Short Contents ************** Emacs Lisp 1 Introduction 2 Lisp Data Types 3 Numbers 4 Strings and Characters 5 Lists 6 Sequences, Arrays, and Vectors 7 Hash Tables 8 Symbols 9 Evaluation 10 Control Structures 11 Variables 12 Functions 13 Macros 14 Customization Settings 15 Loading 16 Byte Compilation 17 Advising Emacs Lisp Functions 18 Debugging Lisp Programs 19 Reading and Printing Lisp Objects 20 Minibuffers 21 Command Loop 22 Keymaps 23 Major and Minor Modes 24 Documentation 25 Files 26 Backups and Auto-Saving 27 Buffers 28 Windows 29 Frames 30 Positions 31 Markers 32 Text 33 Non-ASCII Characters 34 Searching and Matching 35 Syntax Tables 36 Abbrevs and Abbrev Expansion 37 Processes 38 Emacs Display 39 Operating System Interface 40 Preparing Lisp code for distribution Appendix A Emacs 23 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 Table of Contents ***************** Emacs Lisp 1 Introduction 1.1 Caveats 1.2 Lisp History 1.3 Conventions 1.3.1 Some Terms 1.3.2 `nil' and `t' 1.3.3 Evaluation Notation 1.3.4 Printing Notation 1.3.5 Error Messages 1.3.6 Buffer Text Notation 1.3.7 Format of Descriptions 1.3.7.1 A Sample Function Description 1.3.7.2 A Sample Variable Description 1.4 Version Information 1.5 Acknowledgments 2 Lisp Data Types 2.1 Printed Representation and Read Syntax 2.2 Comments 2.3 Programming Types 2.3.1 Integer Type 2.3.2 Floating Point Type 2.3.3 Character Type 2.3.3.1 Basic Char Syntax 2.3.3.2 General Escape Syntax 2.3.3.3 Control-Character Syntax 2.3.3.4 Meta-Character Syntax 2.3.3.5 Other Character Modifier Bits 2.3.4 Symbol Type 2.3.5 Sequence Types 2.3.6 Cons Cell and List Types 2.3.6.1 Drawing Lists as Box Diagrams 2.3.6.2 Dotted Pair Notation 2.3.6.3 Association List Type 2.3.7 Array Type 2.3.8 String Type 2.3.8.1 Syntax for Strings 2.3.8.2 Non-ASCII Characters in Strings 2.3.8.3 Nonprinting Characters in Strings 2.3.8.4 Text Properties in Strings 2.3.9 Vector Type 2.3.10 Char-Table Type 2.3.11 Bool-Vector Type 2.3.12 Hash Table Type 2.3.13 Function Type 2.3.14 Macro Type 2.3.15 Primitive Function Type 2.3.16 Byte-Code Function Type 2.3.17 Autoload Type 2.4 Editing Types 2.4.1 Buffer Type 2.4.2 Marker Type 2.4.3 Window Type 2.4.4 Frame Type 2.4.5 Terminal Type 2.4.6 Window Configuration Type 2.4.7 Frame Configuration Type 2.4.8 Process Type 2.4.9 Stream Type 2.4.10 Keymap Type 2.4.11 Overlay Type 2.4.12 Font Type 2.5 Read Syntax for Circular Objects 2.6 Type Predicates 2.7 Equality Predicates 3 Numbers 3.1 Integer Basics 3.2 Floating Point Basics 3.3 Type Predicates for Numbers 3.4 Comparison of Numbers 3.5 Numeric Conversions 3.6 Arithmetic Operations 3.7 Rounding Operations 3.8 Bitwise Operations on Integers 3.9 Standard Mathematical Functions 3.10 Random Numbers 4 Strings and Characters 4.1 String and Character Basics 4.2 Predicates for Strings 4.3 Creating Strings 4.4 Modifying Strings 4.5 Comparison of Characters and Strings 4.6 Conversion of Characters and Strings 4.7 Formatting Strings 4.8 Case Conversion in Lisp 4.9 The Case Table 5 Lists 5.1 Lists and Cons Cells 5.2 Predicates on Lists 5.3 Accessing Elements of Lists 5.4 Building Cons Cells and Lists 5.5 Modifying List Variables 5.6 Modifying Existing List Structure 5.6.1 Altering List Elements with `setcar' 5.6.2 Altering the CDR of a List 5.6.3 Functions that Rearrange Lists 5.7 Using Lists as Sets 5.8 Association Lists 5.9 Property Lists 5.9.1 Property Lists and Association Lists 5.9.2 Property Lists Outside Symbols 6 Sequences, Arrays, and Vectors 6.1 Sequences 6.2 Arrays 6.3 Functions that Operate on Arrays 6.4 Vectors 6.5 Functions for Vectors 6.6 Char-Tables 6.7 Bool-vectors 6.8 Managing a Fixed-Size Ring of Objects 7 Hash Tables 7.1 Creating Hash Tables 7.2 Hash Table Access 7.3 Defining Hash Comparisons 7.4 Other Hash Table Functions 8 Symbols 8.1 Symbol Components 8.2 Defining Symbols 8.3 Creating and Interning Symbols 8.4 Symbol Properties 8.4.1 Accessing Symbol Properties 8.4.2 Standard Symbol Properties 9 Evaluation 9.1 Introduction to Evaluation 9.2 Kinds of Forms 9.2.1 Self-Evaluating Forms 9.2.2 Symbol Forms 9.2.3 Classification of List Forms 9.2.4 Symbol Function Indirection 9.2.5 Evaluation of Function Forms 9.2.6 Lisp Macro Evaluation 9.2.7 Special Forms 9.2.8 Autoloading 9.3 Quoting 9.4 Backquote 9.5 Eval 10 Control Structures 10.1 Sequencing 10.2 Conditionals 10.2.1 Pattern matching case statement 10.3 Constructs for Combining Conditions 10.4 Iteration 10.5 Nonlocal Exits 10.5.1 Explicit Nonlocal Exits: `catch' and `throw' 10.5.2 Examples of `catch' and `throw' 10.5.3 Errors 10.5.3.1 How to Signal an Error 10.5.3.2 How Emacs Processes Errors 10.5.3.3 Writing Code to Handle Errors 10.5.3.4 Error Symbols and Condition Names 10.5.4 Cleaning Up from Nonlocal Exits 11 Variables 11.1 Global Variables 11.2 Variables that Never Change 11.3 Local Variables 11.4 When a Variable is "Void" 11.5 Defining Global Variables 11.6 Tips for Defining Variables Robustly 11.7 Accessing Variable Values 11.8 Setting Variable Values 11.9 Scoping Rules for Variable Bindings 11.9.1 Dynamic Binding 11.9.2 Proper Use of Dynamic Binding 11.9.3 Lexical Binding 11.9.4 Using Lexical Binding 11.10 Buffer-Local Variables 11.10.1 Introduction to Buffer-Local Variables 11.10.2 Creating and Deleting Buffer-Local Bindings 11.10.3 The Default Value of a Buffer-Local Variable 11.11 File Local Variables 11.12 Directory Local Variables 11.13 Variable Aliases 11.14 Variables with Restricted Values 11.15 Generalized Variables 11.15.1 The `setf' Macro 11.15.2 Defining new `setf' forms 12 Functions 12.1 What Is a Function? 12.2 Lambda Expressions 12.2.1 Components of a Lambda Expression 12.2.2 A Simple Lambda Expression Example 12.2.3 Other Features of Argument Lists 12.2.4 Documentation Strings of Functions 12.3 Naming a Function 12.4 Defining Functions 12.5 Calling Functions 12.6 Mapping Functions 12.7 Anonymous Functions 12.8 Accessing Function Cell Contents 12.9 Closures 12.10 Declaring Functions Obsolete 12.11 Inline Functions 12.12 The `declare' Form 12.13 Telling the Compiler that a Function is Defined 12.14 Determining whether a Function is Safe to Call 12.15 Other Topics Related to Functions 13 Macros 13.1 A Simple Example of a Macro 13.2 Expansion of a Macro Call 13.3 Macros and Byte Compilation 13.4 Defining Macros 13.5 Common Problems Using Macros 13.5.1 Wrong Time 13.5.2 Evaluating Macro Arguments Repeatedly 13.5.3 Local Variables in Macro Expansions 13.5.4 Evaluating Macro Arguments in Expansion 13.5.5 How Many Times is the Macro Expanded? 13.6 Indenting Macros 14 Customization Settings 14.1 Common Item Keywords 14.2 Defining Customization Groups 14.3 Defining Customization Variables 14.4 Customization Types 14.4.1 Simple Types 14.4.2 Composite Types 14.4.3 Splicing into Lists 14.4.4 Type Keywords 14.4.5 Defining New Types 14.5 Applying Customizations 14.6 Custom Themes 15 Loading 15.1 How Programs Do Loading 15.2 Load Suffixes 15.3 Library Search 15.4 Loading Non-ASCII Characters 15.5 Autoload 15.6 Repeated Loading 15.7 Features 15.8 Which File Defined a Certain Symbol 15.9 Unloading 15.10 Hooks for Loading 16 Byte Compilation 16.1 Performance of Byte-Compiled Code 16.2 Byte-Compilation Functions 16.3 Documentation Strings and Compilation 16.4 Dynamic Loading of Individual Functions 16.5 Evaluation During Compilation 16.6 Compiler Errors 16.7 Byte-Code Function Objects 16.8 Disassembled Byte-Code 17 Advising Emacs Lisp Functions 17.1 A Simple Advice Example 17.2 Defining Advice 17.3 Around-Advice 17.4 Computed Advice 17.5 Activation of Advice 17.6 Enabling and Disabling Advice 17.7 Preactivation 17.8 Argument Access in Advice 17.9 The Combined Definition 18 Debugging Lisp Programs 18.1 The Lisp Debugger 18.1.1 Entering the Debugger on an Error 18.1.2 Debugging Infinite Loops 18.1.3 Entering the Debugger on a Function Call 18.1.4 Explicit Entry to the Debugger 18.1.5 Using the Debugger 18.1.6 Debugger Commands 18.1.7 Invoking the Debugger 18.1.8 Internals of the Debugger 18.2 Edebug 18.2.1 Using Edebug 18.2.2 Instrumenting for Edebug 18.2.3 Edebug Execution Modes 18.2.4 Jumping 18.2.5 Miscellaneous Edebug Commands 18.2.6 Breaks 18.2.6.1 Edebug Breakpoints 18.2.6.2 Global Break Condition 18.2.6.3 Source Breakpoints 18.2.7 Trapping Errors 18.2.8 Edebug Views 18.2.9 Evaluation 18.2.10 Evaluation List Buffer 18.2.11 Printing in Edebug 18.2.12 Trace Buffer 18.2.13 Coverage Testing 18.2.14 The Outside Context 18.2.14.1 Checking Whether to Stop 18.2.14.2 Edebug Display Update 18.2.14.3 Edebug Recursive Edit 18.2.15 Edebug and Macros 18.2.15.1 Instrumenting Macro Calls 18.2.15.2 Specification List 18.2.15.3 Backtracking in Specifications 18.2.15.4 Specification Examples 18.2.16 Edebug Options 18.3 Debugging Invalid Lisp Syntax 18.3.1 Excess Open Parentheses 18.3.2 Excess Close Parentheses 18.4 Test Coverage 18.5 Profiling 19 Reading and Printing Lisp Objects 19.1 Introduction to Reading and Printing 19.2 Input Streams 19.3 Input Functions 19.4 Output Streams 19.5 Output Functions 19.6 Variables Affecting Output 20 Minibuffers 20.1 Introduction to Minibuffers 20.2 Reading Text Strings with the Minibuffer 20.3 Reading Lisp Objects with the Minibuffer 20.4 Minibuffer History 20.5 Initial Input 20.6 Completion 20.6.1 Basic Completion Functions 20.6.2 Completion and the Minibuffer 20.6.3 Minibuffer Commands that Do Completion 20.6.4 High-Level Completion Functions 20.6.5 Reading File Names 20.6.6 Completion Variables 20.6.7 Programmed Completion 20.6.8 Completion in Ordinary Buffers 20.7 Yes-or-No Queries 20.8 Asking Multiple Y-or-N Questions 20.9 Reading a Password 20.10 Minibuffer Commands 20.11 Minibuffer Windows 20.12 Minibuffer Contents 20.13 Recursive Minibuffers 20.14 Minibuffer Miscellany 21 Command Loop 21.1 Command Loop Overview 21.2 Defining Commands 21.2.1 Using `interactive' 21.2.2 Code Characters for `interactive' 21.2.3 Examples of Using `interactive' 21.3 Interactive Call 21.4 Distinguish Interactive Calls 21.5 Information from the Command Loop 21.6 Adjusting Point After Commands 21.7 Input Events 21.7.1 Keyboard Events 21.7.2 Function Keys 21.7.3 Mouse Events 21.7.4 Click Events 21.7.5 Drag Events 21.7.6 Button-Down Events 21.7.7 Repeat Events 21.7.8 Motion Events 21.7.9 Focus Events 21.7.10 Miscellaneous System Events 21.7.11 Event Examples 21.7.12 Classifying Events 21.7.13 Accessing Mouse Events 21.7.14 Accessing Scroll Bar Events 21.7.15 Putting Keyboard Events in Strings 21.8 Reading Input 21.8.1 Key Sequence Input 21.8.2 Reading One Event 21.8.3 Modifying and Translating Input Events 21.8.4 Invoking the Input Method 21.8.5 Quoted Character Input 21.8.6 Miscellaneous Event Input Features 21.9 Special Events 21.10 Waiting for Elapsed Time or Input 21.11 Quitting 21.12 Prefix Command Arguments 21.13 Recursive Editing 21.14 Disabling Commands 21.15 Command History 21.16 Keyboard Macros 22 Keymaps 22.1 Key Sequences 22.2 Keymap Basics 22.3 Format of Keymaps 22.4 Creating Keymaps 22.5 Inheritance and Keymaps 22.6 Prefix Keys 22.7 Active Keymaps 22.8 Searching the Active Keymaps 22.9 Controlling the Active Keymaps 22.10 Key Lookup 22.11 Functions for Key Lookup 22.12 Changing Key Bindings 22.13 Remapping Commands 22.14 Keymaps for Translating Sequences of Events 22.14.1 Interaction with normal keymaps 22.15 Commands for Binding Keys 22.16 Scanning Keymaps 22.17 Menu Keymaps 22.17.1 Defining Menus 22.17.1.1 Simple Menu Items 22.17.1.2 Extended Menu Items 22.17.1.3 Menu Separators 22.17.1.4 Alias Menu Items 22.17.2 Menus and the Mouse 22.17.3 Menus and the Keyboard 22.17.4 Menu Example 22.17.5 The Menu Bar 22.17.6 Tool bars 22.17.7 Modifying Menus 22.17.8 Easy Menu 23 Major and Minor Modes 23.1 Hooks 23.1.1 Running Hooks 23.1.2 Setting Hooks 23.2 Major Modes 23.2.1 Major Mode Conventions 23.2.2 How Emacs Chooses a Major Mode 23.2.3 Getting Help about a Major Mode 23.2.4 Defining Derived Modes 23.2.5 Basic Major Modes 23.2.6 Mode Hooks 23.2.7 Tabulated List mode 23.2.8 Generic Modes 23.2.9 Major Mode Examples 23.3 Minor Modes 23.3.1 Conventions for Writing Minor Modes 23.3.2 Keymaps and Minor Modes 23.3.3 Defining Minor Modes 23.4 Mode Line Format 23.4.1 Mode Line Basics 23.4.2 The Data Structure of the Mode Line 23.4.3 The Top Level of Mode Line Control 23.4.4 Variables Used in the Mode Line 23.4.5 `%'-Constructs in the Mode Line 23.4.6 Properties in the Mode Line 23.4.7 Window Header Lines 23.4.8 Emulating Mode Line Formatting 23.5 Imenu 23.6 Font Lock Mode 23.6.1 Font Lock Basics 23.6.2 Search-based Fontification 23.6.3 Customizing Search-Based Fontification 23.6.4 Other Font Lock Variables 23.6.5 Levels of Font Lock 23.6.6 Precalculated Fontification 23.6.7 Faces for Font Lock 23.6.8 Syntactic Font Lock 23.6.9 Multiline Font Lock Constructs 23.6.9.1 Font Lock Multiline 23.6.9.2 Region to Fontify after a Buffer Change 23.7 Automatic Indentation of code 23.7.1 Simple Minded Indentation Engine 23.7.1.1 SMIE Setup and Features 23.7.1.2 Operator Precedence Grammars 23.7.1.3 Defining the Grammar of a Language 23.7.1.4 Defining Tokens 23.7.1.5 Living With a Weak Parser 23.7.1.6 Specifying Indentation Rules 23.7.1.7 Helper Functions for Indentation Rules 23.7.1.8 Sample Indentation Rules 23.8 Desktop Save Mode 24 Documentation 24.1 Documentation Basics 24.2 Access to Documentation Strings 24.3 Substituting Key Bindings in Documentation 24.4 Describing Characters for Help Messages 24.5 Help Functions 25 Files 25.1 Visiting Files 25.1.1 Functions for Visiting Files 25.1.2 Subroutines of Visiting 25.2 Saving Buffers 25.3 Reading from Files 25.4 Writing to Files 25.5 File Locks 25.6 Information about Files 25.6.1 Testing Accessibility 25.6.2 Distinguishing Kinds of Files 25.6.3 Truenames 25.6.4 Other Information about Files 25.6.5 How to Locate Files in Standard Places 25.7 Changing File Names and Attributes 25.8 File Names 25.8.1 File Name Components 25.8.2 Absolute and Relative File Names 25.8.3 Directory Names 25.8.4 Functions that Expand Filenames 25.8.5 Generating Unique File Names 25.8.6 File Name Completion 25.8.7 Standard File Names 25.9 Contents of Directories 25.10 Creating, Copying and Deleting Directories 25.11 Making Certain File Names "Magic" 25.12 File Format Conversion 25.12.1 Overview 25.12.2 Round-Trip Specification 25.12.3 Piecemeal Specification 26 Backups and Auto-Saving 26.1 Backup Files 26.1.1 Making Backup Files 26.1.2 Backup by Renaming or by Copying? 26.1.3 Making and Deleting Numbered Backup Files 26.1.4 Naming Backup Files 26.2 Auto-Saving 26.3 Reverting 27 Buffers 27.1 Buffer Basics 27.2 The Current Buffer 27.3 Buffer Names 27.4 Buffer File Name 27.5 Buffer Modification 27.6 Buffer Modification Time 27.7 Read-Only Buffers 27.8 The Buffer List 27.9 Creating Buffers 27.10 Killing Buffers 27.11 Indirect Buffers 27.12 Swapping Text Between Two Buffers 27.13 The Buffer Gap 28 Windows 28.1 Basic Concepts of Emacs Windows 28.2 Windows and Frames 28.3 Window Sizes 28.4 Resizing Windows 28.5 Splitting Windows 28.6 Deleting Windows 28.7 Recombining Windows 28.8 Selecting Windows 28.9 Cyclic Ordering of Windows 28.10 Buffers and Windows 28.11 Switching to a Buffer in a Window 28.12 Choosing a Window for Display 28.13 Action Functions for `display-buffer' 28.14 Additional Options for Displaying Buffers 28.15 Window History 28.16 Dedicated Windows 28.17 Quitting Windows 28.18 Windows and Point 28.19 The Window Start and End Positions 28.20 Textual Scrolling 28.21 Vertical Fractional Scrolling 28.22 Horizontal Scrolling 28.23 Coordinates and Windows 28.24 Window Configurations 28.25 Window Parameters 28.26 Hooks for Window Scrolling and Changes 29 Frames 29.1 Creating Frames 29.2 Multiple Terminals 29.3 Frame Parameters 29.3.1 Access to Frame Parameters 29.3.2 Initial Frame Parameters 29.3.3 Window Frame Parameters 29.3.3.1 Basic Parameters 29.3.3.2 Position Parameters 29.3.3.3 Size Parameters 29.3.3.4 Layout Parameters 29.3.3.5 Buffer Parameters 29.3.3.6 Window Management Parameters 29.3.3.7 Cursor Parameters 29.3.3.8 Font and Color Parameters 29.3.4 Frame Size And Position 29.3.5 Geometry 29.4 Terminal Parameters 29.5 Frame Titles 29.6 Deleting Frames 29.7 Finding All Frames 29.8 Minibuffers and Frames 29.9 Input Focus 29.10 Visibility of Frames 29.11 Raising and Lowering Frames 29.12 Frame Configurations 29.13 Mouse Tracking 29.14 Mouse Position 29.15 Pop-Up Menus 29.16 Dialog Boxes 29.17 Pointer Shape 29.18 Window System Selections 29.19 Drag and Drop 29.20 Color Names 29.21 Text Terminal Colors 29.22 X Resources 29.23 Display Feature Testing 30 Positions 30.1 Point 30.2 Motion 30.2.1 Motion by Characters 30.2.2 Motion by Words 30.2.3 Motion to an End of the Buffer 30.2.4 Motion by Text Lines 30.2.5 Motion by Screen Lines 30.2.6 Moving over Balanced Expressions 30.2.7 Skipping Characters 30.3 Excursions 30.4 Narrowing 31 Markers 31.1 Overview of Markers 31.2 Predicates on Markers 31.3 Functions that Create Markers 31.4 Information from Markers 31.5 Marker Insertion Types 31.6 Moving Marker Positions 31.7 The Mark 31.8 The Region 32 Text 32.1 Examining Text Near Point 32.2 Examining Buffer Contents 32.3 Comparing Text 32.4 Inserting Text 32.5 User-Level Insertion Commands 32.6 Deleting Text 32.7 User-Level Deletion Commands 32.8 The Kill Ring 32.8.1 Kill Ring Concepts 32.8.2 Functions for Killing 32.8.3 Yanking 32.8.4 Functions for Yanking 32.8.5 Low-Level Kill Ring 32.8.6 Internals of the Kill Ring 32.9 Undo 32.10 Maintaining Undo Lists 32.11 Filling 32.12 Margins for Filling 32.13 Adaptive Fill Mode 32.14 Auto Filling 32.15 Sorting Text 32.16 Counting Columns 32.17 Indentation 32.17.1 Indentation Primitives 32.17.2 Indentation Controlled by Major Mode 32.17.3 Indenting an Entire Region 32.17.4 Indentation Relative to Previous Lines 32.17.5 Adjustable "Tab Stops" 32.17.6 Indentation-Based Motion Commands 32.18 Case Changes 32.19 Text Properties 32.19.1 Examining Text Properties 32.19.2 Changing Text Properties 32.19.3 Text Property Search Functions 32.19.4 Properties with Special Meanings 32.19.5 Formatted Text Properties 32.19.6 Stickiness of Text Properties 32.19.7 Lazy Computation of Text Properties 32.19.8 Defining Clickable Text 32.19.9 Defining and Using Fields 32.19.10 Why Text Properties are not Intervals 32.20 Substituting for a Character Code 32.21 Registers 32.22 Transposition of Text 32.23 Base 64 Encoding 32.24 Checksum/Hash 32.25 Parsing HTML and XML 32.26 Atomic Change Groups 32.27 Change Hooks 33 Non-ASCII Characters 33.1 Text Representations 33.2 Converting Text Representations 33.3 Selecting a Representation 33.4 Character Codes 33.5 Character Properties 33.6 Character Sets 33.7 Scanning for Character Sets 33.8 Translation of Characters 33.9 Coding Systems 33.9.1 Basic Concepts of Coding Systems 33.9.2 Encoding and I/O 33.9.3 Coding Systems in Lisp 33.9.4 User-Chosen Coding Systems 33.9.5 Default Coding Systems 33.9.6 Specifying a Coding System for One Operation 33.9.7 Explicit Encoding and Decoding 33.9.8 Terminal I/O Encoding 33.9.9 MS-DOS File Types 33.10 Input Methods 33.11 Locales 34 Searching and Matching 34.1 Searching for Strings 34.2 Searching and Case 34.3 Regular Expressions 34.3.1 Syntax of Regular Expressions 34.3.1.1 Special Characters in Regular Expressions 34.3.1.2 Character Classes 34.3.1.3 Backslash Constructs in Regular Expressions 34.3.2 Complex Regexp Example 34.3.3 Regular Expression Functions 34.4 Regular Expression Searching 34.5 POSIX Regular Expression Searching 34.6 The Match Data 34.6.1 Replacing the Text that Matched 34.6.2 Simple Match Data Access 34.6.3 Accessing the Entire Match Data 34.6.4 Saving and Restoring the Match Data 34.7 Search and Replace 34.8 Standard Regular Expressions Used in Editing 35 Syntax Tables 35.1 Syntax Table Concepts 35.2 Syntax Descriptors 35.2.1 Table of Syntax Classes 35.2.2 Syntax Flags 35.3 Syntax Table Functions 35.4 Syntax Properties 35.5 Motion and Syntax 35.6 Parsing Expressions 35.6.1 Motion Commands Based on Parsing 35.6.2 Finding the Parse State for a Position 35.6.3 Parser State 35.6.4 Low-Level Parsing 35.6.5 Parameters to Control Parsing 35.7 Syntax Table Internals 35.8 Categories 36 Abbrevs and Abbrev Expansion 36.1 Abbrev Tables 36.2 Defining Abbrevs 36.3 Saving Abbrevs in Files 36.4 Looking Up and Expanding Abbreviations 36.5 Standard Abbrev Tables 36.6 Abbrev Properties 36.7 Abbrev Table Properties 37 Processes 37.1 Functions that Create Subprocesses 37.2 Shell Arguments 37.3 Creating a Synchronous Process 37.4 Creating an Asynchronous Process 37.5 Deleting Processes 37.6 Process Information 37.7 Sending Input to Processes 37.8 Sending Signals to Processes 37.9 Receiving Output from Processes 37.9.1 Process Buffers 37.9.2 Process Filter Functions 37.9.3 Decoding Process Output 37.9.4 Accepting Output from Processes 37.10 Sentinels: Detecting Process Status Changes 37.11 Querying Before Exit 37.12 Accessing Other Processes 37.13 Transaction Queues 37.14 Network Connections 37.15 Network Servers 37.16 Datagrams 37.17 Low-Level Network Access 37.17.1 `make-network-process' 37.17.2 Network Options 37.17.3 Testing Availability of Network Features 37.18 Misc Network Facilities 37.19 Communicating with Serial Ports 37.20 Packing and Unpacking Byte Arrays 37.20.1 Describing Data Layout 37.20.2 Functions to Unpack and Pack Bytes 37.20.3 Examples of Byte Unpacking and Packing 38 Emacs Display 38.1 Refreshing the Screen 38.2 Forcing Redisplay 38.3 Truncation 38.4 The Echo Area 38.4.1 Displaying Messages in the Echo Area 38.4.2 Reporting Operation Progress 38.4.3 Logging Messages in `*Messages*' 38.4.4 Echo Area Customization 38.5 Reporting Warnings 38.5.1 Warning Basics 38.5.2 Warning Variables 38.5.3 Warning Options 38.5.4 Delayed Warnings 38.6 Invisible Text 38.7 Selective Display 38.8 Temporary Displays 38.9 Overlays 38.9.1 Managing Overlays 38.9.2 Overlay Properties 38.9.3 Searching for Overlays 38.10 Width 38.11 Line Height 38.12 Faces 38.12.1 Face Attributes 38.12.2 Defining Faces 38.12.3 Face Attribute Functions 38.12.4 Displaying Faces 38.12.5 Face Remapping 38.12.6 Functions for Working with Faces 38.12.7 Automatic Face Assignment 38.12.8 Basic Faces 38.12.9 Font Selection 38.12.10 Looking Up Fonts 38.12.11 Fontsets 38.12.12 Low-Level Font Representation 38.13 Fringes 38.13.1 Fringe Size and Position 38.13.2 Fringe Indicators 38.13.3 Fringe Cursors 38.13.4 Fringe Bitmaps 38.13.5 Customizing Fringe Bitmaps 38.13.6 The Overlay Arrow 38.14 Scroll Bars 38.15 The `display' Property 38.15.1 Display Specs That Replace The Text 38.15.2 Specified Spaces 38.15.3 Pixel Specification for Spaces 38.15.4 Other Display Specifications 38.15.5 Displaying in the Margins 38.16 Images 38.16.1 Image Formats 38.16.2 Image Descriptors 38.16.3 XBM Images 38.16.4 XPM Images 38.16.5 GIF Images 38.16.6 TIFF Images 38.16.7 PostScript Images 38.16.8 ImageMagick Images 38.16.9 Other Image Types 38.16.10 Defining Images 38.16.11 Showing Images 38.16.12 Animated Images 38.16.13 Image Cache 38.17 Buttons 38.17.1 Button Properties 38.17.2 Button Types 38.17.3 Making Buttons 38.17.4 Manipulating Buttons 38.17.5 Button Buffer Commands 38.18 Abstract Display 38.18.1 Abstract Display Functions 38.18.2 Abstract Display Example 38.19 Blinking Parentheses 38.20 Character Display 38.20.1 Usual Display Conventions 38.20.2 Display Tables 38.20.3 Active Display Table 38.20.4 Glyphs 38.20.5 Glyphless Character Display 38.21 Beeping 38.22 Window Systems 38.23 Bidirectional Display 39 Operating System Interface 39.1 Starting Up Emacs 39.1.1 Summary: Sequence of Actions at Startup 39.1.2 The Init File 39.1.3 Terminal-Specific Initialization 39.1.4 Command-Line Arguments 39.2 Getting Out of Emacs 39.2.1 Killing Emacs 39.2.2 Suspending Emacs 39.3 Operating System Environment 39.4 User Identification 39.5 Time of Day 39.6 Time Conversion 39.7 Parsing and Formatting Times 39.8 Processor Run time 39.9 Time Calculations 39.10 Timers for Delayed Execution 39.11 Idle Timers 39.12 Terminal Input 39.12.1 Input Modes 39.12.2 Recording Input 39.13 Terminal Output 39.14 Sound Output 39.15 Operating on X11 Keysyms 39.16 Batch Mode 39.17 Session Management 39.18 Desktop Notifications 39.19 Dynamically Loaded Libraries 40 Preparing Lisp code for distribution 40.1 Packaging Basics 40.2 Simple Packages 40.3 Multi-file Packages 40.4 Creating and Maintaining Package Archives Appendix A Emacs 23 Antinews A.1 Old Lisp Features in Emacs 23 Appendix B GNU Free Documentation License Appendix C GNU General Public License Appendix D Tips and Conventions D.1 Emacs Lisp Coding Conventions D.2 Key Binding Conventions D.3 Emacs Programming Tips D.4 Tips for Making Compiled Code Fast D.5 Tips for Avoiding Compiler Warnings D.6 Tips for Documentation Strings D.7 Tips on Writing Comments D.8 Conventional Headers for Emacs Libraries Appendix E GNU Emacs Internals E.1 Building Emacs E.2 Pure Storage E.3 Garbage Collection E.4 Memory Usage E.5 Writing Emacs Primitives E.6 Object Internals E.6.1 Buffer Internals E.6.2 Window Internals E.6.3 Process Internals Appendix F Standard Errors Appendix G Standard Keymaps Appendix H Standard Hooks Index Emacs Lisp ********** This is the `GNU Emacs Lisp Reference Manual' corresponding to Emacs version 24.3. Copyright (C) 1990-1996, 1998-2013 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." 1 Introduction ************** Most of the GNU Emacs text editor is written in the programming language called Emacs Lisp. You can write new code in Emacs Lisp and install it as an extension to the editor. However, Emacs Lisp is more than a mere "extension language"; it is a full computer programming language in its own right. You can use it as you would any other programming language. Because Emacs Lisp is designed for use in an editor, it has special features for scanning and parsing text as well as features for handling files, buffers, displays, subprocesses, and so on. Emacs Lisp is closely integrated with the editing facilities; thus, editing commands are functions that can also conveniently be called from Lisp programs, and parameters for customization are ordinary Lisp variables. This manual attempts to be a full description of Emacs Lisp. For a beginner's introduction to Emacs Lisp, see `An Introduction to Emacs Lisp Programming', by Bob Chassell, also published by the Free Software Foundation. This manual presumes considerable familiarity with the use of Emacs for editing; see `The GNU Emacs Manual' for this basic information. Generally speaking, the earlier chapters describe features of Emacs Lisp that have counterparts in many programming languages, and later chapters describe features that are peculiar to Emacs Lisp or relate specifically to editing. This is the `GNU Emacs Lisp Reference Manual', corresponding to Emacs version 24.3. 1.1 Caveats =========== This manual has gone through numerous drafts. It is nearly complete but not flawless. There are a few topics that are not covered, either because we consider them secondary (such as most of the individual modes) or because they are yet to be written. Because we are not able to deal with them completely, we have left out several parts intentionally. The manual should be fully correct in what it does cover, and it is therefore open to criticism on anything it says--from specific examples and descriptive text, to the ordering of chapters and sections. If something is confusing, or you find that you have to look at the sources or experiment to learn something not covered in the manual, then perhaps the manual should be fixed. Please let us know. As you use this manual, we ask that you send corrections as soon as you find them. If you think of a simple, real life example for a function or group of functions, please make an effort to write it up and send it in. Please reference any comments to the node name and function or variable name, as appropriate. Also state the number of the edition you are criticizing. Please send comments and corrections using `M-x report-emacs-bug'. 1.2 Lisp History ================ Lisp (LISt Processing language) was first developed in the late 1950s at the Massachusetts Institute of Technology for research in artificial intelligence. The great power of the Lisp language makes it ideal for other purposes as well, such as writing editing commands. Dozens of Lisp implementations have been built over the years, each with its own idiosyncrasies. Many of them were inspired by Maclisp, which was written in the 1960s at MIT's Project MAC. Eventually the implementers of the descendants of Maclisp came together and developed a standard for Lisp systems, called Common Lisp. In the meantime, Gerry Sussman and Guy Steele at MIT developed a simplified but very powerful dialect of Lisp, called Scheme. GNU Emacs Lisp is largely inspired by Maclisp, and a little by Common Lisp. If you know Common Lisp, you will notice many similarities. However, many features of Common Lisp have been omitted or simplified in order to reduce the memory requirements of GNU Emacs. Sometimes the simplifications are so drastic that a Common Lisp user might be very confused. We will occasionally point out how GNU Emacs Lisp differs from Common Lisp. If you don't know Common Lisp, don't worry about it; this manual is self-contained. A certain amount of Common Lisp emulation is available via the `cl-lib' library. *Note Overview: (cl)Top. 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. 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. *Note Printed Representation::, for more details. You, the person reading this manual, are thought of as "the programmer" and are addressed as "you". "The user" is the person who uses Lisp programs, including those you write. Examples of Lisp code are formatted like this: `(list 1 2 3)'. Names that represent metasyntactic variables, or arguments to a function being described, are formatted like this: FIRST-NUMBER. 1.3.2 `nil' and `t' ------------------- In Emacs Lisp, the symbol `nil' has three separate meanings: it is a symbol with the name `nil'; it is the logical truth value FALSE; and it is the empty list--the list of zero elements. When used as a variable, `nil' always has the value `nil'. As far as the Lisp reader is concerned, `()' and `nil' are identical: they stand for the same object, the symbol `nil'. The different ways of writing the symbol are intended entirely for human readers. After the Lisp reader has read either `()' or `nil', there is no way to determine which representation was actually written by the programmer. In this manual, we write `()' when we wish to emphasize that it means the empty list, and we write `nil' when we wish to emphasize that it means the truth value FALSE. That is a good convention to use in Lisp programs also. (cons 'foo ()) ; Emphasize the empty list (setq foo-flag nil) ; Emphasize the truth value FALSE In contexts where a truth value is expected, any non-`nil' value is considered to be TRUE. However, `t' is the preferred way to represent the truth value TRUE. When you need to choose a value which represents TRUE, and there is no other basis for choosing, use `t'. The symbol `t' always has the value `t'. In Emacs Lisp, `nil' and `t' are special symbols that always evaluate to themselves. This is so that you do not need to quote them to use them as constants in a program. An attempt to change their values results in a `setting-constant' error. *Note Constant Variables::. -- Function: booleanp object Return non-`nil' if OBJECT is one of the two canonical boolean values: `t' or `nil'. 1.3.3 Evaluation Notation ------------------------- A Lisp expression that you can evaluate is called a "form". Evaluating a form always produces a result, which is a Lisp object. In the examples in this manual, this is indicated with `=>': (car '(1 2)) => 1 You can read this as "`(car '(1 2))' evaluates to 1". When a form is a macro call, it expands into a new form for Lisp to evaluate. We show the result of the expansion with `==>'. We may or may not show the result of the evaluation of the expanded form. (third '(a b c)) ==> (car (cdr (cdr '(a b c)))) => c To help describe one form, we sometimes show another form that produces identical results. The exact equivalence of two forms is indicated with `=='. (make-sparse-keymap) == (list 'keymap) 1.3.4 Printing Notation ----------------------- Many of the examples in this manual print text when they are evaluated. If you execute example code in a Lisp Interaction buffer (such as the buffer `*scratch*'), the printed text is inserted into the buffer. If you execute the example by other means (such as by evaluating the function `eval-region'), the printed text is displayed in the echo area. Examples in this manual indicate printed text with `-|', irrespective of where that text goes. The value returned by evaluating the form follows on a separate line with `=>'. (progn (prin1 'foo) (princ "\n") (prin1 'bar)) -| foo -| bar => bar 1.3.5 Error Messages -------------------- Some examples signal errors. This normally displays an error message in the echo area. We show the error message on a line starting with `error-->'. Note that `error-->' itself does not appear in the echo area. (+ 23 'x) error--> Wrong type argument: number-or-marker-p, x 1.3.6 Buffer Text Notation -------------------------- Some examples describe modifications to the contents of a buffer, by showing the "before" and "after" versions of the text. These examples show the contents of the buffer in question between two lines of dashes containing the buffer name. In addition, `-!-' indicates the location of point. (The symbol for point, of course, is not part of the text in the buffer; it indicates the place _between_ two characters where point is currently located.) ---------- Buffer: foo ---------- This is the -!-contents of foo. ---------- Buffer: foo ---------- (insert "changed ") => nil ---------- Buffer: foo ---------- This is the changed -!-contents of foo. ---------- Buffer: foo ---------- 1.3.7 Format of Descriptions ---------------------------- Functions, variables, macros, commands, user options, and special forms are described in this manual in a uniform format. The first line of a description contains the name of the item followed by its arguments, if any. The category--function, variable, or whatever--appears at the beginning of the line. The description follows on succeeding lines, sometimes with examples. 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, *note 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 (*note Documentation::). *Note 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 23.1 (i686-pc-linux-gnu, GTK+ Version 2.14.4) of 2009-06-01 on cyd.mit.edu" If HERE is non-`nil', it inserts the text in the buffer before point, and returns `nil'. When this function is called interactively, it prints the same information in the echo area, but giving a prefix argument makes HERE non-`nil'. -- Variable: emacs-build-time The value of this variable indicates the time at which Emacs was built. It is a list of four integers, like the value of `current-time' (*note Time of Day::). emacs-build-time => (20614 63694 515336 438000) -- Variable: emacs-version The value of this variable is the version of Emacs being run. It is a string such as `"23.1.1"'. The last number in this string is not really part of the Emacs release version number; it is incremented each time Emacs is built in any given directory. A value with four numeric components, such as `"22.0.91.1"', indicates an unreleased test version. -- 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. 1.5 Acknowledgments =================== This manual was originally written by Robert Krawitz, Bil Lewis, Dan LaLiberte, Richard M. Stallman and Chris Welty, the volunteers of the GNU manual group, in an effort extending over several years. Robert J. Chassell helped to review and edit the manual, with the support of the Defense Advanced Research Projects Agency, ARPA Order 6082, arranged by Warren A. Hunt, Jr. of Computational Logic, Inc. Additional sections have since been written by Miles Bader, Lars Brinkhoff, Chong Yidong, Kenichi Handa, Lute Kamstra, Juri Linkov, Glenn Morris, Thien-Thi Nguyen, Dan Nicolaescu, Martin Rudalics, Kim F. Storm, Luc Teirlinck, and Eli Zaretskii, and others. Corrections were supplied by Drew Adams, Juanma Barranquero, Karl Berry, Jim Blandy, Bard Bloom, Stephane Boucher, David Boyes, Alan Carroll, Richard Davis, Lawrence R. Dodd, Peter Doornbosch, David A. Duff, Chris Eich, Beverly Erlebacher, David Eckelkamp, Ralf Fassel, Eirik Fuller, Stephen Gildea, Bob Glickstein, Eric Hanchrow, Jesper Harder, George Hartzell, Nathan Hess, Masayuki Ida, Dan Jacobson, Jak Kirman, Bob Knighten, Frederick M. Korz, Joe Lammens, Glenn M. Lewis, K. Richard Magill, Brian Marick, Roland McGrath, Stefan Monnier, Skip Montanaro, John Gardiner Myers, Thomas A. Peterson, Francesco Potorti, Friedrich Pukelsheim, Arnold D. Robbins, Raul Rockwell, Jason Rumney, Per Starbäck, Shinichirou Sugou, Kimmo Suominen, Edward Tharp, Bill Trost, Rickard Westman, Jean White, Eduard Wiebe, Matthew Wilding, Carl Witty, Dale Worley, Rusty Wright, and David D. Zuhn. For a more complete list of contributors, please see the relevant ChangeLog file in the Emacs sources. 2 Lisp Data Types ***************** A Lisp "object" is a piece of data used and manipulated by Lisp programs. For our purposes, a "type" or "data type" is a set of possible objects. Every object belongs to at least one type. Objects of the same type have similar structures and may usually be used in the same contexts. Types can overlap, and objects can belong to two or more types. Consequently, we can ask whether an object belongs to a particular type, but not for "the" type of an object. A few fundamental object types are built into Emacs. These, from which all other types are constructed, are called "primitive types". Each object belongs to one and only one primitive type. These types include "integer", "float", "cons", "symbol", "string", "vector", "hash-table", "subr", and "byte-code function", plus several special types, such as "buffer", that are related to editing. (*Note 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. *Note 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. 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. *Note Read and Print::. In most cases, an object's printed representation is also a read syntax for the object. However, some types have no read syntax, since it does not make sense to enter objects of these types as constants in a Lisp program. These objects are printed in "hash notation", which consists of the characters `#<', a descriptive string (typically the type name followed by the name of the object), and a closing `>'. For example: (current-buffer) => # 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 (*note 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. *Note Input Functions::, for a description of `read', the basic function for reading objects. 2.2 Comments ============ A "comment" is text that is written in a program only for the sake of humans that read the program, and that has no effect on the meaning of the program. In Lisp, a semicolon (`;') starts a comment if it is not within a string or character constant. The comment continues to the end of line. The Lisp reader discards comments; they do not become part of the Lisp objects which represent the program within the Lisp system. The `#@COUNT' construct, which skips the next COUNT characters, is useful for program-generated comments containing binary data. The Emacs Lisp byte compiler uses this in its output files (*note Byte Compilation::). It isn't meant for source files, however. *Note Comment Tips::, for conventions for formatting comments. 2.3 Programming Types ===================== There are two general categories of types in Emacs Lisp: those having to do with Lisp programming, and those having to do with editing. The former exist in many Lisp implementations, in one form or another. The latter are unique to Emacs Lisp. 2.3.1 Integer Type ------------------ The range of values for integers in Emacs Lisp is -536870912 to 536870911 (30 bits; i.e., -2**29 to 2**29 - 1) on typical 32-bit machines. (Some machines provide a wider range.) Emacs Lisp arithmetic functions do not check for overflow. Thus `(1+ 536870911)' is -536870912 if Emacs integers are 30 bits. The read syntax for integers is a sequence of (base ten) digits with an optional sign at the beginning and an optional period at the end. The printed representation produced by the Lisp interpreter never has a leading `+' or a final `.'. -1 ; The integer -1. 1 ; The integer 1. 1. ; Also the integer 1. +1 ; Also the integer 1. As a special exception, if a sequence of digits specifies an integer too large or too small to be a valid integer object, the Lisp reader reads it as a floating-point number (*note Floating Point Type::). For instance, if Emacs integers are 30 bits, `536870912' is read as the floating-point number `536870912.0'. *Note Numbers::, for more information. 2.3.2 Floating Point Type ------------------------- Floating point numbers are the computer equivalent of scientific notation; you can think of a floating point number as a fraction together with a power of ten. The precise number of significant figures and the range of possible exponents is machine-specific; Emacs uses the C data type `double' to store the value, and internally this records a power of 2 rather than a power of 10. The printed representation for floating point numbers requires either a decimal point (with at least one digit following), an exponent, or both. For example, `1500.0', `15e2', `15.0e2', `1.5e3', and `.15e4' are five ways of writing a floating point number whose value is 1500. They are all equivalent. *Note Numbers::, for more information. 2.3.3 Character Type -------------------- A "character" in Emacs Lisp is nothing more than an integer. In other words, characters are represented by their character codes. For example, the character `A' is represented as the integer 65. Individual characters are used occasionally in programs, but it is more common to work with _strings_, which are sequences composed of characters. *Note String Type::. Characters in strings and buffers are currently limited to the range of 0 to 4194303--twenty two bits (*note Character Codes::). Codes 0 through 127 are ASCII codes; the rest are non-ASCII (*note 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. *Note Describing Characters::. 2.3.3.1 Basic Char Syntax ......................... Since characters are really integers, the printed representation of a character is a decimal number. This is also a possible read syntax for a character, but writing characters that way in Lisp programs is not clear programming. You should _always_ use the special read syntax formats that Emacs Lisp provides for characters. These syntax formats start with a question mark. The usual read syntax for alphanumeric characters is a question mark followed by the character; thus, `?A' for the character `A', `?B' for the character `B', and `?a' for the character `a'. For example: ?Q => 81 ?q => 113 You can use the same syntax for punctuation characters, but it is often a good idea to add a `\' so that the Emacs commands for editing Lisp code don't get confused. For example, `?\(' is the way to write the open-paren character. If the character is `\', you _must_ use a second `\' to quote it: `?\\'. You can express the characters control-g, backspace, tab, newline, vertical tab, formfeed, space, return, del, and escape as `?\a', `?\b', `?\t', `?\n', `?\v', `?\f', `?\s', `?\r', `?\d', and `?\e', respectively. (`?\s' followed by a dash has a different meaning--it applies the "super" modifier to the following character.) Thus, ?\a => 7 ; control-g, `C-g' ?\b => 8 ; backspace, , `C-h' ?\t => 9 ; tab, , `C-i' ?\n => 10 ; newline, `C-j' ?\v => 11 ; vertical tab, `C-k' ?\f => 12 ; formfeed character, `C-l' ?\r => 13 ; carriage return, , `C-m' ?\e => 27 ; escape character, , `C-[' ?\s => 32 ; space character, ?\\ => 92 ; backslash character, `\' ?\d => 127 ; delete character, These sequences which start with backslash are also known as "escape sequences", because backslash plays the role of an "escape character"; this terminology has nothing to do with the character . `\s' is meant for use in character constants; in string constants, just write the space. A backslash is allowed, and harmless, preceding any character without a special escape meaning; thus, `?\+' is equivalent to `?+'. There is no reason to add a backslash before most characters. However, you should add a backslash before any of the characters `()\|;'`"#.,' to avoid confusing the Emacs commands for editing Lisp code. You can also add a backslash before whitespace characters such as space, tab, newline and formfeed. However, it is cleaner to use one of the easily readable escape sequences, such as `\t' or `\s', instead of an actual whitespace character such as a tab or a space. (If you do write backslash followed by a space, you should write an extra space after the character constant to separate it from the following text.) 2.3.3.2 General Escape Syntax ............................. In addition to the specific escape sequences for special important control characters, Emacs provides several types of escape syntax that you can use to specify non-ASCII text characters. Firstly, you can specify characters by their Unicode values. `?\uNNNN' represents a character with Unicode code point `U+NNNN', where NNNN is (by convention) a hexadecimal number with exactly four digits. The backslash indicates that the subsequent characters form an escape sequence, and the `u' specifies a Unicode escape sequence. There is a slightly different syntax for specifying Unicode characters with code points higher than `U+FFFF': `?\U00NNNNNN' represents the character with code point `U+NNNNNN', where NNNNNN is a six-digit hexadecimal number. The Unicode Standard only defines code points up to `U+10FFFF', so if you specify a code point higher than that, Emacs signals an error. Secondly, you can specify characters by their hexadecimal character codes. A hexadecimal escape sequence consists of a backslash, `x', and the hexadecimal character code. Thus, `?\x41' is the character `A', `?\x1' is the character `C-a', and `?\xe0' is the character `a' with grave accent. You can use any number of hex digits, so you can represent any character code in this way. Thirdly, you can specify characters by their character code in octal. An octal escape sequence consists of a backslash followed by up to three octal digits; thus, `?\101' for the character `A', `?\001' for the character `C-a', and `?\002' for the character `C-b'. Only characters up to octal code 777 can be specified this way. These escape sequences may also be used in strings. *Note Non-ASCII in Strings::. 2.3.3.3 Control-Character Syntax ................................ Control characters can be represented using yet another read syntax. This consists of a question mark followed by a backslash, caret, and the corresponding non-control character, in either upper or lower case. For example, both `?\^I' and `?\^i' are valid read syntax for the character `C-i', the character whose value is 9. Instead of the `^', you can use `C-'; thus, `?\C-i' is equivalent to `?\^I' and to `?\^i': ?\^I => 9 ?\C-I => 9 In strings and buffers, the only control characters allowed are those that exist in ASCII; but for keyboard input purposes, you can turn any character into a control character with `C-'. The character codes for these non-ASCII control characters include the 2**26 bit as well as the code for the corresponding non-control character. Ordinary text terminals have no way of generating non-ASCII control characters, but you can generate them straightforwardly using X and other window systems. For historical reasons, Emacs treats the 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 in this way. For representing control characters to be found in files or strings, we recommend the `^' syntax; for control characters in keyboard input, we prefer the `C-' syntax. Which one you use does not affect the meaning of the program, but may guide the understanding of people who read it. 2.3.3.4 Meta-Character Syntax ............................. A "meta character" is a character typed with the 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. *Note Strings of Events::, for details about -handling in strings. The read syntax for meta characters uses `\M-'. For example, `?\M-A' stands for `M-A'. You can use `\M-' together with octal character codes (see below), with `\C-', or with any other syntax for a character. Thus, you can write `M-A' as `?\M-A', or as `?\M-\101'. Likewise, you can write `C-M-b' as `?\M-\C-b', `?\C-\M-b', or `?\M-\002'. 2.3.3.5 Other Character Modifier Bits ..................................... The case of a graphic character is indicated by its character code; for example, ASCII distinguishes between the characters `a' and `A'. But ASCII has no way to represent whether a control character is upper case or lower case. Emacs uses the 2**25 bit to indicate that the shift key was used in typing a control character. This distinction is possible only when you use X terminals or other special terminals; ordinary text terminals do not report the distinction. The Lisp syntax for the shift bit is `\S-'; thus, `?\C-\S-o' or `?\C-\S-O' represents the shifted-control-o character. The X Window System defines three other modifier bits that can be set in a character: "hyper", "super" and "alt". The syntaxes for these bits are `\H-', `\s-' and `\A-'. (Case is significant in these prefixes.) Thus, `?\H-\M-\A-x' represents `Alt-Hyper-Meta-x'. (Note that `\s' with no following `-' represents the space character.) Numerically, the bit values are 2**22 for alt, 2**23 for super and 2**24 for hyper. 2.3.4 Symbol Type ----------------- A "symbol" in GNU Emacs Lisp is an object with a name. The symbol name serves as the printed representation of the symbol. In ordinary Lisp use, with one single obarray (*note Creating Symbols::), a symbol's name is unique--no two symbols have the same name. A symbol can serve as a variable, as a function name, or to hold a property list. Or it may serve only to be distinct from all other Lisp objects, so that its presence in a data structure may be recognized reliably. In a given context, usually only one of these uses is intended. But you can use one symbol in all of these ways, independently. A symbol whose name starts with a colon (`:') is called a "keyword symbol". These symbols automatically act as constants, and are normally used only by comparing an unknown symbol with a few specific alternatives. *Note Constant Variables::. A symbol name can contain any characters whatever. Most symbol names are written with letters, digits, and the punctuation characters `-+=*/'. Such names require no special punctuation; the characters of the name suffice as long as the name does not look like a number. (If it does, write a `\' at the beginning of the name to force interpretation as a symbol.) The characters `_~!@$%^&:<>{}?' are less often used but also require no special punctuation. Any other characters may be included in a symbol's name by escaping them with a backslash. In contrast to its use in strings, however, a backslash in the name of a symbol simply quotes the single character that follows the backslash. For example, in a string, `\t' represents a tab character; in the name of a symbol, however, `\t' merely quotes the letter `t'. To have a symbol with a tab character in its name, you must actually use a tab (preceded with a backslash). But it's rare to do such a thing. Common Lisp note: In Common Lisp, lower case letters are always "folded" to upper case, unless they are explicitly escaped. In Emacs Lisp, upper case and lower case letters are distinct. Here are several examples of symbol names. Note that the `+' in the fourth example is escaped to prevent it from being read as a number. This is not necessary in the sixth example because the rest of the name makes it invalid as a number. foo ; A symbol named `foo'. FOO ; A symbol named `FOO', different from `foo'. 1+ ; A symbol named `1+' ; (not `+1', which is an integer). \+1 ; A symbol named `+1' ; (not a very readable name). \(*\ 1\ 2\) ; A symbol named `(* 1 2)' (a worse name). +-*/_~!@$%^&=:<>{} ; A symbol named `+-*/_~!@$%^&=:<>{}'. ; These characters need not be escaped. As an exception to the rule that a symbol's name serves as its printed representation, `##' is the printed representation for an interned symbol whose name is an empty string. Furthermore, `#:FOO' is the printed representation for an uninterned symbol whose name is FOO. (Normally, the Lisp reader interns all symbols; *note Creating Symbols::.) 2.3.5 Sequence Types -------------------- A "sequence" is a Lisp object that represents an ordered set of elements. There are two kinds of sequence in Emacs Lisp: "lists" and "arrays". Lists are the most commonly-used sequences. A list can hold elements of any type, and its length can be easily changed by adding or removing elements. See the next subsection for more about lists. Arrays are fixed-length sequences. They are further subdivided into strings, vectors, char-tables and bool-vectors. Vectors can hold elements of any type, whereas string elements must be characters, and bool-vector elements must be `t' or `nil'. Char-tables are like vectors except that they are indexed by any valid character code. The characters in a string can have text properties like characters in a buffer (*note 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. *Note Sequences Arrays Vectors::. It is generally impossible to read the same sequence twice, since sequences are always created anew upon reading. If you read the read syntax for a sequence twice, you get two sequences with equal contents. There is one exception: the empty list `()' always stands for the same object, `nil'. 2.3.6 Cons Cell and List Types ------------------------------ A "cons cell" is an object that consists of two slots, called the CAR slot and the CDR slot. Each slot can "hold" any Lisp object. We also say that "the CAR of this cons cell is" whatever object its CAR slot currently holds, and likewise for the CDR. A "list" is a series of cons cells, linked together so that the CDR slot of each cons cell holds either the next cons cell or the empty list. The empty list is actually the symbol `nil'. *Note Lists::, for details. Because most cons cells are used as part of lists, we refer to any structure made out of cons cells as a "list structure". A note to C programmers: a Lisp list thus works as a "linked list" built up of cons cells. Because pointers in Lisp are implicit, we do not distinguish between a cons cell slot "holding" a value versus "pointing to" the value. Because cons cells are so central to Lisp, we also have a word for "an object which is not a cons cell". These objects are called "atoms". The read syntax and printed representation for lists are identical, and consist of a left parenthesis, an arbitrary number of elements, and a right parenthesis. Here are examples of lists: (A 2 "A") ; A list of three elements. () ; A list of no elements (the empty list). nil ; A list of no elements (the empty list). ("A ()") ; A list of one element: the string `"A ()"'. (A ()) ; A list of two elements: `A' and the empty list. (A nil) ; Equivalent to the previous. ((A B C)) ; A list of one element ; (which is a list of three elements). Upon reading, each object inside the parentheses becomes an element of the list. That is, a cons cell is made for each element. The CAR slot of the cons cell holds the element, and its CDR slot refers to the next cons cell of the list, which holds the next element in the list. The CDR slot of the last cons cell is set to hold `nil'. The names CAR and CDR derive from the history of Lisp. The original Lisp implementation ran on an IBM 704 computer which divided words into two parts, called the "address" part and the "decrement"; CAR was an instruction to extract the contents of the address part of a register, and CDR an instruction to extract the contents of the decrement. By contrast, "cons cells" are named for the function `cons' that creates them, which in turn was named for its purpose, the construction of cells. 2.3.6.1 Drawing Lists as Box Diagrams ..................................... A list can be illustrated by a diagram in which the cons cells are shown as pairs of boxes, like dominoes. (The Lisp reader cannot read such an illustration; unlike the textual notation, which can be understood by both humans and computers, the box illustrations can be understood only by humans.) This picture represents the three-element list `(rose violet buttercup)': --- --- --- --- --- --- | | |--> | | |--> | | |--> nil --- --- --- --- --- --- | | | | | | --> rose --> violet --> buttercup In this diagram, each box represents a slot that can hold or refer to any Lisp object. Each pair of boxes represents a cons cell. Each arrow represents a reference to a Lisp object, either an atom or another cons cell. In this example, the first box, which holds the CAR of the first cons cell, refers to or "holds" `rose' (a symbol). The second box, holding the CDR of the first cons cell, refers to the next pair of boxes, the second cons cell. The CAR of the second cons cell is `violet', and its CDR is the third cons cell. The CDR of the third (and last) cons cell is `nil'. Here is another diagram of the same list, `(rose violet buttercup)', sketched in a different manner: --------------- ---------------- ------------------- | car | cdr | | car | cdr | | car | cdr | | rose | o-------->| violet | o-------->| buttercup | nil | | | | | | | | | | --------------- ---------------- ------------------- A list with no elements in it is the "empty list"; it is identical to the symbol `nil'. In other words, `nil' is both a symbol and a list. Here is the list `(A ())', or equivalently `(A nil)', depicted with boxes and arrows: --- --- --- --- | | |--> | | |--> nil --- --- --- --- | | | | --> A --> nil Here is a more complex illustration, showing the three-element list, `((pine needles) oak maple)', the first element of which is a two-element list: --- --- --- --- --- --- | | |--> | | |--> | | |--> nil --- --- --- --- --- --- | | | | | | | --> oak --> maple | | --- --- --- --- --> | | |--> | | |--> nil --- --- --- --- | | | | --> pine --> needles The same list represented in the second box notation looks like this: -------------- -------------- -------------- | car | cdr | | car | cdr | | car | cdr | | o | o------->| oak | o------->| maple | nil | | | | | | | | | | | -- | --------- -------------- -------------- | | | -------------- ---------------- | | car | cdr | | car | cdr | ------>| pine | o------->| needles | nil | | | | | | | -------------- ---------------- 2.3.6.2 Dotted Pair Notation ............................ "Dotted pair notation" is a general syntax for cons cells that represents the CAR and CDR explicitly. In this syntax, `(A . B)' stands for a cons cell whose CAR is the object A and whose CDR is the object B. Dotted pair notation is more general than list syntax because the CDR does not have to be a list. However, it is more cumbersome in cases where list syntax would work. In dotted pair notation, the list `(1 2 3)' is written as `(1 . (2 . (3 . nil)))'. For `nil'-terminated lists, you can use either notation, but list notation is usually clearer and more convenient. When printing a list, the dotted pair notation is only used if the CDR of a cons cell is not a list. Here's an example using boxes to illustrate dotted pair notation. This example shows the pair `(rose . violet)': --- --- | | |--> violet --- --- | | --> rose You can combine dotted pair notation with list notation to represent conveniently a chain of cons cells with a non-`nil' final CDR. You write a dot after the last element of the list, followed by the CDR of the final cons cell. For example, `(rose violet . buttercup)' is equivalent to `(rose . (violet . buttercup))'. The object looks like this: --- --- --- --- | | |--> | | |--> buttercup --- --- --- --- | | | | --> rose --> violet The syntax `(rose . violet . buttercup)' is invalid because there is nothing that it could mean. If anything, it would say to put `buttercup' in the CDR of a cons cell whose CDR is already used for `violet'. The list `(rose violet)' is equivalent to `(rose . (violet))', and looks like this: --- --- --- --- | | |--> | | |--> nil --- --- --- --- | | | | --> rose --> violet Similarly, the three-element list `(rose violet buttercup)' is equivalent to `(rose . (violet . (buttercup)))'. It looks like this: --- --- --- --- --- --- | | |--> | | |--> | | |--> nil --- --- --- --- --- --- | | | | | | --> rose --> violet --> buttercup 2.3.6.3 Association List Type ............................. An "association list" or "alist" is a specially-constructed list whose elements are cons cells. In each element, the CAR is considered a "key", and the CDR is considered an "associated value". (In some cases, the associated value is stored in the CAR of the CDR.) Association lists are often used as stacks, since it is easy to add or remove associations at the front of the list. For example, (setq alist-of-colors '((rose . red) (lily . white) (buttercup . yellow))) sets the variable `alist-of-colors' to an alist of three elements. In the first element, `rose' is the key and `red' is the value. *Note Association Lists::, for a further explanation of alists and for functions that work on alists. *Note Hash Tables::, for another kind of lookup table, which is much faster for handling a large number of keys. 2.3.7 Array Type ---------------- An "array" is composed of an arbitrary number of slots for holding or referring to other Lisp objects, arranged in a contiguous block of memory. Accessing any element of an array takes approximately the same amount of time. In contrast, accessing an element of a list requires time proportional to the position of the element in the list. (Elements at the end of a list take longer to access than elements at the beginning of a list.) Emacs defines four types of array: strings, vectors, bool-vectors, and char-tables. A string is an array of characters and a vector is an array of arbitrary objects. A bool-vector can hold only `t' or `nil'. These kinds of array may have any length up to the largest integer. Char-tables are sparse arrays indexed by any valid character code; they can hold arbitrary objects. The first element of an array has index zero, the second element has index 1, and so on. This is called "zero-origin" indexing. For example, an array of four elements has indices 0, 1, 2, and 3. The largest possible index value is one less than the length of the array. Once an array is created, its length is fixed. All Emacs Lisp arrays are one-dimensional. (Most other programming languages support multidimensional arrays, but they are not essential; you can get the same effect with nested one-dimensional arrays.) Each type of array has its own read syntax; see the following sections for details. The array type is a subset of the sequence type, and contains the string type, the vector type, the bool-vector type, and the char-table type. 2.3.8 String Type ----------------- A "string" is an array of characters. Strings are used for many purposes in Emacs, as can be expected in a text editor; for example, as the names of Lisp symbols, as messages for the user, and to represent text extracted from buffers. Strings in Lisp are constants: evaluation of a string returns the same string. *Note Strings and Characters::, for functions that operate on strings. 2.3.8.1 Syntax for Strings .......................... The read syntax for a string is a double-quote, an arbitrary number of characters, and another double-quote, `"like this"'. To include a double-quote in a string, precede it with a backslash; thus, `"\""' is a string containing just a single double-quote character. Likewise, you can include a backslash by preceding it with another backslash, like this: `"this \\ is a single embedded backslash"'. The newline character is not special in the read syntax for strings; if you write a new line between the double-quotes, it becomes a character in the string. But an escaped newline--one that is preceded by `\'--does not become part of the string; i.e., the Lisp reader ignores an escaped newline while reading a string. An escaped space `\ ' is likewise ignored. "It is useful to include newlines in documentation strings, but the newline is \ ignored if escaped." => "It is useful to include newlines in documentation strings, but the newline is ignored if escaped." 2.3.8.2 Non-ASCII Characters in Strings ....................................... There are two text representations for non-ASCII characters in Emacs strings: multibyte and unibyte (*note 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 (*note 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. *Note General Escape Syntax::, for details about escape sequences. If you use any Unicode-style escape sequence `\uNNNN' or `\U00NNNNNN' in a string constant (even for an ASCII character), Emacs automatically assumes that it is multibyte. You can also use hexadecimal escape sequences (`\xN') and octal escape sequences (`\N') in string constants. *But beware:* If a string constant contains hexadecimal or octal escape sequences, and these escape sequences all specify unibyte characters (i.e., less than 256), and there are no other literal non-ASCII characters or Unicode-style escape sequences in the string, then Emacs automatically assumes that it is a unibyte string. That is to say, it assumes that all non-ASCII characters occurring in the string are 8-bit raw bytes. In hexadecimal and octal escape sequences, the escaped character code may contain a variable number of digits, so the first subsequent character which is not a valid hexadecimal or octal digit terminates the escape sequence. If the next character in a string could be interpreted as a hexadecimal or octal digit, write `\ ' (backslash and space) to terminate the escape sequence. For example, `\xe0\ ' represents one character, `a' with grave accent. `\ ' in a string constant is just like backslash-newline; it does not contribute any character to the string, but it does terminate any preceding hex escape. 2.3.8.3 Nonprinting Characters in Strings ......................................... You can use the same backslash escape-sequences in a string constant as in character literals (but do not use the question mark that begins a character constant). For example, you can write a string containing the nonprinting characters tab and `C-a', with commas and spaces between them, like this: `"\t, \C-a"'. *Note 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. *Note Character Type::. Strings cannot hold characters that have the hyper, super, or alt modifiers. 2.3.8.4 Text Properties in Strings .................................. A string can hold properties for the characters it contains, in addition to the characters themselves. This enables programs that copy text between strings and buffers to copy the text's properties with no special effort. *Note Text Properties::, for an explanation of what text properties mean. Strings with text properties use a special read and print syntax: #("CHARACTERS" PROPERTY-DATA...) where PROPERTY-DATA consists of zero or more elements, in groups of three as follows: BEG END PLIST The elements BEG and END are integers, and together specify a range of indices in the string; PLIST is the property list for that range. For example, #("foo bar" 0 3 (face bold) 3 4 nil 4 7 (face italic)) represents a string whose textual contents are `foo bar', in which the first three characters have a `face' property with value `bold', and the last three have a `face' property with value `italic'. (The fourth character has no text properties, so its property list is `nil'. It is not actually necessary to mention ranges with `nil' as the property list, since any characters not mentioned in any range will default to having no properties.) 2.3.9 Vector Type ----------------- A "vector" is a one-dimensional array of elements of any type. It takes a constant amount of time to access any element of a vector. (In a list, the access time of an element is proportional to the distance of the element from the beginning of the list.) The printed representation of a vector consists of a left square bracket, the elements, and a right square bracket. This is also the read syntax. Like numbers and strings, vectors are considered constants for evaluation. [1 "two" (three)] ; A vector of three elements. => [1 "two" (three)] *Note Vectors::, for functions that work with vectors. 2.3.10 Char-Table Type ---------------------- A "char-table" is a one-dimensional array of elements of any type, indexed by character codes. Char-tables have certain extra features to make them more useful for many jobs that involve assigning information to character codes--for example, a char-table can have a parent to inherit from, a default value, and a small number of extra slots to use for special purposes. A char-table can also specify a single value for a whole character set. The printed representation of a char-table is like a vector except that there is an extra `#^' at the beginning.(1) *Note Char-Tables::, for special functions to operate on char-tables. Uses of char-tables include: * Case tables (*note Case Tables::). * Character category tables (*note Categories::). * Display tables (*note Display Tables::). * Syntax tables (*note Syntax Tables::). ---------- Footnotes ---------- (1) You may also encounter `#^^', used for "sub-char-tables". 2.3.11 Bool-Vector Type ----------------------- A "bool-vector" is a one-dimensional array whose elements must be `t' or `nil'. The printed representation of a bool-vector is like a string, except that it begins with `#&' followed by the length. The string constant that follows actually specifies the contents of the bool-vector as a bitmap--each "character" in the string contains 8 bits, which specify the next 8 elements of the bool-vector (1 stands for `t', and 0 for `nil'). The least significant bits of the character correspond to the lowest indices in the bool-vector. (make-bool-vector 3 t) => #&3"^G" (make-bool-vector 3 nil) => #&3"^@" These results make sense, because the binary code for `C-g' is 111 and `C-@' is the character with code 0. If the length is not a multiple of 8, the printed representation shows extra elements, but these extras really make no difference. For instance, in the next example, the two bool-vectors are equal, because only the first 3 bits are used: (equal #&3"\377" #&3"\007") => t 2.3.12 Hash Table Type ---------------------- A hash table is a very fast kind of lookup table, somewhat like an alist in that it maps keys to corresponding values, but much faster. The printed representation of a hash table specifies its properties and contents, like this: (make-hash-table) => #s(hash-table size 65 test eql rehash-size 1.5 rehash-threshold 0.8 data ()) *Note Hash Tables::, for more information about hash tables. 2.3.13 Function Type -------------------- Lisp functions are executable code, just like functions in other programming languages. In Lisp, unlike most languages, functions are also Lisp objects. A non-compiled function in Lisp is a lambda expression: that is, a list whose first element is the symbol `lambda' (*note 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" (*note Anonymous Functions::). A named function in Lisp is just a symbol with a valid function in its function cell (*note 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'. *Note Calling Functions::. 2.3.14 Macro Type ----------------- A "Lisp macro" is a user-defined construct that extends the Lisp language. It is represented as an object much like a function, but with different argument-passing semantics. A Lisp macro has the form of a list whose first element is the symbol `macro' and whose CDR is a Lisp function object, including the `lambda' symbol. Lisp macro objects are usually defined with the built-in `defmacro' function, but any list that begins with `macro' is a macro as far as Emacs is concerned. *Note Macros::, for an explanation of how to write a macro. *Warning*: Lisp macros and keyboard macros (*note Keyboard Macros::) are entirely different things. When we use the word "macro" without qualification, we mean a Lisp macro, not a keyboard macro. 2.3.15 Primitive Function Type ------------------------------ A "primitive function" is a function callable from Lisp but written in the C programming language. Primitive functions are also called "subrs" or "built-in functions". (The word "subr" is derived from "subroutine".) Most primitive functions evaluate all their arguments when they are called. A primitive function that does not evaluate all its arguments is called a "special form" (*note 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. *Note 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. => # (subrp (symbol-function 'car)) ; Is this a primitive function? => t ; Yes. 2.3.16 Byte-Code Function Type ------------------------------ "Byte-code function objects" are produced by byte-compiling Lisp code (*note 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. *Note Byte-Code Objects::. The printed representation and read syntax for a byte-code function object is like that for a vector, with an additional `#' before the opening `['. 2.3.17 Autoload Type -------------------- An "autoload object" is a list whose first element is the symbol `autoload'. It is stored as the function definition of a symbol, where it serves as a placeholder for the real definition. The autoload object says that the real definition is found in a file of Lisp code that should be loaded when necessary. It contains the name of the file, plus some other information about the real definition. After the file has been loaded, the symbol should have a new function definition that is not an autoload object. The new definition is then called as if it had been there to begin with. From the user's point of view, the function call works as expected, using the function definition in the loaded file. An autoload object is usually created with the function `autoload', which stores the object in the function cell of a symbol. *Note Autoload::, for more details. 2.4 Editing Types ================= The types in the previous section are used for general programming purposes, and most of them are common to most Lisp dialects. Emacs Lisp provides several additional data types for purposes connected with editing. 2.4.1 Buffer Type ----------------- A "buffer" is an object that holds text that can be edited (*note Buffers::). Most buffers hold the contents of a disk file (*note 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 (*note Windows::). But a buffer need not be displayed in any window. Each buffer has a designated position called "point" (*note 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 (*note Text::). Several other data structures are associated with each buffer: * a local syntax table (*note Syntax Tables::); * a local keymap (*note Keymaps::); and, * a list of buffer-local variable bindings (*note Buffer-Local Variables::). * overlays (*note Overlays::). * text properties for the text in the buffer (*note 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. *Note Indirect Buffers::. Buffers have no read syntax. They print in hash notation, showing the buffer name. (current-buffer) => # 2.4.2 Marker Type ----------------- A "marker" denotes a position in a specific buffer. Markers therefore have two components: one for the buffer, and one for the position. Changes in the buffer's text automatically relocate the position value as necessary to ensure that the marker always points between the same two characters in the buffer. Markers have no read syntax. They print in hash notation, giving the current character position and the name of the buffer. (point-marker) => # *Note Markers::, for information on how to test, create, copy, and move markers. 2.4.3 Window Type ----------------- A "window" describes the portion of the terminal screen that Emacs uses to display a buffer. Every window has one associated buffer, whose contents appear in the window. By contrast, a given buffer may appear in one window, no window, or several windows. Though many windows may exist simultaneously, at any time one window is designated the "selected window". This is the window where the cursor is (usually) displayed when Emacs is ready for a command. The selected window usually displays the current buffer, but this is not necessarily the case. Windows are grouped on the screen into frames; each window belongs to one and only one frame. *Note Frame Type::. Windows have no read syntax. They print in hash notation, giving the window number and the name of the buffer being displayed. The window numbers exist to identify windows uniquely, since the buffer displayed in any given window can change frequently. (selected-window) => # *Note Windows::, for a description of the functions that work on windows. 2.4.4 Frame Type ---------------- A "frame" is a screen area that contains one or more Emacs windows; we also use the term "frame" to refer to the Lisp object that Emacs uses to refer to the screen area. Frames have no read syntax. They print in hash notation, giving the frame's title, plus its address in core (useful to identify the frame uniquely). (selected-frame) => # *Note Frames::, for a description of the functions that work on frames. 2.4.5 Terminal Type ------------------- A "terminal" is a device capable of displaying one or more Emacs frames (*note 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) => # 2.4.6 Window Configuration Type ------------------------------- A "window configuration" stores information about the positions, sizes, and contents of the windows in a frame, so you can recreate the same arrangement of windows later. Window configurations do not have a read syntax; their print syntax looks like `#'. *Note Window Configurations::, for a description of several functions related to window configurations. 2.4.7 Frame Configuration Type ------------------------------ A "frame configuration" stores information about the positions, sizes, and contents of the windows in all frames. It is not a primitive type--it is actually a list whose CAR is `frame-configuration' and whose CDR is an alist. Each alist element describes one frame, which appears as the CAR of that element. *Note Frame Configurations::, for a description of several functions related to frame configurations. 2.4.8 Process Type ------------------ The word "process" usually means a running program. Emacs itself runs in a process of this sort. However, in Emacs Lisp, a process is a Lisp object that designates a subprocess created by the Emacs process. Programs such as shells, GDB, ftp, and compilers, running in subprocesses of Emacs, extend the capabilities of Emacs. An Emacs subprocess takes textual input from Emacs and returns textual output to Emacs for further manipulation. Emacs can also send signals to the subprocess. Process objects have no read syntax. They print in hash notation, giving the name of the process: (process-list) => (#) *Note Processes::, for information about functions that create, delete, return information about, send input or signals to, and receive output from processes. 2.4.9 Stream Type ----------------- A "stream" is an object that can be used as a source or sink for characters--either to supply characters for input or to accept them as output. Many different types can be used this way: markers, buffers, strings, and functions. Most often, input streams (character sources) obtain characters from the keyboard, a buffer, or a file, and output streams (character sinks) send characters to a buffer, such as a `*Help*' buffer, or to the echo area. The object `nil', in addition to its other meanings, may be used as a stream. It stands for the value of the variable `standard-input' or `standard-output'. Also, the object `t' as a stream specifies input using the minibuffer (*note Minibuffers::) or output in the echo area (*note The Echo Area::). Streams have no special printed representation or read syntax, and print as whatever primitive type they are. *Note Read and Print::, for a description of functions related to streams, including parsing and printing functions. 2.4.10 Keymap Type ------------------ A "keymap" maps keys typed by the user to commands. This mapping controls how the user's command input is executed. A keymap is actually a list whose CAR is the symbol `keymap'. *Note Keymaps::, for information about creating keymaps, handling prefix keys, local as well as global keymaps, and changing key bindings. 2.4.11 Overlay Type ------------------- An "overlay" specifies properties that apply to a part of a buffer. Each overlay applies to a specified range of the buffer, and contains a property list (a list whose elements are alternating property names and values). Overlay properties are used to present parts of the buffer temporarily in a different display style. Overlays have no read syntax, and print in hash notation, giving the buffer name and range of positions. *Note Overlays::, for information on how you can create and use overlays. 2.4.12 Font Type ---------------- A "font" specifies how to display text on a graphical terminal. There are actually three separate font types--"font objects", "font specs", and "font entities"--each of which has slightly different properties. None of them have a read syntax; their print syntax looks like `#', `#', and `#' respectively. *Note Low-Level Font::, for a description of these Lisp objects. 2.5 Read Syntax for Circular Objects ==================================== To represent shared or circular structures within a complex of Lisp objects, you can use the reader constructs `#N=' and `#N#'. Use `#N=' before an object to label it for later reference; subsequently, you can use `#N#' to refer the same object in another place. Here, N is some integer. For example, here is how to make a list in which the first element recurs as the third element: (#1=(a) b #1#) This differs from ordinary syntax such as this ((a) b (a)) which would result in a list whose first and third elements look alike but are not the same Lisp object. This shows the difference: (prog1 nil (setq x '(#1=(a) b #1#))) (eq (nth 0 x) (nth 2 x)) => t (setq x '((a) b (a))) (eq (nth 0 x) (nth 2 x)) => nil You can also use the same syntax to make a circular structure, which appears as an "element" within itself. Here is an example: #1=(a #1#) This makes a list whose second element is the list itself. Here's how you can see that it really works: (prog1 nil (setq x '#1=(a #1#))) (eq x (cadr x)) => t The Lisp printer can produce this syntax to record circular and shared structure in a Lisp object, if you bind the variable `print-circle' to a non-`nil' value. *Note Output Variables::. 2.6 Type Predicates =================== The Emacs Lisp interpreter itself does not perform type checking on the actual arguments passed to functions when they are called. It could not do so, since function arguments in Lisp do not have declared data types, as they do in other programming languages. It is therefore up to the individual function to test whether each actual argument belongs to a type that the function can use. All built-in functions do check the types of their actual arguments when appropriate, and signal a `wrong-type-argument' error if an argument is of the wrong type. For example, here is what happens if you pass an argument to `+' that it cannot handle: (+ 2 'a) error--> Wrong type argument: number-or-marker-p, a If you want your program to handle different types differently, you must do explicit type checking. The most common way to check the type of an object is to call a "type predicate" function. Emacs has a type predicate for each type, as well as some predicates for combinations of types. A type predicate function takes one argument; it returns `t' if the argument belongs to the appropriate type, and `nil' otherwise. Following a general Lisp convention for predicate functions, most type predicates' names end with `p'. Here is an example which uses the predicates `listp' to check for a list and `symbolp' to check for a symbol. (defun add-on (x) (cond ((symbolp x) ;; If X is a symbol, put it on LIST. (setq list (cons x list))) ((listp x) ;; If X is a list, add its elements to LIST. (setq list (append x list))) (t ;; We handle only symbols and lists. (error "Invalid argument %s in add-on" x)))) Here is a table of predefined type predicates, in alphabetical order, with references to further information. `atom' *Note atom: List-related Predicates. `arrayp' *Note arrayp: Array Functions. `bool-vector-p' *Note bool-vector-p: Bool-Vectors. `bufferp' *Note bufferp: Buffer Basics. `byte-code-function-p' *Note byte-code-function-p: Byte-Code Type. `case-table-p' *Note case-table-p: Case Tables. `char-or-string-p' *Note char-or-string-p: Predicates for Strings. `char-table-p' *Note char-table-p: Char-Tables. `commandp' *Note commandp: Interactive Call. `consp' *Note consp: List-related Predicates. `custom-variable-p' *Note custom-variable-p: Variable Definitions. `display-table-p' *Note display-table-p: Display Tables. `floatp' *Note floatp: Predicates on Numbers. `fontp' *Note Low-Level Font::. `frame-configuration-p' *Note frame-configuration-p: Frame Configurations. `frame-live-p' *Note frame-live-p: Deleting Frames. `framep' *Note framep: Frames. `functionp' *Note functionp: Functions. `hash-table-p' *Note hash-table-p: Other Hash. `integer-or-marker-p' *Note integer-or-marker-p: Predicates on Markers. `integerp' *Note integerp: Predicates on Numbers. `keymapp' *Note keymapp: Creating Keymaps. `keywordp' *Note Constant Variables::. `listp' *Note listp: List-related Predicates. `markerp' *Note markerp: Predicates on Markers. `wholenump' *Note wholenump: Predicates on Numbers. `nlistp' *Note nlistp: List-related Predicates. `numberp' *Note numberp: Predicates on Numbers. `number-or-marker-p' *Note number-or-marker-p: Predicates on Markers. `overlayp' *Note overlayp: Overlays. `processp' *Note processp: Processes. `sequencep' *Note sequencep: Sequence Functions. `stringp' *Note stringp: Predicates for Strings. `subrp' *Note subrp: Function Cells. `symbolp' *Note symbolp: Symbols. `syntax-table-p' *Note syntax-table-p: Syntax Tables. `vectorp' *Note vectorp: Vectors. `window-configuration-p' *Note window-configuration-p: Window Configurations. `window-live-p' *Note window-live-p: Deleting Windows. `windowp' *Note windowp: Basic Windows. `booleanp' *Note booleanp: nil and t. `string-or-null-p' *Note string-or-null-p: Predicates for Strings. 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 (*note 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', `cons', `float', `font-entity', `font-object', `font-spec', `frame', `hash-table', `integer', `marker', `overlay', `process', `string', `subr', `symbol', `vector', `window', or `window-configuration'. (type-of 1) => integer (type-of 'nil) => symbol (type-of '()) ; `()' is `nil'. => symbol (type-of '(x)) => cons 2.7 Equality Predicates ======================= Here we describe functions that test for equality between two objects. Other functions test equality of contents between objects of specific types, e.g., strings. For these predicates, see the appropriate chapter describing the data type. -- Function: eq object1 object2 This function returns `t' if OBJECT1 and OBJECT2 are the same object, and `nil' otherwise. If OBJECT1 and OBJECT2 are integers with the same value, they are considered to be the same object (i.e., `eq' returns `t'). If OBJECT1 and OBJECT2 are symbols with the same name, they are normally the same object--but see *note Creating Symbols:: for exceptions. For other types (e.g., lists, vectors, strings), two arguments with the same contents or elements are not necessarily `eq' to each other: they are `eq' only if they are the same object, meaning that a change in the contents of one will be reflected by the same change in the contents of the other. (eq 'foo 'foo) => t (eq 456 456) => t (eq "asdf" "asdf") => nil (eq "" "") => t ;; This exception occurs because Emacs Lisp ;; makes just one multibyte empty string, to save space. (eq '(1 (2 (3))) '(1 (2 (3)))) => nil (setq foo '(1 (2 (3)))) => (1 (2 (3))) (eq foo foo) => t (eq foo '(1 (2 (3)))) => nil (eq [(1 2) 3] [(1 2) 3]) => nil (eq (point-marker) (point-marker)) => nil The `make-symbol' function returns an uninterned symbol, distinct from the symbol that is used if you write the name in a Lisp expression. Distinct symbols with the same name are not `eq'. *Note Creating Symbols::. (eq (make-symbol "foo") 'foo) => nil -- Function: equal object1 object2 This function returns `t' if OBJECT1 and OBJECT2 have equal components, and `nil' otherwise. Whereas `eq' tests if its arguments are the same object, `equal' looks inside nonidentical arguments to see if their elements or contents are the same. So, if two objects are `eq', they are `equal', but the converse is not always true. (equal 'foo 'foo) => t (equal 456 456) => t (equal "asdf" "asdf") => t (eq "asdf" "asdf") => nil (equal '(1 (2 (3))) '(1 (2 (3)))) => t (eq '(1 (2 (3))) '(1 (2 (3)))) => nil (equal [(1 2) 3] [(1 2) 3]) => t (eq [(1 2) 3] [(1 2) 3]) => nil (equal (point-marker) (point-marker)) => t (eq (point-marker) (point-marker)) => nil Comparison of strings is case-sensitive, but does not take account of text properties--it compares only the characters in the strings. *Note Text Properties::. Use `equal-including-properties' to also compare text properties. For technical reasons, a unibyte string and a multibyte string are `equal' if and only if they contain the same sequence of character codes and all these codes are either in the range 0 through 127 (ASCII) or 160 through 255 (`eight-bit-graphic'). (*note Text Representations::). (equal "asdf" "ASDF") => nil However, two distinct buffers are never considered `equal', even if their textual contents are the same. The test for equality is implemented recursively; for example, given two cons cells X and Y, `(equal X Y)' returns `t' if and only if both the expressions below return `t': (equal (car X) (car Y)) (equal (cdr X) (cdr Y)) Because of this recursive method, circular lists may therefore cause infinite recursion (leading to an error). -- Function: equal-including-properties object1 object2 This function behaves like `equal' in all cases but also requires that for two strings to be equal, they have the same text properties. (equal "asdf" (propertize "asdf" '(asdf t))) => t (equal-including-properties "asdf" (propertize "asdf" '(asdf t))) => nil 3 Numbers ********* GNU Emacs supports two numeric data types: "integers" and "floating point numbers". Integers are whole numbers such as -3, 0, 7, 13, and 511. Their values are exact. Floating point numbers are numbers with fractional parts, such as -4.5, 0.0, or 2.71828. They can also be expressed in exponential notation: 1.5e2 equals 150; in this example, `e2' stands for ten to the second power, and that is multiplied by 1.5. Floating point values are not exact; they have a fixed, limited amount of precision. 