
subrp
See subrp.
symbolp
See symbolp.
syntax-table-p
See syntax-table-p.
threadp
See threadp.
vectorp
See vectorp.
wholenump
See wholenump.
window-configuration-p
window-live-p
See window-live-p.
windowp
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
, orwindow-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 '()) ;
()
isnil
. ⇒ symbol (type-of '(x)) ⇒ cons (type-of (record 'foo)) ⇒ foo
2.8 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, andnil
otherwise.If object1 and object2 are symbols with the same name, they are normally the same object—but see Creating and Interning Symbols for exceptions. For other non-numeric types (e.g., lists, vectors, strings), two arguments with the same contents or elements are not necessarily
eq
to each other: they areeq
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.If object1 and object2 are numbers with differing types or values, then they cannot be the same object and
eq
returnsnil
. If they are fixnums with the same value, then they are the same object andeq
returnst
. If they were computed separately but happen to have the same value and the same non-fixnum numeric type, then they might or might not be the same object, andeq
returnst
ornil
depending on whether the Lisp interpreter created one object or two.(eq 'foo 'foo) ⇒ t
(eq ?A ?A) ⇒ t
(eq 3.0 3.0) ⇒ t or nil ;; Equal floats may or may not be the same object.
(eq (make-string 3 ?A) (make-string 3 ?A)) ⇒ nil
(eq "asdf" "asdf") ⇒ t or nil ;; Equal string constants or may not be the same object.
(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 noteq
. See Creating and Interning 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 eithereq
or noteq
, it should instead use functions that compare object contents such asequal
, 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, andnil
otherwise. Whereaseq
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 areeq
, they areequal
, 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 areequal
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
The
equal
function recursively compares the contents of objects if they are integers, strings, markers, vectors, bool-vectors, byte-code function objects, char-tables, records, or font objects. Other objects are consideredequal
only if they areeq
. For example, two distinct buffers are never consideredequal
, even if their textual contents are the same.
For equal
, equality is defined 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))
Comparing circular lists may therefore cause deep recursion that leads
to an error, and this may result in counterintuitive behavior such as
(equal a b)
returning t
whereas (equal b a)
signals 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
2.9 Mutability
Some Lisp objects should never change. For example, the Lisp
expression "aaa"
yields a string, but you should not change
its contents. And some objects cannot be changed; for example,
although you can create a new number by calculating one, Lisp provides
no operation to change the value of an existing number.
Other Lisp objects are mutable: it is safe to change their values via destructive operations involving side effects. For example, an existing marker can be changed by moving the marker to point to somewhere else.
Although numbers never change and all markers are mutable,
some types have members some of which are mutable and others not. These
types include conses, vectors, and strings. For example,
although "cons"
and (symbol-name 'cons)
both yield
strings that should not be changed, (copy-sequence "cons")
and
(make-string 3 ?a)
both yield mutable strings that can be
changed via later calls to aset
.
A mutable object stops being mutable if it is part of an expression that is evaluated. For example:
(let* ((x (list 0.5)) (y (eval (list 'quote x)))) (setcar x 1.5) ;; The program should not do this. y)
Although the list (0.5)
was mutable when it was created, it should not
have been changed via setcar
because it was given to eval
. The
reverse does not occur: an object that should not be changed never
becomes mutable afterwards.
If a program attempts to change objects that should not be changed, the resulting behavior is undefined: the Lisp interpreter might signal an error, or it might crash or behave unpredictably in other ways.2
When similar constants occur as parts of a program, the Lisp
interpreter might save time or space by reusing existing constants or
their components. For example, (eq "abc" "abc")
returns
t
if the interpreter creates only one instance of the string
literal "abc"
, and returns nil
if it creates two
instances. Lisp programs should be written so that they work
regardless of whether this optimization is in use.
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. Floating-point computations often involve rounding errors, as the numbers have a fixed amount of precision.
- Integer Basics
- Floating-Point Basics
- Type Predicates for Numbers
- Comparison of Numbers
- Numeric Conversions
- Arithmetic Operations
- Rounding Operations
- Bitwise Operations on Integers
- Standard Mathematical Functions
- Random Numbers
3.1 Integer Basics
The Lisp reader reads an integer as a nonempty sequence of decimal digits with optional initial sign and optional final period.
1 ; The integer 1. 1. ; The integer 1. +1 ; Also the integer 1. -1 ; The integer −1. 0 ; The integer 0. -0 ; The integer 0.
The syntax for integers in bases other than 10 consists of ‘#’ followed by a radix indication followed by one or more digits. The radix indications are ‘b’ for binary, ‘o’ for octal, ‘x’ for hex, and ‘radixr’ for radix radix. Thus, ‘#binteger’ reads integer in binary, and ‘#radixrinteger’ reads integer in radix radix. Allowed values of radix run from 2 to 36, and allowed digits are the first radix characters taken from ‘0’–‘9’, ‘A’–‘Z’. Letter case is ignored and there is no initial sign or final period. 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 on Integers), it is often helpful to view the numbers in their binary form.
In binary, the decimal integer 5 looks like this:
…000101
(The ellipsis ‘…’ stands for a conceptually infinite number of bits that match the leading bit; here, an infinite number of 0 bits. Later examples also use this ‘…’ notation.)
The integer −1 looks like this:
…111111
−1 is represented as all 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:
…111011
Many of the functions described in this chapter accept markers for arguments in place of numbers. (See Markers.) Since the actual arguments to such functions may be either numbers or markers, we often give these arguments the name number-or-marker. When the argument value is a marker, its position value is used and its buffer is ignored.
In Emacs Lisp, text characters are represented by integers. Any
integer between zero and the value of (max-char)
, inclusive, is
considered to be valid as a character. See Character Codes.
Integers in Emacs Lisp are not limited to the machine word size.
Under the hood, though, there are two kinds of integers: smaller ones,
called fixnums, and larger ones, called bignums. Although
Emacs Lisp code ordinarily should not depend on whether an integer is
a fixnum or a bignum, older Emacs versions support only fixnums, some
functions in Emacs still accept only fixnums, and older Emacs Lisp
code may have trouble when given bignums. For example, while older
Emacs Lisp code could safely compare integers for numeric equality
with eq
, the presence of bignums means that equality predicates
like eql
and =
should now be used to compare integers.
The range of values for bignums is limited by the amount of main
memory, by machine characteristics such as the size of the word used
to represent a bignum’s exponent, and by the integer-width
variable. These limits are typically much more generous than the
limits for fixnums. A bignum is never numerically equal to a fixnum;
Emacs always represents an integer in fixnum range as a fixnum, not a
bignum.
The range of values for a fixnum 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.
- Variable: most-positive-fixnum ¶
The value of this variable is the greatest “small” 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 numerically least “small” integer that Emacs Lisp can handle. It is negative. Typical values are −2**29 on 32-bit and −2**61 on 64-bit platforms.
- Variable: integer-width ¶
The value of this variable is a nonnegative integer that controls whether Emacs signals a range error when a large integer would be calculated. Integers with absolute values less than 2**n, where n is this variable’s value, do not signal a range error. Attempts to create larger integers typically signal a range error, although there might be no signal if a larger integer can be created cheaply. Setting this variable to a large number can be costly if a computation creates huge integers.
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 almost all computers supported by Emacs, this is IEEE
binary64 floating point format, which is standardized by
IEEE Std
754-2019 and is discussed further in David Goldberg’s paper
“What Every Computer Scientist Should Know About Floating-Point
Arithmetic”. On modern platforms, floating-point operations follow
the IEEE-754 standard closely; however, results are not always rounded
correctly on some systems, notably 32-bit x86.
On some old computer systems, Emacs may not use IEEE floating-point. We know of one such system on which Emacs runs correctly, but does not follow IEEE-754: the VAX running NetBSD using GCC 10.4.0, where the VAX ‘D_Floating’ format is used instead. IBM System/370-derived mainframes and their XL/C compiler are also capable of utilizing a hexadecimal floating point format, but Emacs has not yet been built in such a configuration.
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 a decimal point in a floating-point number that does not have an exponent; ‘1500.’ is an integer, not a floating-point number.
Emacs Lisp treats -0.0
as numerically equal to ordinary zero
with respect to numeric comparisons like =
. 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.
A NaN is never numerically equal to any value, not even to itself.
NaNs carry a sign and a significand, and non-numeric functions treat
two NaNs as equal when their
signs and significands agree. Significands of NaNs are
machine-dependent, as are the digits in their string representation.
NaNs are not available on systems which do not use IEEE floating-point arithmetic; if the read syntax for a NaN is used on a VAX, for example, the reader signals an error.
When NaNs and signed zeros are involved, non-numeric functions like
eql
, equal
, sxhash-eql
, sxhash-equal
and
gethash
determine whether values are indistinguishable, not
whether they are numerically equal. For example, when x and
y are the same NaN, (equal x y)
returns t
whereas
(= x y)
uses numeric comparison and returns nil
;
conversely, (equal 0.0 -0.0)
returns nil
whereas
(= 0.0 -0.0)
returns t
.
Here are read syntaxes for these special floating-point values:
- infinity
‘1.0e+INF’ and ‘-1.0e+INF’
- not-a-number
‘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, if x is finite and nonzero, the value is the logarithm base 2 of |x|, rounded down to an integer. If x is zero or infinite, the value is infinity; if x is a NaN, the value is a NaN.
(logb 10) ⇒ 3 (logb 10.0e20) ⇒ 69 (logb 0) ⇒ -1.0e+INF
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: bignump object ¶
This predicate tests whether its argument is a large integer, and returns
t
if so,nil
otherwise. Unlike small integers, large integers can be=
oreql
even if they are noteq
.
- Function: fixnump object ¶
This predicate tests whether its argument is a small integer, and returns
t
if so,nil
otherwise. Small integers can be compared witheq
.
- 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 fornatnump
.
- 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)
.
3.4 Comparison of Numbers
To test numbers for numerical equality, you should normally use
=
instead of non-numeric comparison predicates like eq
,
eql
and equal
. Distinct floating-point and large
integer objects can be numerically equal. If you use eq
to
compare them, you test whether they are the same object; if you
use eql
or equal
, you test whether their values are
indistinguishable. In contrast, =
uses numeric
comparison, and sometimes returns t
when a non-numeric
comparison would return nil
and vice versa. See Floating-Point Basics.
In Emacs Lisp, if two fixnums are numerically equal, they are the
same Lisp object. That is, eq
is equivalent to =
on
fixnums. It is sometimes convenient to use eq
for comparing
an unknown value with a fixnum, 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 eql
or equal
,
which treat 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))) fuzz-factor)))
- 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)
returnsnil
, but(eql 1.0 1.0)
and(eql 1 1)
both returnt
. This can be used to compare large integers as well as small ones. Floating-point values with the same sign, exponent and fraction areeql
. This differs from numeric comparison:(eql 0.0 -0.0)
returnsnil
and(eql 0.0e+NaN 0.0e+NaN)
returnst
, whereas=
does the opposite.
- Function: /= number-or-marker1 number-or-marker2 ¶
This function tests whether its arguments are numerically equal, and returns
t
if they are not, andnil
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.
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
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.
- 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,foo ⇒ 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). On systems using IEEE-754 floating-point, floating-point division of a nonzero number by zero yields either positive or negative infinity (see Floating-Point Basics); otherwise, anarith-error
is signaled as usual.
- 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 anarith-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. Forfloor
, see Numeric Conversions.
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.
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). Conceptually the bit sequence is infinite on the left, with the most-significant bits being all zeros or all ones. 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: ash integer1 count ¶
-
ash
(arithmetic shift) shifts the bits in integer1 to the left count places, or to the right if count is negative. Left shifts introduce zero bits on the right; right shifts discard the rightmost bits. Considered as an integer operation,ash
multiplies integer1 by 2**count, and then converts the result to an integer by rounding downward, toward minus infinity.Here are examples of
ash
, shifting a pattern of bits one place to the left and to the right. These examples show only the low-order bits of the binary pattern; leading bits all agree with the highest-order bit shown. As you can see, shifting left by one is equivalent to multiplying by two, whereas shifting right by one is equivalent to dividing by two and then rounding toward minus infinity.(ash 7 1) ⇒ 14 ;; Decimal 7 becomes decimal 14. …000111 ⇒ …001110
(ash 7 -1) ⇒ 3 …000111 ⇒ …000011
(ash -7 1) ⇒ -14 …111001 ⇒ …110010
(ash -7 -1) ⇒ -4 …111001 ⇒ …111100
Here are examples of shifting left or right by two bits:
; binary values (ash 5 2) ; 5 = …000101 ⇒ 20 ; = …010100 (ash -5 2) ; -5 = …111011 ⇒ -20 ; = …101100
(ash 5 -2) ⇒ 1 ; = …000001
(ash -5 -2) ⇒ -2 ; = …111110
- 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, then integer1 must be either a fixnum or a positive bignum, andlsh
treats a negative fixnum as if it were unsigned by subtracting twicemost-negative-fixnum
before shifting, producing a nonnegative result. This quirky behavior dates back to when Emacs supported only fixnums; nowadaysash
is a better choice.As
lsh
behaves likeash
except when integer1 and count1 are both negative, the following examples focus on these exceptional cases. These examples assume 30-bit fixnums.; binary values (ash -7 -1) ; -7 = …111111111111111111111111111001 ⇒ -4 ; = …111111111111111111111111111100 (lsh -7 -1) ⇒ 536870908 ; = …011111111111111111111111111100
(ash -5 -2) ; -5 = …111111111111111111111111111011 ⇒ -2 ; = …111111111111111111111111111110 (lsh -5 -2) ⇒ 268435454 ; = …001111111111111111111111111110
- 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.