3.1 Integer Basics ================== The range of values for an integer depends on the machine. The minimum range is -536870912 to 536870911 (30 bits; i.e., -2**29 to 2**29 - 1), but many machines provide a wider range. Many examples in this chapter assume the minimum integer width of 30 bits. The Lisp reader reads an integer as a sequence of digits with optional initial sign and optional final period. An integer that is out of the Emacs range is treated as a floating-point number. 1 ; The integer 1. 1. ; The integer 1. +1 ; Also the integer 1. -1 ; The integer -1. 1073741825 ; The floating point number 1073741825.0. 0 ; The integer 0. -0 ; The integer 0. The syntax for integers in bases other than 10 uses `#' followed by a letter that specifies the radix: `b' for binary, `o' for octal, `x' for hex, or `RADIXr' to specify radix RADIX. Case is not significant for the letter that specifies the radix. Thus, `#bINTEGER' reads INTEGER in binary, and `#RADIXrINTEGER' reads INTEGER in radix RADIX. Allowed values of RADIX run from 2 to 36. For example: #b101100 => 44 #o54 => 44 #x2c => 44 #24r1k => 44 To understand how various functions work on integers, especially the bitwise operators (*note Bitwise Operations::), it is often helpful to view the numbers in their binary form. In 30-bit binary, the decimal integer 5 looks like this: 0000...000101 (30 bits total) (The `...' stands for enough bits to fill out a 30-bit word; in this case, `...' stands for twenty 0 bits. Later examples also use the `...' notation to make binary integers easier to read.) The integer -1 looks like this: 1111...111111 (30 bits total) -1 is represented as 30 ones. (This is called "two's complement" notation.) The negative integer, -5, is creating by subtracting 4 from -1. In binary, the decimal integer 4 is 100. Consequently, -5 looks like this: 1111...111011 (30 bits total) In this implementation, the largest 30-bit binary integer value is 536,870,911 in decimal. In binary, it looks like this: 0111...111111 (30 bits total) Since the arithmetic functions do not check whether integers go outside their range, when you add 1 to 536,870,911, the value is the negative integer -536,870,912: (+ 1 536870911) => -536870912 => 1000...000000 (30 bits total) Many of the functions described in this chapter accept markers for arguments in place of numbers. (*Note Markers::.) Since the actual arguments to such functions may be either numbers or markers, we often give these arguments the name NUMBER-OR-MARKER. When the argument value is a marker, its position value is used and its buffer is ignored. -- Variable: most-positive-fixnum The value of this variable is the largest integer that Emacs Lisp can handle. -- Variable: most-negative-fixnum The value of this variable is the smallest integer that Emacs Lisp can handle. It is negative. 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. *Note String Basics::. 3.2 Floating Point Basics ========================= Floating point numbers are useful for representing numbers that are not integral. The precise range of floating point numbers is machine-specific; it is the same as the range of the C data type `double' on the machine you are using. Emacs uses the IEEE floating point standard, which is supported by all modern computers. The read syntax for floating point numbers requires either a decimal point (with at least one digit following), an exponent, or both. For example, `1500.0', `15e2', `15.0e2', `1.5e3', and `.15e4' are five ways of writing a floating point number whose value is 1500. They are all equivalent. You can also use a minus sign to write negative floating point numbers, as in `-1.0'. Emacs Lisp treats `-0.0' as equal to ordinary zero (with respect to `equal' and `='), even though the two are distinguishable in the IEEE floating point standard. The IEEE floating point standard supports positive infinity and negative infinity as floating point values. It also provides for a class of values called NaN or "not-a-number"; numerical functions return such values in cases where there is no correct answer. For example, `(/ 0.0 0.0)' returns a NaN. (NaN values can also carry a sign, but for practical purposes there's no significant difference between different NaN values in Emacs Lisp.) When a function is documented to return a NaN, it returns an implementation-defined value when Emacs is running on one of the now-rare platforms that do not use IEEE floating point. For example, `(log -1.0)' typically returns a NaN, but on non-IEEE platforms it returns an implementation-defined value. Here are the read syntaxes for these special floating point values: positive infinity `1.0e+INF' negative infinity `-1.0e+INF' Not-a-number `0.0e+NaN' or `-0.0e+NaN'. -- Function: isnan number This predicate tests whether its argument is NaN, and returns `t' if so, `nil' otherwise. The argument must be a number. The following functions are specialized for handling floating point numbers: -- Function: frexp x This function returns a cons cell `(SIG . EXP)', where SIG and EXP are respectively the significand and exponent of the floating point number X: X = SIG * 2^EXP SIG is a floating point number between 0.5 (inclusive) and 1.0 (exclusive). If X is zero, the return value is `(0 . 0)'. -- Function: ldexp sig &optional exp This function returns a floating point number corresponding to the significand SIG and exponent EXP. -- 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 numbers. -- Function: logb number This function returns the binary exponent of NUMBER. More precisely, the value is the logarithm of |NUMBER| base 2, rounded down to an integer. (logb 10) => 3 (logb 10.0e20) => 69 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 *note Predicates on Markers::. -- Function: floatp object This predicate tests whether its argument is a floating point number 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. This is a synonym for `natnump'. -- Function: zerop number This predicate tests whether its argument is zero, and returns `t' if so, `nil' otherwise. The argument must be a number. `(zerop x)' is equivalent to `(= x 0)'. 3.4 Comparison of Numbers ========================= To test numbers for numerical equality, you should normally use `=', not `eq'. There can be many distinct floating point number objects with the same numeric value. If you use `eq' to compare them, then you test whether two values are the same _object_. By contrast, `=' compares only the numeric values of the objects. In Emacs Lisp, each integer value is a unique Lisp object. Therefore, `eq' is equivalent to `=' where integers are concerned. It is sometimes convenient to use `eq' for comparing an unknown value with an integer, because `eq' does not report an error if the unknown value is not a number--it accepts arguments of any type. By contrast, `=' signals an error if the arguments are not numbers or markers. However, it is better programming practice to use `=' if you can, even for comparing integers. Sometimes it is useful to compare numbers with `equal', which treats two numbers as equal if they have the same data type (both integers, or both floating point) and the same value. By contrast, `=' can treat an integer and a floating point number as equal. *Note Equality Predicates::. There is another wrinkle: because floating point arithmetic is not exact, it is often a bad idea to check for equality of two floating point values. Usually it is better to test for approximate equality. Here's a function to do this: (defvar fuzz-factor 1.0e-6) (defun approx-equal (x y) (or (and (= x 0) (= y 0)) (< (/ (abs (- x y)) (max (abs x) (abs y))) fuzz-factor))) Common Lisp note: Comparing numbers in Common Lisp always requires `=' because Common Lisp implements multi-word integers, and two distinct integer objects can have the same numeric value. Emacs Lisp can have just one integer object for any given value because it has a limited range of integer values. -- Function: = number-or-marker1 number-or-marker2 This function tests whether its arguments are numerically equal, and returns `t' if so, `nil' otherwise. -- Function: eql value1 value2 This function acts like `eq' except when both arguments are numbers. It compares numbers by type and numeric value, so that `(eql 1.0 1)' returns `nil', but `(eql 1.0 1.0)' and `(eql 1 1)' both return `t'. -- Function: /= number-or-marker1 number-or-marker2 This function tests whether its arguments are numerically equal, and returns `t' if they are not, and `nil' if they are. -- Function: < number-or-marker1 number-or-marker2 This function tests whether its first argument is strictly less than its second argument. It returns `t' if so, `nil' otherwise. -- Function: <= number-or-marker1 number-or-marker2 This function tests whether its first argument is less than or equal to its second argument. It returns `t' if so, `nil' otherwise. -- Function: > number-or-marker1 number-or-marker2 This function tests whether its first argument is strictly greater than its second argument. It returns `t' if so, `nil' otherwise. -- Function: >= number-or-marker1 number-or-marker2 This function tests whether its first argument is greater than or equal to its second 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. If any of the arguments is floating-point, the value is returned as floating point, even if it was given as an integer. (max 20) => 20 (max 1 2.5) => 2.5 (max 1 3 2.5) => 3.0 -- Function: min number-or-marker &rest numbers-or-markers This function returns the smallest of its arguments. If any of the arguments is floating-point, the value is returned as floating point, even if it was given as an integer. (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 a floating point number, `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. 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 may choose the integer closer to zero, or it may prefer an even integer, depending on your machine. (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 a floating point number. It is important to note that in Emacs Lisp, arithmetic functions do not check for overflow. Thus `(1+ 536870911)' may evaluate to -536870912, depending on your hardware. -- 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: / dividend divisor &rest divisors This function divides DIVIDEND by DIVISOR and returns the quotient. If there are additional arguments DIVISORS, then it divides DIVIDEND by each divisor in turn. 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. (Hypothetically, some machines may have different rounding behavior for negative arguments, because `/' is implemented using the C division operator, which permits machine-dependent rounding; but this does not happen in practice.) (/ 6 2) => 3 (/ 5 2) => 2 (/ 5.0 2) => 2.5 (/ 5 2.0) => 2.5 (/ 5.0 2.0) => 2.5 (/ 25 3 2) => 4 (/ -17 6) => -2 If you divide an integer by the integer 0, Emacs signals an `arith-error' error (*note Errors::). If you divide a floating point number by 0, or divide by the floating point number 0.0, the result is either positive or negative infinity (*note Float Basics::). -- Function: % dividend divisor This function returns the integer remainder after division of DIVIDEND by DIVISOR. The arguments must be integers or markers. For any two integers DIVIDEND and DIVISOR, (+ (% DIVIDEND DIVISOR) (* (/ DIVIDEND DIVISOR) DIVISOR)) always equals DIVIDEND. If DIVISOR is zero, Emacs signals an `arith-error' error. (% 9 4) => 1 (% -9 4) => -1 (% 9 -4) => 1 (% -9 -4) => -1 -- Function: mod dividend divisor This function returns the value of DIVIDEND modulo DIVISOR; in other words, the remainder after division of DIVIDEND by DIVISOR, but with the same sign as DIVISOR. The arguments must be numbers or markers. Unlike `%', `mod' permits floating point arguments; it rounds the quotient downward (towards minus infinity) to an integer, and uses that quotient to compute the remainder. If DIVISOR is zero, `mod' signals an `arith-error' error if both arguments are integers, and returns a NaN otherwise. (mod 9 4) => 1 (mod -9 4) => 3 (mod 9 -4) => -3 (mod -9 -4) => -1 (mod 5.5 2.5) => .5 For any two numbers DIVIDEND and DIVISOR, (+ (mod DIVIDEND DIVISOR) (* (floor DIVIDEND DIVISOR) DIVISOR)) always equals DIVIDEND, subject to rounding error if either argument is floating point. For `floor', see *note 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. 3.8 Bitwise Operations on Integers ================================== In a computer, an integer is represented as a binary number, a sequence of "bits" (digits which are either zero or one). A bitwise operation acts on the individual bits of such a sequence. For example, "shifting" moves the whole sequence left or right one or more places, reproducing the same pattern "moved over". The bitwise operations in Emacs Lisp apply only to integers. -- Function: lsh integer1 count `lsh', which is an abbreviation for "logical shift", shifts the bits in INTEGER1 to the left COUNT places, or to the right if COUNT is negative, bringing zeros into the vacated bits. If COUNT is negative, `lsh' shifts zeros into the leftmost (most-significant) bit, producing a positive result even if INTEGER1 is negative. Contrast this with `ash', below. Here are two examples of `lsh', shifting a pattern of bits one place to the left. We show only the low-order eight bits of the binary pattern; the rest are all zero. (lsh 5 1) => 10 ;; Decimal 5 becomes decimal 10. 00000101 => 00001010 (lsh 7 1) => 14 ;; Decimal 7 becomes decimal 14. 00000111 => 00001110 As the examples illustrate, shifting the pattern of bits one place to the left produces a number that is twice the value of the previous number. Shifting a pattern of bits two places to the left produces results like this (with 8-bit binary numbers): (lsh 3 2) => 12 ;; Decimal 3 becomes decimal 12. 00000011 => 00001100 On the other hand, shifting one place to the right looks like this: (lsh 6 -1) => 3 ;; Decimal 6 becomes decimal 3. 00000110 => 00000011 (lsh 5 -1) => 2 ;; Decimal 5 becomes decimal 2. 00000101 => 00000010 As the example illustrates, shifting one place to the right divides the value of a positive integer by two, rounding downward. The function `lsh', like all Emacs Lisp arithmetic functions, does not check for overflow, so shifting left can discard significant bits and change the sign of the number. For example, left shifting 536,870,911 produces -2 in the 30-bit implementation: (lsh 536870911 1) ; left shift => -2 In binary, the argument looks like this: ;; Decimal 536,870,911 0111...111111 (30 bits total) which becomes the following when left shifted: ;; Decimal -2 1111...111110 (30 bits total) -- Function: ash integer1 count `ash' ("arithmetic shift") shifts the bits in INTEGER1 to the left COUNT places, or to the right if COUNT is negative. `ash' gives the same results as `lsh' except when INTEGER1 and COUNT are both negative. In that case, `ash' puts ones in the empty bit positions on the left, while `lsh' puts zeros in those bit positions. Thus, with `ash', shifting the pattern of bits one place to the right looks like this: (ash -6 -1) => -3 ;; Decimal -6 becomes decimal -3. 1111...111010 (30 bits total) => 1111...111101 (30 bits total) In contrast, shifting the pattern of bits one place to the right with `lsh' looks like this: (lsh -6 -1) => 536870909 ;; Decimal -6 becomes decimal 536,870,909. 1111...111010 (30 bits total) => 0111...111101 (30 bits total) Here are other examples: ; 30-bit binary values (lsh 5 2) ; 5 = 0000...000101 => 20 ; = 0000...010100 (ash 5 2) => 20 (lsh -5 2) ; -5 = 1111...111011 => -20 ; = 1111...101100 (ash -5 2) => -20 (lsh 5 -2) ; 5 = 0000...000101 => 1 ; = 0000...000001 (ash 5 -2) => 1 (lsh -5 -2) ; -5 = 1111...111011 => 268435454 ; = 0011...111110 (ash -5 -2) ; -5 = 1111...111011 => -2 ; = 1111...111110 -- Function: logand &rest ints-or-markers This function returns the "logical and" of the arguments: the Nth bit is set in the result if, and only if, the Nth bit is set in all the arguments. ("Set" means that the value of the bit is 1 rather than 0.) For example, using 4-bit binary numbers, the "logical and" of 13 and 12 is 12: 1101 combined with 1100 produces 1100. In both the binary numbers, the leftmost two bits are set (i.e., they are 1's), so the leftmost two bits of the returned value are set. However, for the rightmost two bits, each is zero in at least one of the arguments, so the rightmost two bits of the returned value are 0's. Therefore, (logand 13 12) => 12 If `logand' is not passed any argument, it returns a value of -1. This number is an identity element for `logand' because its binary representation consists entirely of ones. If `logand' is passed just one argument, it returns that argument. ; 30-bit binary values (logand 14 13) ; 14 = 0000...001110 ; 13 = 0000...001101 => 12 ; 12 = 0000...001100 (logand 14 13 4) ; 14 = 0000...001110 ; 13 = 0000...001101 ; 4 = 0000...000100 => 4 ; 4 = 0000...000100 (logand) => -1 ; -1 = 1111...111111 -- Function: logior &rest ints-or-markers This function returns the "inclusive or" of its arguments: the Nth bit is set in the result if, and only if, the Nth bit is set in at least one of the arguments. If there are no arguments, the result is zero, which is an identity element for this operation. If `logior' is passed just one argument, it returns that argument. ; 30-bit binary values (logior 12 5) ; 12 = 0000...001100 ; 5 = 0000...000101 => 13 ; 13 = 0000...001101 (logior 12 5 7) ; 12 = 0000...001100 ; 5 = 0000...000101 ; 7 = 0000...000111 => 15 ; 15 = 0000...001111 -- Function: logxor &rest ints-or-markers This function returns the "exclusive or" of its arguments: the Nth bit is set in the result if, and only if, the Nth bit is set in an odd number of the arguments. If there are no arguments, the result is 0, which is an identity element for this operation. If `logxor' is passed just one argument, it returns that argument. ; 30-bit binary values (logxor 12 5) ; 12 = 0000...001100 ; 5 = 0000...000101 => 9 ; 9 = 0000...001001 (logxor 12 5 7) ; 12 = 0000...001100 ; 5 = 0000...000101 ; 7 = 0000...000111 => 14 ; 14 = 0000...001110 -- Function: lognot integer This function returns the logical complement of its argument: the Nth bit is one in the result if, and only if, the Nth bit is zero in INTEGER, and vice-versa. (lognot 5) => -6 ;; 5 = 0000...000101 (30 bits total) ;; becomes ;; -6 = 1111...111010 (30 bits total) 3.9 Standard Mathematical Functions =================================== These mathematical functions allow integers as well as floating point numbers as arguments. -- Function: sin arg -- Function: cos arg -- Function: tan arg These are the basic trigonometric functions, with argument ARG measured in radians. -- Function: asin arg The value of `(asin ARG)' is a number between -pi/2 and pi/2 (inclusive) whose sine is ARG. If ARG is out of range (outside [-1, 1]), `asin' returns a NaN. -- Function: acos arg The value of `(acos ARG)' is a number between 0 and pi (inclusive) whose cosine is ARG. If ARG is out of range (outside [-1, 1]), `acos' returns a NaN. -- Function: atan y &optional x The value of `(atan Y)' is a number between -pi/2 and pi/2 (exclusive) whose tangent is Y. If the optional second argument X is given, the value of `(atan y x)' is the angle in radians between the vector `[X, Y]' and the `X' axis. -- Function: exp arg This is the exponential function; it returns e to the power ARG. -- Function: log arg &optional base This function returns the logarithm of ARG, with base BASE. If you don't specify BASE, the natural base e is used. If ARG or BASE is negative, `log' returns a NaN. -- Function: log10 arg This function returns the logarithm of ARG, with base 10: `(log10 X)' == `(log X 10)'. -- Function: expt x y This function returns X raised to power Y. If both arguments are integers and Y is positive, the result is an integer; in this case, overflow causes truncation, so watch out. If X is a finite negative number and Y is a finite non-integer, `expt' returns a NaN. -- Function: sqrt arg This returns the square root of ARG. If ARG is negative, `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". Starting from any given seed, the `random' function always generates the same sequence of numbers. By default, Emacs initializes the random seed at startup, in such a way that the sequence of values of `random' (with overwhelming likelihood) differs in each Emacs run. Sometimes you want the random number sequence to be repeatable. For example, when debugging a program whose behavior depends on the random number sequence, it is helpful to get the same behavior in each program run. To make the sequence repeat, execute `(random "")'. This sets the seed to a constant value for your particular Emacs executable (though it may differ for other Emacs builds). You can use other strings to choose various seed values. -- Function: random &optional limit This function returns a pseudo-random integer. Repeated calls return a series of pseudo-random integers. If LIMIT is a positive integer, the value is chosen to be nonnegative and less than LIMIT. Otherwise, the value might be any integer representable in Lisp, i.e., an integer between `most-negative-fixnum' and `most-positive-fixnum' (*note Integer Basics::). If LIMIT is `t', it means to choose a new seed based on the current time of day and on Emacs's process ID number. If LIMIT is a string, it means to choose a new seed based on the string's contents. 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. *Note Strings of Events::, for special considerations for strings of keyboard character events. 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. *Note 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 (*note Sequences Arrays Vectors::). Unlike in C, Emacs Lisp strings are _not_ terminated by a distinguished character code. Since strings are arrays, and therefore sequences as well, you can operate on them with the general array and sequence functions documented in *note Sequences Arrays Vectors::. For example, you can access or change individual characters in a string using the functions `aref' and `aset' (*note Array Functions::). However, note that `length' should _not_ be used for computing the width of a string on display; use `string-width' (*note Width::) instead. There are two text representations for non-ASCII characters in Emacs strings (and in buffers): unibyte and multibyte. For most Lisp programming, you don't need to be concerned with these two representations. *Note 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. *Note 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' (*note Regexp Search::). The functions `match-string' (*note Simple Match Data::) and `replace-match' (*note Replacing Match::) are useful for decomposing and modifying strings after matching regular expressions against them. Like a buffer, a string can contain text properties for the characters in it, as well as the characters themselves. *Note 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. *Note Text::, for information about functions that display strings or copy them into buffers. *Note Character Type::, and *note String Type::, for information about the syntax of characters and strings. *Note Non-ASCII Characters::, for functions to convert between text representations and to encode and decode character codes. 4.2 Predicates for Strings ========================== For more information about general sequence and array predicates, see *note Sequences Arrays Vectors::, and *note Arrays::. -- Function: stringp object This function returns `t' if OBJECT is a string, `nil' otherwise. -- Function: string-or-null-p object This function returns `t' if OBJECT is a string or `nil'. It returns `nil' otherwise. -- Function: char-or-string-p object This function returns `t' if OBJECT is a string or a character (i.e., an integer), `nil' otherwise. 4.3 Creating Strings ==================== The following functions create strings, either from scratch, or by putting strings together, or by taking them apart. -- Function: make-string count character This function returns a string made up of COUNT repetitions of CHARACTER. If COUNT is negative, an error is signaled. (make-string 5 ?x) => "xxxxx" (make-string 0 ?x) => "" Other functions to compare with this one include `make-vector' (*note Vectors::) and `make-list' (*note Building Lists::). -- Function: string &rest characters This returns a string containing the characters CHARACTERS. (string ?a ?b ?c) => "abc" -- Function: substring string start &optional 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. (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 (*note Sequence Functions::). If the characters copied from STRING have text properties, the properties are copied into the new string also. *Note Text Properties::. `substring' also accepts a vector for the first argument. For example: (substring [a b (c) "d"] 1 3) => [b (c)] A `wrong-type-argument' error is signaled if START is not an integer or if END is neither an integer nor `nil'. An `args-out-of-range' error is signaled if START indicates a character following END, or if either integer is out of range for STRING. Contrast this function with `buffer-substring' (*note Buffer Contents::), which returns a string containing a portion of the text in the current buffer. The beginning of a string is at index 0, but the beginning of a buffer is at index 1. -- Function: substring-no-properties string &optional start end This works like `substring' but discards all text properties from the value. Also, START may be omitted or `nil', which is equivalent to 0. Thus, `(substring-no-properties STRING)' returns a copy of STRING, with all text properties removed. -- Function: concat &rest sequences This function returns a new string consisting of the characters in the arguments passed to it (along with their text properties, if any). The arguments may be strings, lists of numbers, or vectors of numbers; they are not themselves changed. If `concat' receives no arguments, it returns an empty string. (concat "abc" "-def") => "abc-def" (concat "abc" (list 120 121) [122]) => "abcxyz" ;; `nil' is an empty sequence. (concat "abc" nil "-def") => "abc-def" (concat "The " "quick brown " "fox.") => "The quick brown fox." (concat) => "" This function always constructs a new string that is not `eq' to any existing string, except when the result is the empty string (to save space, Emacs makes only one empty multibyte string). For information about other concatenation functions, see the description of `mapconcat' in *note Mapping Functions::, `vconcat' in *note Vector Functions::, and `append' in *note Building Lists::. For concatenating individual command-line arguments into a string to be used as a shell command, see *note combine-and-quote-strings: Shell Arguments. -- Function: split-string string &optional separators omit-nulls This function splits STRING into substrings based on the regular expression SEPARATORS (*note Regular Expressions::). Each match for SEPARATORS defines a splitting point; the substrings between splitting points are made into a list, which is returned. If OMIT-NULLS is `nil' (or omitted), the result contains null strings whenever there are two consecutive matches for SEPARATORS, or a match is adjacent to the beginning or end of STRING. If OMIT-NULLS is `t', these null strings are omitted from the result. If SEPARATORS is `nil' (or omitted), the default is the value of `split-string-default-separators'. As a special case, when SEPARATORS is `nil' (or omitted), null strings are always omitted from the result. Thus: (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" "") More examples: (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") If you need to split a string into a list of individual command-line arguments suitable for `call-process' or `start-process', see *note split-string-and-unquote: Shell Arguments. -- Variable: split-string-default-separators The default value of SEPARATORS for `split-string'. Its usual value is `"[ \f\t\n\r\v]+"'. 4.4 Modifying Strings ===================== The most basic way to alter the contents of an existing string is with `aset' (*note Array Functions::). `(aset STRING IDX CHAR)' stores CHAR into STRING at index IDX. Each character occupies one or more bytes, and if CHAR needs a different number of bytes from the character already present at that index, `aset' signals an error. A more powerful function is `store-substring': -- Function: store-substring string idx obj This function alters part of the contents of the string STRING, by storing OBJ starting at index IDX. The argument OBJ may be either a character or a (smaller) string. Since it is impossible to change the length of an existing string, it is an error if OBJ doesn't fit within STRING's actual length, or if any new character requires a different number of bytes from the character currently present at that point in STRING. To clear out a string that contained a password, use `clear-string': -- Function: clear-string string This makes STRING a unibyte string and clears its contents to zeros. It may also change STRING's length. 4.5 Comparison of Characters and Strings ======================================== -- Function: char-equal character1 character2 This function returns `t' if the arguments represent the same character, `nil' otherwise. This function ignores differences in case if `case-fold-search' is non-`nil'. (char-equal ?x ?x) => t (let ((case-fold-search nil)) (char-equal ?x ?X)) => nil -- Function: string= string1 string2 This function returns `t' if the characters of the two strings match exactly. Symbols are also allowed as arguments, in which case the symbol names are used. Case is always significant, regardless of `case-fold-search'. This function is equivalent to `equal' for comparing two strings (*note Equality Predicates::). In particular, the text properties of the two strings are ignored. But if either argument is not a string or symbol, an error is signaled. (string= "abc" "abc") => t (string= "abc" "ABC") => nil (string= "ab" "ABC") => nil For technical reasons, a unibyte and a multibyte string are `equal' if and only if they contain the same sequence of character codes and all these codes are either in the range 0 through 127 (ASCII) or 160 through 255 (`eight-bit-graphic'). However, when a unibyte string is converted to a multibyte string, all characters with codes in the range 160 through 255 are converted to characters with higher codes, whereas ASCII characters remain unchanged. Thus, a unibyte string and its conversion to multibyte are only `equal' if the string is all ASCII. Character codes 160 through 255 are not entirely proper in multibyte text, even though they can occur. As a consequence, the situation where a unibyte and a multibyte string are `equal' without both being all ASCII is a technical oddity that very few Emacs Lisp programmers ever get confronted with. *Note Text Representations::. -- Function: string-equal string1 string2 `string-equal' is another name for `string='. -- Function: string< string1 string2 This function compares two strings a character at a time. It scans both the strings at the same time to find the first pair of corresponding characters that do not match. If the lesser character of these two is the character from STRING1, then STRING1 is less, and this function returns `t'. If the lesser character is the one from STRING2, then STRING1 is greater, and this function returns `nil'. If the two strings match entirely, the value is `nil'. Pairs of characters are compared according to their character codes. Keep in mind that lower case letters have higher numeric values in the ASCII character set than their upper case counterparts; digits and many punctuation characters have a lower numeric value than upper case letters. An ASCII character is less than any non-ASCII character; a unibyte non-ASCII character is always less than any multibyte non-ASCII character (*note Text Representations::). (string< "abc" "abd") => t (string< "abd" "abc") => nil (string< "123" "abc") => t When the strings have different lengths, and they match up to the length of STRING1, then the result is `t'. If they match up to the length of STRING2, the result is `nil'. A string of no characters is less than any other string. (string< "" "abc") => t (string< "ab" "abc") => t (string< "abc" "") => nil (string< "abc" "ab") => nil (string< "" "") => nil Symbols are also allowed as arguments, in which case their print names are used. -- Function: string-lessp string1 string2 `string-lessp' is another name for `string<'. -- 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: compare-strings string1 start1 end1 string2 start2 end2 &optional ignore-case This function compares a specified part of STRING1 with a specified part of STRING2. The specified part of STRING1 runs from index START1 (inclusive) up to index END1 (exclusive); `nil' for START1 means the start of the string, while `nil' for END1 means the length of the string. Likewise, the specified part of STRING2 runs from index START2 up to index END2. The strings are compared by the numeric values of their characters. For instance, STR1 is considered "smaller than" STR2 if its first differing character has a smaller numeric value. If IGNORE-CASE is non-`nil', characters are converted to lower-case before comparing them. Unibyte strings are converted to multibyte for comparison (*note Text Representations::), so that a unibyte string and its conversion to multibyte are always regarded as equal. If the specified portions of the two strings match, the value is `t'. Otherwise, the value is an integer which indicates how many leading characters agree, and which string is less. Its absolute value is one plus the number of characters that agree at the beginning of the two strings. The sign is negative if STRING1 (or its specified portion) is less. -- Function: assoc-string key alist &optional case-fold This function works like `assoc', except that KEY must be a string or symbol, and comparison is done using `compare-strings'. Symbols are converted to strings before testing. If CASE-FOLD is non-`nil', it ignores case differences. Unlike `assoc', this function can also match elements of the alist that are strings or symbols rather than conses. In particular, ALIST can be a list of strings or symbols rather than an actual alist. *Note Association Lists::. See also the function `compare-buffer-substrings' in *note 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 *note Regexp Search::. 4.6 Conversion of Characters and Strings ======================================== This section describes functions for converting between characters, strings and integers. `format' (*note Formatting Strings::) and `prin1-to-string' (*note Output Functions::) can also convert Lisp objects into strings. `read-from-string' (*note 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 (*note Converting Representations::). *Note 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, which may be an integer or a floating point 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 *note Formatting Strings::. -- Function: string-to-number string &optional base This function returns the numeric value of the characters in STRING. If BASE is non-`nil', it must be an integer between 2 and 16 (inclusive), and integers are converted in that base. If BASE is `nil', then base ten is used. Floating point conversion only works in base ten; we have not implemented other radices for floating point numbers, because that would be much more work and does not seem useful. If STRING looks like an integer but its value is too large to fit into a Lisp integer, `string-to-number' returns a floating point result. The parsing skips spaces and tabs at the beginning of STRING, then reads as much of STRING as it can interpret as a number in the given base. (On some systems it ignores other whitespace at the beginning, not just spaces and tabs.) If the first character after the ignored whitespace is neither a digit in the given base, nor a plus or minus sign, nor the leading dot of a floating point 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. *Note Creating Strings::. -- Function: string-to-char string This function returns the first character in STRING. This mostly identical to `(aref string 0)', except that it returns 0 if the string is empty. (The value is also 0 when the first character of STRING is the null character, ASCII code 0.) This function may be eliminated in the future if it does not seem useful enough to retain. Here are some other functions that can convert to or from a string: `concat' This function converts a vector or a list into a string. *Note Creating Strings::. `vconcat' This function converts a string into a vector. *Note Vector Functions::. `append' This function converts a string into a list. *Note Building Lists::. `byte-to-string' This function converts a byte of character data into a unibyte string. *Note Converting Representations::. 4.7 Formatting Strings ====================== "Formatting" means constructing a string by substituting computed values at various places in a constant string. This constant string controls how the other values are printed, as well as where they appear; it is called a "format string". Formatting is often useful for computing messages to be displayed. In fact, the functions `message' and `error' provide the same formatting feature described here; they differ from `format' only in how they use the result of formatting. -- Function: format string &rest objects This function returns a new string that is made by copying STRING and then replacing any format specification in the copy 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. A format specification is a sequence of characters beginning with a `%'. Thus, if there is a `%d' in STRING, the `format' function replaces it with the printed representation of one of the values to be formatted (one of the arguments OBJECTS). For example: (format "The value of fill-column is %d." fill-column) => "The value of fill-column is 72." Since `format' interprets `%' characters as format specifications, you should _never_ pass an arbitrary string as the first argument. This is particularly true when the string is generated by some Lisp code. Unless the string is _known_ to never include any `%' characters, pass `"%s"', described below, as the first argument, and the string as the second, like this: (format "%s" ARBITRARY-STRING) If STRING contains more than one format specification, the format specifications correspond to successive values from OBJECTS. Thus, the first format specification in STRING uses the first such value, the second format specification uses the second such value, and so on. Any extra format specifications (those for which there are no corresponding values) cause an error. Any extra values to be formatted are ignored. Certain format specifications require values of particular types. If you supply a value that doesn't fit the requirements, an error is signaled. Here is a table of valid format specifications: `%s' Replace the specification with the printed representation of the object, made without quoting (that is, using `princ', not `prin1'--*note 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'--*note Output Functions::). Thus, strings are enclosed in `"' characters, and `\' characters appear where necessary before special characters. `%o' Replace the specification with the base-eight representation of an integer. `%d' Replace the specification with the base-ten representation of an integer. `%x' `%X' Replace the specification with the base-sixteen representation of an integer. `%x' uses lower case and `%X' uses upper case. `%c' Replace the specification with the character which is the value given. `%e' Replace the specification with the exponential notation for a floating point number. `%f' Replace the specification with the decimal-point notation for a floating point number. `%g' Replace the specification with notation for a floating point number, using either exponential notation or decimal-point notation, whichever is shorter. `%%' Replace the specification with a single `%'. This format specification is unusual in that it does not use a value. For example, `(format "%% %d" 30)' returns `"% 30"'. Any other format character results in an `Invalid format operation' error. Here are several examples: (format "The name of this buffer is %s." (buffer-name)) => "The name of this buffer is strings.texi." (format "The buffer object prints as %s." (current-buffer)) => "The buffer object prints as strings.texi." (format "The octal value of %d is %o, and the hex value is %x." 18 18 18) => "The octal value of 18 is 22, and the hex value is 12." A specification can have a "width", which is a decimal number between the `%' and the specification character. If the printed representation of the object contains fewer characters than this width, `format' extends it with padding. The width specifier is ignored for the `%%' specification. Any padding introduced by the width specifier normally consists of spaces inserted on the left: (format "%5d is padded on the left with spaces" 123) => " 123 is padded on the left with spaces" If the width is too small, `format' does not truncate the object's printed representation. Thus, you can use a width to specify a minimum spacing between columns with no risk of losing information. In the following three examples, `%7s' specifies a minimum width of 7. In the first case, the string inserted in place of `%7s' has only 3 letters, and needs 4 blank spaces as padding. In the second case, the string `"specification"' is 13 letters wide but is not truncated. (format "The word `%7s' has %d letters in it." "foo" (length "foo")) => "The word ` foo' has 3 letters in it." (format "The word `%7s' has %d letters in it." "specification" (length "specification")) => "The word `specification' has 13 letters in it." Immediately after the `%' and before the optional width specifier, you can also put certain "flag characters". The flag `+' inserts a plus sign before a positive number, so that it always has a sign. A space character as flag inserts a space before a positive number. (Otherwise, positive numbers start with the first digit.) These flags are useful for ensuring that positive numbers and negative numbers use the same number of columns. They are ignored except for `%d', `%e', `%f', `%g', and if both flags are used, `+' takes precedence. The flag `#' specifies an "alternate form" which depends on the format in use. For `%o', it ensures that the result begins with a `0'. For `%x' and `%X', it prefixes the result with `0x' or `0X'. For `%e', `%f', and `%g', the `#' flag means include a decimal point even if the precision is zero. The flag `0' ensures that the padding consists of `0' characters instead of spaces. This flag is ignored for non-numerical specification characters like `%s', `%S' and `%c'. These specification characters accept the `0' flag, but still pad with _spaces_. The flag `-' causes the padding inserted by the width specifier, if any, to be inserted on the right rather than the left. If both `-' and `0' are present, the `0' flag is ignored. (format "%06d is padded on the left with zeros" 123) => "000123 is padded on the left with zeros" (format "%-6d is padded on the right" 123) => "123 is padded on the right" (format "The word `%-7s' actually has %d letters in it." "foo" (length "foo")) => "The word `foo ' actually has 3 letters in it." All the specification characters allow an optional "precision" before the character (after the width, if present). The precision is a decimal-point `.' followed by a digit-string. For the floating-point specifications (`%e', `%f', `%g'), the precision specifies how many decimal places to show; if zero, the decimal-point itself is also omitted. For `%s' and `%S', the precision truncates the string to the given width, so `%.3s' shows only the first three characters of the representation for OBJECT. Precision has no effect for other specification characters. 4.8 Case Conversion in Lisp =========================== The character case functions change the case of single characters or of the contents of strings. The functions normally convert only alphabetic characters (the letters `A' through `Z' and `a' through `z', as well as non-ASCII letters); other characters are not altered. You can specify a different case conversion mapping by specifying a case table (*note Case Tables::). These functions do not modify the strings that are passed to them as arguments. The examples below use the characters `X' and `x' which have ASCII codes 88 and 120 respectively. -- Function: downcase string-or-char This function converts STRING-OR-CHAR, which should be either a character or a string, to lower case. When STRING-OR-CHAR is a string, this function returns a new string in which each letter in the argument that is upper case is converted to lower case. When STRING-OR-CHAR is a character, this function returns the corresponding lower case character (an integer); if the original character is lower case, or is not a letter, the return value is equal to the original character. (downcase "The cat in the hat") => "the cat in the hat" (downcase ?X) => 120 -- Function: upcase string-or-char This function converts STRING-OR-CHAR, which should be either a character or a string, to upper case. When STRING-OR-CHAR is a string, this function returns a new string in which each letter in the argument that is lower case is converted to upper case. When STRING-OR-CHAR is a character, this function returns the corresponding upper case character (an integer); if the original character is upper case, or is not a letter, the return value is equal to the original character. (upcase "The cat in the hat") => "THE CAT IN THE HAT" (upcase ?x) => 88 -- Function: capitalize string-or-char This function capitalizes strings or characters. If STRING-OR-CHAR is a string, the function returns a new string whose contents are a copy of STRING-OR-CHAR in which each word has been capitalized. This means that the first character of each word is converted to upper case, and the rest are converted to lower case. The definition of a word is any sequence of consecutive characters that are assigned to the word constituent syntax class in the current syntax table (*note Syntax Class Table::). When STRING-OR-CHAR is a character, this function does the same thing as `upcase'. (capitalize "The cat in the hat") => "The Cat In The Hat" (capitalize "THE 77TH-HATTED CAT") => "The 77th-Hatted Cat" (capitalize ?x) => 88 -- Function: upcase-initials string-or-char If STRING-OR-CHAR is a string, this function capitalizes the initials of the words in STRING-OR-CHAR, without altering any letters other than the initials. It returns a new string whose contents are a copy of STRING-OR-CHAR, in which each word has had its initial letter converted to upper case. The definition of a word is any sequence of consecutive characters that are assigned to the word constituent syntax class in the current syntax table (*note Syntax Class Table::). When the argument to `upcase-initials' is a character, `upcase-initials' has the same result as `upcase'. (upcase-initials "The CAT in the hAt") => "The CAT In The HAt" *Note Text Comparison::, for functions that compare strings; some of them ignore case differences, or can optionally ignore case differences. 4.9 The Case Table ================== You can customize case conversion by installing a special "case table". A case table specifies the mapping between upper case and lower case letters. It affects both the case conversion functions for Lisp objects (see the previous section) and those that apply to text in the buffer (*note 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 (*note 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 via `throw' or error (*note Nonlocal Exits::). Some language environments modify the case conversions of ASCII characters; for example, in the Turkish language environment, the ASCII character `I' is downcased into a Turkish "dotless i". This can interfere with code that requires ordinary ASCII case conversion, such as implementations of ASCII-based network protocols. In that case, use the `with-case-table' macro with the variable ASCII-CASE-TABLE, which stores the unmodified case table for the ASCII character set. -- 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. *Note 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. 5.1 Lists and Cons Cells ======================== Lists in Lisp are not a primitive data type; they are built up from "cons cells" (*note Cons Cell Type::). A cons cell is a data object that represents an ordered pair. That is, it has two slots, and each slot "holds", or "refers to", some Lisp object. One slot is known as the CAR, and the other is known as the CDR. (These names are traditional; see *note Cons Cell Type::.) CDR is pronounced "could-er". We say that "the CAR of this cons cell is" whatever object its CAR slot currently holds, and likewise for the CDR. A list is a series of cons cells "chained together", so that each cell refers to the next one. There is one cons cell for each element of the list. By convention, the CARs of the cons cells hold the elements of the list, and the CDRs are used to chain the list (this asymmetry between CAR and CDR is entirely a matter of convention; at the level of cons cells, the CAR and CDR slots have similar properties). Hence, the CDR slot of each cons cell in a list refers to the following cons cell. Also by convention, the CDR of the last cons cell in a list is `nil'. We call such a `nil'-terminated structure a "true list". In Emacs Lisp, the symbol `nil' is both a symbol and a list with no elements. For convenience, the symbol `nil' is considered to have `nil' as its CDR (and also as its CAR). Hence, the CDR of a true list is always a true list. The CDR of a nonempty true list is a true list containing all the elements except the first. If the CDR of a list's last cons cell is some value other than `nil', we call the structure a "dotted list", since its printed representation would use dotted pair notation (*note Dotted Pair Notation::). There is one other possibility: some cons cell's CDR could point to one of the previous cons cells in the list. We call that structure a "circular list". For some purposes, it does not matter whether a list is true, circular or dotted. If a program doesn't look far enough down the list to see the CDR of the final cons cell, it won't care. However, some functions that operate on lists demand true lists and signal errors if given a dotted list. Most functions that try to find the end of a list enter infinite loops if given a circular list. Because most cons cells are used as part of lists, we refer to any structure made out of cons cells as a "list structure". 5.2 Predicates on Lists ======================= The following predicates test whether a Lisp object is an atom, whether it is a cons cell or is a list, or whether it is the distinguished object `nil'. (Many of these predicates can be defined in terms of the others, but they are used so often that it is worth having them.) -- Function: consp object This function returns `t' if OBJECT is a cons cell, `nil' otherwise. `nil' is not a cons cell, although it _is_ a list. -- Function: atom object This function returns `t' if OBJECT is an atom, `nil' otherwise. All objects except cons cells are atoms. The symbol `nil' is an atom and is also a list; it is the only Lisp object that is both. (atom OBJECT) == (not (consp OBJECT)) -- Function: listp object This function returns `t' if OBJECT is a cons cell or `nil'. Otherwise, it returns `nil'. (listp '(1)) => t (listp '()) => t -- Function: nlistp object This function is the opposite of `listp': it returns `t' if OBJECT is not a list. Otherwise, it returns `nil'. (listp OBJECT) == (not (nlistp OBJECT)) -- Function: null object This function returns `t' if OBJECT is `nil', and returns `nil' otherwise. This function is identical to `not', but as a matter of clarity we use `null' when OBJECT is considered a list and `not' when it is considered a truth value (see `not' in *note Combining Conditions::). (null '(1)) => nil (null '()) => t 5.3 Accessing Elements of Lists =============================== -- Function: car cons-cell This function returns the value referred to by the first slot of the cons cell CONS-CELL. In other words, it returns the CAR of CONS-CELL. As a special case, if CONS-CELL is `nil', this function returns `nil'. Therefore, any list is a valid argument. An error is signaled if the argument is not a cons cell or `nil'. (car '(a b c)) => a (car '()) => nil -- Function: cdr cons-cell This function returns the value referred to by the second slot of the cons cell CONS-CELL. In other words, it returns the CDR of CONS-CELL. As a special case, if CONS-CELL is `nil', this function returns `nil'; therefore, any list is a valid argument. An error is signaled if the argument is not a cons cell or `nil'. (cdr '(a b c)) => (b c) (cdr '()) => nil -- Function: car-safe object This function lets you take the CAR of a cons cell while avoiding errors for other data types. It returns the CAR of OBJECT if OBJECT is a cons cell, `nil' otherwise. This is in contrast to `car', which signals an error if OBJECT is not a list. (car-safe OBJECT) == (let ((x OBJECT)) (if (consp x) (car x) 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 to `cdr', 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'. *Note Generalized Variables::. For the `push' macro, which adds an element to a list, *Note 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'. If N is negative, `nth' returns the first element of LIST. (nth 2 '(1 2 3 4)) => 3 (nth 10 '(1 2 3 4)) => nil (nth -3 '(1 2 3 4)) => 1 (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. *Note Sequence Functions::. -- Function: nthcdr n list This function returns the Nth CDR of LIST. In other words, it skips past the first N links of LIST and returns what follows. If N is zero or negative, `nthcdr' returns all of LIST. If the length of LIST is N or less, `nthcdr' returns `nil'. (nthcdr 1 '(1 2 3 4)) => (2 3 4) (nthcdr 10 '(1 2 3 4)) => nil (nthcdr -3 '(1 2 3 4)) => (1 2 3 4) -- Function: last list &optional n This function returns the last link of LIST. The `car' of this link is the list's last element. If LIST is null, `nil' is returned. If N is non-`nil', the Nth-to-last link is returned instead, or the whole of LIST if N is bigger than LIST's length. -- Function: safe-length list This function returns the length of LIST, with no risk of either an error or an infinite loop. It generally returns the number of distinct cons cells in the list. However, for circular lists, the value is just an upper bound; it is often too large. If LIST is not `nil' or a cons cell, `safe-length' returns 0. The most common way to compute the length of a list, when you are not worried that it may be circular, is with `length'. *Note Sequence Functions::. -- Function: caar cons-cell This is the same as `(car (car CONS-CELL))'. -- Function: cadr cons-cell This is the same as `(car (cdr CONS-CELL))' or `(nth 1 CONS-CELL)'. -- Function: cdar cons-cell This is the same as `(cdr (car CONS-CELL))'. -- Function: cddr cons-cell This is the same as `(cdr (cdr CONS-CELL))' or `(nthcdr 2 CONS-CELL)'. -- Function: butlast x &optional n This function returns the list X with the last element, or the last N elements, removed. If N is greater than zero it makes a copy of the list so as not to damage the original list. In general, `(append (butlast X N) (last X N))' will return a list equal to X. -- Function: nbutlast x &optional n This is a version of `butlast' that works by destructively modifying the `cdr' of the appropriate element, rather than making a copy of the list. 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". (1) For example: (setq list (cons newelt list)) Note that there is no conflict between the variable named `list' used in this example and the function named `list' described below; any symbol can serve both purposes. -- Function: list &rest objects This function creates a list with OBJECTS as its elements. The resulting list is always `nil'-terminated. If no OBJECTS are given, the empty list is returned. (list 1 2 3 4 5) => (1 2 3 4 5) (list 1 2 '(3 4 5) 'foo) => (1 2 (3 4 5) foo) (list) => nil -- Function: make-list length object This function creates a list of LENGTH elements, in which each element is OBJECT. Compare `make-list' with `make-string' (*note 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 *note Rearrangement::, for a way to join lists with no copying.) More generally, the final argument to `append' may be any Lisp object. The final argument is not copied or converted; it becomes the CDR of the last cons cell in the new list. If the final argument is itself a list, then its elements become in effect elements of the result list. If the final element is not a list, the result is a dotted list since its final CDR is not `nil' as required in a true list. 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. *Note Sequences Arrays Vectors::. Here we show the use of vectors and strings as arguments to `append': (append [a b] "cd" nil) => (a b 99 100) With the help of `apply' (*note 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: reverse list This function creates a new list whose elements are the elements of LIST, but in reverse order. The original argument LIST is _not_ altered. (setq x '(1 2 3 4)) => (1 2 3 4) (reverse x) => (4 3 2 1) x => (1 2 3 4) -- Function: copy-tree tree &optional vecp This function returns a copy of the tree `tree'. If TREE is a cons cell, this makes a new cons cell with the same CAR and CDR, then recursively copies the CAR and CDR in the same way. Normally, when TREE is anything other than a cons cell, `copy-tree' simply returns TREE. However, if VECP is non-`nil', it copies vectors too (and operates recursively on their elements). -- Function: number-sequence from &optional to separation This returns a list of numbers starting with FROM and incrementing by SEPARATION, and ending at or just before TO. SEPARATION can be positive or negative and defaults to 1. If TO is `nil' or numerically equal to FROM, the value is the one-element list `(FROM)'. If TO is less than FROM with a positive SEPARATION, or greater than FROM with a negative SEPARATION, the value is `nil' because those arguments specify an empty sequence. If SEPARATION is 0 and TO is neither `nil' nor numerically equal to FROM, `number-sequence' signals an error, since those arguments specify an infinite sequence. All arguments can be integers or floating point numbers. However, 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) ---------- Footnotes ---------- (1) There is no strictly equivalent way to add an element to the end of a list. You can use `(append LISTNAME (list NEWELT))', which creates a whole new list by copying LISTNAME and adding NEWELT to its end. Or you can use `(nconc LISTNAME (list NEWELT))', which modifies LISTNAME by following all the CDRs and then replacing the terminating `nil'. Compare this to adding an element to the beginning of a list with `cons', which neither copies nor modifies the list. 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))'. *Note Generalized Variables::. For the `pop' macro, which removes the first element from a list, *Note List Elements::. Two functions modify lists that are the values of variables. -- Function: add-to-list symbol element &optional append compare-fn This function sets the variable SYMBOL by consing ELEMENT onto the old value, if ELEMENT is not already a member of that value. It returns the resulting list, whether updated or not. The value of SYMBOL had better be a list already before the call. `add-to-list' uses COMPARE-FN to compare ELEMENT against existing list members; if COMPARE-FN is `nil', it uses `equal'. Normally, if ELEMENT is added, it is added to the front of SYMBOL, but if the optional argument APPEND is non-`nil', it is added at the end. The argument SYMBOL is not implicitly quoted; `add-to-list' is an ordinary function, like `set' and unlike `setq'. Quote the argument yourself if that is what you want. Here's a scenario showing how to use `add-to-list': (setq foo '(a b)) => (a b) (add-to-list 'foo 'c) ;; Add `c'. => (c a b) (add-to-list 'foo 'b) ;; No effect. => (c a b) foo ;; `foo' was changed. => (c a b) An equivalent expression for `(add-to-list 'VAR VALUE)' is this: (or (member VALUE VAR) (setq VAR (cons VALUE VAR))) -- Function: add-to-ordered-list symbol element &optional order This function sets the variable SYMBOL by inserting ELEMENT into the old value, which must be a list, at the position specified by ORDER. If ELEMENT is already a member of the list, its position in the list is adjusted according to ORDER. Membership is tested using `eq'. This function returns the resulting list, whether updated or not. The ORDER is typically a number (integer or float), and the elements of the list are sorted in non-decreasing numerical order. ORDER may also be omitted or `nil'. Then the numeric order of ELEMENT stays unchanged if it already has one; otherwise, ELEMENT has no numeric order. Elements without a numeric list order are placed at the end of the list, in no particular order. Any other value for ORDER removes the numeric order of ELEMENT if it already has one; otherwise, it is equivalent to `nil'. The argument SYMBOL is not implicitly quoted; `add-to-ordered-list' is an ordinary function, like `set' and unlike `setq'. Quote the argument yourself if necessary. The ordering information is stored in a hash table on SYMBOL's `list-order' property. Here's a scenario showing how to use `add-to-ordered-list': (setq foo '()) => nil (add-to-ordered-list 'foo 'a 1) ;; Add `a'. => (a) (add-to-ordered-list 'foo 'c 3) ;; Add `c'. => (a c) (add-to-ordered-list 'foo 'b 2) ;; Add `b'. => (a b c) (add-to-ordered-list 'foo 'b 4) ;; Move `b'. => (a c b) (add-to-ordered-list 'foo 'd) ;; Append `d'. => (a c b d) (add-to-ordered-list 'foo 'e) ;; Add `e'. => (a c b e d) foo ;; `foo' was changed. => (a c b e d) 5.6 Modifying Existing List Structure ===================================== You can modify the CAR and CDR contents of a cons cell with the primitives `setcar' and `setcdr'. We call these "destructive" operations because they change existing list structure. Common Lisp note: Common Lisp uses functions `rplaca' and `rplacd' to alter list structure; they change structure the same way as `setcar' and `setcdr', but the Common Lisp functions return the cons cell while `setcar' and `setcdr' return the new CAR or CDR. 5.6.1 Altering List Elements with `setcar' ------------------------------------------ Changing the CAR of a cons cell is done with `setcar'. When used on a list, `setcar' replaces one element of a list with a different element. -- Function: setcar cons object This function stores OBJECT as the new CAR of CONS, replacing its previous CAR. In other words, it changes the CAR slot of CONS to refer to OBJECT. It returns the value OBJECT. For example: (setq x '(1 2)) => (1 2) (setcar x 4) => 4 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 '(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 '(1 2 3)) => (1 2 3) (setcdr x '(4)) => (4) x => (1 4) You can delete elements from the middle of a list by altering the CDRs of the cons cells in the list. For example, here we delete the second element, `b', from the list `(a b c)', by changing the CDR of the first cons cell: (setq x1 '(a b c)) => (a b c) (setcdr x1 (cdr (cdr x1))) => (c) x1 => (a c) Here is the result in box notation: -------------------- | | -------------- | -------------- | -------------- | car | cdr | | | car | cdr | -->| car | cdr | | a | o----- | b | o-------->| c | nil | | | | | | | | | | -------------- -------------- -------------- The second cons cell, which previously held the element `b', still exists and its CAR is still `b', but it no longer forms part of this list. It is equally easy to insert a new element by changing CDRs: (setq x1 '(a b c)) => (a b c) (setcdr x1 (cons 'd (cdr x1))) => (d b c) x1 => (a d b c) Here is this result in box notation: -------------- ------------- ------------- | car | cdr | | car | cdr | | car | cdr | | a | o | -->| b | o------->| c | nil | | | | | | | | | | | | --------- | -- | ------------- ------------- | | ----- -------- | | | --------------- | | | car | cdr | | -->| d | o------ | | | --------------- 5.6.3 Functions that Rearrange Lists ------------------------------------ Here are some functions that rearrange lists "destructively" by modifying the CDRs of their component cons cells. We call these functions "destructive" because they chew up the original lists passed to them as arguments, relinking their cons cells to form a new list that is the returned value. See `delq', in *note Sets And Lists::, for another function that modifies cons cells. -- Function: nconc &rest lists This function returns a list containing all the elements of LISTS. Unlike `append' (*note Building Lists::), the LISTS are _not_ copied. Instead, the last CDR of each of the LISTS is changed to refer to the following list. The last of the LISTS is not altered. For example: (setq x '(1 2 3)) => (1 2 3) (nconc x '(4 5)) => (1 2 3 4 5) 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 '(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) must be lists. A common pitfall is to use a quoted constant list as a non-last argument to `nconc'. If you do this, your program will change each time you run it! Here is what happens: (defun add-foo (x) ; We want this function to add (nconc '(foo) x)) ; `foo' to the front of its arg. (symbol-function 'add-foo) => (lambda (x) (nconc (quote (foo)) x)) (setq xx (add-foo '(1 2))) ; It seems to work. => (foo 1 2) (setq xy (add-foo '(3 4))) ; What happened? => (foo 1 2 3 4) (eq xx xy) => t (symbol-function 'add-foo) => (lambda (x) (nconc (quote (foo 1 2 3 4) x))) -- Function: nreverse list This function reverses the order of the elements of LIST. Unlike `reverse', `nreverse' alters its argument by reversing the CDRs in the cons cells forming the list. The cons cell that used to be the last one in LIST becomes the first cons cell of the value. For example: (setq x '(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 | | | | | | | | | | | | | | ------------- | --------- | - | -------- | - | | | | ------------- ------------ -- Function: sort list predicate This function sorts LIST stably, though destructively, and returns the sorted list. It 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 LIST. To get an increasing order sort, the PREDICATE should return non-`nil' if the first element is "less than" the second, or `nil' if not. The comparison function PREDICATE must give reliable results for any given pair of arguments, at least within a single call to `sort'. It must be "antisymmetric"; that is, if A is less than B, B must not be less than A. It must be "transitive"--that is, if A is less than B, and B is less than C, then A must be less than C. If you use a comparison function which does not meet these requirements, the result of `sort' is unpredictable. The destructive aspect of `sort' is that it rearranges the cons cells forming LIST by changing CDRs. A nondestructive sort function would create new cons cells to store the elements in their sorted order. If you wish to make a sorted copy without destroying the original, copy it first with `copy-sequence' and then sort. Sorting does not change the CARs of the cons cells in LIST; the cons cell that originally contained the element `a' in LIST still has `a' in its CAR after sorting, but it now appears in a different position in the list due to the change of CDRs. For example: (setq nums '(1 3 2 6 5 4 0)) => (1 3 2 6 5 4 0) (sort nums '<) => (0 1 2 3 4 5 6) nums => (1 2 3 4 5 6) *Warning*: Note that the list in `nums' no longer contains 0; this is the same cons cell that it was before, but it is no longer the first one in the list. Don't assume a variable that formerly held the argument now holds the entire sorted list! Instead, save the result of `sort' and use that. Most often we store the result back into the variable that held the original list: (setq nums (sort nums '<)) *Note Sorting::, for more functions that perform sorting. See `documentation' in *note Accessing Documentation::, for a useful example of `sort'. 5.7 Using Lists as Sets ======================= A list can represent an unordered mathematical set--simply consider a value an element of a set if it appears in the list, and ignore the order of the list. To form the union of two sets, use `append' (as long as you don't mind having duplicate elements). You can remove `equal' duplicates using `delete-dups'. Other useful functions for sets include `memq' and `delq', and their `equal' versions, `member' and `delete'. Common Lisp note: Common Lisp has functions `union' (which avoids duplicate elements) and `intersection' for set operations. Although standard GNU Emacs Lisp does not have them, the `cl-lib' library provides versions. *Note Lists as Sets: (cl)Lists as Sets. -- Function: memq object list This function tests to see whether OBJECT is a member of LIST. If it is, `memq' returns a list starting with the first occurrence of OBJECT. Otherwise, it returns `nil'. The letter `q' in `memq' says that it uses `eq' to compare OBJECT against the elements of the list. For example: (memq 'b '(a b c b a)) => (b c b a) (memq '(2) '((1) (2))) ; `(2)' and `(2)' are not `eq'. => nil -- Function: delq object list This function destructively removes all elements `eq' to OBJECT from LIST, and returns the resulting list. The letter `q' in `delq' says that it uses `eq' to compare OBJECT against the elements of the list, like `memq' and `remq'. Typically, when you invoke `delq', you should use the return value by assigning it to the variable which held the original list. The reason for this is explained below. The `delq' function deletes elements from the front of the list by simply advancing down the list, and returning a sublist that starts after those elements. For example: (delq 'a '(a b c)) == (cdr '(a b c)) When an element to be deleted appears in the middle of the list, removing it involves changing the CDRs (*note Setcdr::). (setq sample-list '(a b c (4))) => (a b c (4)) (delq 'a sample-list) => (b c (4)) sample-list => (a b c (4)) (delq 'c sample-list) => (a b (4)) sample-list => (a b (4)) Note that `(delq 'c sample-list)' modifies `sample-list' to splice out the third element, but `(delq 'a sample-list)' does not splice anything--it just returns a shorter list. Don't assume that a variable which formerly held the argument LIST now has fewer elements, or that it still holds the original list! Instead, save the result of `delq' and use that. Most often we store the result back into the variable that held the original list: (setq flowers (delq 'rose flowers)) In the following example, the `(4)' that `delq' attempts to match and the `(4)' in the `sample-list' are not `eq': (delq '(4) sample-list) => (a c (4)) If you want to delete elements that are `equal' to a given value, use `delete' (see below). -- Function: remq object list This function returns a copy of LIST, with all elements removed which are `eq' to OBJECT. The letter `q' in `remq' says that it uses `eq' to compare OBJECT against the elements of `list'. (setq sample-list '(a b c a b c)) => (a b c a b c) (remq 'a sample-list) => (b c b c) 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 using `eql', so floating point elements are compared by value. If OBJECT is a member, `memql' returns a list starting with its first occurrence in LIST. Otherwise, it returns `nil'. Compare this with `memq': (memql 1.2 '(1.1 1.2 1.3)) ; `1.2' and `1.2' are `eql'. => (1.2 1.3) (memq 1.2 '(1.1 1.2 1.3)) ; `1.2' and `1.2' are not `eq'. => nil The following three functions are like `memq', `delq' and `remq', but use `equal' rather than `eq' to compare elements. *Note Equality Predicates::. -- Function: member object list The function `member' tests to see whether OBJECT is a member of LIST, comparing members with OBJECT using `equal'. If OBJECT is a member, `member' returns a list starting with its first occurrence in LIST. Otherwise, it returns `nil'. Compare this with `memq': (member '(2) '((1) (2))) ; `(2)' and `(2)' are `equal'. => ((2)) (memq '(2) '((1) (2))) ; `(2)' and `(2)' are not `eq'. => nil ;; Two strings with the same contents are `equal'. (member "foo" '("foo" "bar")) => ("foo" "bar") -- Function: delete object sequence This function removes all elements `equal' to OBJECT from SEQUENCE, and returns the resulting sequence. If SEQUENCE is a list, `delete' is to `delq' as `member' is to `memq': it uses `equal' to compare elements with OBJECT, like `member'; when it finds an element that matches, it cuts the element out just as `delq' would. As with `delq', you should typically use the return value by assigning it to the variable which held the original list. If `sequence' is a vector or string, `delete' returns a copy of `sequence' with all elements `equal' to `object' removed. For example: (setq l '((2) (1) (2))) (delete '(2) l) => ((1)) l => ((2) (1)) ;; If you want to change `l' reliably, ;; write `(setq l (delete '(2) l))'. (setq l '((2) (1) (2))) (delete '(1) l) => ((2) (2)) 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 of `sequence', a list, vector, or string, with elements `equal' to `object' removed. For example: (remove '(2) '((2) (1) (2))) => ((1)) (remove '(2) [(2) (1) (2)]) => [(1)] Common Lisp note: The functions `member', `delete' and `remove' in GNU Emacs Lisp are derived from Maclisp, not Common Lisp. The Common Lisp versions do not use `equal' to compare elements. -- Function: member-ignore-case object list This function is like `member', except that OBJECT should be a string and that it ignores differences in letter-case and text representation: upper-case and lower-case letters are treated as equal, and unibyte strings are converted to multibyte prior to comparison. -- Function: delete-dups list This function destructively removes all `equal' duplicates from LIST, stores the result in LIST and returns it. Of several `equal' occurrences of an element in LIST, `delete-dups' keeps the first one. See also the function `add-to-list', in *note 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".(1) 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. *Note Property Lists::, for a comparison of property lists and association lists. -- Function: assoc key alist This function returns the first association for KEY in ALIST, comparing KEY against the alist elements using `equal' (*note Equality Predicates::). It returns `nil' if no association in ALIST has a CAR `equal' to KEY. For example: (setq trees '((pine . cones) (oak . acorns) (maple . seeds))) => ((pine . cones) (oak . acorns) (maple . seeds)) (assoc 'oak trees) => (oak . acorns) (cdr (assoc 'oak trees)) => acorns (assoc 'birch trees) => nil Here is another example, in which the keys and values are not symbols: (setq needles-per-cluster '((2 "Austrian Pine" "Red Pine") (3 "Pitch Pine") (5 "White Pine"))) (cdr (assoc 3 needles-per-cluster)) => ("Pitch Pine") (cdr (assoc 2 needles-per-cluster)) => ("Austrian Pine" "Red Pine") The function `assoc-string' is much like `assoc' except that it ignores certain differences between strings. *Note Text Comparison::. -- Function: rassoc value alist This function returns the first association with value VALUE in ALIST. It returns `nil' if no association in ALIST has a CDR `equal' to VALUE. `rassoc' is like `assoc' except that it compares the CDR of each ALIST association instead of the CAR. You can think of this as "reverse `assoc'", finding the key for a given value. -- Function: assq key alist This function is like `assoc' in that it returns the first association for KEY in ALIST, but it makes the comparison using `eq' instead of `equal'. `assq' returns `nil' if no association in ALIST has a CAR `eq' to KEY. This function is used more often than `assoc', since `eq' is faster than `equal' and most alists use symbols as keys. *Note Equality Predicates::. (setq trees '((pine . cones) (oak . acorns) (maple . seeds))) => ((pine . cones) (oak . acorns) (maple . seeds)) (assq 'pine trees) => (pine . cones) On the other hand, `assq' is not usually useful in alists where the keys may not be symbols: (setq leaves '(("simple leaves" . oak) ("compound leaves" . horsechestnut))) (assq "simple leaves" leaves) => nil (assoc "simple leaves" leaves) => ("simple leaves" . oak) -- Function: rassq value alist This function returns the first association with value VALUE in ALIST. It returns `nil' if no association in ALIST has a CDR `eq' to VALUE. `rassq' is like `assq' except that it compares the CDR of each ALIST association instead of the CAR. You can think of this as "reverse `assq'", finding the key for a given value. For example: (setq trees '((pine . cones) (oak . acorns) (maple . seeds))) (rassq 'acorns trees) => (oak . acorns) (rassq 'spores trees) => nil `rassq' cannot search for a value stored in the CAR of the CDR of an element: (setq colors '((rose red) (lily white) (buttercup yellow))) (rassq 'white colors) => nil In this case, the CDR of the association `(lily white)' is not the symbol `white', but rather the list `(white)'. This becomes clearer if the association is written in dotted pair notation: (lily white) == (lily . (white)) -- Function: assoc-default key alist &optional test default This function searches ALIST for a match for KEY. For each element of ALIST, it compares the element (if it is an atom) or the element's CAR (if it is a cons) against KEY, by calling TEST with two arguments: the element or its CAR, and KEY. The arguments are passed in that order so that you can get useful results using `string-match' with an alist that contains regular expressions (*note Regexp Search::). If TEST is omitted or `nil', `equal' is used for comparison. If an alist element matches KEY by this criterion, then `assoc-default' returns a value based on this element. If the element is a cons, then the value is the element's CDR. Otherwise, the return value is DEFAULT. If no alist element matches KEY, `assoc-default' returns `nil'. -- Function: copy-alist alist This function returns a two-level deep copy of ALIST: it creates a new copy of each association, so that you can alter the associations of the new alist without changing the old one. (setq needles-per-cluster '((2 . ("Austrian Pine" "Red Pine")) (3 . ("Pitch Pine")) (5 . ("White Pine")))) => ((2 "Austrian Pine" "Red Pine") (3 "Pitch Pine") (5 "White Pine")) (setq copy (copy-alist needles-per-cluster)) => ((2 "Austrian Pine" "Red Pine") (3 "Pitch Pine") (5 "White Pine")) (eq needles-per-cluster copy) => nil (equal needles-per-cluster copy) => t (eq (car needles-per-cluster) (car copy)) => nil (cdr (car (cdr needles-per-cluster))) => ("Pitch Pine") (eq (cdr (car (cdr needles-per-cluster))) (cdr (car (cdr copy)))) => t This example shows how `copy-alist' makes it possible to change the associations of one copy without affecting the other: (setcdr (assq 3 copy) '("Martian Vacuum Pine")) (cdr (assq 3 needles-per-cluster)) => ("Pitch Pine") -- Function: assq-delete-all key alist This function deletes from ALIST all the elements whose CAR is `eq' to KEY, much as if you used `delq' to delete each such element one by one. It returns the shortened alist, and often modifies the original list structure of ALIST. For correct results, use the return value of `assq-delete-all' rather than looking at the saved value of ALIST. (setq alist '((foo 1) (bar 2) (foo 3) (lose 4))) => ((foo 1) (bar 2) (foo 3) (lose 4)) (assq-delete-all 'foo alist) => ((bar 2) (lose 4)) alist => ((foo 1) (bar 2) (lose 4)) -- Function: rassq-delete-all value alist This function deletes from ALIST all the elements whose CDR is `eq' to VALUE. It returns the shortened alist, and often modifies the original list structure of ALIST. `rassq-delete-all' is like `assq-delete-all' except that it compares the CDR of each ALIST association instead of the CAR. ---------- Footnotes ---------- (1) This usage of "key" is not related to the term "key sequence"; it means a value used to look up an item in a table. In this case, the table is the alist, and the alist associations are the items. 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. *Note 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. *Note Symbol Properties::. 5.9.1 Property Lists and Association Lists ------------------------------------------ Association lists (*note Association Lists::) are very similar to property lists. In contrast to association lists, the order of the pairs in the property list is not significant, since the property names must be distinct. Property lists are better than association lists for attaching information to various Lisp function names or variables. If your program keeps all such information in one association list, it will typically need to search that entire list each time it checks for an association for a particular Lisp function name or variable, which could be slow. By contrast, if you keep the same information in the property lists of the function names or variables themselves, each search will scan only the length of one property list, which is usually short. This is why the documentation for a variable is recorded in a property named `variable-documentation'. The byte compiler likewise uses properties to record those functions needing special treatment. However, association lists have their own advantages. Depending on your application, it may be faster to add an association to the front of an association list than to update a property. All properties for a symbol are stored in the same property list, so there is a possibility of a conflict between different uses of a property name. (For this reason, it is a good idea to choose property names that are probably unique, such as by beginning the property name with the program's usual name-prefix for variables and functions.) An association list may be used like a stack where associations are pushed on the front of the list and later discarded; this is not possible with a property list. 5.9.2 Property Lists Outside Symbols ------------------------------------ The following functions can be used to manipulate property lists. They all compare property names using `eq'. -- Function: plist-get plist property This returns the value of the PROPERTY property stored in the property list PLIST. It accepts a malformed PLIST argument. If PROPERTY is not found in the PLIST, it returns `nil'. For example, (plist-get '(foo 4) 'foo) => 4 (plist-get '(foo 4 bad) 'foo) => 4 (plist-get '(foo 4 bad) 'bad) => nil (plist-get '(foo 4 bad) 'bar) => nil -- Function: plist-put plist property value This stores VALUE as the value of the PROPERTY property in the property list PLIST. It may modify PLIST destructively, or it may construct a new list structure without altering the old. The function returns the modified property list, so you can store that back in the place where you got PLIST. For example, (setq my-plist '(bar t foo 4)) => (bar t foo 4) (setq my-plist (plist-put my-plist 'foo 69)) => (bar t foo 69) (setq my-plist (plist-put my-plist 'quux '(a))) => (bar t foo 69 quux (a)) -- Function: lax-plist-get plist property Like `plist-get' except that it compares properties using `equal' instead of `eq'. -- Function: lax-plist-put plist property value Like `plist-put' except that it compares properties using `equal' instead of `eq'. -- Function: plist-member plist property This returns non-`nil' if PLIST contains the given PROPERTY. Unlike `plist-get', this allows you to distinguish between a missing property and a property with the value `nil'. The value is actually the tail of PLIST whose `car' is PROPERTY. 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 | | | | | |____________| |_____________| | | | |________________________________| | |_____________________________________________| 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. -- Function: length sequence This function returns the number of elements in SEQUENCE. If SEQUENCE is a dotted list, a `wrong-type-argument' error is signaled. Circular lists may cause an infinite loop. For a char-table, the value returned is always one more than the maximum Emacs character code. *Note 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 *note Text Representations::. If you need to compute the width of a string on display, you should use `string-width' (*note Width::), not `length', since `length' only counts the number of characters, but does not account for the display width of each character. -- Function: elt sequence index This function returns the element of SEQUENCE indexed by INDEX. Legitimate values of INDEX are integers ranging from 0 up to one less than the length of SEQUENCE. If SEQUENCE is a list, out-of-range values behave as for `nth'. *Note Definition of nth::. Otherwise, out-of-range values trigger an `args-out-of-range' error. (elt [1 2 3 4] 2) => 3 (elt '(1 2 3 4) 2) => 3 ;; We use `string' to show clearly which character `elt' returns. (string (elt "1234" 2)) => "3" (elt [1 2 3 4] 4) error--> Args out of range: [1 2 3 4], 4 (elt [1 2 3 4] -1) error--> Args out of range: [1 2 3 4], -1 This function generalizes `aref' (*note Array Functions::) and `nth' (*note Definition of nth::). -- Function: copy-sequence sequence This function returns a copy of SEQUENCE. The copy is the same type of object as the original sequence, and it has the same elements in the same order. Storing a new element into the copy does not affect the original SEQUENCE, and vice versa. However, the elements of the new sequence are not copies; they are identical (`eq') to the elements of the original. Therefore, changes made within these elements, as found via the copied sequence, are also visible in the original sequence. If the sequence 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. *Note 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 *note Building Lists::, `concat' in *note Creating Strings::, and `vconcat' in *note Vector Functions::, for other ways to copy sequences. (setq bar '(1 2)) => (1 2) (setq x (vector 'foo bar)) => [foo (1 2)] (setq y (copy-sequence x)) => [foo (1 2)] (eq x y) => nil (equal x y) => t (eq (elt x 1) (elt y 1)) => t ;; Replacing an element of one sequence. (aset x 0 'quux) x => [quux (1 2)] y => [foo (1 2)] ;; Modifying the inside of a shared element. (setcar (aref x 1) 69) x => [quux (69 2)] y => [foo (69 2)] 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" (*note String Type::), "vectors" (*note Vector Type::), "bool-vectors" (*note Bool-Vector Type::), and "char-tables" (*note Char-Table Type::). Vectors and char-tables can hold elements of any type, but strings can only hold characters, and bool-vectors can only hold `t' and `nil'. All four kinds of array share these characteristics: * The first element of an array has index zero, the second element has index 1, and so on. This is called "zero-origin" indexing. For example, an array of four elements has indices 0, 1, 2, and 3. * The length of the array is fixed once you create it; you cannot change the length of an existing array. * For purposes of evaluation, the array is a constant--i.e., it evaluates to itself. * The elements of an array may be referenced or changed with the functions `aref' and `aset', respectively (*note Array Functions::). When you create an array, other than a char-table, you must specify its length. You cannot specify the length of a char-table, because that is determined by the range of character codes. In principle, if you want an array of text characters, you could use either a string or a vector. In practice, we always choose strings for such applications, for four reasons: * They occupy one-fourth the space of a vector of the same elements. * Strings are printed in a way that shows the contents more clearly as text. * Strings can hold text properties. *Note 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. *Note 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. *Note 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 array index This function returns the INDEXth element of ARRAY. 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 *note Sequence Functions::. -- Function: aset array index object This function sets the INDEXth element of ARRAY to be OBJECT. It returns OBJECT. (setq w [foo bar baz]) => [foo bar baz] (aset w 0 'fu) => fu w => [fu bar baz] (setq x "asdfasfd") => "asdfasfd" (aset x 3 ?Z) => 90 x => "asdZasfd" If ARRAY is a string and OBJECT is not a character, a `wrong-type-argument' error results. The function converts a unibyte string to multibyte if necessary to insert a character. -- Function: fillarray array object This function fills the array ARRAY with OBJECT, so that each element of ARRAY is OBJECT. It returns ARRAY. (setq a [a b c d e f g]) => [a b c d e f g] (fillarray a 0) => [0 0 0 0 0 0 0] a => [0 0 0 0 0 0 0] (setq s "When in the course") => "When in the course" (fillarray s ?-) => "------------------" If ARRAY is a string and OBJECT is not a character, a `wrong-type-argument' error results. The general sequence functions `copy-sequence' and `length' are often useful for objects known to be arrays. *Note Sequence Functions::. 6.4 Vectors =========== A "vector" is a general-purpose array whose elements can be any Lisp objects. (By contrast, the elements of a string can only be characters. *Note Strings and Characters::.) Vectors are used in Emacs for many purposes: as key sequences (*note Key Sequences::), as symbol-lookup tables (*note Creating Symbols::), as part of the representation of a byte-compiled function (*note 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. *Note Self-Evaluating Forms::. Here are examples illustrating these principles: (setq avector [1 two '(three) "four" [five]]) => [1 two (quote (three)) "four" [five]] (eval avector) => [1 two (quote (three)) "four" [five]] (eq avector (eval avector)) => t 6.5 Functions for Vectors ========================= Here are some functions that relate to vectors: -- 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 true lists, vectors, strings or bool-vectors. If no SEQUENCES are given, an empty vector is returned. The value is a newly constructed 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. *Note Byte-Code Objects::. For other concatenation functions, see `mapconcat' in *note Mapping Functions::, `concat' in *note Creating Strings::, and `append' in *note Building Lists::. The `append' function also provides a way to convert a vector into a list with the same elements: (setq avector [1 two (quote (three)) "four" [five]]) => [1 two (quote (three)) "four" [five]] (append avector nil) => (1 two (quote (three)) "four" [five]) 6.6 Char-Tables =============== A char-table is much like a vector, except that it is indexed by character codes. Any valid character code, without modifiers, can be used as an index in a char-table. You can access a char-table's elements with `aref' and `aset', as with any array. In addition, a char-table can have "extra slots" to hold additional data not associated with particular character codes. Like vectors, char-tables are constants when evaluated, and can hold elements of any type. Each char-table has a "subtype", a symbol, which serves two purposes: * The subtype provides an easy way to tell what the char-table is for. For instance, display tables are char-tables with `display-table' as the subtype, and syntax tables are char-tables with `syntax-table' as the subtype. The subtype can be queried using the function `char-table-subtype', described below. * The subtype controls the number of "extra slots" in the char-table. This number is specified by the subtype's `char-table-extra-slots' symbol property (*note 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 no `char-table-extra-slots' symbol property (*note Property Lists::), the char-table has no extra slots. -- Function: char-table-p object This function returns `t' if OBJECT is a char-table, and `nil' otherwise. -- Function: char-table-subtype char-table This function returns the subtype symbol of CHAR-TABLE. There is no special function to access default values in a char-table. To do that, use `char-table-range' (see below). -- Function: char-table-parent char-table This function returns the parent of CHAR-TABLE. The parent is always either `nil' or another char-table. -- Function: set-char-table-parent char-table new-parent This function sets the parent of CHAR-TABLE to NEW-PARENT. -- Function: char-table-extra-slot char-table n This function returns the contents of extra slot N 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 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 for `char-table-range'--either a valid character or a cons cell `(FROM . TO)', specifying a range of characters that share the same value. The value is what `(char-table-range CHAR-TABLE KEY)' returns. Overall, the key-value pairs passed to FUNCTION describe all the values stored in CHAR-TABLE. The return value is always `nil'; to make calls to `map-char-table' useful, FUNCTION should have side effects. For example, here is how to examine the elements of the syntax table: (let (accumulator) (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. There are two special functions for working with bool-vectors; aside from that, you manipulate them with same functions used for other kinds of arrays. -- Function: make-bool-vector length initial Return a new bool-vector of LENGTH elements, each one initialized to INITIAL. -- Function: bool-vector-p object This returns `t' if OBJECT is a bool-vector, and `nil' otherwise. Here is an example of creating, examining, and updating a bool-vector. Note that the printed form represents up to 8 boolean values as a single character. (setq bv (make-bool-vector 5 t)) => #&5"^_" (aref bv 1) => t (aset bv 3 nil) => nil bv => #&5"^W" These results make sense because the binary codes for control-_ and control-W are 11111 and 10111, respectively. 6.8 Managing a Fixed-Size Ring of Objects ========================================= A "ring" is a fixed-size data structure that supports insertion, deletion, rotation, and modulo-indexed reference and traversal. An efficient ring data structure is implemented by the `ring' package. It provides the functions listed in this section. Note that several "rings" in Emacs, like the kill ring and the mark ring, are actually implemented as simple lists, _not_ using the `ring' package; thus the following functions won't work on them. -- Function: make-ring size This returns a new ring capable of holding SIZE objects. SIZE should be an integer. -- Function: ring-p object This returns `t' if OBJECT is a ring, `nil' otherwise. -- Function: ring-size ring This returns the maximum capacity of the RING. -- Function: ring-length ring This returns the number of objects that RING currently contains. The value will never exceed that returned by `ring-size'. -- Function: ring-elements ring This returns a list of the objects in RING, in order, newest first. -- Function: ring-copy ring This returns a new ring which is a copy of RING. The new ring contains the same (`eq') objects as RING. -- Function: ring-empty-p ring This returns `t' if RING is empty, `nil' otherwise. The newest element in the ring always has index 0. Higher indices correspond to older elements. Indices are computed modulo the ring length. Index -1 corresponds to the oldest element, -2 to the next-oldest, and so forth. -- Function: ring-ref ring index This returns the object in RING found at index INDEX. INDEX may be negative or greater than the ring length. If RING is empty, `ring-ref' signals an error. -- Function: ring-insert ring object This inserts OBJECT into RING, making it the newest element, and returns OBJECT. If the ring is full, insertion removes the oldest element to make room for the new element. -- Function: ring-remove ring &optional index Remove an object from RING, and return that object. The argument INDEX specifies which item to remove; if it is `nil', that means to remove the oldest item. If RING is empty, `ring-remove' signals an error. -- Function: ring-insert-at-beginning ring object This inserts OBJECT into RING, treating it as the oldest element. The return value is not significant. If the ring is full, this function removes the newest element to make room for the inserted element. If you are careful not to exceed the ring size, you can use the ring as a first-in-first-out queue. For example: (let ((fifo (make-ring 5))) (mapc (lambda (obj) (ring-insert fifo obj)) '(0 one "two")) (list (ring-remove fifo) t (ring-remove fifo) t (ring-remove fifo))) => (0 t one t "two") 7 Hash Tables ************* A hash table is a very fast kind of lookup table, somewhat like an alist (*note 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. *Note Creating Hash::. (Note that the term "hash notation", which refers to the initial `#' character used in the printed representations of objects with no read representation, has nothing to do with the term "hash table". *Note Printed Representation::.) Obarrays are also a kind of hash table, but they are a different type of object and are used only for recording interned symbols (*note Creating Symbols::). 7.1 Creating Hash Tables ======================== The principal function for creating a hash table is `make-hash-table'. -- Function: make-hash-table &rest keyword-args This function creates a new hash table according to the specified arguments. The arguments should consist of alternating keywords (particular symbols recognized specially) and values corresponding to them. Several keywords make sense in `make-hash-table', but the only two that you really need to know about are `:test' and `:weakness'. `:test TEST' This specifies the method of key lookup for this hash table. The default is `eql'; `eq' and `equal' are other alternatives: `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 numbers; 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' (*note Defining Hash::) to define additional possibilities for TEST. `:weakness WEAK' The weakness of a hash table specifies whether the presence of a key or value in the hash table preserves it from garbage collection. The value, WEAK, must be one of `nil', `key', `value', `key-or-value', `key-and-value', or `t' which is an alias for `key-and-value'. If WEAK is `key' then the hash table does not prevent its keys from being collected as garbage (if they are not referenced anywhere else); if a particular key does get collected, the corresponding association is removed from the hash table. If WEAK is `value', then the hash table does not prevent values from being collected as garbage (if they are not referenced anywhere else); if a particular value does get collected, the corresponding association is removed from the hash table. If WEAK is `key-and-value' or `t', both the key and the value must be live in order to preserve the association. Thus, the hash table does not protect either keys or values from garbage collection; if either one is collected as garbage, that removes the association. If WEAK is `key-or-value', either the key or the value can preserve the association. Thus, associations are removed from the hash table when both their key and value would be collected as garbage (if not for references from weak hash tables). The default for WEAK is `nil', so that all keys and values referenced in the hash table are preserved from garbage collection. `:size SIZE' This specifies a hint for how many associations you plan to store in the hash table. If you know the approximate number, you can make things a little more efficient by specifying it this way. If you specify too small a size, the hash table will grow automatically when necessary, but doing that takes some extra time. The default size is 65. `:rehash-size REHASH-SIZE' When you add an association to a hash table and the table is "full", it grows automatically. This value specifies how to make the hash table larger, at that time. If REHASH-SIZE is an integer, it should be positive, and the hash table grows by adding that much to the nominal size. If REHASH-SIZE is a floating point number, it had better be greater than 1, and the hash table grows by multiplying the old size by 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 this fraction of the nominal size. The default for THRESHOLD is 0.8. -- Function: makehash &optional test This is equivalent to `make-hash-table', but with a different style argument list. The argument TEST specifies the method of key lookup. This function is obsolete. Use `make-hash-table' instead. You can also create a new hash table using the printed representation for hash tables. The Lisp reader can read this printed representation, provided each element in the specified hash table has a valid read syntax (*note Printed Representation::). For instance, the following specifies a new hash table containing the keys `key1' and `key2' (both symbols) associated with `val1' (a symbol) and `300' (a number) respectively. #s(hash-table size 30 data (key1 val1 key2 300)) The printed representation for a hash table consists of `#s' followed by a list beginning with `hash-table'. The rest of the list should consist of zero or more property-value pairs specifying the hash table's properties and initial contents. The properties and values are read literally. Valid property names are `size', `test', `weakness', `rehash-size', `rehash-threshold', and `data'. The `data' property should be a list of key-value pairs for the initial contents; the other properties have the same meanings as the matching `make-hash-table' keywords (`:size', `:test', etc.), described above. Note that you cannot specify a hash table whose initial contents include objects that have no read syntax, such as buffers and frames. Such objects may be added to the hash table after it is created. 7.2 Hash Table Access ===================== This section describes the functions for accessing and storing associations in a hash table. In general, any Lisp object can be used as a hash key, unless the comparison method imposes limits. Any Lisp object can also be used as the value. -- Function: gethash key table &optional default This function looks up KEY in TABLE, and returns its associated VALUE--or DEFAULT, if KEY has no association in TABLE. -- Function: puthash key value table This function enters an association for KEY in TABLE, with value VALUE. If KEY already has an association in TABLE, VALUE replaces the old associated value. -- Function: remhash key table This function removes the association for KEY from TABLE, if there is one. If KEY has no association, `remhash' does nothing. Common Lisp note: In Common Lisp, `remhash' returns non-`nil' if it actually removed an association and `nil' otherwise. In Emacs Lisp, `remhash' always returns `nil'. -- Function: clrhash table This function removes all the associations from hash table TABLE, so that it becomes empty. This is also called "clearing" the hash table. Common Lisp note: In Common Lisp, `clrhash' returns the empty TABLE. In Emacs Lisp, it returns `nil'. -- Function: maphash function table This function calls FUNCTION once for each of the associations in TABLE. The function FUNCTION should accept two arguments--a KEY listed in TABLE, and its associated VALUE. `maphash' returns `nil'. 7.3 Defining Hash Comparisons ============================= You can define new methods of key lookup by means of `define-hash-table-test'. In order to use this feature, you need to understand how hash tables work, and what a "hash code" means. You can think of a hash table conceptually as a large array of many slots, each capable of holding one association. To look up a key, `gethash' first computes an integer, the hash code, from the key. It reduces this integer modulo the length of the array, to produce an index in the array. Then it looks in that slot, and if necessary in other nearby slots, to see if it has found the key being sought. Thus, to define a new method of key lookup, you need to specify both a function to compute the hash code from a key, and a function to compare two keys directly. -- 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 integer values for hash codes, including negative integers. The specified functions are stored in the property list of NAME under the property `hash-table-test'; the property value's form is `(TEST-FN HASH-FN)'. -- Function: sxhash obj This function returns a hash code for Lisp object OBJ. This is an integer which reflects the contents of OBJ and the other Lisp objects it points to. If two objects OBJ1 and OBJ2 are equal, then `(sxhash OBJ1)' and `(sxhash OBJ2)' are the same integer. If the two objects are not equal, the values returned by `sxhash' are usually different, but not always; once in a rare while, by luck, you will encounter two distinct-looking objects that give the same result from `sxhash'. This example creates a hash table whose keys are strings that are compared case-insensitively. (defun case-fold-string= (a b) (eq t (compare-strings a nil nil b nil nil t))) (defun case-fold-string-hash (a) (sxhash (upcase a))) (define-hash-table-test 'case-fold 'case-fold-string= 'case-fold-string-hash) (make-hash-table :test 'case-fold) Here is how you could define a hash table test equivalent to the predefined test value `equal'. The keys can be any Lisp object, and equal-looking objects are considered the same key. (define-hash-table-test 'contents-hash 'equal 'sxhash) (make-hash-table :test 'contents-hash) 7.4 Other Hash Table Functions ============================== Here are some other functions for working with hash tables. -- 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' (*note Creating Hash::). -- Function: hash-table-weakness table This function returns the WEAK value that was specified for hash table TABLE. -- Function: hash-table-rehash-size table This returns the rehash size of TABLE. -- Function: hash-table-rehash-threshold table This returns the rehash threshold of TABLE. -- Function: hash-table-size table This returns the current nominal size of TABLE. 8 Symbols ********* A "symbol" is an object with a unique name. This chapter describes symbols, their components, their property lists, and how they are created and interned. Separate chapters describe the use of symbols as variables and as function names; see *note Variables::, and *note Functions::. For the precise read syntax for symbols, see *note 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. 8.1 Symbol Components ===================== Each symbol has four components (or "cells"), each of which references another object: Print name The symbol's name. Value The symbol's current value as a variable. Function The symbol's function definition. It can also hold a symbol, a keymap, or a keyboard macro. Property list The symbol's property list. The print name cell always holds a string, and cannot be changed. Each of the other three cells can be set to any Lisp object. The print name cell holds the string that is the name of a symbol. Since symbols are represented textually by their names, it is important not to have two symbols with the same name. The Lisp reader ensures this: every time it reads a symbol, it looks for an existing symbol with the specified name before it creates a new one. To get a symbol's name, use the function `symbol-name' (*note Creating Symbols::). The value cell holds a symbol's value as a variable, which is what you get if the symbol itself is evaluated as a Lisp expression. *Note 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"). *Note Constant Variables::. The function cell holds a symbol's function definition. Often, we refer to "the function `foo'" when we really mean the function stored in the function cell of `foo'; we make the distinction explicit only when necessary. Typically, the function cell is used to hold a function (*note Functions::) or a macro (*note Macros::). However, it can also be used to hold a symbol (*note Function Indirection::), keyboard macro (*note Keyboard Macros::), keymap (*note Keymaps::), or autoload object (*note Autoloading::). To get the contents of a symbol's function cell, use the function `symbol-function' (*note Function Cells::). The property list cell normally should hold a correctly formatted property list. To get a symbol's property list, use the function `symbol-plist'. *Note 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) => # 8.2 Defining Symbols ==================== A "definition" is a special kind of Lisp expression that announces your intention to use a symbol in a particular way. It typically specifies a value or meaning for the symbol for one kind of use, plus documentation for its meaning when used in this way. Thus, when you define a symbol as a variable, you can supply an initial value for the variable, plus documentation for the variable. `defvar' and `defconst' are special forms that define a symbol as a "global variable"--a variable that can be accessed at any point in a Lisp program. *Note Variables::, for details about variables. To define a customizable variable, use the `defcustom' macro, which also calls `defvar' as a subroutine (*note Customization::). In principle, you can assign a variable value to any symbol with `setq', whether not it has first been defined as a variable. However, you ought to write a variable definition for each global variable that you want to use; otherwise, your Lisp program may not act correctly if it is evaluated with lexical scoping enabled (*note Variable Scoping::). `defun' defines a symbol as a function, creating a lambda expression and storing it in the function cell of the symbol. This lambda expression thus becomes the function definition of the symbol. (The term "function definition", meaning the contents of the function cell, is derived from the idea that `defun' gives the symbol its definition as a function.) `defsubst' and `defalias' are two other ways of defining a function. *Note 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. *Note Macros::. As previously noted, Emacs Lisp allows the same symbol to be defined both as a variable (e.g., with `defvar') and as a function or macro (e.g., with `defun'). Such definitions do not conflict. These definition also act as guides for programming tools. For example, the `C-h f' and `C-h v' commands create help buffers containing links to the relevant variable, function, or macro definitions. *Note Name Help: (emacs)Name Help. 8.3 Creating and Interning Symbols ================================== To understand how symbols are created in GNU Emacs Lisp, you must know how Lisp reads them. Lisp must ensure that it finds the same symbol every time it reads the same set of characters. Failure to do so would cause complete confusion. When the Lisp reader encounters a symbol, it reads all the characters of the name. Then it "hashes" those characters to find an index in a table called an "obarray". Hashing is an efficient method of looking something up. For example, instead of searching a telephone book cover to cover when looking up Jan Jones, you start with the J's and go from there. That is a simple version of hashing. Each element of the obarray is a "bucket" which holds all the symbols with a given hash code; to look for a given name, it is sufficient to look through all the symbols in the bucket for that name's hash code. (The same idea is used for general Emacs hash tables, but they are a different data type; see *note Hash Tables::.) If a symbol with the desired name is found, the reader uses that symbol. If the obarray does not contain a symbol with that name, the reader makes a new symbol and adds it to the obarray. Finding or adding a symbol with a certain name is called "interning" it, and the symbol is then called an "interned symbol". Interning ensures that each obarray has just one symbol with any particular name. Other like-named symbols may exist, but not in the same obarray. Thus, the reader gets the same symbols for the same names, as long as you keep reading with the same obarray. Interning usually happens automatically in the reader, but sometimes other programs need to do it. For example, after the `M-x' command obtains the command name as a string using the minibuffer, it then interns the string, to get the interned symbol with that name. No obarray contains all symbols; in fact, some symbols are not in any obarray. They are called "uninterned symbols". An uninterned symbol has the same four cells as other symbols; however, the only way to gain access to it is by finding it in some other object or as the value of a variable. Creating an uninterned symbol is useful in generating Lisp code, because an uninterned symbol used as a variable in the code you generate cannot clash with any variables used in other Lisp programs. In Emacs Lisp, an obarray is actually a vector. Each element of the vector is a bucket; its value is either an interned symbol whose name hashes to that bucket, or 0 if the bucket is empty. Each interned symbol has an internal link (invisible to the user) to the next symbol in the bucket. Because these links are invisible, there is no way to find all the symbols in an obarray except using `mapatoms' (below). The order of symbols in a bucket is not significant. In an empty obarray, every element is 0, so you can create an obarray with `(make-vector LENGTH 0)'. *This is the only valid way to create an obarray.* Prime numbers as lengths tend to result in good hashing; lengths one less than a power of two are also good. *Do not try to put symbols in an obarray yourself.* This does not work--only `intern' can enter a symbol in an obarray properly. Common Lisp note: Unlike Common Lisp, Emacs Lisp does not provide for interning a single symbol in several obarrays. Most of the functions below take a name and sometimes an obarray as arguments. A `wrong-type-argument' error is signaled if the name is not a string, or if the obarray is not a vector. -- Function: symbol-name symbol This function returns the string that is SYMBOL's name. For example: (symbol-name 'foo) => "foo" *Warning:* Changing the string by substituting characters does change the name of the symbol, but fails to update the obarray, so don't do it! -- Function: make-symbol name This function returns a newly-allocated, uninterned symbol whose name is NAME (which must be a string). Its value and function definition are void, and its property list is `nil'. In the example below, the value of `sym' is not `eq' to `foo' because it is a distinct uninterned symbol whose name is also `foo'. (setq sym (make-symbol "foo")) => foo (eq sym 'foo) => nil -- Function: intern name &optional obarray This function returns the interned symbol whose name is NAME. If there is no such symbol in the obarray OBARRAY, `intern' creates a new one, adds it to the obarray, and returns it. If OBARRAY is omitted, the value of the global variable `obarray' is used. (setq sym (intern "foo")) => foo (eq sym 'foo) => t (setq sym1 (intern "foo" other-obarray)) => foo (eq sym1 'foo) => nil Common Lisp note: In Common Lisp, you can intern an existing symbol in an obarray. In Emacs Lisp, you cannot do this, because the argument to `intern' must be a string, not a symbol. -- Function: intern-soft name &optional obarray This function returns the symbol in OBARRAY whose name is NAME, or `nil' if OBARRAY has no symbol with that name. Therefore, you can use `intern-soft' to test whether a symbol with a given name is already interned. If OBARRAY is omitted, the value of the global variable `obarray' is used. The argument NAME may also be a symbol; in that case, the function returns NAME if NAME is interned in the specified obarray, and otherwise `nil'. (intern-soft "frazzle") ; No such symbol exists. => nil (make-symbol "frazzle") ; Create an uninterned one. => frazzle (intern-soft "frazzle") ; That one cannot be found. => nil (setq sym (intern "frazzle")) ; Create an interned one. => frazzle (intern-soft "frazzle") ; That one can be found! => frazzle (eq sym 'frazzle) ; And it is the same one. => t -- Variable: obarray This variable is the standard obarray for use by `intern' and `read'. -- Function: mapatoms function &optional obarray This function calls FUNCTION once with each symbol in the obarray OBARRAY. Then it returns `nil'. If OBARRAY is omitted, it defaults to the value of `obarray', the standard obarray for ordinary symbols. (setq count 0) => 0 (defun count-syms (s) (setq count (1+ count))) => count-syms (mapatoms 'count-syms) => nil count => 1871 See `documentation' in *note Accessing Documentation::, for another example using `mapatoms'. -- Function: unintern symbol obarray This function deletes SYMBOL from the obarray OBARRAY. If `symbol' is not actually in the obarray, `unintern' does nothing. If OBARRAY is `nil', the current obarray is used. If you provide a string instead of a symbol as SYMBOL, it stands for a symbol name. Then `unintern' deletes the symbol (if any) in the obarray which has that name. If there is no such symbol, `unintern' does nothing. If `unintern' does delete a symbol, it returns `t'. Otherwise it returns `nil'. 8.4 Symbol Properties ===================== A symbol may possess any number of "symbol properties", which can be used to record miscellaneous information about the symbol. For example, when a symbol has a `risky-local-variable' property with a non-`nil' value, that means the variable which the symbol names is a risky file-local variable (*note File Local Variables::). Each symbol's properties and property values are stored in the symbol's property list cell (*note Symbol Components::), in the form of a property list (*note Property Lists::). 8.4.1 Accessing Symbol Properties --------------------------------- The following functions can be used to access symbol properties. -- Function: get symbol property This function returns the value of the property named PROPERTY in SYMBOL's property list. If there is no such property, it returns `nil'. Thus, there is no distinction between a value of `nil' and the absence of the property. The name PROPERTY is compared with the existing property names using `eq', so any object is a legitimate property. See `put' for an example. -- Function: put symbol property value This function puts VALUE onto SYMBOL's property list under the property name PROPERTY, replacing any previous property value. The `put' function returns VALUE. (put 'fly 'verb 'transitive) =>'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 (*note Abbrevs::). You could define `put' in terms of `setplist' and `plist-put', as follows: (defun put (symbol prop value) (setplist symbol (plist-put (symbol-plist symbol) prop value))) -- Function: function-get symbol property 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. *Note Defining Functions::. 8.4.2 Standard Symbol Properties -------------------------------- Here, we list the symbol properties which are used for special purposes in Emacs. In the following table, whenever we say "the named function", that means the function whose name is the relevant symbol; similarly for "the named variable" etc. `:advertised-binding' This property value specifies the preferred key binding, when showing documentation, for the named function. *Note Keys in Documentation::. `char-table-extra-slots' The value, if non-`nil', specifies the number of extra slots in the named char-table type. *Note 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. *Note 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. *Note Variable Definitions::. `disabled' If the value is non-`nil', the named function is disabled as a command. *Note Disabling Commands::. `face-documentation' The value stores the documentation string of the named face. This is set automatically by `defface'. *Note Defining Faces::. `history-length' The value, if non-`nil', specifies the maximum minibuffer history length for the named history list variable. *Note 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. *Note Interactive Call::. `menu-enable' The value is an expression for determining whether the named menu item should be enabled in menus. *Note Simple Menu Items::. `mode-class' If the value is `special', the named major mode is "special". *Note 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. *Note Creating Buffer-Local::. `permanent-local-hook' If the value is non-`nil', the named function should not be deleted from the local value of a hook variable when changing major modes. *Note Setting Hooks::. `pure' This property is used internally to mark certain named functions for byte compiler optimization. Do not set it. `risky-local-variable' If the value is non-`nil', the named variable is considered risky as a file-local variable. *Note File Local Variables::. `safe-function' If the value is non-`nil', the named function is considered generally safe for evaluation. *Note Function Safety::. `safe-local-eval-function' If the value is non-`nil', the named function is safe to call in file-local evaluation forms. *Note File Local Variables::. `safe-local-variable' The value specifies a function for determining safe file-local values for the named variable. *Note File Local Variables::. `side-effect-free' A non-`nil' value indicates that the named function is free of side-effects, for determining function safety (*note Function Safety::) as well as for byte compiler optimizations. Do not set it. `variable-documentation' If non-`nil', this specifies the named vaariable's documentation string. This is set automatically by `defvar' and related functions. *Note Defining Faces::. 9 Evaluation ************ The "evaluation" of expressions in Emacs Lisp is performed by the "Lisp interpreter"--a program that receives a Lisp object as input and computes its "value as an expression". How it does this depends on the data type of the object, according to rules described in this chapter. The interpreter runs automatically to evaluate portions of your program, but can also be called explicitly via the Lisp primitive function `eval'. 9.1 Introduction to Evaluation ============================== The Lisp interpreter, or evaluator, is the part of Emacs that computes the value of an expression that is given to it. When a function written in Lisp is called, the evaluator computes the value of the function by evaluating the expressions in the function body. Thus, running any Lisp program really means running the Lisp interpreter. A Lisp object that is intended for evaluation is called a "form" or "expression"(1). 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. *Note 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). *Note 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 (*note Variables::).(2) 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 (*note 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. *Note Command Loop::. ---------- Footnotes ---------- (1) It is sometimes also referred to as an "S-expression" or "sexp", but we generally do not use this terminology in this manual. (2) This definition of "environment" is specifically not intended to include all the data that can affect the result of a program. 9.2 Kinds of Forms ================== A Lisp object that is intended to be evaluated is called a "form" (or an "expression"). How Emacs evaluates a form depends on its data type. Emacs has three different kinds of form that are evaluated differently: symbols, lists, and "all other types". This section describes all three kinds, one by one, starting with the "all other types" which are self-evaluating forms. 9.2.1 Self-Evaluating Forms --------------------------- A "self-evaluating form" is any form that is not a list or symbol. Self-evaluating forms evaluate to themselves: the result of evaluation is the same object that was evaluated. Thus, the number 25 evaluates to 25, and the string `"foo"' evaluates to the string `"foo"'. Likewise, evaluating a vector does not cause evaluation of the elements of the vector--it returns the same vector with its contents unchanged. '123 ; A number, shown without evaluation. => 123 123 ; Evaluated as usual--result is the same. => 123 (eval '123) ; Evaluated "by hand"--result is the same. => 123 (eval (eval '123)) ; Evaluating twice changes nothing. => 123 It is common to write numbers, characters, strings, and even vectors in Lisp code, taking advantage of the fact that they self-evaluate. However, it is quite unusual to do this for types that lack a read syntax, because there's no way to write them textually. It is possible to construct Lisp expressions containing these types by means of a Lisp program. Here is an example: ;; Build an expression containing a buffer object. (setq print-exp (list 'print (current-buffer))) => (print #) ;; Evaluate it. (eval print-exp) -| # => # 9.2.2 Symbol Forms ------------------ When a symbol is evaluated, it is treated as a variable. The result is the variable's value, if it has one. If the symbol has no value as a variable, the Lisp interpreter signals an error. For more information on the use of variables, see *note 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. *Note Constant Variables::. 9.2.3 Classification of List Forms ---------------------------------- A form that is a nonempty list is either a function call, a macro call, or a special form, according to its first element. These three kinds of forms are evaluated in different ways, described below. The remaining list elements constitute the "arguments" for the function, macro, or special form. The first step in evaluating a nonempty list is to examine its first element. This element alone determines what kind of form the list is and how the rest of the list is to be processed. The first element is _not_ evaluated, as it would be in some Lisp dialects such as Scheme. 9.2.4 Symbol Function Indirection --------------------------------- If the first element of the list is a symbol then evaluation examines the symbol's function cell, and uses its contents instead of the original symbol. If the contents are another symbol, this process, called "symbol function indirection", is repeated until it obtains a non-symbol. *Note Function Names::, for more information about symbol function indirection. One possible consequence of this process is an infinite loop, in the event that a symbol's function cell refers to the same symbol. Or a symbol may have a void function cell, in which case the subroutine `symbol-function' signals a `void-function' error. But if neither of these things happens, we eventually obtain a non-symbol, which ought to be a function or other suitable object. More precisely, we should now have a Lisp function (a lambda expression), a byte-code function, a primitive function, a Lisp macro, a special form, or an autoload object. Each of these types is a case described in one of the following sections. If the object is not one of these types, Emacs signals an `invalid-function' error. The following example illustrates the symbol indirection process. We use `fset' to set the function cell of a symbol and `symbol-function' to get the function cell contents (*note Function Cells::). Specifically, we store the symbol `car' into the function cell of `first', and the symbol `first' into the function cell of `erste'. ;; Build this function cell linkage: ;; ------------- ----- ------- ------- ;; | # | <-- | car | <-- | first | <-- | erste | ;; ------------- ----- ------- ------- (symbol-function '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 signals a `void-function' error if the final symbol is unbound and optional argument NOERROR is `nil' or omitted. Otherwise, if NOERROR is non-`nil', it returns `nil' if the final symbol is unbound. It signals a `cyclic-function-indirection' error if there is a loop in the chain of symbols. Here is how you could define `indirect-function' in Lisp: (defun indirect-function (function) (if (symbolp function) (indirect-function (symbol-function function)) function)) 9.2.5 Evaluation of Function Forms ---------------------------------- If the first element of a list being evaluated is a Lisp function object, byte-code object or primitive function object, then that list is a "function call". For example, here is a call to the function `+': (+ 1 x) The first step in evaluating a function call is to evaluate the remaining elements of the list from left to right. The results are the actual argument values, one value for each list element. The next step is to call the function with this list of arguments, effectively using the function `apply' (*note Calling Functions::). If the function is written in Lisp, the arguments are used to bind the argument variables of the function (*note Lambda Expressions::); then the forms in the function body are evaluated in order, and the value of the last body form becomes the value of the function call. 9.2.6 Lisp Macro Evaluation --------------------------- If the first element of a list being evaluated is a macro object, then the list is a "macro call". When a macro call is evaluated, the elements of the rest of the list are _not_ initially evaluated. Instead, these elements themselves are used as the arguments of the macro. The macro definition computes a replacement form, called the "expansion" of the macro, to be evaluated in place of the original form. The expansion may be any sort of form: a self-evaluating constant, a symbol, or a list. If the expansion is itself a macro call, this process of expansion repeats until some other sort of form results. Ordinary evaluation of a macro call finishes by evaluating the expansion. However, the macro expansion is not necessarily evaluated right away, or at all, because other programs also expand macro calls, and they may or may not evaluate the expansions. Normally, the argument expressions are not evaluated as part of computing the macro expansion, but instead appear as part of the expansion, so they are computed when the expansion is evaluated. For example, given a macro defined as follows: (defmacro cadr (x) (list 'car (list 'cdr x))) an expression such as `(cadr (assq 'handler list))' is a macro call, and its expansion is: (car (cdr (assq 'handler list))) Note that the argument `(assq 'handler list)' appears in the expansion. *Note Macros::, for a complete description of Emacs Lisp macros. 9.2.7 Special Forms ------------------- A "special form" is a primitive function specially marked so that its arguments are not all evaluated. Most special forms define control structures or perform variable bindings--things which functions cannot do. Each special form has its own rules for which arguments are evaluated and which are used without evaluation. Whether a particular argument is evaluated may depend on the results of evaluating other arguments. Here is a list, in alphabetical order, of all of the special forms in Emacs Lisp with a reference to where each is described. `and' *note Combining Conditions:: `catch' *note Catch and Throw:: `cond' *note Conditionals:: `condition-case' *note Handling Errors:: `defconst' *note Defining Variables:: `defvar' *note Defining Variables:: `function' *note Anonymous Functions:: `if' *note Conditionals:: `interactive' *note Interactive Call:: `let' `let*' *note Local Variables:: `or' *note Combining Conditions:: `prog1' `prog2' `progn' *note Sequencing:: `quote' *note Quoting:: `save-current-buffer' *note Current Buffer:: `save-excursion' *note Excursions:: `save-restriction' *note Narrowing:: `setq' *note Setting Variables:: `setq-default' *note Creating Buffer-Local:: `track-mouse' *note Mouse Tracking:: `unwind-protect' *note Nonlocal Exits:: `while' *note Iteration:: Common Lisp note: Here are some comparisons of special forms in GNU Emacs Lisp and Common Lisp. `setq', `if', and `catch' are special forms in both Emacs Lisp and Common Lisp. `save-excursion' is a special form in Emacs Lisp, but doesn't exist in Common Lisp. `throw' is a special form in Common Lisp (because it must be able to throw multiple values), but it is a function in Emacs Lisp (which doesn't have multiple values). 9.2.8 Autoloading ----------------- The "autoload" feature allows you to call a function or macro whose function definition has not yet been loaded into Emacs. It specifies which file contains the definition. When an autoload object appears as a symbol's function definition, calling that symbol as a function automatically loads the specified file; then it calls the real definition loaded from that file. The way to arrange for an autoload object to appear as a symbol's function definition is described in *note Autoload::. 9.3 Quoting =========== The special form `quote' returns its single argument, as written, without evaluating it. This provides a way to include constant symbols and lists, which are not self-evaluating objects, in a program. (It is not necessary to quote self-evaluating objects such as numbers, strings, and vectors.) -- Special Form: quote object This special form returns OBJECT, without evaluating it. Because `quote' is used so often in programs, Lisp provides a convenient read syntax for it. An apostrophe character (`'') followed by a Lisp object (in read syntax) expands to a list whose first element is `quote', and whose second element is the object. Thus, the read syntax `'x' is an abbreviation for `(quote x)'. Here are some examples of expressions that use `quote': (quote (+ 1 2)) => (+ 1 2) (quote foo) => foo 'foo => foo ''foo => (quote foo) '(quote foo) => (quote foo) ['foo] => [(quote foo)] Other quoting constructs include `function' (*note Anonymous Functions::), which causes an anonymous lambda expression written in Lisp to be compiled, and ``' (*note Backquote::), which is used to quote only part of a list, while computing and substituting other parts. 9.4 Backquote ============= "Backquote constructs" allow you to quote a list, but selectively evaluate elements of that list. In the simplest case, it is identical to the special form `quote' (described in the previous section; *note Quoting::). For example, these two forms yield identical results: `(a list of (+ 2 3) elements) => (a list of (+ 2 3) elements) '(a list of (+ 2 3) elements) => (a list of (+ 2 3) elements) The special marker `,' inside of the argument to backquote indicates a value that isn't constant. The Emacs Lisp evaluator evaluates the argument of `,', and puts the value in the list structure: `(a list of ,(+ 2 3) elements) => (a list of 5 elements) Substitution with `,' is allowed at deeper levels of the list structure also. For example: `(1 2 (3 ,(+ 4 5))) => (1 2 (3 9)) You can also "splice" an evaluated value into the resulting list, using the special marker `,@'. The elements of the spliced list become elements at the same level as the other elements of the resulting list. The equivalent code without using ``' is often unreadable. Here are some examples: (setq some-list '(2 3)) => (2 3) (cons 1 (append some-list '(4) some-list)) => (1 2 3 4 2 3) `(1 ,@some-list 4 ,@some-list) => (1 2 3 4 2 3) (setq list '(hack foo bar)) => (hack foo bar) (cons 'use (cons 'the (cons 'words (append (cdr list) '(as elements))))) => (use the words foo bar as elements) `(use the words ,@(cdr list) as elements) => (use the words foo bar as elements) 9.5 Eval ======== Most often, forms are evaluated automatically, by virtue of their occurrence in a program being run. On rare occasions, you may need to write code that evaluates a form that is computed at run time, such as after reading a form from text being edited or getting one from a property list. On these occasions, use the `eval' function. Often `eval' is not needed and something else should be used instead. For example, to get the value of a variable, while `eval' works, `symbol-value' is preferable; or rather than store expressions in a property list that then need to go through `eval', it is better to store functions instead that are then passed to `funcall'. The functions and variables described in this section evaluate forms, specify limits to the evaluation process, or record recently returned values. Loading a file also does evaluation (*note 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. How the evaluation proceeds depends on the type of the object (*note Forms::). The argument LEXICAL, if non-`nil', means to evaluate FORM using lexical scoping rules for variables, instead of the default dynamic scoping rules. *Note Lexical Binding::. Since `eval' is a function, the argument expression that appears in a call to `eval' is evaluated twice: once as preparation before `eval' is called, and again by the `eval' function itself. Here is an example: (setq foo 'bar) => bar (setq bar 'baz) => baz ;; Here `eval' receives argument `foo' (eval 'foo) => bar ;; Here `eval' receives argument `bar', which is the value of `foo' (eval foo) => baz The number of currently active calls to `eval' is limited to `max-lisp-eval-depth' (see below). -- Command: eval-region start end &optional stream read-function This function evaluates the forms in the current buffer in the region defined by the positions START and END. It reads forms from the region and calls `eval' on them until the end of the region is reached, or until an error is signaled and not handled. By default, `eval-region' does not produce any output. However, if STREAM is non-`nil', any output produced by output functions (*note Output Functions::), as well as the values that result from evaluating the expressions in the region are printed using STREAM. *Note Output Streams::. If READ-FUNCTION is non-`nil', it should be a function, which is used instead of `read' to read expressions one by one. This function is called with one argument, the stream for reading input. You can also use the variable `load-read-function' (*note How Programs Do Loading: Definition of load-read-function.) to specify this function, but it is more robust to use the READ-FUNCTION argument. `eval-region' does not move point. It always returns `nil'. -- Command: eval-buffer &optional buffer-or-name stream filename unibyte print This is similar to `eval-region', but the arguments provide different optional features. `eval-buffer' operates on the entire accessible portion of buffer BUFFER-OR-NAME. BUFFER-OR-NAME can be a buffer, a buffer name (a string), or `nil' (or omitted), which means to use the current buffer. STREAM is used as in `eval-region', unless STREAM is `nil' and PRINT non-`nil'. In that case, values that result from evaluating the expressions are still discarded, but the output of the output functions is printed in the echo area. FILENAME is the file name to use for `load-history' (*note Unloading::), and defaults to `buffer-file-name' (*note Buffer File Name::). If UNIBYTE is non-`nil', `read' converts strings to unibyte whenever possible. `eval-current-buffer' is an alias for this command. -- User Option: max-lisp-eval-depth This variable defines the maximum depth allowed in calls to `eval', `apply', and `funcall' before an error is signaled (with error message `"Lisp nesting exceeds max-lisp-eval-depth"'). This limit, with the associated error when it is exceeded, is one way Emacs Lisp avoids infinite recursion on an ill-defined function. If you increase the value of `max-lisp-eval-depth' too much, such code can cause stack overflow instead. The depth limit counts internal uses of `eval', `apply', and `funcall', such as for calling the functions mentioned in Lisp expressions, and recursive evaluation of function call arguments and function body forms, as well as explicit calls in Lisp code. The default value of this variable is 400. If you set it to a value less than 100, Lisp will reset it to 100 if the given value is reached. Entry to the Lisp debugger increases the value, if there is little room left, to make sure the debugger itself has room to execute. `max-specpdl-size' provides another limit on nesting. *Note Local Variables: Definition of max-specpdl-size. -- 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' in `lisp-interaction-mode'.) The elements are ordered most recent first. (setq x 1) => 1 (list 'A (1+ 2) auto-save-default) => (A 3 t) values => ((A 3 t) 1 ...) This variable is useful for referring back to values of forms recently evaluated. It is generally a bad idea to print the value of `values' itself, since this may be very long. Instead, examine particular elements, like this: ;; Refer to the most recent evaluation result. (nth 0 values) => (A 3 t) ;; That put a new element on, ;; so all elements move back one. (nth 1 values) => (A 3 t) ;; This gets the element that was next-to-most-recent ;; before this example. (nth 3 values) => 1 10 Control Structures ********************* A Lisp program consists of a set of "expressions", or "forms" (*note 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 (*note Macros::). 10.1 Sequencing =============== Evaluating forms in the order they appear is the most common way control passes from one form to another. In some contexts, such as in a function body, this happens automatically. Elsewhere you must use a control structure construct to do this: `progn', the simplest control construct of Lisp. A `progn' special form looks like this: (progn A B C ...) and it says to execute the forms A, B, C, and so on, in that order. These forms are called the "body" of the `progn' form. The value of the last form in the body becomes the value of the entire `progn'. `(progn)' returns `nil'. In the early days of Lisp, `progn' was the only way to execute two or more forms in succession and use the value of the last of them. But programmers found they often needed to use a `progn' in the body of a function, where (at that time) only one form was allowed. So the body of a function was made into an "implicit `progn'": several forms are allowed just as in the body of an actual `progn'. Many other control structures likewise contain an implicit `progn'. As a result, `progn' is not used as much as it was many years ago. It is needed now most often inside an `unwind-protect', `and', `or', or in the THEN-part of an `if'. -- 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" 10.2 Conditionals ================= Conditional control structures choose among alternatives. Emacs Lisp has four conditional forms: `if', which is much the same as in other languages; `when' and `unless', which are variants of `if'; and `cond', which is a generalized case statement. -- Special Form: if condition then-form else-forms... `if' chooses between the THEN-FORM and the ELSE-FORMS based on the value of CONDITION. If the evaluated CONDITION is non-`nil', THEN-FORM is evaluated and the result returned. Otherwise, the ELSE-FORMS are evaluated in textual order, and the value of the last one is returned. (The ELSE part of `if' is an example of an implicit `progn'. *Note Sequencing::.) If CONDITION has the value `nil', and no ELSE-FORMS are given, `if' returns `nil'. `if' is a special form because the branch that is not selected is never evaluated--it is ignored. Thus, in this example, `true' is not printed because `print' is never called: (if nil (print 'true) 'very-false) => very-false -- Macro: when condition then-forms... This is a variant of `if' where there are no ELSE-FORMS, and possibly several THEN-FORMS. In particular, (when CONDITION A B C) is entirely equivalent to (if CONDITION (progn A B C) nil) -- Macro: unless condition forms... This is a variant of `if' where there is no THEN-FORM: (unless CONDITION A B C) is entirely equivalent to (if CONDITION nil A B C) -- Special Form: cond clause... `cond' chooses among an arbitrary number of alternatives. Each CLAUSE in the `cond' must be a list. The CAR of this list is the CONDITION; the remaining elements, if any, the BODY-FORMS. Thus, a clause looks like this: (CONDITION BODY-FORMS...) `cond' tries the clauses in textual order, by evaluating the CONDITION of each clause. If the value of CONDITION is non-`nil', the clause "succeeds"; then `cond' evaluates its BODY-FORMS, and the value of the last of BODY-FORMS becomes the value of the `cond'. The remaining clauses are ignored. If the value of CONDITION is `nil', the clause "fails", so the `cond' moves on to the following clause, trying its CONDITION. If every CONDITION evaluates to `nil', so that every clause fails, `cond' returns `nil'. A clause may also look like this: (CONDITION) Then, if CONDITION is non-`nil' when tested, the value of CONDITION becomes the value of the `cond' form. The following example has four clauses, which test for the cases where the value of `x' is a number, string, buffer and symbol, respectively: (cond ((numberp x) x) ((stringp x) x) ((bufferp x) (setq temporary-hack x) ; multiple body-forms (buffer-name x)) ; in one clause ((symbolp x) (symbol-value x))) Often we want to execute the last clause whenever none of the previous clauses was successful. To do this, we use `t' as the CONDITION of the last clause, like this: `(t BODY-FORMS)'. The form `t' evaluates to `t', which is never `nil', so this clause never fails, provided the `cond' gets to it at all. For example: (setq a 5) (cond ((eq a 'hack) 'foo) (t "default")) => "default" This `cond' expression returns `foo' if the value of `a' is `hack', and returns the string `"default"' otherwise. Any conditional construct can be expressed with `cond' or with `if'. Therefore, the choice between them is a matter of style. For example: (if A B C) == (cond (A B) (t C)) 10.2.1 Pattern matching case statement -------------------------------------- To compare a particular value against various possible cases, the macro `pcase' can come handy. It takes the following form: (pcase EXP BRANCH1 BRANCH2 BRANCH3 ...) where each BRANCH takes the form `(UPATTERN BODY-FORMS...)'. It will first evaluate EXP and then compare the value against each UPATTERN to see which BRANCH to use, after which it will run the corresponding BODY-FORMS. A common use case is to distinguish between a few different constant values: (pcase (get-return-code x) (`success (message "Done!")) (`would-block (message "Sorry, can't do it now")) (`read-only (message "The shmliblick is read-only")) (`access-denied (message "You do not have the needed rights")) (code (message "Unknown return code %S" code))) In the last clause, `code' is a variable that gets bound to the value that was returned by `(get-return-code x)'. To give a more complex example, a simple interpreter for a little expression language could look like: (defun evaluate (exp env) (pcase exp (`(add ,x ,y) (+ (evaluate x env) (evaluate y env))) (`(call ,fun ,arg) (funcall (evaluate fun) (evaluate arg env))) (`(fn ,arg ,body) (lambda (val) (evaluate body (cons (cons arg val) env)))) ((pred numberp) exp) ((pred symbolp) (cdr (assq exp env))) (_ (error "Unknown expression %S" exp)))) Where ``(add ,x ,y)' is a pattern that checks that `exp' is a three element list starting with the symbol `add', then extracts the second and third elements and binds them to the variables `x' and `y'. `(pred numberp)' is a pattern that simply checks that `exp' is a number, and `_' is the catch-all pattern that matches anything. There are two kinds of patterns involved in `pcase', called _U-patterns_ and _Q-patterns_. The UPATTERN mentioned above are U-patterns and can take the following forms: ``QPATTERN' This is one of the most common form of patterns. The intention is to mimic the backquote macro: this pattern matches those values that could have been built by such a backquote expression. Since we're pattern matching rather than building a value, the unquote does not indicate where to plug an expression, but instead it lets one specify a U-pattern that should match the value at that location. More specifically, a Q-pattern can take the following forms: `(QPATTERN1 . QPATTERN2)' This pattern matches any cons cell whose `car' matches QPATTERN1 and whose `cdr' matches PATTERN2. `ATOM' This pattern matches any atom `equal' to ATOM. `,UPATTERN' This pattern matches any object that matches the UPATTERN. `SYMBOL' A mere symbol in a U-pattern matches anything, and additionally let-binds this symbol to the value that it matched, so that you can later refer to it, either in the BODY-FORMS or also later in the pattern. `_' This so-called _don't care_ pattern matches anything, like the previous one, but unless symbol patterns it does not bind any variable. `(pred PRED)' This pattern matches if the function PRED returns non-`nil' when called with the object being matched. `(or UPATTERN1 UPATTERN2...)' This pattern matches as soon as one of the argument patterns succeeds. All argument patterns should let-bind the same variables. `(and UPATTERN1 UPATTERN2...)' This pattern matches only if all the argument patterns succeed. `(guard EXP)' This pattern ignores the object being examined and simply succeeds if EXP evaluates to non-`nil' and fails otherwise. It is typically used inside an `and' pattern. For example, `(and x (guard (< x 10)))' is a pattern which matches any number smaller than 10 and let-binds it to the variable `x'. 10.3 Constructs for Combining Conditions ======================================== This section describes three constructs that are often used together with `if' and `cond' to express complicated conditions. The constructs `and' and `or' can also be used individually as kinds of multiple conditional constructs. -- Function: not condition This function tests for the falsehood of CONDITION. It returns `t' if CONDITION is `nil', and `nil' otherwise. The function `not' is identical to `null', and we recommend using the name `null' if you are testing for an empty list. -- Special Form: and conditions... The `and' special form tests whether all the CONDITIONS are true. It works by evaluating the CONDITIONS one by one in the order written. If any of the CONDITIONS evaluates to `nil', then the result of the `and' must be `nil' regardless of the remaining CONDITIONS; so `and' returns `nil' right away, ignoring the remaining CONDITIONS. If all the CONDITIONS turn out non-`nil', then the value of the last of them becomes the value of the `and' form. Just `(and)', with no CONDITIONS, returns `t', appropriate because all the CONDITIONS turned out non-`nil'. (Think about it; which one did not?) Here is an example. The first condition returns the integer 1, which is not `nil'. Similarly, the second condition returns the integer 2, which is not `nil'. The third condition is `nil', so the remaining condition is never evaluated. (and (print 1) (print 2) nil (print 3)) -| 1 -| 2 => nil Here is a more realistic example of using `and': (if (and (consp foo) (eq (car foo) 'x)) (message "foo is a list starting with x")) Note that `(car foo)' is not executed if `(consp foo)' returns `nil', thus avoiding an error. `and' expressions can also be written using either `if' or `cond'. Here's how: (and ARG1 ARG2 ARG3) == (if ARG1 (if ARG2 ARG3)) == (cond (ARG1 (cond (ARG2 ARG3)))) -- Special Form: or conditions... The `or' special form tests whether at least one of the CONDITIONS is true. It works by evaluating all the CONDITIONS one by one in the order written. If any of the CONDITIONS evaluates to a non-`nil' value, then the result of the `or' must be non-`nil'; so `or' returns right away, ignoring the remaining CONDITIONS. The value it returns is the non-`nil' value of the condition just evaluated. If all the CONDITIONS turn out `nil', then the `or' expression returns `nil'. Just `(or)', with no CONDITIONS, returns `nil', appropriate because all the CONDITIONS turned out `nil'. (Think about it; which one did not?) For example, this expression tests whether `x' is either `nil' or the integer zero: (or (eq x nil) (eq x 0)) Like the `and' construct, `or' can be written in terms of `cond'. For example: (or ARG1 ARG2 ARG3) == (cond (ARG1) (ARG2) (ARG3)) You could almost write `or' in terms of `if', 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. 10.4 Iteration ============== Iteration means executing part of a program repetitively. For example, you might want to repeat some computation once for each element of a list, or once for each integer from 0 to N. You can do this in Emacs Lisp with the special form `while': -- Special Form: while condition forms... `while' first evaluates CONDITION. If the result is non-`nil', it evaluates FORMS in textual order. Then it reevaluates CONDITION, and if the result is non-`nil', it evaluates FORMS again. This process repeats until CONDITION evaluates to `nil'. There is no limit on the number of iterations that may occur. The loop will continue until either CONDITION evaluates to `nil' or until an error or `throw' jumps out of it (*note Nonlocal Exits::). The value of a `while' form is always `nil'. (setq num 0) => 0 (while (< num 4) (princ (format "Iteration %d." num)) (setq num (1+ num))) -| Iteration 0. -| Iteration 1. -| Iteration 2. -| Iteration 3. => nil To write a "repeat...until" loop, which will execute something on each iteration and then do the end-test, put the body followed by the end-test in a `progn' as the first argument of `while', as shown here: (while (progn (forward-line 1) (not (looking-at "^$")))) This moves forward one line and continues moving by lines until it reaches an empty line. It is peculiar in that the `while' has no body, just the end test (which also does the real work of moving point). The `dolist' and `dotimes' macros provide convenient ways to write two common kinds of loops. -- Macro: dolist (var list [result]) body... This construct executes BODY once for each element of LIST, binding the variable VAR locally to hold the current element. Then it returns the value of evaluating RESULT, or `nil' if RESULT is omitted. For example, here is how you could use `dolist' to define the `reverse' function: (defun reverse (list) (let (value) (dolist (elt list value) (setq value (cons elt value))))) -- Macro: dotimes (var count [result]) body... This construct executes BODY once for each integer from 0 (inclusive) to COUNT (exclusive), binding the variable VAR to the integer for the current iteration. Then it returns the value of evaluating RESULT, or `nil' if RESULT is omitted. Here is an example of using `dotimes' to do something 100 times: (dotimes (i 100) (insert "I will not obey absurd orders\n")) 10.5 Nonlocal Exits =================== A "nonlocal exit" is a transfer of control from one point in a program to another remote point. Nonlocal exits can occur in Emacs Lisp as a result of errors; you can also use them under explicit control. Nonlocal exits unbind all variable bindings made by the constructs being exited. 10.5.1 Explicit Nonlocal Exits: `catch' and `throw' --------------------------------------------------- Most control constructs affect only the flow of control within the construct itself. The function `throw' is the exception to this rule of normal program execution: it performs a nonlocal exit on request. (There are other exceptions, but they are for error handling only.) `throw' is used inside a `catch', and jumps back to that `catch'. For example: (defun foo-outer () (catch 'foo (foo-inner))) (defun foo-inner () ... (if x (throw 'foo t)) ...) The `throw' form, if executed, transfers control straight back to the corresponding `catch', which returns immediately. The code following the `throw' is not executed. The second argument of `throw' is used as the return value of the `catch'. The function `throw' finds the matching `catch' based on the first argument: it searches for a `catch' whose first argument is `eq' to the one specified in the `throw'. If there is more than one applicable `catch', the innermost one takes precedence. Thus, in the above example, the `throw' specifies `foo', and the `catch' in `foo-outer' specifies the same symbol, so that `catch' is the applicable one (assuming there is no other matching `catch' in between). Executing `throw' exits all Lisp constructs up to the matching `catch', including function calls. When binding constructs such as `let' or function calls are exited in this way, the bindings are unbound, just as they are when these constructs exit normally (*note Local Variables::). Likewise, `throw' restores the buffer and position saved by `save-excursion' (*note 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 (*note Cleanups::). The `throw' need not appear lexically within the `catch' that it jumps to. It can equally well be called from another function called within the `catch'. As long as the `throw' takes place chronologically after entry to the `catch', and chronologically before exit from it, it has access to that `catch'. This is why `throw' can be used in commands such as `exit-recursive-edit' that throw back to the editor command loop (*note Recursive Editing::). Common Lisp note: Most other versions of Lisp, including Common Lisp, have several ways of transferring control nonsequentially: `return', `return-from', and `go', for example. Emacs Lisp has only `throw'. The `cl-lib' library provides versions of some of these. *Note Blocks and Exits: (cl)Blocks and Exits. -- Special Form: catch tag body... `catch' establishes a return point for the `throw' function. The return point is distinguished from other such return points by TAG, which may be any Lisp object except `nil'. The argument TAG is evaluated normally before the return point is established. With the return point in effect, `catch' evaluates the forms of the BODY in textual order. If the forms execute normally (without error or nonlocal exit) the value of the last body form is returned from the `catch'. If a `throw' is executed during the execution of BODY, specifying the same value TAG, the `catch' form exits immediately; the value it returns is whatever was specified as the second argument of `throw'. -- Function: throw tag value The purpose of `throw' is to return from a return point previously established with `catch'. The argument TAG is used to choose among the various existing return points; it must be `eq' to the value specified in the `catch'. If multiple return points match TAG, the innermost one is used. The argument VALUE is used as the value to return from that `catch'. If no return point is in effect with tag TAG, then a `no-catch' error is signaled with data `(TAG VALUE)'. 10.5.2 Examples of `catch' and `throw' -------------------------------------- One way to use `catch' and `throw' is to exit from a doubly nested loop. (In most languages, this would be done with a "goto".) Here we compute `(foo I J)' for I and J varying from 0 to 9: (defun search-foo () (catch 'loop (let ((i 0)) (while (< i 10) (let ((j 0)) (while (< j 10) (if (foo i j) (throw 'loop (list i j))) (setq j (1+ j)))) (setq i (1+ i)))))) If `foo' ever returns non-`nil', we stop immediately and return a list of I and J. If `foo' always returns `nil', the `catch' returns normally, and the value is `nil', since that is the result of the `while'. Here are two tricky examples, slightly different, showing two return points at once. First, two return points with the same tag, `hack': (defun catch2 (tag) (catch tag (throw 'hack 'yes))) => catch2 (catch 'hack (print (catch2 'hack)) 'no) -| yes => no Since both return points have tags that match the `throw', it goes to the inner one, the one established in `catch2'. Therefore, `catch2' returns normally with value `yes', and this value is printed. Finally the second body form in the outer `catch', which is `'no', is evaluated and returned from the outer `catch'. Now let's change the argument given to `catch2': (catch 'hack (print (catch2 'quux)) 'no) => yes We still have two return points, but this time only the outer one has the tag `hack'; the inner one has the tag `quux' instead. Therefore, `throw' makes the outer `catch' return the value `yes'. The function `print' is never called, and the body-form `'no' is never evaluated. 10.5.3 Errors ------------- When Emacs Lisp attempts to evaluate a form that, for some reason, cannot be evaluated, it "signals" an "error". When an error is signaled, Emacs's default reaction is to print an error message and terminate execution of the current command. This is the right thing to do in most cases, such as if you type `C-f' at the end of the buffer. In complicated programs, simple termination may not be what you want. For example, the program may have made temporary changes in data structures, or created temporary buffers that should be deleted before the program is finished. In such cases, you would use `unwind-protect' to establish "cleanup expressions" to be evaluated in case of error. (*Note Cleanups::.) Occasionally, you may wish the program to continue execution despite an error in a subroutine. In these cases, you would use `condition-case' to establish "error handlers" to recover control in case of error. Resist the temptation to use error handling to transfer control from one part of the program to another; use `catch' and `throw' instead. *Note Catch and Throw::. 10.5.3.1 How to Signal an Error ............................... "Signaling" an error means beginning error processing. Error processing normally aborts all or part of the running program and returns to a point that is set up to handle the error (*note Processing of Errors::). Here we describe how to signal an error. Most errors are signaled "automatically" within Lisp primitives which you call for other purposes, such as if you try to take the CAR of an integer or move forward a character at the end of the buffer. You can also signal errors explicitly with the functions `error' and `signal'. Quitting, which happens when the user types `C-g', is not considered an error, but it is handled almost like an error. *Note 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' (*note 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 "You have committed %d errors" 10) error--> You have committed 10 errors `error' works by calling `signal' with two arguments: the error symbol `error', and a list containing the string returned by `format'. *Warning:* If you want to use your own string as an error message verbatim, don't just write `(error STRING)'. If STRING contains `%', it will be interpreted as a format specifier, 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 bearing a property `error-conditions' whose value is a list of condition names. This is how Emacs Lisp classifies different sorts of errors. *Note Error Symbols::, for a description of error symbols, error conditions and condition names. If the error is not handled, the two arguments are used in printing the error message. Normally, this error message is provided by the `error-message' property of ERROR-SYMBOL. If DATA is non-`nil', this is followed by a colon and a comma separated list of the unevaluated elements of DATA. For `error', the error message is the CAR of DATA (that must be a string). Subcategories of `file-error' are handled specially. The number and significance of the objects in DATA depends on ERROR-SYMBOL. For example, with a `wrong-type-argument' error, there should be two objects in the list: a predicate that describes the type that was expected, and the object that failed to fit that type. Both ERROR-SYMBOL and DATA are available to any error handlers that handle the error: `condition-case' binds a local variable to a list of the form `(ERROR-SYMBOL . DATA)' (*note Handling Errors::). The function `signal' never returns. (signal 'wrong-number-of-arguments '(x y)) error--> Wrong number of arguments: x, y (signal 'no-such-error '("My unknown error condition")) error--> peculiar error: "My unknown error condition" -- Function: user-error format-string &rest args This function behaves exactly like `error', except that it uses the error symbol `user-error' rather than `error'. As the name suggests, this is intended to report errors on the part of the user, rather than errors in the code itself. For example, if you try to use the command `Info-history-back' (`l') to move back beyond the start of your Info browsing history, Emacs signals a `user-error'. Such errors do not cause entry to the debugger, even when `debug-on-error' is non-`nil'. *Note Error Debugging::. Common Lisp note: Emacs Lisp has nothing like the Common Lisp concept of continuable errors. 10.5.3.2 How Emacs Processes Errors ................................... When an error is signaled, `signal' searches for an active "handler" for the error. A handler is a sequence of Lisp expressions designated to be executed if an error happens in part of the Lisp program. If the error has an applicable handler, the handler is executed, and control resumes following the handler. The handler executes in the environment of the `condition-case' that established it; all functions called within that `condition-case' have already been exited, and the handler cannot return to them. If there is no applicable handler for the error, it terminates the current command and returns control to the editor command loop. (The command loop has an implicit handler for all kinds of errors.) The command loop's handler uses the error symbol and associated data to print an error message. You can use the variable `command-error-function' to control how this is done: -- Variable: command-error-function This variable, if non-`nil', specifies a function to use to handle errors that return control to the Emacs command loop. The function should take three arguments: DATA, a list of the same form that `condition-case' would bind to its variable; CONTEXT, a string describing the situation in which the error occurred, or (more often) `nil'; and CALLER, the Lisp function which called the primitive that signaled the error. An error that has no explicit handler may call the Lisp debugger. The debugger is enabled if the variable `debug-on-error' (*note Error Debugging::) is non-`nil'. Unlike error handlers, the debugger runs in the environment of the error, so that you can examine values of variables precisely as they were at the time of the error. 10.5.3.3 Writing Code to Handle Errors ...................................... The usual effect of signaling an error is to terminate the command that is running and return immediately to the Emacs editor command loop. You can arrange to trap errors occurring in a part of your program by establishing an error handler, with the special form `condition-case'. A simple example looks like this: (condition-case nil (delete-file filename) (error nil)) This deletes the file named FILENAME, catching any error and returning `nil' if an error occurs. (You can use the macro `ignore-errors' for a simple case like this; see below.) The `condition-case' construct is often used to trap errors that are predictable, such as failure to open a file in a call to `insert-file-contents'. It is also used to trap errors that are totally unpredictable, such as when the program evaluates an expression read from the user. The second argument of `condition-case' is called the "protected form". (In the example above, the protected form is a call to `delete-file'.) The error handlers go into effect when this form begins execution and are deactivated when this form returns. They remain in effect for all the intervening time. In particular, they are in effect during the execution of functions called by this form, in their subroutines, and so on. This is a good thing, since, strictly speaking, errors can be signaled only by Lisp primitives (including `signal' and `error') called by the protected form, not by the protected form itself. The arguments after the protected form are handlers. Each handler lists one or more "condition names" (which are symbols) to specify which errors it will handle. The error symbol specified when an error is signaled also defines a list of condition names. A handler applies to an error if they have any condition names in common. In the example above, there is one handler, and it specifies one condition name, `error', which covers all errors. The search for an applicable handler checks all the established handlers starting with the most recently established one. Thus, if two nested `condition-case' forms offer to handle the same error, the inner of the two gets to handle it. If an error is handled by some `condition-case' form, this ordinarily prevents the debugger from being run, even if `debug-on-error' says this error should invoke the debugger. If you want to be able to debug errors that are caught by a `condition-case', set the variable `debug-on-signal' to a non-`nil' value. You can also specify that a particular handler should let the debugger run first, by writing `debug' among the conditions, like this: (condition-case nil (delete-file filename) ((debug error) nil)) The effect of `debug' here is only to prevent `condition-case' from suppressing the call to the debugger. Any given error will invoke the debugger only if `debug-on-error' and the other usual filtering mechanisms say it should. *Note Error Debugging::. -- Macro: condition-case-unless-debug var protected-form handlers... The macro `condition-case-unless-debug' provides another way to handle debugging of such forms. It behaves exactly like `condition-case', unless the variable `debug-on-error' is non-`nil', in which case it does not handle any errors at all. Once Emacs decides that a certain handler handles the error, it returns control to that handler. To do so, Emacs unbinds all variable bindings made by binding constructs that are being exited, and executes the cleanups of all `unwind-protect' forms that are being exited. Once control arrives at the handler, the body of the handler executes normally. After execution of the handler body, execution returns from the `condition-case' form. Because the protected form is exited completely before execution of the handler, the handler cannot resume execution at the point of the error, nor can it examine variable bindings that were made within the protected form. All it can do is clean up and proceed. Error signaling and handling have some resemblance to `throw' and `catch' (*note Catch and Throw::), but they are entirely separate facilities. An error cannot be caught by a `catch', and a `throw' cannot be handled by an error handler (though using `throw' when there is no suitable `catch' signals an error that can be handled). -- Special Form: condition-case var protected-form handlers... This special form establishes the error handlers HANDLERS around the execution of PROTECTED-FORM. If PROTECTED-FORM executes without error, the value it returns becomes the value of the `condition-case' form; in this case, the `condition-case' has no effect. The `condition-case' form makes a difference when an error occurs during PROTECTED-FORM. Each of the HANDLERS is a list of the form `(CONDITIONS BODY...)'. Here CONDITIONS is an error condition name to be handled, or a list of condition names (which can include `debug' to allow the debugger to run before the handler); BODY is one or more Lisp expressions to be executed when this handler handles an error. Here are examples of handlers: (error nil) (arith-error (message "Division by zero")) ((arith-error file-error) (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. The `error-conditions' property of this symbol is a list of condition names (*note Error Symbols::). Emacs searches all the active `condition-case' forms for a handler that specifies one or more of these condition names; the innermost matching `condition-case' handles the error. Within this `condition-case', the first applicable handler handles the error. After executing the body of the handler, the `condition-case' returns normally, using the value of the last form in the handler body as the overall value. The argument VAR is a variable. `condition-case' does not bind this variable when executing the PROTECTED-FORM, only when it handles an error. At that time, it binds VAR locally to an "error description", which is a list giving the particulars of the error. The error description has the form `(ERROR-SYMBOL . DATA)'. The handler can refer to this list to decide what to do. For example, if the error is for failure opening a file, the file name is the second element of DATA--the third element of the error description. If VAR is `nil', that means no variable is bound. Then the error symbol and associated data are not available to the handler. Sometimes it is necessary to re-throw a signal caught by `condition-case', for some outer-level handler to catch. Here's how to do that: (signal (car err) (cdr err)) where `err' is the error description variable, the first argument to `condition-case' whose error condition you want to re-throw. *Note 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. *Note 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 returns `nil'. Here's the example at the beginning of this subsection rewritten using `ignore-errors': (ignore-errors (delete-file filename)) -- Macro: with-demoted-errors body... This macro is like a milder version of `ignore-errors'. Rather than suppressing errors altogether, it converts them into messages. Use this form around code that is not expected to signal errors, but should be robust if one does occur. Note that this macro uses `condition-case-unless-debug' rather than `condition-case'. 10.5.3.4 Error Symbols and Condition Names .......................................... When you signal an error, you specify an "error symbol" to specify the kind of error you have in mind. Each error has one and only one error symbol to categorize it. This is the finest classification of errors defined by the Emacs Lisp language. These narrow classifications are grouped into a hierarchy of wider classes called "error conditions", identified by "condition names". The narrowest such classes belong to the error symbols themselves: each error symbol is also a condition name. There are also condition names for more extensive classes, up to the condition name `error' which takes in all kinds of errors (but not `quit'). Thus, each error has one or more condition names: `error', the error symbol if that is distinct from `error', and perhaps some intermediate classifications. In order for a symbol to be an error symbol, it must have an `error-conditions' property which gives a list of condition names. This list defines the conditions that this kind of error belongs to. (The error symbol itself, and the symbol `error', should always be members of this list.) Thus, the hierarchy of condition names is defined by the `error-conditions' properties of the error symbols. Because quitting is not considered an error, the value of the `error-conditions' property of `quit' is just `(quit)'. In addition to the `error-conditions' list, the error symbol should have an `error-message' property whose value is a string to be printed when that error is signaled but not handled. If the error symbol has no `error-message' property or if the `error-message' property exists, but is not a string, the error message `peculiar error' is used. *Note Definition of signal::. Here is how we define a new error symbol, `new-error': (put 'new-error 'error-conditions '(error my-own-errors new-error)) => (error my-own-errors new-error) (put 'new-error 'error-message "A new error") => "A new error" This error has three condition names: `new-error', the narrowest classification; `my-own-errors', which we imagine is a wider classification; and `error', which is the widest of all. The error string should start with a capital letter but it should not end with a period. This is for consistency with the rest of Emacs. Naturally, Emacs will never signal `new-error' on its own; only an explicit call to `signal' (*note Definition of signal::) in your code can do this: (signal 'new-error '(x y)) error--> A new error: x, y This error can be handled through any of the three condition names. This example handles `new-error' and any other errors in the class `my-own-errors': (condition-case foo (bar nil t) (my-own-errors nil)) The significant way that errors are classified is by their condition names--the names used to match errors with handlers. An error symbol serves only as a convenient way to specify the intended error message and list of condition names. It would be cumbersome to give `signal' a list of condition names rather than one error symbol. By contrast, using only error symbols without condition names would seriously decrease the power of `condition-case'. Condition names make it possible to categorize errors at various levels of generality when you write an error handler. Using error symbols alone would eliminate all but the narrowest level of classification. *Note Standard Errors::, for a list of the main error symbols and their conditions. 10.5.4 Cleaning Up from Nonlocal Exits -------------------------------------- The `unwind-protect' construct is essential whenever you temporarily put a data structure in an inconsistent state; it permits you to make the data consistent again in the event of an error or throw. (Another more specific cleanup construct that is used only for changes in buffer contents is the atomic change group; *note Atomic Changes::.) -- Special Form: unwind-protect body-form cleanup-forms... `unwind-protect' executes BODY-FORM with a guarantee that the CLEANUP-FORMS will be evaluated if control leaves BODY-FORM, no matter how that happens. BODY-FORM may complete normally, or execute a `throw' out of the `unwind-protect', or cause an error; in all cases, the CLEANUP-FORMS will be evaluated. If BODY-FORM finishes normally, `unwind-protect' returns the value of BODY-FORM, after it evaluates the CLEANUP-FORMS. If BODY-FORM does not finish, `unwind-protect' does not return any value in the normal sense. Only BODY-FORM is protected by the `unwind-protect'. If any of the CLEANUP-FORMS themselves exits nonlocally (via a `throw' or an error), `unwind-protect' is _not_ guaranteed to evaluate the rest of them. If the failure of one of the CLEANUP-FORMS has the potential to cause trouble, then protect it with another `unwind-protect' around that form. The number of currently active `unwind-protect' forms counts, together with the number of local variable bindings, against the limit `max-specpdl-size' (*note Local Variables: Definition of max-specpdl-size.). 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 (*note Current Buffer: Definition of with-temp-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 (*note Processes::) to try to establish a connection to a remote machine. As the function `ftp-login' is highly susceptible to numerous problems that the writer of the function cannot anticipate, it is protected with a form that guarantees deletion of the process in the event of failure. Otherwise, Emacs might fill up with useless subprocesses. (let ((win nil)) (unwind-protect (progn (setq process (ftp-setup-buffer host file)) (if (setq win (ftp-login process host user password)) (message "Logged in") (error "Ftp login failed"))) (or win (and process (delete-process process))))) This example has a small bug: if the user types `C-g' to quit, and the quit happens immediately after the function `ftp-setup-buffer' returns but before the variable `process' is set, the process will not be killed. There is no easy way to fix this bug, but at least it is very unlikely. 11 Variables ************ A "variable" is a name used in a program to stand for a value. In Lisp, each variable is represented by a Lisp symbol (*note Symbols::). The variable name is simply the symbol's name, and the variable's value is stored in the symbol's value cell(1). *Note 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. ---------- Footnotes ---------- (1) To be precise, under the default "dynamic binding" rules the value cell always holds the variable's current value, but this is not the case under "lexical binding" rules. *Note Variable Scoping::, for details. 