Therefore,
(logand 13 12) ⇒ 12
If
logand
is not passed any argument, it returns a value of −1. This number is an identity element forlogand
because its binary representation consists entirely of ones. Iflogand
is passed just one argument, it returns that argument.; binary values (logand 14 13) ; 14 = …001110 ; 13 = …001101 ⇒ 12 ; 12 = …001100
(logand 14 13 4) ; 14 = …001110 ; 13 = …001101 ; 4 = …000100 ⇒ 4 ; 4 = …000100
(logand) ⇒ -1 ; -1 = …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.; binary values (logior 12 5) ; 12 = …001100 ; 5 = …000101 ⇒ 13 ; 13 = …001101
(logior 12 5 7) ; 12 = …001100 ; 5 = …000101 ; 7 = …000111 ⇒ 15 ; 15 = …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.; binary values (logxor 12 5) ; 12 = …001100 ; 5 = …000101 ⇒ 9 ; 9 = …001001
(logxor 12 5 7) ; 12 = …001100 ; 5 = …000101 ; 7 = …000111 ⇒ 14 ; 14 = …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. The result equals −1 − integer.
(lognot 5) ⇒ -6 ;; 5 = …000101 ;; becomes ;; -6 = …111010
- Function: logcount integer ¶
This function returns the Hamming weight of integer: the number of ones in the binary representation of integer. If integer is negative, it returns the number of zero bits in its two’s complement binary representation. The result is always nonnegative.
(logcount 43) ; 43 = …000101011 ⇒ 4 (logcount -43) ; -43 = …111010101 ⇒ 3
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 theX
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 nonnegative, the result is an integer; in this case, overflow signals an error, 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…).
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.
The random seed is typically initialized from system entropy;
however, on obsolescent platforms lacking entropy pools,
the seed is taken from less-random volatile data such as the current time.
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 fixnum, i.e., any integer from
most-negative-fixnum
throughmost-positive-fixnum
(see Integer Basics).If limit is a string, it means to choose a new seed based on the string’s contents. This causes later calls to
random
to return a reproducible sequence of results.If limit is
t
, it means to choose a new seed as if Emacs were restarting. This causes later calls torandom
to return an unpredictable sequence of results.
If you need a random nonce for cryptographic purposes, using
random
is typically not the best approach, for several reasons:
- Although you can use
(random t)
to consult system entropy, doing so can adversely affect other parts of your program that benefit from reproducible results. - The system-dependent pseudo-random number generator (PRNG) used by
random
is not necessarily suitable for cryptography. - A call to
(random t)
does not give direct access to system entropy; the entropy is passed through the system-dependent PRNG, thus possibly biasing the results. - On typical platforms the random seed contains only 32 bits, which is typically narrower than an Emacs fixnum, and is not nearly enough for cryptographic purposes.
- A
(random t)
call leaves information about the nonce scattered about Emacs’s internal state, increasing the size of the internal attack surface. - On obsolescent platforms lacking entropy pools,
(random t)
is seeded from a cryptographically weak source.
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 Putting Keyboard Events in Strings, for special considerations for strings of keyboard character events.
- String and Character Basics
- Predicates for Strings
- Creating Strings
- Modifying Strings
- Comparison of Characters and Strings
- Conversion of Characters and Strings
- Formatting Strings
- Custom Format Strings
- Case Conversion in Lisp
- The Case Table
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, and 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, and Vectors. For example, you can access
individual characters in a string using the function aref
(see Functions that Operate on Arrays).
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 Regular Expression Searching). The functions match-string
(see Simple Match Data Access) and replace-match
(see Replacing the Text that Matched) 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.
Also, 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.
4.2 Predicates for Strings
For more information about general sequence and array predicates, see Sequences, Arrays, and 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 ornil
. It returnsnil
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.
4.3 Creating Strings
The following functions create strings, either from scratch, or by
putting strings together, or by taking them apart. (For functions
that create strings based on the modified contents of other strings,
like string-replace
and replace-regexp-in-string
, see
Search and Replace.)
- Function: make-string count character &optional multibyte ¶
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) ⇒ ""
Normally, if character is an ASCII character, the result is a unibyte string. But if the optional argument multibyte is non-
nil
, the function will produce a multibyte string instead. This is useful when you later need to concatenate the result with non-ASCII strings or replace some of its characters with non-ASCII characters.Other functions to compare with this one include
make-vector
(see Vectors) andmake-list
(see Building Cons Cells and 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 Sequences).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 nornil
. Anargs-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 Examining 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 ornil
, 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 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." (concat) ⇒ ""This function does not always allocate a new string. Callers are advised not rely on the result being a new string nor on it being
eq
to an existing string.In particular, mutating the returned value may inadvertently change another string, alter a constant string in the program, or even raise an error. To obtain a string that you can safely mutate, use
copy-sequence
on the result.For information about other concatenation functions, see the description of
mapconcat
in Mapping Functions,vconcat
in Functions for Vectors, andappend
in Building Cons Cells and 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 ofsplit-string-default-separators
and the function behaves as if omit-nulls weret
.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 ist
, 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
orstart-process
, see split-string-and-unquote.Examples:
(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 " split-string-default-separators) ⇒ ("" "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]+"
.
- Function: string-clean-whitespace string ¶
Clean up the whitespace in string by collapsing stretches of whitespace to a single space character, as well as removing all whitespace from the start and the end of string.
- Function: string-trim-left string &optional regexp ¶
Remove the leading text that matches regexp from string. regexp defaults to ‘[ \t\n\r]+’.
- Function: string-trim-right string &optional regexp ¶
Remove the trailing text that matches regexp from string. regexp defaults to ‘[ \t\n\r]+’.
- Function: string-trim string &optional trim-left trim-right ¶
Remove the leading text that matches trim-left and trailing text that matches trim-right from string. Both regexps default to ‘[ \t\n\r]+’.
- Function: string-fill string length ¶
Attempt to Word-wrap string so that no lines are longer than length. Filling is done on whitespace boundaries only. If there are individual words that are longer than length, these will not be shortened.
- Function: string-limit string length &optional end coding-system ¶
If string is shorter than length characters, string is returned as is. Otherwise, return a substring of string consisting of the first length characters. If the optional end parameter is given, return a string of the length last characters instead.
If coding-system is non-
nil
, string will be encoded before limiting, and the result will be a unibyte string that’s shorter thanlength
bytes. If string contains characters that are encoded into several bytes (for instance, when usingutf-8
), the resulting unibyte string is never truncated in the middle of a character representation.This function measures the string length in characters or bytes, and thus is generally inappropriate if you need to shorten strings for display purposes; use
truncate-string-to-width
orwindow-text-pixel-size
orstring-glyph-split
instead (see Size of Displayed Text).
- Function: string-lines string &optional omit-nulls keep-newlines ¶
Split string into a list of strings on newline boundaries. If the optional argument omit-nulls is non-
nil
, remove empty lines from the results. If the optional argument keep-newlines is non-nil
, don’t remove the trailing newlines from the result strings.
- Function: string-pad string length &optional padding start ¶
Pad string to be of the given length using padding as the padding character. padding defaults to the space character. If string is longer than length, no padding is done. If start is
nil
or omitted, the padding is appended to the characters of string, and if it’s non-nil
, the padding is prepended to string’s characters.
- Function: string-chop-newline string ¶
Remove the final newline, if any, from string.
4.4 Modifying Strings
You can alter the contents of a mutable string via operations described in this section. See Mutability.
The most basic way to alter the contents of an existing string is with
aset
(see Functions that Operate on Arrays). (aset string idx char)
stores char into string at character
index idx. It will automatically convert a pure-ASCII
string to a multibyte string (see Text Representations) if
needed, but we recommend to always make sure string is multibyte
(e.g., by using string-to-multibyte
, see Converting Text Representations), if char is a non-ASCII character, not
a raw byte.
A more powerful function is store-substring
:
- Function: store-substring string idx obj ¶
This function alters part of the contents of the specified string, by storing obj starting at character index idx. The argument obj may be either a character (in which case the function behaves exactly as
aset
) or a (smaller) string. If obj is a multibyte string, we recommend to make sure string is also multibyte, even if it’s pure-ASCII.Since it is impossible to change the number of characters in an existing string, it is an error if obj consists of more characters than would fit in string starting at character index idx.
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.
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 ifcase-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 ofcase-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; useequal-including-properties
if you need to distinguish between strings that differ only in their text properties. However, unlikeequal
, 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
A unibyte and a multibyte string are equal in the sense of
string=
if and only if they contain the same sequence of character codes all being in the range 0–127 (ASCII). See Text Representations.
- Function: string-equal string1 string2 ¶
string-equal
is another name forstring=
.
- Function: string-equal-ignore-case string1 string2 ¶
string-equal-ignore-case
compares strings ignoring case differences, likechar-equal
whencase-fold-search
ist
.
- Function: string-collate-equalp string1 string2 &optional locale ignore-case ¶
This function returns
t
if string1 and string2 are equal with respect to the collation rules of the specified locale, which defaults to your current system locale. A collation rule is not only determined by the lexicographic order of the characters contained in string1 and string2, but also by further rules about relations between these characters. Usually, it is defined by the locale environment with which Emacs is running and by the Standard C library against which Emacs was linked3.For example, characters with different code points but the same meaning, like different grave accent Unicode characters, might, in some locales, be considered as equal:
(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 compared case-insensitively, by converting them to lower-case. However, if the underlying system library doesn’t provide locale-specific collation rules, this function falls back tostring-equal
, in which case the ignore-case argument is ignored, and the comparison will always be case-sensitive.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 returnsnil
. If the two strings match entirely, the value isnil
.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 isnil
. 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 forstring<
.
- 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 of the specified locale, which defaults to your current system locale. A collation order is not only determined by the lexicographic order of the characters contained in string1 and string2, but also by further rules about relations between these characters. Usually, it is defined by the locale environment with which Emacs is running, and by the Standard C library against which Emacs was linked.For example, punctuation and whitespace characters might be ignored for sorting (see Sequences):
(sort (list "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"
letsstring-collate-lessp
behave likestring-lessp
:(sort (list "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 compared case-insensitively, by converting them to lower-case. However, if the underlying system library doesn’t provide locale-specific collation rules, this function falls back tostring-lessp
, in which case the ignore-case argument is ignored, and the comparison will always be case-sensitive.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: string-search needle haystack &optional start-pos ¶
Return the position of the first instance of needle in haystack, both of which are strings. If start-pos is non-
nil
, start searching from that position in needle. Returnnil
if no match was found. This function only considers the characters in the strings when doing the comparison; text properties are ignored. Matching is always case-sensitive.
- 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, whilenil
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, using the current buffer’s case-table (see The Case Table), 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: string-distance string1 string2 &optional bytecompare ¶
This function returns the Levenshtein distance between the source string string1 and the target string string2. The Levenshtein distance is the number of single-character changes—deletions, insertions, or replacements—required to transform the source string into the target string; it is one possible definition of the edit distance between strings.
Letter-case of the strings is significant for the computed distance, but their text properties are ignored. If the optional argument bytecompare is non-
nil
, the function calculates the distance in terms of bytes instead of characters. The byte-wise comparison uses the internal Emacs representation of characters, so it will produce inaccurate results for multibyte strings that include raw bytes (see Text Representations); make the strings unibyte by encoding them (see Explicit Encoding and Decoding) if you need accurate results with raw bytes.
- 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 usingcompare-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. Unlikeassoc
, 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
Regular Expression Searching.
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 Text 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 isnil
, 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.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 is 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:
concat
This function converts a vector or a list into a string. See Creating Strings.
vconcat
This function converts a string into a vector. See Functions for Vectors.
append
This function converts a string into a list. See Building Cons Cells and Lists.
byte-to-string
This function converts a byte of character data into a unibyte string. See Converting Text Representations.
4.7 Formatting Strings
Formatting means constructing a string by substituting computed values at various places in a constant string. This constant string controls how the other values are printed, as well as where they appear; it is called a format string.
Formatting is often useful for computing messages to be displayed. In
fact, the functions message
and error
provide the same
formatting feature described here; they differ from format-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 yieldt
.
- 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 oftext-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:
- ‘%s’
Replace the specification with the printed representation of the object, made without quoting (that is, using
princ
, notprin1
—see Output Functions). Thus, strings are represented by their contents alone, with no ‘"’ characters, and symbols appear without ‘\’ characters.If the object is a string, its text properties are copied into the output. The text properties of the ‘%s’ itself are also copied, but those of the object take priority.
- ‘%S’
Replace the specification with the printed representation of the object, made with quoting (that is, using
prin1
—see Output Functions). Thus, strings are enclosed in ‘"’ characters, and ‘\’ characters appear where necessary before special characters.- ‘%o’ ¶
Replace the specification with the base-eight representation of an integer. Negative integers are formatted in a platform-dependent way. The object can also be a floating-point number that is formatted as an integer, dropping any fraction.
- ‘%d’
Replace the specification with the base-ten representation of a signed integer. The object can also be a floating-point number that is formatted as an integer, dropping any fraction.
- ‘%x’ ¶
- ‘%X’
Replace the specification with the base-sixteen representation of an integer. Negative integers are formatted in a platform-dependent way. ‘%x’ uses lower case and ‘%X’ uses upper case. The object can also be a floating-point number that is formatted as an integer, dropping any fraction.
- ‘%c’
Replace the specification with the character which is the value given.
- ‘%e’
Replace the specification with the exponential notation for a floating-point number.
- ‘%f’
Replace the specification with the decimal-point notation for a floating-point number.
- ‘%g’
Replace the specification with notation for a floating-point number, using either exponential notation or decimal-point notation. 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." (format-message "The name of this buffer is ‘%s’." (buffer-name)) ⇒ "The name of this buffer is ‘strings.texi’." (format-message "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 nonnegative number, so that it always has a sign. A space character as flag inserts a space before a nonnegative number. (Otherwise, nonnegative numbers start with the first digit.) These flags are useful for ensuring that nonnegative 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 nonzero results 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.
If you plan to use read
later on the formatted string to
retrieve a copy of the formatted value, use a specification that lets
read
reconstruct the value. To format numbers in this
reversible way you can use ‘%s’ and ‘%S’, to format just
integers you can also use ‘%d’, and to format just nonnegative
integers you can also use ‘#x%x’ and ‘#o%o’. Other formats
may be problematic; for example, ‘%d’ and ‘%g’ can mishandle
NaNs and can lose precision and type, and ‘#x%x’ and ‘#o%o’
can mishandle negative integers. See Input Functions.
The functions described in this section accept a fixed set of
specification characters. The next section describes a function
format-spec
which can accept custom specification characters,
such as ‘%a’ or ‘%z’.