11.1 Global Variables ===================== The simplest way to use a variable is "globally". This means that the variable has just one value at a time, and this value is in effect (at least for the moment) throughout the Lisp system. The value remains in effect until you specify a new one. When a new value replaces the old one, no trace of the old value remains in the variable. You specify a value for a symbol with `setq'. For example, (setq x '(a b)) gives the variable `x' the value `(a b)'. Note that `setq' is a special form (*note Special Forms::); it does not evaluate its first argument, the name of the variable, but it does evaluate the second argument, the new value. Once the variable has a value, you can refer to it by using the symbol itself as an expression. Thus, x => (a b) assuming the `setq' form shown above has already been executed. If you do set the same variable again, the new value replaces the old one: x => (a b) (setq x 4) => 4 x => 4 11.2 Variables that Never Change ================================ In Emacs Lisp, certain symbols normally evaluate to themselves. These include `nil' and `t', as well as any symbol whose name starts with `:' (these are called "keywords"). These symbols cannot be rebound, nor can their values be changed. Any attempt to set or bind `nil' or `t' signals a `setting-constant' error. The same is true for a keyword (a symbol whose name starts with `:'), if it is interned in the standard obarray, except that setting such a symbol to itself is not an error. nil == 'nil => nil (setq nil 500) error--> Attempt to set constant symbol: nil -- Function: keywordp object function returns `t' if OBJECT is a symbol whose name starts with `:', interned in the standard obarray, and returns `nil' otherwise. These constants are fundamentally different from the "constants" defined using the `defconst' special form (*note Defining Variables::). A `defconst' form serves to inform human readers that you do not intend to change the value of a variable, but Emacs does not raise an error if you actually change it. 11.3 Local Variables ==================== Global variables have values that last until explicitly superseded with new values. Sometimes it is useful to give a variable a "local value"--a value that takes effect only within a certain part of a Lisp program. When a variable has a local value, we say that it is "locally bound" to that value, and that it is a "local variable". For example, when a function is called, its argument variables receive local values, which are the actual arguments supplied to the function call; these local bindings take effect within the body of the function. To take another example, the `let' special form explicitly establishes local bindings for specific variables, which take effect within the body of the `let' form. We also speak of the "global binding", which is where (conceptually) the global value is kept. Establishing a local binding saves away the variable's previous value (or lack of one). We say that the previous value is "shadowed". Both global and local values may be shadowed. If a local binding is in effect, using `setq' on the local variable stores the specified value in the local binding. When that local binding is no longer in effect, the previously shadowed value (or lack of one) comes back. A variable can have more than one local binding at a time (e.g., if there are nested `let' forms that bind the variable). The "current binding" is the local binding that is actually in effect. It determines the value returned by evaluating the variable symbol, and it is the binding acted on by `setq'. For most purposes, you can think of the current binding as the "innermost" local binding, or the global binding if there is no local binding. To be more precise, a rule called the "scoping rule" determines where in a program a local binding takes effect. The default scoping rule in Emacs Lisp is called "dynamic scoping", which simply states that the current binding at any given point in the execution of a program is the most recently-created binding for that variable that still exists. For details about dynamic scoping, and an alternative scoping rule called "lexical scoping", *Note Variable Scoping::. The special forms `let' and `let*' exist to create local bindings: -- Special Form: let (bindings...) forms... This special form sets up local bindings for a certain set of variables, as specified by BINDINGS, and then evaluates all of the FORMS in textual order. Its return value is the value of the last form in FORMS. Each of the BINDINGS is either (i) a symbol, in which case that symbol is locally bound to `nil'; or (ii) a list of the form `(SYMBOL VALUE-FORM)', in which case SYMBOL is locally bound to the result of evaluating VALUE-FORM. If VALUE-FORM is omitted, `nil' is used. All of the VALUE-FORMs in BINDINGS are evaluated in the order they appear and _before_ binding any of the symbols to them. Here is an example of this: `z' is bound to the old value of `y', which is 2, not the new value of `y', which is 1. (setq y 2) => 2 (let ((y 1) (z y)) (list y z)) => (1 2) -- Special Form: let* (bindings...) forms... This special form is like `let', but it binds each variable right after computing its local value, before computing the local value for the next variable. Therefore, an expression in BINDINGS can refer to the preceding symbols bound in this `let*' form. Compare the following example with the example above for `let'. (setq y 2) => 2 (let* ((y 1) (z y)) ; Use the just-established value of `y'. (list y z)) => (1 1) Here is a complete list of the other facilities that create local bindings: * Function calls (*note Functions::). * Macro calls (*note Macros::). * `condition-case' (*note Errors::). Variables can also have buffer-local bindings (*note Buffer-Local Variables::); a few variables have terminal-local bindings (*note Multiple Terminals::). These kinds of bindings work somewhat like ordinary local bindings, but they are localized depending on "where" you are in Emacs. -- User Option: max-specpdl-size This variable defines the limit on the total number of local variable bindings and `unwind-protect' cleanups (see *note Cleaning Up from Nonlocal Exits: Cleanups.) that are allowed before Emacs signals an error (with data `"Variable binding depth exceeds max-specpdl-size"'). This limit, with the associated error when it is exceeded, is one way that Lisp avoids infinite recursion on an ill-defined function. `max-lisp-eval-depth' provides another limit on depth of nesting. *Note Eval: Definition of max-lisp-eval-depth. The default value is 1300. Entry to the Lisp debugger increases the value, if there is little room left, to make sure the debugger itself has room to execute. 11.4 When a Variable is "Void" ============================== We say that a variable is void if its symbol has an unassigned value cell (*note Symbol Components::). Under Emacs Lisp's default dynamic binding rules (*note Variable Scoping::), the value cell stores the variable's current (local or global) value. Note that an unassigned value cell is _not_ the same as having `nil' in the value cell. The symbol `nil' is a Lisp object and can be the value of a variable, just as any other object can be; but it is still a value. If a variable is void, trying to evaluate the variable signals a `void-variable' error rather than a value. Under lexical binding rules, the value cell only holds the variable's global value, i.e., the value outside of any lexical binding construct. When a variable is lexically bound, the local value is determined by the lexical environment; the variable may have a local value if its symbol's value cell is unassigned. -- Function: makunbound symbol This function empties out the value cell of SYMBOL, making the variable void. It returns SYMBOL. If SYMBOL has a dynamic local binding, `makunbound' voids the current binding, and this voidness lasts only as long as the local binding is in effect. Afterwards, the previously shadowed local or global binding is reexposed; then the variable will no longer be void, unless the reexposed binding is void too. Here are some examples (assuming dynamic binding is in effect): (setq x 1) ; Put a value in the global binding. => 1 (let ((x 2)) ; Locally bind it. (makunbound 'x) ; Void the local binding. x) error--> Symbol's value as variable is void: x x ; The global binding is unchanged. => 1 (let ((x 2)) ; Locally bind it. (let ((x 3)) ; And again. (makunbound 'x) ; Void the innermost-local binding. x)) ; And refer: it's void. error--> Symbol's value as variable is void: x (let ((x 2)) (let ((x 3)) (makunbound 'x)) ; Void inner binding, then remove it. x) ; Now outer `let' binding is visible. => 2 -- Function: boundp variable This function returns `t' if VARIABLE (a symbol) is not void, and `nil' if it is void. Here are some examples (assuming dynamic binding is in effect): (boundp 'abracadabra) ; Starts out void. => nil (let ((abracadabra 5)) ; Locally bind it. (boundp 'abracadabra)) => t (boundp 'abracadabra) ; Still globally void. => nil (setq abracadabra 5) ; Make it globally nonvoid. => 5 (boundp 'abracadabra) => t 11.5 Defining Global Variables ============================== A "variable definition" is a construct that announces your intention to use a symbol as a global variable. It uses the special forms `defvar' or `defconst', which are documented below. A variable definition serves three purposes. First, it informs people who read the code that the symbol is _intended_ to be used a certain way (as a variable). Second, it informs the Lisp system of this, optionally supplying an initial value and a documentation string. Third, it provides information to programming tools such as `etags', allowing them to find where the variable was defined. The difference between `defconst' and `defvar' is mainly a matter of intent, serving to inform human readers of whether the value should ever change. Emacs Lisp does not actually prevent you from changing the value of a variable defined with `defconst'. One notable difference between the two forms is that `defconst' unconditionally initializes the variable, whereas `defvar' initializes it only if it is originally void. To define a customizable variable, you should use `defcustom' (which calls `defvar' as a subroutine). *Note Variable Definitions::. -- Special Form: defvar symbol [value [doc-string]] This special form defines SYMBOL as a variable. Note that SYMBOL is not evaluated; the symbol to be defined should appear explicitly in the `defvar' form. The variable is marked as "special", meaning that it should always be dynamically bound (*note Variable Scoping::). If SYMBOL is void and VALUE is specified, `defvar' evaluates VALUE and sets SYMBOL to the result. But if SYMBOL already has a value (i.e., it is not void), VALUE is not even evaluated, and SYMBOL's value remains unchanged. If VALUE is omitted, the value of SYMBOL is not changed in any case. If SYMBOL has a buffer-local binding in the current buffer, `defvar' operates on the default value, which is buffer-independent, not the current (buffer-local) binding. It sets the default value if the default value is void. *Note Buffer-Local Variables::. When you evaluate a top-level `defvar' form with `C-M-x' in Emacs Lisp mode (`eval-defun'), a special feature of `eval-defun' arranges to set the variable unconditionally, without testing whether its value is void. If the DOC-STRING argument is supplied, it specifies the documentation string for the variable (stored in the symbol's `variable-documentation' property). *Note Documentation::. Here are some examples. This form defines `foo' but does not initialize it: (defvar foo) => foo This example initializes the value of `bar' to `23', and gives it a documentation string: (defvar bar 23 "The normal weight of a bar.") => bar The `defvar' form returns SYMBOL, but it is normally used at top level in a file where its value does not matter. -- Special Form: defconst symbol value [doc-string] This special form defines SYMBOL as a value and initializes it. It informs a person reading your code that SYMBOL has a standard global value, established here, that should not be changed by the user or by other programs. Note that SYMBOL is not evaluated; the symbol to be defined must appear explicitly in the `defconst'. The `defconst' form, like `defvar', marks the variable as "special", meaning that it should always be dynamically bound (*note Variable Scoping::). In addition, it marks the variable as risky (*note File Local Variables::). `defconst' always evaluates VALUE, and sets the value of SYMBOL to the result. If SYMBOL does have a buffer-local binding in the current buffer, `defconst' sets the default value, not the buffer-local value. (But you should not be making buffer-local bindings for a symbol that is defined with `defconst'.) An example of the use of `defconst' is Emacs's definition of `float-pi'--the mathematical constant pi, which ought not to be changed by anyone (attempts by the Indiana State Legislature notwithstanding). As the second form illustrates, however, `defconst' is only advisory. (defconst float-pi 3.141592653589793 "The value of Pi.") => float-pi (setq float-pi 3) => float-pi float-pi => 3 *Warning:* If you use a `defconst' or `defvar' special form while the variable has a local binding (made with `let', or a function argument), it sets the local binding rather than the global binding. This is not what you usually want. To prevent this, use these special forms at top level in a file, where normally no local binding is in effect, and make sure to load the file before making a local binding for the variable. 11.6 Tips for Defining Variables Robustly ========================================= When you define a variable whose value is a function, or a list of functions, use a name that ends in `-function' or `-functions', respectively. There are several other variable name conventions; here is a complete list: `...-hook' The variable is a normal hook (*note Hooks::). `...-function' The value is a function. `...-functions' The value is a list of functions. `...-form' The value is a form (an expression). `...-forms' The value is a list of forms (expressions). `...-predicate' The value is a predicate--a function of one argument that returns non-`nil' for "good" arguments and `nil' for "bad" arguments. `...-flag' The value is significant only as to whether it is `nil' or not. Since such variables often end up acquiring more values over time, this convention is not strongly recommended. `...-program' The value is a program name. `...-command' The value is a whole shell command. `...-switches' The value specifies options for a command. When you define a variable, always consider whether you should mark it as "safe" or "risky"; see *note File Local Variables::. When defining and initializing a variable that holds a complicated value (such as a keymap with bindings in it), it's best to put the entire computation of the value into the `defvar', like this: (defvar my-mode-map (let ((map (make-sparse-keymap))) (define-key map "\C-c\C-a" 'my-command) ... map) DOCSTRING) This method has several benefits. First, if the user quits while loading the file, the variable is either still uninitialized or initialized properly, never in-between. If it is still uninitialized, reloading the file will initialize it properly. Second, reloading the file once the variable is initialized will not alter it; that is important if the user has run hooks to alter part of the contents (such as, to rebind keys). Third, evaluating the `defvar' form with `C-M-x' will reinitialize the map completely. Putting so much code in the `defvar' form has one disadvantage: it puts the documentation string far away from the line which names the variable. Here's a safe way to avoid that: (defvar my-mode-map nil DOCSTRING) (unless my-mode-map (let ((map (make-sparse-keymap))) (define-key map "\C-c\C-a" 'my-command) ... (setq my-mode-map map))) This has all the same advantages as putting the initialization inside the `defvar', except that you must type `C-M-x' twice, once on each form, if you do want to reinitialize the variable. 11.7 Accessing Variable Values ============================== The usual way to reference a variable is to write the symbol which names it. *Note Symbol Forms::. Occasionally, you may want to reference a variable which is only determined at run time. In that case, you cannot specify the variable name in the text of the program. You can use the `symbol-value' function to extract the value. -- Function: symbol-value symbol This function returns the value stored in SYMBOL's value cell. This is where the variable's current (dynamic) value is stored. If the variable has no local binding, this is simply its global value. If the variable is void, a `void-variable' error is signaled. If the variable is lexically bound, the value reported by `symbol-value' is not necessarily the same as the variable's lexical value, which is determined by the lexical environment rather than the symbol's value cell. *Note Variable Scoping::. (setq abracadabra 5) => 5 (setq foo 9) => 9 ;; Here the symbol `abracadabra' ;; is the symbol whose value is examined. (let ((abracadabra 'foo)) (symbol-value 'abracadabra)) => foo ;; Here, the value of `abracadabra', ;; which is `foo', ;; is the symbol whose value is examined. (let ((abracadabra 'foo)) (symbol-value abracadabra)) => 9 (symbol-value 'abracadabra) => 5 11.8 Setting Variable Values ============================ The usual way to change the value of a variable is with the special form `setq'. When you need to compute the choice of variable at run time, use the function `set'. -- Special Form: setq [symbol form]... This special form is the most common method of changing a variable's value. Each SYMBOL is given a new value, which is the result of evaluating the corresponding FORM. The current binding of the symbol is changed. `setq' does not evaluate SYMBOL; it sets the symbol that you write. We say that this argument is "automatically quoted". The `q' in `setq' stands for "quoted". The value of the `setq' form is the value of the last FORM. (setq x (1+ 2)) => 3 x ; `x' now has a global value. => 3 (let ((x 5)) (setq x 6) ; The local binding of `x' is set. x) => 6 x ; The global value is unchanged. => 3 Note that the first FORM is evaluated, then the first SYMBOL is set, then the second FORM is evaluated, then the second SYMBOL is set, and so on: (setq x 10 ; Notice that `x' is set before y (1+ x)) ; the value of `y' is computed. => 11 -- Function: set symbol value This function puts VALUE in the value cell of SYMBOL. Since it is a function rather than a special form, the expression written for SYMBOL is evaluated to obtain the symbol to set. The return value is VALUE. When dynamic variable binding is in effect (the default), `set' has the same effect as `setq', apart from the fact that `set' evaluates its SYMBOL argument whereas `setq' does not. But when a variable is lexically bound, `set' affects its _dynamic_ value, whereas `setq' affects its current (lexical) value. *Note Variable Scoping::. (set one 1) error--> Symbol's value as variable is void: one (set 'one 1) => 1 (set 'two 'one) => one (set two 2) ; `two' evaluates to symbol `one'. => 2 one ; So it is `one' that was set. => 2 (let ((one 1)) ; This binding of `one' is set, (set 'one 3) ; not the global value. one) => 3 one => 2 If SYMBOL is not actually a symbol, a `wrong-type-argument' error is signaled. (set '(x y) 'z) error--> Wrong type argument: symbolp, (x y) 11.9 Scoping Rules for Variable Bindings ======================================== When you create a local binding for a variable, that binding takes effect only within a limited portion of the program (*note Local Variables::). This section describes exactly what this means. Each local binding has a certain "scope" and "extent". "Scope" refers to _where_ in the textual source code the binding can be accessed. "Extent" refers to _when_, as the program is executing, the binding exists. By default, the local bindings that Emacs creates are "dynamic bindings". Such a binding has "indefinite scope", meaning that any part of the program can potentially access the variable binding. It also has "dynamic extent", meaning that the binding lasts only while the binding construct (such as the body of a `let' form) is being executed. Emacs can optionally create "lexical bindings". A lexical binding has "lexical scope", meaning that any reference to the variable must be located textually within the binding construct. It also has "indefinite extent", meaning that under some circumstances the binding can live on even after the binding construct has finished executing, by means of special objects called "closures". The following subsections describe dynamic binding and lexical binding in greater detail, and how to enable lexical binding in Emacs Lisp programs. 11.9.1 Dynamic Binding ---------------------- By default, the local variable bindings made by Emacs are dynamic bindings. When a variable is dynamically bound, its current binding at any point in the execution of the Lisp program is simply the most recently-created dynamic local binding for that symbol, or the global binding if there is no such local binding. Dynamic bindings have indefinite scope and dynamic extent, as shown by the following example: (defvar x -99) ; `x' receives an initial value of -99. (defun getx () x) ; `x' is used "free" in this function. (let ((x 1)) ; `x' is dynamically bound. (getx)) => 1 ;; After the `let' form finishes, `x' reverts to its ;; previous value, which is -99. (getx) => -99 The function `getx' refers to `x'. This is a "free" reference, in the sense that there is no binding for `x' within that `defun' construct itself. When we call `getx' from within a `let' form in which `x' is (dynamically) bound, it retrieves the local value of `x' (i.e., 1). But when we call `getx' outside the `let' form, it retrieves the global value of `x' (i.e., -99). Here is another example, which illustrates setting a dynamically bound variable using `setq': (defvar x -99) ; `x' receives an initial value of -99. (defun addx () (setq x (1+ x))) ; Add 1 to `x' and return its new value. (let ((x 1)) (addx) (addx)) => 3 ; The two `addx' calls add to `x' twice. ;; After the `let' form finishes, `x' reverts to its ;; previous value, which is -99. (addx) => -98 Dynamic binding is implemented in Emacs Lisp in a simple way. Each symbol has a value cell, which specifies its current dynamic value (or absence of value). *Note Symbol Components::. When a symbol is given a dynamic local binding, Emacs records the contents of the value cell (or absence thereof) in a stack, and stores the new local value in the value cell. When the binding construct finishes executing, Emacs pops the old value off the stack, and puts it in the value cell. 11.9.2 Proper Use of Dynamic Binding ------------------------------------ Dynamic binding is a powerful feature, as it allows programs to refer to variables that are not defined within their local textual scope. However, if used without restraint, this can also make programs hard to understand. There are two clean ways to use this technique: * If a variable has no global definition, use it as a local variable only within a binding construct, e.g., the body of the `let' form where the variable was bound, or the body of the function for an argument variable. If this convention is followed consistently throughout a program, the value of the variable will not affect, nor be affected by, any uses of the same variable symbol elsewhere in the program. * Otherwise, define the variable with `defvar', `defconst', or `defcustom'. *Note Defining Variables::. Usually, the definition should be at top-level in an Emacs Lisp file. As far as possible, it should include a documentation string which explains the meaning and purpose of the variable. You should also choose the variable's name to avoid name conflicts (*note Coding Conventions::). Then you can bind the variable anywhere in a program, knowing reliably what the effect will be. Wherever you encounter the variable, it will be easy to refer back to the definition, e.g., via the `C-h v' command (provided the variable definition has been loaded into Emacs). *Note Name Help: (emacs)Name Help. For example, it is common to use local bindings for customizable variables like `case-fold-search': (defun search-for-abc () "Search for the string \"abc\", ignoring case differences." (let ((case-fold-search nil)) (re-search-forward "abc"))) 11.9.3 Lexical Binding ---------------------- Optionally, you can create lexical bindings in Emacs Lisp. A lexically bound variable has "lexical scope", meaning that any reference to the variable must be located textually within the binding construct. Here is an example (*note Using Lexical Binding::, for how to actually enable lexical binding): (let ((x 1)) ; `x' is lexically bound. (+ x 3)) => 4 (defun getx () x) ; `x' is used "free" in this function. (let ((x 1)) ; `x' is lexically bound. (getx)) error--> Symbol's value as variable is void: x Here, the variable `x' has no global value. When it is lexically bound within a `let' form, it can be used in the textual confines of that `let' form. But it can _not_ be used from within a `getx' function called from the `let' form, since the function definition of `getx' occurs outside the `let' form itself. Here is how lexical binding works. Each binding construct defines a "lexical environment", specifying the symbols that are bound within the construct and their local values. When the Lisp evaluator wants the current value of a variable, it looks first in the lexical environment; if the variable is not specified in there, it looks in the symbol's value cell, where the dynamic value is stored. Lexical bindings have indefinite extent. Even after a binding construct has finished executing, its lexical environment can be "kept around" in Lisp objects called "closures". A closure is created when you define a named or anonymous function with lexical binding enabled. *Note Closures::, for details. When a closure is called as a function, any lexical variable references within its definition use the retained lexical environment. Here is an example: (defvar my-ticker nil) ; We will use this dynamically bound ; variable to store a closure. (let ((x 0)) ; `x' is lexically bound. (setq my-ticker (lambda () (setq x (1+ x))))) => (closure ((x . 0) t) () (1+ x)) (funcall my-ticker) => 1 (funcall my-ticker) => 2 (funcall my-ticker) => 3 x ; Note that `x' has no global value. error--> Symbol's value as variable is void: x The `let' binding defines a lexical environment in which the variable `x' is locally bound to 0. Within this binding construct, we define a lambda expression which increments `x' by one and returns the incremented value. This lambda expression is automatically turned into a closure, in which the lexical environment lives on even after the `let' binding construct has exited. Each time we evaluate the closure, it increments `x', using the binding of `x' in that lexical environment. Note that functions like `symbol-value', `boundp', and `set' only retrieve or modify a variable's dynamic binding (i.e., the contents of its symbol's value cell). Also, the code in the body of a `defun' or `defmacro' cannot refer to surrounding lexical variables. Currently, lexical binding is not much used within the Emacs sources. However, we expect its importance to increase in the future. Lexical binding opens up a lot more opportunities for optimization, so Emacs Lisp code that makes use of lexical binding is likely to run faster in future Emacs versions. Such code is also much more friendly to concurrency, which we want to add to Emacs in the near future. 11.9.4 Using Lexical Binding ---------------------------- When loading an Emacs Lisp file or evaluating a Lisp buffer, lexical binding is enabled if the buffer-local variable `lexical-binding' is non-`nil': -- Variable: lexical-binding If this buffer-local variable is non-`nil', Emacs Lisp files and buffers are evaluated using lexical binding instead of dynamic binding. (However, special variables are still dynamically bound; see below.) If `nil', dynamic binding is used for all local variables. This variable is typically set for a whole Emacs Lisp file, as a file local variable (*note File Local Variables::). Note that unlike other such variables, this one must be set in the first line of a file. When evaluating Emacs Lisp code directly using an `eval' call, lexical binding is enabled if the LEXICAL argument to `eval' is non-`nil'. *Note Eval::. Even when lexical binding is enabled, certain variables will continue to be dynamically bound. These are called "special variables". Every variable that has been defined with `defvar', `defcustom' or `defconst' is a special variable (*note Defining Variables::). All other variables are subject to lexical binding. -- Function: special-variable-p symbol This function returns non-`nil' if SYMBOL is a special variable (i.e., it has a `defvar', `defcustom', or `defconst' variable definition). Otherwise, the return value is `nil'. The use of a special variable as a formal argument in a function is discouraged. Doing so gives rise to unspecified behavior when lexical binding mode is enabled (it may use lexical binding sometimes, and dynamic binding other times). Converting an Emacs Lisp program to lexical binding is pretty easy. First, add a file-local variable setting of `lexical-binding' to `t' in the Emacs Lisp source file. Second, check that every variable in the program which needs to be dynamically bound has a variable definition, so that it is not inadvertently bound lexically. A simple way to find out which variables need a variable definition is to byte-compile the source file. *Note Byte Compilation::. If a non-special variable is used outside of a `let' form, the byte-compiler will warn about reference or assignment to a "free variable". If a non-special variable is bound but not used within a `let' form, the byte-compiler will warn about an "unused lexical variable". The byte-compiler will also issue a warning if you use a special variable as a function argument. (To silence byte-compiler warnings about unused variables, just use a variable name that start with an underscore. The byte-compiler interprets this as an indication that this is a variable known not to be used.) 11.10 Buffer-Local Variables ============================ Global and local variable bindings are found in most programming languages in one form or another. Emacs, however, also supports additional, unusual kinds of variable binding, such as "buffer-local" bindings, which apply only in one buffer. Having different values for a variable in different buffers is an important customization method. (Variables can also have bindings that are local to each terminal. *Note Multiple Terminals::.) 11.10.1 Introduction to Buffer-Local Variables ---------------------------------------------- A buffer-local variable has a buffer-local binding associated with a particular buffer. The binding is in effect when that buffer is current; otherwise, it is not in effect. If you set the variable while a buffer-local binding is in effect, the new value goes in that binding, so its other bindings are unchanged. This means that the change is visible only in the buffer where you made it. The variable's ordinary binding, which is not associated with any specific buffer, is called the "default binding". In most cases, this is the global binding. A variable can have buffer-local bindings in some buffers but not in other buffers. The default binding is shared by all the buffers that don't have their own bindings for the variable. (This includes all newly-created buffers.) If you set the variable in a buffer that does not have a buffer-local binding for it, this sets the default binding, so the new value is visible in all the buffers that see the default binding. The most common use of buffer-local bindings is for major modes to change variables that control the behavior of commands. For example, C mode and Lisp mode both set the variable `paragraph-start' to specify that only blank lines separate paragraphs. They do this by making the variable buffer-local in the buffer that is being put into C mode or Lisp mode, and then setting it to the new value for that mode. *Note Major Modes::. The usual way to make a buffer-local binding is with `make-local-variable', which is what major mode commands typically use. This affects just the current buffer; all other buffers (including those yet to be created) will continue to share the default value unless they are explicitly given their own buffer-local bindings. A more powerful operation is to mark the variable as "automatically buffer-local" by calling `make-variable-buffer-local'. You can think of this as making the variable local in all buffers, even those yet to be created. More precisely, the effect is that setting the variable automatically makes the variable local to the current buffer if it is not already so. All buffers start out by sharing the default value of the variable as usual, but setting the variable creates a buffer-local binding for the current buffer. The new value is stored in the buffer-local binding, leaving the default binding untouched. This means that the default value cannot be changed with `setq' in any buffer; the only way to change it is with `setq-default'. *Warning:* When a variable has buffer-local bindings in one or more buffers, `let' rebinds the binding that's currently in effect. For instance, if the current buffer has a buffer-local value, `let' temporarily rebinds that. If no buffer-local bindings are in effect, `let' rebinds the default value. If inside the `let' you then change to a different current buffer in which a different binding is in effect, you won't see the `let' binding any more. And if you exit the `let' while still in the other buffer, you won't see the unbinding occur (though it will occur properly). Here is an example to illustrate: (setq foo 'g) (set-buffer "a") (make-local-variable 'foo) (setq foo 'a) (let ((foo 'temp)) ;; foo => 'temp ; let binding in buffer `a' (set-buffer "b") ;; foo => 'g ; the global value since foo is not local in `b' BODY...) foo => 'g ; exiting restored the local value in buffer `a', ; but we don't see that in buffer `b' (set-buffer "a") ; verify the local value was restored foo => 'a Note that references to `foo' in BODY access the buffer-local binding of buffer `b'. When a file specifies local variable values, these become buffer-local values when you visit the file. *Note File Variables: (emacs)File Variables. A buffer-local variable cannot be made terminal-local (*note Multiple Terminals::). 11.10.2 Creating and Deleting Buffer-Local Bindings --------------------------------------------------- -- Command: make-local-variable variable This function creates a buffer-local binding in the current buffer for VARIABLE (a symbol). Other buffers are not affected. The value returned is VARIABLE. The buffer-local value of VARIABLE starts out as the same value VARIABLE previously had. If VARIABLE was void, it remains void. ;; In buffer `b1': (setq foo 5) ; Affects all buffers. => 5 (make-local-variable 'foo) ; Now it is local in `b1'. => foo foo ; That did not change => 5 ; the value. (setq foo 6) ; Change the value => 6 ; in `b1'. foo => 6 ;; In buffer `b2', the value hasn't changed. (with-current-buffer "b2" foo) => 5 Making a variable buffer-local within a `let'-binding for that variable does not work reliably, unless the buffer in which you do this is not current either on entry to or exit from the `let'. This is because `let' does not distinguish between different kinds of bindings; it knows only which variable the binding was made for. If the variable is terminal-local (*note Multiple Terminals::), this function signals an error. Such variables cannot have buffer-local bindings as well. *Warning:* do not use `make-local-variable' for a hook variable. The hook variables are automatically made buffer-local as needed if you use the LOCAL argument to `add-hook' or `remove-hook'. -- Macro: setq-local variable value This macro creates a buffer-local binding in the current buffer for VARIABLE, and gives it the buffer-local value VALUE. It is equivalent to calling `make-local-variable' followed by `setq'. VARIABLE should be an unquoted symbol. -- Command: make-variable-buffer-local variable This function marks VARIABLE (a symbol) automatically buffer-local, so that any subsequent attempt to set it will make it local to the current buffer at the time. Unlike `make-local-variable', with which it is often confused, this cannot be undone, and affects the behavior of the variable in all buffers. A peculiar wrinkle of this feature is that binding the variable (with `let' or other binding constructs) does not create a buffer-local binding for it. Only setting the variable (with `set' or `setq'), while the variable does not have a `let'-style binding that was made in the current buffer, does so. If VARIABLE does not have a default value, then calling this command will give it a default value of `nil'. If VARIABLE already has a default value, that value remains unchanged. Subsequently calling `makunbound' on VARIABLE will result in a void buffer-local value and leave the default value unaffected. The value returned is VARIABLE. *Warning:* Don't assume that you should use `make-variable-buffer-local' for user-option variables, simply because users _might_ want to customize them differently in different buffers. Users can make any variable local, when they wish to. It is better to leave the choice to them. The time to use `make-variable-buffer-local' is when it is crucial that no two buffers ever share the same binding. For example, when a variable is used for internal purposes in a Lisp program which depends on having separate values in separate buffers, then using `make-variable-buffer-local' can be the best solution. -- Macro: defvar-local variable value &optional docstring This macro defines VARIABLE as a variable with initial value VALUE and DOCSTRING, and marks it as automatically buffer-local. It is equivalent to calling `defvar' followed by `make-variable-buffer-local'. VARIABLE should be an unquoted symbol. -- Function: local-variable-p variable &optional buffer This returns `t' if VARIABLE is buffer-local in buffer BUFFER (which defaults to the current buffer); otherwise, `nil'. -- Function: local-variable-if-set-p variable &optional buffer This returns `t' if VARIABLE either has a buffer-local value in buffer BUFFER, or is automatically buffer-local. Otherwise, it returns `nil'. If omitted or `nil', BUFFER defaults to the current buffer. -- Function: buffer-local-value variable buffer This function returns the buffer-local binding of VARIABLE (a symbol) in buffer BUFFER. If VARIABLE does not have a buffer-local binding in buffer BUFFER, it returns the default value (*note Default Value::) of VARIABLE instead. -- Function: buffer-local-variables &optional buffer This function returns a list describing the buffer-local variables in buffer BUFFER. (If BUFFER is omitted, the current buffer is used.) Normally, each list element has the form `(SYM . VAL)', where SYM is a buffer-local variable (a symbol) and VAL is its buffer-local value. But when a variable's buffer-local binding in BUFFER is void, its list element is just SYM. (make-local-variable 'foobar) (makunbound 'foobar) (make-local-variable 'bind-me) (setq bind-me 69) (setq lcl (buffer-local-variables)) ;; First, built-in variables local in all buffers: => ((mark-active . nil) (buffer-undo-list . nil) (mode-name . "Fundamental") ... ;; Next, non-built-in buffer-local variables. ;; This one is buffer-local and void: foobar ;; This one is buffer-local and nonvoid: (bind-me . 69)) Note that storing new values into the CDRs of cons cells in this list does _not_ change the buffer-local values of the variables. -- Command: kill-local-variable variable This function deletes the buffer-local binding (if any) for VARIABLE (a symbol) in the current buffer. As a result, the default binding of VARIABLE becomes visible in this buffer. This typically results in a change in the value of VARIABLE, since the default value is usually different from the buffer-local value just eliminated. If you kill the buffer-local binding of a variable that automatically becomes buffer-local when set, this makes the default value visible in the current buffer. However, if you set the variable again, that will once again create a buffer-local binding for it. `kill-local-variable' returns VARIABLE. This function is a command because it is sometimes useful to kill one buffer-local variable interactively, just as it is useful to create buffer-local variables interactively. -- Function: kill-all-local-variables This function eliminates all the buffer-local variable bindings of the current buffer except for variables marked as "permanent" and local hook functions that have a non-`nil' `permanent-local-hook' property (*note Setting Hooks::). As a result, the buffer will see the default values of most variables. This function also resets certain other information pertaining to the buffer: it sets the local keymap to `nil', the syntax table to the value of `(standard-syntax-table)', the case table to `(standard-case-table)', and the abbrev table to the value of `fundamental-mode-abbrev-table'. The very first thing this function does is run the normal hook `change-major-mode-hook' (see below). Every major mode command begins by calling this function, which has the effect of switching to Fundamental mode and erasing most of the effects of the previous major mode. To ensure that this does its job, the variables that major modes set should not be marked permanent. `kill-all-local-variables' returns `nil'. -- Variable: change-major-mode-hook The function `kill-all-local-variables' runs this normal hook before it does anything else. This gives major modes a way to arrange for something special to be done if the user switches to a different major mode. It is also useful for buffer-specific minor modes that should be forgotten if the user changes the major mode. For best results, make this variable buffer-local, so that it will disappear after doing its job and will not interfere with the subsequent major mode. *Note Hooks::. A buffer-local variable is "permanent" if the variable name (a symbol) has a `permanent-local' property that is non-`nil'. Such variables are unaffected by `kill-all-local-variables', and their local bindings are therefore not cleared by changing major modes. Permanent locals are appropriate for data pertaining to where the file came from or how to save it, rather than with how to edit the contents. 11.10.3 The Default Value of a Buffer-Local Variable ---------------------------------------------------- The global value of a variable with buffer-local bindings is also called the "default" value, because it is the value that is in effect whenever neither the current buffer nor the selected frame has its own binding for the variable. The functions `default-value' and `setq-default' access and change a variable's default value regardless of whether the current buffer has a buffer-local binding. For example, you could use `setq-default' to change the default setting of `paragraph-start' for most buffers; and this would work even when you are in a C or Lisp mode buffer that has a buffer-local value for this variable. The special forms `defvar' and `defconst' also set the default value (if they set the variable at all), rather than any buffer-local value. -- Function: default-value symbol This function returns SYMBOL's default value. This is the value that is seen in buffers and frames that do not have their own values for this variable. If SYMBOL is not buffer-local, this is equivalent to `symbol-value' (*note Accessing Variables::). -- Function: default-boundp symbol The function `default-boundp' tells you whether SYMBOL's default value is nonvoid. If `(default-boundp 'foo)' returns `nil', then `(default-value 'foo)' would get an error. `default-boundp' is to `default-value' as `boundp' is to `symbol-value'. -- Special Form: setq-default [symbol form]... This special form gives each SYMBOL a new default value, which is the result of evaluating the corresponding FORM. It does not evaluate SYMBOL, but does evaluate FORM. The value of the `setq-default' form is the value of the last FORM. If a SYMBOL is not buffer-local for the current buffer, and is not marked automatically buffer-local, `setq-default' has the same effect as `setq'. If SYMBOL is buffer-local for the current buffer, then this changes the value that other buffers will see (as long as they don't have a buffer-local value), but not the value that the current buffer sees. ;; In buffer `foo': (make-local-variable 'buffer-local) => buffer-local (setq buffer-local 'value-in-foo) => value-in-foo (setq-default buffer-local 'new-default) => new-default buffer-local => value-in-foo (default-value 'buffer-local) => new-default ;; In (the new) buffer `bar': buffer-local => new-default (default-value 'buffer-local) => new-default (setq buffer-local 'another-default) => another-default (default-value 'buffer-local) => another-default ;; Back in buffer `foo': buffer-local => value-in-foo (default-value 'buffer-local) => another-default -- Function: set-default symbol value This function is like `setq-default', except that SYMBOL is an ordinary evaluated argument. (set-default (car '(a b c)) 23) => 23 (default-value 'a) => 23 11.11 File Local Variables ========================== A file can specify local variable values; Emacs uses these to create buffer-local bindings for those variables in the buffer visiting that file. *Note Local Variables in Files: (emacs)File Variables, for basic information about file-local variables. This section describes the functions and variables that affect how file-local variables are processed. If a file-local variable could specify an arbitrary function or Lisp expression that would be called later, visiting a file could take over your Emacs. Emacs protects against this by automatically setting only those file-local variables whose specified values are known to be safe. Other file-local variables are set only if the user agrees. For additional safety, `read-circle' is temporarily bound to `nil' when Emacs reads file-local variables (*note Input Functions::). This prevents the Lisp reader from recognizing circular and shared Lisp structures (*note Circular Objects::). -- User Option: enable-local-variables This variable controls whether to process file-local variables. The possible values are: `t' (the default) Set the safe variables, and query (once) about any unsafe variables. `:safe' Set only the safe variables and do not query. `:all' Set all the variables and do not query. `nil' Don't set any variables. anything else Query (once) about all the variables. -- Variable: inhibit-local-variables-regexps This is a list of regular expressions. If a file has a name matching an element of this list, then it is not scanned for any form of file-local variable. For examples of why you might want to use this, *note Auto Major Mode::. -- Function: hack-local-variables &optional mode-only This function parses, and binds or evaluates as appropriate, any local variables specified by the contents of the current buffer. The variable `enable-local-variables' has its effect here. However, this function does not look for the `mode:' local variable in the `-*-' line. `set-auto-mode' does that, also taking `enable-local-variables' into account (*note Auto Major Mode::). This function works by walking the alist stored in `file-local-variables-alist' and applying each local variable in turn. It calls `before-hack-local-variables-hook' and `hack-local-variables-hook' before and after applying the variables, respectively. It only calls the before-hook if the alist is non-`nil'; it always calls the other hook. This function ignores a `mode' element if it specifies the same major mode as the buffer already has. If the optional argument MODE-ONLY is non-`nil', then all this function does is return a symbol specifying the major mode, if the `-*-' line or the local variables list specifies one, and `nil' otherwise. It does not set the mode nor any other file-local variable. -- Variable: file-local-variables-alist This buffer-local variable holds the alist of file-local variable settings. Each element of the alist is of the form `(VAR . VALUE)', where VAR is a symbol of the local variable and VALUE is its value. When Emacs visits a file, it first collects all the file-local variables into this alist, and then the `hack-local-variables' function applies them one by one. -- Variable: before-hack-local-variables-hook Emacs calls this hook immediately before applying file-local variables stored in `file-local-variables-alist'. -- Variable: hack-local-variables-hook Emacs calls this hook immediately after it finishes applying file-local variables stored in `file-local-variables-alist'. You can specify safe values for a variable with a `safe-local-variable' property. The property has to be a function of one argument; any value is safe if the function returns non-`nil' given that value. Many commonly-encountered file variables have `safe-local-variable' properties; these include `fill-column', `fill-prefix', and `indent-tabs-mode'. For boolean-valued variables that are safe, use `booleanp' as the property value. Lambda expressions should be quoted so that `describe-variable' can display the predicate. When defining a user option using `defcustom', you can set its `safe-local-variable' property by adding the arguments `:safe FUNCTION' to `defcustom' (*note Variable Definitions::). -- User Option: safe-local-variable-values This variable provides another way to mark some variable values as safe. It is a list of cons cells `(VAR . VAL)', where VAR is a variable name and VAL is a value which is safe for that variable. When Emacs asks the user whether or not to obey a set of file-local variable specifications, the user can choose to mark them as safe. Doing so adds those variable/value pairs to `safe-local-variable-values', and saves it to the user's custom file. -- Function: safe-local-variable-p sym val This function returns non-`nil' if it is safe to give SYM the value VAL, based on the above criteria. Some variables are considered "risky". If a variable is risky, it is never entered automatically into `safe-local-variable-values'; Emacs always queries before setting a risky variable, unless the user explicitly allows a value by customizing `safe-local-variable-values' directly. Any variable whose name has a non-`nil' `risky-local-variable' property is considered risky. When you define a user option using `defcustom', you can set its `risky-local-variable' property by adding the arguments `:risky VALUE' to `defcustom' (*note Variable Definitions::). In addition, any variable whose name ends in any of `-command', `-frame-alist', `-function', `-functions', `-hook', `-hooks', `-form', `-forms', `-map', `-map-alist', `-mode-alist', `-program', or `-predicate' is automatically considered risky. The variables `font-lock-keywords', `font-lock-keywords' followed by a digit, and `font-lock-syntactic-keywords' are also considered risky. -- Function: risky-local-variable-p sym This function returns non-`nil' if SYM is a risky variable, based on the above criteria. -- Variable: ignored-local-variables This variable holds a list of variables that should not be given local values by files. Any value specified for one of these variables is completely ignored. The `Eval:' "variable" is also a potential loophole, so Emacs normally asks for confirmation before handling it. -- User Option: enable-local-eval This variable controls processing of `Eval:' in `-*-' lines or local variables lists in files being visited. A value of `t' means process them unconditionally; `nil' means ignore them; anything else means ask the user what to do for each file. The default value is `maybe'. -- User Option: safe-local-eval-forms This variable holds a list of expressions that are safe to evaluate when found in the `Eval:' "variable" in a file local variables list. If the expression is a function call and the function has a `safe-local-eval-function' property, the property value determines whether the expression is safe to evaluate. The property value can be a predicate to call to test the expression, a list of such predicates (it's safe if any predicate succeeds), or `t' (always safe provided the arguments are constant). Text properties are also potential loopholes, since their values could include functions to call. So Emacs discards all text properties from string values specified for file-local variables. 11.12 Directory Local Variables =============================== A directory can specify local variable values common to all files in that directory; Emacs uses these to create buffer-local bindings for those variables in buffers visiting any file in that directory. This is useful when the files in the directory belong to some "project" and therefore share the same local variables. There are two different methods for specifying directory local variables: by putting them in a special file, or by defining a "project class" for that directory. -- Constant: dir-locals-file This constant is the name of the file where Emacs expects to find the directory-local variables. The name of the file is `.dir-locals.el'(1). A file by that name in a directory causes Emacs to apply its settings to any file in that directory or any of its subdirectories (optionally, you can exclude subdirectories; see below). If some of the subdirectories have their own `.dir-locals.el' files, Emacs uses the settings from the deepest file it finds starting from the file's directory and moving up the directory tree. The file specifies local variables as a specially formatted list; see *note Per-directory Local Variables: (emacs)Directory Variables, for more details. -- Function: hack-dir-local-variables This function reads the `.dir-locals.el' file and stores the directory-local variables in `file-local-variables-alist' that is local to the buffer visiting any file in the directory, without applying them. It also stores the directory-local settings in `dir-locals-class-alist', where it defines a special class for the directory in which `.dir-locals.el' file was found. This function works by calling `dir-locals-set-class-variables' and `dir-locals-set-directory-class', described below. -- Function: hack-dir-local-variables-non-file-buffer This function looks for directory-local variables, and immediately applies them in the current buffer. It is intended to be called in the mode commands for non-file buffers, such as Dired buffers, to let them obey directory-local variable settings. For non-file buffers, Emacs looks for directory-local variables in `default-directory' and its parent directories. -- Function: dir-locals-set-class-variables class variables This function defines a set of variable settings for the named CLASS, which is a symbol. You can later assign the class to one or more directories, and Emacs will apply those variable settings to all files in those directories. The list in VARIABLES can be of one of the two forms: `(MAJOR-MODE . ALIST)' or `(DIRECTORY . LIST)'. With the first form, if the file's buffer turns on a mode that is derived from MAJOR-MODE, then the all the variables in the associated ALIST are applied; ALIST should be of the form `(NAME . VALUE)'. A special value `nil' for MAJOR-MODE means the settings are applicable to any mode. In ALIST, you can use a special NAME: `subdirs'. If the associated value is `nil', the alist is only applied to files in the relevant directory, not to those in any subdirectories. With the second form of VARIABLES, if DIRECTORY is the initial substring of the file's directory, then LIST is applied recursively by following the above rules; LIST should be of one of the two forms accepted by this function in VARIABLES. -- Function: dir-locals-set-directory-class directory class &optional mtime This function assigns CLASS to all the files in `directory' and its subdirectories. Thereafter, all the variable settings specified for CLASS will be applied to any visited file in DIRECTORY and its children. CLASS must have been already defined by `dir-locals-set-class-variables'. Emacs uses this function internally when it loads directory variables from a `.dir-locals.el' file. In that case, the optional argument MTIME holds the file modification time (as returned by `file-attributes'). Emacs uses this time to check stored local variables are still valid. If you are assigning a class directly, not via a file, this argument should be `nil'. -- Variable: dir-locals-class-alist This alist holds the class symbols and the associated variable settings. It is updated by `dir-locals-set-class-variables'. -- Variable: dir-locals-directory-cache This alist holds directory names, their assigned class names, and modification times of the associated directory local variables file (if there is one). The function `dir-locals-set-directory-class' updates this list. ---------- Footnotes ---------- (1) The MS-DOS version of Emacs uses `_dir-locals.el' instead, due to limitations of the DOS filesystems. 11.13 Variable Aliases ====================== It is sometimes useful to make two variables synonyms, so that both variables always have the same value, and changing either one also changes the other. Whenever you change the name of a variable--either because you realize its old name was not well chosen, or because its meaning has partly changed--it can be useful to keep the old name as an _alias_ of the new one for compatibility. You can do this with `defvaralias'. -- Function: defvaralias new-alias base-variable &optional docstring This function defines the symbol NEW-ALIAS as a variable alias for symbol BASE-VARIABLE. This means that retrieving the value of NEW-ALIAS returns the value of BASE-VARIABLE, and changing the value of NEW-ALIAS changes the value of BASE-VARIABLE. The two aliased variable names always share the same value and the same bindings. If the DOCSTRING argument is non-`nil', it specifies the documentation for NEW-ALIAS; otherwise, the alias gets the same documentation as BASE-VARIABLE has, if any, unless BASE-VARIABLE is itself an alias, in which case NEW-ALIAS gets the documentation of the variable at the end of the chain of aliases. This function returns BASE-VARIABLE. Variable aliases are convenient for replacing an old name for a variable with a new name. `make-obsolete-variable' declares that the old name is obsolete and therefore that it may be removed at some stage in the future. -- Function: make-obsolete-variable obsolete-name current-name when &optional access-type This function makes the byte compiler warn that the variable OBSOLETE-NAME is obsolete. If CURRENT-NAME is a symbol, it is the variable's new name; then the warning message says to use CURRENT-NAME instead of OBSOLETE-NAME. If CURRENT-NAME is a string, this is the message and there is no replacement variable. WHEN should be a string indicating when the variable was first made obsolete (usually a version number string). The optional argument ACCESS-TYPE, if non-`nil', should should specify the kind of access that will trigger obsolescence warnings; it can be either `get' or `set'. You can make two variables synonyms and declare one obsolete at the same time using the macro `define-obsolete-variable-alias'. -- Macro: define-obsolete-variable-alias obsolete-name current-name &optional when docstring This macro marks the variable OBSOLETE-NAME as obsolete and also makes it an alias for the variable CURRENT-NAME. It is equivalent to the following: (defvaralias OBSOLETE-NAME CURRENT-NAME DOCSTRING) (make-obsolete-variable OBSOLETE-NAME CURRENT-NAME WHEN) -- Function: indirect-variable variable This function returns the variable at the end of the chain of aliases of VARIABLE. If VARIABLE is not a symbol, or if VARIABLE is not defined as an alias, the function returns VARIABLE. This function signals a `cyclic-variable-indirection' error if there is a loop in the chain of symbols. (defvaralias 'foo 'bar) (indirect-variable 'foo) => bar (indirect-variable 'bar) => bar (setq bar 2) bar => 2 foo => 2 (setq foo 0) bar => 0 foo => 0 11.14 Variables with Restricted Values ====================================== Ordinary Lisp variables can be assigned any value that is a valid Lisp object. However, certain Lisp variables are not defined in Lisp, but in C. Most of these variables are defined in the C code using `DEFVAR_LISP'. Like variables defined in Lisp, these can take on any value. However, some variables are defined using `DEFVAR_INT' or `DEFVAR_BOOL'. *Note Writing Emacs Primitives: Defining Lisp variables in C, in particular the description of functions of the type `syms_of_FILENAME', for a brief discussion of the C implementation. Variables of type `DEFVAR_BOOL' can only take on the values `nil' or `t'. Attempting to assign them any other value will set them to `t': (let ((display-hourglass 5)) display-hourglass) => t -- Variable: byte-boolean-vars This variable holds a list of all variables of type `DEFVAR_BOOL'. Variables of type `DEFVAR_INT' can only take on integer values. Attempting to assign them any other value will result in an error: (setq undo-limit 1000.0) error--> Wrong type argument: integerp, 1000.0 11.15 Generalized Variables =========================== A "generalized variable" or "place form" is one of the many places in Lisp memory where values can be stored. The simplest place form is a regular Lisp variable. But the CARs and CDRs of lists, elements of arrays, properties of symbols, and many other locations are also places where Lisp values are stored. Generalized variables are analogous to "lvalues" in the C language, where `x = a[i]' gets an element from an array and `a[i] = x' stores an element using the same notation. Just as certain forms like `a[i]' can be lvalues in C, there is a set of forms that can be generalized variables in Lisp. 11.15.1 The `setf' Macro ------------------------ The `setf' macro is the most basic way to operate on generalized variables. The `setf' form is like `setq', except that it accepts arbitrary place forms on the left side rather than just symbols. For example, `(setf (car a) b)' sets the car of `a' to `b', doing the same operation as `(setcar a b)', but without having to remember two separate functions for setting and accessing every type of place. -- Macro: setf [place form]... This macro evaluates FORM and stores it in PLACE, which must be a valid generalized variable form. If there are several PLACE and FORM pairs, the assignments are done sequentially just as with `setq'. `setf' returns the value of the last FORM. The following Lisp forms will work as generalized variables, and so may appear in the PLACE argument of `setf': * A symbol naming a variable. In other words, `(setf x y)' is exactly equivalent to `(setq x y)', and `setq' itself is strictly speaking redundant given that `setf' exists. Many programmers continue to prefer `setq' for setting simple variables, though, purely for stylistic or historical reasons. The macro `(setf x y)' actually expands to `(setq x y)', so there is no performance penalty for using it in compiled code. * A call to any of the following standard Lisp functions: aref cddr symbol-function car elt symbol-plist caar get symbol-value cadr gethash cdr nth cdar nthcdr * A call to any of the following Emacs-specific functions: default-value process-get frame-parameter process-sentinel terminal-parameter window-buffer keymap-parent window-display-table match-data window-dedicated-p overlay-get window-hscroll overlay-start window-parameter overlay-end window-point process-buffer window-start process-filter `setf' signals an error if you pass a PLACE form that it does not know how to handle. Note that for `nthcdr', the list argument of the function must itself be a valid PLACE form. For example, `(setf (nthcdr 0 foo) 7)' will set `foo' itself to 7. The macros `push' (*note List Variables::) and `pop' (*note List Elements::) can manipulate generalized variables, not just lists. `(pop PLACE)' removes and returns the first element of the list stored in PLACE. It is analogous to `(prog1 (car PLACE) (setf PLACE (cdr PLACE)))', except that it takes care to evaluate all subforms only once. `(push X PLACE)' inserts X at the front of the list stored in PLACE. It is analogous to `(setf PLACE (cons X PLACE))', except for evaluation of the subforms. Note that `push' and `pop' on an `nthcdr' place can be used to insert or delete at any position in a list. The `cl-lib' library defines various extensions for generalized variables, including additional `setf' places. *Note Generalized Variables: (cl)Generalized Variables. 11.15.2 Defining new `setf' forms --------------------------------- This section describes how to define new forms that `setf' can operate on. -- Macro: gv-define-simple-setter name setter &optional fix-return This macro enables you to easily define `setf' methods for simple cases. NAME is the name of a function, macro, or special form. You can use this macro whenever NAME has a directly corresponding SETTER function that updates it, e.g., `(gv-define-simple-setter car setcar)'. This macro translates a call of the form (setf (NAME ARGS...) VALUE) into (SETTER ARGS... VALUE) Such a `setf' call is documented to return VALUE. This is no problem with, e.g., `car' and `setcar', because `setcar' returns the value that it set. If your SETTER function does not return VALUE, use a non-`nil' value for the FIX-RETURN argument of `gv-define-simple-setter'. This expands into something equivalent to (let ((temp VALUE)) (SETTER ARGS... temp) temp) so ensuring that it returns the correct result. -- Macro: gv-define-setter name arglist &rest body This macro allows for more complex `setf' expansions than the previous form. You may need to use this form, for example, if there is no simple setter function to call, or if there is one but it requires different arguments to the place form. This macro expands the form `(setf (NAME ARGS...) VALUE)' by first binding the `setf' argument forms `(VALUE ARGS...)' according to ARGLIST, and then executing BODY. BODY should return a Lisp form that does the assignment, and finally returns the value that was set. An example of using this macro is: (gv-define-setter caar (val x) `(setcar (car ,x) ,val)) For more control over the expansion, see the macro `gv-define-expander'. The macro `gv-letplace' can be useful in defining macros that perform similarly to `setf'; for example, the `incf' macro of Common Lisp. Consult the source file `gv.el' for more details. Common Lisp note: Common Lisp defines another way to specify the `setf' behavior of a function, namely "`setf' functions", whose names are lists `(setf NAME)' rather than symbols. For example, `(defun (setf foo) ...)' defines the function that is used when `setf' is applied to `foo'. Emacs does not support this. It is a compile-time error to use `setf' on a form that has not already had an appropriate expansion defined. In Common Lisp, this is not an error since the function `(setf FUNC)' might be defined later. 12 Functions ************ A Lisp program is composed mainly of Lisp functions. This chapter explains what functions are, how they accept arguments, and how to define them. 12.1 What Is a Function? ======================== In a general sense, a function is a rule for carrying out a computation given input values called "arguments". The result of the computation is called the "value" or "return value" of the function. The computation can also have side effects, such as lasting changes in the values of variables or the contents of data structures. In most computer languages, every function has a name. But in Lisp, a function in the strictest sense has no name: it is an object which can _optionally_ be associated with a symbol (e.g., `car') that serves as the function name. *Note Function Names::. When a function has been given a name, we usually also refer to that symbol as a "function" (e.g., we refer to "the function `car'"). In this manual, the distinction between a function name and the function object itself is usually unimportant, but we will take note wherever it is relevant. Certain function-like objects, called "special forms" and "macros", also accept arguments to carry out computations. However, as explained below, these are not considered functions in Emacs Lisp. Here are important terms for functions and function-like objects: "lambda expression" A function (in the strict sense, i.e., a function object) which is written in Lisp. These are described in the following section. *Note Lambda Expressions::. "primitive" A function which is callable from Lisp but is actually written in C. Primitives are also called "built-in functions", or "subrs". Examples include functions like `car' and `append'. In addition, all special forms (see below) are also considered primitives. Usually, a function is implemented as a primitive because it is a fundamental part of Lisp (e.g., `car'), or because it provides a low-level interface to operating system services, or because it needs to run fast. Unlike functions defined in Lisp, primitives can be modified or added only by changing the C sources and recompiling Emacs. See *note Writing Emacs Primitives::. "special form" A primitive that is like a function but does not evaluate all of its arguments in the usual way. It may evaluate only some of the arguments, or may evaluate them in an unusual order, or several times. Examples include `if', `and', and `while'. *Note Special Forms::. "macro" A construct defined in Lisp, which differs from a function in that it translates a Lisp expression into another expression which is to be evaluated instead of the original expression. Macros enable Lisp programmers to do the sorts of things that special forms can do. *Note Macros::. "command" An object which can be invoked via the `command-execute' primitive, usually due to the user typing in a key sequence "bound" to that command. *Note Interactive Call::. A command is usually a function; if the function is written in Lisp, it is made into a command by an `interactive' form in the function definition (*note Defining Commands::). Commands that are functions can also be called from Lisp expressions, just like other functions. Keyboard macros (strings and vectors) are commands also, even though they are not functions. *Note Keyboard Macros::. We say that a symbol is a command if its function cell contains a command (*note Symbol Components::); such a "named command" can be invoked with `M-x'. "closure" A function object that is much like a lambda expression, except that it also encloses an "environment" of lexical variable bindings. *Note Closures::. "byte-code function" A function that has been compiled by the byte compiler. *Note Byte-Code Type::. "autoload object" A place-holder for a real function. If the autoload object is called, Emacs loads the file containing the definition of the real function, and then calls the real function. *Note Autoload::. You can use the function `functionp' to test if an object is a function: -- Function: functionp object This function returns `t' if OBJECT is any kind of function, i.e., can be passed to `funcall'. Note that `functionp' returns `t' for symbols that are function names, and returns `nil' for special forms. Unlike `functionp', the next three functions do _not_ treat a symbol as its function definition. -- Function: subrp object This function returns `t' if OBJECT is a built-in function (i.e., a Lisp primitive). (subrp 'message) ; `message' is a symbol, => nil ; not a subr object. (subrp (symbol-function 'message)) => t -- Function: byte-code-function-p object This function returns `t' if OBJECT is a byte-code function. For example: (byte-code-function-p (symbol-function 'next-line)) => t -- Function: subr-arity subr This function provides information about the argument list of a primitive, SUBR. The returned value is a pair `(MIN . MAX)'. MIN is the minimum number of args. MAX is the maximum number or the symbol `many', for a function with `&rest' arguments, or the symbol `unevalled' if SUBR is a special form. 12.2 Lambda Expressions ======================= A lambda expression is a function object written in Lisp. Here is an example: (lambda (x) "Return the hyperbolic cosine of X." (* 0.5 (+ (exp x) (exp (- x))))) In Emacs Lisp, such a list is valid as an expression--it evaluates to itself. But its main use is not to be evaluated as an expression, but to be called as a function. A lambda expression, by itself, has no name; it is an "anonymous function". Although lambda expressions can be used this way (*note Anonymous Functions::), they are more commonly associated with symbols to make "named functions" (*note Function Names::). Before going into these details, the following subsections describe the components of a lambda expression and what they do. 12.2.1 Components of a Lambda Expression ---------------------------------------- A lambda expression is a list that looks like this: (lambda (ARG-VARIABLES...) [DOCUMENTATION-STRING] [INTERACTIVE-DECLARATION] BODY-FORMS...) The first element of a lambda expression is always the symbol `lambda'. This indicates that the list represents a function. The reason functions are defined to start with `lambda' is so that other lists, intended for other uses, will not accidentally be valid as functions. The second element is a list of symbols--the argument variable names. This is called the "lambda list". When a Lisp function is called, the argument values are matched up against the variables in the lambda list, which are given local bindings with the values provided. *Note Local Variables::. The documentation string is a Lisp string object placed within the function definition to describe the function for the Emacs help facilities. *Note Function Documentation::. The interactive declaration is a list of the form `(interactive CODE-STRING)'. This declares how to provide arguments if the function is used interactively. Functions with this declaration are called "commands"; they can be called using `M-x' or bound to a key. Functions not intended to be called in this way should not have interactive declarations. *Note Defining Commands::, for how to write an interactive declaration. The rest of the elements are the "body" of the function: the Lisp code to do the work of the function (or, as a Lisp programmer would say, "a list of Lisp forms to evaluate"). The value returned by the function is the value returned by the last element of the body. 12.2.2 A Simple Lambda Expression Example ----------------------------------------- Consider the following example: (lambda (a b c) (+ a b c)) We can call this function by passing it to `funcall', like this: (funcall (lambda (a b c) (+ a b c)) 1 2 3) This call evaluates the body of the lambda expression with the variable `a' bound to 1, `b' bound to 2, and `c' bound to 3. Evaluation of the body adds these three numbers, producing the result 6; therefore, this call to the function returns the value 6. Note that the arguments can be the results of other function calls, as in this example: (funcall (lambda (a b c) (+ a b c)) 1 (* 2 3) (- 5 4)) This evaluates the arguments `1', `(* 2 3)', and `(- 5 4)' from left to right. Then it applies the lambda expression to the argument values 1, 6 and 1 to produce the value 8. As these examples show, you can use a form with a lambda expression as its CAR to make local variables and give them values. In the old days of Lisp, this technique was the only way to bind and initialize local variables. But nowadays, it is clearer to use the special form `let' for this purpose (*note Local Variables::). Lambda expressions are mainly used as anonymous functions for passing as arguments to other functions (*note Anonymous Functions::), or stored as symbol function definitions to produce named functions (*note Function Names::). 12.2.3 Other Features of Argument Lists --------------------------------------- Our simple sample function, `(lambda (a b c) (+ a b c))', specifies three argument variables, so it must be called with three arguments: if you try to call it with only two arguments or four arguments, you get a `wrong-number-of-arguments' error. It is often convenient to write a function that allows certain arguments to be omitted. For example, the function `substring' accepts three arguments--a string, the start index and the end index--but the third argument defaults to the LENGTH of the string if you omit it. It is also convenient for certain functions to accept an indefinite number of arguments, as the functions `list' and `+' do. To specify optional arguments that may be omitted when a function is called, simply include the keyword `&optional' before the optional arguments. To specify a list of zero or more extra arguments, include the keyword `&rest' before one final argument. Thus, the complete syntax for an argument list is as follows: (REQUIRED-VARS... [&optional OPTIONAL-VARS...] [&rest REST-VAR]) The square brackets indicate that the `&optional' and `&rest' clauses, and the variables that follow them, are optional. A call to the function requires one actual argument for each of the REQUIRED-VARS. There may be actual arguments for zero or more of the OPTIONAL-VARS, and there cannot be any actual arguments beyond that unless the lambda list uses `&rest'. In that case, there may be any number of extra actual arguments. If actual arguments for the optional and rest variables are omitted, then they always default to `nil'. There is no way for the function to distinguish between an explicit argument of `nil' and an omitted argument. However, the body of the function is free to consider `nil' an abbreviation for some other meaningful value. This is what `substring' does; `nil' as the third argument to `substring' means to use the length of the string supplied. Common Lisp note: Common Lisp allows the function to specify what default value to use when an optional argument is omitted; Emacs Lisp always uses `nil'. Emacs Lisp does not support "supplied-p" variables that tell you whether an argument was explicitly passed. For example, an argument list that looks like this: (a b &optional c d &rest e) binds `a' and `b' to the first two actual arguments, which are required. If one or two more arguments are provided, `c' and `d' are bound to them respectively; any arguments after the first four are collected into a list and `e' is bound to that list. If there are only two arguments, `c' is `nil'; if two or three arguments, `d' is `nil'; if four arguments or fewer, `e' is `nil'. There is no way to have required arguments following optional ones--it would not make sense. To see why this must be so, suppose that `c' in the example were optional and `d' were required. Suppose three actual arguments are given; which variable would the third argument be for? Would it be used for the C, or for D? One can argue for both possibilities. Similarly, it makes no sense to have any more arguments (either required or optional) after a `&rest' argument. Here are some examples of argument lists and proper calls: (funcall (lambda (n) (1+ n)) ; One required: 1) ; requires exactly one argument. => 2 (funcall (lambda (n &optional n1) ; One required and one optional: (if n1 (+ n n1) (1+ n))) ; 1 or 2 arguments. 1 2) => 3 (funcall (lambda (n &rest ns) ; One required and one rest: (+ n (apply '+ ns))) ; 1 or more arguments. 1 2 3 4 5) => 15 12.2.4 Documentation Strings of Functions ----------------------------------------- A lambda expression may optionally have a "documentation string" just after the lambda list. This string does not affect execution of the function; it is a kind of comment, but a systematized comment which actually appears inside the Lisp world and can be used by the Emacs help facilities. *Note Documentation::, for how the documentation string is accessed. It is a good idea to provide documentation strings for all the functions in your program, even those that are called only from within your program. Documentation strings are like comments, except that they are easier to access. The first line of the documentation string should stand on its own, because `apropos' displays just this first line. It should consist of one or two complete sentences that summarize the function's purpose. The start of the documentation string is usually indented in the source file, but since these spaces come before the starting double-quote, they are not part of the string. Some people make a practice of indenting any additional lines of the string so that the text lines up in the program source. _That is a mistake._ The indentation of the following lines is inside the string; what looks nice in the source code will look ugly when displayed by the help commands. You may wonder how the documentation string could be optional, since there are required components of the function that follow it (the body). Since evaluation of a string returns that string, without any side effects, it has no effect if it is not the last form in the body. Thus, in practice, there is no confusion between the first form of the body and the documentation string; if the only body form is a string then it serves both as the return value and as the documentation. The last line of the documentation string can specify calling conventions different from the actual function arguments. Write text like this: \(fn ARGLIST) following a blank line, at the beginning of the line, with no newline following it inside the documentation string. (The `\' is used to avoid confusing the Emacs motion commands.) The calling convention specified in this way appears in help messages in place of the one derived from the actual arguments of the function. This feature is particularly useful for macro definitions, since the arguments written in a macro definition often do not correspond to the way users think of the parts of the macro call. 12.3 Naming a Function ====================== A symbol can serve as the name of a function. This happens when the symbol's "function cell" (*note Symbol Components::) contains a function object (e.g., a lambda expression). Then the symbol itself becomes a valid, callable function, equivalent to the function object in its function cell. The contents of the function cell are also called the symbol's "function definition". The procedure of using a symbol's function definition in place of the symbol is called "symbol function indirection"; see *note Function Indirection::. If you have not given a symbol a function definition, its function cell is said to be "void", and it cannot be used as a function. In practice, nearly all functions have names, and are referred to by their names. You can create a named Lisp function by defining a lambda expression and putting it in a function cell (*note Function Cells::). However, it is more common to use the `defun' special form, described in the next section. *Note Defining Functions::. We give functions names because it is convenient to refer to them by their names in Lisp expressions. Also, a named Lisp function can easily refer to itself--it can be recursive. Furthermore, primitives can only be referred to textually by their names, since primitive function objects (*note Primitive Function Type::) have no read syntax. A function need not have a unique name. A given function object _usually_ appears in the function cell of only one symbol, but this is just a convention. It is easy to store it in several symbols using `fset'; then each of the symbols is a valid name for the same function. Note that a symbol used as a function name may also be used as a variable; these two uses of a symbol are independent and do not conflict. (This is not the case in some dialects of Lisp, like Scheme.) 12.4 Defining Functions ======================= We usually give a name to a function when it is first created. This is called "defining a function", and it is done with the `defun' macro. -- Macro: defun name args [doc] [declare] [interactive] body... `defun' is the usual way to define new Lisp functions. It defines the symbol NAME as a function with argument list ARGS and body forms given by BODY. Neither NAME nor ARGS should be quoted. DOC, if present, should be a string specifying the function's documentation string (*note Function Documentation::). DECLARE, if present, should be a `declare' form specifying function metadata (*note Declare Form::). INTERACTIVE, if present, should be an `interactive' form specifying how the function is to be called interactively (*note Interactive Call::). The return value of `defun' is undefined. Here are some examples: (defun foo () 5) (foo) => 5 (defun bar (a &optional b &rest c) (list a b c)) (bar 1 2 3 4 5) => (1 2 (3 4 5)) (bar 1) => (1 nil nil) (bar) error--> Wrong number of arguments. (defun capitalize-backwards () "Upcase the last letter of the word at point." (interactive) (backward-word 1) (forward-word 1) (backward-char 1) (capitalize-word 1)) Be careful not to redefine existing functions unintentionally. `defun' redefines even primitive functions such as `car' without any hesitation or notification. Emacs does not prevent you from doing this, because redefining a function is sometimes done deliberately, and there is no way to distinguish deliberate redefinition from unintentional redefinition. -- Function: defalias name definition &optional doc This function defines the symbol NAME as a function, with definition DEFINITION (which can be any valid Lisp function). Its return value is _undefined_. If DOC is non-`nil', it becomes the function documentation of NAME. Otherwise, any documentation provided by DEFINITION is used. The proper place to use `defalias' is where a specific function name is being defined--especially where that name appears explicitly in the source file being loaded. This is because `defalias' records which file defined the function, just like `defun' (*note Unloading::). By contrast, in programs that manipulate function definitions for other purposes, it is better to use `fset', which does not keep such records. *Note Function Cells::. You cannot create a new primitive function with `defun' or `defalias', but you can use them to change the function definition of any symbol, even one such as `car' or `x-popup-menu' whose normal definition is a primitive. However, this is risky: for instance, it is next to impossible to redefine `car' without breaking Lisp completely. Redefining an obscure function such as `x-popup-menu' is less dangerous, but it still may not work as you expect. If there are calls to the primitive from C code, they call the primitive's C definition directly, so changing the symbol's definition will have no effect on them. See also `defsubst', which defines a function like `defun' and tells the Lisp compiler to perform inline expansion on it. *Note Inline Functions::. 12.5 Calling Functions ====================== Defining functions is only half the battle. Functions don't do anything until you "call" them, i.e., tell them to run. Calling a function is also known as "invocation". The most common way of invoking a function is by evaluating a list. For example, evaluating the list `(concat "a" "b")' calls the function `concat' with arguments `"a"' and `"b"'. *Note Evaluation::, for a description of evaluation. When you write a list as an expression in your program, you specify which function to call, and how many arguments to give it, in the text of the program. Usually that's just what you want. Occasionally you need to compute at run time which function to call. To do that, use the function `funcall'. When you also need to determine at run time how many arguments to pass, use `apply'. -- Function: funcall function &rest arguments `funcall' calls FUNCTION with ARGUMENTS, and returns whatever FUNCTION returns. Since `funcall' is a function, all of its arguments, including FUNCTION, are evaluated before `funcall' is called. This means that you can use any expression to obtain the function to be called. It also means that `funcall' does not see the expressions you write for the ARGUMENTS, only their values. These values are _not_ evaluated a second time in the act of calling FUNCTION; the operation of `funcall' is like the normal procedure for calling a function, once its arguments have already been evaluated. The argument FUNCTION must be either a Lisp function or a primitive function. Special forms and macros are not allowed, because they make sense only when given the "unevaluated" argument expressions. `funcall' cannot provide these because, as we saw above, it never knows them in the first place. (setq f 'list) => list (funcall f 'x 'y 'z) => (x y z) (funcall f 'x 'y '(z)) => (x y (z)) (funcall 'and t nil) error--> Invalid function: # Compare these examples with the examples of `apply'. -- Function: apply function &rest arguments `apply' calls FUNCTION with ARGUMENTS, just like `funcall' but with one difference: the last of ARGUMENTS is a list of objects, which are passed to FUNCTION as separate arguments, rather than a single list. We say that `apply' "spreads" this list so that each individual element becomes an argument. `apply' returns the result of calling FUNCTION. As with `funcall', FUNCTION must either be a Lisp function or a primitive function; special forms and macros do not make sense in `apply'. (setq f 'list) => list (apply f 'x 'y 'z) error--> Wrong type argument: listp, z (apply '+ 1 2 '(3 4)) => 10 (apply '+ '(1 2 3 4)) => 10 (apply 'append '((a b c) nil (x y z) nil)) => (a b c x y z) For an interesting example of using `apply', see *note Definition of mapcar::. Sometimes it is useful to fix some of the function's arguments at certain values, and leave the rest of arguments for when the function is actually called. The act of fixing some of the function's arguments is called "partial application" of the function(1). The result is a new function that accepts the rest of arguments and calls the original function with all the arguments combined. Here's how to do partial application in Emacs Lisp: -- Function: apply-partially func &rest args This function returns a new function which, when called, will call FUNC with the list of arguments composed from ARGS and additional arguments specified at the time of the call. If FUNC accepts N arguments, then a call to `apply-partially' with `M < N' arguments will produce a new function of `N - M' arguments. Here's how we could define the built-in function `1+', if it didn't exist, using `apply-partially' and `+', another built-in function: (defalias '1+ (apply-partially '+ 1) "Increment argument by one.") (1+ 10) => 11 It is common for Lisp functions to accept functions as arguments or find them in data structures (especially in hook variables and property lists) and call them using `funcall' or `apply'. Functions that accept function arguments are often called "functionals". Sometimes, when you call a functional, it is useful to supply a no-op function as the argument. Here are two different kinds of no-op function: -- Function: identity arg This function returns ARG and has no side effects. -- Function: ignore &rest args This function ignores any arguments and returns `nil'. Some functions are user-visible "commands", which can be called interactively (usually by a key sequence). It is possible to invoke such a command exactly as though it was called interactively, by using the `call-interactively' function. *Note Interactive Call::. ---------- Footnotes ---------- (1) This is related to, but different from "currying", which transforms a function that takes multiple arguments in such a way that it can be called as a chain of functions, each one with a single argument. 12.6 Mapping Functions ====================== A "mapping function" applies a given function (_not_ a special form or macro) to each element of a list or other collection. Emacs Lisp has several such functions; this section describes `mapcar', `mapc', and `mapconcat', which map over a list. *Note Definition of mapatoms::, for the function `mapatoms' which maps over the symbols in an obarray. *Note Definition of maphash::, for the function `maphash' which maps over key/value associations in a hash table. These mapping functions do not allow char-tables because a char-table is a sparse array whose nominal range of indices is very large. To map over a char-table in a way that deals properly with its sparse nature, use the function `map-char-table' (*note Char-Tables::). -- Function: mapcar function sequence `mapcar' applies FUNCTION to each element of SEQUENCE in turn, and returns a list of the results. The argument SEQUENCE can be any kind of sequence except a char-table; that is, a list, a vector, a bool-vector, or a string. The result is always a list. The length of the result is the same as the length of SEQUENCE. For example: (mapcar 'car '((a b) (c d) (e f))) => (a c e) (mapcar '1+ [1 2 3]) => (2 3 4) (mapcar 'string "abc") => ("a" "b" "c") ;; Call each function in `my-hooks'. (mapcar 'funcall my-hooks) (defun mapcar* (function &rest args) "Apply FUNCTION to successive cars of all ARGS. Return the list of results." ;; If no list is exhausted, (if (not (memq nil args)) ;; apply function to CARs. (cons (apply function (mapcar 'car args)) (apply 'mapcar* function ;; Recurse for rest of elements. (mapcar 'cdr args))))) (mapcar* 'cons '(a b c) '(1 2 3 4)) => ((a . 1) (b . 2) (c . 3)) -- Function: mapc function sequence `mapc' is like `mapcar' except that FUNCTION is used for side-effects only--the values it returns are ignored, not collected into a list. `mapc' always returns SEQUENCE. -- Function: mapconcat function sequence separator `mapconcat' applies FUNCTION to each element of SEQUENCE: the results, which must be strings, are concatenated. Between each pair of result strings, `mapconcat' inserts the string SEPARATOR. Usually SEPARATOR contains a space or comma or other suitable punctuation. The argument FUNCTION must be a function that can take one argument and return a string. The argument SEQUENCE can be any kind of sequence except a char-table; that is, a list, a vector, a bool-vector, or a string. (mapconcat 'symbol-name '(The cat in the hat) " ") => "The cat in the hat" (mapconcat (function (lambda (x) (format "%c" (1+ x)))) "HAL-8000" "") => "IBM.9111" 12.7 Anonymous Functions ======================== Although functions are usually defined with `defun' and given names at the same time, it is sometimes convenient to use an explicit lambda expression--an "anonymous function". Anonymous functions are valid wherever function names are. They are often assigned as variable values, or as arguments to functions; for instance, you might pass one as the FUNCTION argument to `mapcar', which applies that function to each element of a list (*note Mapping Functions::). *Note describe-symbols example::, for a realistic example of this. When defining a lambda expression that is to be used as an anonymous function, you can in principle use any method to construct the list. But typically you should use the `lambda' macro, or the `function' special form, or the `#'' read syntax: -- Macro: lambda args [doc] [interactive] body... This macro returns an anonymous function with argument list ARGS, documentation string DOC (if any), interactive spec INTERACTIVE (if any), and body forms given by BODY. In effect, this macro makes `lambda' forms "self-quoting": evaluating a form whose CAR is `lambda' yields the form itself: (lambda (x) (* x x)) => (lambda (x) (* x x)) The `lambda' form has one other effect: it tells the Emacs evaluator and byte-compiler that its argument is a function, by using `function' as a subroutine (see below). -- Special Form: function function-object This special form returns FUNCTION-OBJECT without evaluating it. In this, it is similar to `quote' (*note Quoting::). But unlike `quote', it also serves as a note to the Emacs evaluator and byte-compiler that FUNCTION-OBJECT is intended to be used as a function. Assuming FUNCTION-OBJECT is a valid lambda expression, this has two effects: * When the code is byte-compiled, FUNCTION-OBJECT is compiled into a byte-code function object (*note Byte Compilation::). * When lexical binding is enabled, FUNCTION-OBJECT is converted into a closure. *Note Closures::. The read syntax `#'' is a short-hand for using `function'. The following forms are all equivalent: (lambda (x) (* x x)) (function (lambda (x) (* x x))) #'(lambda (x) (* x x)) In the following example, we define a `change-property' function that takes a function as its third argument, followed by a `double-property' function that makes use of `change-property' by passing it an anonymous function: (defun change-property (symbol prop function) (let ((value (get symbol prop))) (put symbol prop (funcall function value)))) (defun double-property (symbol prop) (change-property symbol prop (lambda (x) (* 2 x)))) Note that we do not quote the `lambda' form. If you compile the above code, the anonymous function is also compiled. This would not happen if, say, you had constructed the anonymous function by quoting it as a list: (defun double-property (symbol prop) (change-property symbol prop (lambda (x) (* 2 x)))) In that case, the anonymous function is kept as a lambda expression in the compiled code. The byte-compiler cannot assume this list is a function, even though it looks like one, since it does not know that `change-property' intends to use it as a function. 12.8 Accessing Function Cell Contents ===================================== The "function definition" of a symbol is the object stored in the function cell of the symbol. The functions described here access, test, and set the function cell of symbols. See also the function `indirect-function'. *Note Definition of indirect-function::. -- Function: symbol-function symbol This returns the object in the function cell of SYMBOL. If the symbol's function cell is void, a `void-function' error is signaled. This function does not check that the returned object is a legitimate function. (defun bar (n) (+ n 2)) (symbol-function 'bar) => (lambda (n) (+ n 2)) (fset 'baz 'bar) => bar (symbol-function 'baz) => bar If you have never given a symbol any function definition, we say that that symbol's function cell is "void". In other words, the function cell does not have any Lisp object in it. If you try to call such a symbol as a function, it signals a `void-function' error. Note that void is not the same as `nil' or the symbol `void'. The symbols `nil' and `void' are Lisp objects, and can be stored into a function cell just as any other object can be (and they can be valid functions if you define them in turn with `defun'). A void function cell contains no object whatsoever. You can test the voidness of a symbol's function definition with `fboundp'. After you have given a symbol a function definition, you can make it void once more using `fmakunbound'. -- Function: fboundp symbol This function returns `t' if the symbol has an object in its function cell, `nil' otherwise. It does not check that the object is a legitimate function. -- Function: fmakunbound symbol This function makes SYMBOL's function cell void, so that a subsequent attempt to access this cell will cause a `void-function' error. It returns SYMBOL. (See also `makunbound', in *note Void Variables::.) (defun foo (x) x) (foo 1) =>1 (fmakunbound 'foo) => foo (foo 1) error--> Symbol's function definition is void: foo -- Function: fset symbol definition This function stores DEFINITION in the function cell of SYMBOL. The result is DEFINITION. Normally DEFINITION should be a function or the name of a function, but this is not checked. The argument SYMBOL is an ordinary evaluated argument. The primary use of this function is as a subroutine by constructs that define or alter functions, like `defadvice' (*note Advising Functions::). (If `defun' were not a primitive, it could be written as a Lisp macro using `fset'.) You can also use it to give a symbol a function definition that is not a list, e.g., a keyboard macro (*note Keyboard Macros::): ;; Define a named keyboard macro. (fset 'kill-two-lines "\^u2\^k") => "\^u2\^k" It you wish to use `fset' to make an alternate name for a function, consider using `defalias' instead. *Note Definition of defalias::. 12.9 Closures ============= As explained in *note Variable Scoping::, Emacs can optionally enable lexical binding of variables. When lexical binding is enabled, any named function that you create (e.g., with `defun'), as well as any anonymous function that you create using the `lambda' macro or the `function' special form or the `#'' syntax (*note Anonymous Functions::), is automatically converted into a "closure". A closure is a function that also carries a record of the lexical environment that existed when the function was defined. When it is invoked, any lexical variable references within its definition use the retained lexical environment. In all other respects, closures behave much like ordinary functions; in particular, they can be called in the same way as ordinary functions. *Note Lexical Binding::, for an example of using a closure. Currently, an Emacs Lisp closure object is represented by a list with the symbol `closure' as the first element, a list representing the lexical environment as the second element, and the argument list and body forms as the remaining elements: ;; lexical binding is enabled. (lambda (x) (* x x)) => (closure (t) (x) (* x x)) However, the fact that the internal structure of a closure is "exposed" to the rest of the Lisp world is considered an internal implementation detail. For this reason, we recommend against directly examining or altering the structure of closure objects. 12.10 Declaring Functions Obsolete ================================== You can mark a named function as "obsolete", meaning that it may be removed at some point in the future. This causes Emacs to warn that the function is obsolete whenever it byte-compiles code containing that function, and whenever it displays the documentation for that function. In all other respects, an obsolete function behaves like any other function. The easiest way to mark a function as obsolete is to put a `(declare (obsolete ...))' form in the function's `defun' definition. *Note Declare Form::. Alternatively, you can use the `make-obsolete' function, described below. A macro (*note Macros::) can also be marked obsolete with `make-obsolete'; this has the same effects as for a function. An alias for a function or macro can also be marked as obsolete; this makes the alias itself obsolete, not the function or macro which it resolves to. -- Function: make-obsolete obsolete-name current-name &optional when This function marks OBSOLETE-NAME as obsolete. OBSOLETE-NAME should be a symbol naming a function or macro, or an alias for a function or macro. If CURRENT-NAME is a symbol, the warning message says to use CURRENT-NAME instead of OBSOLETE-NAME. CURRENT-NAME does not need to be an alias for OBSOLETE-NAME; it can be a different function with similar functionality. CURRENT-NAME can also be a string, which serves as the warning message. The message should begin in lower case, and end with a period. It can also be `nil', in which case the warning message provides no additional details. If provided, WHEN should be a string indicating when the function was first made obsolete--for example, a date or a release number. -- Macro: define-obsolete-function-alias obsolete-name current-name &optional when doc This convenience macro marks the function OBSOLETE-NAME obsolete and also defines it as an alias for the function CURRENT-NAME. It is equivalent to the following: (defalias OBSOLETE-NAME CURRENT-NAME DOC) (make-obsolete OBSOLETE-NAME CURRENT-NAME WHEN) In addition, you can mark a certain a particular calling convention for a function as obsolete: -- Function: set-advertised-calling-convention function signature when This function specifies the argument list SIGNATURE as the correct way to call FUNCTION. This causes the Emacs byte compiler to issue a warning whenever it comes across an Emacs Lisp program that calls FUNCTION any other way (however, it will still allow the code to be byte compiled). WHEN should be a string indicating when the variable was first made obsolete (usually a version number string). For instance, in old versions of Emacs the `sit-for' function accepted three arguments, like this (sit-for seconds milliseconds nodisp) However, calling `sit-for' this way is considered obsolete (*note Waiting::). The old calling convention is deprecated like this: (set-advertised-calling-convention 'sit-for '(seconds &optional nodisp) "22.1") 12.11 Inline Functions ====================== An "inline function" is a function that works just like an ordinary function, except for one thing: when you byte-compile a call to the function (*note Byte Compilation::), the function's definition is expanded into the caller. To define an inline function, use `defsubst' instead of `defun'. -- Macro: defsubst name args [doc] [declare] [interactive] body... This macro defines an inline function. Its syntax is exactly the same as `defun' (*note Defining Functions::). Making a function inline often makes its function calls run faster. But it also has disadvantages. For one thing, it reduces flexibility; if you change the definition of the function, calls already inlined still use the old definition until you recompile them. Another disadvantage is that making a large function inline can increase the size of compiled code both in files and in memory. Since the speed advantage of inline functions is greatest for small functions, you generally should not make large functions inline. Also, inline functions do not behave well with respect to debugging, tracing, and advising (*note Advising Functions::). Since ease of debugging and the flexibility of redefining functions are important features of Emacs, you should not make a function inline, even if it's small, unless its speed is really crucial, and you've timed the code to verify that using `defun' actually has performance problems. It's possible to define a macro to expand into the same code that an inline function would execute (*note Macros::). But the macro would be limited to direct use in expressions--a macro cannot be called with `apply', `mapcar' and so on. Also, it takes some work to convert an ordinary function into a macro. To convert it into an inline function is easy; just replace `defun' with `defsubst'. Since each argument of an inline function is evaluated exactly once, you needn't worry about how many times the body uses the arguments, as you do for macros. After an inline function is defined, its inline expansion can be performed later on in the same file, just like macros. 12.12 The `declare' Form ======================== `declare' is a special macro which can be used to add "meta" properties to a function or macro: for example, marking it as obsolete, or giving its forms a special indentation convention in Emacs Lisp mode. -- Macro: declare specs... This macro ignores its arguments and evaluates to `nil'; it has no run-time effect. However, when a `declare' form occurs in the DECLARE argument of a `defun' or `defsubst' function definition (*note Defining Functions::) or a `defmacro' macro definition (*note Defining Macros::), it appends the properties specified by SPECS to the function or macro. This work is specially performed by `defun', `defsubst', and `defmacro'. Each element in SPECS should have the form `(PROPERTY ARGS...)', which should not be quoted. These have the following effects: `(advertised-calling-convention SIGNATURE WHEN)' This acts like a call to `set-advertised-calling-convention' (*note Obsolete Functions::); SIGNATURE specifies the correct argument list for calling the function or macro, and WHEN should be a string indicating when the variable was first made obsolete. `(debug EDEBUG-FORM-SPEC)' This is valid for macros only. When stepping through the macro with Edebug, use EDEBUG-FORM-SPEC. *Note Instrumenting Macro Calls::. `(doc-string N)' Use element number N, if any, as the documentation string. `(indent INDENT-SPEC)' Indent calls to this function or macro according to INDENT-SPEC. This is typically used for macros, though it works for functions too. *Note Indenting Macros::. `(obsolete CURRENT-NAME WHEN)' Mark the function or macro as obsolete, similar to a call to `make-obsolete' (*note Obsolete Functions::). CURRENT-NAME should be a symbol (in which case the warning message says to use that instead), a string (specifying the warning message), or `nil' (in which case the warning message gives no extra details). WHEN should be a string indicating when the function or macro was first made obsolete. 12.13 Telling the Compiler that a Function is Defined ===================================================== Byte-compiling a file often produces warnings about functions that the compiler doesn't know about (*note Compiler Errors::). Sometimes this indicates a real problem, but usually the functions in question are defined in other files which would be loaded if that code is run. For example, byte-compiling `fortran.el' used to warn: In end of data: fortran.el:2152:1:Warning: the function `gud-find-c-expr' is not known to be defined. In fact, `gud-find-c-expr' is only used in the function that Fortran mode uses for the local value of `gud-find-expr-function', which is a callback from GUD; if it is called, the GUD functions will be loaded. When you know that such a warning does not indicate a real problem, it is good to suppress the warning. That makes new warnings which might mean real problems more visible. You do that with `declare-function'. All you need to do is add a `declare-function' statement before the first use of the function in question: (declare-function gud-find-c-expr "gud.el" nil) This says that `gud-find-c-expr' is defined in `gud.el' (the `.el' can be omitted). The compiler takes for granted that that file really defines the function, and does not check. The optional third argument specifies the argument list of `gud-find-c-expr'. In this case, it takes no arguments (`nil' is different from not specifying a value). In other cases, this might be something like `(file &optional overwrite)'. You don't have to specify the argument list, but if you do the byte compiler can check that the calls match the declaration. -- Macro: declare-function function file &optional arglist fileonly Tell the byte compiler to assume that FUNCTION is defined, with arguments ARGLIST, and that the definition should come from the file FILE. FILEONLY non-`nil' means only check that FILE exists, not that it actually defines FUNCTION. To verify that these functions really are declared where `declare-function' says they are, use `check-declare-file' to check all `declare-function' calls in one source file, or use `check-declare-directory' check all the files in and under a certain directory. These commands find the file that ought to contain a function's definition using `locate-library'; if that finds no file, they expand the definition file name relative to the directory of the file that contains the `declare-function' call. You can also say that a function is a primitive by specifying a file name ending in `.c' or `.m'. This is useful only when you call a primitive that is defined only on certain systems. Most primitives are always defined, so they will never give you a warning. Sometimes a file will optionally use functions from an external package. If you prefix the filename in the `declare-function' statement with `ext:', then it will be checked if it is found, otherwise skipped without error. There are some function definitions that `check-declare' does not understand (e.g., `defstruct' and some other macros). In such cases, you can pass a non-`nil' FILEONLY argument to `declare-function', meaning to only check that the file exists, not that it actually defines the function. Note that to do this without having to specify an argument list, you should set the ARGLIST argument to `t' (because `nil' means an empty argument list, as opposed to an unspecified one). 12.14 Determining whether a Function is Safe to Call ==================================================== Some major modes, such as SES, call functions that are stored in user files. (*note (ses)Top::, for more information on SES.) User files sometimes have poor pedigrees--you can get a spreadsheet from someone you've just met, or you can get one through email from someone you've never met. So it is risky to call a function whose source code is stored in a user file until you have determined that it is safe. -- Function: unsafep form &optional unsafep-vars Returns `nil' if FORM is a "safe" Lisp expression, or returns a list that describes why it might be unsafe. The argument UNSAFEP-VARS is a list of symbols known to have temporary bindings at this point; it is mainly used for internal recursive calls. The current buffer is an implicit argument, which provides a list of buffer-local bindings. Being quick and simple, `unsafep' does a very light analysis and rejects many Lisp expressions that are actually safe. There are no known cases where `unsafep' returns `nil' for an unsafe expression. However, a "safe" Lisp expression can return a string with a `display' property, containing an associated Lisp expression to be executed after the string is inserted into a buffer. This associated expression can be a virus. In order to be safe, you must delete properties from all strings calculated by user code before inserting them into buffers. 12.15 Other Topics Related to Functions ======================================= Here is a table of several functions that do things related to function calling and function definitions. They are documented elsewhere, but we provide cross references here. `apply' See *note Calling Functions::. `autoload' See *note Autoload::. `call-interactively' See *note Interactive Call::. `called-interactively-p' See *note Distinguish Interactive::. `commandp' See *note Interactive Call::. `documentation' See *note Accessing Documentation::. `eval' See *note Eval::. `funcall' See *note Calling Functions::. `function' See *note Anonymous Functions::. `ignore' See *note Calling Functions::. `indirect-function' See *note Function Indirection::. `interactive' See *note Using Interactive::. `interactive-p' See *note Distinguish Interactive::. `mapatoms' See *note Creating Symbols::. `mapcar' See *note Mapping Functions::. `map-char-table' See *note Char-Tables::. `mapconcat' See *note Mapping Functions::. `undefined' See *note Functions for Key Lookup::. 13 Macros ********* "Macros" enable you to define new control constructs and other language features. A macro is defined much like a function, but instead of telling how to compute a value, it tells how to compute another Lisp expression which will in turn compute the value. We call this expression the "expansion" of the macro. Macros can do this because they operate on the unevaluated expressions for the arguments, not on the argument values as functions do. They can therefore construct an expansion containing these argument expressions or parts of them. If you are using a macro to do something an ordinary function could do, just for the sake of speed, consider using an inline function instead. *Note Inline Functions::. 13.1 A Simple Example of a Macro ================================ Suppose we would like to define a Lisp construct to increment a variable value, much like the `++' operator in C. We would like to write `(inc x)' and have the effect of `(setq x (1+ x))'. Here's a macro definition that does the job: (defmacro inc (var) (list 'setq var (list '1+ var))) When this is called with `(inc x)', the argument VAR is the symbol `x'--_not_ the _value_ of `x', as it would be in a function. The body of the macro uses this to construct the expansion, which is `(setq x (1+ x))'. Once the macro definition returns this expansion, Lisp proceeds to evaluate it, thus incrementing `x'. 13.2 Expansion of a Macro Call ============================== A macro call looks just like a function call in that it is a list which starts with the name of the macro. The rest of the elements of the list are the arguments of the macro. Evaluation of the macro call begins like evaluation of a function call except for one crucial difference: the macro arguments are the actual expressions appearing in the macro call. They are not evaluated before they are given to the macro definition. By contrast, the arguments of a function are results of evaluating the elements of the function call list. Having obtained the arguments, Lisp invokes the macro definition just as a function is invoked. The argument variables of the macro are bound to the argument values from the macro call, or to a list of them in the case of a `&rest' argument. And the macro body executes and returns its value just as a function body does. The second crucial difference between macros and functions is that the value returned by the macro body is an alternate Lisp expression, also known as the "expansion" of the macro. The Lisp interpreter proceeds to evaluate the expansion as soon as it comes back from the macro. Since the expansion is evaluated in the normal manner, it may contain calls to other macros. It may even be a call to the same macro, though this is unusual. Note that Emacs tries to expand macros when loading an uncompiled Lisp file. This is not always possible, but if it is, it speeds up subsequent execution. *Note How Programs Do Loading::. You can see the expansion of a given macro call by calling `macroexpand'. -- Function: macroexpand form &optional environment This function expands FORM, if it is a macro call. If the result is another macro call, it is expanded in turn, until something which is not a macro call results. That is the value returned by `macroexpand'. If FORM is not a macro call to begin with, it is returned as given. Note that `macroexpand' does not look at the subexpressions of FORM (although some macro definitions may do so). Even if they are macro calls themselves, `macroexpand' does not expand them. The function `macroexpand' does not expand calls to inline functions. Normally there is no need for that, since a call to an inline function is no harder to understand than a call to an ordinary function. If ENVIRONMENT is provided, it specifies an alist of macro definitions that shadow the currently defined macros. Byte compilation uses this feature. (defmacro inc (var) (list 'setq var (list '1+ var))) (macroexpand '(inc r)) => (setq r (1+ r)) (defmacro inc2 (var1 var2) (list 'progn (list 'inc var1) (list 'inc var2))) (macroexpand '(inc2 r s)) => (progn (inc r) (inc s)) ; `inc' not expanded here. -- Function: macroexpand-all form &optional environment `macroexpand-all' expands macros like `macroexpand', but will look for and expand all macros in FORM, not just at the top-level. If no macros are expanded, the return value is `eq' to FORM. Repeating the example used for `macroexpand' above with `macroexpand-all', we see that `macroexpand-all' _does_ expand the embedded calls to `inc': (macroexpand-all '(inc2 r s)) => (progn (setq r (1+ r)) (setq s (1+ s))) 13.3 Macros and Byte Compilation ================================ You might ask why we take the trouble to compute an expansion for a macro and then evaluate the expansion. Why not have the macro body produce the desired results directly? The reason has to do with compilation. When a macro call appears in a Lisp program being compiled, the Lisp compiler calls the macro definition just as the interpreter would, and receives an expansion. But instead of evaluating this expansion, it compiles the expansion as if it had appeared directly in the program. As a result, the compiled code produces the value and side effects intended for the macro, but executes at full compiled speed. This would not work if the macro body computed the value and side effects itself--they would be computed at compile time, which is not useful. In order for compilation of macro calls to work, the macros must already be defined in Lisp when the calls to them are compiled. The compiler has a special feature to help you do this: if a file being compiled contains a `defmacro' form, the macro is defined temporarily for the rest of the compilation of that file. Byte-compiling a file also executes any `require' calls at top-level in the file, so you can ensure that necessary macro definitions are available during compilation by requiring the files that define them (*note Named Features::). To avoid loading the macro definition files when someone _runs_ the compiled program, write `eval-when-compile' around the `require' calls (*note Eval During Compile::). 13.4 Defining Macros ==================== A Lisp macro object is a list whose CAR is `macro', and whose CDR is a lambda expression. Expansion of the macro works by applying the lambda expression (with `apply') to the list of _unevaluated_ arguments from the macro call. It is possible to use an anonymous Lisp macro just like an anonymous function, but this is never done, because it does not make sense to pass an anonymous macro to functionals such as `mapcar'. In practice, all Lisp macros have names, and they are almost always defined with the `defmacro' macro. -- Macro: defmacro name args [doc] [declare] body... `defmacro' defines the symbol NAME (which should not be quoted) as a macro that looks like this: (macro lambda ARGS . BODY) (Note that the CDR of this list is a lambda expression.) This macro object is stored in the function cell of NAME. The meaning of ARGS is the same as in a function, and the keywords `&rest' and `&optional' may be used (*note Argument List::). Neither NAME nor ARGS should be quoted. The return value of `defmacro' is undefined. DOC, if present, should be a string specifying the macro's documentation string. DECLARE, if present, should be a `declare' form specifying metadata for the macro (*note Declare Form::). Note that macros cannot have interactive declarations, since they cannot be called interactively. Macros often need to construct large list structures from a mixture of constants and nonconstant parts. To make this easier, use the ``' syntax (*note Backquote::). For example: (defmacro t-becomes-nil (variable) `(if (eq ,variable t) (setq ,variable nil))) (t-becomes-nil foo) == (if (eq foo t) (setq foo nil)) The body of a macro definition can include a `declare' form, which specifies additional properties about the macro. *Note Declare Form::. 13.5 Common Problems Using Macros ================================= Macro expansion can have counterintuitive consequences. This section describes some important consequences that can lead to trouble, and rules to follow to avoid trouble. 13.5.1 Wrong Time ----------------- The most common problem in writing macros is doing some of the real work prematurely--while expanding the macro, rather than in the expansion itself. For instance, one real package had this macro definition: (defmacro my-set-buffer-multibyte (arg) (if (fboundp 'set-buffer-multibyte) (set-buffer-multibyte arg))) With this erroneous macro definition, the program worked fine when interpreted but failed when compiled. This macro definition called `set-buffer-multibyte' during compilation, which was wrong, and then did nothing when the compiled package was run. The definition that the programmer really wanted was this: (defmacro my-set-buffer-multibyte (arg) (if (fboundp 'set-buffer-multibyte) `(set-buffer-multibyte ,arg))) This macro expands, if appropriate, into a call to `set-buffer-multibyte' that will be executed when the compiled program is actually run. 13.5.2 Evaluating Macro Arguments Repeatedly -------------------------------------------- When defining a macro you must pay attention to the number of times the arguments will be evaluated when the expansion is executed. The following macro (used to facilitate iteration) illustrates the problem. This macro allows us to write a "for" loop construct. (defmacro for (var from init to final do &rest body) "Execute a simple \"for\" loop. For example, (for i from 1 to 10 do (print i))." (list 'let (list (list var init)) (cons 'while (cons (list '<= var final) (append body (list (list 'inc var))))))) (for i from 1 to 3 do (setq square (* i i)) (princ (format "\n%d %d" i square))) ==> (let ((i 1)) (while (<= i 3) (setq square (* i i)) (princ (format "\n%d %d" i square)) (inc i))) -|1 1 -|2 4 -|3 9 => nil The arguments `from', `to', and `do' in this macro are "syntactic sugar"; they are entirely ignored. The idea is that you will write noise words (such as `from', `to', and `do') in those positions in the macro call. Here's an equivalent definition simplified through use of backquote: (defmacro for (var from init to final do &rest body) "Execute a simple \"for\" loop. For example, (for i from 1 to 10 do (print i))." `(let ((,var ,init)) (while (<= ,var ,final) ,@body (inc ,var)))) Both forms of this definition (with backquote and without) suffer from the defect that FINAL is evaluated on every iteration. If FINAL is a constant, this is not a problem. If it is a more complex form, say `(long-complex-calculation x)', this can slow down the execution significantly. If FINAL has side effects, executing it more than once is probably incorrect. A well-designed macro definition takes steps to avoid this problem by producing an expansion that evaluates the argument expressions exactly once unless repeated evaluation is part of the intended purpose of the macro. Here is a correct expansion for the `for' macro: (let ((i 1) (max 3)) (while (<= i max) (setq square (* i i)) (princ (format "%d %d" i square)) (inc i))) Here is a macro definition that creates this expansion: (defmacro for (var from init to final do &rest body) "Execute a simple for loop: (for i from 1 to 10 do (print i))." `(let ((,var ,init) (max ,final)) (while (<= ,var max) ,@body (inc ,var)))) Unfortunately, this fix introduces another problem, described in the following section. 13.5.3 Local Variables in Macro Expansions ------------------------------------------ In the previous section, the definition of `for' was fixed as follows to make the expansion evaluate the macro arguments the proper number of times: (defmacro for (var from init to final do &rest body) "Execute a simple for loop: (for i from 1 to 10 do (print i))." `(let ((,var ,init) (max ,final)) (while (<= ,var max) ,@body (inc ,var)))) The new definition of `for' has a new problem: it introduces a local variable named `max' which the user does not expect. This causes trouble in examples such as the following: (let ((max 0)) (for x from 0 to 10 do (let ((this (frob x))) (if (< max this) (setq max this))))) The references to `max' inside the body of the `for', which are supposed to refer to the user's binding of `max', really access the binding made by `for'. The way to correct this is to use an uninterned symbol instead of `max' (*note Creating Symbols::). The uninterned symbol can be bound and referred to just like any other symbol, but since it is created by `for', we know that it cannot already appear in the user's program. Since it is not interned, there is no way the user can put it into the program later. It will never appear anywhere except where put by `for'. Here is a definition of `for' that works this way: (defmacro for (var from init to final do &rest body) "Execute a simple for loop: (for i from 1 to 10 do (print i))." (let ((tempvar (make-symbol "max"))) `(let ((,var ,init) (,tempvar ,final)) (while (<= ,var ,tempvar) ,@body (inc ,var))))) This creates an uninterned symbol named `max' and puts it in the expansion instead of the usual interned symbol `max' that appears in expressions ordinarily. 13.5.4 Evaluating Macro Arguments in Expansion ---------------------------------------------- Another problem can happen if the macro definition itself evaluates any of the macro argument expressions, such as by calling `eval' (*note Eval::). If the argument is supposed to refer to the user's variables, you may have trouble if the user happens to use a variable with the same name as one of the macro arguments. Inside the macro body, the macro argument binding is the most local binding of this variable, so any references inside the form being evaluated do refer to it. Here is an example: (defmacro foo (a) (list 'setq (eval a) t)) (setq x 'b) (foo x) ==> (setq b t) => t ; and `b' has been set. ;; but (setq a 'c) (foo a) ==> (setq a t) => t ; but this set `a', not `c'. It makes a difference whether the user's variable is named `a' or `x', because `a' conflicts with the macro argument variable `a'. Another problem with calling `eval' in a macro definition is that it probably won't do what you intend in a compiled program. The byte compiler runs macro definitions while compiling the program, when the program's own computations (which you might have wished to access with `eval') don't occur and its local variable bindings don't exist. To avoid these problems, *don't evaluate an argument expression while computing the macro expansion*. Instead, substitute the expression into the macro expansion, so that its value will be computed as part of executing the expansion. This is how the other examples in this chapter work. 13.5.5 How Many Times is the Macro Expanded? -------------------------------------------- Occasionally problems result from the fact that a macro call is expanded each time it is evaluated in an interpreted function, but is expanded only once (during compilation) for a compiled function. If the macro definition has side effects, they will work differently depending on how many times the macro is expanded. Therefore, you should avoid side effects in computation of the macro expansion, unless you really know what you are doing. One special kind of side effect can't be avoided: constructing Lisp objects. Almost all macro expansions include constructed lists; that is the whole point of most macros. This is usually safe; there is just one case where you must be careful: when the object you construct is part of a quoted constant in the macro expansion. If the macro is expanded just once, in compilation, then the object is constructed just once, during compilation. But in interpreted execution, the macro is expanded each time the macro call runs, and this means a new object is constructed each time. In most clean Lisp code, this difference won't matter. It can matter only if you perform side-effects on the objects constructed by the macro definition. Thus, to avoid trouble, *avoid side effects on objects constructed by macro definitions*. Here is an example of how such side effects can get you into trouble: (defmacro empty-object () (list 'quote (cons nil nil))) (defun initialize (condition) (let ((object (empty-object))) (if condition (setcar object condition)) object)) If `initialize' is interpreted, a new list `(nil)' is constructed each time `initialize' is called. Thus, no side effect survives between calls. If `initialize' is compiled, then the macro `empty-object' is expanded during compilation, producing a single "constant" `(nil)' that is reused and altered each time `initialize' is called. One way to avoid pathological cases like this is to think of `empty-object' as a funny kind of constant, not as a memory allocation construct. You wouldn't use `setcar' on a constant such as `'(nil)', so naturally you won't use it on `(empty-object)' either. 13.6 Indenting Macros ===================== Within a macro definition, you can use the `declare' form (*note Defining Macros::) to specify how should indent calls to the macro. An indentation specification is written like this: (declare (indent INDENT-SPEC)) Here are the possibilities for INDENT-SPEC: `nil' This is the same as no property--use the standard indentation pattern. `defun' Handle this function like a `def' construct: treat the second line as the start of a "body". an integer, NUMBER The first NUMBER arguments of the function are "distinguished" arguments; the rest are considered the body of the expression. A line in the expression is indented according to whether the first argument on it is distinguished or not. If the argument is part of the body, the line is indented `lisp-body-indent' more columns than the open-parenthesis starting the containing expression. If the argument is distinguished and is either the first or second argument, it is indented _twice_ that many extra columns. If the argument is distinguished and not the first or second argument, the line uses the standard pattern. a symbol, SYMBOL SYMBOL should be a function name; that function is called to calculate the indentation of a line within this expression. The function receives two arguments: POS The position at which the line being indented begins. STATE The value returned by `parse-partial-sexp' (a Lisp primitive for indentation and nesting computation) when it parses up to the beginning of this line. It should return either a number, which is the number of columns of indentation for that line, or a list whose car is such a number. The difference between returning a number and returning a list is that a number says that all following lines at the same nesting level should be indented just like this one; a list says that following lines might call for different indentations. This makes a difference when the indentation is being computed by `C-M-q'; if the value is a number, `C-M-q' need not recalculate indentation for the following lines until the end of the list. 14 Customization Settings ************************* Users of Emacs can customize variables and faces without writing Lisp code, by using the Customize interface. *Note Easy Customization: (emacs)Easy Customization. This chapter describes how to define "customization items" that users can interact with through the Customize interface. Customization items include customizable variables, which are defined with the `defcustom' macro (*note Variable Definitions::); customizable faces, which are defined with `defface' (described separately in *note Defining Faces::); and "customization groups", defined with `defgroup' (*note Group Definitions::), which act as containers for groups of related customization items. 14.1 Common Item Keywords ========================= The customization declarations that we will describe in the next few sections--`defcustom', `defgroup', etc.--all accept keyword arguments (*note Constant Variables::) for specifying various information. This section describes keywords that apply to all types of customization declarations. All of these keywords, except `:tag', can be used more than once in a given item. Each use of the keyword has an independent effect. The keyword `:tag' is an exception because any given item can only display one name. `:tag LABEL' Use LABEL, a string, instead of the item's name, to label the item in customization menus and buffers. *Don't use a tag which is substantially different from the item's real name; that would cause confusion.* `:group GROUP' Put this customization item in group GROUP. When you use `:group' in a `defgroup', it makes the new group a subgroup of GROUP. If you use this keyword more than once, you can put a single item into more than one group. Displaying any of those groups will show this item. Please don't overdo this, since the result would be annoying. `:link LINK-DATA' Include an external link after the documentation string for this item. This is a sentence containing a button that references some other documentation. There are several alternatives you can use for LINK-DATA: `(custom-manual INFO-NODE)' Link to an Info node; INFO-NODE is a string which specifies the node name, as in `"(emacs)Top"'. The link appears as `[Manual]' in the customization buffer and enters the built-in Info reader on INFO-NODE. `(info-link INFO-NODE)' Like `custom-manual' except that the link appears in the customization buffer with the Info node name. `(url-link URL)' Link to a web page; URL is a string which specifies the URL. The link appears in the customization buffer as URL and invokes the WWW browser specified by `browse-url-browser-function'. `(emacs-commentary-link LIBRARY)' Link to the commentary section of a library; LIBRARY is a string which specifies the library name. *Note Library Headers::. `(emacs-library-link LIBRARY)' Link to an Emacs Lisp library file; LIBRARY is a string which specifies the library name. `(file-link FILE)' Link to a file; FILE is a string which specifies the name of the file to visit with `find-file' when the user invokes this link. `(function-link FUNCTION)' Link to the documentation of a function; FUNCTION is a string which specifies the name of the function to describe with `describe-function' when the user invokes this link. `(variable-link VARIABLE)' Link to the documentation of a variable; VARIABLE is a string which specifies the name of the variable to describe with `describe-variable' when the user invokes this link. `(custom-group-link GROUP)' Link to another customization group. Invoking it creates a new customization buffer for GROUP. You can specify the text to use in the customization buffer by adding `:tag NAME' after the first element of the LINK-DATA; for example, `(info-link :tag "foo" "(emacs)Top")' makes a link to the Emacs manual which appears in the buffer as `foo'. You can use this keyword more than once, to add multiple links. `:load FILE' Load file FILE (a string) before displaying this customization item (*note Loading::). Loading is done with `load', and only if the file is not already loaded. `:require FEATURE' Execute `(require 'FEATURE)' when your saved customizations set the value of this item. FEATURE should be a symbol. The most common reason to use `:require' is when a variable enables a feature such as a minor mode, and just setting the variable won't have any effect unless the code which implements the mode is loaded. `:version VERSION' This keyword specifies that the item was first introduced in Emacs version VERSION, or that its default value was changed in that version. The value VERSION must be a string. `:package-version '(PACKAGE . VERSION)' This keyword specifies that the item was first introduced in PACKAGE version VERSION, or that its meaning or default value was changed in that version. This keyword takes priority over `:version'. PACKAGE should be the official name of the package, as a symbol (e.g., `MH-E'). VERSION should be a string. If the package PACKAGE is released as part of Emacs, PACKAGE and VERSION should appear in the value of `customize-package-emacs-version-alist'. Packages distributed as part of Emacs that use the `:package-version' keyword must also update the `customize-package-emacs-version-alist' variable. -- Variable: customize-package-emacs-version-alist This alist provides a mapping for the versions of Emacs that are associated with versions of a package listed in the `:package-version' keyword. Its elements are: (PACKAGE (PVERSION . EVERSION)...) For each PACKAGE, which is a symbol, there are one or more elements that contain a package version PVERSION with an associated Emacs version EVERSION. These versions are strings. For example, the MH-E package updates this alist with the following: (add-to-list 'customize-package-emacs-version-alist '(MH-E ("6.0" . "22.1") ("6.1" . "22.1") ("7.0" . "22.1") ("7.1" . "22.1") ("7.2" . "22.1") ("7.3" . "22.1") ("7.4" . "22.1") ("8.0" . "22.1"))) The value of PACKAGE needs to be unique and it needs to match the PACKAGE value appearing in the `:package-version' keyword. Since the user might see the value in an error message, a good choice is the official name of the package, such as MH-E or Gnus. 14.2 Defining Customization Groups ================================== Each Emacs Lisp package should have one main customization group which contains all the options, faces and other groups in the package. If the package has a small number of options and faces, use just one group and put everything in it. When there are more than twenty or so options and faces, then you should structure them into subgroups, and put the subgroups under the package's main customization group. It is OK to put some of the options and faces in the package's main group alongside the subgroups. The package's main or only group should be a member of one or more of the standard customization groups. (To display the full list of them, use `M-x customize'.) Choose one or more of them (but not too many), and add your group to each of them using the `:group' keyword. The way to declare new customization groups is with `defgroup'. -- Macro: defgroup group members doc [keyword value]... Declare GROUP as a customization group containing MEMBERS. Do not quote the symbol GROUP. The argument DOC specifies the documentation string for the group. The argument MEMBERS is a list specifying an initial set of customization items to be members of the group. However, most often MEMBERS is `nil', and you specify the group's members by using the `:group' keyword when defining those members. If you want to specify group members through MEMBERS, each element should have the form `(NAME WIDGET)'. Here NAME is a symbol, and WIDGET is a widget type for editing that symbol. Useful widgets are `custom-variable' for a variable, `custom-face' for a face, and `custom-group' for a group. When you introduce a new group into Emacs, use the `:version' keyword in the `defgroup'; then you need not use it for the individual members of the group. In addition to the common keywords (*note Common Keywords::), you can also use this keyword in `defgroup': `:prefix PREFIX' If the name of an item in the group starts with PREFIX, and the customizable variable `custom-unlispify-remove-prefixes' is non-`nil', the item's tag will omit PREFIX. A group can have any number of prefixes. -- User Option: custom-unlispify-remove-prefixes If this variable is non-`nil', the prefixes specified by a group's `:prefix' keyword are omitted from tag names, whenever the user customizes the group. The default value is `nil', i.e., the prefix-discarding feature is disabled. This is because discarding prefixes often leads to confusing names for options and faces. 14.3 Defining Customization Variables ===================================== "Customizable variables", also called "user options", are global Lisp variables whose values can be set through the Customize interface. Unlike other global variables, which are defined with `defvar' (*note Defining Variables::), customizable variables are defined using the `defcustom' macro. In addition to calling `defvar' as a subroutine, `defcustom' states how the variable should be displayed in the Customize interface, the values it is allowed to take, etc. -- Macro: defcustom option standard doc [keyword value]... This macro declares OPTION as a user option (i.e., a customizable variable). You should not quote OPTION. The argument STANDARD is an expression that specifies the standard value for OPTION. Evaluating the `defcustom' form evaluates STANDARD, but does not necessarily install the standard value. If OPTION already has a default value, `defcustom' does not change it. If the user has saved a customization for OPTION, `defcustom' installs the user's customized value as OPTION's default value. If neither of those cases applies, `defcustom' installs the result of evaluating STANDARD as the default value. The expression STANDARD can be evaluated at various other times, too--whenever the customization facility needs to know OPTION's standard value. So be sure to use an expression which is harmless to evaluate at any time. The argument DOC specifies the documentation string for the variable. If a `defcustom' does not specify any `:group', the last group defined with `defgroup' in the same file will be used. This way, most `defcustom' do not need an explicit `:group'. When you evaluate a `defcustom' form with `C-M-x' in Emacs Lisp mode (`eval-defun'), a special feature of `eval-defun' arranges to set the variable unconditionally, without testing whether its value is void. (The same feature applies to `defvar'.) *Note Defining Variables::. If you put a `defcustom' in a pre-loaded Emacs Lisp file (*note Building Emacs::), the standard value installed at dump time might be incorrect, e.g., because another variable that it depends on has not been assigned the right value yet. In that case, use `custom-reevaluate-setting', described below, to re-evaluate the standard value after Emacs starts up. In addition to the keywords listed in *note Common Keywords::, this macro accepts the following keywords: `:type TYPE' Use TYPE as the data type for this option. It specifies which values are legitimate, and how to display the value (*note Customization Types::). `:options VALUE-LIST' Specify the list of reasonable values for use in this option. The user is not restricted to using only these values, but they are offered as convenient alternatives. This is meaningful only for certain types, currently including `hook', `plist' and `alist'. See the definition of the individual types for a description of how to use `:options'. `:set SETFUNCTION' Specify SETFUNCTION as the way to change the value of this option when using the Customize interface. The function SETFUNCTION should take two arguments, a symbol (the option name) and the new value, and should do whatever is necessary to update the value properly for this option (which may not mean simply setting the option as a Lisp variable). The default for SETFUNCTION is `set-default'. If you specify this keyword, the variable's documentation string should describe how to do the same job in hand-written Lisp code. `:get GETFUNCTION' Specify GETFUNCTION as the way to extract the value of this option. The function GETFUNCTION should take one argument, a symbol, and should return whatever customize should use as the "current value" for that symbol (which need not be the symbol's Lisp value). The default is `default-value'. You have to really understand the workings of Custom to use `:get' correctly. It is meant for values that are treated in Custom as variables but are not actually stored in Lisp variables. It is almost surely a mistake to specify GETFUNCTION for a value that really is stored in a Lisp variable. `:initialize FUNCTION' FUNCTION should be a function used to initialize the variable when the `defcustom' is evaluated. It should take two arguments, the option name (a symbol) and the value. Here are some predefined functions meant for use in this way: `custom-initialize-set' Use the variable's `:set' function to initialize the variable, but do not reinitialize it if it is already non-void. `custom-initialize-default' Like `custom-initialize-set', but use the function `set-default' to set the variable, instead of the variable's `:set' function. This is the usual choice for a variable whose `:set' function enables or disables a minor mode; with this choice, defining the variable will not call the minor mode function, but customizing the variable will do so. `custom-initialize-reset' Always use the `:set' function to initialize the variable. If the variable is already non-void, reset it by calling the `:set' function using the current value (returned by the `:get' method). This is the default `:initialize' function. `custom-initialize-changed' Use the `:set' function to initialize the variable, if it is already set or has been customized; otherwise, just use `set-default'. `custom-initialize-safe-set' `custom-initialize-safe-default' These functions behave like `custom-initialize-set' (`custom-initialize-default', respectively), but catch errors. If an error occurs during initialization, they set the variable to `nil' using `set-default', and signal no error. These functions are meant for options defined in pre-loaded files, where the STANDARD expression may signal an error because some required variable or function is not yet defined. The value normally gets updated in `startup.el', ignoring the value computed by `defcustom'. After startup, if one unsets the value and reevaluates the `defcustom', the STANDARD expression can be evaluated without error. `:risky VALUE' Set the variable's `risky-local-variable' property to VALUE (*note File Local Variables::). `:safe FUNCTION' Set the variable's `safe-local-variable' property to FUNCTION (*note File Local Variables::). `:set-after VARIABLES' When setting variables according to saved customizations, make sure to set the variables VARIABLES before this one; i.e., delay setting this variable until after those others have been handled. Use `:set-after' if setting this variable won't work properly unless those other variables already have their intended values. It is useful to specify the `:require' keyword for an option that "turns on" a certain feature. This causes Emacs to load the feature, if it is not already loaded, whenever the option is set. *Note Common Keywords::. Here is an example, from the library `saveplace.el': (defcustom save-place nil "Non-nil means automatically save place in each file..." :type 'boolean :require 'saveplace :group 'save-place) If a customization item has a type such as `hook' or `alist', which supports `:options', you can add additional values to the list from outside the `defcustom' declaration by calling `custom-add-frequent-value'. For example, if you define a function `my-lisp-mode-initialization' intended to be called from `emacs-lisp-mode-hook', you might want to add that to the list of reasonable values for `emacs-lisp-mode-hook', but not by editing its definition. You can do it thus: (custom-add-frequent-value 'emacs-lisp-mode-hook 'my-lisp-mode-initialization) -- Function: custom-add-frequent-value symbol value For the customization option SYMBOL, add VALUE to the list of reasonable values. The precise effect of adding a value depends on the customization type of SYMBOL. Internally, `defcustom' uses the symbol property `standard-value' to record the expression for the standard value, `saved-value' to record the value saved by the user with the customization buffer, and `customized-value' to record the value set by the user with the customization buffer, but not saved. *Note Symbol Properties::. These properties are lists, the car of which is an expression that evaluates to the value. -- Function: custom-reevaluate-setting symbol This function re-evaluates the standard value of SYMBOL, which should be a user option declared via `defcustom'. If the variable was customized, this function re-evaluates the saved value instead. Then it sets the user option to that value (using the option's `:set' property if that is defined). This is useful for customizable options that are defined before their value could be computed correctly. For example, during startup Emacs calls this function for some user options that were defined in pre-loaded Emacs Lisp files, but whose initial values depend on information available only at run-time. -- Function: custom-variable-p arg This function returns non-`nil' if ARG is a customizable variable. A customizable variable is either a variable that has a `standard-value' or `custom-autoload' property (usually meaning it was declared with `defcustom'), or an alias for another customizable variable. 14.4 Customization Types ======================== When you define a user option with `defcustom', you must specify its "customization type". That is a Lisp object which describes (1) which values are legitimate and (2) how to display the value in the customization buffer for editing. You specify the customization type in `defcustom' with the `:type' keyword. The argument of `:type' is evaluated, but only once when the `defcustom' is executed, so it isn't useful for the value to vary. Normally we use a quoted constant. For example: (defcustom diff-command "diff" "The command to use to run diff." :type '(string) :group 'diff) In general, a customization type is a list whose first element is a symbol, one of the customization type names defined in the following sections. After this symbol come a number of arguments, depending on the symbol. Between the type symbol and its arguments, you can optionally write keyword-value pairs (*note Type Keywords::). Some type symbols do not use any arguments; those are called "simple types". For a simple type, if you do not use any keyword-value pairs, you can omit the parentheses around the type symbol. For example just `string' as a customization type is equivalent to `(string)'. All customization types are implemented as widgets; see *note Introduction: (widget)Top, for details. 14.4.1 Simple Types ------------------- This section describes all the simple customization types. For several of these customization types, the customization widget provides inline completion with `C-M-i' or `M-'. `sexp' The value may be any Lisp object that can be printed and read back. You can use `sexp' as a fall-back for any option, if you don't want to take the time to work out a more specific type to use. `integer' The value must be an integer. `number' The value must be a number (floating point or integer). `float' The value must be a floating point number. `string' The value must be a string. The customization buffer shows the string without delimiting `"' characters or `\' quotes. `regexp' Like `string' except that the string must be a valid regular expression. `character' The value must be a character code. A character code is actually an integer, but this type shows the value by inserting the character in the buffer, rather than by showing the number. `file' The value must be a file name. The widget provides completion. `(file :must-match t)' The value must be a file name for an existing file. The widget provides completion. `directory' The value must be a directory name. The widget provides completion. `hook' The value must be a list of functions. This customization type is used for hook variables. You can use the `:options' keyword in a hook variable's `defcustom' to specify a list of functions recommended for use in the hook; *Note Variable Definitions::. `symbol' The value must be a symbol. It appears in the customization buffer as the symbol name. The widget provides completion. `function' The value must be either a lambda expression or a function name. The widget provides completion for function names. `variable' The value must be a variable name. The widget provides completion. `face' The value must be a symbol which is a face name. The widget provides completion. `boolean' The value is boolean--either `nil' or `t'. Note that by using `choice' and `const' together (see the next section), you can specify that the value must be `nil' or `t', but also specify the text to describe each value in a way that fits the specific meaning of the alternative. `key-sequence' The value is a key sequence. The customization buffer shows the key sequence using the same syntax as the `kbd' function. *Note Key Sequences::. `coding-system' The value must be a coding-system name, and you can do completion with `M-'. `color' The value must be a valid color name. The widget provides completion for color names, as well as a sample and a button for selecting a color name from a list of color names shown in a `*Colors*' buffer. 14.4.2 Composite Types ---------------------- When none of the simple types is appropriate, you can use composite types, which build new types from other types or from specified data. The specified types or data are called the "arguments" of the composite type. The composite type normally looks like this: (CONSTRUCTOR ARGUMENTS...) but you can also add keyword-value pairs before the arguments, like this: (CONSTRUCTOR {KEYWORD VALUE}... ARGUMENTS...) Here is a table of constructors and how to use them to write composite types: `(cons CAR-TYPE CDR-TYPE)' The value must be a cons cell, its CAR must fit CAR-TYPE, and its CDR must fit CDR-TYPE. For example, `(cons string symbol)' is a customization type which matches values such as `("foo" . foo)'. In the customization buffer, the CAR and CDR are displayed and edited separately, each according to their specified type. `(list ELEMENT-TYPES...)' The value must be a list with exactly as many elements as the ELEMENT-TYPES given; and each element must fit the corresponding ELEMENT-TYPE. For example, `(list integer string function)' describes a list of three elements; the first element must be an integer, the second a string, and the third a function. In the customization buffer, each element is displayed and edited separately, according to the type specified for it. `(group ELEMENT-TYPES...)' This works like `list' except for the formatting of text in the Custom buffer. `list' labels each element value with its tag; `group' does not. `(vector ELEMENT-TYPES...)' Like `list' except that the value must be a vector instead of a list. The elements work the same as in `list'. `(alist :key-type KEY-TYPE :value-type VALUE-TYPE)' The value must be a list of cons-cells, the CAR of each cell representing a key of customization type KEY-TYPE, and the CDR of the same cell representing a value of customization type VALUE-TYPE. The user can add and delete key/value pairs, and edit both the key and the value of each pair. If omitted, KEY-TYPE and VALUE-TYPE default to `sexp'. The user can add any key matching the specified key type, but you can give some keys a preferential treatment by specifying them with the `:options' (see *note Variable Definitions::). The specified keys will always be shown in the customize buffer (together with a suitable value), with a checkbox to include or exclude or disable the key/value pair from the alist. The user will not be able to edit the keys specified by the `:options' keyword argument. The argument to the `:options' keywords should be a list of specifications for reasonable keys in the alist. Ordinarily, they are simply atoms, which stand for themselves. For example: :options '("foo" "bar" "baz") specifies that there are three "known" keys, namely `"foo"', `"bar"' and `"baz"', which will always be shown first. You may want to restrict the value type for specific keys, for example, the value associated with the `"bar"' key can only be an integer. You can specify this by using a list instead of an atom in the list. The first element will specify the key, like before, while the second element will specify the value type. For example: :options '("foo" ("bar" integer) "baz") Finally, you may want to change how the key is presented. By default, the key is simply shown as a `const', since the user cannot change the special keys specified with the `:options' keyword. However, you may want to use a more specialized type for presenting the key, like `function-item' if you know it is a symbol with a function binding. This is done by using a customization type specification instead of a symbol for the key. :options '("foo" ((function-item some-function) integer) "baz") Many alists use lists with two elements, instead of cons cells. For example, (defcustom list-alist '(("foo" 1) ("bar" 2) ("baz" 3)) "Each element is a list of the form (KEY VALUE).") instead of (defcustom cons-alist '(("foo" . 1) ("bar" . 2) ("baz" . 3)) "Each element is a cons-cell (KEY . VALUE).") Because of the way lists are implemented on top of cons cells, you can treat `list-alist' in the example above as a cons cell alist, where the value type is a list with a single element containing the real value. (defcustom list-alist '(("foo" 1) ("bar" 2) ("baz" 3)) "Each element is a list of the form (KEY VALUE)." :type '(alist :value-type (group integer))) The `group' widget is used here instead of `list' only because the formatting is better suited for the purpose. Similarly, you can have alists with more values associated with each key, using variations of this trick: (defcustom person-data '(("brian" 50 t) ("dorith" 55 nil) ("ken" 52 t)) "Alist of basic info about people. Each element has the form (NAME AGE MALE-FLAG)." :type '(alist :value-type (group integer boolean))) `(plist :key-type KEY-TYPE :value-type VALUE-TYPE)' This customization type is similar to `alist' (see above), except that (i) the information is stored as a property list, (*note Property Lists::), and (ii) KEY-TYPE, if omitted, defaults to `symbol' rather than `sexp'. `(choice ALTERNATIVE-TYPES...)' The value must fit one of ALTERNATIVE-TYPES. For example, `(choice integer string)' allows either an integer or a string. In the customization buffer, the user selects an alternative using a menu, and can then edit the value in the usual way for that alternative. Normally the strings in this menu are determined automatically from the choices; however, you can specify different strings for the menu by including the `:tag' keyword in the alternatives. For example, if an integer stands for a number of spaces, while a string is text to use verbatim, you might write the customization type this way, (choice (integer :tag "Number of spaces") (string :tag "Literal text")) so that the menu offers `Number of spaces' and `Literal text'. In any alternative for which `nil' is not a valid value, other than a `const', you should specify a valid default for that alternative using the `:value' keyword. *Note Type Keywords::. If some values are covered by more than one of the alternatives, customize will choose the first alternative that the value fits. This means you should always list the most specific types first, and the most general last. Here's an example of proper usage: (choice (const :tag "Off" nil) symbol (sexp :tag "Other")) This way, the special value `nil' is not treated like other symbols, and symbols are not treated like other Lisp expressions. `(radio ELEMENT-TYPES...)' This is similar to `choice', except that the choices are displayed using `radio buttons' rather than a menu. This has the advantage of displaying documentation for the choices when applicable and so is often a good choice for a choice between constant functions (`function-item' customization types). `(const VALUE)' The value must be VALUE--nothing else is allowed. The main use of `const' is inside of `choice'. For example, `(choice integer (const nil))' allows either an integer or `nil'. `:tag' is often used with `const', inside of `choice'. For example, (choice (const :tag "Yes" t) (const :tag "No" nil) (const :tag "Ask" foo)) describes a variable for which `t' means yes, `nil' means no, and `foo' means "ask". `(other VALUE)' This alternative can match any Lisp value, but if the user chooses this alternative, that selects the value VALUE. The main use of `other' is as the last element of `choice'. For example, (choice (const :tag "Yes" t) (const :tag "No" nil) (other :tag "Ask" foo)) describes a variable for which `t' means yes, `nil' means no, and anything else means "ask". If the user chooses `Ask' from the menu of alternatives, that specifies the value `foo'; but any other value (not `t', `nil' or `foo') displays as `Ask', just like `foo'. `(function-item FUNCTION)' Like `const', but used for values which are functions. This displays the documentation string as well as the function name. The documentation string is either the one you specify with `:doc', or FUNCTION's own documentation string. `(variable-item VARIABLE)' Like `const', but used for values which are variable names. This displays the documentation string as well as the variable name. The documentation string is either the one you specify with `:doc', or VARIABLE's own documentation string. `(set TYPES...)' The value must be a list, and each element of the list must match one of the TYPES specified. This appears in the customization buffer as a checklist, so that each of TYPES may have either one corresponding element or none. It is not possible to specify two different elements that match the same one of TYPES. For example, `(set integer symbol)' allows one integer and/or one symbol in the list; it does not allow multiple integers or multiple symbols. As a result, it is rare to use nonspecific types such as `integer' in a `set'. Most often, the TYPES in a `set' are `const' types, as shown here: (set (const :bold) (const :italic)) Sometimes they describe possible elements in an alist: (set (cons :tag "Height" (const height) integer) (cons :tag "Width" (const width) integer)) That lets the user specify a height value optionally and a width value optionally. `(repeat ELEMENT-TYPE)' The value must be a list and each element of the list must fit the type ELEMENT-TYPE. This appears in the customization buffer as a list of elements, with `[INS]' and `[DEL]' buttons for adding more elements or removing elements. `(restricted-sexp :match-alternatives CRITERIA)' This is the most general composite type construct. The value may be any Lisp object that satisfies one of CRITERIA. CRITERIA should be a list, and each element should be one of these possibilities: * A predicate--that is, a function of one argument that has no side effects, and returns either `nil' or non-`nil' according to the argument. Using a predicate in the list says that objects for which the predicate returns non-`nil' are acceptable. * A quoted constant--that is, `'OBJECT'. This sort of element in the list says that OBJECT itself is an acceptable value. For example, (restricted-sexp :match-alternatives (integerp 't 'nil)) allows integers, `t' and `nil' as legitimate values. The customization buffer shows all legitimate values using their read syntax, and the user edits them textually. Here is a table of the keywords you can use in keyword-value pairs in a composite type: `:tag TAG' Use TAG as the name of this alternative, for user communication purposes. This is useful for a type that appears inside of a `choice'. `:match-alternatives CRITERIA' Use CRITERIA to match possible values. This is used only in `restricted-sexp'. `:args ARGUMENT-LIST' Use the elements of ARGUMENT-LIST as the arguments of the type construct. For instance, `(const :args (foo))' is equivalent to `(const foo)'. You rarely need to write `:args' explicitly, because normally the arguments are recognized automatically as whatever follows the last keyword-value pair. 14.4.3 Splicing into Lists -------------------------- The `:inline' feature lets you splice a variable number of elements into the middle of a `list' or `vector' customization type. You use it by adding `:inline t' to a type specification which is contained in a `list' or `vector' specification. Normally, each entry in a `list' or `vector' type specification describes a single element type. But when an entry contains `:inline t', the value it matches is merged directly into the containing sequence. For example, if the entry matches a list with three elements, those become three elements of the overall sequence. This is analogous to `,@' in a backquote construct (*note Backquote::). For example, to specify a list whose first element must be `baz' and whose remaining arguments should be zero or more of `foo' and `bar', use this customization type: (list (const baz) (set :inline t (const foo) (const bar))) This matches values such as `(baz)', `(baz foo)', `(baz bar)' and `(baz foo bar)'. When the element-type is a `choice', you use `:inline' not in the `choice' itself, but in (some of) the alternatives of the `choice'. For example, to match a list which must start with a file name, followed either by the symbol `t' or two strings, use this customization type: (list file (choice (const t) (list :inline t string string))) If the user chooses the first alternative in the choice, then the overall list has two elements and the second element is `t'. If the user chooses the second alternative, then the overall list has three elements and the second and third must be strings. 14.4.4 Type Keywords -------------------- You can specify keyword-argument pairs in a customization type after the type name symbol. Here are the keywords you can use, and their meanings: `:value DEFAULT' Provide a default value. If `nil' is not a valid value for the alternative, then it is essential to specify a valid default with `:value'. If you use this for a type that appears as an alternative inside of `choice'; it specifies the default value to use, at first, if and when the user selects this alternative with the menu in the customization buffer. Of course, if the actual value of the option fits this alternative, it will appear showing the actual value, not DEFAULT. `:format FORMAT-STRING' This string will be inserted in the buffer to represent the value corresponding to the type. The following `%' escapes are available for use in FORMAT-STRING: `%[BUTTON%]' Display the text BUTTON marked as a button. The `:action' attribute specifies what the button will do if the user invokes it; its value is a function which takes two arguments--the widget which the button appears in, and the event. There is no way to specify two different buttons with different actions. `%{SAMPLE%}' Show SAMPLE in a special face specified by `:sample-face'. `%v' Substitute the item's value. How the value is represented depends on the kind of item, and (for variables) on the customization type. `%d' Substitute the item's documentation string. `%h' Like `%d', but if the documentation string is more than one line, add a button to control whether to show all of it or just the first line. `%t' Substitute the tag here. You specify the tag with the `:tag' keyword. `%%' Display a literal `%'. `:action ACTION' Perform ACTION if the user clicks on a button. `:button-face FACE' Use the face FACE (a face name or a list of face names) for button text displayed with `%[...%]'. `:button-prefix PREFIX' `:button-suffix SUFFIX' These specify the text to display before and after a button. Each can be: `nil' No text is inserted. a string The string is inserted literally. a symbol The symbol's value is used. `:tag TAG' Use TAG (a string) as the tag for the value (or part of the value) that corresponds to this type. `:doc DOC' Use DOC as the documentation string for this value (or part of the value) that corresponds to this type. In order for this to work, you must specify a value for `:format', and use `%d' or `%h' in that value. The usual reason to specify a documentation string for a type is to provide more information about the meanings of alternatives inside a `:choice' type or the parts of some other composite type. `:help-echo MOTION-DOC' When you move to this item with `widget-forward' or `widget-backward', it will display the string MOTION-DOC in the echo area. In addition, MOTION-DOC is used as the mouse `help-echo' string and may actually be a function or form evaluated to yield a help string. If it is a function, it is called with one argument, the widget. `:match FUNCTION' Specify how to decide whether a value matches the type. The corresponding value, FUNCTION, should be a function that accepts two arguments, a widget and a value; it should return non-`nil' if the value is acceptable. `:validate FUNCTION' Specify a validation function for input. FUNCTION takes a widget as an argument, and should return `nil' if the widget's current value is valid for the widget. Otherwise, it should return the widget containing the invalid data, and set that widget's `:error' property to a string explaining the error. 14.4.5 Defining New Types ------------------------- In the previous sections we have described how to construct elaborate type specifications for `defcustom'. In some cases you may want to give such a type specification a name. The obvious case is when you are using the same type for many user options: rather than repeat the specification for each option, you can give the type specification a name, and use that name each `defcustom'. The other case is when a user option's value is a recursive data structure. To make it possible for a datatype to refer to itself, it needs to have a name. Since custom types are implemented as widgets, the way to define a new customize type is to define a new widget. We are not going to describe the widget interface here in details, see *note Introduction: (widget)Top, for that. Instead we are going to demonstrate the minimal functionality needed for defining new customize types by a simple example. (define-widget 'binary-tree-of-string 'lazy "A binary tree made of cons-cells and strings." :offset 4 :tag "Node" :type '(choice (string :tag "Leaf" :value "") (cons :tag "Interior" :value ("" . "") binary-tree-of-string binary-tree-of-string))) (defcustom foo-bar "" "Sample variable holding a binary tree of strings." :type 'binary-tree-of-string) The function to define a new widget is called `define-widget'. The first argument is the symbol we want to make a new widget type. The second argument is a symbol representing an existing widget, the new widget is going to be defined in terms of difference from the existing widget. For the purpose of defining new customization types, the `lazy' widget is perfect, because it accepts a `:type' keyword argument with the same syntax as the keyword argument to `defcustom' with the same name. The third argument is a documentation string for the new widget. You will be able to see that string with the `M-x widget-browse binary-tree-of-string ' command. After these mandatory arguments follow the keyword arguments. The most important is `:type', which describes the data type we want to match with this widget. Here a `binary-tree-of-string' is described as being either a string, or a cons-cell whose car and cdr are themselves both `binary-tree-of-string'. Note the reference to the widget type we are currently in the process of defining. The `:tag' attribute is a string to name the widget in the user interface, and the `:offset' argument is there to ensure that child nodes are indented four spaces relative to the parent node, making the tree structure apparent in the customization buffer. The `defcustom' shows how the new widget can be used as an ordinary customization type. The reason for the name `lazy' is that the other composite widgets convert their inferior widgets to internal form when the widget is instantiated in a buffer. This conversion is recursive, so the inferior widgets will convert _their_ inferior widgets. If the data structure is itself recursive, this conversion is an infinite recursion. The `lazy' widget prevents the recursion: it convert its `:type' argument only when needed. 14.5 Applying Customizations ============================ The following functions are responsible for installing the user's customization settings for variables and faces, respectively. When the user invokes `Save for future sessions' in the Customize interface, that takes effect by writing a `custom-set-variables' and/or a `custom-set-faces' form into the custom file, to be evaluated the next time Emacs starts. -- Function: custom-set-variables &rest args This function installs the variable customizations specified by ARGS. Each argument in ARGS should have the form (VAR EXPRESSION [NOW [REQUEST [COMMENT]]]) VAR is a variable name (a symbol), and EXPRESSION is an expression which evaluates to the desired customized value. If the `defcustom' form for VAR has been evaluated prior to this `custom-set-variables' call, EXPRESSION is immediately evaluated, and the variable's value is set to the result. Otherwise, EXPRESSION is stored into the variable's `saved-value' property, to be evaluated when the relevant `defcustom' is called (usually when the library defining that variable is loaded into Emacs). The NOW, REQUEST, and COMMENT entries are for internal use only, and may be omitted. NOW, if non-`nil', means to set the variable's value now, even if the variable's `defcustom' form has not been evaluated. REQUEST is a list of features to be loaded immediately (*note Named Features::). COMMENT is a string describing the customization. -- Function: custom-set-faces &rest args This function installs the face customizations specified by ARGS. Each argument in ARGS should have the form (FACE SPEC [NOW [COMMENT]]) FACE is a face name (a symbol), and SPEC is the customized face specification for that face (*note Defining Faces::). The NOW and COMMENT entries are for internal use only, and may be omitted. NOW, if non-`nil', means to install the face specification now, even if the `defface' form has not been evaluated. COMMENT is a string describing the customization. 14.6 Custom Themes ================== "Custom themes" are collections of settings that can be enabled or disabled as a unit. *Note Custom Themes: (emacs)Custom Themes. Each Custom theme is defined by an Emacs Lisp source file, which should follow the conventions described in this section. (Instead of writing a Custom theme by hand, you can also create one using a Customize-like interface; *note Creating Custom Themes: (emacs)Creating Custom Themes.) A Custom theme file should be named `FOO-theme.el', where FOO is the theme name. The first Lisp form in the file should be a call to `deftheme', and the last form should be a call to `provide-theme'. -- Macro: deftheme theme &optional doc This macro declares THEME (a symbol) as the name of a Custom theme. The optional argument DOC should be a string describing the theme; this is the description shown when the user invokes the `describe-theme' command or types `?' in the `*Custom Themes*' buffer. Two special theme names are disallowed (using them causes an error): `user' is a "dummy" theme that stores the user's direct customization settings, and `changed' is a "dummy" theme that stores changes made outside of the Customize system. -- Macro: provide-theme theme This macro declares that the theme named THEME has been fully specified. In between `deftheme' and `provide-theme' are Lisp forms specifying the theme settings: usually a call to `custom-theme-set-variables' and/or a call to `custom-theme-set-faces'. -- Function: custom-theme-set-variables theme &rest args This function specifies the Custom theme THEME's variable settings. THEME should be a symbol. Each argument in ARGS should be a list of the form (VAR EXPRESSION [NOW [REQUEST [COMMENT]]]) where the list entries have the same meanings as in `custom-set-variables'. *Note Applying Customizations::. -- Function: custom-theme-set-faces theme &rest args This function specifies the Custom theme THEME's face settings. THEME should be a symbol. Each argument in ARGS should be a list of the form (FACE SPEC [NOW [COMMENT]]) where the list entries have the same meanings as in `custom-set-faces'. *Note Applying Customizations::. In theory, a theme file can also contain other Lisp forms, which would be evaluated when loading the theme, but that is "bad form". To protect against loading themes containing malicious code, Emacs displays the source file and asks for confirmation from the user before loading any non-built-in theme for the first time. The following functions are useful for programmatically enabling and disabling themes: -- Function: custom-theme-p theme This function return a non-`nil' value if THEME (a symbol) is the name of a Custom theme (i.e., a Custom theme which has been loaded into Emacs, whether or not the theme is enabled). Otherwise, it returns `nil'. -- Command: load-theme theme &optional no-confirm no-enable This function loads the Custom theme named THEME from its source file, looking for the source file in the directories specified by the variable `custom-theme-load-path'. *Note Custom Themes: (emacs)Custom Themes. It also "enables" the theme (unless the optional argument NO-ENABLE is non-`nil'), causing its variable and face settings to take effect. It prompts the user for confirmation before loading the theme, unless the optional argument NO-CONFIRM is non-`nil'. -- Command: enable-theme theme This function enables the Custom theme named THEME. It signals an error if no such theme has been loaded. -- Command: disable-theme theme This function disables the Custom theme named THEME. The theme remains loaded, so that a subsequent call to `enable-theme' will re-enable it. 15 Loading ********** Loading a file of Lisp code means bringing its contents into the Lisp environment in the form of Lisp objects. Emacs finds and opens the file, reads the text, evaluates each form, and then closes the file. Such a file is also called a "Lisp library". The load functions evaluate all the expressions in a file just as the `eval-buffer' function evaluates all the expressions in a buffer. The difference is that the load functions read and evaluate the text in the file as found on disk, not the text in an Emacs buffer. The loaded file must contain Lisp expressions, either as source code or as byte-compiled code. Each form in the file is called a "top-level form". There is no special format for the forms in a loadable file; any form in a file may equally well be typed directly into a buffer and evaluated there. (Indeed, most code is tested this way.) Most often, the forms are function definitions and variable definitions. 