4.8 Custom Format Strings
Sometimes it is useful to allow users and Lisp programs alike to
control how certain text is generated via custom format control
strings. For example, a format string could control how to display
someone’s forename, surname, and email address. Using the function
format
described in the previous section, the format string
could be something like "%s %s <%s>"
. This approach
quickly becomes impractical, however, as it can be unclear which
specification character corresponds to which piece of information.
A more convenient format string for such cases would be something like
"%f %l <%e>"
, where each specification character carries
more semantic information and can easily be rearranged relative to
other specification characters, making such format strings more easily
customizable by the user.
The function format-spec
described in this section performs a
similar function to format
, except it operates on format
control strings that use arbitrary specification characters.
- Function: format-spec template spec-alist &optional ignore-missing split ¶
This function returns a string produced from the format string template according to conversions specified in spec-alist, which is an alist (see Association Lists) of the form
(letter . replacement)
. Each specification%letter
in template will be replaced by replacement when formatting the resulting string.The characters in template, 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 their replacements.
Using an alist to specify conversions gives rise to some useful properties:
- If spec-alist contains more unique letter keys than there are unique specification characters in template, the unused keys are simply ignored.
- If spec-alist contains more than one association with the same letter, the closest one to the start of the list is used.
- If template contains the same specification character more than once, then the same replacement found in spec-alist is used as a basis for all of that character’s substitutions.
- The order of specifications in template need not correspond to the order of associations in spec-alist.
REPLACEMENT can also be a function taking no arguments, and returning a string to be used for the replacement. It will only be called when the corresponding LETTER is used in the TEMPLATE. This is useful, for example, to avoid prompting for input unless it is needed.
The optional argument ignore-missing indicates how to handle specification characters in template that are not found in spec-alist. If it is
nil
or omitted, the function signals an error; if it isignore
, those format specifications are left verbatim in the output, including their text properties, if any; if it isdelete
, those format specifications are removed from the output; any other non-nil
value is handled likeignore
, but any occurrences of ‘%%’ are also left verbatim in the output.If the optional argument split is non-
nil
, instead of returning a single string,format-spec
will split the result into a list of strings, based on where the substitutions were performed. For instance:(format-spec "foo %b bar" '((?b . "zot")) nil t) ⇒ ("foo " "zot" " bar")
The syntax of format specifications accepted by format-spec
is
similar, but not identical, to that accepted by format
. In
both cases, a format specification is a sequence of characters
beginning with ‘%’ and ending with an alphabetic letter such as
‘s’.
Unlike format
, which assigns specific meanings to a fixed set
of specification characters, format-spec
accepts arbitrary
specification characters and treats them all equally. For example:
(setq my-site-info (list (cons ?s system-name) (cons ?t (symbol-name system-type)) (cons ?c system-configuration) (cons ?v emacs-version) (cons ?e invocation-name) (cons ?p (number-to-string (emacs-pid))) (cons ?a user-mail-address) (cons ?n user-full-name))) (format-spec "%e %v (%c)" my-site-info) ⇒ "emacs 27.1 (x86_64-pc-linux-gnu)" (format-spec "%n <%a>" my-site-info) ⇒ "Emacs Developers <emacs-devel@gnu.org>"
A format specification can include any number of the following flag characters immediately after the ‘%’ to modify aspects of the substitution.
- ‘0’
This flag causes any padding specified by the width to consist of ‘0’ characters instead of spaces.
- ‘-’
This flag causes any padding specified by the width to be inserted on the right rather than the left.
- ‘<’
This flag causes the substitution to be truncated on the left to the given width and precision, if specified.
- ‘>’
This flag causes the substitution to be truncated on the right to the given width, if specified.
- ‘^’
This flag converts the substituted text to upper case (see Case Conversion in Lisp).
- ‘_ (underscore)’
This flag converts the substituted text to lower case (see Case Conversion in Lisp).
The result of using contradictory flags (for instance, both upper and lower case) is undefined.
As is the case with format
, a format specification can include
a width, which is a decimal number that appears after any flags, and a
precision, which is a decimal-point ‘.’ followed by a decimal
number that appears after any flags and width.
If a substitution contains fewer characters than its specified width, it is padded on the left:
(format-spec "%8a is padded on the left with spaces" '((?a . "alpha"))) ⇒ " alpha is padded on the left with spaces"
If a substitution contains more characters than its specified precision, it is truncated on the right:
(format-spec "%.2a is truncated on the right" '((?a . "alpha"))) ⇒ "al is truncated on the right"
Here is a more complicated example that combines several aforementioned features:
(setq my-battery-info (list (cons ?p "73") ; Percentage (cons ?L "Battery") ; Status (cons ?t "2:23") ; Remaining time (cons ?c "24330") ; Capacity (cons ?r "10.6"))) ; Rate of discharge (format-spec "%>^-3L : %3p%% (%05t left)" my-battery-info) ⇒ "BAT : 73% (02:23 left)" (format-spec "%>^-3L : %3p%% (%05t left)" (cons (cons ?L "AC") my-battery-info)) ⇒ "AC : 73% (02:23 left)"
As the examples in this section illustrate, format-spec
is
often used for selectively formatting an assortment of different
pieces of information. This is useful in programs that provide
user-customizable format strings, as the user can choose to format
with a regular syntax and in any desired order only a subset of the
information that the program makes available.
4.9 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 The Case Table).
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 Table of Syntax Classes).
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 Table of Syntax Classes).
When the argument to
upcase-initials
is a character,upcase-initials
has the same result asupcase
.(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 Comparison of Characters and Strings, for functions that compare strings; some of them ignore case differences, or can optionally ignore case differences.
4.10 The Case Table
You can customize case conversion by installing a special case table. A case table specifies the mapping between upper case and lower case letters. It affects both the case conversion functions for Lisp objects (see the previous section) and those that apply to text in the buffer (see Case Changes). Each buffer has a case table; there is also a standard case table which is used to initialize the case table of new buffers.
A case table is a char-table (see Char-Tables) whose subtype is
case-table
. This char-table maps each character into the
corresponding lower case character. It has three extra slots, which
hold related tables:
- upcase
The upcase table maps each character into the corresponding upper case character.
- canonicalize
The canonicalize table maps all of a set of case-related characters into a particular member of that set.
- equivalences
The equivalences table maps each one of a set of case-related characters into the next character in that set.
In simple cases, all you need to specify is the mapping to lower-case; the three related tables will be calculated automatically from that one.
For some languages, upper and lower case letters are not in one-to-one correspondence. There may be two different lower case letters with the same upper case equivalent. In these cases, you need to specify the maps for both lower case and upper case.
The extra table canonicalize maps each character to a canonical equivalent; any two characters that are related by case-conversion have the same canonical equivalent character. For example, since ‘a’ and ‘A’ are related by case-conversion, they should have the same canonical equivalent character (which should be either ‘a’ for both of them, or ‘A’ for both of them).
The extra table equivalences is a map that cyclically permutes each equivalence class (of characters with the same canonical equivalent). (For ordinary ASCII, this would map ‘a’ into ‘A’ and ‘A’ into ‘a’, and likewise for each set of equivalent characters.)
When constructing a case table, you can provide nil
for
canonicalize; then Emacs fills in this slot from the lower case
and upper case mappings. You can also provide nil
for
equivalences; then Emacs fills in this slot from
canonicalize. In a case table that is actually in use, those
components are non-nil
. Do not try to specify
equivalences without also specifying canonicalize.
Here are the functions for working with case tables:
- 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 viathrow
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.
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.
- Lists and Cons Cells
- Predicates on Lists
- Accessing Elements of Lists
- Building Cons Cells and Lists
- Modifying List Variables
- Modifying Existing List Structure
- Using Lists as Sets
- Association Lists
- Property 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 and List Types). 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 and List Types.) 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 list4. 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. You
can use the function proper-list-p
, described in the next
section (see proper-list-p), to determine
whether a list is a proper one.
Because most cons cells are used as part of lists, we refer to any structure made out of cons cells as a list structure.
5.3 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 returnsnil
. Therefore, any list is a valid argument. An error is signaled if the argument is not a cons cell ornil
.(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 returnsnil
; therefore, any list is a valid argument. An error is signaled if the argument is not a cons cell ornil
.(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 tocar
, which signals an error if object is not a list.(car-safe object) ≡ (let ((x object)) (if (consp x) (car x) nil))
- 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 tocdr
, which signals an error if object is not a list.(cdr-safe object) ≡ (let ((x object)) (if (consp x) (cdr x) nil))
- 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)))
.x ⇒ (a b c) (pop x) ⇒ a x ⇒ (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 Modifying 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 Sequences.
- 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
returnsnil
.(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: take n list ¶
This function returns the n first elements of list. Essentially, it returns the part of list that
nthcdr
skips.take
returns list if shorter than n elements; it returnsnil
if n is zero or negative.(take 3 '(a b c d)) ⇒ (a b c)
(take 10 '(a b c d)) ⇒ (a b c d)
(take 0 '(a b c d)) ⇒ nil
- Function: ntake n list ¶
This is a version of
take
that works by destructively modifying the list structure of the argument. That makes it faster, but the original value of list may be lost.ntake
returns list unmodified if shorter than n elements; it returnsnil
if n is zero or negative. Otherwise, it returns list truncated to its first n elements.This means that it is usually a good idea to use the return value and not just rely on the truncation effect unless n is known to be positive.
- 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 Sequences.
- 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 in Common Lisp
Extensions.
- 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 thecdr
of the appropriate element, rather than making a copy of the list.
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. 5 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 namedlist
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)
(list) ⇒ nil
- Function: make-list length object ¶
This function creates a list of length elements, in which each element is object. Compare
make-list
withmake-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 Functions that Rearrange Lists, 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 notnil
as required in a proper list (see Lists and 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)
trees ⇒ (pine oak) more-trees ⇒ (maple birch pine oak)
(eq trees (cdr (cdr more-trees))) ⇒ t
You can see how append
works by looking at a box diagram. The
variable trees
is set to the list (pine oak)
and then the
variable more-trees
is set to the list (maple birch pine
oak)
. However, the variable trees
continues to refer to the
original list:
more-trees trees | | | --- --- --- --- -> --- --- --- --- --> | | |--> | | |--> | | |--> | | |--> nil --- --- --- --- --- --- --- --- | | | | | | | | --> maple -->birch --> pine --> oak
An empty sequence contributes nothing to the value returned by
append
. As a consequence of this, a final nil
argument
forces a copy of the previous argument:
trees ⇒ (pine oak)
(setq wood (append trees nil)) ⇒ (pine oak)
wood ⇒ (pine oak)
(eq wood trees) ⇒ nil
This once was the usual way to copy a list, before the function
copy-sequence
was invented. See Sequences, Arrays, and Vectors.
Here we show the use of vectors and strings as arguments to append
:
(append [a b] "cd" nil) ⇒ (a b 99 100)
With the help of apply
(see Calling Functions), we can append
all the lists in a list of lists:
(apply 'append '((a b c) nil (x y z) nil)) ⇒ (a b c x y z)
If no sequences are given, nil
is returned:
(append) ⇒ nil
Here are some examples where the final argument is not a list:
(append '(x y) 'z) ⇒ (x y . z) (append '(x y) [z]) ⇒ (x y . [z])
The second example shows that when the final argument is a sequence but not a list, the sequence’s elements do not become elements of the resulting list. Instead, the sequence becomes the final CDR, like any other non-list final argument.
- 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: flatten-tree tree ¶
This function returns a “flattened” copy of tree, that is, a list containing all the non-
nil
terminal nodes, or leaves, of the tree of cons cells rooted at tree. Leaves in the returned list are in the same order as in tree.
(flatten-tree '(1 (2 . 3) nil (4 5 (6)) 7)) ⇒(1 2 3 4 5 6 7)
- Function: ensure-list object ¶
This function returns object as a list. If object is already a list, the function returns it; otherwise, the function returns a one-element list containing object.
This is usually useful if you have a variable that may or may not be a list, and you can then say, for instance:
(dolist (elem (ensure-list foo)) (princ elem))
- Function: number-sequence from &optional to separation ¶
This function 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 isnil
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)
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) l ⇒ (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 Accessing Elements of Lists.
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 isnil
, it usesequal
.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, likeset
and unlikesetq
. Quote the argument yourself if that is what you want.Do not use this function when symbol refers to a lexical variable.
Here’s a scenario showing how to use add-to-list
:
(setq foo '(a b)) ⇒ (a b) (add-to-list 'foo 'c) ;; Addc
. ⇒ (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:
(if (member value var) 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, likeset
and unlikesetq
. Quote the argument yourself if necessary.The ordering information is stored in a hash table on symbol’s
list-order
property. symbol cannot refer to a lexical variable.
Here’s a scenario showing how to use add-to-ordered-list
:
(setq foo '()) ⇒ nil (add-to-ordered-list 'foo 'a 1) ;; Adda
. ⇒ (a) (add-to-ordered-list 'foo 'c 3) ;; Addc
. ⇒ (a c) (add-to-ordered-list 'foo 'b 2) ;; Addb
. ⇒ (a b c) (add-to-ordered-list 'foo 'b 4) ;; Moveb
. ⇒ (a c b) (add-to-ordered-list 'foo 'd) ;; Appendd
. ⇒ (a c b d) (add-to-ordered-list 'foo 'e) ;; Adde
. ⇒ (a c b e d) foo ;;foo
was changed. ⇒ (a c b e d)
5.6 Modifying Existing List Structure
You can modify the CAR and CDR contents of a cons cell with the
primitives setcar
and setcdr
. These are destructive
operations because they change existing list structure.
Destructive operations should be applied only to mutable lists,
that is, lists constructed via cons
, list
or similar
operations. Lists created by quoting are part of the program and
should not be changed by destructive operations. See Mutability.
Common Lisp note: Common Lisp uses functions
rplaca
andrplacd
to alter list structure; they change structure the same way assetcar
andsetcdr
, but the Common Lisp functions return the cons cell whilesetcar
andsetcdr
return the new CAR or CDR.
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 (list 1 2)) ⇒ (1 2)
(setcar x 4) ⇒ 4
x ⇒ (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 (list 'a 'b 'c))
⇒ (a b c)
(setq x2 (cons 'z (cdr x1)))
⇒ (z b c)
;; Replace the CAR of a shared link. (setcar (cdr x1) 'foo) ⇒ foo x1 ; Both lists are changed. ⇒ (a foo c) x2 ⇒ (z foo c)
;; Replace the CAR of a link that is not shared. (setcar x1 'baz) ⇒ baz x1 ; Only one list is changed. ⇒ (baz foo c) x2 ⇒ (z foo c)
Here is a graphical depiction of the shared structure of the two lists
in the variables x1
and x2
, showing why replacing b
changes them both:
--- --- --- --- --- --- x1---> | | |----> | | |--> | | |--> nil --- --- --- --- --- --- | --> | | | | | | --> a | --> b --> c | --- --- | x2--> | | |-- --- --- | | --> z
Here is an alternative form of box diagram, showing the same relationship:
x1: -------------- -------------- -------------- | car | cdr | | car | cdr | | car | cdr | | a | o------->| b | o------->| c | nil | | | | -->| | | | | | -------------- | -------------- -------------- | x2: | -------------- | | car | cdr | | | z | o---- | | | --------------
5.6.2 Altering the CDR of a List
The lowest-level primitive for modifying a CDR is setcdr
:
- 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 (list 1 2 3)) ⇒ (1 2 3)
(setcdr x '(4)) ⇒ (4)
x ⇒ (1 4)
You can delete elements from the middle of a list by altering the
CDRs of the cons cells in the list. For example, here we delete
the second element, b
, from the list (a b c)
, by changing
the CDR of the first cons cell:
(setq x1 (list 'a 'b 'c)) ⇒ (a b c) (setcdr x1 (cdr (cdr x1))) ⇒ (c) x1 ⇒ (a c)
Here is the result in box notation:
-------------------- | | -------------- | -------------- | -------------- | car | cdr | | | car | cdr | -->| car | cdr | | a | o----- | b | o-------->| c | nil | | | | | | | | | | -------------- -------------- --------------
The second cons cell, which previously held the element b
, still
exists and its CAR is still b
, but it no longer forms part
of this list.
It is equally easy to insert a new element by changing CDRs:
(setq x1 (list 'a 'b 'c)) ⇒ (a b c) (setcdr x1 (cons 'd (cdr x1))) ⇒ (d b c) x1 ⇒ (a d b c)
Here is this result in box notation:
-------------- ------------- ------------- | car | cdr | | car | cdr | | car | cdr | | a | o | -->| b | o------->| c | nil | | | | | | | | | | | | --------- | -- | ------------- ------------- | | ----- -------- | | | --------------- | | | car | cdr | | -->| d | o------ | | | ---------------
5.6.3 Functions that Rearrange Lists
Here are some functions that rearrange lists destructively by modifying the CDRs of their component cons cells. 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 Using Lists as Sets, 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 Cons Cells and 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 (list 1 2 3)) ⇒ (1 2 3)
(nconc x '(4 5)) ⇒ (1 2 3 4 5)
x ⇒ (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 (list 1 2 3)) ⇒ (1 2 3)
(nconc x 'z) ⇒ (1 2 3 . z)
x ⇒ (1 2 3 . z)
However, the other arguments (all but the last) should be mutable lists.
A common pitfall is to use a constant list as a non-last argument to
nconc
. If you do this, the resulting behavior is undefined (see Self-Evaluating Forms). It is possible that your program will change each time you run it! Here is what might happen (though this is not guaranteed to happen):(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 '(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 '(foo 1 2 3 4) x))
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
or seq-uniq
.
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) andintersection
for set operations. In Emacs Lisp, variants of these facilities are provided by the cl-lib library. See Lists as Sets in Common Lisp Extensions.
- 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 returnsnil
. The letter ‘q’ inmemq
says that it useseq
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))) ; The two
(2)
s need not beeq
. ⇒ Unspecified; might benil
or((2))
.
- Function: delq object list ¶
-
This function destructively removes all elements
eq
to object from list, and returns the resulting list. The letter ‘q’ indelq
says that it useseq
to compare object against the elements of the list, likememq
andremq
.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 Altering the CDR of a List).
(setq sample-list (list 'a 'b 'c '(4))) ⇒ (a b c (4))
(delq 'a sample-list) ⇒ (b c (4))
sample-list ⇒ (a b c (4))
(delq 'c sample-list) ⇒ (a b (4))
sample-list ⇒ (a b (4))
Note that (delq 'c sample-list)
modifies sample-list
to
splice out the third element, but (delq 'a sample-list)
does not
splice anything—it just returns a shorter list. Don’t assume that a
variable which formerly held the argument list now has fewer
elements, or that it still holds the original list! Instead, save the
result of delq
and use that. Most often we store the result back
into the variable that held the original list:
(setq flowers (delq 'rose flowers))
In the following example, the (list 4)
that delq
attempts to match
and the (4)
in the sample-list
are equal
but not eq
:
(delq (list 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’ inremq
says that it useseq
to compare object against the elements oflist
.(setq sample-list (list 'a 'b 'c 'a 'b 'c)) ⇒ (a b c a b c)
(remq 'a sample-list) ⇒ (b c b c)
sample-list ⇒ (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 usingeql
, 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 returnsnil
.Compare this with
memq
:(memql 1.2 '(1.1 1.2 1.3)) ;
1.2
and1.2
areeql
. ⇒ (1.2 1.3)(memq 1.2 '(1.1 1.2 1.3)) ; The two
1.2
s need not beeq
. ⇒ Unspecified; might benil
or(1.2 1.3)
.
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 usingequal
. If object is a member,member
returns a list starting with its first occurrence in list. Otherwise, it returnsnil
.Compare this with
memq
:(member '(2) '((1) (2))) ;
(2)
and(2)
areequal
. ⇒ ((2))(memq '(2) '((1) (2))) ; The two
(2)
s need not beeq
. ⇒ Unspecified; might benil
or(2)
.;; 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 todelq
asmember
is tomemq
: it usesequal
to compare elements with object, likemember
; when it finds an element that matches, it cuts the element out just asdelq
would. As withdelq
, 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 ofsequence
with all elementsequal
toobject
removed.For example:
(setq l (list '(2) '(1) '(2))) (delete '(2) l) ⇒ ((1)) l ⇒ ((2) (1)) ;; If you want to change
l
reliably, ;; write(setq l (delete '(2) l))
.(setq l (list '(2) '(1) '(2))) (delete '(1) l) ⇒ ((2) (2)) l ⇒ ((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 ofsequence
, a list, vector, or string, with elementsequal
toobject
removed. For example:(remove '(2) '((2) (1) (2))) ⇒ ((1))
(remove '(2) [(2) (1) (2)]) ⇒ [(1)]
Common Lisp note: The functions
member
,delete
andremove
in GNU Emacs Lisp are derived from Maclisp, not Common Lisp. The Common Lisp versions do not useequal
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 severalequal
occurrences of an element in list,delete-dups
keeps the first one. Seeseq-uniq
for non-destructive operation (see Sequences).
See also the function add-to-list
, in Modifying List Variables,
for a way to add an element to a list stored in a variable and used as a
set.
5.8 Association Lists
An association list, or alist for short, records a mapping from keys to values. It is a list of cons cells called associations: the CAR of each cons cell is the key, and the CDR is the associated value.6
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 a function, and
equal
otherwise (see Equality Predicates). If testfn is a function, it is called with two arguments: the CAR of an element from alist and key. The function returnsnil
if no association in alist has a CAR equal to key, as tested by testfn. 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 Comparison of Characters and Strings.
- 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 CDRequal
to value.rassoc
is likeassoc
except that it compares the CDR of each alist association instead of the CAR. You can think of this as reverseassoc
, 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 usingeq
.assq
returnsnil
if no association in alist has a CAReq
to key. This function is used more often thanassoc
, sinceeq
is faster thanequal
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) ⇒ Unspecified; might be
nil
or("simple leaves" . oak)
. (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 toeq
.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 iseql
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 CDReq
to value.rassq
is likeassq
except that it compares the CDR of each alist association instead of the CAR. You can think of this as reverseassq
, 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 symbolwhite
, 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 Regular Expression Searching). If test is omitted ornil
,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
returnsnil
.
- 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 useddelq
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 ofassq-delete-all
rather than looking at the saved value of alist.(setq alist (list '(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)) alist ⇒ ((foo 1) (bar 2) (lose 4))
- Function: assoc-delete-all key alist &optional test ¶
This function is like
assq-delete-all
except that it accepts an optional argument test, a predicate function to compare the keys in alist. If omitted ornil
, test defaults toequal
. Asassq-delete-all
, this function often modifies the original list structure of alist.
- 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 likeassq-delete-all
except that it compares the CDR of each alist association instead of the CAR.
- Macro: let-alist alist body ¶
Creates a binding for each symbol used as keys the association list alist, prefixed with dot. This can be useful when accessing several items in the same association list, and it’s best understood through a simple example:
(setq colors '((rose . red) (lily . white) (buttercup . yellow))) (let-alist colors (if (eq .rose 'red) .lily)) ⇒ white
The body is inspected at compilation time, and only the symbols that appear in body with a ‘.’ as the first character in the symbol name will be bound. Finding the keys is done with
assq
, and thecdr
of the return value of thisassq
is assigned as the value for the binding.Nested association lists is supported:
(setq colors '((rose . red) (lily (belladonna . yellow) (brindisi . pink)))) (let-alist colors (if (eq .rose 'red) .lily.belladonna)) ⇒ yellow
Nesting
let-alist
inside each other is allowed, but the code in the innerlet-alist
can’t access the variables bound by the outerlet-alist
.
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.
- Function: plistp object ¶
This predicate function returns non-
nil
if object is a valid property list.
5.9.1 Property Lists and Association Lists
Association lists (see Association Lists) are very similar to property lists. In contrast to association lists, the order of the pairs in the property list is not significant, since the property names must be distinct.
Property lists are better than association lists for attaching
information to various Lisp function names or variables. If your
program keeps all such information in one association list, it will
typically need to search that entire list each time it checks for an
association for a particular Lisp function name or variable, which
could be slow. By contrast, if you keep the same information in the
property lists of the function names or variables themselves, each
search will scan only the length of one property list, which is
usually short. This is why the documentation for a variable is
recorded in a property named variable-documentation
. The byte
compiler likewise uses properties to record those functions needing
special treatment.
However, association lists have their own advantages. Depending on your application, it may be faster to add an association to the front of an association list than to update a property. All properties for a symbol are stored in the same property list, so there is a possibility of a conflict between different uses of a property name. (For this reason, it is a good idea to choose property names that are probably unique, such as by beginning the property name with the program’s usual name-prefix for variables and functions.) An association list may be used like a stack where associations are pushed on the front of the list and later discarded; this is not possible with a property list.
5.9.2 Property Lists Outside Symbols
The following functions can be used to manipulate property lists.
They all default to comparing property names using eq
.
- Function: plist-get plist property &optional predicate ¶
This returns the value of the property property stored in the property list plist. Comparisons are done with predicate, which defaults to
eq
. It accepts a malformed plist argument. If property is not found in the plist, it returnsnil
. 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 &optional predicate ¶
This stores value as the value of the property property in the property list plist. Comparisons are done with predicate, which defaults to
eq
. 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 (list '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 ¶
This obsolete function is like
plist-get
except that it compares properties usingequal
instead ofeq
.
- Function: lax-plist-put plist property value ¶
This obsolete function is like
plist-put
except that it compares properties usingequal
instead ofeq
.
- Function: plist-member plist property &optional predicate ¶
This returns non-
nil
if plist contains the given property. Comparisons are done with predicate, which defaults toeq
. Unlikeplist-get
, this allows you to distinguish between a missing property and a property with the valuenil
. The value is actually the tail of plist whosecar
is property.
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 | | | | | |____________| |_____________| | | | |________________________________| | |_____________________________________________|
- Sequences
- Arrays
- Functions that Operate on Arrays
- Vectors
- Functions for Vectors
- Char-Tables
- Bool-vectors
- Managing a Fixed-Size Ring of Objects
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 alsoseqp
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 thecircular-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: length< sequence length ¶
Return non-
nil
if sequence is shorter than length. This may be more efficient than computing the length of sequence if sequence is a long list.
- Function: length> sequence length ¶
Return non-
nil
if sequence is longer than length.
- Function: length= sequence length ¶
Return non-
nil
if the length of sequence is equal to length.
- 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 anargs-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 characterelt
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 Functions that Operate on Arrays) andnth
(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 Cons Cells and Lists,concat
in Creating Strings, andvconcat
in Functions for Vectors, for other ways to copy sequences.(setq bar (list 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) x ⇒ (1 2 3 4)
(setq x [1 2 3 4]) ⇒ [1 2 3 4]
(reverse x) ⇒ [4 3 2 1] x ⇒ [1 2 3 4]
(setq x "xyzzy") ⇒ "xyzzy"
(reverse x) ⇒ "yzzyx" x ⇒ "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 (list 'a 'b 'c)) ⇒ (a b c)
x ⇒ (a b c) (nreverse x) ⇒ (c b a)
;; The cons cell that was first is now last. x ⇒ (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 (copy-sequence [1 2 3 4])) ⇒ [1 2 3 4] (nreverse x) ⇒ [4 3 2 1] x ⇒ [4 3 2 1]
Note that unlike
reverse
, this function doesn’t work with strings. Although you can alter string data by usingaset
, it is strongly encouraged to treat strings as immutable even when they are mutable. See Mutability.
- 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, ornil
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 ofsort
is unpredictable.The destructive aspect of
sort
for lists is that it reuses the cons cells forming sequence by changing their contents, possibly rearranging them in a different order. This means that the value of the input list is undefined after sorting; only the list returned bysort
has a well-defined value. Example:(setq nums (list 2 1 4 3 0)) (sort nums #'<) ⇒ (0 1 2 3 4) ; nums is unpredictable at this point
Most often we store the result back into the variable that held the original list:
(setq nums (sort nums #'<))
If you wish to make a sorted copy without destroying the original, copy it first and then sort:
(setq nums (list 2 1 4 3 0)) (sort (copy-sequence nums) #'<) ⇒ (0 1 2 3 4)
nums ⇒ (2 1 4 3 0)
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 ofvector
, but their relative order is preserved. All items whosecar
is 9 are grouped at the end ofvector
, but their relative order is also preserved:(setq vector (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 Text, for more functions that perform sorting. See
documentation
in Access to Documentation Strings, for a useful example ofsort
.