15.1 How Programs Do Loading ============================ Emacs Lisp has several interfaces for loading. For example, `autoload' creates a placeholder object for a function defined in a file; trying to call the autoloading function loads the file to get the function's real definition (*note Autoload::). `require' loads a file if it isn't already loaded (*note Named Features::). Ultimately, all these facilities call the `load' function to do the work. -- Function: load filename &optional missing-ok nomessage nosuffix must-suffix This function finds and opens a file of Lisp code, evaluates all the forms in it, and closes the file. To find the file, `load' first looks for a file named `FILENAME.elc', that is, for a file whose name is FILENAME with the extension `.elc' appended. If such a file exists, it is loaded. If there is no file by that name, then `load' looks for a file named `FILENAME.el'. If that file exists, it is loaded. Finally, if neither of those names is found, `load' looks for a file named FILENAME with nothing appended, and loads it if it exists. (The `load' function is not clever about looking at FILENAME. In the perverse case of a file named `foo.el.el', evaluation of `(load "foo.el")' will indeed find it.) If Auto Compression mode is enabled, as it is by default, then if `load' can not find a file, it searches for a compressed version of the file before trying other file names. It decompresses and loads it if it exists. It looks for compressed versions by appending each of the suffixes in `jka-compr-load-suffixes' to the file name. The value of this variable must be a list of strings. Its standard value is `(".gz")'. If the optional argument NOSUFFIX is non-`nil', then `load' does not try the suffixes `.elc' and `.el'. In this case, you must specify the precise file name you want, except that, if Auto Compression mode is enabled, `load' will still use `jka-compr-load-suffixes' to find compressed versions. By specifying the precise file name and using `t' for NOSUFFIX, you can prevent file names like `foo.el.el' from being tried. If the optional argument MUST-SUFFIX is non-`nil', then `load' insists that the file name used must end in either `.el' or `.elc' (possibly extended with a compression suffix), unless it contains an explicit directory name. If FILENAME is a relative file name, such as `foo' or `baz/foo.bar', `load' searches for the file using the variable `load-path'. It appends FILENAME to each of the directories listed in `load-path', and loads the first file it finds whose name matches. The current default directory is tried only if it is specified in `load-path', where `nil' stands for the default directory. `load' tries all three possible suffixes in the first directory in `load-path', then all three suffixes in the second directory, and so on. *Note Library Search::. Whatever the name under which the file is eventually found, and the directory where Emacs found it, Emacs sets the value of the variable `load-file-name' to that file's name. If you get a warning that `foo.elc' is older than `foo.el', it means you should consider recompiling `foo.el'. *Note Byte Compilation::. When loading a source file (not compiled), `load' performs character set translation just as Emacs would do when visiting the file. *Note Coding Systems::. When loading an uncompiled file, Emacs tries to expand any macros that the file contains (*note Macros::). We refer to this as "eager macro expansion". Doing this (rather than deferring the expansion until the relevant code runs) can significantly speed up the execution of uncompiled code. Sometimes, this macro expansion cannot be done, owing to a cyclic dependency. In the simplest example of this, the file you are loading refers to a macro defined in another file, and that file in turn requires the file you are loading. This is generally harmless. Emacs prints a warning (`Eager macro-expansion skipped due to cycle...') giving details of the problem, but it still loads the file, just leaving the macro unexpanded for now. You may wish to restructure your code so that this does not happen. Loading a compiled file does not cause macroexpansion, because this should already have happened during compilation. *Note Compiling Macros::. Messages like `Loading foo...' and `Loading foo...done' appear in the echo area during loading unless NOMESSAGE is non-`nil'. Any unhandled errors while loading a file terminate loading. If the load was done for the sake of `autoload', any function definitions made during the loading are undone. If `load' can't find the file to load, then normally it signals the error `file-error' (with `Cannot open load file FILENAME'). But if MISSING-OK is non-`nil', then `load' just returns `nil'. You can use the variable `load-read-function' to specify a function for `load' to use instead of `read' for reading expressions. See below. `load' returns `t' if the file loads successfully. -- Command: load-file filename This command loads the file FILENAME. If FILENAME is a relative file name, then the current default directory is assumed. This command does not use `load-path', and does not append suffixes. However, it does look for compressed versions (if Auto Compression Mode is enabled). Use this command if you wish to specify precisely the file name to load. -- Command: load-library library This command loads the library named LIBRARY. It is equivalent to `load', except for the way it reads its argument interactively. *Note Lisp Libraries: (emacs)Lisp Libraries. -- Variable: load-in-progress This variable is non-`nil' if Emacs is in the process of loading a file, and it is `nil' otherwise. -- Variable: load-file-name When Emacs is in the process of loading a file, this variable's value is the name of that file, as Emacs found it during the search described earlier in this section. -- Variable: load-read-function This variable specifies an alternate expression-reading function for `load' and `eval-region' to use instead of `read'. The function should accept one argument, just as `read' does. Normally, the variable's value is `nil', which means those functions should use `read'. Instead of using this variable, it is cleaner to use another, newer feature: to pass the function as the READ-FUNCTION argument to `eval-region'. *Note Eval: Definition of eval-region. For information about how `load' is used in building Emacs, see *note Building Emacs::. 15.2 Load Suffixes ================== We now describe some technical details about the exact suffixes that `load' tries. -- Variable: load-suffixes This is a list of suffixes indicating (compiled or source) Emacs Lisp files. It should not include the empty string. `load' uses these suffixes in order when it appends Lisp suffixes to the specified file name. The standard value is `(".elc" ".el")' which produces the behavior described in the previous section. -- Variable: load-file-rep-suffixes This is a list of suffixes that indicate representations of the same file. This list should normally start with the empty string. When `load' searches for a file it appends the suffixes in this list, in order, to the file name, before searching for another file. Enabling Auto Compression mode appends the suffixes in `jka-compr-load-suffixes' to this list and disabling Auto Compression mode removes them again. The standard value of `load-file-rep-suffixes' if Auto Compression mode is disabled is `("")'. Given that the standard value of `jka-compr-load-suffixes' is `(".gz")', the standard value of `load-file-rep-suffixes' if Auto Compression mode is enabled is `("" ".gz")'. -- Function: get-load-suffixes This function returns the list of all suffixes that `load' should try, in order, when its MUST-SUFFIX argument is non-`nil'. This takes both `load-suffixes' and `load-file-rep-suffixes' into account. If `load-suffixes', `jka-compr-load-suffixes' and `load-file-rep-suffixes' all have their standard values, this function returns `(".elc" ".elc.gz" ".el" ".el.gz")' if Auto Compression mode is enabled and `(".elc" ".el")' if Auto Compression mode is disabled. To summarize, `load' normally first tries the suffixes in the value of `(get-load-suffixes)' and then those in `load-file-rep-suffixes'. If NOSUFFIX is non-`nil', it skips the former group, and if MUST-SUFFIX is non-`nil', it skips the latter group. 15.3 Library Search =================== When Emacs loads a Lisp library, it searches for the library in a list of directories specified by the variable `load-path'. -- Variable: load-path The value of this variable is a list of directories to search when loading files with `load'. Each element is a string (which must be a directory name) or `nil' (which stands for the current working directory). Each time Emacs starts up, it sets up the value of `load-path' in several steps. First, it initializes `load-path' to the directories specified by the environment variable `EMACSLOADPATH', if that exists. The syntax of `EMACSLOADPATH' is the same as used for `PATH'; directory names are separated by `:' (or `;', on some operating systems), and `.' stands for the current default directory. Here is an example of how to set `EMACSLOADPATH' variable from `sh': export EMACSLOADPATH EMACSLOADPATH=/home/foo/.emacs.d/lisp:/opt/emacs/lisp Here is how to set it from `csh': setenv EMACSLOADPATH /home/foo/.emacs.d/lisp:/opt/emacs/lisp If `EMACSLOADPATH' is not set (which is usually the case), Emacs initializes `load-path' with the following two directories: "/usr/local/share/emacs/VERSION/site-lisp" and "/usr/local/share/emacs/site-lisp" The first one is for locally installed packages for a particular Emacs version; the second is for locally installed packages meant for use with all installed Emacs versions. If you run Emacs from the directory where it was built--that is, an executable that has not been formally installed--Emacs puts two more directories in `load-path'. These are the `lisp' and `site-lisp' subdirectories of the main build directory. (Both are represented as absolute file names.) Next, Emacs "expands" the initial list of directories in `load-path' by adding the subdirectories of those directories. Both immediate subdirectories and subdirectories multiple levels down are added. But it excludes subdirectories whose names do not start with a letter or digit, and subdirectories named `RCS' or `CVS', and subdirectories containing a file named `.nosearch'. Next, Emacs adds any extra load directory that you specify using the `-L' command-line option (*note Action Arguments: (emacs)Action Arguments.). It also adds the directories where optional packages are installed, if any (*note Packaging Basics::). It is common to add code to one's init file (*note Init File::) to add one or more directories to `load-path'. For example: (push "~/.emacs.d/lisp" load-path) Dumping Emacs uses a special value of `load-path'. If the value of `load-path' at the end of dumping is unchanged (that is, still the same special value), the dumped Emacs switches to the ordinary `load-path' value when it starts up, as described above. But if `load-path' has any other value at the end of dumping, that value is used for execution of the dumped Emacs also. -- Command: locate-library library &optional nosuffix path interactive-call This command finds the precise file name for library LIBRARY. It searches for the library in the same way `load' does, and the argument NOSUFFIX has the same meaning as in `load': don't add suffixes `.elc' or `.el' to the specified name LIBRARY. If the PATH is non-`nil', that list of directories is used instead of `load-path'. When `locate-library' is called from a program, it returns the file name as a string. When the user runs `locate-library' interactively, the argument INTERACTIVE-CALL is `t', and this tells `locate-library' to display the file name in the echo area. -- Command: list-load-path-shadows &optional stringp This command shows a list of "shadowed" Emacs Lisp files. A shadowed file is one that will not normally be loaded, despite being in a directory on `load-path', due to the existence of another similarly-named file in a directory earlier on `load-path'. For instance, suppose `load-path' is set to ("/opt/emacs/site-lisp" "/usr/share/emacs/23.3/lisp") and that both these directories contain a file named `foo.el'. Then `(require 'foo)' never loads the file in the second directory. Such a situation might indicate a problem in the way Emacs was installed. When called from Lisp, this function prints a message listing the shadowed files, instead of displaying them in a buffer. If the optional argument `stringp' is non-`nil', it instead returns the shadowed files as a string. 15.4 Loading Non-ASCII Characters ================================= When Emacs Lisp programs contain string constants with non-ASCII characters, these can be represented within Emacs either as unibyte strings or as multibyte strings (*note Text Representations::). Which representation is used depends on how the file is read into Emacs. If it is read with decoding into multibyte representation, the text of the Lisp program will be multibyte text, and its string constants will be multibyte strings. If a file containing Latin-1 characters (for example) is read without decoding, the text of the program will be unibyte text, and its string constants will be unibyte strings. *Note Coding Systems::. In most Emacs Lisp programs, the fact that non-ASCII strings are multibyte strings should not be noticeable, since inserting them in unibyte buffers converts them to unibyte automatically. However, if this does make a difference, you can force a particular Lisp file to be interpreted as unibyte by writing `coding: raw-text' in a local variables section. With that designator, the file will unconditionally be interpreted as unibyte. This can matter when making keybindings to non-ASCII characters written as `?vLITERAL'. 15.5 Autoload ============= The "autoload" facility lets you register the existence of a function or macro, but put off loading the file that defines it. The first call to the function automatically loads the proper library, in order to install the real definition and other associated code, then runs the real definition as if it had been loaded all along. Autoloading can also be triggered by looking up the documentation of the function or macro (*note Documentation Basics::). There are two ways to set up an autoloaded function: by calling `autoload', and by writing a special "magic" comment in the source before the real definition. `autoload' is the low-level primitive for autoloading; any Lisp program can call `autoload' at any time. Magic comments are the most convenient way to make a function autoload, for packages installed along with Emacs. These comments do nothing on their own, but they serve as a guide for the command `update-file-autoloads', which constructs calls to `autoload' and arranges to execute them when Emacs is built. -- Function: autoload function filename &optional docstring interactive type This function defines the function (or macro) named FUNCTION so as to load automatically from FILENAME. The string FILENAME specifies the file to load to get the real definition of FUNCTION. If FILENAME does not contain either a directory name, or the suffix `.el' or `.elc', this function insists on adding one of these suffixes, and it will not load from a file whose name is just FILENAME with no added suffix. (The variable `load-suffixes' specifies the exact required suffixes.) The argument DOCSTRING is the documentation string for the function. Specifying the documentation string in the call to `autoload' makes it possible to look at the documentation without loading the function's real definition. Normally, this should be identical to the documentation string in the function definition itself. If it isn't, the function definition's documentation string takes effect when it is loaded. If INTERACTIVE is non-`nil', that says FUNCTION can be called interactively. This lets completion in `M-x' work without loading FUNCTION's real definition. The complete interactive specification is not given here; it's not needed unless the user actually calls FUNCTION, and when that happens, it's time to load the real definition. You can autoload macros and keymaps as well as ordinary functions. Specify TYPE as `macro' if FUNCTION is really a macro. Specify TYPE as `keymap' if FUNCTION is really a keymap. Various parts of Emacs need to know this information without loading the real definition. An autoloaded keymap loads automatically during key lookup when a prefix key's binding is the symbol FUNCTION. Autoloading does not occur for other kinds of access to the keymap. In particular, it does not happen when a Lisp program gets the keymap from the value of a variable and calls `define-key'; not even if the variable name is the same symbol FUNCTION. if FUNCTION already has non-void function definition that is not an autoload object, this function does nothing and returns `nil'. Otherwise, it constructs an autoload object (*note Autoload Type::), and stores it as the function definition for FUNCTION. The autoload object has this form: (autoload FILENAME DOCSTRING INTERACTIVE TYPE) For example, (symbol-function 'run-prolog) => (autoload "prolog" 169681 t nil) In this case, `"prolog"' is the name of the file to load, 169681 refers to the documentation string in the `emacs/etc/DOC-VERSION' file (*note Documentation Basics::), `t' means the function is interactive, and `nil' that it is not a macro or a keymap. -- Function: autoloadp object This function returns non-`nil' if OBJECT is an autoload object. For example, to check if `run-prolog' is defined as an autoloaded function, evaluate (autoloadp (symbol-function 'run-prolog)) The autoloaded file usually contains other definitions and may require or provide one or more features. If the file is not completely loaded (due to an error in the evaluation of its contents), any function definitions or `provide' calls that occurred during the load are undone. This is to ensure that the next attempt to call any function autoloading from this file will try again to load the file. If not for this, then some of the functions in the file might be defined by the aborted load, but fail to work properly for the lack of certain subroutines not loaded successfully because they come later in the file. If the autoloaded file fails to define the desired Lisp function or macro, then an error is signaled with data `"Autoloading failed to define function FUNCTION-NAME"'. A magic autoload comment (often called an "autoload cookie") consists of `;;;###autoload', on a line by itself, just before the real definition of the function in its autoloadable source file. The command `M-x update-file-autoloads' writes a corresponding `autoload' call into `loaddefs.el'. (The string that serves as the autoload cookie and the name of the file generated by `update-file-autoloads' can be changed from the above defaults, see below.) Building Emacs loads `loaddefs.el' and thus calls `autoload'. `M-x update-directory-autoloads' is even more powerful; it updates autoloads for all files in the current directory. The same magic comment can copy any kind of form into `loaddefs.el'. The form following the magic comment is copied verbatim, _except_ if it is one of the forms which the autoload facility handles specially (e.g., by conversion into an `autoload' call). The forms which are not copied verbatim are the following: Definitions for function or function-like objects: `defun' and `defmacro'; also `cl-defun' and `cl-defmacro' (*note Argument Lists: (cl)Argument Lists.), and `define-overloadable-function' (see the commentary in `mode-local.el'). Definitions for major or minor modes: `define-minor-mode', `define-globalized-minor-mode', `define-generic-mode', `define-derived-mode', `easy-mmode-define-minor-mode', `easy-mmode-define-global-mode', `define-compilation-mode', and `define-global-minor-mode'. Other definition types: `defcustom', `defgroup', `defclass' (*note EIEIO: (eieio)Top.), and `define-skeleton' (see the commentary in `skeleton.el'). You can also use a magic comment to execute a form at build time _without_ executing it when the file itself is loaded. To do this, write the form _on the same line_ as the magic comment. Since it is in a comment, it does nothing when you load the source file; but `M-x update-file-autoloads' copies it to `loaddefs.el', where it is executed while building Emacs. The following example shows how `doctor' is prepared for autoloading with a magic comment: ;;;###autoload (defun doctor () "Switch to *doctor* buffer and start giving psychotherapy." (interactive) (switch-to-buffer "*doctor*") (doctor-mode)) Here's what that produces in `loaddefs.el': (autoload (quote doctor) "doctor" "\ Switch to *doctor* buffer and start giving psychotherapy. \(fn)" t nil) The backslash and newline immediately following the double-quote are a convention used only in the preloaded uncompiled Lisp files such as `loaddefs.el'; they tell `make-docfile' to put the documentation string in the `etc/DOC' file. *Note Building Emacs::. See also the commentary in `lib-src/make-docfile.c'. `(fn)' in the usage part of the documentation string is replaced with the function's name when the various help functions (*note Help Functions::) display it. If you write a function definition with an unusual macro that is not one of the known and recognized function definition methods, use of an ordinary magic autoload comment would copy the whole definition into `loaddefs.el'. That is not desirable. You can put the desired `autoload' call into `loaddefs.el' instead by writing this: ;;;###autoload (autoload 'foo "myfile") (mydefunmacro foo ...) You can use a non-default string as the autoload cookie and have the corresponding autoload calls written into a file whose name is different from the default `loaddefs.el'. Emacs provides two variables to control this: -- Variable: generate-autoload-cookie The value of this variable should be a string whose syntax is a Lisp comment. `M-x update-file-autoloads' copies the Lisp form that follows the cookie into the autoload file it generates. The default value of this variable is `";;;###autoload"'. -- Variable: generated-autoload-file The value of this variable names an Emacs Lisp file where the autoload calls should go. The default value is `loaddefs.el', but you can override that, e.g., in the "Local Variables" section of a `.el' file (*note File Local Variables::). The autoload file is assumed to contain a trailer starting with a formfeed character. The following function may be used to explicitly load the library specified by an autoload object: -- Function: autoload-do-load autoload &optional name macro-only This function performs the loading specified by AUTOLOAD, which should be an autoload object. The optional argument NAME, if non-`nil', should be a symbol whose function value is AUTOLOAD; in that case, the return value of this function is the symbol's new function value. If the value of the optional argument MACRO-ONLY is `macro', this function avoids loading a function, only a macro. 15.6 Repeated Loading ===================== You can load a given file more than once in an Emacs session. For example, after you have rewritten and reinstalled a function definition by editing it in a buffer, you may wish to return to the original version; you can do this by reloading the file it came from. When you load or reload files, bear in mind that the `load' and `load-library' functions automatically load a byte-compiled file rather than a non-compiled file of similar name. If you rewrite a file that you intend to save and reinstall, you need to byte-compile the new version; otherwise Emacs will load the older, byte-compiled file instead of your newer, non-compiled file! If that happens, the message displayed when loading the file includes, `(compiled; note, source is newer)', to remind you to recompile it. When writing the forms in a Lisp library file, keep in mind that the file might be loaded more than once. For example, think about whether each variable should be reinitialized when you reload the library; `defvar' does not change the value if the variable is already initialized. (*Note Defining Variables::.) The simplest way to add an element to an alist is like this: (push '(leif-mode " Leif") minor-mode-alist) But this would add multiple elements if the library is reloaded. To avoid the problem, use `add-to-list' (*note List Variables::): (add-to-list 'minor-mode-alist '(leif-mode " Leif")) Occasionally you will want to test explicitly whether a library has already been loaded. If the library uses `provide' to provide a named feature, you can use `featurep' earlier in the file to test whether the `provide' call has been executed before (*note Named Features::). Alternatively, you could use something like this: (defvar foo-was-loaded nil) (unless foo-was-loaded EXECUTE-FIRST-TIME-ONLY (setq foo-was-loaded t)) 15.7 Features ============= `provide' and `require' are an alternative to `autoload' for loading files automatically. They work in terms of named "features". Autoloading is triggered by calling a specific function, but a feature is loaded the first time another program asks for it by name. A feature name is a symbol that stands for a collection of functions, variables, etc. The file that defines them should "provide" the feature. Another program that uses them may ensure they are defined by "requiring" the feature. This loads the file of definitions if it hasn't been loaded already. To require the presence of a feature, call `require' with the feature name as argument. `require' looks in the global variable `features' to see whether the desired feature has been provided already. If not, it loads the feature from the appropriate file. This file should call `provide' at the top level to add the feature to `features'; if it fails to do so, `require' signals an error. For example, in `idlwave.el', the definition for `idlwave-complete-filename' includes the following code: (defun idlwave-complete-filename () "Use the comint stuff to complete a file name." (require 'comint) (let* ((comint-file-name-chars "~/A-Za-z0-9+_.$#%={}\\-") (comint-completion-addsuffix nil) ...) (comint-dynamic-complete-filename))) The expression `(require 'comint)' loads the file `comint.el' if it has not yet been loaded, ensuring that `comint-dynamic-complete-filename' is defined. Features are normally named after the files that provide them, so that `require' need not be given the file name. (Note that it is important that the `require' statement be outside the body of the `let'. Loading a library while its variables are let-bound can have unintended consequences, namely the variables becoming unbound after the let exits.) The `comint.el' file contains the following top-level expression: (provide 'comint) This adds `comint' to the global `features' list, so that `(require 'comint)' will henceforth know that nothing needs to be done. When `require' is used at top level in a file, it takes effect when you byte-compile that file (*note Byte Compilation::) as well as when you load it. This is in case the required package contains macros that the byte compiler must know about. It also avoids byte compiler warnings for functions and variables defined in the file loaded with `require'. Although top-level calls to `require' are evaluated during byte compilation, `provide' calls are not. Therefore, you can ensure that a file of definitions is loaded before it is byte-compiled by including a `provide' followed by a `require' for the same feature, as in the following example. (provide 'my-feature) ; Ignored by byte compiler, ; evaluated by `load'. (require 'my-feature) ; Evaluated by byte compiler. The compiler ignores the `provide', then processes the `require' by loading the file in question. Loading the file does execute the `provide' call, so the subsequent `require' call does nothing when the file is loaded. -- Function: provide feature &optional subfeatures This function announces that FEATURE is now loaded, or being loaded, into the current Emacs session. This means that the facilities associated with FEATURE are or will be available for other Lisp programs. The direct effect of calling `provide' is if not already in FEATURES then to add FEATURE to the front of that list and call any `eval-after-load' code waiting for it (*note Hooks for Loading::). The argument FEATURE must be a symbol. `provide' returns FEATURE. If provided, SUBFEATURES should be a list of symbols indicating a set of specific subfeatures provided by this version of FEATURE. You can test the presence of a subfeature using `featurep'. The idea of subfeatures is that you use them when a package (which is one FEATURE) is complex enough to make it useful to give names to various parts or functionalities of the package, which might or might not be loaded, or might or might not be present in a given version. *Note Network Feature Testing::, for an example. features => (bar bish) (provide 'foo) => foo features => (foo bar bish) When a file is loaded to satisfy an autoload, and it stops due to an error in the evaluation of its contents, any function definitions or `provide' calls that occurred during the load are undone. *Note Autoload::. -- Function: require feature &optional filename noerror This function checks whether FEATURE is present in the current Emacs session (using `(featurep FEATURE)'; see below). The argument FEATURE must be a symbol. If the feature is not present, then `require' loads FILENAME with `load'. If FILENAME is not supplied, then the name of the symbol FEATURE is used as the base file name to load. However, in this case, `require' insists on finding FEATURE with an added `.el' or `.elc' suffix (possibly extended with a compression suffix); a file whose name is just FEATURE won't be used. (The variable `load-suffixes' specifies the exact required Lisp suffixes.) If NOERROR is non-`nil', that suppresses errors from actual loading of the file. In that case, `require' returns `nil' if loading the file fails. Normally, `require' returns FEATURE. If loading the file succeeds but does not provide FEATURE, `require' signals an error, `Required feature FEATURE was not provided'. -- Function: featurep feature &optional subfeature This function returns `t' if FEATURE has been provided in the current Emacs session (i.e., if FEATURE is a member of `features'.) If SUBFEATURE is non-`nil', then the function returns `t' only if that subfeature is provided as well (i.e., if SUBFEATURE is a member of the `subfeature' property of the FEATURE symbol.) -- Variable: features The value of this variable is a list of symbols that are the features loaded in the current Emacs session. Each symbol was put in this list with a call to `provide'. The order of the elements in the `features' list is not significant. 15.8 Which File Defined a Certain Symbol ======================================== -- Function: symbol-file symbol &optional type This function returns the name of the file that defined SYMBOL. If TYPE is `nil', then any kind of definition is acceptable. If TYPE is `defun', `defvar', or `defface', that specifies function definition, variable definition, or face definition only. The value is normally an absolute file name. It can also be `nil', if the definition is not associated with any file. If SYMBOL specifies an autoloaded function, the value can be a relative file name without extension. The basis for `symbol-file' is the data in the variable `load-history'. -- Variable: load-history The value of this variable is an alist that associates the names of loaded library files with the names of the functions and variables they defined, as well as the features they provided or required. Each element in this alist describes one loaded library (including libraries that are preloaded at startup). It is a list whose CAR is the absolute file name of the library (a string). The rest of the list elements have these forms: `VAR' The symbol VAR was defined as a variable. `(defun . FUN)' The function FUN was defined. `(t . FUN)' The function FUN was previously an autoload before this library redefined it as a function. The following element is always `(defun . FUN)', which represents defining FUN as a function. `(autoload . FUN)' The function FUN was defined as an autoload. `(defface . FACE)' The face FACE was defined. `(require . FEATURE)' The feature FEATURE was required. `(provide . FEATURE)' The feature FEATURE was provided. The value of `load-history' may have one element whose CAR is `nil'. This element describes definitions made with `eval-buffer' on a buffer that is not visiting a file. The command `eval-region' updates `load-history', but does so by adding the symbols defined to the element for the file being visited, rather than replacing that element. *Note Eval::. 15.9 Unloading ============== You can discard the functions and variables loaded by a library to reclaim memory for other Lisp objects. To do this, use the function `unload-feature': -- Command: unload-feature feature &optional force This command unloads the library that provided feature FEATURE. It undefines all functions, macros, and variables defined in that library with `defun', `defalias', `defsubst', `defmacro', `defconst', `defvar', and `defcustom'. It then restores any autoloads formerly associated with those symbols. (Loading saves these in the `autoload' property of the symbol.) Before restoring the previous definitions, `unload-feature' runs `remove-hook' to remove functions in the library from certain hooks. These hooks include variables whose names end in `-hook' (or the deprecated suffix `-hooks'), plus those listed in `unload-feature-special-hooks', as well as `auto-mode-alist'. This is to prevent Emacs from ceasing to function because important hooks refer to functions that are no longer defined. Standard unloading activities also undoes ELP profiling of functions in that library, unprovides any features provided by the library, and cancels timers held in variables defined by the library. If these measures are not sufficient to prevent malfunction, a library can define an explicit unloader named `FEATURE-unload-function'. If that symbol is defined as a function, `unload-feature' calls it with no arguments before doing anything else. It can do whatever is appropriate to unload the library. If it returns `nil', `unload-feature' proceeds to take the normal unload actions. Otherwise it considers the job to be done. Ordinarily, `unload-feature' refuses to unload a library on which other loaded libraries depend. (A library A depends on library B if A contains a `require' for B.) If the optional argument FORCE is non-`nil', dependencies are ignored and you can unload any library. The `unload-feature' function is written in Lisp; its actions are based on the variable `load-history'. -- Variable: unload-feature-special-hooks This variable holds a list of hooks to be scanned before unloading a library, to remove functions defined in the library. 15.10 Hooks for Loading ======================= You can ask for code to be executed each time Emacs loads a library, by using the variable `after-load-functions': -- Variable: after-load-functions This abnormal hook is run after loading a file. Each function in the hook is called with a single argument, the absolute filename of the file that was just loaded. If you want code to be executed when a _particular_ library is loaded, use the function `eval-after-load': -- Function: eval-after-load library form This function arranges to evaluate FORM at the end of loading the file LIBRARY, each time LIBRARY is loaded. If LIBRARY is already loaded, it evaluates FORM right away. Don't forget to quote FORM! You don't need to give a directory or extension in the file name LIBRARY. Normally, you just give a bare file name, like this: (eval-after-load "edebug" '(def-edebug-spec c-point t)) To restrict which files can trigger the evaluation, include a directory or an extension or both in LIBRARY. Only a file whose absolute true name (i.e., the name with all symbolic links chased out) matches all the given name components will match. In the following example, `my_inst.elc' or `my_inst.elc.gz' in some directory `..../foo/bar' will trigger the evaluation, but not `my_inst.el': (eval-after-load "foo/bar/my_inst.elc" ...) LIBRARY can also be a feature (i.e., a symbol), in which case FORM is evaluated at the end of any file where `(provide LIBRARY)' is called. An error in FORM does not undo the load, but does prevent execution of the rest of FORM. Normally, well-designed Lisp programs should not use `eval-after-load'. If you need to examine and set the variables defined in another library (those meant for outside use), you can do it immediately--there is no need to wait until the library is loaded. If you need to call functions defined by that library, you should load the library, preferably with `require' (*note Named Features::). -- Variable: after-load-alist This variable stores an alist built by `eval-after-load', containing the expressions to evaluate when certain libraries are loaded. Each element looks like this: (REGEXP-OR-FEATURE FORMS...) The key REGEXP-OR-FEATURE is either a regular expression or a symbol, and the value is a list of forms. The forms are evaluated when the key matches the absolute true name or feature name of the library being loaded. 16 Byte Compilation ******************* Emacs Lisp has a "compiler" that translates functions written in Lisp into a special representation called "byte-code" that can be executed more efficiently. The compiler replaces Lisp function definitions with byte-code. When a byte-code function is called, its definition is evaluated by the "byte-code interpreter". Because the byte-compiled code is evaluated by the byte-code interpreter, instead of being executed directly by the machine's hardware (as true compiled code is), byte-code is completely transportable from machine to machine without recompilation. It is not, however, as fast as true compiled code. In general, any version of Emacs can run byte-compiled code produced by recent earlier versions of Emacs, but the reverse is not true. If you do not want a Lisp file to be compiled, ever, put a file-local variable binding for `no-byte-compile' into it, like this: ;; -*-no-byte-compile: t; -*- 16.1 Performance of Byte-Compiled Code ====================================== A byte-compiled function is not as efficient as a primitive function written in C, but runs much faster than the version written in Lisp. Here is an example: (defun silly-loop (n) "Return the time, in seconds, to run N iterations of a loop." (let ((t1 (float-time))) (while (> (setq n (1- n)) 0)) (- (float-time) t1))) => silly-loop (silly-loop 50000000) => 10.235304117202759 (byte-compile 'silly-loop) => [Compiled code not shown] (silly-loop 50000000) => 3.705854892730713 In this example, the interpreted code required 10 seconds to run, whereas the byte-compiled code required less than 4 seconds. These results are representative, but actual results may vary. 16.2 Byte-Compilation Functions =============================== You can byte-compile an individual function or macro definition with the `byte-compile' function. You can compile a whole file with `byte-compile-file', or several files with `byte-recompile-directory' or `batch-byte-compile'. Sometimes, the byte compiler produces warning and/or error messages (*note Compiler Errors::, for details). These messages are recorded in a buffer called `*Compile-Log*', which uses Compilation mode. *Note Compilation Mode: (emacs)Compilation Mode. Be careful when writing macro calls in files that you intend to byte-compile. Since macro calls are expanded when they are compiled, the macros need to be loaded into Emacs or the byte compiler will not do the right thing. The usual way to handle this is with `require' forms which specify the files containing the needed macro definitions (*note Named Features::). Normally, the byte compiler does not evaluate the code that it is compiling, but it handles `require' forms specially, by loading the specified libraries. To avoid loading the macro definition files when someone _runs_ the compiled program, write `eval-when-compile' around the `require' calls (*note Eval During Compile::). For more details, *Note Compiling Macros::. Inline (`defsubst') functions are less troublesome; if you compile a call to such a function before its definition is known, the call will still work right, it will just run slower. -- Function: byte-compile symbol This function byte-compiles the function definition of SYMBOL, replacing the previous definition with the compiled one. The function definition of SYMBOL must be the actual code for the function; `byte-compile' does not handle function indirection. The return value is the byte-code function object which is the compiled definition of SYMBOL (*note Byte-Code Objects::). (defun factorial (integer) "Compute factorial of INTEGER." (if (= 1 integer) 1 (* integer (factorial (1- integer))))) => factorial (byte-compile 'factorial) => #[(integer) "^H\301U\203^H^@\301\207\302^H\303^HS!\"\207" [integer 1 * factorial] 4 "Compute factorial of INTEGER."] If SYMBOL's definition is a byte-code function object, `byte-compile' does nothing and returns `nil'. It does not "compile the symbol's definition again", since the original (non-compiled) code has already been replaced in the symbol's function cell by the byte-compiled code. The argument to `byte-compile' can also be a `lambda' expression. In that case, the function returns the corresponding compiled code but does not store it anywhere. -- Command: compile-defun &optional arg This command reads the defun containing point, compiles it, and evaluates the result. If you use this on a defun that is actually a function definition, the effect is to install a compiled version of that function. `compile-defun' normally displays the result of evaluation in the echo area, but if ARG is non-`nil', it inserts the result in the current buffer after the form it compiled. -- Command: byte-compile-file filename &optional load This function compiles a file of Lisp code named FILENAME into a file of byte-code. The output file's name is made by changing the `.el' suffix into `.elc'; if FILENAME does not end in `.el', it adds `.elc' to the end of FILENAME. Compilation works by reading the input file one form at a time. If it is a definition of a function or macro, the compiled function or macro definition is written out. Other forms are batched together, then each batch is compiled, and written so that its compiled code will be executed when the file is read. All comments are discarded when the input file is read. This command returns `t' if there were no errors and `nil' otherwise. When called interactively, it prompts for the file name. If LOAD is non-`nil', this command loads the compiled file after compiling it. Interactively, LOAD is the prefix argument. % ls -l push* -rw-r--r-- 1 lewis 791 Oct 5 20:31 push.el (byte-compile-file "~/emacs/push.el") => t % ls -l push* -rw-r--r-- 1 lewis 791 Oct 5 20:31 push.el -rw-rw-rw- 1 lewis 638 Oct 8 20:25 push.elc -- Command: byte-recompile-directory directory &optional flag force This command recompiles every `.el' file in DIRECTORY (or its subdirectories) that needs recompilation. A file needs recompilation if a `.elc' file exists but is older than the `.el' file. When a `.el' file has no corresponding `.elc' file, FLAG says what to do. If it is `nil', this command ignores these files. If FLAG is 0, it compiles them. If it is neither `nil' nor 0, it asks the user whether to compile each such file, and asks about each subdirectory as well. Interactively, `byte-recompile-directory' prompts for DIRECTORY and FLAG is the prefix argument. If FORCE is non-`nil', this command recompiles every `.el' file that has a `.elc' file. The returned value is unpredictable. -- Function: batch-byte-compile &optional noforce This function runs `byte-compile-file' on files specified on the command line. This function must be used only in a batch execution of Emacs, as it kills Emacs on completion. An error in one file does not prevent processing of subsequent files, but no output file will be generated for it, and the Emacs process will terminate with a nonzero status code. If NOFORCE is non-`nil', this function does not recompile files that have an up-to-date `.elc' file. % emacs -batch -f batch-byte-compile *.el 16.3 Documentation Strings and Compilation ========================================== Functions and variables loaded from a byte-compiled file access their documentation strings dynamically from the file whenever needed. This saves space within Emacs, and makes loading faster because the documentation strings themselves need not be processed while loading the file. Actual access to the documentation strings becomes slower as a result, but this normally is not enough to bother users. Dynamic access to documentation strings does have drawbacks: * If you delete or move the compiled file after loading it, Emacs can no longer access the documentation strings for the functions and variables in the file. * If you alter the compiled file (such as by compiling a new version), then further access to documentation strings in this file will probably give nonsense results. These problems normally occur only if you build Emacs yourself and use it from the directory where you built it, and you happen to edit and/or recompile the Lisp source files. They can be easily cured by reloading each file after recompiling it. The dynamic documentation string feature writes compiled files that use a special Lisp reader construct, `#@COUNT'. This construct skips the next COUNT characters. It also uses the `#$' construct, which stands for "the name of this file, as a string". It is usually best not to use these constructs in Lisp source files, since they are not designed to be clear to humans reading the file. You can disable the dynamic documentation string feature at compile time by setting `byte-compile-dynamic-docstrings' to `nil'; this is useful mainly if you expect to change the file, and you want Emacs processes that have already loaded it to keep working when the file changes. You can do this globally, or for one source file by specifying a file-local binding for the variable. One way to do that is by adding this string to the file's first line: -*-byte-compile-dynamic-docstrings: nil;-*- -- User Option: byte-compile-dynamic-docstrings If this is non-`nil', the byte compiler generates compiled files that are set up for dynamic loading of documentation strings. 16.4 Dynamic Loading of Individual Functions ============================================ When you compile a file, you can optionally enable the "dynamic function loading" feature (also known as "lazy loading"). With dynamic function loading, loading the file doesn't fully read the function definitions in the file. Instead, each function definition contains a place-holder which refers to the file. The first time each function is called, it reads the full definition from the file, to replace the place-holder. The advantage of dynamic function loading is that loading the file becomes much faster. This is a good thing for a file which contains many separate user-callable functions, if using one of them does not imply you will probably also use the rest. A specialized mode which provides many keyboard commands often has that usage pattern: a user may invoke the mode, but use only a few of the commands it provides. The dynamic loading feature has certain disadvantages: * If you delete or move the compiled file after loading it, Emacs can no longer load the remaining function definitions not already loaded. * If you alter the compiled file (such as by compiling a new version), then trying to load any function not already loaded will usually yield nonsense results. These problems will never happen in normal circumstances with installed Emacs files. But they are quite likely to happen with Lisp files that you are changing. The easiest way to prevent these problems is to reload the new compiled file immediately after each recompilation. The byte compiler uses the dynamic function loading feature if the variable `byte-compile-dynamic' is non-`nil' at compilation time. Do not set this variable globally, since dynamic loading is desirable only for certain files. Instead, enable the feature for specific source files with file-local variable bindings. For example, you could do it by writing this text in the source file's first line: -*-byte-compile-dynamic: t;-*- -- Variable: byte-compile-dynamic If this is non-`nil', the byte compiler generates compiled files that are set up for dynamic function loading. -- Function: fetch-bytecode function If FUNCTION is a byte-code function object, this immediately finishes loading the byte code of FUNCTION from its byte-compiled file, if it is not fully loaded already. Otherwise, it does nothing. It always returns FUNCTION. 16.5 Evaluation During Compilation ================================== These features permit you to write code to be evaluated during compilation of a program. -- Special Form: eval-and-compile body... This form marks BODY to be evaluated both when you compile the containing code and when you run it (whether compiled or not). You can get a similar result by putting BODY in a separate file and referring to that file with `require'. That method is preferable when BODY is large. Effectively `require' is automatically `eval-and-compile', the package is loaded both when compiling and executing. `autoload' is also effectively `eval-and-compile' too. It's recognized when compiling, so uses of such a function don't produce "not known to be defined" warnings. Most uses of `eval-and-compile' are fairly sophisticated. If a macro has a helper function to build its result, and that macro is used both locally and outside the package, then `eval-and-compile' should be used to get the helper both when compiling and then later when running. If functions are defined programmatically (with `fset' say), then `eval-and-compile' can be used to have that done at compile-time as well as run-time, so calls to those functions are checked (and warnings about "not known to be defined" suppressed). -- Special Form: eval-when-compile body... This form marks BODY to be evaluated at compile time but not when the compiled program is loaded. The result of evaluation by the compiler becomes a constant which appears in the compiled program. If you load the source file, rather than compiling it, BODY is evaluated normally. If you have a constant that needs some calculation to produce, `eval-when-compile' can do that at compile-time. For example, (defvar my-regexp (eval-when-compile (regexp-opt '("aaa" "aba" "abb")))) If you're using another package, but only need macros from it (the byte compiler will expand those), then `eval-when-compile' can be used to load it for compiling, but not executing. For example, (eval-when-compile (require 'my-macro-package)) The same sort of thing goes for macros and `defsubst' functions defined locally and only for use within the file. They are needed for compiling the file, but in most cases they are not needed for execution of the compiled file. For example, (eval-when-compile (unless (fboundp 'some-new-thing) (defmacro 'some-new-thing () (compatibility code)))) This is often good for code that's only a fallback for compatibility with other versions of Emacs. *Common Lisp Note:* At top level, `eval-when-compile' is analogous to the Common Lisp idiom `(eval-when (compile eval) ...)'. Elsewhere, the Common Lisp `#.' reader macro (but not when interpreting) is closer to what `eval-when-compile' does. 16.6 Compiler Errors ==================== Byte compilation outputs all errors and warnings into the buffer `*Compile-Log*'. The messages include file names and line numbers that identify the location of the problem. The usual Emacs commands for operating on compiler diagnostics work properly on these messages. When an error is due to invalid syntax in the program, the byte compiler might get confused about the errors' exact location. One way to investigate is to switch to the buffer ` *Compiler Input*'. (This buffer name starts with a space, so it does not show up in `M-x list-buffers'.) This buffer contains the program being compiled, and point shows how far the byte compiler was able to read; the cause of the error might be nearby. *Note Syntax Errors::, for some tips for locating syntax errors. When the byte compiler warns about functions that were used but not defined, it always reports the line number for the end of the file, not the locations where the missing functions were called. To find the latter, you must search for the function names. You can suppress the compiler warning for calling an undefined function FUNC by conditionalizing the function call on an `fboundp' test, like this: (if (fboundp 'FUNC) ...(FUNC ...)...) The call to FUNC must be in the THEN-FORM of the `if', and FUNC must appear quoted in the call to `fboundp'. (This feature operates for `cond' as well.) You can tell the compiler that a function is defined using `declare-function' (*note Declaring Functions::). Likewise, you can tell the compiler that a variable is defined using `defvar' with no initial value. You can suppress the compiler warning for a specific use of an undefined variable VARIABLE by conditionalizing its use on a `boundp' test, like this: (if (boundp 'VARIABLE) ...VARIABLE...) The reference to VARIABLE must be in the THEN-FORM of the `if', and VARIABLE must appear quoted in the call to `boundp'. You can suppress any and all compiler warnings within a certain expression using the construct `with-no-warnings': -- Special Form: with-no-warnings body... In execution, this is equivalent to `(progn BODY...)', but the compiler does not issue warnings for anything that occurs inside BODY. We recommend that you use this construct around the smallest possible piece of code, to avoid missing possible warnings other than one you intend to suppress. More precise control of warnings is possible by setting the variable `byte-compile-warnings'. 16.7 Byte-Code Function Objects =============================== Byte-compiled functions have a special data type: they are "byte-code function objects". Whenever such an object appears as a function to be called, Emacs uses the byte-code interpreter to execute the byte-code. Internally, a byte-code function object is much like a vector; its elements can be accessed using `aref'. Its printed representation is like that for a vector, with an additional `#' before the opening `['. It must have at least four elements; there is no maximum number, but only the first six elements have any normal use. They are: ARGLIST The list of argument symbols. BYTE-CODE The string containing the byte-code instructions. CONSTANTS The vector of Lisp objects referenced by the byte code. These include symbols used as function names and variable names. STACKSIZE The maximum stack size this function needs. DOCSTRING The documentation string (if any); otherwise, `nil'. The value may be a number or a list, in case the documentation string is stored in a file. Use the function `documentation' to get the real documentation string (*note Accessing Documentation::). INTERACTIVE The interactive spec (if any). This can be a string or a Lisp expression. It is `nil' for a function that isn't interactive. Here's an example of a byte-code function object, in printed representation. It is the definition of the command `backward-sexp'. #[(&optional arg) "^H\204^F^@\301^P\302^H[!\207" [arg 1 forward-sexp] 2 254435 "^p"] The primitive way to create a byte-code object is with `make-byte-code': -- Function: make-byte-code &rest elements This function constructs and returns a byte-code function object with ELEMENTS as its elements. You should not try to come up with the elements for a byte-code function yourself, because if they are inconsistent, Emacs may crash when you call the function. Always leave it to the byte compiler to create these objects; it makes the elements consistent (we hope). 16.8 Disassembled Byte-Code =========================== People do not write byte-code; that job is left to the byte compiler. But we provide a disassembler to satisfy a cat-like curiosity. The disassembler converts the byte-compiled code into human-readable form. The byte-code interpreter is implemented as a simple stack machine. It pushes values onto a stack of its own, then pops them off to use them in calculations whose results are themselves pushed back on the stack. When a byte-code function returns, it pops a value off the stack and returns it as the value of the function. In addition to the stack, byte-code functions can use, bind, and set ordinary Lisp variables, by transferring values between variables and the stack. -- Command: disassemble object &optional buffer-or-name This command displays the disassembled code for OBJECT. In interactive use, or if BUFFER-OR-NAME is `nil' or omitted, the output goes in a buffer named `*Disassemble*'. If BUFFER-OR-NAME is non-`nil', it must be a buffer or the name of an existing buffer. Then the output goes there, at point, and point is left before the output. The argument OBJECT can be a function name, a lambda expression or a byte-code object. If it is a lambda expression, `disassemble' compiles it and disassembles the resulting compiled code. Here are two examples of using the `disassemble' function. We have added explanatory comments to help you relate the byte-code to the Lisp source; these do not appear in the output of `disassemble'. (defun factorial (integer) "Compute factorial of an integer." (if (= 1 integer) 1 (* integer (factorial (1- integer))))) => factorial (factorial 4) => 24 (disassemble 'factorial) -| byte-code for factorial: doc: Compute factorial of an integer. args: (integer) 0 varref integer ; Get the value of `integer' and ; push it onto the stack. 1 constant 1 ; Push 1 onto stack. 2 eqlsign ; Pop top two values off stack, compare ; them, and push result onto stack. 3 goto-if-nil 1 ; Pop and test top of stack; ; if `nil', go to 1, else continue. 6 constant 1 ; Push 1 onto top of stack. 7 return ; Return the top element of the stack. 8:1 varref integer ; Push value of `integer' onto stack. 9 constant factorial ; Push `factorial' onto stack. 10 varref integer ; Push value of `integer' onto stack. 11 sub1 ; Pop `integer', decrement value, ; push new value onto stack. 12 call 1 ; Call function `factorial' using first ; (i.e., top) stack element as argument; ; push returned value onto stack. 13 mult ; Pop top two values off stack, multiply ; them, and push result onto stack. 14 return ; Return the top element of the stack. The `silly-loop' function is somewhat more complex: (defun silly-loop (n) "Return time before and after N iterations of a loop." (let ((t1 (current-time-string))) (while (> (setq n (1- n)) 0)) (list t1 (current-time-string)))) => silly-loop (disassemble 'silly-loop) -| byte-code for silly-loop: doc: Return time before and after N iterations of a loop. args: (n) 0 constant current-time-string ; Push `current-time-string' ; onto top of stack. 1 call 0 ; Call `current-time-string' with no ; argument, push result onto stack. 2 varbind t1 ; Pop stack and bind `t1' to popped value. 3:1 varref n ; Get value of `n' from the environment ; and push the value on the stack. 4 sub1 ; Subtract 1 from top of stack. 5 dup ; Duplicate top of stack; i.e., copy the top ; of the stack and push copy onto stack. 6 varset n ; Pop the top of the stack, ; and bind `n' to the value. ;; (In effect, the sequence `dup varset' copies the top of the stack ;; into the value of `n' without popping it.) 7 constant 0 ; Push 0 onto stack. 8 gtr ; Pop top two values off stack, ; test if N is greater than 0 ; and push result onto stack. 9 goto-if-not-nil 1 ; Goto 1 if `n' > 0 ; (this continues the while loop) ; else continue. 12 varref t1 ; Push value of `t1' onto stack. 13 constant current-time-string ; Push `current-time-string' ; onto the top of the stack. 14 call 0 ; Call `current-time-string' again. 15 unbind 1 ; Unbind `t1' in local environment. 16 list2 ; Pop top two elements off stack, create a ; list of them, and push it onto stack. 17 return ; Return value of the top of stack. 17 Advising Emacs Lisp Functions ******************************** The "advice" feature lets you add to the existing definition of a function, by "advising the function". This is a cleaner method for a library to customize functions defined within Emacs--cleaner than redefining the whole function. Each function can have multiple "pieces of advice", each of which can be separately defined and then "enabled" or "disabled". All the enabled pieces of advice for any given function actually take effect when you "activate advice" for that function, or when you define or redefine the function. Note that enabling a piece of advice and activating advice for a function are not the same thing. Advice is useful for altering the behavior of existing calls to an existing function. If you want the new behavior for new function calls or new key bindings, you should define a new function or command, and have it use the existing function as a subroutine. Advising a function can cause confusion in debugging, since people who debug calls to the original function may not notice that it has been modified with advice. Therefore, if you have the possibility to change the code of that function to run a hook, please solve the problem that way. Advice should be reserved for the cases where you cannot get the function changed. In particular, Emacs's own source files should not put advice on functions in Emacs. There are currently a few exceptions to this convention, but we aim to correct them. Unless you know what you are doing, do _not_ advise a primitive (*note What Is a Function::). Some primitives are used by the advice mechanism; advising them could cause an infinite recursion. Also, many primitives are called directly from C code. Calls to the primitive from Lisp code will take note of the advice, but calls from C code will ignore the advice. 17.1 A Simple Advice Example ============================ The command `next-line' moves point down vertically one or more lines; it is the standard binding of `C-n'. When used on the last line of the buffer, this command inserts a newline to create a line to move to if `next-line-add-newlines' is non-`nil' (its default is `nil'.) Suppose you wanted to add a similar feature to `previous-line', which would insert a new line at the beginning of the buffer for the command to move to (when `next-line-add-newlines' is non-`nil'). How could you do this? You could do it by redefining the whole function, but that is not modular. The advice feature provides a cleaner alternative: you can effectively add your code to the existing function definition, without actually changing or even seeing that definition. Here is how to do this: (defadvice previous-line (before next-line-at-end (&optional arg try-vscroll)) "Insert an empty line when moving up from the top line." (if (and next-line-add-newlines (= arg 1) (save-excursion (beginning-of-line) (bobp))) (progn (beginning-of-line) (newline)))) This expression defines a "piece of advice" for the function `previous-line'. This piece of advice is named `next-line-at-end', and the symbol `before' says that it is "before-advice" which should run before the regular definition of `previous-line'. `(&optional arg try-vscroll)' specifies how the advice code can refer to the function's arguments. When this piece of advice runs, it creates an additional line, in the situation where that is appropriate, but does not move point to that line. This is the correct way to write the advice, because the normal definition will run afterward and will move back to the newly inserted line. Defining the advice doesn't immediately change the function `previous-line'. That happens when you "activate" the advice, like this: (ad-activate 'previous-line) This is what actually begins to use the advice that has been defined so far for the function `previous-line'. Henceforth, whenever that function is run, whether invoked by the user with `C-p' or `M-x', or called from Lisp, it runs the advice first, and its regular definition second. This example illustrates before-advice, which is one "class" of advice: it runs before the function's base definition. There are two other advice classes: "after-advice", which runs after the base definition, and "around-advice", which lets you specify an expression to wrap around the invocation of the base definition. 17.2 Defining Advice ==================== To define a piece of advice, use the macro `defadvice'. A call to `defadvice' has the following syntax, which is based on the syntax of `defun' and `defmacro', but adds more: (defadvice FUNCTION (CLASS NAME [POSITION] [ARGLIST] FLAGS...) [DOCUMENTATION-STRING] [INTERACTIVE-FORM] BODY-FORMS...) Here, FUNCTION is the name of the function (or macro or special form) to be advised. From now on, we will write just "function" when describing the entity being advised, but this always includes macros and special forms. In place of the argument list in an ordinary definition, an advice definition calls for several different pieces of information. CLASS specifies the "class" of the advice--one of `before', `after', or `around'. Before-advice runs before the function itself; after-advice runs after the function itself; around-advice is wrapped around the execution of the function itself. After-advice and around-advice can override the return value by setting `ad-return-value'. -- Variable: ad-return-value While advice is executing, after the function's original definition has been executed, this variable holds its return value, which will ultimately be returned to the caller after finishing all the advice. After-advice and around-advice can arrange to return some other value by storing it in this variable. The argument NAME is the name of the advice, a non-`nil' symbol. The advice name uniquely identifies one piece of advice, within all the pieces of advice in a particular class for a particular FUNCTION. The name allows you to refer to the piece of advice--to redefine it, or to enable or disable it. The optional POSITION specifies where, in the current list of advice of the specified CLASS, this new advice should be placed. It should be either `first', `last' or a number that specifies a zero-based position (`first' is equivalent to 0). If no position is specified, the default is `first'. Position values outside the range of existing positions in this class are mapped to the beginning or the end of the range, whichever is closer. The POSITION value is ignored when redefining an existing piece of advice. The optional ARGLIST can be used to define the argument list for the sake of advice. This becomes the argument list of the combined definition that is generated in order to run the advice (*note Combined Definition::). Therefore, the advice expressions can use the argument variables in this list to access argument values. The argument list used in advice need not be the same as the argument list used in the original function, but must be compatible with it, so that it can handle the ways the function is actually called. If two pieces of advice for a function both specify an argument list, they must specify the same argument list. *Note Argument Access in Advice::, for more information about argument lists and advice, and a more flexible way for advice to access the arguments. The remaining elements, FLAGS, are symbols that specify further information about how to use this piece of advice. Here are the valid symbols and their meanings: `activate' Activate the advice for FUNCTION now. Changes in a function's advice always take effect the next time you activate advice for the function; this flag says to do so, for FUNCTION, immediately after defining this piece of advice. This flag has no immediate effect if FUNCTION itself is not defined yet (a situation known as "forward advice"), because it is impossible to activate an undefined function's advice. However, defining FUNCTION will automatically activate its advice. `protect' Protect this piece of advice against non-local exits and errors in preceding code and advice. Protecting advice places it as a cleanup in an `unwind-protect' form, so that it will execute even if the previous code gets an error or uses `throw'. *Note Cleanups::. `compile' Compile the combined definition that is used to run the advice. This flag is ignored unless `activate' is also specified. *Note Combined Definition::. `disable' Initially disable this piece of advice, so that it will not be used unless subsequently explicitly enabled. *Note Enabling Advice::. `preactivate' Activate advice for FUNCTION when this `defadvice' is compiled or macroexpanded. This generates a compiled advised definition according to the current advice state, which will be used during activation if appropriate. *Note Preactivation::. This is useful only if this `defadvice' is byte-compiled. The optional DOCUMENTATION-STRING serves to document this piece of advice. When advice is active for FUNCTION, the documentation for FUNCTION (as returned by `documentation') combines the documentation strings of all the advice for FUNCTION with the documentation string of its original function definition. The optional INTERACTIVE-FORM form can be supplied to change the interactive behavior of the original function. If more than one piece of advice has an INTERACTIVE-FORM, then the first one (the one with the smallest position) found among all the advice takes precedence. The possibly empty list of BODY-FORMS specifies the body of the advice. The body of an advice can access or change the arguments, the return value, the binding environment, and perform any other kind of side effect. *Warning:* When you advise a macro, keep in mind that macros are expanded when a program is compiled, not when a compiled program is run. All subroutines used by the advice need to be available when the byte compiler expands the macro. -- Command: ad-unadvise function This command deletes all pieces of advice from FUNCTION. -- Command: ad-unadvise-all This command deletes all pieces of advice from all functions. 17.3 Around-Advice ================== Around-advice lets you "wrap" a Lisp expression "around" the original function definition. You specify where the original function definition should go by means of the special symbol `ad-do-it'. Where this symbol occurs inside the around-advice body, it is replaced with a `progn' containing the forms of the surrounded code. Here is an example: (defadvice foo (around foo-around) "Ignore case in `foo'." (let ((case-fold-search t)) ad-do-it)) Its effect is to make sure that case is ignored in searches when the original definition of `foo' is run. -- Variable: ad-do-it This is not really a variable, rather a place-holder that looks like a variable. You use it in around-advice to specify the place to run the function's original definition and other "earlier" around-advice. If the around-advice does not use `ad-do-it', then it does not run the original function definition. This provides a way to override the original definition completely. (It also overrides lower-positioned pieces of around-advice). If the around-advice uses `ad-do-it' more than once, the original definition is run at each place. In this way, around-advice can execute the original definition (and lower-positioned pieces of around-advice) several times. Another way to do that is by using `ad-do-it' inside of a loop. 17.4 Computed Advice ==================== The macro `defadvice' resembles `defun' in that the code for the advice, and all other information about it, are explicitly stated in the source code. You can also create advice whose details are computed, using the function `ad-add-advice'. -- Function: ad-add-advice function advice class position Calling `ad-add-advice' adds ADVICE as a piece of advice to FUNCTION in class CLASS. The argument ADVICE has this form: (NAME PROTECTED ENABLED DEFINITION) Here, PROTECTED and ENABLED are flags; if PROTECTED is non-`nil', the advice is protected against non-local exits (*note Defining Advice::), and if ENABLED is `nil' the advice is initially disabled (*note Enabling Advice::). DEFINITION should have the form (advice . LAMBDA) where LAMBDA is a lambda expression; this lambda expression is called in order to perform the advice. *Note Lambda Expressions::. If the FUNCTION argument to `ad-add-advice' already has one or more pieces of advice in the specified CLASS, then POSITION specifies where in the list to put the new piece of advice. The value of POSITION can either be `first', `last', or a number (counting from 0 at the beginning of the list). Numbers outside the range are mapped to the beginning or the end of the range, whichever is closer. The POSITION value is ignored when redefining an existing piece of advice. If FUNCTION already has a piece of ADVICE with the same name, then the position argument is ignored and the old advice is replaced with the new one. 17.5 Activation of Advice ========================= By default, advice does not take effect when you define it--only when you "activate" advice for the function. However, the advice will be activated automatically if you define or redefine the function later. You can request the activation of advice for a function when you define the advice, by specifying the `activate' flag in the `defadvice'; or you can activate the advice separately by calling the function `ad-activate' or one of the other activation commands listed below. Separating the activation of advice from the act of defining it permits you to add several pieces of advice to one function efficiently, without redefining the function over and over as each advice is added. More importantly, it permits defining advice for a function before that function is actually defined. When a function's advice is first activated, the function's original definition is saved, and all enabled pieces of advice for that function are combined with the original definition to make a new definition. (Pieces of advice that are currently disabled are not used; see *note Enabling Advice::.) This definition is installed, and optionally byte-compiled as well, depending on conditions described below. In all of the commands to activate advice, if COMPILE is `t' (or anything but `nil' or a negative number), the command also compiles the combined definition which implements the advice. If it is `nil' or a negative number, what happens depends on `ad-default-compilation-action' as described below. -- Command: ad-activate function &optional compile This command activates all the advice defined for FUNCTION. Activating advice does nothing if FUNCTION's advice is already active. But if there is new advice, added since the previous time you activated advice for FUNCTION, it activates the new advice. -- Command: ad-deactivate function This command deactivates the advice for FUNCTION. -- Command: ad-update function &optional compile This command activates the advice for FUNCTION if its advice is already activated. This is useful if you change the advice. -- Command: ad-activate-all &optional compile This command activates the advice for all functions. -- Command: ad-deactivate-all This command deactivates the advice for all functions. -- Command: ad-update-all &optional compile This command activates the advice for all functions whose advice is already activated. This is useful if you change the advice of some functions. -- Command: ad-activate-regexp regexp &optional compile This command activates all pieces of advice whose names match REGEXP. More precisely, it activates all advice for any function which has at least one piece of advice that matches REGEXP. -- Command: ad-deactivate-regexp regexp This command deactivates all pieces of advice whose names match REGEXP. More precisely, it deactivates all advice for any function which has at least one piece of advice that matches REGEXP. -- Command: ad-update-regexp regexp &optional compile This command activates pieces of advice whose names match REGEXP, but only those for functions whose advice is already activated. Reactivating a function's advice is useful for putting into effect all the changes that have been made in its advice (including enabling and disabling specific pieces of advice; *note Enabling Advice::) since the last time it was activated. -- Command: ad-start-advice Turn on automatic advice activation when a function is defined or redefined. This is the default mode. -- Command: ad-stop-advice Turn off automatic advice activation when a function is defined or redefined. -- User Option: ad-default-compilation-action This variable controls whether to compile the combined definition that results from activating advice for a function. A value of `always' specifies to compile unconditionally. A value of `never' specifies never compile the advice. A value of `maybe' specifies to compile if the byte compiler is already loaded. A value of `like-original' specifies to compile the advice if the original definition of the advised function is compiled or a built-in function. This variable takes effect only if the COMPILE argument of `ad-activate' (or any of the above functions) did not force compilation. If the advised definition was constructed during "preactivation" (*note Preactivation::), then that definition must already be compiled, because it was constructed during byte-compilation of the file that contained the `defadvice' with the `preactivate' flag. 17.6 Enabling and Disabling Advice ================================== Each piece of advice has a flag that says whether it is enabled or not. By enabling or disabling a piece of advice, you can turn it on and off without having to undefine and redefine it. For example, here is how to disable a particular piece of advice named `my-advice' for the function `foo': (ad-disable-advice 'foo 'before 'my-advice) This function by itself only changes the enable flag for a piece of advice. To make the change take effect in the advised definition, you must activate the advice for `foo' again: (ad-activate 'foo) -- Command: ad-disable-advice function class name This command disables the piece of advice named NAME in class CLASS on FUNCTION. -- Command: ad-enable-advice function class name This command enables the piece of advice named NAME in class CLASS on FUNCTION. You can also disable many pieces of advice at once, for various functions, using a regular expression. As always, the changes take real effect only when you next reactivate advice for the functions in question. -- Command: ad-disable-regexp regexp This command disables all pieces of advice whose names match REGEXP, in all classes, on all functions. -- Command: ad-enable-regexp regexp This command enables all pieces of advice whose names match REGEXP, in all classes, on all functions. 17.7 Preactivation ================== Constructing a combined definition to execute advice is moderately expensive. When a library advises many functions, this can make loading the library slow. In that case, you can use "preactivation" to construct suitable combined definitions in advance. To use preactivation, specify the `preactivate' flag when you define the advice with `defadvice'. This `defadvice' call creates a combined definition which embodies this piece of advice (whether enabled or not) plus any other currently enabled advice for the same function, and the function's own definition. If the `defadvice' is compiled, that compiles the combined definition also. When the function's advice is subsequently activated, if the enabled advice for the function matches what was used to make this combined definition, then the existing combined definition is used, thus avoiding the need to construct one. Thus, preactivation never causes wrong results--but it may fail to do any good, if the enabled advice at the time of activation doesn't match what was used for preactivation. Here are some symptoms that can indicate that a preactivation did not work properly, because of a mismatch. * Activation of the advised function takes longer than usual. * The byte compiler gets loaded while an advised function gets activated. * `byte-compile' is included in the value of `features' even though you did not ever explicitly use the byte compiler. Compiled preactivated advice works properly even if the function itself is not defined until later; however, the function needs to be defined when you _compile_ the preactivated advice. There is no elegant way to find out why preactivated advice is not being used. What you can do is to trace the function `ad-cache-id-verification-code' (with the function `trace-function-background') before the advised function's advice is activated. After activation, check the value returned by `ad-cache-id-verification-code' for that function: `verified' means that the preactivated advice was used, while other values give some information about why they were considered inappropriate. *Warning:* There is one known case that can make preactivation fail, in that a preconstructed combined definition is used even though it fails to match the current state of advice. This can happen when two packages define different pieces of advice with the same name, in the same class, for the same function. But you should avoid that anyway. 17.8 Argument Access in Advice ============================== The simplest way to access the arguments of an advised function in the body of a piece of advice is to use the same names that the function definition uses. To do this, you need to know the names of the argument variables of the original function. While this simple method is sufficient in many cases, it has a disadvantage: it is not robust, because it hard-codes the argument names into the advice. If the definition of the original function changes, the advice might break. Another method is to specify an argument list in the advice itself. This avoids the need to know the original function definition's argument names, but it has a limitation: all the advice on any particular function must use the same argument list, because the argument list actually used for all the advice comes from the first piece of advice for that function. A more robust method is to use macros that are translated into the proper access forms at activation time, i.e., when constructing the advised definition. Access macros access actual arguments by their (zero-based) position, regardless of how these actual arguments get distributed onto the argument variables of a function. This is robust because in Emacs Lisp the meaning of an argument is strictly determined by its position in the argument list. -- Macro: ad-get-arg position This returns the actual argument that was supplied at POSITION. -- Macro: ad-get-args position This returns the list of actual arguments supplied starting at POSITION. -- Macro: ad-set-arg position value This sets the value of the actual argument at POSITION to VALUE -- Macro: ad-set-args position value-list This sets the list of actual arguments starting at POSITION to VALUE-LIST. Now an example. Suppose the function `foo' is defined as (defun foo (x y &optional z &rest r) ...) and is then called with (foo 0 1 2 3 4 5 6) which means that X is 0, Y is 1, Z is 2 and R is `(3 4 5 6)' within the body of `foo'. Here is what `ad-get-arg' and `ad-get-args' return in this case: (ad-get-arg 0) => 0 (ad-get-arg 1) => 1 (ad-get-arg 2) => 2 (ad-get-arg 3) => 3 (ad-get-args 2) => (2 3 4 5 6) (ad-get-args 4) => (4 5 6) Setting arguments also makes sense in this example: (ad-set-arg 5 "five") has the effect of changing the sixth argument to `"five"'. If this happens in advice executed before the body of `foo' is run, then R will be `(3 4 "five" 6)' within that body. Here is an example of setting a tail of the argument list: (ad-set-args 0 '(5 4 3 2 1 0)) If this happens in advice executed before the body of `foo' is run, then within that body, X will be 5, Y will be 4, Z will be 3, and R will be `(2 1 0)' inside the body of `foo'. These argument constructs are not really implemented as Lisp macros. Instead they are implemented specially by the advice mechanism. 17.9 The Combined Definition ============================ Suppose that a function has N pieces of before-advice (numbered from 0 through N-1), M pieces of around-advice and K pieces of after-advice. Assuming no piece of advice is protected, the combined definition produced to implement the advice for a function looks like this: (lambda ARGLIST [ [ADVISED-DOCSTRING] [(interactive ...)] ] (let (ad-return-value) before-0-body-form... .... before-N-1-body-form... around-0-body-form... around-1-body-form... .... around-M-1-body-form... (setq ad-return-value apply original definition to ARGLIST) end-of-around-M-1-body-form... .... end-of-around-1-body-form... end-of-around-0-body-form... after-0-body-form... .... after-K-1-body-form... ad-return-value)) Macros are redefined as macros, which means adding `macro' to the beginning of the combined definition. The interactive form is present if the original function or some piece of advice specifies one. When an interactive primitive function is advised, advice uses a special method: it calls the primitive with `call-interactively' so that it will read its own arguments. In this case, the advice cannot access the arguments. The body forms of the various advice in each class are assembled according to their specified order. The forms of around-advice L are included in one of the forms of around-advice L - 1. The innermost part of the around advice onion is apply original definition to ARGLIST whose form depends on the type of the original function. The variable `ad-return-value' is set to whatever this returns. The variable is visible to all pieces of advice, which can access and modify it before it is actually returned from the advised function. The semantic structure of advised functions that contain protected pieces of advice is the same. The only difference is that `unwind-protect' forms ensure that the protected advice gets executed even if some previous piece of advice had an error or a non-local exit. If any around-advice is protected, then the whole around-advice onion is protected as a result. 18 Debugging Lisp Programs ************************** There are several ways to find and investigate problems in an Emacs Lisp program. * If a problem occurs when you run the program, you can use the built-in Emacs Lisp debugger to suspend the Lisp evaluator, and examine and/or alter its internal state. * You can use Edebug, a source-level debugger for Emacs Lisp. * If a syntactic problem is preventing Lisp from even reading the program, you can locate it using Lisp editing commands. * You can look at the error and warning messages produced by the byte compiler when it compiles the program. *Note Compiler Errors::. * You can use the Testcover package to perform coverage testing on the program. * You can use the ERT package to write regression tests for the program. *Note the ERT manual: (ERT)Top. * You can profile the program to get hints about how to make it more efficient. Other useful tools for debugging input and output problems are the dribble file (*note Terminal Input::) and the `open-termscript' function (*note Terminal Output::). 18.1 The Lisp Debugger ====================== The ordinary "Lisp debugger" provides the ability to suspend evaluation of a form. While evaluation is suspended (a state that is commonly known as a "break"), you may examine the run time stack, examine the values of local or global variables, or change those values. Since a break is a recursive edit, all the usual editing facilities of Emacs are available; you can even run programs that will enter the debugger recursively. *Note Recursive Editing::. 18.1.1 Entering the Debugger on an Error ---------------------------------------- The most important time to enter the debugger is when a Lisp error happens. This allows you to investigate the immediate causes of the error. However, entry to the debugger is not a normal consequence of an error. Many commands signal Lisp errors when invoked inappropriately, and during ordinary editing it would be very inconvenient to enter the debugger each time this happens. So if you want errors to enter the debugger, set the variable `debug-on-error' to non-`nil'. (The command `toggle-debug-on-error' provides an easy way to do this.) -- User Option: debug-on-error This variable determines whether the debugger is called when an error is signaled and not handled. If `debug-on-error' is `t', all kinds of errors call the debugger, except those listed in `debug-ignored-errors' (see below). If it is `nil', none call the debugger. The value can also be a list of error conditions (*note Signaling Errors::). Then the debugger is called only for error conditions in this list (except those also listed in `debug-ignored-errors'). For example, if you set `debug-on-error' to the list `(void-variable)', the debugger is only called for errors about a variable that has no value. Note that `eval-expression-debug-on-error' overrides this variable in some cases; see below. When this variable is non-`nil', Emacs does not create an error handler around process filter functions and sentinels. Therefore, errors in these functions also invoke the debugger. *Note Processes::. -- User Option: debug-ignored-errors This variable specifies errors which should not enter the debugger, regardless of the value of `debug-on-error'. Its value is a list of error condition symbols and/or regular expressions. If the error has any of those condition symbols, or if the error message matches any of the regular expressions, then that error does not enter the debugger. The normal value of this variable includes `user-error', as well as several errors that happen often during editing but rarely result from bugs in Lisp programs. However, "rarely" is not "never"; if your program fails with an error that matches this list, you may try changing this list to debug the error. The easiest way is usually to set `debug-ignored-errors' to `nil'. -- User Option: eval-expression-debug-on-error If this variable has a non-`nil' value (the default), running the command `eval-expression' causes `debug-on-error' to be temporarily bound to to `t'. *Note Evaluating Emacs-Lisp Expressions: (emacs)Lisp Eval. If `eval-expression-debug-on-error' is `nil', then the value of `debug-on-error' is not changed during `eval-expression'. -- Variable: debug-on-signal Normally, errors caught by `condition-case' never invoke the debugger. The `condition-case' gets a chance to handle the error before the debugger gets a chance. If you change `debug-on-signal' to a non-`nil' value, the debugger gets the first chance at every error, regardless of the presence of `condition-case'. (To invoke the debugger, the error must still fulfill the criteria specified by `debug-on-error' and `debug-ignored-errors'.) *Warning:* Setting this variable to non-`nil' may have annoying effects. Various parts of Emacs catch errors in the normal course of affairs, and you may not even realize that errors happen there. If you need to debug code wrapped in `condition-case', consider using `condition-case-unless-debug' (*note Handling Errors::). -- User Option: debug-on-event If you set `debug-on-event' to a special event (*note Special Events::), Emacs will try to enter the debugger as soon as it receives this event, bypassing `special-event-map'. At present, the only supported values correspond to the signals `SIGUSR1' and `SIGUSR2' (this is the default). This can be helpful when `inhibit-quit' is set and Emacs is not otherwise responding. -- Variable: debug-on-message If you set `debug-on-message' to a regular expression, Emacs will enter the debugger if it displays a matching message in the echo area. For example, this can be useful when trying to find the cause of a particular message. To debug an error that happens during loading of the init file, use the option `--debug-init'. This binds `debug-on-error' to `t' while loading the init file, and bypasses the `condition-case' which normally catches errors in the init file. 18.1.2 Debugging Infinite Loops ------------------------------- When a program loops infinitely and fails to return, your first problem is to stop the loop. On most operating systems, you can do this with `C-g', which causes a "quit". *Note Quitting::. Ordinary quitting gives no information about why the program was looping. To get more information, you can set the variable `debug-on-quit' to non-`nil'. Once you have the debugger running in the middle of the infinite loop, you can proceed from the debugger using the stepping commands. If you step through the entire loop, you may get enough information to solve the problem. Quitting with `C-g' is not considered an error, and `debug-on-error' has no effect on the handling of `C-g'. Likewise, `debug-on-quit' has no effect on errors. -- User Option: debug-on-quit This variable determines whether the debugger is called when `quit' is signaled and not handled. If `debug-on-quit' is non-`nil', then the debugger is called whenever you quit (that is, type `C-g'). If `debug-on-quit' is `nil' (the default), then the debugger is not called when you quit. 18.1.3 Entering the Debugger on a Function Call ----------------------------------------------- To investigate a problem that happens in the middle of a program, one useful technique is to enter the debugger whenever a certain function is called. You can do this to the function in which the problem occurs, and then step through the function, or you can do this to a function called shortly before the problem, step quickly over the call to that function, and then step through its caller. -- Command: debug-on-entry function-name This function requests FUNCTION-NAME to invoke the debugger each time it is called. It works by inserting the form `(implement-debug-on-entry)' into the function definition as the first form. Any function or macro defined as Lisp code may be set to break on entry, regardless of whether it is interpreted code or compiled code. If the function is a command, it will enter the debugger when called from Lisp and when called interactively (after the reading of the arguments). You can also set debug-on-entry for primitive functions (i.e., those written in C) this way, but it only takes effect when the primitive is called from Lisp code. Debug-on-entry is not allowed for special forms. When `debug-on-entry' is called interactively, it prompts for FUNCTION-NAME in the minibuffer. If the function is already set up to invoke the debugger on entry, `debug-on-entry' does nothing. `debug-on-entry' always returns FUNCTION-NAME. *Warning:* if you redefine a function after using `debug-on-entry' on it, the code to enter the debugger is discarded by the redefinition. In effect, redefining the function cancels the break-on-entry feature for that function. Here's an example to illustrate use of this function: (defun fact (n) (if (zerop n) 1 (* n (fact (1- n))))) => fact (debug-on-entry 'fact) => fact (fact 3) ------ Buffer: *Backtrace* ------ Debugger entered--entering a function: * fact(3) eval((fact 3)) eval-last-sexp-1(nil) eval-last-sexp(nil) call-interactively(eval-last-sexp) ------ Buffer: *Backtrace* ------ (symbol-function 'fact) => (lambda (n) (debug (quote debug)) (if (zerop n) 1 (* n (fact (1- n))))) -- Command: cancel-debug-on-entry &optional function-name This function undoes the effect of `debug-on-entry' on FUNCTION-NAME. When called interactively, it prompts for FUNCTION-NAME in the minibuffer. If FUNCTION-NAME is omitted or `nil', it cancels break-on-entry for all functions. Calling `cancel-debug-on-entry' does nothing to a function which is not currently set up to break on entry. 