The seq.el library provides the following additional sequence
manipulation macros and functions, prefixed with seq-
.
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 likeelt
. For the details, see Definition of elt.(seq-elt [1 2 3 4] 2) ⇒ 3
seq-elt
returns places settable usingsetf
(see Thesetf
Macro).(setq vec [1 2 3 4]) (setf (seq-elt vec 2) 5) vec ⇒ [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 likelength
. 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 ofsequencep
.(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-split sequence length ¶
This function returns a list consisting of sub-sequences of sequence of (at most) length length. (The final element may be shorter than length if the length of sequence isn’t a multiple of length.
(seq-split [0 1 2 3 4] 2) ⇒ ([0 1] [2 3] [4])
- 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) (1 b) (2 c))
- 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-remove-at-position sequence n ¶
-
This function returns a copy of sequence where the element at (zero-based) index n got removed. The result is a sequence of the same type as sequence.
(seq-remove-at-position [1 -1 3 -3 5] 0) ⇒ [-1 3 -3 5]
(seq-remove-at-position [1 -1 3 -3 5] 3) ⇒ [1 -1 3 5]
- Function: seq-keep function sequence ¶
This function returns a list of all non-
nil
results from calling function on the elements in sequence.(seq-keep #'cl-digit-char-p '(?6 ?a ?7)) ⇒ (6 7)
- 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.
function is called with two arguments. initial-value (and then the accumulated value) is used as the first argument, and the elements in sequence are used for the second argument.
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-p sequence elt &optional function ¶
This function returns non-
nil
if at least one element in sequence is equal to elt. If the optional argument function is non-nil
, it is a function of two arguments to use instead of the defaultequal
.(seq-contains-p '(symbol1 symbol2) 'symbol1) ⇒ t
(seq-contains-p '(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 defaultequal
.(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 (zero-based) 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 defaultequal
.(seq-position '(a b c) 'b) ⇒ 1
(seq-position '(a b c) 'd) ⇒ nil
- Function: seq-positions sequence elt &optional testfn ¶
This function returns a list of the (zero-based) indices of the elements in sequence for which testfn returns non-
nil
when passed the element and elt as arguments. testfn defaults toequal
.(seq-positions '(a b c a d) 'a) ⇒ (0 3)
(seq-positions '(a b c a d) 'z) ⇒ nil
(seq-positions '(11 5 7 12 9 15) 10 #'>=) ⇒ (0 3 5)
- 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 defaultequal
.(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
orstring
.(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 isnil
.(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-union sequence1 sequence2 &optional function ¶
-
This function returns a list of the elements that appear either in sequence1 or sequence2. The elements of the returned list are all unique, in the sense that no two elements there will compare equal. If the optional argument function is non-
nil
, it should be a function of two arguments to use to compare elements, instead of the defaultequal
.(seq-union [1 2 3] [3 5]) ⇒ (1 2 3 5)
- 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 defaultequal
.(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 defaultequal
.(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
orlist
.(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] others)
⇒ [3 4]
The
pcase
patterns provide an alternative facility for destructuring binding, see Destructuring withpcase
Patterns.
- Macro: seq-setq var-sequence val-sequence ¶
-
This macro works similarly to
seq-let
, except that values are assigned to variables as if bysetq
instead of as in alet
binding.(let ((a nil) (b nil)) (seq-setq (_ a _ b) '(1 2 3 4)) (list a b)) ⇒ (2 4)
- 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.
6.2 Arrays
An array object has slots that hold a number of other Lisp objects, called the elements of the array. Any element of an array may be accessed in constant time. In contrast, the time to access an element of a list is proportional to the position of that element in the list.
Emacs defines four types of array, all one-dimensional:
strings (see String Type), vectors (see Vector Type), bool-vectors (see Bool-Vector Type), and
char-tables (see Char-Table Type). Vectors and char-tables
can hold elements of any type, but strings can only hold characters,
and bool-vectors can only hold t
and nil
.
All four kinds of array share these characteristics:
- The first element of an array has index zero, the second element has index 1, and so on. This is called zero-origin indexing. For example, an array of four elements has indices 0, 1, 2, and 3.
- The length of the array is fixed once you create it; you cannot change the length of an existing array.
- For purposes of evaluation, the array is a constant—i.e., it evaluates to itself.
- The elements of an array may be referenced or changed with the functions
aref
andaset
, respectively (see Functions that Operate on Arrays).
When you create an array, other than a char-table, you must specify its length. You cannot specify the length of a char-table, because that is determined by the range of character codes.
In principle, if you want an array of text characters, you could use either a string or a vector. In practice, we always choose strings for such applications, for four reasons:
- They occupy one-fourth the space of a vector of the same elements.
- Strings are printed in a way that shows the contents more clearly as text.
- Strings can hold text properties. See Text Properties.
- Many of the specialized editing and I/O facilities of Emacs accept only strings. For example, you cannot insert a vector of characters into a buffer the way you can insert a string. See Strings and Characters.
By contrast, for an array of keyboard input characters (such as a key sequence), a vector may be necessary, because many keyboard input characters are outside the range that will fit in a string. See Key Sequence Input.
6.3 Functions that Operate on Arrays
In this section, we describe the functions that accept all types of arrays.
- 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 Sequences.
- Function: aset array index object ¶
This function sets the indexth element of array to be object. It returns object.
(setq w (vector 'foo 'bar 'baz)) ⇒ [foo bar baz] (aset w 0 'fu) ⇒ fu w ⇒ [fu bar baz]
;;
copy-sequence
copies the string to be modified later. (setq x (copy-sequence "asdfasfd")) ⇒ "asdfasfd" (aset x 3 ?Z) ⇒ 90 x ⇒ "asdZasfd"The array should be mutable. See Mutability.
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 (copy-sequence [a b c d e f g])) ⇒ [a b c d e f g] (fillarray a 0) ⇒ [0 0 0 0 0 0 0] a ⇒ [0 0 0 0 0 0 0]
(setq s (copy-sequence "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 Sequences.
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 and Interning 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. Vectors written with square brackets
should not be modified via aset
or other destructive
operations. See Mutability.
Here are examples illustrating these principles:
(setq avector [1 two '(three) "four" [five]]) ⇒ [1 two '(three) "four" [five]] (eval avector) ⇒ [1 two '(three) "four" [five]] (eq avector (eval avector)) ⇒ t
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"] (vector) ⇒ []
- 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) ⇒ [] (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 Function Objects.For other concatenation functions, see
mapconcat
in Mapping Functions,concat
in Creating Strings, andappend
in Building Cons Cells and 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 '(three) "four" [five]] (append avector nil) ⇒ (1 two '(three) "four" [five])
6.6 Char-Tables
A char-table is much like a vector, except that it is indexed by
character codes. Any valid character code, without modifiers, can be
used as an index in a char-table. You can access a char-table’s
elements with aref
and aset
, as with any array. In
addition, a char-table can have extra slots to hold additional
data not associated with particular character codes. Like vectors,
char-tables are constants when evaluated, and can hold elements of any
type.
Each char-table has a subtype, a symbol, which serves two purposes:
- The subtype provides an easy way to tell what the char-table is for.
For instance, display tables are char-tables with
display-table
as the subtype, and syntax tables are char-tables withsyntax-table
as the subtype. The subtype can be queried using the functionchar-table-subtype
, described below. - The subtype controls the number of extra slots in the
char-table. This number is specified by the subtype’s
char-table-extra-slots
symbol property (see Symbol Properties), whose value should be an integer between 0 and 10. If the subtype has no such symbol property, the char-table has no extra slots.
A char-table can have a parent, which is another char-table. If
it does, then whenever the char-table specifies nil
for a
particular character c, it inherits the value specified in the
parent. In other words, (aref char-table c)
returns
the value from the parent of char-table if char-table itself
specifies nil
.
A char-table can also have a default value. If so, then
(aref char-table c)
returns the default value
whenever the char-table does not specify any other non-nil
value.
- 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 nochar-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, andnil
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:
nil
Refers to the default value.
- char
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:
nil
Refers to the default value.
t
Refers to the whole range of character codes.
- char
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 forchar-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 tomap-char-table
useful, function should have side effects. For example, here is how to examine the elements of the syntax table:(let (accumulator) (map-char-table (lambda (key value) (setq accumulator (cons (list (if (consp key) (list (car key) (cdr key)) key) value) accumulator))) (syntax-table)) accumulator) ⇒ (((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)))
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, andnil
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 everyt
value in a is alsot
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 ist
ornil
, and i is an index intoa
.
- 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" (bool-vector) ⇒ #&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 bv ⇒ #&5"^W"
These results make sense because the binary codes for control-_ and control-W are 11111 and 10111, respectively.
6.8 Managing a Fixed-Size Ring of Objects
A ring is a fixed-size data structure that supports insertion,
deletion, rotation, and modulo-indexed reference and traversal. An
efficient ring data structure is implemented by the ring
package. It provides the functions listed in this section.
Note that several rings in Emacs, like the kill ring and the
mark ring, are actually implemented as simple lists, not using
the ring
package; thus the following functions won’t work on
them.
- 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.
- Function: ring-resize ring size ¶
Set the size of ring to size. If the new size is smaller, then the oldest items in the ring are discarded.
If you are careful not to exceed the ring size, you can use the ring as a first-in-first-out queue. For example:
(let ((fifo (make-ring 5))) (mapc (lambda (obj) (ring-insert fifo obj)) '(0 one "two")) (list (ring-remove fifo) t (ring-remove fifo) t (ring-remove fifo))) ⇒ (0 t one t "two")
7 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.
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)
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.
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:
- Lookup in a hash table is extremely fast for large tables—in fact, the time required is essentially independent of how many elements are stored in the table. For smaller tables (a few tens of elements) alists may still be faster because hash tables have a more-or-less constant overhead.
- The correspondences in a hash table are in no particular order.
- There is no way to share structure between two hash tables, the way two alists can share a common tail.
Emacs Lisp provides a general-purpose hash table data type, along with a series of functions for operating on them. Hash tables have a special printed representation, which consists of ‘#s’ followed by a list specifying the hash table properties and contents. See Creating Hash Tables. (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 and Read Syntax.)
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 and Interning Symbols).
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
andequal
are other alternatives:eql
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.eq
Any two distinct Lisp objects are different as keys.
equal
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 Comparisons) 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
, ort
which is an alias forkey-and-value
. If weak iskey
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
ort
, 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 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 and Read Syntax). For instance,
the following specifies a 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))
Note, however, that when using this in Emacs Lisp code, it’s
undefined whether this creates a new hash table or not. If you want
to create a new hash table, you should always use
make-hash-table
(see Self-Evaluating Forms).
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.
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. This function always returns 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 andnil
otherwise. In Emacs Lisp,remhash
always returnsnil
.
- 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.
clrhash
returns the empty table.
- 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
returnsnil
.
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 can reduce 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. The two functions should be consistent with each other: that is, two keys’ hash codes should be the same if the keys compare as equal. Also, since the two functions can be called at any time (such as by the garbage collector), the functions should be free of side effects and should return quickly, and their behavior should depend on only on properties of the keys that do not change.
- 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 fixnums for hash codes, including negative fixnums.
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 that 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 bysxhash-equal
are usually different, but not always.sxhash-equal
is designed to be reasonably fast (since it’s used for indexing hash tables) so it won’t recurse deeply into nested structures. In addition; once in a rare while, by luck, you will encounter two distinct-looking simple objects that give the same result fromsxhash-equal
. So you can’t, in general, usesxhash-equal
to check whether an object has changed.Common Lisp note: In Common Lisp a similar function is called
sxhash
. Emacs provides this name as a compatibility alias forsxhash-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 bignum or a float number, in which case a 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 string-hash-ignore-case (a) (sxhash-equal (upcase a))) (define-hash-table-test 'ignore-case 'string-equal-ignore-case 'string-hash-ignore-case) (make-hash-table :test 'ignore-case)
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)
Lisp programs should not rely on hash codes being preserved between Emacs sessions, as the implementation of the hash functions uses some details of the object storage that can change between sessions and between different architectures.
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 Tables).
- 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.
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.
- Symbol Components
- Defining Symbols
- Creating and Interning Symbols
- Symbol Properties
- Shorthands
- Symbols with Position
9.1 Symbol Components
Each symbol has four components (or “cells”), each of which references another object:
- Print name ¶
The symbol’s name.
- Value ¶
The symbol’s current value as a variable.
- Function ¶
The symbol’s function definition. It can also hold a symbol, a keymap, or a keyboard macro.
- Property list ¶
The symbol’s property list.
The print name cell always holds a string, and cannot be changed. Each of the other three cells can be set to any Lisp object.
The print name cell holds the string that is the name of a symbol.
Since symbols are represented textually by their names, it is
important not to have two symbols with the same name. The Lisp reader
ensures this: every time it reads a symbol, it looks for an existing
symbol with the specified name before it creates a new one. To get a
symbol’s name, use the function symbol-name
(see Creating and Interning Symbols). However, although each symbol has only one unique
print name, it is nevertheless possible to refer to that same
symbol via different alias names called “shorthands”
(see Shorthands).
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 Variables that Never Change.
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 Symbol 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 Accessing Function Cell Contents).
The property list cell normally should hold a correctly formatted
property list. To get a symbol’s property list, use the function
symbol-plist
. See Symbol Properties.
The function cell or the value cell may be void, which means
that the cell does not reference any object. (This is not the same
thing as holding the symbol void
, nor the same as holding the
symbol nil
.) Examining a function or value cell that is void
results in an error, such as ‘Symbol's value as variable is void’.
Because each symbol has separate value and function cells, variables
names and function names do not conflict. For example, the symbol
buffer-file-name
has a value (the name of the file being
visited in the current buffer) as well as a function definition (a
primitive function that returns the name of the file):
buffer-file-name ⇒ "/gnu/elisp/symbols.texi" (symbol-function 'buffer-file-name) ⇒ #<subr buffer-file-name>
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 Settings).