18.1.4 Explicit Entry to the Debugger ------------------------------------- You can cause the debugger to be called at a certain point in your program by writing the expression `(debug)' at that point. To do this, visit the source file, insert the text `(debug)' at the proper place, and type `C-M-x' (`eval-defun', a Lisp mode key binding). *Warning:* if you do this for temporary debugging purposes, be sure to undo this insertion before you save the file! The place where you insert `(debug)' must be a place where an additional form can be evaluated and its value ignored. (If the value of `(debug)' isn't ignored, it will alter the execution of the program!) The most common suitable places are inside a `progn' or an implicit `progn' (*note Sequencing::). If you don't know exactly where in the source code you want to put the debug statement, but you want to display a backtrace when a certain message is displayed, you can set `debug-on-message' to a regular expression matching the desired message. 18.1.5 Using the Debugger ------------------------- When the debugger is entered, it displays the previously selected buffer in one window and a buffer named `*Backtrace*' in another window. The backtrace buffer contains one line for each level of Lisp function execution currently going on. At the beginning of this buffer is a message describing the reason that the debugger was invoked (such as the error message and associated data, if it was invoked due to an error). The backtrace buffer is read-only and uses a special major mode, Debugger mode, in which letters are defined as debugger commands. The usual Emacs editing commands are available; thus, you can switch windows to examine the buffer that was being edited at the time of the error, switch buffers, visit files, or do any other sort of editing. However, the debugger is a recursive editing level (*note Recursive Editing::) and it is wise to go back to the backtrace buffer and exit the debugger (with the `q' command) when you are finished with it. Exiting the debugger gets out of the recursive edit and buries the backtrace buffer. (You can customize what the `q' command does with the backtrace buffer by setting the variable `debugger-bury-or-kill'. For example, set it to `kill' if you prefer to kill the buffer rather than bury it. Consult the variable's documentation for more possibilities.) When the debugger has been entered, the `debug-on-error' variable is temporarily set according to `eval-expression-debug-on-error'. If the latter variable is non-`nil', `debug-on-error' will temporarily be set to `t'. This means that any further errors that occur while doing a debugging session will (by default) trigger another backtrace. If this is not what you want, you can either set `eval-expression-debug-on-error' to `nil', or set `debug-on-error' to `nil' in `debugger-mode-hook'. The backtrace buffer shows you the functions that are executing and their argument values. It also allows you to specify a stack frame by moving point to the line describing that frame. (A stack frame is the place where the Lisp interpreter records information about a particular invocation of a function.) The frame whose line point is on is considered the "current frame". Some of the debugger commands operate on the current frame. If a line starts with a star, that means that exiting that frame will call the debugger again. This is useful for examining the return value of a function. If a function name is underlined, that means the debugger knows where its source code is located. You can click with the mouse on that name, or move to it and type , to visit the source code. The debugger itself must be run byte-compiled, since it makes assumptions about how many stack frames are used for the debugger itself. These assumptions are false if the debugger is running interpreted. 18.1.6 Debugger Commands ------------------------ The debugger buffer (in Debugger mode) provides special commands in addition to the usual Emacs commands. The most important use of debugger commands is for stepping through code, so that you can see how control flows. The debugger can step through the control structures of an interpreted function, but cannot do so in a byte-compiled function. If you would like to step through a byte-compiled function, replace it with an interpreted definition of the same function. (To do this, visit the source for the function and type `C-M-x' on its definition.) You cannot use the Lisp debugger to step through a primitive function. Here is a list of Debugger mode commands: `c' Exit the debugger and continue execution. This resumes execution of the program as if the debugger had never been entered (aside from any side-effects that you caused by changing variable values or data structures while inside the debugger). `d' Continue execution, but enter the debugger the next time any Lisp function is called. This allows you to step through the subexpressions of an expression, seeing what values the subexpressions compute, and what else they do. The stack frame made for the function call which enters the debugger in this way will be flagged automatically so that the debugger will be called again when the frame is exited. You can use the `u' command to cancel this flag. `b' Flag the current frame so that the debugger will be entered when the frame is exited. Frames flagged in this way are marked with stars in the backtrace buffer. `u' Don't enter the debugger when the current frame is exited. This cancels a `b' command on that frame. The visible effect is to remove the star from the line in the backtrace buffer. `j' Flag the current frame like `b'. Then continue execution like `c', but temporarily disable break-on-entry for all functions that are set up to do so by `debug-on-entry'. `e' Read a Lisp expression in the minibuffer, evaluate it, and print the value in the echo area. The debugger alters certain important variables, and the current buffer, as part of its operation; `e' temporarily restores their values from outside the debugger, so you can examine and change them. This makes the debugger more transparent. By contrast, `M-:' does nothing special in the debugger; it shows you the variable values within the debugger. `R' Like `e', but also save the result of evaluation in the buffer `*Debugger-record*'. `q' Terminate the program being debugged; return to top-level Emacs command execution. If the debugger was entered due to a `C-g' but you really want to quit, and not debug, use the `q' command. `r' Return a value from the debugger. The value is computed by reading an expression with the minibuffer and evaluating it. The `r' command is useful when the debugger was invoked due to exit from a Lisp call frame (as requested with `b' or by entering the frame with `d'); then the value specified in the `r' command is used as the value of that frame. It is also useful if you call `debug' and use its return value. Otherwise, `r' has the same effect as `c', and the specified return value does not matter. You can't use `r' when the debugger was entered due to an error. `l' Display a list of functions that will invoke the debugger when called. This is a list of functions that are set to break on entry by means of `debug-on-entry'. *Warning:* if you redefine such a function and thus cancel the effect of `debug-on-entry', it may erroneously show up in this list. 18.1.7 Invoking the Debugger ---------------------------- Here we describe in full detail the function `debug' that is used to invoke the debugger. -- Command: debug &rest debugger-args This function enters the debugger. It switches buffers to a buffer named `*Backtrace*' (or `*Backtrace*<2>' if it is the second recursive entry to the debugger, etc.), and fills it with information about the stack of Lisp function calls. It then enters a recursive edit, showing the backtrace buffer in Debugger mode. The Debugger mode `c', `d', `j', and `r' commands exit the recursive edit; then `debug' switches back to the previous buffer and returns to whatever called `debug'. This is the only way the function `debug' can return to its caller. The use of the DEBUGGER-ARGS is that `debug' displays the rest of its arguments at the top of the `*Backtrace*' buffer, so that the user can see them. Except as described below, this is the _only_ way these arguments are used. However, certain values for first argument to `debug' have a special significance. (Normally, these values are used only by the internals of Emacs, and not by programmers calling `debug'.) Here is a table of these special values: `lambda' A first argument of `lambda' means `debug' was called because of entry to a function when `debug-on-next-call' was non-`nil'. The debugger displays `Debugger entered--entering a function:' as a line of text at the top of the buffer. `debug' `debug' as first argument means `debug' was called because of entry to a function that was set to debug on entry. The debugger displays the string `Debugger entered--entering a function:', just as in the `lambda' case. It also marks the stack frame for that function so that it will invoke the debugger when exited. `t' When the first argument is `t', this indicates a call to `debug' due to evaluation of a function call form when `debug-on-next-call' is non-`nil'. The debugger displays `Debugger entered--beginning evaluation of function call form:' as the top line in the buffer. `exit' When the first argument is `exit', it indicates the exit of a stack frame previously marked to invoke the debugger on exit. The second argument given to `debug' in this case is the value being returned from the frame. The debugger displays `Debugger entered--returning value:' in the top line of the buffer, followed by the value being returned. `error' When the first argument is `error', the debugger indicates that it is being entered because an error or `quit' was signaled and not handled, by displaying `Debugger entered--Lisp error:' followed by the error signaled and any arguments to `signal'. For example, (let ((debug-on-error t)) (/ 1 0)) ------ Buffer: *Backtrace* ------ Debugger entered--Lisp error: (arith-error) /(1 0) ... ------ Buffer: *Backtrace* ------ If an error was signaled, presumably the variable `debug-on-error' is non-`nil'. If `quit' was signaled, then presumably the variable `debug-on-quit' is non-`nil'. `nil' Use `nil' as the first of the DEBUGGER-ARGS when you want to enter the debugger explicitly. The rest of the DEBUGGER-ARGS are printed on the top line of the buffer. You can use this feature to display messages--for example, to remind yourself of the conditions under which `debug' is called. 18.1.8 Internals of the Debugger -------------------------------- This section describes functions and variables used internally by the debugger. -- Variable: debugger The value of this variable is the function to call to invoke the debugger. Its value must be a function of any number of arguments, or, more typically, the name of a function. This function should invoke some kind of debugger. The default value of the variable is `debug'. The first argument that Lisp hands to the function indicates why it was called. The convention for arguments is detailed in the description of `debug' (*note Invoking the Debugger::). -- Command: backtrace This function prints a trace of Lisp function calls currently active. This is the function used by `debug' to fill up the `*Backtrace*' buffer. It is written in C, since it must have access to the stack to determine which function calls are active. The return value is always `nil'. In the following example, a Lisp expression calls `backtrace' explicitly. This prints the backtrace to the stream `standard-output', which, in this case, is the buffer `backtrace-output'. Each line of the backtrace represents one function call. The line shows the values of the function's arguments if they are all known; if they are still being computed, the line says so. The arguments of special forms are elided. (with-output-to-temp-buffer "backtrace-output" (let ((var 1)) (save-excursion (setq var (eval '(progn (1+ var) (list 'testing (backtrace)))))))) => (testing nil) ----------- Buffer: backtrace-output ------------ backtrace() (list ...computing arguments...) (progn ...) eval((progn (1+ var) (list (quote testing) (backtrace)))) (setq ...) (save-excursion ...) (let ...) (with-output-to-temp-buffer ...) eval((with-output-to-temp-buffer ...)) eval-last-sexp-1(nil) eval-last-sexp(nil) call-interactively(eval-last-sexp) ----------- Buffer: backtrace-output ------------ -- Variable: debug-on-next-call If this variable is non-`nil', it says to call the debugger before the next `eval', `apply' or `funcall'. Entering the debugger sets `debug-on-next-call' to `nil'. The `d' command in the debugger works by setting this variable. -- Function: backtrace-debug level flag This function sets the debug-on-exit flag of the stack frame LEVEL levels down the stack, giving it the value FLAG. If FLAG is non-`nil', this will cause the debugger to be entered when that frame later exits. Even a nonlocal exit through that frame will enter the debugger. This function is used only by the debugger. -- Variable: command-debug-status This variable records the debugging status of the current interactive command. Each time a command is called interactively, this variable is bound to `nil'. The debugger can set this variable to leave information for future debugger invocations during the same command invocation. The advantage of using this variable rather than an ordinary global variable is that the data will never carry over to a subsequent command invocation. -- Function: backtrace-frame frame-number The function `backtrace-frame' is intended for use in Lisp debuggers. It returns information about what computation is happening in the stack frame FRAME-NUMBER levels down. If that frame has not evaluated the arguments yet, or is a special form, the value is `(nil FUNCTION ARG-FORMS...)'. If that frame has evaluated its arguments and called its function already, the return value is `(t FUNCTION ARG-VALUES...)'. In the return value, FUNCTION is whatever was supplied as the CAR of the evaluated list, or a `lambda' expression in the case of a macro call. If the function has a `&rest' argument, that is represented as the tail of the list ARG-VALUES. If FRAME-NUMBER is out of range, `backtrace-frame' returns `nil'. 18.2 Edebug =========== Edebug is a source-level debugger for Emacs Lisp programs, with which you can: * Step through evaluation, stopping before and after each expression. * Set conditional or unconditional breakpoints. * Stop when a specified condition is true (the global break event). * Trace slow or fast, stopping briefly at each stop point, or at each breakpoint. * Display expression results and evaluate expressions as if outside of Edebug. * Automatically re-evaluate a list of expressions and display their results each time Edebug updates the display. * Output trace information on function calls and returns. * Stop when an error occurs. * Display a backtrace, omitting Edebug's own frames. * Specify argument evaluation for macros and defining forms. * Obtain rudimentary coverage testing and frequency counts. The first three sections below should tell you enough about Edebug to start using it. 18.2.1 Using Edebug ------------------- To debug a Lisp program with Edebug, you must first "instrument" the Lisp code that you want to debug. A simple way to do this is to first move point into the definition of a function or macro and then do `C-u C-M-x' (`eval-defun' with a prefix argument). See *note Instrumenting::, for alternative ways to instrument code. Once a function is instrumented, any call to the function activates Edebug. Depending on which Edebug execution mode you have selected, activating Edebug may stop execution and let you step through the function, or it may update the display and continue execution while checking for debugging commands. The default execution mode is step, which stops execution. *Note Edebug Execution Modes::. Within Edebug, you normally view an Emacs buffer showing the source of the Lisp code you are debugging. This is referred to as the "source code buffer", and it is temporarily read-only. An arrow in the left fringe indicates the line where the function is executing. Point initially shows where within the line the function is executing, but this ceases to be true if you move point yourself. If you instrument the definition of `fac' (shown below) and then execute `(fac 3)', here is what you would normally see. Point is at the open-parenthesis before `if'. (defun fac (n) =>-!-(if (< 0 n) (* n (fac (1- n))) 1)) The places within a function where Edebug can stop execution are called "stop points". These occur both before and after each subexpression that is a list, and also after each variable reference. Here we use periods to show the stop points in the function `fac': (defun fac (n) .(if .(< 0 n.). .(* n. .(fac .(1- n.).).). 1).) The special commands of Edebug are available in the source code buffer in addition to the commands of Emacs Lisp mode. For example, you can type the Edebug command to execute until the next stop point. If you type once after entry to `fac', here is the display you will see: (defun fac (n) =>(if -!-(< 0 n) (* n (fac (1- n))) 1)) When Edebug stops execution after an expression, it displays the expression's value in the echo area. Other frequently used commands are `b' to set a breakpoint at a stop point, `g' to execute until a breakpoint is reached, and `q' to exit Edebug and return to the top-level command loop. Type `?' to display a list of all Edebug commands. 18.2.2 Instrumenting for Edebug ------------------------------- In order to use Edebug to debug Lisp code, you must first "instrument" the code. Instrumenting code inserts additional code into it, to invoke Edebug at the proper places. When you invoke command `C-M-x' (`eval-defun') with a prefix argument on a function definition, it instruments the definition before evaluating it. (This does not modify the source code itself.) If the variable `edebug-all-defs' is non-`nil', that inverts the meaning of the prefix argument: in this case, `C-M-x' instruments the definition _unless_ it has a prefix argument. The default value of `edebug-all-defs' is `nil'. The command `M-x edebug-all-defs' toggles the value of the variable `edebug-all-defs'. If `edebug-all-defs' is non-`nil', then the commands `eval-region', `eval-current-buffer', and `eval-buffer' also instrument any definitions they evaluate. Similarly, `edebug-all-forms' controls whether `eval-region' should instrument _any_ form, even non-defining forms. This doesn't apply to loading or evaluations in the minibuffer. The command `M-x edebug-all-forms' toggles this option. Another command, `M-x edebug-eval-top-level-form', is available to instrument any top-level form regardless of the values of `edebug-all-defs' and `edebug-all-forms'. `edebug-defun' is an alias for `edebug-eval-top-level-form'. While Edebug is active, the command `I' (`edebug-instrument-callee') instruments the definition of the function or macro called by the list form after point, if it is not already instrumented. This is possible only if Edebug knows where to find the source for that function; for this reason, after loading Edebug, `eval-region' records the position of every definition it evaluates, even if not instrumenting it. See also the `i' command (*note Jumping::), which steps into the call after instrumenting the function. Edebug knows how to instrument all the standard special forms, `interactive' forms with an expression argument, anonymous lambda expressions, and other defining forms. However, Edebug cannot determine on its own what a user-defined macro will do with the arguments of a macro call, so you must provide that information using Edebug specifications; for details, *note Edebug and Macros::. When Edebug is about to instrument code for the first time in a session, it runs the hook `edebug-setup-hook', then sets it to `nil'. You can use this to load Edebug specifications associated with a package you are using, but only when you use Edebug. To remove instrumentation from a definition, simply re-evaluate its definition in a way that does not instrument. There are two ways of evaluating forms that never instrument them: from a file with `load', and from the minibuffer with `eval-expression' (`M-:'). If Edebug detects a syntax error while instrumenting, it leaves point at the erroneous code and signals an `invalid-read-syntax' error. *Note Edebug Eval::, for other evaluation functions available inside of Edebug. 18.2.3 Edebug Execution Modes ----------------------------- Edebug supports several execution modes for running the program you are debugging. We call these alternatives "Edebug execution modes"; do not confuse them with major or minor modes. The current Edebug execution mode determines how far Edebug continues execution before stopping--whether it stops at each stop point, or continues to the next breakpoint, for example--and how much Edebug displays the progress of the evaluation before it stops. Normally, you specify the Edebug execution mode by typing a command to continue the program in a certain mode. Here is a table of these commands; all except for `S' resume execution of the program, at least for a certain distance. `S' Stop: don't execute any more of the program, but wait for more Edebug commands (`edebug-stop'). `' Step: stop at the next stop point encountered (`edebug-step-mode'). `n' Next: stop at the next stop point encountered after an expression (`edebug-next-mode'). Also see `edebug-forward-sexp' in *note Jumping::. `t' Trace: pause (normally one second) at each Edebug stop point (`edebug-trace-mode'). `T' Rapid trace: update the display at each stop point, but don't actually pause (`edebug-Trace-fast-mode'). `g' Go: run until the next breakpoint (`edebug-go-mode'). *Note Breakpoints::. `c' Continue: pause one second at each breakpoint, and then continue (`edebug-continue-mode'). `C' Rapid continue: move point to each breakpoint, but don't pause (`edebug-Continue-fast-mode'). `G' Go non-stop: ignore breakpoints (`edebug-Go-nonstop-mode'). You can still stop the program by typing `S', or any editing command. In general, the execution modes earlier in the above list run the program more slowly or stop sooner than the modes later in the list. While executing or tracing, you can interrupt the execution by typing any Edebug command. Edebug stops the program at the next stop point and then executes the command you typed. For example, typing `t' during execution switches to trace mode at the next stop point. You can use `S' to stop execution without doing anything else. If your function happens to read input, a character you type intending to interrupt execution may be read by the function instead. You can avoid such unintended results by paying attention to when your program wants input. Keyboard macros containing the commands in this section do not completely work: exiting from Edebug, to resume the program, loses track of the keyboard macro. This is not easy to fix. Also, defining or executing a keyboard macro outside of Edebug does not affect commands inside Edebug. This is usually an advantage. See also the `edebug-continue-kbd-macro' option in *note Edebug Options::. When you enter a new Edebug level, the initial execution mode comes from the value of the variable `edebug-initial-mode' (*note Edebug Options::). By default, this specifies step mode. Note that you may reenter the same Edebug level several times if, for example, an instrumented function is called several times from one command. -- User Option: edebug-sit-for-seconds This option specifies how many seconds to wait between execution steps in trace mode or continue mode. The default is 1 second. 18.2.4 Jumping -------------- The commands described in this section execute until they reach a specified location. All except `i' make a temporary breakpoint to establish the place to stop, then switch to go mode. Any other breakpoint reached before the intended stop point will also stop execution. *Note Breakpoints::, for the details on breakpoints. These commands may fail to work as expected in case of nonlocal exit, as that can bypass the temporary breakpoint where you expected the program to stop. `h' Proceed to the stop point near where point is (`edebug-goto-here'). `f' Run the program for one expression (`edebug-forward-sexp'). `o' Run the program until the end of the containing sexp (`edebug-step-out'). `i' Step into the function or macro called by the form after point (`edebug-step-in'). The `h' command proceeds to the stop point at or after the current location of point, using a temporary breakpoint. The `f' command runs the program forward over one expression. More precisely, it sets a temporary breakpoint at the position that `forward-sexp' would reach, then executes in go mode so that the program will stop at breakpoints. With a prefix argument N, the temporary breakpoint is placed N sexps beyond point. If the containing list ends before N more elements, then the place to stop is after the containing expression. You must check that the position `forward-sexp' finds is a place that the program will really get to. In `cond', for example, this may not be true. For flexibility, the `f' command does `forward-sexp' starting at point, rather than at the stop point. If you want to execute one expression _from the current stop point_, first type `w' (`edebug-where') to move point there, and then type `f'. The `o' command continues "out of" an expression. It places a temporary breakpoint at the end of the sexp containing point. If the containing sexp is a function definition itself, `o' continues until just before the last sexp in the definition. If that is where you are now, it returns from the function and then stops. In other words, this command does not exit the currently executing function unless you are positioned after the last sexp. The `i' command steps into the function or macro called by the list form after point, and stops at its first stop point. Note that the form need not be the one about to be evaluated. But if the form is a function call about to be evaluated, remember to use this command before any of the arguments are evaluated, since otherwise it will be too late. The `i' command instruments the function or macro it's supposed to step into, if it isn't instrumented already. This is convenient, but keep in mind that the function or macro remains instrumented unless you explicitly arrange to deinstrument it. 18.2.5 Miscellaneous Edebug Commands ------------------------------------ Some miscellaneous Edebug commands are described here. `?' Display the help message for Edebug (`edebug-help'). `C-]' Abort one level back to the previous command level (`abort-recursive-edit'). `q' Return to the top level editor command loop (`top-level'). This exits all recursive editing levels, including all levels of Edebug activity. However, instrumented code protected with `unwind-protect' or `condition-case' forms may resume debugging. `Q' Like `q', but don't stop even for protected code (`edebug-top-level-nonstop'). `r' Redisplay the most recently known expression result in the echo area (`edebug-previous-result'). `d' Display a backtrace, excluding Edebug's own functions for clarity (`edebug-backtrace'). You cannot use debugger commands in the backtrace buffer in Edebug as you would in the standard debugger. The backtrace buffer is killed automatically when you continue execution. You can invoke commands from Edebug that activate Edebug again recursively. Whenever Edebug is active, you can quit to the top level with `q' or abort one recursive edit level with `C-]'. You can display a backtrace of all the pending evaluations with `d'. 18.2.6 Breaks ------------- Edebug's step mode stops execution when the next stop point is reached. There are three other ways to stop Edebug execution once it has started: breakpoints, the global break condition, and source breakpoints. 18.2.6.1 Edebug Breakpoints ........................... While using Edebug, you can specify "breakpoints" in the program you are testing: these are places where execution should stop. You can set a breakpoint at any stop point, as defined in *note Using Edebug::. For setting and unsetting breakpoints, the stop point that is affected is the first one at or after point in the source code buffer. Here are the Edebug commands for breakpoints: `b' Set a breakpoint at the stop point at or after point (`edebug-set-breakpoint'). If you use a prefix argument, the breakpoint is temporary--it turns off the first time it stops the program. `u' Unset the breakpoint (if any) at the stop point at or after point (`edebug-unset-breakpoint'). `x CONDITION ' Set a conditional breakpoint which stops the program only if evaluating CONDITION produces a non-`nil' value (`edebug-set-conditional-breakpoint'). With a prefix argument, the breakpoint is temporary. `B' Move point to the next breakpoint in the current definition (`edebug-next-breakpoint'). While in Edebug, you can set a breakpoint with `b' and unset one with `u'. First move point to the Edebug stop point of your choice, then type `b' or `u' to set or unset a breakpoint there. Unsetting a breakpoint where none has been set has no effect. Re-evaluating or reinstrumenting a definition removes all of its previous breakpoints. A "conditional breakpoint" tests a condition each time the program gets there. Any errors that occur as a result of evaluating the condition are ignored, as if the result were `nil'. To set a conditional breakpoint, use `x', and specify the condition expression in the minibuffer. Setting a conditional breakpoint at a stop point that has a previously established conditional breakpoint puts the previous condition expression in the minibuffer so you can edit it. You can make a conditional or unconditional breakpoint "temporary" by using a prefix argument with the command to set the breakpoint. When a temporary breakpoint stops the program, it is automatically unset. Edebug always stops or pauses at a breakpoint, except when the Edebug mode is Go-nonstop. In that mode, it ignores breakpoints entirely. To find out where your breakpoints are, use the `B' command, which moves point to the next breakpoint following point, within the same function, or to the first breakpoint if there are no following breakpoints. This command does not continue execution--it just moves point in the buffer. 18.2.6.2 Global Break Condition ............................... A "global break condition" stops execution when a specified condition is satisfied, no matter where that may occur. Edebug evaluates the global break condition at every stop point; if it evaluates to a non-`nil' value, then execution stops or pauses depending on the execution mode, as if a breakpoint had been hit. If evaluating the condition gets an error, execution does not stop. The condition expression is stored in `edebug-global-break-condition'. You can specify a new expression using the `X' command from the source code buffer while Edebug is active, or using `C-x X X' from any buffer at any time, as long as Edebug is loaded (`edebug-set-global-break-condition'). The global break condition is the simplest way to find where in your code some event occurs, but it makes code run much more slowly. So you should reset the condition to `nil' when not using it. 18.2.6.3 Source Breakpoints ........................... All breakpoints in a definition are forgotten each time you reinstrument it. If you wish to make a breakpoint that won't be forgotten, you can write a "source breakpoint", which is simply a call to the function `edebug' in your source code. You can, of course, make such a call conditional. For example, in the `fac' function, you can insert the first line as shown below, to stop when the argument reaches zero: (defun fac (n) (if (= n 0) (edebug)) (if (< 0 n) (* n (fac (1- n))) 1)) When the `fac' definition is instrumented and the function is called, the call to `edebug' acts as a breakpoint. Depending on the execution mode, Edebug stops or pauses there. If no instrumented code is being executed when `edebug' is called, that function calls `debug'. 18.2.7 Trapping Errors ---------------------- Emacs normally displays an error message when an error is signaled and not handled with `condition-case'. While Edebug is active and executing instrumented code, it normally responds to all unhandled errors. You can customize this with the options `edebug-on-error' and `edebug-on-quit'; see *note Edebug Options::. When Edebug responds to an error, it shows the last stop point encountered before the error. This may be the location of a call to a function which was not instrumented, and within which the error actually occurred. For an unbound variable error, the last known stop point might be quite distant from the offending variable reference. In that case, you might want to display a full backtrace (*note Edebug Misc::). If you change `debug-on-error' or `debug-on-quit' while Edebug is active, these changes will be forgotten when Edebug becomes inactive. Furthermore, during Edebug's recursive edit, these variables are bound to the values they had outside of Edebug. 18.2.8 Edebug Views ------------------- These Edebug commands let you view aspects of the buffer and window status as they were before entry to Edebug. The outside window configuration is the collection of windows and contents that were in effect outside of Edebug. `v' Switch to viewing the outside window configuration (`edebug-view-outside'). Type `C-x X w' to return to Edebug. `p' Temporarily display the outside current buffer with point at its outside position (`edebug-bounce-point'), pausing for one second before returning to Edebug. With a prefix argument N, pause for N seconds instead. `w' Move point back to the current stop point in the source code buffer (`edebug-where'). If you use this command in a different window displaying the same buffer, that window will be used instead to display the current definition in the future. `W' Toggle whether Edebug saves and restores the outside window configuration (`edebug-toggle-save-windows'). With a prefix argument, `W' only toggles saving and restoring of the selected window. To specify a window that is not displaying the source code buffer, you must use `C-x X W' from the global keymap. You can view the outside window configuration with `v' or just bounce to the point in the current buffer with `p', even if it is not normally displayed. After moving point, you may wish to jump back to the stop point. You can do that with `w' from a source code buffer. You can jump back to the stop point in the source code buffer from any buffer using `C-x X w'. Each time you use `W' to turn saving _off_, Edebug forgets the saved outside window configuration--so that even if you turn saving back _on_, the current window configuration remains unchanged when you next exit Edebug (by continuing the program). However, the automatic redisplay of `*edebug*' and `*edebug-trace*' may conflict with the buffers you wish to see unless you have enough windows open. 18.2.9 Evaluation ----------------- While within Edebug, you can evaluate expressions as if Edebug were not running. Edebug tries to be invisible to the expression's evaluation and printing. Evaluation of expressions that cause side effects will work as expected, except for changes to data that Edebug explicitly saves and restores. *Note The Outside Context::, for details on this process. `e EXP ' Evaluate expression EXP in the context outside of Edebug (`edebug-eval-expression'). That is, Edebug tries to minimize its interference with the evaluation. `M-: EXP ' Evaluate expression EXP in the context of Edebug itself (`eval-expression'). `C-x C-e' Evaluate the expression before point, in the context outside of Edebug (`edebug-eval-last-sexp'). Edebug supports evaluation of expressions containing references to lexically bound symbols created by the following constructs in `cl.el': `lexical-let', `macrolet', and `symbol-macrolet'. 18.2.10 Evaluation List Buffer ------------------------------ You can use the "evaluation list buffer", called `*edebug*', to evaluate expressions interactively. You can also set up the "evaluation list" of expressions to be evaluated automatically each time Edebug updates the display. `E' Switch to the evaluation list buffer `*edebug*' (`edebug-visit-eval-list'). In the `*edebug*' buffer you can use the commands of Lisp Interaction mode (*note Lisp Interaction: (emacs)Lisp Interaction.) as well as these special commands: `C-j' Evaluate the expression before point, in the outside context, and insert the value in the buffer (`edebug-eval-print-last-sexp'). `C-x C-e' Evaluate the expression before point, in the context outside of Edebug (`edebug-eval-last-sexp'). `C-c C-u' Build a new evaluation list from the contents of the buffer (`edebug-update-eval-list'). `C-c C-d' Delete the evaluation list group that point is in (`edebug-delete-eval-item'). `C-c C-w' Switch back to the source code buffer at the current stop point (`edebug-where'). You can evaluate expressions in the evaluation list window with `C-j' or `C-x C-e', just as you would in `*scratch*'; but they are evaluated in the context outside of Edebug. The expressions you enter interactively (and their results) are lost when you continue execution; but you can set up an "evaluation list" consisting of expressions to be evaluated each time execution stops. To do this, write one or more "evaluation list groups" in the evaluation list buffer. An evaluation list group consists of one or more Lisp expressions. Groups are separated by comment lines. The command `C-c C-u' (`edebug-update-eval-list') rebuilds the evaluation list, scanning the buffer and using the first expression of each group. (The idea is that the second expression of the group is the value previously computed and displayed.) Each entry to Edebug redisplays the evaluation list by inserting each expression in the buffer, followed by its current value. It also inserts comment lines so that each expression becomes its own group. Thus, if you type `C-c C-u' again without changing the buffer text, the evaluation list is effectively unchanged. If an error occurs during an evaluation from the evaluation list, the error message is displayed in a string as if it were the result. Therefore, expressions using variables that are not currently valid do not interrupt your debugging. Here is an example of what the evaluation list window looks like after several expressions have been added to it: (current-buffer) # ;--------------------------------------------------------------- (selected-window) # ;--------------------------------------------------------------- (point) 196 ;--------------------------------------------------------------- bad-var "Symbol's value as variable is void: bad-var" ;--------------------------------------------------------------- (recursion-depth) 0 ;--------------------------------------------------------------- this-command eval-last-sexp ;--------------------------------------------------------------- To delete a group, move point into it and type `C-c C-d', or simply delete the text for the group and update the evaluation list with `C-c C-u'. To add a new expression to the evaluation list, insert the expression at a suitable place, insert a new comment line, then type `C-c C-u'. You need not insert dashes in the comment line--its contents don't matter. After selecting `*edebug*', you can return to the source code buffer with `C-c C-w'. The `*edebug*' buffer is killed when you continue execution, and recreated next time it is needed. 18.2.11 Printing in Edebug -------------------------- If an expression in your program produces a value containing circular list structure, you may get an error when Edebug attempts to print it. One way to cope with circular structure is to set `print-length' or `print-level' to truncate the printing. Edebug does this for you; it binds `print-length' and `print-level' to the values of the variables `edebug-print-length' and `edebug-print-level' (so long as they have non-`nil' values). *Note Output Variables::. -- User Option: edebug-print-length If non-`nil', Edebug binds `print-length' to this value while printing results. The default value is `50'. -- User Option: edebug-print-level If non-`nil', Edebug binds `print-level' to this value while printing results. The default value is `50'. You can also print circular structures and structures that share elements more informatively by binding `print-circle' to a non-`nil' value. Here is an example of code that creates a circular structure: (setq a '(x y)) (setcar a a) Custom printing prints this as `Result: #1=(#1# y)'. The `#1=' notation labels the structure that follows it with the label `1', and the `#1#' notation references the previously labeled structure. This notation is used for any shared elements of lists or vectors. -- User Option: edebug-print-circle If non-`nil', Edebug binds `print-circle' to this value while printing results. The default value is `t'. Other programs can also use custom printing; see `cust-print.el' for details. 18.2.12 Trace Buffer -------------------- Edebug can record an execution trace, storing it in a buffer named `*edebug-trace*'. This is a log of function calls and returns, showing the function names and their arguments and values. To enable trace recording, set `edebug-trace' to a non-`nil' value. Making a trace buffer is not the same thing as using trace execution mode (*note Edebug Execution Modes::). When trace recording is enabled, each function entry and exit adds lines to the trace buffer. A function entry record consists of `::::{', followed by the function name and argument values. A function exit record consists of `::::}', followed by the function name and result of the function. The number of `:'s in an entry shows its recursion depth. You can use the braces in the trace buffer to find the matching beginning or end of function calls. You can customize trace recording for function entry and exit by redefining the functions `edebug-print-trace-before' and `edebug-print-trace-after'. -- Macro: edebug-tracing string body... This macro requests additional trace information around the execution of the BODY forms. The argument STRING specifies text to put in the trace buffer, after the `{' or `}'. All the arguments are evaluated, and `edebug-tracing' returns the value of the last form in BODY. -- Function: edebug-trace format-string &rest format-args This function inserts text in the trace buffer. It computes the text with `(apply 'format FORMAT-STRING FORMAT-ARGS)'. It also appends a newline to separate entries. `edebug-tracing' and `edebug-trace' insert lines in the trace buffer whenever they are called, even if Edebug is not active. Adding text to the trace buffer also scrolls its window to show the last lines inserted. 18.2.13 Coverage Testing ------------------------ Edebug provides rudimentary coverage testing and display of execution frequency. Coverage testing works by comparing the result of each expression with the previous result; each form in the program is considered "covered" if it has returned two different values since you began testing coverage in the current Emacs session. Thus, to do coverage testing on your program, execute it under various conditions and note whether it behaves correctly; Edebug will tell you when you have tried enough different conditions that each form has returned two different values. Coverage testing makes execution slower, so it is only done if `edebug-test-coverage' is non-`nil'. Frequency counting is performed for all executions of an instrumented function, even if the execution mode is Go-nonstop, and regardless of whether coverage testing is enabled. Use `C-x X =' (`edebug-display-freq-count') to display both the coverage information and the frequency counts for a definition. Just `=' (`edebug-temp-display-freq-count') displays the same information temporarily, only until you type another key. -- Command: edebug-display-freq-count This command displays the frequency count data for each line of the current definition. It inserts frequency counts as comment lines after each line of code. You can undo all insertions with one `undo' command. The counts appear under the `(' before an expression or the `)' after an expression, or on the last character of a variable. To simplify the display, a count is not shown if it is equal to the count of an earlier expression on the same line. The character `=' following the count for an expression says that the expression has returned the same value each time it was evaluated. In other words, it is not yet "covered" for coverage testing purposes. To clear the frequency count and coverage data for a definition, simply reinstrument it with `eval-defun'. For example, after evaluating `(fac 5)' with a source breakpoint, and setting `edebug-test-coverage' to `t', when the breakpoint is reached, the frequency data looks like this: (defun fac (n) (if (= n 0) (edebug)) ;#6 1 = =5 (if (< 0 n) ;#5 = (* n (fac (1- n))) ;# 5 0 1)) ;# 0 The comment lines show that `fac' was called 6 times. The first `if' statement returned 5 times with the same result each time; the same is true of the condition on the second `if'. The recursive call of `fac' did not return at all. 18.2.14 The Outside Context --------------------------- Edebug tries to be transparent to the program you are debugging, but it does not succeed completely. Edebug also tries to be transparent when you evaluate expressions with `e' or with the evaluation list buffer, by temporarily restoring the outside context. This section explains precisely what context Edebug restores, and how Edebug fails to be completely transparent. 18.2.14.1 Checking Whether to Stop .................................. Whenever Edebug is entered, it needs to save and restore certain data before even deciding whether to make trace information or stop the program. * `max-lisp-eval-depth' and `max-specpdl-size' are both increased to reduce Edebug's impact on the stack. You could, however, still run out of stack space when using Edebug. * The state of keyboard macro execution is saved and restored. While Edebug is active, `executing-kbd-macro' is bound to `nil' unless `edebug-continue-kbd-macro' is non-`nil'. 18.2.14.2 Edebug Display Update ............................... When Edebug needs to display something (e.g., in trace mode), it saves the current window configuration from "outside" Edebug (*note Window Configurations::). When you exit Edebug, it restores the previous window configuration. Emacs redisplays only when it pauses. Usually, when you continue execution, the program re-enters Edebug at a breakpoint or after stepping, without pausing or reading input in between. In such cases, Emacs never gets a chance to redisplay the "outside" configuration. Consequently, what you see is the same window configuration as the last time Edebug was active, with no interruption. Entry to Edebug for displaying something also saves and restores the following data (though some of them are deliberately not restored if an error or quit signal occurs). * Which buffer is current, and the positions of point and the mark in the current buffer, are saved and restored. * The outside window configuration is saved and restored if `edebug-save-windows' is non-`nil' (*note Edebug Options::). The window configuration is not restored on error or quit, but the outside selected window _is_ reselected even on error or quit in case a `save-excursion' is active. If the value of `edebug-save-windows' is a list, only the listed windows are saved and restored. The window start and horizontal scrolling of the source code buffer are not restored, however, so that the display remains coherent within Edebug. * The value of point in each displayed buffer is saved and restored if `edebug-save-displayed-buffer-points' is non-`nil'. * The variables `overlay-arrow-position' and `overlay-arrow-string' are saved and restored, so you can safely invoke Edebug from the recursive edit elsewhere in the same buffer. * `cursor-in-echo-area' is locally bound to `nil' so that the cursor shows up in the window. 18.2.14.3 Edebug Recursive Edit ............................... When Edebug is entered and actually reads commands from the user, it saves (and later restores) these additional data: * The current match data. *Note Match Data::. * The variables `last-command', `this-command', `last-command-event', `last-input-event', `last-event-frame', `last-nonmenu-event', and `track-mouse'. Commands in Edebug do not affect these variables outside of Edebug. Executing commands within Edebug can change the key sequence that would be returned by `this-command-keys', and there is no way to reset the key sequence from Lisp. Edebug cannot save and restore the value of `unread-command-events'. Entering Edebug while this variable has a nontrivial value can interfere with execution of the program you are debugging. * Complex commands executed while in Edebug are added to the variable `command-history'. In rare cases this can alter execution. * Within Edebug, the recursion depth appears one deeper than the recursion depth outside Edebug. This is not true of the automatically updated evaluation list window. * `standard-output' and `standard-input' are bound to `nil' by the `recursive-edit', but Edebug temporarily restores them during evaluations. * The state of keyboard macro definition is saved and restored. While Edebug is active, `defining-kbd-macro' is bound to `edebug-continue-kbd-macro'. 18.2.15 Edebug and Macros ------------------------- To make Edebug properly instrument expressions that call macros, some extra care is needed. This subsection explains the details. 18.2.15.1 Instrumenting Macro Calls ................................... When Edebug instruments an expression that calls a Lisp macro, it needs additional information about the macro to do the job properly. This is because there is no a-priori way to tell which subexpressions of the macro call are forms to be evaluated. (Evaluation may occur explicitly in the macro body, or when the resulting expansion is evaluated, or any time later.) Therefore, you must define an Edebug specification for each macro that Edebug will encounter, to explain the format of calls to that macro. To do this, add a `debug' declaration to the macro definition. Here is a simple example that shows the specification for the `for' example macro (*note Argument Evaluation::). (defmacro for (var from init to final do &rest body) "Execute a simple \"for\" loop. For example, (for i from 1 to 10 do (print i))." (declare (debug (symbolp "from" form "to" form "do" &rest form))) ...) The Edebug specification says which parts of a call to the macro are forms to be evaluated. For simple macros, the specification often looks very similar to the formal argument list of the macro definition, but specifications are much more general than macro arguments. *Note Defining Macros::, for more explanation of the `declare' form. Take care to ensure that the specifications are known to Edebug when you instrument code. If you are instrumenting a function from a file that uses `eval-when-compile' to require another file containing macro definitions, you may need to explicitly load that file. You can also define an edebug specification for a macro separately from the macro definition with `def-edebug-spec'. Adding `debug' declarations is preferred, and more convenient, for macro definitions in Lisp, but `def-edebug-spec' makes it possible to define Edebug specifications for special forms implemented in C. -- Macro: def-edebug-spec macro specification Specify which expressions of a call to macro MACRO are forms to be evaluated. SPECIFICATION should be the edebug specification. Neither argument is evaluated. The MACRO argument can actually be any symbol, not just a macro name. Here is a table of the possibilities for SPECIFICATION and how each directs processing of arguments. `t' All arguments are instrumented for evaluation. `0' None of the arguments is instrumented. a symbol The symbol must have an Edebug specification, which is used instead. This indirection is repeated until another kind of specification is found. This allows you to inherit the specification from another macro. a list The elements of the list describe the types of the arguments of a calling form. The possible elements of a specification list are described in the following sections. If a macro has no Edebug specification, neither through a `debug' declaration nor through a `def-edebug-spec' call, the variable `edebug-eval-macro-args' comes into play. -- User Option: edebug-eval-macro-args This controls the way Edebug treats macro arguments with no explicit Edebug specification. If it is `nil' (the default), none of the arguments is instrumented for evaluation. Otherwise, all arguments are instrumented. 18.2.15.2 Specification List ............................ A "specification list" is required for an Edebug specification if some arguments of a macro call are evaluated while others are not. Some elements in a specification list match one or more arguments, but others modify the processing of all following elements. The latter, called "specification keywords", are symbols beginning with `&' (such as `&optional'). A specification list may contain sublists, which match arguments that are themselves lists, or it may contain vectors used for grouping. Sublists and groups thus subdivide the specification list into a hierarchy of levels. Specification keywords apply only to the remainder of the sublist or group they are contained in. When a specification list involves alternatives or repetition, matching it against an actual macro call may require backtracking. For more details, *note Backtracking::. Edebug specifications provide the power of regular expression matching, plus some context-free grammar constructs: the matching of sublists with balanced parentheses, recursive processing of forms, and recursion via indirect specifications. Here's a table of the possible elements of a specification list, with their meanings (see *note Specification Examples::, for the referenced examples): `sexp' A single unevaluated Lisp object, which is not instrumented. `form' A single evaluated expression, which is instrumented. `place' A generalized variable. *Note Generalized Variables::. `body' Short for `&rest form'. See `&rest' below. `function-form' A function form: either a quoted function symbol, a quoted lambda expression, or a form (that should evaluate to a function symbol or lambda expression). This is useful when an argument that's a lambda expression might be quoted with `quote' rather than `function', since it instruments the body of the lambda expression either way. `lambda-expr' A lambda expression with no quoting. `&optional' All following elements in the specification list are optional; as soon as one does not match, Edebug stops matching at this level. To make just a few elements optional, followed by non-optional elements, use `[&optional SPECS...]'. To specify that several elements must all match or none, use `&optional [SPECS...]'. See the `defun' example. `&rest' All following elements in the specification list are repeated zero or more times. In the last repetition, however, it is not a problem if the expression runs out before matching all of the elements of the specification list. To repeat only a few elements, use `[&rest SPECS...]'. To specify several elements that must all match on every repetition, use `&rest [SPECS...]'. `&or' Each of the following elements in the specification list is an alternative. One of the alternatives must match, or the `&or' specification fails. Each list element following `&or' is a single alternative. To group two or more list elements as a single alternative, enclose them in `[...]'. `¬' Each of the following elements is matched as alternatives as if by using `&or', but if any of them match, the specification fails. If none of them match, nothing is matched, but the `¬' specification succeeds. `&define' Indicates that the specification is for a defining form. The defining form itself is not instrumented (that is, Edebug does not stop before and after the defining form), but forms inside it typically will be instrumented. The `&define' keyword should be the first element in a list specification. `nil' This is successful when there are no more arguments to match at the current argument list level; otherwise it fails. See sublist specifications and the backquote example. `gate' No argument is matched but backtracking through the gate is disabled while matching the remainder of the specifications at this level. This is primarily used to generate more specific syntax error messages. See *note Backtracking::, for more details. Also see the `let' example. `OTHER-SYMBOL' Any other symbol in a specification list may be a predicate or an indirect specification. If the symbol has an Edebug specification, this "indirect specification" should be either a list specification that is used in place of the symbol, or a function that is called to process the arguments. The specification may be defined with `def-edebug-spec' just as for macros. See the `defun' example. Otherwise, the symbol should be a predicate. The predicate is called with the argument, and if the predicate returns `nil', the specification fails and the argument is not instrumented. Some suitable predicates include `symbolp', `integerp', `stringp', `vectorp', and `atom'. `[ELEMENTS...]' A vector of elements groups the elements into a single "group specification". Its meaning has nothing to do with vectors. `"STRING"' The argument should be a symbol named STRING. This specification is equivalent to the quoted symbol, `'SYMBOL', where the name of SYMBOL is the STRING, but the string form is preferred. `(vector ELEMENTS...)' The argument should be a vector whose elements must match the ELEMENTS in the specification. See the backquote example. `(ELEMENTS...)' Any other list is a "sublist specification" and the argument must be a list whose elements match the specification ELEMENTS. A sublist specification may be a dotted list and the corresponding list argument may then be a dotted list. Alternatively, the last CDR of a dotted list specification may be another sublist specification (via a grouping or an indirect specification, e.g., `(spec . [(more specs...)])') whose elements match the non-dotted list arguments. This is useful in recursive specifications such as in the backquote example. Also see the description of a `nil' specification above for terminating such recursion. Note that a sublist specification written as `(specs . nil)' is equivalent to `(specs)', and `(specs . (sublist-elements...))' is equivalent to `(specs sublist-elements...)'. Here is a list of additional specifications that may appear only after `&define'. See the `defun' example. `name' The argument, a symbol, is the name of the defining form. A defining form is not required to have a name field; and it may have multiple name fields. `:name' This construct does not actually match an argument. The element following `:name' should be a symbol; it is used as an additional name component for the definition. You can use this to add a unique, static component to the name of the definition. It may be used more than once. `arg' The argument, a symbol, is the name of an argument of the defining form. However, lambda-list keywords (symbols starting with `&') are not allowed. `lambda-list' This matches a lambda list--the argument list of a lambda expression. `def-body' The argument is the body of code in a definition. This is like `body', described above, but a definition body must be instrumented with a different Edebug call that looks up information associated with the definition. Use `def-body' for the highest level list of forms within the definition. `def-form' The argument is a single, highest-level form in a definition. This is like `def-body', except it is used to match a single form rather than a list of forms. As a special case, `def-form' also means that tracing information is not output when the form is executed. See the `interactive' example. 18.2.15.3 Backtracking in Specifications ........................................ If a specification fails to match at some point, this does not necessarily mean a syntax error will be signaled; instead, "backtracking" will take place until all alternatives have been exhausted. Eventually every element of the argument list must be matched by some element in the specification, and every required element in the specification must match some argument. When a syntax error is detected, it might not be reported until much later, after higher-level alternatives have been exhausted, and with the point positioned further from the real error. But if backtracking is disabled when an error occurs, it can be reported immediately. Note that backtracking is also reenabled automatically in several situations; when a new alternative is established by `&optional', `&rest', or `&or', or at the start of processing a sublist, group, or indirect specification. The effect of enabling or disabling backtracking is limited to the remainder of the level currently being processed and lower levels. Backtracking is disabled while matching any of the form specifications (that is, `form', `body', `def-form', and `def-body'). These specifications will match any form so any error must be in the form itself rather than at a higher level. Backtracking is also disabled after successfully matching a quoted symbol or string specification, since this usually indicates a recognized construct. But if you have a set of alternative constructs that all begin with the same symbol, you can usually work around this constraint by factoring the symbol out of the alternatives, e.g., `["foo" &or [first case] [second case] ...]'. Most needs are satisfied by these two ways that backtracking is automatically disabled, but occasionally it is useful to explicitly disable backtracking by using the `gate' specification. This is useful when you know that no higher alternatives could apply. See the example of the `let' specification. 18.2.15.4 Specification Examples ................................ It may be easier to understand Edebug specifications by studying the examples provided here. A `let' special form has a sequence of bindings and a body. Each of the bindings is either a symbol or a sublist with a symbol and optional expression. In the specification below, notice the `gate' inside of the sublist to prevent backtracking once a sublist is found. (def-edebug-spec let ((&rest &or symbolp (gate symbolp &optional form)) body)) Edebug uses the following specifications for `defun' and the associated argument list and `interactive' specifications. It is necessary to handle interactive forms specially since an expression argument is actually evaluated outside of the function body. (The specification for `defmacro' is very similar to that for `defun', but allows for the `declare' statement.) (def-edebug-spec defun (&define name lambda-list [&optional stringp] ; Match the doc string, if present. [&optional ("interactive" interactive)] def-body)) (def-edebug-spec lambda-list (([&rest arg] [&optional ["&optional" arg &rest arg]] &optional ["&rest" arg] ))) (def-edebug-spec interactive (&optional &or stringp def-form)) ; Notice: `def-form' The specification for backquote below illustrates how to match dotted lists and use `nil' to terminate recursion. It also illustrates how components of a vector may be matched. (The actual specification defined by Edebug is a little different, and does not support dotted lists because doing so causes very deep recursion that could fail.) (def-edebug-spec \` (backquote-form)) ; Alias just for clarity. (def-edebug-spec backquote-form (&or ([&or "," ",@"] &or ("quote" backquote-form) form) (backquote-form . [&or nil backquote-form]) (vector &rest backquote-form) sexp)) 18.2.16 Edebug Options ---------------------- These options affect the behavior of Edebug: -- User Option: edebug-setup-hook Functions to call before Edebug is used. Each time it is set to a new value, Edebug will call those functions once and then reset `edebug-setup-hook' to `nil'. You could use this to load up Edebug specifications associated with a package you are using, but only when you also use Edebug. *Note Instrumenting::. -- User Option: edebug-all-defs If this is non-`nil', normal evaluation of defining forms such as `defun' and `defmacro' instruments them for Edebug. This applies to `eval-defun', `eval-region', `eval-buffer', and `eval-current-buffer'. Use the command `M-x edebug-all-defs' to toggle the value of this option. *Note Instrumenting::. -- User Option: edebug-all-forms If this is non-`nil', the commands `eval-defun', `eval-region', `eval-buffer', and `eval-current-buffer' instrument all forms, even those that don't define anything. This doesn't apply to loading or evaluations in the minibuffer. Use the command `M-x edebug-all-forms' to toggle the value of this option. *Note Instrumenting::. -- User Option: edebug-save-windows If this is non-`nil', Edebug saves and restores the window configuration. That takes some time, so if your program does not care what happens to the window configurations, it is better to set this variable to `nil'. If the value is a list, only the listed windows are saved and restored. You can use the `W' command in Edebug to change this variable interactively. *Note Edebug Display Update::. -- User Option: edebug-save-displayed-buffer-points If this is non-`nil', Edebug saves and restores point in all displayed buffers. Saving and restoring point in other buffers is necessary if you are debugging code that changes the point of a buffer that is displayed in a non-selected window. If Edebug or the user then selects the window, point in that buffer will move to the window's value of point. Saving and restoring point in all buffers is expensive, since it requires selecting each window twice, so enable this only if you need it. *Note Edebug Display Update::. -- User Option: edebug-initial-mode If this variable is non-`nil', it specifies the initial execution mode for Edebug when it is first activated. Possible values are `step', `next', `go', `Go-nonstop', `trace', `Trace-fast', `continue', and `Continue-fast'. The default value is `step'. *Note Edebug Execution Modes::. -- User Option: edebug-trace If this is non-`nil', trace each function entry and exit. Tracing output is displayed in a buffer named `*edebug-trace*', one function entry or exit per line, indented by the recursion level. Also see `edebug-tracing', in *note Trace Buffer::. -- User Option: edebug-test-coverage If non-`nil', Edebug tests coverage of all expressions debugged. *Note Coverage Testing::. -- User Option: edebug-continue-kbd-macro If non-`nil', continue defining or executing any keyboard macro that is executing outside of Edebug. Use this with caution since it is not debugged. *Note Edebug Execution Modes::. -- User Option: edebug-unwrap-results If non-`nil', Edebug tries to remove any of its own instrumentation when showing the results of expressions. This is relevant when debugging macros where the results of expressions are themselves instrumented expressions. As a very artificial example, suppose that the example function `fac' has been instrumented, and consider a macro of the form: (defmacro test () "Edebug example." (if (symbol-function 'fac) ...)) If you instrument the `test' macro and step through it, then by default the result of the `symbol-function' call has numerous `edebug-after' and `edebug-before' forms, which can make it difficult to see the "actual" result. If `edebug-unwrap-results' is non-`nil', Edebug tries to remove these forms from the result. -- User Option: edebug-on-error Edebug binds `debug-on-error' to this value, if `debug-on-error' was previously `nil'. *Note Trapping Errors::. -- User Option: edebug-on-quit Edebug binds `debug-on-quit' to this value, if `debug-on-quit' was previously `nil'. *Note Trapping Errors::. If you change the values of `edebug-on-error' or `edebug-on-quit' while Edebug is active, their values won't be used until the _next_ time Edebug is invoked via a new command. -- User Option: edebug-global-break-condition If non-`nil', an expression to test for at every stop point. If the result is non-`nil', then break. Errors are ignored. *Note Global Break Condition::. 18.3 Debugging Invalid Lisp Syntax ================================== The Lisp reader reports invalid syntax, but cannot say where the real problem is. For example, the error "End of file during parsing" in evaluating an expression indicates an excess of open parentheses (or square brackets). The reader detects this imbalance at the end of the file, but it cannot figure out where the close parenthesis should have been. Likewise, "Invalid read syntax: ")"" indicates an excess close parenthesis or missing open parenthesis, but does not say where the missing parenthesis belongs. How, then, to find what to change? If the problem is not simply an imbalance of parentheses, a useful technique is to try `C-M-e' at the beginning of each defun, and see if it goes to the place where that defun appears to end. If it does not, there is a problem in that defun. However, unmatched parentheses are the most common syntax errors in Lisp, and we can give further advice for those cases. (In addition, just moving point through the code with Show Paren mode enabled might find the mismatch.) 18.3.1 Excess Open Parentheses ------------------------------ The first step is to find the defun that is unbalanced. If there is an excess open parenthesis, the way to do this is to go to the end of the file and type `C-u C-M-u'. This will move you to the beginning of the first defun that is unbalanced. The next step is to determine precisely what is wrong. There is no way to be sure of this except by studying the program, but often the existing indentation is a clue to where the parentheses should have been. The easiest way to use this clue is to reindent with `C-M-q' and see what moves. *But don't do this yet!* Keep reading, first. Before you do this, make sure the defun has enough close parentheses. Otherwise, `C-M-q' will get an error, or will reindent all the rest of the file until the end. So move to the end of the defun and insert a close parenthesis there. Don't use `C-M-e' to move there, since that too will fail to work until the defun is balanced. Now you can go to the beginning of the defun and type `C-M-q'. Usually all the lines from a certain point to the end of the function will shift to the right. There is probably a missing close parenthesis, or a superfluous open parenthesis, near that point. (However, don't assume this is true; study the code to make sure.) Once you have found the discrepancy, undo the `C-M-q' with `C-_', since the old indentation is probably appropriate to the intended parentheses. After you think you have fixed the problem, use `C-M-q' again. If the old indentation actually fit the intended nesting of parentheses, and you have put back those parentheses, `C-M-q' should not change anything. 18.3.2 Excess Close Parentheses ------------------------------- To deal with an excess close parenthesis, first go to the beginning of the file, then type `C-u -1 C-M-u' to find the end of the first unbalanced defun. Then find the actual matching close parenthesis by typing `C-M-f' at the beginning of that defun. This will leave you somewhere short of the place where the defun ought to end. It is possible that you will find a spurious close parenthesis in that vicinity. If you don't see a problem at that point, the next thing to do is to type `C-M-q' at the beginning of the defun. A range of lines will probably shift left; if so, the missing open parenthesis or spurious close parenthesis is probably near the first of those lines. (However, don't assume this is true; study the code to make sure.) Once you have found the discrepancy, undo the `C-M-q' with `C-_', since the old indentation is probably appropriate to the intended parentheses. After you think you have fixed the problem, use `C-M-q' again. If the old indentation actually fits the intended nesting of parentheses, and you have put back those parentheses, `C-M-q' should not change anything. 18.4 Test Coverage ================== You can do coverage testing for a file of Lisp code by loading the `testcover' library and using the command `M-x testcover-start FILE ' to instrument the code. Then test your code by calling it one or more times. Then use the command `M-x testcover-mark-all' to display colored highlights on the code to show where coverage is insufficient. The command `M-x testcover-next-mark' will move point forward to the next highlighted spot. Normally, a red highlight indicates the form was never completely evaluated; a brown highlight means it always evaluated to the same value (meaning there has been little testing of what is done with the result). However, the red highlight is skipped for forms that can't possibly complete their evaluation, such as `error'. The brown highlight is skipped for forms that are expected to always evaluate to the same value, such as `(setq x 14)'. For difficult cases, you can add do-nothing macros to your code to give advice to the test coverage tool. -- Macro: 1value form Evaluate FORM and return its value, but inform coverage testing that FORM's value should always be the same. -- Macro: noreturn form Evaluate FORM, informing coverage testing that FORM should never return. If it ever does return, you get a run-time error. Edebug also has a coverage testing feature (*note Coverage Testing::). These features partly duplicate each other, and it would be cleaner to combine them. 18.5 Profiling ============== If your program is working correctly, but you want to make it run more quickly or efficiently, the first thing to do is "profile" your code so that you know how it is using resources. If you find that one particular function is responsible for a significant portion of the runtime, you can start looking for ways to optimize that piece. Emacs has built-in support for this. To begin profiling, type `M-x profiler-start'. You can choose to profile by processor usage, memory usage, or both. After doing some work, type `M-x profiler-report' to display a summary buffer for each resource that you chose to profile. The names of the report buffers include the times at which the reports were generated, so you can generate another report later on without erasing previous results. When you have finished profiling, type `M-x profiler-stop' (there is a small overhead associated with profiling). The profiler report buffer shows, on each line, a function that was called, followed by how much resource (processor or memory) it used in absolute and percentage times since profiling started. If a given line has a `+' symbol at the left-hand side, you can expand that line by typing , in order to see the function(s) called by the higher-level function. Pressing again will collapse back to the original state. Press `j' or `mouse-2' to jump to the definition of a function. Press `d' to view a function's documentation. You can save a profile to a file using `C-x C-w'. You can compare two profiles using `='. The `elp' library offers an alternative approach. See the file `elp.el' for instructions. You can check the speed of individual Emacs Lisp forms using the `benchmark' library. See the functions `benchmark-run' and `benchmark-run-compiled' in `benchmark.el'. To profile Emacs at the level of its C code, you can build it using the `--enable-profiling' option of `configure'. When Emacs exits, it generates a file `gmon.out' that you can examine using the `gprof' utility. This feature is mainly useful for debugging Emacs. It actually stops the Lisp-level `M-x profiler-...' commands described above from working. 19 Reading and Printing Lisp Objects ************************************ "Printing" and "reading" are the operations of converting Lisp objects to textual form and vice versa. They use the printed representations and read syntax described in *note Lisp Data Types::. This chapter describes the Lisp functions for reading and printing. It also describes "streams", which specify where to get the text (if reading) or where to put it (if printing). 19.1 Introduction to Reading and Printing ========================================= "Reading" a Lisp object means parsing a Lisp expression in textual form and producing a corresponding Lisp object. This is how Lisp programs get into Lisp from files of Lisp code. We call the text the "read syntax" of the object. For example, the text `(a . 5)' is the read syntax for a cons cell whose CAR is `a' and whose CDR is the number 5. "Printing" a Lisp object means producing text that represents that object--converting the object to its "printed representation" (*note Printed Representation::). Printing the cons cell described above produces the text `(a . 5)'. Reading and printing are more or less inverse operations: printing the object that results from reading a given piece of text often produces the same text, and reading the text that results from printing an object usually produces a similar-looking object. For example, printing the symbol `foo' produces the text `foo', and reading that text returns the symbol `foo'. Printing a list whose elements are `a' and `b' produces the text `(a b)', and reading that text produces a list (but not the same list) with elements `a' and `b'. However, these two operations are not precisely inverse to each other. There are three kinds of exceptions: * Printing can produce text that cannot be read. For example, buffers, windows, frames, subprocesses and markers print as text that starts with `#'; if you try to read this text, you get an error. There is no way to read those data types. * One object can have multiple textual representations. For example, `1' and `01' represent the same integer, and `(a b)' and `(a . (b))' represent the same list. Reading will accept any of the alternatives, but printing must choose one of them. * Comments can appear at certain points in the middle of an object's read sequence without affecting the result of reading it. 19.2 Input Streams ================== Most of the Lisp functions for reading text take an "input stream" as an argument. The input stream specifies where or how to get the characters of the text to be read. Here are the possible types of input stream: BUFFER The input characters are read from BUFFER, starting with the character directly after point. Point advances as characters are read. MARKER The input characters are read from the buffer that MARKER is in, starting with the character directly after the marker. The marker position advances as characters are read. The value of point in the buffer has no effect when the stream is a marker. STRING The input characters are taken from STRING, starting at the first character in the string and using as many characters as required. FUNCTION The input characters are generated by FUNCTION, which must support two kinds of calls: * When it is called with no arguments, it should return the next character. * When it is called with one argument (always a character), FUNCTION should save the argument and arrange to return it on the next call. This is called "unreading" the character; it happens when the Lisp reader reads one character too many and wants to "put it back where it came from". In this case, it makes no difference what value FUNCTION returns. `t' `t' used as a stream means that the input is read from the minibuffer. In fact, the minibuffer is invoked once and the text given by the user is made into a string that is then used as the input stream. If Emacs is running in batch mode, standard input is used instead of the minibuffer. For example, (message "%s" (read t)) will read a Lisp expression from standard input and print the result to standard output. `nil' `nil' supplied as an input stream means to use the value of `standard-input' instead; that value is the "default input stream", and must be a non-`nil' input stream. SYMBOL A symbol as input stream is equivalent to the symbol's function definition (if any). Here is an example of reading from a stream that is a buffer, showing where point is located before and after: ---------- Buffer: foo ---------- This-!- is the contents of foo. ---------- Buffer: foo ---------- (read (get-buffer "foo")) => is (read (get-buffer "foo")) => the ---------- Buffer: foo ---------- This is the-!- contents of foo. ---------- Buffer: foo ---------- Note that the first read skips a space. Reading skips any amount of whitespace preceding the significant text. Here is an example of reading from a stream that is a marker, initially positioned at the beginning of the buffer shown. The value read is the symbol `This'. ---------- Buffer: foo ---------- This is the contents of foo. ---------- Buffer: foo ---------- (setq m (set-marker (make-marker) 1 (get-buffer "foo"))) => # (read m) => This m => # ;; Before the first space. Here we read from the contents of a string: (read "(When in) the course") => (When in) The following example reads from the minibuffer. The prompt is: `Lisp expression: '. (That is always the prompt used when you read from the stream `t'.) The user's input is shown following the prompt. (read t) => 23 ---------- Buffer: Minibuffer ---------- Lisp expression: 23 ---------- Buffer: Minibuffer ---------- Finally, here is an example of a stream that is a function, named `useless-stream'. Before we use the stream, we initialize the variable `useless-list' to a list of characters. Then each call to the function `useless-stream' obtains the next character in the list or unreads a character by adding it to the front of the list. (setq useless-list (append "XY()" nil)) => (88 89 40 41) (defun useless-stream (&optional unread) (if unread (setq useless-list (cons unread useless-list)) (prog1 (car useless-list) (setq useless-list (cdr useless-list))))) => useless-stream Now we read using the stream thus constructed: (read 'useless-stream) => XY useless-list => (40 41) Note that the open and close parentheses remain in the list. The Lisp reader encountered the open parenthesis, decided that it ended the input, and unread it. Another attempt to read from the stream at this point would read `()' and return `nil'. 19.3 Input Functions ==================== This section describes the Lisp functions and variables that pertain to reading. In the functions below, STREAM stands for an input stream (see the previous section). If STREAM is `nil' or omitted, it defaults to the value of `standard-input'. An `end-of-file' error is signaled if reading encounters an unterminated list, vector, or string. -- Function: read &optional stream This function reads one textual Lisp expression from STREAM, returning it as a Lisp object. This is the basic Lisp input function. -- Function: read-from-string string &optional start end This function reads the first textual Lisp expression from the text in STRING. It returns a cons cell whose CAR is that expression, and whose CDR is an integer giving the position of the next remaining character in the string (i.e., the first one not read). If START is supplied, then reading begins at index START in the string (where the first character is at index 0). If you specify END, then reading is forced to stop just before that index, as if the rest of the string were not there. For example: (read-from-string "(setq x 55) (setq y 5)") => ((setq x 55) . 11) (read-from-string "\"A short string\"") => ("A short string" . 16) ;; Read starting at the first character. (read-from-string "(list 112)" 0) => ((list 112) . 10) ;; Read starting at the second character. (read-from-string "(list 112)" 1) => (list . 5) ;; Read starting at the seventh character, ;; and stopping at the ninth. (read-from-string "(list 112)" 6 8) => (11 . 8) -- Variable: standard-input This variable holds the default input stream--the stream that `read' uses when the STREAM argument is `nil'. The default is `t', meaning use the minibuffer. -- Variable: read-circle If non-`nil', this variable enables the reading of circular and shared structures. *Note Circular Objects::. Its default value is `t'. 19.4 Output Streams =================== An output stream specifies what to do with the characters produced by printing. Most print functions accept an output stream as an optional argument. Here are the possible types of output stream: BUFFER The output characters are inserted into BUFFER at point. Point advances as characters are inserted. MARKER The output characters are inserted into the buffer that MARKER points into, at the marker position. The marker position advances as characters are inserted. The value of point in the buffer has no effect on printing when the stream is a marker, and this kind of printing does not move point (except that if the marker points at or before the position of point, point advances with the surrounding text, as usual). FUNCTION The output characters are passed to FUNCTION, which is responsible for storing them away. It is called with a single character as argument, as many times as there are characters to be output, and is responsible for storing the characters wherever you want to put them. `t' The output characters are displayed in the echo area. `nil' `nil' specified as an output stream means to use the value of `standard-output' instead; that value is the "default output stream", and must not be `nil'. SYMBOL A symbol as output stream is equivalent to the symbol's function definition (if any). Many of the valid output streams are also valid as input streams. The difference between input and output streams is therefore more a matter of how you use a Lisp object, than of different types of object. Here is an example of a buffer used as an output stream. Point is initially located as shown immediately before the `h' in `the'. At the end, point is located directly before that same `h'. ---------- Buffer: foo ---------- This is t-!-he contents of foo. ---------- Buffer: foo ---------- (print "This is the output" (get-buffer "foo")) => "This is the output" ---------- Buffer: foo ---------- This is t "This is the output" -!-he contents of foo. ---------- Buffer: foo ---------- Now we show a use of a marker as an output stream. Initially, the marker is in buffer `foo', between the `t' and the `h' in the word `the'. At the end, the marker has advanced over the inserted text so that it remains positioned before the same `h'. Note that the location of point, shown in the usual fashion, has no effect. ---------- Buffer: foo ---------- This is the -!-output ---------- Buffer: foo ---------- (setq m (copy-marker 10)) => # (print "More output for foo." m) => "More output for foo." ---------- Buffer: foo ---------- This is t "More output for foo." he -!-output ---------- Buffer: foo ---------- m => # The following example shows output to the echo area: (print "Echo Area output" t) => "Echo Area output" ---------- Echo Area ---------- "Echo Area output" ---------- Echo Area ---------- Finally, we show the use of a function as an output stream. The function `eat-output' takes each character that it is given and conses it onto the front of the list `last-output' (*note Building Lists::). At the end, the list contains all the characters output, but in reverse order. (setq last-output nil) => nil (defun eat-output (c) (setq last-output (cons c last-output))) => eat-output (print "This is the output" 'eat-output) => "This is the output" last-output => (10 34 116 117 112 116 117 111 32 101 104 116 32 115 105 32 115 105 104 84 34 10) Now we can put the output in the proper order by reversing the list: (concat (nreverse last-output)) => " \"This is the output\" " Calling `concat' converts the list to a string so you can see its contents more clearly. 