In principle, you can assign a variable value to any symbol with
setq
, whether or 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 Scoping Rules for Variable Bindings).
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 in The GNU Emacs Manual.
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 sequence of characters in the same context. Failure to do so would cause complete confusion.
When the Lisp reader encounters a name that references a symbol in the source code, it reads all the characters of that name. Then it looks up that name in a table called an obarray to find the symbol that the programmer meant. The technique used in this lookup is called “hashing”, an efficient method of looking something up by converting a sequence of characters to a number, known as a “hash code”. 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.)
When looking up names, the Lisp reader also considers “shorthands”. If the programmer supplied them, this allows the reader to find a symbol even if its name isn’t present in its full form in the source code. Of course, the reader needs to be aware of some pre-established context about such shorthands, much as one needs context to be to able to refer uniquely to Jan Jones by just the name “Jan”: it’s probably fine when amongst the Joneses, or when Jan has been mentioned recently, but very ambiguous in any other situation. See Shorthands.
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 may want 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. As another example, a hypothetical telephone book program could intern the name of each looked up person’s name as a symbol, even if the obarray did not contain it, so that it could attach information to that new symbol, such as the last time someone looked it up.
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. Uninterned symbols are sometimes useful in generating Lisp code, see below.
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 the same name in several different “packages”, thus creating multiple symbols with the same name but different packages. Emacs Lisp provides a different namespacing system called “shorthands” (see Shorthands).
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: Never alter the string returned by that function. Doing that might make Emacs dysfunctional, and might even crash Emacs.
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.
- 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 ofsym
is noteq
tofoo
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 appendinggensym-counter
to prefix and incrementing that counter, guaranteeing that no two calls to this function will generate a symbol with the same name. The prefix defaults to"g"
.
To avoid problems when accidentally interning printed representation
of generated code (see Printed Representation and Read Syntax), it is recommended
to use gensym
instead of make-symbol
.
- 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 variableobarray
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 useintern-soft
to test whether a symbol with a given name is already interned. If obarray is omitted, the value of the global variableobarray
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
andread
.
- 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 ofobarray
, the standard obarray for ordinary symbols.(setq count 0) ⇒ 0 (defun count-syms (s) (setq count (1+ count))) ⇒ count-syms (mapatoms 'count-syms) ⇒ nil count ⇒ 1871
See
documentation
in Access to Documentation Strings, for another example usingmapatoms
.
- 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 isnil
, 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 returnst
. Otherwise it returnsnil
.
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).
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 ofnil
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) ⇒'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 and Abbrev Expansion).
You could define
put
in terms ofsetplist
andplist-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 symbolmacro
, 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.
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.
:advertised-binding
This property value specifies the preferred key binding, when showing documentation, for the named function. See Substituting Key Bindings in Documentation.
char-table-extra-slots
The value, if non-
nil
, specifies the number of extra slots in the named char-table type. See Char-Tables.customized-face
face-defface-spec
saved-face
theme-face
These properties are used to record a face’s standard, saved, customized, and themed face specs. Do not set them directly; they are managed by
defface
and related functions. See Defining Faces.customized-value
saved-value
standard-value
theme-value
These properties are used to record a customizable variable’s standard value, saved value, customized-but-unsaved value, and themed values. Do not set them directly; they are managed by
defcustom
and related functions. See Defining Customization Variables.definition-name
This property is used to find the definition of a symbol in the source code, when it might be hard to find the definition by textual search of the source file. For example, a
define-derived-mode
(see Defining Derived Modes) might define a mode-specific function or a variable implicitly; or your Lisp program might generate a run-time call todefun
to define a function (see Defining Functions). In these and similar cases, thedefinition-name
property of the symbol should be another symbol whose definition can be found by textual search and whose code defines the original symbol. In the example withdefine-derived-mode
, the value of this property of the functions and variables it defines should be the mode symbol. The Emacs Help commands such as C-h f (see Help in The GNU Emacs Manual) use this property to show the definition of a symbol via a button in the *Help* buffer where the symbol’s documentation is shown.disabled
If the value is non-
nil
, the named function is disabled as a command. See Disabling Commands.face-documentation
The value stores the documentation string of the named face. This is set automatically by
defface
. See Defining Faces.history-length
The value, if non-
nil
, specifies the maximum minibuffer history length for the named history list variable. See Minibuffer History.interactive-form
The value is an interactive form for the named function. Normally, you should not set this directly; use the
interactive
special form instead. See Interactive Call.menu-enable
The value is an expression for determining whether the named menu item should be enabled in menus. See Simple Menu Items.
mode-class
If the value is
special
, the named major mode is special. See Major Mode Conventions.permanent-local
If the value is non-
nil
, the named variable is a buffer-local variable whose value should not be reset when changing major modes. See Creating and Deleting Buffer-Local Bindings.permanent-local-hook
If the value is non-
nil
, the named function should not be deleted from the local value of a hook variable when changing major modes. See Setting Hooks.pure
¶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).risky-local-variable
If the value is non-
nil
, the named variable is considered risky as a file-local variable. See File Local Variables.safe-function
If the value is non-
nil
, the named function is considered generally safe for evaluation. See Determining whether a Function is Safe to Call.safe-local-eval-function
If the value is non-
nil
, the named function is safe to call in file-local evaluation forms. See File Local Variables.safe-local-variable
The value specifies a function for determining safe file-local values for the named variable. See File Local Variables. Since this value is consulted when loading files, the function should be efficient and should ideally not lead to loading any libraries to determine the safeness (e.g., it should not be an autoloaded function).
side-effect-free
¶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 iserror-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 Determining whether a Function is Safe to Call).undo-inhibit-region
If non-
nil
, the named function prevents theundo
operation from being restricted to the active region, ifundo
is invoked immediately after the function. See Undo.variable-documentation
If non-
nil
, this specifies the named variable’s documentation string. This is set automatically bydefvar
and related functions. See Defining Faces.
9.5 Shorthands
The symbol shorthands, sometimes known as “renamed symbols”, are symbolic forms found in Lisp source. They’re just like regular symbolic forms, except that when the Lisp reader encounters them, it produces symbols which have a different and usually longer print name (see Symbol Components).
It is useful to think of shorthands as abbreviating the full names of intended symbols. Despite this, do not confuse shorthands with the Abbrev system see Abbrevs and Abbrev Expansion.
Shorthands make Emacs Lisp’s namespacing etiquette easier to work
with. Since all symbols are stored in a single obarray
(see Creating and Interning Symbols), programmers commonly prefix each symbol
name with the name of the library where it originates. For example,
the functions text-property-search-forward
and
text-property-search-backward
both belong to the
text-property-search.el library (see Loading). By properly
prefixing symbol names, one effectively prevents clashes between
similarly named symbols which belong to different libraries and thus do
different things. However, this practice commonly originates very
long symbols names, which are inconvenient to type and read after a
while. Shorthands solve these issues in a clean way.
- Variable: read-symbol-shorthands ¶
This variable’s value is an alist whose elements have the form
(shorthand-prefix . longhand-prefix)
. Each element instructs the Lisp reader to read every symbol form which starts with shorthand-prefix as if it started with longhand-prefix instead.This variable may only be set in file-local variables (see Local Variables in Files in The GNU Emacs Manual).
Here’s an example of shorthands usage in a hypothetical string manipulating library some-nice-string-utils.el.
(defun some-nice-string-utils-split (separator s &optional omit-nulls) "A match-data saving variant of `split-string'." (save-match-data (split-string s separator omit-nulls))) (defun some-nice-string-utils-lines (s) "Split string S at newline characters into a list of strings." (some-nice-string-utils-split "\\(\r\n\\|[\n\r]\\)" s))
As can be seen, it’s quite tedious to read or develop this code since the symbol names to type are so long. We can use shorthands to alleviate that.
(defun snu-split (separator s &optional omit-nulls) "A match-data saving variation on `split-string'." (save-match-data (split-string s separator omit-nulls))) (defun snu-lines (s) "Split string S into a list of strings on newline characters." (snu-split "\\(\r\n\\|[\n\r]\\)" s)) ;; Local Variables: ;; read-symbol-shorthands: (("snu-" . "some-nice-string-utils-")) ;; End:
Even though the two excerpts look different, they are quite identical
after the Lisp reader processes them. Both will lead to the very same
symbols being interned (see Creating and Interning Symbols). Thus loading or
byte-compiling any of the two files has equivalent results. The
shorthands snu-split
and snu-lines
used in the second
version are not interned in the obarray. This is easily seen
by moving point to the location where the shorthands are used and
waiting for ElDoc (see Local Variables
in Files in The GNU Emacs Manual) to hint at the true full name
of the symbol under point in the echo area.
Since read-symbol-shorthands
is a file-local variable, it is
possible that multiple libraries depending on
some-nice-string-utils-lines.el refer to the same symbols under
different shorthands, or not using shorthands at all. In the
next example, the my-tricks.el library refers to the symbol
some-nice-string-utils-lines
using the sns-
prefix
instead of snu-
.
(defun t-reverse-lines (s) (string-join (reverse (sns-lines s)) "\n") ;; Local Variables: ;; read-symbol-shorthands: (("t-" . "my-tricks-") ;; ("sns-" . "some-nice-string-utils-")) ;; End:
9.5.1 Exceptions
There are two exceptions to rules governing Shorthand transformations:
- Symbol forms comprised entirely of characters in the Emacs Lisp symbol
constituent class (see Table of Syntax Classes) are not transformed.
For example, it’s possible to use
-
or/=
as shorthand prefixes, but that won’t shadow the arithmetic functions of those names. - Symbol forms whose names start with ‘#_’ are not transformed.
9.6 Symbols with Position
A symbol with position is a symbol, the bare symbol, together with an unsigned integer called the position. These objects are intended for use by the byte compiler, which records in them the position of each symbol occurrence and uses those positions in warning and error messages.
The printed representation of a symbol with position uses the hash
notation outlined in Printed Representation and Read Syntax. It looks like
‘#<symbol foo at 12345>’. It has no read syntax. You can cause
just the bare symbol to be printed by binding the variable
print-symbols-bare
to non-nil
around the print
operation. The byte compiler does this before writing its output to
the compiled Lisp file.
For most purposes, when the flag variable
symbols-with-pos-enabled
is non-nil
, symbols with
positions behave just as bare symbols do. For example, ‘(eq
#<symbol foo at 12345> foo)’ has a value t
when that variable
is set (but nil
when it isn’t set). Most of the time in Emacs this
variable is nil
, but the byte compiler binds it to t
when it runs.
Typically, symbols with position are created by the byte compiler
calling the reader function read-positioning-symbols
(see Input Functions). One can also be created with the function
position-symbol
.
- Variable: symbols-with-pos-enabled ¶
When this variable is non-
nil
, symbols with position behave like the contained bare symbol. Emacs runs a little more slowly in this case.
- Variable: print-symbols-bare ¶
When bound to non-
nil
, the Lisp printer prints only the bare symbol of a symbol with position, ignoring the position.
- Function: symbol-with-pos-p symbol. ¶
This function returns
t
if symbol is a symbol with position,nil
otherwise.
- Function: bare-symbol symbol ¶
This function returns the bare symbol contained in symbol, or symbol itself if it is already a bare symbol. For any other type of object, it signals an error.
- Function: symbol-with-pos-pos symbol ¶
This function returns the position, a number, from a symbol with position. For any other type of object, it signals an error.
- Function: position-symbol sym pos ¶
Make a new symbol with position. sym is either a bare symbol or a symbol with position, and supplies the symbol part of the new object. pos is either an integer which becomes the number part of the new object, or a symbol with position whose position is used. Emacs signals an error if either argument is invalid.
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
.
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 expression7. 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).8 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.
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.
- Self-Evaluating Forms
- Symbol Forms
- Classification of List Forms
- Symbol Function Indirection
- Evaluation of Function Forms
- Lisp Macro Evaluation
- Special Forms
- Autoloading
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
A self-evaluating form yields a value that becomes part of the program,
and you should not try to modify it via setcar
, aset
or
similar operations. The Lisp interpreter might unify the constants
yielded by your program’s self-evaluating forms, so that these
constants might share structure. See Mutability.
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>
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
a ⇒ 123
The symbols nil
and t
are treated specially, so that the
value of nil
is always nil
, and the value of t
is
always t
; you cannot set or bind them to any other values. Thus,
these two symbols act like self-evaluating forms, even though
eval
treats them like any other symbol. A symbol whose name
starts with ‘:’ also self-evaluates in the same way; likewise,
its value ordinarily cannot be changed. See Variables that Never Change.
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.
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 Naming a Function, 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 Accessing Function Cell Contents). 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 acyclic-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 (and function (symbolp function)) (indirect-function (symbol-function function)) function))
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.
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.
10.2.7 Special Forms
A special form is a primitive 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.
and
catch
cond
see Conditionals
condition-case
defconst
defvar
function
if
see Conditionals
interactive
see Interactive Call
lambda
let
let*
see Local Variables
or
prog1
prog2
progn
see Sequencing
quote
see Quoting
save-current-buffer
save-excursion
see Excursions
save-restriction
see Narrowing
setq
setq-default
unwind-protect
see Nonlocal Exits
while
see Iteration
Common Lisp note: Here are some comparisons of special forms in GNU Emacs Lisp and Common Lisp.
setq
,if
, andcatch
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).
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.
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. The returned value might be shared and should not be modified. See Self-Evaluating Forms.
Because quote
is used so often in programs, Lisp provides a
convenient read syntax for it. An apostrophe character (‘'’)
followed by a Lisp object (in read syntax) expands to a list whose first
element is quote
, and whose second element is the object. Thus,
the read syntax 'x
is an abbreviation for (quote x)
.
Here are some examples of expressions that use quote
:
(quote (+ 1 2)) ⇒ (+ 1 2)
(quote foo) ⇒ foo
'foo ⇒ foo
''foo ⇒ 'foo
'(quote foo) ⇒ 'foo
['foo] ⇒ ['foo]
Although the expressions (list '+ 1 2)
and '(+ 1 2)
both yield lists equal to (+ 1 2)
, the former yields a
freshly-minted mutable list whereas the latter yields a list
built from conses that might be shared and should not be modified.
See Self-Evaluating Forms.
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.
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)
If a subexpression of a backquote construct has no substitutions or
splices, it acts like quote
in that it yields conses,
vectors and strings that might be shared and should not be modified.
See Self-Evaluating Forms.
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 Kinds of Forms.
The argument lexical specifies the scoping rule for local variables (see Scoping Rules for Variable Bindings). If it is omitted or
nil
, that means to evaluate form using the default dynamic scoping rule. If it ist
, 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 toeval
is evaluated twice: once as preparation beforeeval
is called, and again by theeval
function itself. Here is an example:(setq foo 'bar) ⇒ bar
(setq bar 'baz) ⇒ baz ;; Here
eval
receives argumentfoo
(eval 'foo) ⇒ bar ;; Hereeval
receives argumentbar
, which is the value offoo
(eval foo) ⇒ bazThe number of currently active calls to
eval
is limited tomax-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 ofread
to read expressions one by one. This function is called with one argument, the stream for reading input. You can also use the variableload-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 returnsnil
.
- 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 in The GNU Emacs Manual). buffer-or-name can be a buffer, a buffer name (a string), ornil
(or omitted), which means to use the current buffer. stream is used as ineval-region
, unless stream isnil
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 forload-history
(see Unloading), and defaults tobuffer-file-name
(see Buffer File Name). If unibyte is non-nil
,read
converts strings to unibyte whenever possible.
- User Option: max-lisp-eval-depth ¶
This variable defines the maximum depth allowed in calls to
eval
,apply
, andfuncall
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 how 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 Entering the Debugger on an Error.The depth limit counts internal uses of
eval
,apply
, andfuncall
, 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 1600. 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.
- 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
.)This variable is obsolete, and will be removed in a future version, since it constantly enlarges the memory footprint of the Emacs process. For that reason, we recommend against using it.
The elements of
values
are ordered most recent first.(setq x 1) ⇒ 1
(list 'A (1+ 2) auto-save-default) ⇒ (A 3 t)
values ⇒ ((A 3 t) 1 …)
This variable could be 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
10.6 Deferred and Lazy Evaluation
Sometimes it is useful to delay the evaluation of an expression, for example if you want to avoid performing a time-consuming calculation if it turns out that the result is not needed in the future of the program. The thunk library provides the following functions and macros to support such deferred evaluation:
- Macro: thunk-delay forms… ¶
Return a thunk for evaluating the forms. A thunk is a closure (see Closures) that inherits the lexical environment of the
thunk-delay
call. Using this macro requireslexical-binding
.
- Function: thunk-force thunk ¶
Force thunk to perform the evaluation of the forms specified in the
thunk-delay
that created the thunk. The result of the evaluation of the last form is returned. The thunk also “remembers” that it has been forced: Any further calls ofthunk-force
with the same thunk will just return the same result without evaluating the forms again.
- Macro: thunk-let (bindings…) forms… ¶
This macro is analogous to
let
but creates “lazy” variable bindings. Any binding has the form(symbol value-form)
. Unlikelet
, the evaluation of any value-form is deferred until the binding of the according symbol is used for the first time when evaluating the forms. Any value-form is evaluated at most once. Using this macro requireslexical-binding
.
Example:
(defun f (number) (thunk-let ((derived-number (progn (message "Calculating 1 plus 2 times %d" number) (1+ (* 2 number))))) (if (> number 10) derived-number number)))
(f 5) ⇒ 5
(f 12) -| Calculating 1 plus 2 times 12 ⇒ 25
Because of the special nature of lazily bound variables, it is an error
to set them (e.g. with setq
).
- Macro: thunk-let* (bindings…) forms… ¶
This is like
thunk-let
but any expression in bindings is allowed to refer to preceding bindings in thisthunk-let*
form. Using this macro requireslexical-binding
.
(thunk-let* ((x (prog2 (message "Calculating x...") (+ 1 1) (message "Finished calculating x"))) (y (prog2 (message "Calculating y...") (+ x 1) (message "Finished calculating y"))) (z (prog2 (message "Calculating z...") (+ y 1) (message "Finished calculating z"))) (a (prog2 (message "Calculating a...") (+ z 1) (message "Finished calculating a")))) (* z x)) -| Calculating z... -| Calculating y... -| Calculating x... -| Finished calculating x -| Finished calculating y -| Finished calculating z ⇒ 8
thunk-let
and thunk-let*
use thunks implicitly: their
expansion creates helper symbols and binds them to thunks wrapping the
binding expressions. All references to the original variables in the
body forms are then replaced by an expression that calls
thunk-force
with the according helper variable as the argument.
So, any code using thunk-let
or thunk-let*
could be
rewritten to use thunks, but in many cases using these macros results
in nicer code than using thunks explicitly.
11 Control Structures
A Lisp program consists of a set of expressions, or forms (see Kinds of Forms). We control the order of execution of these forms by enclosing them in control structures. Control structures are special forms which control when, whether, or how many times to execute the forms they contain.
The simplest order of execution is sequential execution: first form a, then form b, and so on. This is what happens when you write several forms in succession in the body of a function, or at top level in a file of Lisp code—the forms are executed in the order written. We call this textual order. For example, if a function body consists of two forms a and b, evaluation of the function evaluates first a and then b. The result of evaluating b becomes the value of the function.
Explicit control structures make possible an order of execution other than sequential.
Emacs Lisp provides several kinds of control structure, including other varieties of sequencing, conditionals, iteration, and (controlled) jumps—all discussed below. The built-in control structures are special forms since their subforms are not necessarily evaluated or not evaluated sequentially. You can use macros to define your own control structure constructs (see Macros).
- Sequencing
- Conditionals
- Constructs for Combining Conditions
- Pattern-Matching Conditional
- Iteration
- Generators
- Nonlocal Exits
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"
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 ofif
is an example of an implicitprogn
. See Sequencing.)If condition has the value
nil
, and no else-forms are given,if
returnsnil
.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 becauseprint
is never called:(if nil (print 'true) 'very-false) ⇒ 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 thecond
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; thencond
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 thecond
moves on to the following clause, trying its condition.A clause may also look like this:
(condition)
Then, if condition is non-
nil
when tested, thecond
form returns the value of condition.If every condition evaluates to
nil
, so that every clause fails,cond
returnsnil
.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 formt
evaluates tot
, which is nevernil
, so this clause never fails, provided thecond
gets to it at all. For example:(setq a 5) (cond ((eq a 'hack) 'foo) (t "default")) ⇒ "default"
This
cond
expression returnsfoo
if the value ofa
ishack
, 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))
It can be convenient to bind variables in conjunction with using a
conditional. It’s often the case that you compute a value, and then
want to do something with that value if it’s non-nil
. The
straightforward way to do that is to just write, for instance:
(let ((result1 (do-computation))) (when result1 (let ((result2 (do-more result1))) (when result2 (do-something result2)))))
Since this is a very common pattern, Emacs provides a number of macros to make this easier and more readable. The above can be written the following way instead:
(when-let ((result1 (do-computation)) (result2 (do-more result1))) (do-something result2))
There’s a number of variations on this theme, and they’re briefly described below.
- Macro: if-let spec then-form else-forms... ¶
Evaluate each binding in spec in turn, like in
let*
(see Local Variables, stopping if a binding value isnil
. If all are non-nil
, return the value of then-form, otherwise the last form in else-forms.
- Macro: when-let spec then-forms... ¶
Like
if-let
, but without else-forms.
- Macro: while-let spec then-forms... ¶
Like
when-let
, but repeat until a binding in spec isnil
. The return value is alwaysnil
.
11.3 Constructs for Combining Conditions
This section describes 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 isnil
, andnil
otherwise. The functionnot
is identical tonull
, and we recommend using the namenull
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 theand
must benil
regardless of the remaining conditions; soand
returnsnil
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 theand
form. Just(and)
, with no conditions, returnst
, 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 notnil
. The third condition isnil
, 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)
returnsnil
, thus avoiding an error.and
expressions can also be written using eitherif
orcond
. 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 theor
must be non-nil
; soor
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 theor
expression returnsnil
. Just(or)
, with no conditions, returnsnil
, appropriate because all the conditions turned outnil
. (Think about it; which one did not?)For example, this expression tests whether
x
is eithernil
or the integer zero:(or (eq x nil) (eq x 0))
Like the
and
construct,or
can be written in terms ofcond
. For example:(or arg1 arg2 arg3) ≡ (cond (arg1) (arg2) (arg3))
You could almost write
or
in terms ofif
, but not quite:(if arg1 arg1 (if arg2 arg2 arg3))
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.
- Function: xor condition1 condition2 ¶
This function returns the boolean exclusive-or of condition1 and condition2. That is,
xor
returnsnil
if either both arguments arenil
, or both are non-nil
. Otherwise, it returns the value of that argument which is non-nil
.Note that in contrast to
or
, both arguments are always evaluated.
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 in Common Lisp Extensions)
that overcomes their limitations and introduces
the pattern matching programming style.
The limitations that pcase
overcomes are:
- The
cond
form chooses among alternatives by evaluating the predicate condition of each of its clauses (see Conditionals). The primary limitation is that variables let-bound in condition are not available to the clause’s body-forms.Another annoyance (more an inconvenience than a limitation) is that when a series of condition predicates implement equality tests, there is a lot of repeated code. (
cl-case
solves this inconvenience.) - The
cl-case
macro chooses among alternatives by evaluating the equality of its first argument against a set of specific values.Its limitations are two-fold:
- The equality tests use
eql
. - The values must be known and written in advance.
These render
cl-case
unsuitable for strings or compound data structures (e.g., lists or vectors). (cond
doesn’t have these limitations, but it has others, see above.) - The equality tests use
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”).
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 tonil
.
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:
_ (underscore)
Matches any expval. This is also known as don’t care or wildcard.
'val
Matches if expval equals val. The comparison is done as if by
equal
(see Equality Predicates).keyword
integer
string
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.symbol
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, andnil
, which signals an error. Likewise, it makes no sense to bind keyword symbols (see Variables that Never Change).(cl-type type)
Matches if expval is of type type, which is a type descriptor as accepted by
cl-typep
(see Type Predicates in Common Lisp Extensions). Examples:(cl-type integer) (cl-type (integer 0 10))
(pred function)
Matches if the predicate function returns non-
nil
when called on expval. The test can be negated with the syntax(pred (not function))
. 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. Unlikepred
, 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 Constructs for 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.To present a consistent environment (see Introduction to Evaluation) to body-forms (thus avoiding an evaluation error on match), the set of variables bound by the pattern is the union of the variables bound by each sub-pattern. If a variable is not bound by the sub-pattern that matched, then it is bound to
nil
.(rx rx-expr…)
Matches strings against the regexp rx-expr…, using the
rx
regexp notation (see Therx
Structured Regexp Notation), as if bystring-match
.In addition to the usual
rx
syntax, rx-expr… can contain the following constructs:(let ref rx-expr…)
Bind the symbol ref to a submatch that matches rx-expr.... ref is bound in body-forms to the string of the submatch or
nil
, but can also be used inbackref
.(backref ref)
Like the standard
backref
construct, but ref can here also be a name introduced by a previous(let ref …)
construct.
Example: Advantage Over cl-case
Here’s an example that highlights some advantages pcase
has over cl-case
(see Conditionals in Common Lisp Extensions).
(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.
(and
(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.
- 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 (usingpred
), and then bind symbols to thecar
andcdr
of expval (using oneapp
each).For A, because symbol
st
is mentioned twice, the second mention becomes an equality test usingeq
. On the other hand, B uses two separate symbols,s1
ands2
, 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))
- Side-effecting code referencing symbol is undefined.
Avoid.
For example, here are two similar functions.
Both use
and
, symbol andguard
:(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
referencesn
simply and directly, whileMAYBE
referencesn
with a side-effect, in the expression(incf n)
. Wheninteger
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 referencesn
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 (inlet
) and function (inpred
andapp
). - The first
- 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, forand
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:(require 'cl-lib) (pcase (read-number "Enter an integer: ") ((or (and (pred cl-evenp) e-num) ; bind
e-num
to expval o-num) ; bindo-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:(require 'cl-lib) (pcase (read-number "Enter an integer: ") ((and num ; line 1 (or (and (pred cl-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, useslet
twice (lines 3-4) to bind the same symbolspin
in both sub-patterns. The value ofspin
distinguishes the sub-patterns. The body form references both symbols (line 5).
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)
. Thepcase
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 useEXPVAL
to stand for the result of evaluating expression (first arg topcase
).
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).
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) ls
(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 whosecdr
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.symbol
keyword
number
string
Matches if the corresponding element of expval is
equal
to the specified literal object.,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-bindssecond-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) env))))
((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
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-Style 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 apcase
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.
Here’s a trivial example:
(pcase-let ((`(,major ,minor) (split-string "image/png" "/"))) minor) ⇒ "png"
- 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 apcase
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 tolet*
, 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 todolist
(see Iteration).
- Macro: pcase-setq pattern value… ¶
Assign values to variables in a
setq
form, destructuring each value according to its respective pattern.
- Macro: pcase-lambda lambda-list &rest body ¶
This is like
lambda
, but allows each argument to be a pattern. For instance, here’s a simple function that takes a cons cell as the argument:(setq fun (pcase-lambda (`(,key . ,val)) (vector key (* val 10)))) (funcall fun '(foo . 2)) ⇒ [foo 20]
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 tonil
.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 orthrow
jumps out of it (see Nonlocal Exits).The value of a
while
form is alwaysnil
.(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 ofwhile
, 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 usedolist
to define thereverse
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. Use of result is deprecated. Here is an example of usingdotimes
to do something 100 times:(dotimes (i 100) (insert "I will not obey absurd orders\n"))
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 whereiter-yield
oriter-yield-from
appears. When body returns normally,iter-next
signalsiter-end-of-sequence
with body’s result as its condition data.Any kind of Lisp code is valid inside body, but
iter-yield
anditer-yield-from
cannot appear insideunwind-protect
forms.
- Macro: iter-lambda args [doc] [interactive] body… ¶
iter-lambda
produces an unnamed generator function that works just like a generator function produced withiter-defun
.
- Macro: iter-yield value ¶
When it appears inside a generator function,
iter-yield
indicates that the current iterator should pause and return value fromiter-next
.iter-yield
evaluates to thevalue
parameter of next call toiter-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 usingiter-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 &optional 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 theiter-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 firstiter-next
call to a given iterator, since at the start of iterator’s generator function, the generator function is not evaluating anyiter-yield
form.
- Function: iter-close iterator ¶
If iterator is suspended inside an
unwind-protect
’sbodyform
and becomes unreachable, Emacs will eventually run unwind handlers after a garbage collection pass. (Note thatiter-yield
is illegal inside anunwind-protect
’sunwindforms
.) To ensure that these handlers are run before then, useiter-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 in Common Lisp Extensions.
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 -1) (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) (iter-end-of-sequence ;; Prints -1, which my-iter returned normally (print (cdr x)))))
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.
- Explicit Nonlocal Exits:
catch
andthrow
- Examples of
catch
andthrow
- Errors
- Cleaning Up from 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 (foo-inner))) (defun foo-inner () … (if x (throw 'foo t)) …)
The throw
form, if executed, transfers control straight back to
the corresponding catch
, which returns immediately. The code
following the throw
is not executed. The second argument of
throw
is used as the return value of the catch
.
The function throw
finds the matching catch
based on the
first argument: it searches for a catch
whose first argument is
eq
to the one specified in the throw
. If there is more
than one applicable catch
, the innermost one takes precedence.
Thus, in the above example, the throw
specifies foo
, and
the catch
in foo-outer
specifies the same symbol, so that
catch
is the applicable one (assuming there is no other matching
catch
in between).
Executing throw
exits all Lisp constructs up to the matching
catch
, including function calls. When binding constructs such
as let
or function calls are exited in this way, the bindings
are unbound, just as they are when these constructs exit normally
(see Local Variables). Likewise, throw
restores the buffer
and position saved by save-excursion
(see Excursions), and
the narrowing status saved by save-restriction
. It also runs
any cleanups established with the unwind-protect
special form
when it exits that form (see Cleaning Up from Nonlocal Exits).
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
, andgo
, for example. Emacs Lisp has onlythrow
. The cl-lib library provides versions of some of these. See Blocks and Exits in Common Lisp Extensions.
- Special Form: catch tag body… ¶
-
catch
establishes a return point for thethrow
function. The return point is distinguished from other such return points by tag, which may be any Lisp object exceptnil
. 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 thecatch
.If a
throw
is executed during the execution of body, specifying the same value tag, thecatch
form exits immediately; the value it returns is whatever was specified as the second argument ofthrow
.
- Function: throw tag value ¶
The purpose of
throw
is to return from a return point previously established withcatch
. The argument tag is used to choose among the various existing return points; it must beeq
to the value specified in thecatch
. 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)
.
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)) 'no) -| yes ⇒ no
Since both return points have tags that match the throw
, it goes to
the inner one, the one established in catch2
. Therefore,
catch2
returns normally with value yes
, and this value is
printed. Finally the second body form in the outer catch
, which is
'no
, is evaluated and returned from the outer catch
.
Now let’s change the argument given to catch2
:
(catch 'hack (print (catch2 'quux)) 'no) ⇒ yes
We still have two return points, but this time only the outer one has
the tag hack
; the inner one has the tag quux
instead.
Therefore, throw
makes the outer catch
return the value
yes
. The function print
is never called, and the
body-form 'no
is never evaluated.
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 Cleaning Up from Nonlocal Exits.) 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.
For reporting problems without terminating the execution of the current command, consider issuing a warning instead. See Reporting Warnings.
Resist the temptation to use error handling to transfer control from
one part of the program to another; use catch
and throw
instead. See Explicit Nonlocal Exits: catch
and throw
.
- How to Signal an Error
- How Emacs Processes Errors
- Writing Code to Handle Errors
- Error Symbols and Condition Names
11.7.3.1 How to Signal an Error
Signaling an error means beginning error processing. Error processing normally aborts all or part of the running program and returns to a point that is set up to handle the error (see How Emacs Processes 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 callingsignal
with two arguments: the error symbolerror
, and a list containing the string returned byformat-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 and Condition Names, 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. Forerror
, the error message is the CAR of data (that must be a string). Subcategories offile-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 Writing Code to Handle 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 symboluser-error
rather thanerror
. 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 commandInfo-history-back
(l) to move back beyond the start of your Info browsing history, Emacs signals auser-error
. Such errors do not cause entry to the debugger, even whendebug-on-error
is non-nil
. See Entering the Debugger on an Error.
Common Lisp note: Emacs Lisp has nothing like the Common Lisp concept of continuable errors.
11.7.3.2 How Emacs Processes Errors
When an error is signaled, signal
searches for an active
handler for the error. A handler is a sequence of Lisp
expressions designated to be executed if an error happens in part of the
Lisp program. If the error has an applicable handler, the handler is
executed, and control resumes following the handler. The handler
executes in the environment of the condition-case
that
established it; all functions called within that condition-case
have already been exited, and the handler cannot return to them.
If there is no applicable handler for the error, it terminates the
current command and returns control to the editor command loop. (The
command loop has an implicit handler for all kinds of errors.) The
command loop’s handler uses the error symbol and associated data to
print an error message. You can use the variable
command-error-function
to control how this is done:
- 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 thatcondition-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 Entering the Debugger on an Error) 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.
11.7.3.3 Writing Code to Handle Errors
The usual effect of signaling an error is to terminate the command
that is running and return immediately to the Emacs editor command loop.
You can arrange to trap errors occurring in a part of your program by
establishing an error handler, with the special form
condition-case
. A simple example looks like this:
(condition-case nil (delete-file filename) (error nil))
This deletes the file named filename, catching any error and
returning nil
if an error occurs. (You can use the macro
ignore-errors
for a simple case like this; see below.)
The condition-case
construct is often used to trap errors that
are predictable, such as failure to open a file in a call to
insert-file-contents
. It is also used to trap errors that are
totally unpredictable, such as when the program evaluates an expression
read from the user.
The second argument of condition-case
is called the
protected form. (In the example above, the protected form is a
call to delete-file
.) The error handlers go into effect when
this form begins execution and are deactivated when this form returns.
They remain in effect for all the intervening time. In particular, they
are in effect during the execution of functions called by this form, in
their subroutines, and so on. This is a good thing, since, strictly
speaking, errors can be signaled only by Lisp primitives (including
signal
and error
) called by the protected form, not by the
protected form itself.
The arguments after the protected form are handlers. Each handler
lists one or more condition names (which are symbols) to specify
which errors it will handle. The error symbol specified when an error
is signaled also defines a list of condition names. A handler applies
to an error if they have any condition names in common. In the example
above, there is one handler, and it specifies one condition name,
error
, which covers all errors.
The search for an applicable handler checks all the established handlers
starting with the most recently established one. Thus, if two nested
condition-case
forms offer to handle the same error, the inner of
the two gets to handle it.
If an error is handled by some condition-case
form, this
ordinarily prevents the debugger from being run, even if
debug-on-error
says this error should invoke the debugger.
If you want to be able to debug errors that are caught by a
condition-case
, set the variable debug-on-signal
to a
non-nil
value. You can also specify that a particular handler
should let the debugger run first, by writing debug
among the
conditions, like this:
(condition-case nil (delete-file filename) ((debug error) nil))
The effect of debug
here is only to prevent
condition-case
from suppressing the call to the debugger. Any
given error will invoke the debugger only if debug-on-error
and
the other usual filtering mechanisms say it should. See Entering the Debugger on an Error.
- 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 likecondition-case
, unless the variabledebug-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 Explicit Nonlocal Exits: 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 the absence of a success handler; see below). In this case, thecondition-case
has no effect. Thecondition-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 includedebug
to allow the debugger to run before the handler). A condition name oft
matches any condition. 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) (message "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 and Condition Names). Emacs searches all the active
condition-case
forms for a handler that specifies one or more of these condition names; the innermost matchingcondition-case
handles the error. Within thiscondition-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.As a special case, one of the handlers can be a list of the form
(:success body…)
, where body is executed with var (if non-nil
) bound to the return value of protected-form when that expression terminates without error.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 tocondition-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)) 1000000))) ⇒ safe-divide
(safe-divide 5 0) -| Arithmetic error: (arith-error) ⇒ 1000000
The handler specifies condition name arith-error
so that it
will handle only division-by-zero errors. Other kinds of errors will
not be handled (by this condition-case
). Thus:
(safe-divide nil 3) error→ Wrong type argument: number-or-marker-p, nil
Here is a condition-case
that catches all kinds of errors,
including those from error
:
(setq baz 34) ⇒ 34
(condition-case err
(if (eq baz 35)
t
;; This is a call to the function error
.
(error "Rats! The variable %s was %s, not 35" 'baz baz))
;; This is the handler; it is not a form.
(error (princ (format "The error was: %s" err))
2))
-| The error was: (error "Rats! The variable baz was 34, not 35")
⇒ 2
- 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 returnsnil
.Here’s the example at the beginning of this subsection rewritten using
ignore-errors
:(ignore-errors (delete-file filename))
- Macro: ignore-error condition body… ¶
This macro is like
ignore-errors
, but will only ignore the specific error condition specified.(ignore-error end-of-file (read ""))
condition can also be a list of error conditions.
- 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"
. Usewith-demoted-errors
around code that is not expected to signal errors, but should be robust if one does occur. Note that this macro usescondition-case-unless-debug
rather thancondition-case
.
11.7.3.4 Error Symbols and Condition Names
When you signal an error, you specify an error symbol to specify the kind of error you have in mind. Each error has one and only one error symbol to categorize it. This is the finest classification of errors defined by the Emacs Lisp language.
These narrow classifications are grouped into a hierarchy of wider
classes called error conditions, identified by condition
names. The narrowest such classes belong to the error symbols
themselves: each error symbol is also a condition name. There are also
condition names for more extensive classes, up to the condition name
error
which takes in all kinds of errors (but not quit
).
Thus, each error has one or more condition names: error
, the
error symbol if that is distinct from error
, and perhaps some
intermediate classifications.
- 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 toerror
). 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 symbolerror
. Because quitting is not considered an error, the set of parents ofquit
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.
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 Change Groups.)
- 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 athrow
out of theunwind-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 athrow
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 anotherunwind-protect
around that form.
For example, here we make an invisible buffer for temporary use, and make sure to kill it before finishing:
(let ((buffer (get-buffer-create " *temp*"))) (with-current-buffer buffer (unwind-protect body-form (kill-buffer buffer))))
You might think that we could just as well write (kill-buffer
(current-buffer))
and dispense with the variable buffer
.
However, the way shown above is safer, if body-form happens to
get an error after switching to a different buffer! (Alternatively,
you could write a save-current-buffer
around body-form,
to ensure that the temporary buffer becomes current again in time to
kill it.)
Emacs includes a standard macro called with-temp-buffer
which
expands into more or less the code shown above (see Current Buffer). Several of the macros defined in
this manual use unwind-protect
in this way.
Here is an actual example derived from an FTP package. It creates a
process (see Processes) to try to establish a connection to a remote
machine. As the function ftp-login
is highly susceptible to
numerous problems that the writer of the function cannot anticipate, it
is protected with a form that guarantees deletion of the process in the
event of failure. Otherwise, Emacs might fill up with useless
subprocesses.
(let ((win nil)) (unwind-protect (progn (setq process (ftp-setup-buffer host file)) (if (setq win (ftp-login process host user password)) (message "Logged in") (error "Ftp login failed"))) (or win (and process (delete-process process)))))
This example has a small bug: if the user types C-g to
quit, and the quit happens immediately after the function
ftp-setup-buffer
returns but before the variable process
is
set, the process will not be killed. There is no easy way to fix this bug,
but at least it is very unlikely.
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 cell9. See Symbol Components. In Emacs Lisp, the use of a symbol as a variable is independent of its use as a function name.
As previously noted in this manual, a Lisp program is represented primarily by Lisp objects, and only secondarily as text. The textual form of a Lisp program is given by the read syntax of the Lisp objects that constitute the program. Hence, the textual form of a variable in a Lisp program is written using the read syntax for the symbol representing the variable.
- Global Variables
- Variables that Never Change
- Local Variables
- When a Variable is Void
- Defining Global Variables
- Tips for Defining Variables Robustly
- Accessing Variable Values
- Setting Variable Values
- Running a function when a variable is changed.
- Scoping Rules for Variable Bindings
- Buffer-Local Variables
- File Local Variables
- Directory Local Variables
- Connection Local Variables
- Variable Aliases
- Variables with Restricted Values
- Generalized Variables
- Multisession 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:
x ⇒ (a b)
(setq x 4) ⇒ 4
x ⇒ 4
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 returnsnil
otherwise.
These constants are fundamentally different from the constants
defined using the defconst
special form (see Defining Global 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.
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 Scoping Rules for Variable Bindings. Lately Emacs is moving towards using lexical binding in more and more places, with the goal of eventually making lexical binding the default. In particular, all Emacs Lisp source files and the *scratch* buffer use lexical 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 ofy
, which is 2, not the new value ofy
, 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 thislet*
form. Compare the following example with the example above forlet
.(setq y 2) ⇒ 2
(let* ((y 1) (z y)) ; Use the just-established value of
y
. (list y z)) ⇒ (1 1)Basically, the
let*
binding ofx
andy
in the previous example is equivalent to using nestedlet
bindings:(let ((y 1)) (let ((z y)) (list y z)))
- Special Form: letrec (bindings…) forms… ¶
This special form is like
let*
, but all the variables are bound before any of the local values are computed. The values are then assigned to the locally bound variables. This is only useful when lexical binding is in effect, and you want to create closures that refer to bindings that would otherwise not yet be in effect when usinglet*
.For instance, here’s a closure that removes itself from a hook after being run once:
(letrec ((hookfun (lambda () (message "Run once") (remove-hook 'post-command-hook hookfun)))) (add-hook 'post-command-hook hookfun))
- Special Form: dlet (bindings…) forms… ¶
This special form is like
let
, but it binds all variables dynamically. This is rarely useful—you usual