19.5 Output Functions ===================== This section describes the Lisp functions for printing Lisp objects--converting objects into their printed representation. Some of the Emacs printing functions add quoting characters to the output when necessary so that it can be read properly. The quoting characters used are `"' and `\'; they distinguish strings from symbols, and prevent punctuation characters in strings and symbols from being taken as delimiters when reading. *Note Printed Representation::, for full details. You specify quoting or no quoting by the choice of printing function. If the text is to be read back into Lisp, then you should print with quoting characters to avoid ambiguity. Likewise, if the purpose is to describe a Lisp object clearly for a Lisp programmer. However, if the purpose of the output is to look nice for humans, then it is usually better to print without quoting. Lisp objects can refer to themselves. Printing a self-referential object in the normal way would require an infinite amount of text, and the attempt could cause infinite recursion. Emacs detects such recursion and prints `#LEVEL' instead of recursively printing an object already being printed. For example, here `#0' indicates a recursive reference to the object at level 0 of the current print operation: (setq foo (list nil)) => (nil) (setcar foo foo) => (#0) In the functions below, STREAM stands for an output stream. (See the previous section for a description of output streams.) If STREAM is `nil' or omitted, it defaults to the value of `standard-output'. -- Function: print object &optional stream The `print' function is a convenient way of printing. It outputs the printed representation of OBJECT to STREAM, printing in addition one newline before OBJECT and another after it. Quoting characters are used. `print' returns OBJECT. For example: (progn (print 'The\ cat\ in) (print "the hat") (print " came back")) -| -| The\ cat\ in -| -| "the hat" -| -| " came back" => " came back" -- Function: prin1 object &optional stream This function outputs the printed representation of OBJECT to STREAM. It does not print newlines to separate output as `print' does, but it does use quoting characters just like `print'. It returns OBJECT. (progn (prin1 'The\ cat\ in) (prin1 "the hat") (prin1 " came back")) -| The\ cat\ in"the hat"" came back" => " came back" -- Function: princ object &optional stream This function outputs the printed representation of OBJECT to STREAM. It returns OBJECT. This function is intended to produce output that is readable by people, not by `read', so it doesn't insert quoting characters and doesn't put double-quotes around the contents of strings. It does not add any spacing between calls. (progn (princ 'The\ cat) (princ " in the \"hat\"")) -| The cat in the "hat" => " in the \"hat\"" -- Function: terpri &optional stream This function outputs a newline to STREAM. The name stands for "terminate print". -- Function: write-char character &optional stream This function outputs CHARACTER to STREAM. It returns CHARACTER. -- Function: prin1-to-string object &optional noescape This function returns a string containing the text that `prin1' would have printed for the same argument. (prin1-to-string 'foo) => "foo" (prin1-to-string (mark-marker)) => "#" If NOESCAPE is non-`nil', that inhibits use of quoting characters in the output. (This argument is supported in Emacs versions 19 and later.) (prin1-to-string "foo") => "\"foo\"" (prin1-to-string "foo" t) => "foo" See `format', in *note Formatting Strings::, for other ways to obtain the printed representation of a Lisp object as a string. -- Macro: with-output-to-string body... This macro executes the BODY forms with `standard-output' set up to feed output into a string. Then it returns that string. For example, if the current buffer name is `foo', (with-output-to-string (princ "The buffer is ") (princ (buffer-name))) returns `"The buffer is foo"'. -- Function: pp object &optional stream This function outputs OBJECT to STREAM, just like `prin1', but does it in a more "pretty" way. That is, it'll indent and fill the object to make it more readable for humans. 19.6 Variables Affecting Output =============================== -- Variable: standard-output The value of this variable is the default output stream--the stream that print functions use when the STREAM argument is `nil'. The default is `t', meaning display in the echo area. -- Variable: print-quoted If this is non-`nil', that means to print quoted forms using abbreviated reader syntax, e.g., `(quote foo)' prints as `'foo', and `(function foo)' as `#'foo'. -- Variable: print-escape-newlines If this variable is non-`nil', then newline characters in strings are printed as `\n' and formfeeds are printed as `\f'. Normally these characters are printed as actual newlines and formfeeds. This variable affects the print functions `prin1' and `print' that print with quoting. It does not affect `princ'. Here is an example using `prin1': (prin1 "a\nb") -| "a -| b" => "a b" (let ((print-escape-newlines t)) (prin1 "a\nb")) -| "a\nb" => "a b" In the second expression, the local binding of `print-escape-newlines' is in effect during the call to `prin1', but not during the printing of the result. -- Variable: print-escape-nonascii If this variable is non-`nil', then unibyte non-ASCII characters in strings are unconditionally printed as backslash sequences by the print functions `prin1' and `print' that print with quoting. Those functions also use backslash sequences for unibyte non-ASCII characters, regardless of the value of this variable, when the output stream is a multibyte buffer or a marker pointing into one. -- Variable: print-escape-multibyte If this variable is non-`nil', then multibyte non-ASCII characters in strings are unconditionally printed as backslash sequences by the print functions `prin1' and `print' that print with quoting. Those functions also use backslash sequences for multibyte non-ASCII characters, regardless of the value of this variable, when the output stream is a unibyte buffer or a marker pointing into one. -- Variable: print-length The value of this variable is the maximum number of elements to print in any list, vector or bool-vector. If an object being printed has more than this many elements, it is abbreviated with an ellipsis. If the value is `nil' (the default), then there is no limit. (setq print-length 2) => 2 (print '(1 2 3 4 5)) -| (1 2 ...) => (1 2 ...) -- Variable: print-level The value of this variable is the maximum depth of nesting of parentheses and brackets when printed. Any list or vector at a depth exceeding this limit is abbreviated with an ellipsis. A value of `nil' (which is the default) means no limit. -- User Option: eval-expression-print-length -- User Option: eval-expression-print-level These are the values for `print-length' and `print-level' used by `eval-expression', and thus, indirectly, by many interactive evaluation commands (*note Evaluating Emacs-Lisp Expressions: (emacs)Lisp Eval.). These variables are used for detecting and reporting circular and shared structure: -- Variable: print-circle If non-`nil', this variable enables detection of circular and shared structure in printing. *Note Circular Objects::. -- Variable: print-gensym If non-`nil', this variable enables detection of uninterned symbols (*note Creating Symbols::) in printing. When this is enabled, uninterned symbols print with the prefix `#:', which tells the Lisp reader to produce an uninterned symbol. -- Variable: print-continuous-numbering If non-`nil', that means number continuously across print calls. This affects the numbers printed for `#N=' labels and `#M#' references. Don't set this variable with `setq'; you should only bind it temporarily to `t' with `let'. When you do that, you should also bind `print-number-table' to `nil'. -- Variable: print-number-table This variable holds a vector used internally by printing to implement the `print-circle' feature. You should not use it except to bind it to `nil' when you bind `print-continuous-numbering'. -- Variable: float-output-format This variable specifies how to print floating point numbers. The default is `nil', meaning use the shortest output that represents the number without losing information. To control output format more precisely, you can put a string in this variable. The string should hold a `%'-specification to be used in the C function `sprintf'. For further restrictions on what you can use, see the variable's documentation string. 20 Minibuffers ************** A "minibuffer" is a special buffer that Emacs commands use to read arguments more complicated than the single numeric prefix argument. These arguments include file names, buffer names, and command names (as in `M-x'). The minibuffer is displayed on the bottom line of the frame, in the same place as the echo area (*note The Echo Area::), but only while it is in use for reading an argument. 20.1 Introduction to Minibuffers ================================ In most ways, a minibuffer is a normal Emacs buffer. Most operations _within_ a buffer, such as editing commands, work normally in a minibuffer. However, many operations for managing buffers do not apply to minibuffers. The name of a minibuffer always has the form ` *Minibuf-NUMBER*', and it cannot be changed. Minibuffers are displayed only in special windows used only for minibuffers; these windows always appear at the bottom of a frame. (Sometimes frames have no minibuffer window, and sometimes a special kind of frame contains nothing but a minibuffer window; see *note Minibuffers and Frames::.) The text in the minibuffer always starts with the "prompt string", the text that was specified by the program that is using the minibuffer to tell the user what sort of input to type. This text is marked read-only so you won't accidentally delete or change it. It is also marked as a field (*note Fields::), so that certain motion functions, including `beginning-of-line', `forward-word', `forward-sentence', and `forward-paragraph', stop at the boundary between the prompt and the actual text. The minibuffer's window is normally a single line; it grows automatically if the contents require more space. Whilst it is active, you can explicitly resize it temporarily with the window sizing commands; it reverts to its normal size when the minibuffer is exited. When the minibuffer is not active, you can resize it permanently by using the window sizing commands in the frame's other window, or dragging the mode line with the mouse. (Due to details of the current implementation, for this to work `resize-mini-windows' must be `nil'.) If the frame contains just a minibuffer, you can change the minibuffer's size by changing the frame's size. Use of the minibuffer reads input events, and that alters the values of variables such as `this-command' and `last-command' (*note Command Loop Info::). Your program should bind them around the code that uses the minibuffer, if you do not want that to change them. Under some circumstances, a command can use a minibuffer even if there is an active minibuffer; such a minibuffer is called a "recursive minibuffer". The first minibuffer is named ` *Minibuf-1*'. Recursive minibuffers are named by incrementing the number at the end of the name. (The names begin with a space so that they won't show up in normal buffer lists.) Of several recursive minibuffers, the innermost (or most recently entered) is the active minibuffer. We usually call this "the" minibuffer. You can permit or forbid recursive minibuffers by setting the variable `enable-recursive-minibuffers', or by putting properties of that name on command symbols (*Note Recursive Mini::.) Like other buffers, a minibuffer uses a local keymap (*note Keymaps::) to specify special key bindings. The function that invokes the minibuffer also sets up its local map according to the job to be done. *Note Text from Minibuffer::, for the non-completion minibuffer local maps. *Note Completion Commands::, for the minibuffer local maps for completion. When a minibuffer is inactive, its major mode is `minibuffer-inactive-mode', with keymap `minibuffer-inactive-mode-map'. This is only really useful if the minibuffer is in a separate frame. *Note Minibuffers and Frames::. When Emacs is running in batch mode, any request to read from the minibuffer actually reads a line from the standard input descriptor that was supplied when Emacs was started. 20.2 Reading Text Strings with the Minibuffer ============================================= The most basic primitive for minibuffer input is `read-from-minibuffer', which can be used to read either a string or a Lisp object in textual form. The function `read-regexp' is used for reading regular expressions (*note Regular Expressions::), which are a special kind of string. There are also specialized functions for reading commands, variables, file names, etc. (*note Completion::). In most cases, you should not call minibuffer input functions in the middle of a Lisp function. Instead, do all minibuffer input as part of reading the arguments for a command, in the `interactive' specification. *Note Defining Commands::. -- Function: read-from-minibuffer prompt &optional initial keymap read history default inherit-input-method This function is the most general way to get input from the minibuffer. By default, it accepts arbitrary text and returns it as a string; however, if READ is non-`nil', then it uses `read' to convert the text into a Lisp object (*note Input Functions::). The first thing this function does is to activate a minibuffer and display it with PROMPT (which must be a string) as the prompt. Then the user can edit text in the minibuffer. When the user types a command to exit the minibuffer, `read-from-minibuffer' constructs the return value from the text in the minibuffer. Normally it returns a string containing that text. However, if READ is non-`nil', `read-from-minibuffer' reads the text and returns the resulting Lisp object, unevaluated. (*Note Input Functions::, for information about reading.) The argument DEFAULT specifies default values to make available through the history commands. It should be a string, a list of strings, or `nil'. The string or strings become the minibuffer's "future history", available to the user with `M-n'. If READ is non-`nil', then DEFAULT is also used as the input to `read', if the user enters empty input. If DEFAULT is a list of strings, the first string is used as the input. If DEFAULT is `nil', empty input results in an `end-of-file' error. However, in the usual case (where READ is `nil'), `read-from-minibuffer' ignores DEFAULT when the user enters empty input and returns an empty string, `""'. In this respect, it differs from all the other minibuffer input functions in this chapter. If KEYMAP is non-`nil', that keymap is the local keymap to use in the minibuffer. If KEYMAP is omitted or `nil', the value of `minibuffer-local-map' is used as the keymap. Specifying a keymap is the most important way to customize the minibuffer for various applications such as completion. The argument HISTORY specifies a history list variable to use for saving the input and for history commands used in the minibuffer. It defaults to `minibuffer-history'. You can optionally specify a starting position in the history list as well. *Note Minibuffer History::. If the variable `minibuffer-allow-text-properties' is non-`nil', then the string that is returned includes whatever text properties were present in the minibuffer. Otherwise all the text properties are stripped when the value is returned. If the argument INHERIT-INPUT-METHOD is non-`nil', then the minibuffer inherits the current input method (*note Input Methods::) and the setting of `enable-multibyte-characters' (*note Text Representations::) from whichever buffer was current before entering the minibuffer. Use of INITIAL is mostly deprecated; we recommend using a non-`nil' value only in conjunction with specifying a cons cell for HISTORY. *Note Initial Input::. -- Function: read-string prompt &optional initial history default inherit-input-method This function reads a string from the minibuffer and returns it. The arguments PROMPT, INITIAL, HISTORY and INHERIT-INPUT-METHOD are used as in `read-from-minibuffer'. The keymap used is `minibuffer-local-map'. The optional argument DEFAULT is used as in `read-from-minibuffer', except that, if non-`nil', it also specifies a default value to return if the user enters null input. As in `read-from-minibuffer' it should be a string, a list of strings, or `nil', which is equivalent to an empty string. When DEFAULT is a string, that string is the default value. When it is a list of strings, the first string is the default value. (All these strings are available to the user in the "future minibuffer history".) This function works by calling the `read-from-minibuffer' function: (read-string PROMPT INITIAL HISTORY DEFAULT INHERIT) == (let ((value (read-from-minibuffer PROMPT INITIAL nil nil HISTORY DEFAULT INHERIT))) (if (and (equal value "") DEFAULT) (if (consp DEFAULT) (car DEFAULT) DEFAULT) value)) -- Function: read-regexp prompt &optional default history This function reads a regular expression as a string from the minibuffer and returns it. The argument PROMPT is used as in `read-from-minibuffer'. The optional argument DEFAULT specifies a default value to return if the user enters null input; it should be a string, or `nil', which is equivalent to an empty string. The optional argument HISTORY, if non-`nil', is a symbol specifying a minibuffer history list to use (*note Minibuffer History::). If it is omitted or `nil', the history list defaults to `regexp-history'. `read-regexp' also collects a few useful candidates for input and passes them to `read-from-minibuffer', to make them available to the user as the "future minibuffer history list" (*note future list: (emacs)Minibuffer History.). These candidates are: - The word or symbol at point. - The last regexp used in an incremental search. - The last string used in an incremental search. - The last string or pattern used in query-replace commands. This function works by calling the `read-from-minibuffer' function, after computing the list of defaults as described above. -- Variable: minibuffer-allow-text-properties If this variable is `nil', then `read-from-minibuffer' and `read-string' strip all text properties from the minibuffer input before returning it. However, `read-no-blanks-input' (see below), as well as `read-minibuffer' and related functions (*note Reading Lisp Objects With the Minibuffer: Object from Minibuffer.), and all functions that do minibuffer input with completion, discard text properties unconditionally, regardless of the value of this variable. -- Variable: minibuffer-local-map This is the default local keymap for reading from the minibuffer. By default, it makes the following bindings: `C-j' `exit-minibuffer' `exit-minibuffer' `C-g' `abort-recursive-edit' `M-n' `next-history-element' `M-p' `previous-history-element' `M-s' `next-matching-history-element' `M-r' `previous-matching-history-element' -- Function: read-no-blanks-input prompt &optional initial inherit-input-method This function reads a string from the minibuffer, but does not allow whitespace characters as part of the input: instead, those characters terminate the input. The arguments PROMPT, INITIAL, and INHERIT-INPUT-METHOD are used as in `read-from-minibuffer'. This is a simplified interface to the `read-from-minibuffer' function, and passes the value of the `minibuffer-local-ns-map' keymap as the KEYMAP argument for that function. Since the keymap `minibuffer-local-ns-map' does not rebind `C-q', it _is_ possible to put a space into the string, by quoting it. This function discards text properties, regardless of the value of `minibuffer-allow-text-properties'. (read-no-blanks-input PROMPT INITIAL) == (let (minibuffer-allow-text-properties) (read-from-minibuffer PROMPT INITIAL minibuffer-local-ns-map)) -- Variable: minibuffer-local-ns-map This built-in variable is the keymap used as the minibuffer local keymap in the function `read-no-blanks-input'. By default, it makes the following bindings, in addition to those of `minibuffer-local-map': `exit-minibuffer' `exit-minibuffer' `?' `self-insert-and-exit' 20.3 Reading Lisp Objects with the Minibuffer ============================================= This section describes functions for reading Lisp objects with the minibuffer. -- Function: read-minibuffer prompt &optional initial This function reads a Lisp object using the minibuffer, and returns it without evaluating it. The arguments PROMPT and INITIAL are used as in `read-from-minibuffer'. This is a simplified interface to the `read-from-minibuffer' function: (read-minibuffer PROMPT INITIAL) == (let (minibuffer-allow-text-properties) (read-from-minibuffer PROMPT INITIAL nil t)) Here is an example in which we supply the string `"(testing)"' as initial input: (read-minibuffer "Enter an expression: " (format "%s" '(testing))) ;; Here is how the minibuffer is displayed: ---------- Buffer: Minibuffer ---------- Enter an expression: (testing)-!- ---------- Buffer: Minibuffer ---------- The user can type immediately to use the initial input as a default, or can edit the input. -- Function: eval-minibuffer prompt &optional initial This function reads a Lisp expression using the minibuffer, evaluates it, then returns the result. The arguments PROMPT and INITIAL are used as in `read-from-minibuffer'. This function simply evaluates the result of a call to `read-minibuffer': (eval-minibuffer PROMPT INITIAL) == (eval (read-minibuffer PROMPT INITIAL)) -- Function: edit-and-eval-command prompt form This function reads a Lisp expression in the minibuffer, evaluates it, then returns the result. The difference between this command and `eval-minibuffer' is that here the initial FORM is not optional and it is treated as a Lisp object to be converted to printed representation rather than as a string of text. It is printed with `prin1', so if it is a string, double-quote characters (`"') appear in the initial text. *Note Output Functions::. In the following example, we offer the user an expression with initial text that is already a valid form: (edit-and-eval-command "Please edit: " '(forward-word 1)) ;; After evaluation of the preceding expression, ;; the following appears in the minibuffer: ---------- Buffer: Minibuffer ---------- Please edit: (forward-word 1)-!- ---------- Buffer: Minibuffer ---------- Typing right away would exit the minibuffer and evaluate the expression, thus moving point forward one word. 20.4 Minibuffer History ======================= A "minibuffer history list" records previous minibuffer inputs so the user can reuse them conveniently. It is a variable whose value is a list of strings (previous inputs), most recent first. There are many separate minibuffer history lists, used for different kinds of inputs. It's the Lisp programmer's job to specify the right history list for each use of the minibuffer. You specify a minibuffer history list with the optional HISTORY argument to `read-from-minibuffer' or `completing-read'. Here are the possible values for it: VARIABLE Use VARIABLE (a symbol) as the history list. (VARIABLE . STARTPOS) Use VARIABLE (a symbol) as the history list, and assume that the initial history position is STARTPOS (a nonnegative integer). Specifying 0 for STARTPOS is equivalent to just specifying the symbol VARIABLE. `previous-history-element' will display the most recent element of the history list in the minibuffer. If you specify a positive STARTPOS, the minibuffer history functions behave as if `(elt VARIABLE (1- STARTPOS))' were the history element currently shown in the minibuffer. For consistency, you should also specify that element of the history as the initial minibuffer contents, using the INITIAL argument to the minibuffer input function (*note Initial Input::). If you don't specify HISTORY, then the default history list `minibuffer-history' is used. For other standard history lists, see below. You can also create your own history list variable; just initialize it to `nil' before the first use. Both `read-from-minibuffer' and `completing-read' add new elements to the history list automatically, and provide commands to allow the user to reuse items on the list. The only thing your program needs to do to use a history list is to initialize it and to pass its name to the input functions when you wish. But it is safe to modify the list by hand when the minibuffer input functions are not using it. Emacs functions that add a new element to a history list can also delete old elements if the list gets too long. The variable `history-length' specifies the maximum length for most history lists. To specify a different maximum length for a particular history list, put the length in the `history-length' property of the history list symbol. The variable `history-delete-duplicates' specifies whether to delete duplicates in history. -- Function: add-to-history history-var newelt &optional maxelt keep-all This function adds a new element NEWELT, if it isn't the empty string, to the history list stored in the variable HISTORY-VAR, and returns the updated history list. It limits the list length to the value of MAXELT (if non-`nil') or `history-length' (described below). The possible values of MAXELT have the same meaning as the values of `history-length'. Normally, `add-to-history' removes duplicate members from the history list if `history-delete-duplicates' is non-`nil'. However, if KEEP-ALL is non-`nil', that says not to remove duplicates, and to add NEWELT to the list even if it is empty. -- Variable: history-add-new-input If the value of this variable is `nil', standard functions that read from the minibuffer don't add new elements to the history list. This lets Lisp programs explicitly manage input history by using `add-to-history'. The default value is `t'. -- User Option: history-length The value of this variable specifies the maximum length for all history lists that don't specify their own maximum lengths. If the value is `t', that means there is no maximum (don't delete old elements). If a history list variable's symbol has a non-`nil' `history-length' property, it overrides this variable for that particular history list. -- User Option: history-delete-duplicates If the value of this variable is `t', that means when adding a new history element, all previous identical elements are deleted. Here are some of the standard minibuffer history list variables: -- Variable: minibuffer-history The default history list for minibuffer history input. -- Variable: query-replace-history A history list for arguments to `query-replace' (and similar arguments to other commands). -- Variable: file-name-history A history list for file-name arguments. -- Variable: buffer-name-history A history list for buffer-name arguments. -- Variable: regexp-history A history list for regular expression arguments. -- Variable: extended-command-history A history list for arguments that are names of extended commands. -- Variable: shell-command-history A history list for arguments that are shell commands. -- Variable: read-expression-history A history list for arguments that are Lisp expressions to evaluate. -- Variable: face-name-history A history list for arguments that are faces. 20.5 Initial Input ================== Several of the functions for minibuffer input have an argument called INITIAL. This is a mostly-deprecated feature for specifying that the minibuffer should start out with certain text, instead of empty as usual. If INITIAL is a string, the minibuffer starts out containing the text of the string, with point at the end, when the user starts to edit the text. If the user simply types to exit the minibuffer, it will use the initial input string to determine the value to return. *We discourage use of a non-`nil' value for INITIAL*, because initial input is an intrusive interface. History lists and default values provide a much more convenient method to offer useful default inputs to the user. There is just one situation where you should specify a string for an INITIAL argument. This is when you specify a cons cell for the HISTORY argument. *Note Minibuffer History::. INITIAL can also be a cons cell of the form `(STRING . POSITION)'. This means to insert STRING in the minibuffer but put point at POSITION within the string's text. As a historical accident, POSITION was implemented inconsistently in different functions. In `completing-read', POSITION's value is interpreted as origin-zero; that is, a value of 0 means the beginning of the string, 1 means after the first character, etc. In `read-minibuffer', and the other non-completion minibuffer input functions that support this argument, 1 means the beginning of the string, 2 means after the first character, etc. Use of a cons cell as the value for INITIAL arguments is deprecated. 20.6 Completion =============== "Completion" is a feature that fills in the rest of a name starting from an abbreviation for it. Completion works by comparing the user's input against a list of valid names and determining how much of the name is determined uniquely by what the user has typed. For example, when you type `C-x b' (`switch-to-buffer') and then type the first few letters of the name of the buffer to which you wish to switch, and then type (`minibuffer-complete'), Emacs extends the name as far as it can. Standard Emacs commands offer completion for names of symbols, files, buffers, and processes; with the functions in this section, you can implement completion for other kinds of names. The `try-completion' function is the basic primitive for completion: it returns the longest determined completion of a given initial string, with a given set of strings to match against. The function `completing-read' provides a higher-level interface for completion. A call to `completing-read' specifies how to determine the list of valid names. The function then activates the minibuffer with a local keymap that binds a few keys to commands useful for completion. Other functions provide convenient simple interfaces for reading certain kinds of names with completion. 20.6.1 Basic Completion Functions --------------------------------- The following completion functions have nothing in themselves to do with minibuffers. We describe them here to keep them near the higher-level completion features that do use the minibuffer. -- Function: try-completion string collection &optional predicate This function returns the longest common substring of all possible completions of STRING in COLLECTION. COLLECTION is called the "completion table". Its value must be a list of strings or cons cells, an obarray, a hash table, or a completion function. `try-completion' compares STRING against each of the permissible completions specified by the completion table. If no permissible completions match, it returns `nil'. If there is just one matching completion, and the match is exact, it returns `t'. Otherwise, it returns the longest initial sequence common to all possible matching completions. If COLLECTION is an list, the permissible completions are specified by the elements of the list, each of which should be either a string, or a cons cell whose CAR is either a string or a symbol (a symbol is converted to a string using `symbol-name'). If the list contains elements of any other type, those are ignored. If COLLECTION is an obarray (*note Creating Symbols::), the names of all symbols in the obarray form the set of permissible completions. If COLLECTION is a hash table, then the keys that are strings are the possible completions. Other keys are ignored. You can also use a function as COLLECTION. Then the function is solely responsible for performing completion; `try-completion' returns whatever this function returns. The function is called with three arguments: STRING, PREDICATE and `nil' (the third argument is so that the same function can be used in `all-completions' and do the appropriate thing in either case). *Note Programmed Completion::. If the argument PREDICATE is non-`nil', then it must be a function of one argument, unless COLLECTION is a hash table, in which case it should be a function of two arguments. It is used to test each possible match, and the match is accepted only if PREDICATE returns non-`nil'. The argument given to PREDICATE is either a string or a cons cell (the CAR of which is a string) from the alist, or a symbol (_not_ a symbol name) from the obarray. If COLLECTION is a hash table, PREDICATE is called with two arguments, the string key and the associated value. In addition, to be acceptable, a completion must also match all the regular expressions in `completion-regexp-list'. (Unless COLLECTION is a function, in which case that function has to handle `completion-regexp-list' itself.) In the first of the following examples, the string `foo' is matched by three of the alist CARs. All of the matches begin with the characters `fooba', so that is the result. In the second example, there is only one possible match, and it is exact, so the return value is `t'. (try-completion "foo" '(("foobar1" 1) ("barfoo" 2) ("foobaz" 3) ("foobar2" 4))) => "fooba" (try-completion "foo" '(("barfoo" 2) ("foo" 3))) => t In the following example, numerous symbols begin with the characters `forw', and all of them begin with the word `forward'. In most of the symbols, this is followed with a `-', but not in all, so no more than `forward' can be completed. (try-completion "forw" obarray) => "forward" Finally, in the following example, only two of the three possible matches pass the predicate `test' (the string `foobaz' is too short). Both of those begin with the string `foobar'. (defun test (s) (> (length (car s)) 6)) => test (try-completion "foo" '(("foobar1" 1) ("barfoo" 2) ("foobaz" 3) ("foobar2" 4)) 'test) => "foobar" -- Function: all-completions string collection &optional predicate This function returns a list of all possible completions of STRING. The arguments to this function are the same as those of `try-completion', and it uses `completion-regexp-list' in the same way that `try-completion' does. If COLLECTION is a function, it is called with three arguments: STRING, PREDICATE and `t'; then `all-completions' returns whatever the function returns. *Note Programmed Completion::. Here is an example, using the function `test' shown in the example for `try-completion': (defun test (s) (> (length (car s)) 6)) => test (all-completions "foo" '(("foobar1" 1) ("barfoo" 2) ("foobaz" 3) ("foobar2" 4)) 'test) => ("foobar1" "foobar2") -- Function: test-completion string collection &optional predicate This function returns non-`nil' if STRING is a valid completion alternative specified by COLLECTION and PREDICATE. The arguments are the same as in `try-completion'. For instance, if COLLECTION is a list of strings, this is true if STRING appears in the list and PREDICATE is satisfied. This function uses `completion-regexp-list' in the same way that `try-completion' does. If PREDICATE is non-`nil' and if COLLECTION contains several strings that are equal to each other, as determined by `compare-strings' according to `completion-ignore-case', then PREDICATE should accept either all or none of them. Otherwise, the return value of `test-completion' is essentially unpredictable. If COLLECTION is a function, it is called with three arguments, the values STRING, PREDICATE and `lambda'; whatever it returns, `test-completion' returns in turn. -- Function: completion-boundaries string collection predicate suffix This function returns the boundaries of the field on which COLLECTION will operate, assuming that STRING holds the text before point and SUFFIX holds the text after point. Normally completion operates on the whole string, so for all normal collections, this will always return `(0 . (length SUFFIX))'. But more complex completion such as completion on files is done one field at a time. For example, completion of `"/usr/sh"' will include `"/usr/share/"' but not `"/usr/share/doc"' even if `"/usr/share/doc"' exists. Also `all-completions' on `"/usr/sh"' will not include `"/usr/share/"' but only `"share/"'. So if STRING is `"/usr/sh"' and SUFFIX is `"e/doc"', `completion-boundaries' will return `(5 . 1)' which tells us that the COLLECTION will only return completion information that pertains to the area after `"/usr/"' and before `"/doc"'. If you store a completion alist in a variable, you should mark the variable as "risky" by giving it a non-`nil' `risky-local-variable' property. *Note File Local Variables::. -- Variable: completion-ignore-case If the value of this variable is non-`nil', case is not considered significant in completion. Within `read-file-name', this variable is overridden by `read-file-name-completion-ignore-case' (*note Reading File Names::); within `read-buffer', it is overridden by `read-buffer-completion-ignore-case' (*note High-Level Completion::). -- Variable: completion-regexp-list This is a list of regular expressions. The completion functions only consider a completion acceptable if it matches all regular expressions in this list, with `case-fold-search' (*note Searching and Case::) bound to the value of `completion-ignore-case'. -- Macro: lazy-completion-table var fun This macro provides a way to initialize the variable VAR as a collection for completion in a lazy way, not computing its actual contents until they are first needed. You use this macro to produce a value that you store in VAR. The actual computation of the proper value is done the first time you do completion using VAR. It is done by calling FUN with no arguments. The value FUN returns becomes the permanent value of VAR. Here is an example: (defvar foo (lazy-completion-table foo make-my-alist)) There are several functions that take an existing completion table and return a modified version. `completion-table-case-fold' returns a case-insensitive table. `completion-table-in-turn' combines multiple input tables. `completion-table-subvert' alters a table to use a different initial prefix. `completion-table-with-quoting' returns a table suitable for operating on quoted text. `completion-table-with-predicate' filters a table with a predicate function. `completion-table-with-terminator' adds a terminating string. 20.6.2 Completion and the Minibuffer ------------------------------------ This section describes the basic interface for reading from the minibuffer with completion. -- Function: completing-read prompt collection &optional predicate require-match initial history default inherit-input-method This function reads a string in the minibuffer, assisting the user by providing completion. It activates the minibuffer with prompt PROMPT, which must be a string. The actual completion is done by passing the completion table COLLECTION and the completion predicate PREDICATE to the function `try-completion' (*note Basic Completion::). This happens in certain commands bound in the local keymaps used for completion. Some of these commands also call `test-completion'. Thus, if PREDICATE is non-`nil', it should be compatible with COLLECTION and `completion-ignore-case'. *Note Definition of test-completion::. The value of the optional argument REQUIRE-MATCH determines how the user may exit the minibuffer: * If `nil', the usual minibuffer exit commands work regardless of the input in the minibuffer. * If `t', the usual minibuffer exit commands won't exit unless the input completes to an element of COLLECTION. * If `confirm', the user can exit with any input, but is asked for confirmation if the input is not an element of COLLECTION. * If `confirm-after-completion', the user can exit with any input, but is asked for confirmation if the preceding command was a completion command (i.e., one of the commands in `minibuffer-confirm-exit-commands') and the resulting input is not an element of COLLECTION. *Note Completion Commands::. * Any other value of REQUIRE-MATCH behaves like `t', except that the exit commands won't exit if it performs completion. However, empty input is always permitted, regardless of the value of REQUIRE-MATCH; in that case, `completing-read' returns the first element of DEFAULT, if it is a list; `""', if DEFAULT is `nil'; or DEFAULT. The string or strings in DEFAULT are also available to the user through the history commands. The function `completing-read' uses `minibuffer-local-completion-map' as the keymap if REQUIRE-MATCH is `nil', and uses `minibuffer-local-must-match-map' if REQUIRE-MATCH is non-`nil'. *Note Completion Commands::. The argument HISTORY specifies which history list variable to use for saving the input and for minibuffer history commands. It defaults to `minibuffer-history'. *Note Minibuffer History::. The argument INITIAL is mostly deprecated; we recommend using a non-`nil' value only in conjunction with specifying a cons cell for HISTORY. *Note Initial Input::. For default input, use DEFAULT instead. If the argument INHERIT-INPUT-METHOD is non-`nil', then the minibuffer inherits the current input method (*note Input Methods::) and the setting of `enable-multibyte-characters' (*note Text Representations::) from whichever buffer was current before entering the minibuffer. If the variable `completion-ignore-case' is non-`nil', completion ignores case when comparing the input against the possible matches. *Note Basic Completion::. In this mode of operation, PREDICATE must also ignore case, or you will get surprising results. Here's an example of using `completing-read': (completing-read "Complete a foo: " '(("foobar1" 1) ("barfoo" 2) ("foobaz" 3) ("foobar2" 4)) nil t "fo") ;; After evaluation of the preceding expression, ;; the following appears in the minibuffer: ---------- Buffer: Minibuffer ---------- Complete a foo: fo-!- ---------- Buffer: Minibuffer ---------- If the user then types ` b ', `completing-read' returns `barfoo'. The `completing-read' function binds variables to pass information to the commands that actually do completion. They are described in the following section. -- Variable: completing-read-function The value of this variable must be a function, which is called by `completing-read' to actually do its work. It should accept the same arguments as `completing-read'. This can be bound to a different function to completely override the normal behavior of `completing-read'. 20.6.3 Minibuffer Commands that Do Completion --------------------------------------------- This section describes the keymaps, commands and user options used in the minibuffer to do completion. -- Variable: minibuffer-completion-table The value of this variable is the completion table used for completion in the minibuffer. This is the global variable that contains what `completing-read' passes to `try-completion'. It is used by minibuffer completion commands such as `minibuffer-complete-word'. -- Variable: minibuffer-completion-predicate This variable's value is the predicate that `completing-read' passes to `try-completion'. The variable is also used by the other minibuffer completion functions. -- Variable: minibuffer-completion-confirm This variable determines whether Emacs asks for confirmation before exiting the minibuffer; `completing-read' binds this variable, and the function `minibuffer-complete-and-exit' checks the value before exiting. If the value is `nil', confirmation is not required. If the value is `confirm', the user may exit with an input that is not a valid completion alternative, but Emacs asks for confirmation. If the value is `confirm-after-completion', the user may exit with an input that is not a valid completion alternative, but Emacs asks for confirmation if the user submitted the input right after any of the completion commands in `minibuffer-confirm-exit-commands'. -- Variable: minibuffer-confirm-exit-commands This variable holds a list of commands that cause Emacs to ask for confirmation before exiting the minibuffer, if the REQUIRE-MATCH argument to `completing-read' is `confirm-after-completion'. The confirmation is requested if the user attempts to exit the minibuffer immediately after calling any command in this list. -- Command: minibuffer-complete-word This function completes the minibuffer contents by at most a single word. Even if the minibuffer contents have only one completion, `minibuffer-complete-word' does not add any characters beyond the first character that is not a word constituent. *Note Syntax Tables::. -- Command: minibuffer-complete This function completes the minibuffer contents as far as possible. -- Command: minibuffer-complete-and-exit This function completes the minibuffer contents, and exits if confirmation is not required, i.e., if `minibuffer-completion-confirm' is `nil'. If confirmation _is_ required, it is given by repeating this command immediately--the command is programmed to work without confirmation when run twice in succession. -- Command: minibuffer-completion-help This function creates a list of the possible completions of the current minibuffer contents. It works by calling `all-completions' using the value of the variable `minibuffer-completion-table' as the COLLECTION argument, and the value of `minibuffer-completion-predicate' as the PREDICATE argument. The list of completions is displayed as text in a buffer named `*Completions*'. -- Function: display-completion-list completions &optional common-substring This function displays COMPLETIONS to the stream in `standard-output', usually a buffer. (*Note Read and Print::, for more information about streams.) The argument COMPLETIONS is normally a list of completions just returned by `all-completions', but it does not have to be. Each element may be a symbol or a string, either of which is simply printed. It can also be a list of two strings, which is printed as if the strings were concatenated. The first of the two strings is the actual completion, the second string serves as annotation. The argument COMMON-SUBSTRING is the prefix that is common to all the completions. With normal Emacs completion, it is usually the same as the string that was completed. `display-completion-list' uses this to highlight text in the completion list for better visual feedback. This is not needed in the minibuffer; for minibuffer completion, you can pass `nil'. This function is called by `minibuffer-completion-help'. A common way to use it is together with `with-output-to-temp-buffer', like this: (with-output-to-temp-buffer "*Completions*" (display-completion-list (all-completions (buffer-string) my-alist) (buffer-string))) -- User Option: completion-auto-help If this variable is non-`nil', the completion commands automatically display a list of possible completions whenever nothing can be completed because the next character is not uniquely determined. -- Variable: minibuffer-local-completion-map `completing-read' uses this value as the local keymap when an exact match of one of the completions is not required. By default, this keymap makes the following bindings: `?' `minibuffer-completion-help' `minibuffer-complete-word' `minibuffer-complete' and uses `minibuffer-local-map' as its parent keymap (*note Definition of minibuffer-local-map::). -- Variable: minibuffer-local-must-match-map `completing-read' uses this value as the local keymap when an exact match of one of the completions is required. Therefore, no keys are bound to `exit-minibuffer', the command that exits the minibuffer unconditionally. By default, this keymap makes the following bindings: `C-j' `minibuffer-complete-and-exit' `minibuffer-complete-and-exit' and uses `minibuffer-local-completion-map' as its parent keymap. -- Variable: minibuffer-local-filename-completion-map This is a sparse keymap that simply unbinds ; because filenames can contain spaces. The function `read-file-name' combines this keymap with either `minibuffer-local-completion-map' or `minibuffer-local-must-match-map'. 20.6.4 High-Level Completion Functions -------------------------------------- This section describes the higher-level convenience functions for reading certain sorts of names with completion. In most cases, you should not call these functions in the middle of a Lisp function. When possible, do all minibuffer input as part of reading the arguments for a command, in the `interactive' specification. *Note Defining Commands::. -- Function: read-buffer prompt &optional default require-match This function reads the name of a buffer and returns it as a string. The argument DEFAULT is the default name to use, the value to return if the user exits with an empty minibuffer. If non-`nil', it should be a string, a list of strings, or a buffer. If it is a list, the default value is the first element of this list. It is mentioned in the prompt, but is not inserted in the minibuffer as initial input. The argument PROMPT should be a string ending with a colon and a space. If DEFAULT is non-`nil', the function inserts it in PROMPT before the colon to follow the convention for reading from the minibuffer with a default value (*note Programming Tips::). The optional argument REQUIRE-MATCH has the same meaning as in `completing-read'. *Note Minibuffer Completion::. In the following example, the user enters `minibuffer.t', and then types . The argument REQUIRE-MATCH is `t', and the only buffer name starting with the given input is `minibuffer.texi', so that name is the value. (read-buffer "Buffer name: " "foo" t) ;; After evaluation of the preceding expression, ;; the following prompt appears, ;; with an empty minibuffer: ---------- Buffer: Minibuffer ---------- Buffer name (default foo): -!- ---------- Buffer: Minibuffer ---------- ;; The user types `minibuffer.t '. => "minibuffer.texi" -- User Option: read-buffer-function This variable, if non-`nil', specifies a function for reading buffer names. `read-buffer' calls this function instead of doing its usual work, with the same arguments passed to `read-buffer'. -- User Option: read-buffer-completion-ignore-case If this variable is non-`nil', `read-buffer' ignores case when performing completion. -- Function: read-command prompt &optional default This function reads the name of a command and returns it as a Lisp symbol. The argument PROMPT is used as in `read-from-minibuffer'. Recall that a command is anything for which `commandp' returns `t', and a command name is a symbol for which `commandp' returns `t'. *Note Interactive Call::. The argument DEFAULT specifies what to return if the user enters null input. It can be a symbol, a string or a list of strings. If it is a string, `read-command' interns it before returning it. If it is a list, `read-command' interns the first element of this list. If DEFAULT is `nil', that means no default has been specified; then if the user enters null input, the return value is `(intern "")', that is, a symbol whose name is an empty string. (read-command "Command name? ") ;; After evaluation of the preceding expression, ;; the following prompt appears with an empty minibuffer: ---------- Buffer: Minibuffer ---------- Command name? ---------- Buffer: Minibuffer ---------- If the user types `forward-c ', then this function returns `forward-char'. The `read-command' function is a simplified interface to `completing-read'. It uses the variable `obarray' so as to complete in the set of extant Lisp symbols, and it uses the `commandp' predicate so as to accept only command names: (read-command PROMPT) == (intern (completing-read PROMPT obarray 'commandp t nil)) -- Function: read-variable prompt &optional default This function reads the name of a customizable variable and returns it as a symbol. Its arguments have the same form as those of `read-command'. It behaves just like `read-command', except that it uses the predicate `custom-variable-p' instead of `commandp'. -- Command: read-color &optional prompt convert allow-empty display This function reads a string that is a color specification, either the color's name or an RGB hex value such as `#RRRGGGBBB'. It prompts with PROMPT (default: `"Color (name or #RGB triplet):"') and provides completion for color names, but not for hex RGB values. In addition to names of standard colors, completion candidates include the foreground and background colors at point. Valid RGB values are described in *note Color Names::. The function's return value is the string typed by the user in the minibuffer. However, when called interactively or if the optional argument CONVERT is non-`nil', it converts any input color name into the corresponding RGB value string and instead returns that. This function requires a valid color specification to be input. Empty color names are allowed when ALLOW-EMPTY is non-`nil' and the user enters null input. Interactively, or when DISPLAY is non-`nil', the return value is also displayed in the echo area. See also the functions `read-coding-system' and `read-non-nil-coding-system', in *note User-Chosen Coding Systems::, and `read-input-method-name', in *note Input Methods::. 20.6.5 Reading File Names ------------------------- The high-level completion functions `read-file-name', `read-directory-name', and `read-shell-command' are designed to read file names, directory names, and shell commands, respectively. They provide special features, including automatic insertion of the default directory. -- Function: read-file-name prompt &optional directory default require-match initial predicate This function reads a file name, prompting with PROMPT and providing completion. As an exception, this function reads a file name using a graphical file dialog instead of the minibuffer, if all of the following are true: 1. It is invoked via a mouse command. 2. The selected frame is on a graphical display supporting such dialogs. 3. The variable `use-dialog-box' is non-`nil'. *Note Dialog Boxes: (emacs)Dialog Boxes. 4. The DIRECTORY argument, described below, does not specify a remote file. *Note Remote Files: (emacs)Remote Files. The exact behavior when using a graphical file dialog is platform-dependent. Here, we simply document the behavior when using the minibuffer. `read-file-name' does not automatically expand the returned file name. You must call `expand-file-name' yourself if an absolute file name is required. The optional argument REQUIRE-MATCH has the same meaning as in `completing-read'. *Note Minibuffer Completion::. The argument DIRECTORY specifies the directory to use for completing relative file names. It should be an absolute directory name. If the variable `insert-default-directory' is non-`nil', DIRECTORY is also inserted in the minibuffer as initial input. It defaults to the current buffer's value of `default-directory'. If you specify INITIAL, that is an initial file name to insert in the buffer (after DIRECTORY, if that is inserted). In this case, point goes at the beginning of INITIAL. The default for INITIAL is `nil'--don't insert any file name. To see what INITIAL does, try the command `C-x C-v' in a buffer visiting a file. *Please note:* we recommend using DEFAULT rather than INITIAL in most cases. If DEFAULT is non-`nil', then the function returns DEFAULT if the user exits the minibuffer with the same non-empty contents that `read-file-name' inserted initially. The initial minibuffer contents are always non-empty if `insert-default-directory' is non-`nil', as it is by default. DEFAULT is not checked for validity, regardless of the value of REQUIRE-MATCH. However, if REQUIRE-MATCH is non-`nil', the initial minibuffer contents should be a valid file (or directory) name. Otherwise `read-file-name' attempts completion if the user exits without any editing, and does not return DEFAULT. DEFAULT is also available through the history commands. If DEFAULT is `nil', `read-file-name' tries to find a substitute default to use in its place, which it treats in exactly the same way as if it had been specified explicitly. If DEFAULT is `nil', but INITIAL is non-`nil', then the default is the absolute file name obtained from DIRECTORY and INITIAL. If both DEFAULT and INITIAL are `nil' and the buffer is visiting a file, `read-file-name' uses the absolute file name of that file as default. If the buffer is not visiting a file, then there is no default. In that case, if the user types without any editing, `read-file-name' simply returns the pre-inserted contents of the minibuffer. If the user types in an empty minibuffer, this function returns an empty string, regardless of the value of REQUIRE-MATCH. This is, for instance, how the user can make the current buffer visit no file using `M-x set-visited-file-name'. If PREDICATE is non-`nil', it specifies a function of one argument that decides which file names are acceptable completion alternatives. A file name is an acceptable value if PREDICATE returns non-`nil' for it. Here is an example of using `read-file-name': (read-file-name "The file is ") ;; After evaluation of the preceding expression, ;; the following appears in the minibuffer: ---------- Buffer: Minibuffer ---------- The file is /gp/gnu/elisp/-!- ---------- Buffer: Minibuffer ---------- Typing `manual ' results in the following: ---------- Buffer: Minibuffer ---------- The file is /gp/gnu/elisp/manual.texi-!- ---------- Buffer: Minibuffer ---------- If the user types , `read-file-name' returns the file name as the string `"/gp/gnu/elisp/manual.texi"'. -- Variable: read-file-name-function If non-`nil', this should be a function that accepts the same arguments as `read-file-name'. When `read-file-name' is called, it calls this function with the supplied arguments instead of doing its usual work. -- User Option: read-file-name-completion-ignore-case If this variable is non-`nil', `read-file-name' ignores case when performing completion. -- Function: read-directory-name prompt &optional directory default require-match initial This function is like `read-file-name' but allows only directory names as completion alternatives. If DEFAULT is `nil' and INITIAL is non-`nil', `read-directory-name' constructs a substitute default by combining DIRECTORY (or the current buffer's default directory if DIRECTORY is `nil') and INITIAL. If both DEFAULT and INITIAL are `nil', this function uses DIRECTORY as substitute default, or the current buffer's default directory if DIRECTORY is `nil'. -- User Option: insert-default-directory This variable is used by `read-file-name', and thus, indirectly, by most commands reading file names. (This includes all commands that use the code letters `f' or `F' in their interactive form. *Note Code Characters for interactive: Interactive Codes.) Its value controls whether `read-file-name' starts by placing the name of the default directory in the minibuffer, plus the initial file name, if any. If the value of this variable is `nil', then `read-file-name' does not place any initial input in the minibuffer (unless you specify initial input with the INITIAL argument). In that case, the default directory is still used for completion of relative file names, but is not displayed. If this variable is `nil' and the initial minibuffer contents are empty, the user may have to explicitly fetch the next history element to access a default value. If the variable is non-`nil', the initial minibuffer contents are always non-empty and the user can always request a default value by immediately typing in an unedited minibuffer. (See above.) For example: ;; Here the minibuffer starts out with the default directory. (let ((insert-default-directory t)) (read-file-name "The file is ")) ---------- Buffer: Minibuffer ---------- The file is ~lewis/manual/-!- ---------- Buffer: Minibuffer ---------- ;; Here the minibuffer is empty and only the prompt ;; appears on its line. (let ((insert-default-directory nil)) (read-file-name "The file is ")) ---------- Buffer: Minibuffer ---------- The file is -!- ---------- Buffer: Minibuffer ---------- -- Function: read-shell-command prompt &optional initial history &rest args This function reads a shell command from the minibuffer, prompting with PROMPT and providing intelligent completion. It completes the first word of the command using candidates that are appropriate for command names, and the rest of the command words as file names. This function uses `minibuffer-local-shell-command-map' as the keymap for minibuffer input. The HISTORY argument specifies the history list to use; if is omitted or `nil', it defaults to `shell-command-history' (*note shell-command-history: Minibuffer History.). The optional argument INITIAL specifies the initial content of the minibuffer (*note Initial Input::). The rest of ARGS, if present, are used as the DEFAULT and INHERIT-INPUT-METHOD arguments in `read-from-minibuffer' (*note Text from Minibuffer::). -- Variable: minibuffer-local-shell-command-map This keymap is used by `read-shell-command' for completing command and file names that are part of a shell command. It uses `minibuffer-local-map' as its parent keymap, and binds to `completion-at-point'. 20.6.6 Completion Variables --------------------------- Here are some variables that can be used to alter the default completion behavior. -- User Option: completion-styles The value of this variable is a list of completion style (symbols) to use for performing completion. A "completion style" is a set of rules for generating completions. Each symbol occurring this list must have a corresponding entry in `completion-styles-alist'. -- Variable: completion-styles-alist This variable stores a list of available completion styles. Each element in the list has the form (STYLE TRY-COMPLETION ALL-COMPLETIONS DOC) Here, STYLE is the name of the completion style (a symbol), which may be used in the `completion-styles' variable to refer to this style; TRY-COMPLETION is the function that does the completion; ALL-COMPLETIONS is the function that lists the completions; and DOC is a string describing the completion style. The TRY-COMPLETION and ALL-COMPLETIONS functions should each accept four arguments: STRING, COLLECTION, PREDICATE, and POINT. The STRING, COLLECTION, and PREDICATE arguments have the same meanings as in `try-completion' (*note Basic Completion::), and the POINT argument is the position of point within STRING. Each function should return a non-`nil' value if it performed its job, and `nil' if it did not (e.g., if there is no way to complete STRING according to the completion style). When the user calls a completion command like `minibuffer-complete' (*note Completion Commands::), Emacs looks for the first style listed in `completion-styles' and calls its TRY-COMPLETION function. If this function returns `nil', Emacs moves to the next listed completion style and calls its TRY-COMPLETION function, and so on until one of the TRY-COMPLETION functions successfully performs completion and returns a non-`nil' value. A similar procedure is used for listing completions, via the ALL-COMPLETIONS functions. *Note Completion Styles: (emacs)Completion Styles, for a description of the available completion styles. -- User Option: completion-category-overrides This variable specifies special completion styles and other completion behaviors to use when completing certain types of text. Its value should be an alist with elements of the form `(CATEGORY . ALIST)'. CATEGORY is a symbol describing what is being completed; currently, the `buffer', `file', and `unicode-name' categories are defined, but others can be defined via specialized completion functions (*note Programmed Completion::). ALIST is an association list describing how completion should behave for the corresponding category. The following alist keys are supported: `styles' The value should be a list of completion styles (symbols). `cycle' The value should be a value for `completion-cycle-threshold' (*note Completion Options: (emacs)Completion Options.) for this category. Additional alist entries may be defined in the future. -- Variable: completion-extra-properties This variable is used to specify extra properties of the current completion command. It is intended to be let-bound by specialized completion commands. Its value should be a list of property and value pairs. The following properties are supported: `:annotation-function' The value should be a function to add annotations in the completions buffer. This function must accept one argument, a completion, and should either return `nil' or a string to be displayed next to the completion. `:exit-function' The value should be a function to run after performing completion. The function should accept two arguments, STRING and STATUS, where STRING is the text to which the field was completed, and STATUS indicates what kind of operation happened: `finished' if text is now complete, `sole' if the text cannot be further completed but completion is not finished, or `exact' if the text is a valid completion but may be further completed. 20.6.7 Programmed Completion ---------------------------- Sometimes it is not possible or convenient to create an alist or an obarray containing all the intended possible completions ahead of time. In such a case, you can supply your own function to compute the completion of a given string. This is called "programmed completion". Emacs uses programmed completion when completing file names (*note File Name Completion::), among many other cases. To use this feature, pass a function as the COLLECTION argument to `completing-read'. The function `completing-read' arranges to pass your completion function along to `try-completion', `all-completions', and other basic completion functions, which will then let your function do all the work. The completion function should accept three arguments: * The string to be completed. * A predicate function with which to filter possible matches, or `nil' if none. The function should call the predicate for each possible match, and ignore the match if the predicate returns `nil'. * A flag specifying the type of completion operation to perform. This is one of the following four values: `nil' This specifies a `try-completion' operation. The function should return `t' if the specified string is a unique and exact match; if there is more than one match, it should return the common substring of all matches (if the string is an exact match for one completion alternative but also matches other longer alternatives, the return value is the string); if there are no matches, it should return `nil'. `t' This specifies an `all-completions' operation. The function should return a list of all possible completions of the specified string. `lambda' This specifies a `test-completion' operation. The function should return `t' if the specified string is an exact match for some completion alternative; `nil' otherwise. `(boundaries . SUFFIX)' This specifies a `completion-boundaries' operation. The function should return `(boundaries START . END)', where START is the position of the beginning boundary in the specified string, and END is the position of the end boundary in SUFFIX. `metadata' This specifies a request for information about the state of the current completion. The return value should have the form `(metadata . ALIST)', where ALIST is an alist whose elements are described below. If the flag has any other value, the completion function should return `nil'. The following is a list of metadata entries that a completion function may return in response to a `metadata' flag argument: `category' The value should be a symbol describing what kind of text the completion function is trying to complete. If the symbol matches one of the keys in `completion-category-overrides', the usual completion behavior is overridden. *Note Completion Variables::. `annotation-function' The value should be a function for "annotating" completions. The function should take one argument, STRING, which is a possible completion. It should return a string, which is displayed after the completion STRING in the `*Completions*' buffer. `display-sort-function' The value should be a function for sorting completions. The function should take one argument, a list of completion strings, and return a sorted list of completion strings. It is allowed to alter the input list destructively. `cycle-sort-function' The value should be a function for sorting completions, when `completion-cycle-threshold' is non-`nil' and the user is cycling through completion alternatives. *Note Completion Options: (emacs)Completion Options. Its argument list and return value are the same as for `display-sort-function'. -- Function: completion-table-dynamic function This function is a convenient way to write a function that can act as a programmed completion function. The argument FUNCTION should be a function that takes one argument, a string, and returns an alist of possible completions of it. You can think of `completion-table-dynamic' as a transducer between that interface and the interface for programmed completion functions. 20.6.8 Completion in Ordinary Buffers ------------------------------------- Although completion is usually done in the minibuffer, the completion facility can also be used on the text in ordinary Emacs buffers. In many major modes, in-buffer completion is performed by the `C-M-i' or `M-' command, bound to `completion-at-point'. *Note Symbol Completion: (emacs)Symbol Completion. This command uses the abnormal hook variable `completion-at-point-functions': -- Variable: completion-at-point-functions The value of this abnormal hook should be a list of functions, which are used to compute a completion table for completing the text at point. It can be used by major modes to provide mode-specific completion tables (*note Major Mode Conventions::). When the command `completion-at-point' runs, it calls the functions in the list one by one, without any argument. Each function should return `nil' if it is unable to produce a completion table for the text at point. Otherwise it should return a list of the form (START END COLLECTION . PROPS) START and END delimit the text to complete (which should enclose point). COLLECTION is a completion table for completing that text, in a form suitable for passing as the second argument to `try-completion' (*note Basic Completion::); completion alternatives will be generated from this completion table in the usual way, via the completion styles defined in `completion-styles' (*note Completion Variables::). PROPS is a property list for additional information; any of the properties in `completion-extra-properties' are recognized (*note Completion Variables::), as well as the following additional ones: `:predicate' The value should be a predicate that completion candidates need to satisfy. `:exclusive' If the value is `no', then if the completion table fails to match the text at point, `completion-at-point' moves on to the next function in `completion-at-point-functions' instead of reporting a completion failure. A function in `completion-at-point-functions' may also return a function. In that case, that returned function is called, with no argument, and it is entirely responsible for performing the completion. We discourage this usage; it is intended to help convert old code to using `completion-at-point'. The first function in `completion-at-point-functions' to return a non-`nil' value is used by `completion-at-point'. The remaining functions are not called. The exception to this is when there is an `:exclusive' specification, as described above. The following function provides a convenient way to perform completion on an arbitrary stretch of text in an Emacs buffer: -- Function: completion-in-region start end collection &optional predicate This function completes the text in the current buffer between the positions START and END, using COLLECTION. The argument COLLECTION has the same meaning as in `try-completion' (*note Basic Completion::). This function inserts the completion text directly into the current buffer. Unlike `completing-read' (*note Minibuffer Completion::), it does not activate the minibuffer. For this function to work, point must be somewhere between START and END. 20.7 Yes-or-No Queries ====================== This section describes functions used to ask the user a yes-or-no question. The function `y-or-n-p' can be answered with a single character; it is useful for questions where an inadvertent wrong answer will not have serious consequences. `yes-or-no-p' is suitable for more momentous questions, since it requires three or four characters to answer. If either of these functions is called in a command that was invoked using the mouse--more precisely, if `last-nonmenu-event' (*note Command Loop Info::) is either `nil' or a list--then it uses a dialog box or pop-up menu to ask the question. Otherwise, it uses keyboard input. You can force use either of the mouse or of keyboard input by binding `last-nonmenu-event' to a suitable value around the call. Strictly speaking, `yes-or-no-p' uses the minibuffer and `y-or-n-p' does not; but it seems best to describe them together. -- Function: y-or-n-p prompt This function asks the user a question, expecting input in the echo area. It returns `t' if the user types `y', `nil' if the user types `n'. This function also accepts to mean yes and to mean no. It accepts `C-]' to mean "quit", like `C-g', because the question might look like a minibuffer and for that reason the user might try to use `C-]' to get out. The answer is a single character, with no needed to terminate it. Upper and lower case are equivalent. "Asking the question" means printing PROMPT in the echo area, followed by the string `(y or n) '. If the input is not one of the expected answers (`y', `n', `', `', or something that quits), the function responds `Please answer y or n.', and repeats the request. This function does not actually use the minibuffer, since it does not allow editing of the answer. It actually uses the echo area (*note The Echo Area::), which uses the same screen space as the minibuffer. The cursor moves to the echo area while the question is being asked. The answers and their meanings, even `y' and `n', are not hardwired, and are specified by the keymap `query-replace-map' (*note Search and Replace::). In particular, if the user enters the special responses `recenter', `scroll-up', `scroll-down', `scroll-other-window', or `scroll-other-window-down' (respectively bound to `C-l', `C-v', `M-v', `C-M-v' and `C-M-S-v' in `query-replace-map'), this function performs the specified window recentering or scrolling operation, and poses the question again. We show successive lines of echo area messages, but only one actually appears on the screen at a time. -- Function: y-or-n-p-with-timeout prompt seconds default Like `y-or-n-p', except that if the user fails to answer within SECONDS seconds, this function stops waiting and returns DEFAULT. It works by setting up a timer; see *note Timers::. The argument SECONDS may be an integer or a floating point number. -- Function: yes-or-no-p prompt This function asks the user a question, expecting input in the minibuffer. It returns `t' if the user enters `yes', `nil' if the user types `no'. The user must type to finalize the response. Upper and lower case are equivalent. `yes-or-no-p' starts by displaying PROMPT in the echo area, followed by `(yes or no) '. The user must type one of the expected responses; otherwise, the function responds `Please answer yes or no.', waits about two seconds and repeats the request. `yes-or-no-p' requires more work from the user than `y-or-n-p' and is appropriate for more crucial decisions. Here is an example: (yes-or-no-p "Do you really want to remove everything? ") ;; After evaluation of the preceding expression, ;; the following prompt appears, ;; with an empty minibuffer: ---------- Buffer: minibuffer ---------- Do you really want to remove everything? (yes or no) ---------- Buffer: minibuffer ---------- If the user first types `y ', which is invalid because this function demands the entire word `yes', it responds by displaying these prompts, with a brief pause between them: ---------- Buffer: minibuffer ---------- Please answer yes or no. Do you really want to remove everything? (yes or no) ---------- Buffer: minibuffer ---------- 20.8 Asking Multiple Y-or-N Questions ===================================== When you have a series of similar questions to ask, such as "Do you want to save this buffer" for each buffer in turn, you should use `map-y-or-n-p' to ask the collection of questions, rather than asking each question individually. This gives the user certain convenient facilities such as the ability to answer the whole series at once. -- Function: map-y-or-n-p prompter actor list &optional help action-alist no-cursor-in-echo-area This function asks the user a series of questions, reading a single-character answer in the echo area for each one. The value of LIST specifies the objects to ask questions about. It should be either a list of objects or a generator function. If it is a function, it should expect no arguments, and should return either the next object to ask about, or `nil', meaning to stop asking questions. The argument PROMPTER specifies how to ask each question. If PROMPTER is a string, the question text is computed like this: (format PROMPTER OBJECT) where OBJECT is the next object to ask about (as obtained from LIST). If not a string, PROMPTER should be a function of one argument (the next object to ask about) and should return the question text. If the value is a string, that is the question to ask the user. The function can also return `t', meaning do act on this object (and don't ask the user), or `nil', meaning ignore this object (and don't ask the user). The argument ACTOR says how to act on the answers that the user gives. It should be a function of one argument, and it is called with each object that the user says yes for. Its argument is always an object obtained from LIST. If the argument HELP is given, it should be a list of this form: (SINGULAR PLURAL ACTION) where SINGULAR is a string containing a singular noun that describes the objects conceptually being acted on, PLURAL is the corresponding plural noun, and ACTION is a transitive verb describing what ACTOR does. If you don't specify HELP, the default is `("object" "objects" "act on")'. Each time a question is asked, the user may enter `y', `Y', or to act on that object; `n', `N', or to skip that object; `!' to act on all following objects; or `q' to exit (skip all following objects); `.' (period) to act on the current object and then exit; or `C-h' to get help. These are the same answers that `query-replace' accepts. The keymap `query-replace-map' defines their meaning for `map-y-or-n-p' as well as for `query-replace'; see *note Search and Replace::. You can use ACTION-ALIST to specify additional possible answers and what they mean. It is an alist of elements of the form `(CHAR FUNCTION HELP)', each of which defines one additional answer. In this element, CHAR is a character (the answer); FUNCTION is a function of one argument (an object from LIST); HELP is a string. When the user responds with CHAR, `map-y-or-n-p' calls FUNCTION. If it returns non-`nil', the object is considered "acted upon", and `map-y-or-n-p' advances to the next object in LIST. If it returns `nil', the prompt is repeated for the same object. Normally, `map-y-or-n-p' binds `cursor-in-echo-area' while prompting. But if NO-CURSOR-IN-ECHO-AREA is non-`nil', it does not do that. If `map-y-or-n-p' is called in a command that was invoked using the mouse--more precisely, if `last-nonmenu-event' (*note Command Loop Info::) is either `nil' or a list--then it uses a dialog box or pop-up menu to ask the question. In this case, it does not use keyboard input or the echo area. You can force use either of the mouse or of keyboard input by binding `last-nonmenu-event' to a suitable value around the call. The return value of `map-y-or-n-p' is the number of objects acted on. 20.9 Reading a Password ======================= To read a password to pass to another program, you can use the function `read-passwd'. -- Function: read-passwd prompt &optional confirm default This function reads a password, prompting with PROMPT. It does not echo the password as the user types it; instead, it echoes `.' for each character in the password. The optional argument CONFIRM, if non-`nil', says to read the password twice and insist it must be the same both times. If it isn't the same, the user has to type it over and over until the last two times match. The optional argument DEFAULT specifies the default password to return if the user enters empty input. If DEFAULT is `nil', then `read-passwd' returns the null string in that case. 20.10 Minibuffer Commands ========================= This section describes some commands meant for use in the minibuffer. -- Command: exit-minibuffer This command exits the active minibuffer. It is normally bound to keys in minibuffer local keymaps. -- Command: self-insert-and-exit This command exits the active minibuffer after inserting the last character typed on the keyboard (found in `last-command-event'; *note Command Loop Info::). -- Command: previous-history-element n This command replaces the minibuffer contents with the value of the Nth previous (older) history element. -- Command: next-history-element n This command replaces the minibuffer contents with the value of the Nth more recent history element. -- Command: previous-matching-history-element pattern n This command replaces the minibuffer contents with the value of the Nth previous (older) history element that matches PATTERN (a regular expression). -- Command: next-matching-history-element pattern n This command replaces the minibuffer contents with the value of the Nth next (newer) history element that matches PATTERN (a regular expression). -- Command: previous-complete-history-element n This command replaces the minibuffer contents with the value of the Nth previous (older) history element that completes the current contents of the minibuffer before the point. -- Command: next-complete-history-element n This command replaces the minibuffer contents with the value of the Nth next (newer) history element that completes the current contents of the minibuffer before the point. 20.11 Minibuffer Windows ======================== These functions access and select minibuffer windows and test whether they are active. -- Function: active-minibuffer-window This function returns the currently active minibuffer window, or `nil' if there is none. -- Function: minibuffer-window &optional frame This function returns the minibuffer window used for frame FRAME. If FRAME is `nil', that stands for the current frame. Note that the minibuffer window used by a frame need not be part of that frame--a frame that has no minibuffer of its own necessarily uses some other frame's minibuffer window. -- Function: set-minibuffer-window window This function specifies WINDOW as the minibuffer window to use. This affects where the minibuffer is displayed if you put text in it without invoking the usual minibuffer commands. It has no effect on the usual minibuffer input functions because they all start by choosing the minibuffer window according to the current frame. -- Function: window-minibuffer-p &optional window This function returns non-`nil' if WINDOW is a minibuffer window. WINDOW defaults to the selected window. It is not correct to determine whether a given window is a minibuffer by comparing it with the result of `(minibuffer-window)', because there can be more than one minibuffer window if there is more than one frame. -- Function: minibuffer-window-active-p window This function returns non-`nil' if WINDOW is the currently active minibuffer window. 20.12 Minibuffer Contents ========================= These functions access the minibuffer prompt and contents. -- Function: minibuffer-prompt This function returns the prompt string of the currently active minibuffer. If no minibuffer is active, it returns `nil'. -- Function: minibuffer-prompt-end This function returns the current position of the end of the minibuffer prompt, if a minibuffer is current. Otherwise, it returns the minimum valid buffer position. -- Function: minibuffer-prompt-width This function returns the current display-width of the minibuffer prompt, if a minibuffer is current. Otherwise, it returns zero. -- Function: minibuffer-contents This function returns the editable contents of the minibuffer (that is, everything except the prompt) as a string, if a minibuffer is current. Otherwise, it returns the entire contents of the current buffer. -- Function: minibuffer-contents-no-properties This is like `minibuffer-contents', except that it does not copy text properties, just the characters themselves. *Note Text Properties::. -- Function: minibuffer-completion-contents This is like `minibuffer-contents', except that it returns only the contents before point. That is the part that completion commands operate on. *Note Minibuffer Completion::. -- Function: delete-minibuffer-contents This function erases the editable contents of the minibuffer (that is, everything except the prompt), if a minibuffer is current. Otherwise, it erases the entire current buffer. 20.13 Recursive Minibuffers =========================== These functions and variables deal with recursive minibuffers (*note Recursive Editing::): -- Function: minibuffer-depth This function returns the current depth of activations of the minibuffer, a nonnegative integer. If no minibuffers are active, it returns zero. -- User Option: enable-recursive-minibuffers If this variable is non-`nil', you can invoke commands (such as `find-file') that use minibuffers even while the minibuffer window is active. Such invocation produces a recursive editing level for a new minibuffer. The outer-level minibuffer is invisible while you are editing the inner one. If this variable is `nil', you cannot invoke minibuffer commands when the minibuffer window is active, not even if you switch to another window to do it. If a command name has a property `enable-recursive-minibuffers' that is non-`nil', then the command can use the minibuffer to read arguments even if it is invoked from the minibuffer. A command can also achieve this by binding `enable-recursive-minibuffers' to `t' in the interactive declaration (*note Using Interactive::). The minibuffer command `next-matching-history-element' (normally `M-s' in the minibuffer) does the latter. 20.14 Minibuffer Miscellany =========================== -- Function: minibufferp &optional buffer-or-name This function returns non-`nil' if BUFFER-OR-NAME is a minibuffer. If BUFFER-OR-NAME is omitted, it tests the current buffer. -- Variable: minibuffer-setup-hook This is a normal hook that is run whenever the minibuffer is entered. *Note Hooks::. -- Variable: minibuffer-exit-hook This is a normal hook that is run whenever the minibuffer is exited. *Note Hooks::. -- Variable: minibuffer-help-form The current value of this variable is used to rebind `help-form' locally inside the minibuffer (*note Help Functions::). -- Variable: minibuffer-scroll-window If the value of this variable is non-`nil', it should be a window object. When the function `scroll-other-window' is called in the minibuffer, it scrolls this window. -- Function: minibuffer-selected-window This function returns the window that was selected when the minibuffer was entered. If selected window is not a minibuffer window, it returns `nil'. -- User Option: max-mini-window-height This variable specifies the maximum height for resizing minibuffer windows. If a float, it specifies a fraction of the height of the frame. If an integer, it specifies a number of lines. -- Function: minibuffer-message string &rest args This function displays STRING temporarily at the end of the minibuffer text, for a few seconds, or until the next input event arrives, whichever comes first. The variable `minibuffer-message-timeout' specifies the number of seconds to wait in the absence of input. It defaults to 2. If ARGS is non-`nil', the actual message is obtained by passing STRING and ARGS through `format'. *Note Formatting Strings::. -- Command: minibuffer-inactive-mode This is the major mode used in inactive minibuffers. It uses keymap `minibuffer-inactive-mode-map'. This can be useful if the minibuffer is in a separate frame. *Note Minibuffers and Frames::. 21 Command Loop *************** When you run Emacs, it enters the "editor command loop" almost immediately. This loop reads key sequences, executes their definitions, and displays the results. In this chapter, we describe how these things are done, and the subroutines that allow Lisp programs to do them. 21.1 Command Loop Overview ========================== The first thing the command loop must do is read a key sequence, which is a sequence of input events that translates into a command. It does this by calling the function `read-key-sequence'. Lisp programs can also call this function (*note Key Sequence Input::). They can also read input at a lower level with `read-key' or `read-event' (*note Reading One Event::), or discard pending input with `discard-input' (*note Event Input Misc::). The key sequence is translated into a command through the currently active keymaps. *Note Key Lookup::, for information on how this is done. The result should be a keyboard macro or an interactively callable function. If the key is `M-x', then it reads the name of another command, which it then calls. This is done by the command `execute-extended-command' (*note Interactive Call::). Prior to executing the command, Emacs runs `undo-boundary' to create an undo boundary. *Note Maintaining Undo::. To execute a command, Emacs first reads its arguments by calling `command-execute' (*note Interactive Call::). For commands written in Lisp, the `interactive' specification says how to read the arguments. This may use the prefix argument (*note Prefix Command Arguments::) or may read with prompting in the minibuffer (*note Minibuffers::). For example, the command `find-file' has an `interactive' specification which says to read a file name using the minibuffer. The function body of `find-file' does not use the minibuffer, so if you call `find-file' as a function from Lisp code, you must supply the file name string as an ordinary Lisp function argument. If the command is a keyboard macro (i.e., a string or vector), Emacs executes it using `execute-kbd-macro' (*note Keyboard Macros::). -- Variable: pre-command-hook This normal hook is run by the editor command loop before it executes each command. At that time, `this-command' contains the command that is about to run, and `last-command' describes the previous command. *Note Command Loop Info::. -- Variable: post-command-hook This normal hook is run by the editor command loop after it executes each command (including commands terminated prematurely by quitting or by errors). At that time, `this-command' refers to the command that just ran, and `last-command' refers to the command before that. This hook is also run when Emacs first enters the command loop (at which point `this-command' and `last-command' are both `nil'). Quitting is suppressed while running `pre-command-hook' and `post-command-hook'. If an error happens while executing one of these hooks, it does not terminate execution of the hook; instead the error is silenced and the function in which the error occurred is removed from the hook. A request coming into the Emacs server (*note Emacs Server: (emacs)Emacs Server.) runs these two hooks just as a keyboard command does. 21.2 Defining Commands ====================== The special form `interactive' turns a Lisp function into a command. The `interactive' form must be located at top-level in the function body (usually as the first form in the body), or in the `interactive-form' property of the function symbol. When the `interactive' form is located in the function body, it does nothing when actually executed. Its presence serves as a flag, which tells the Emacs command loop that the function can be called interactively. The argument of the `interactive' form controls the reading of arguments for an interactive call. 21.2.1 Using `interactive' -------------------------- This section describes how to write the `interactive' form that makes a Lisp function an interactively-callable command, and how to examine a command's `interactive' form. -- Special Form: interactive arg-descriptor This special form declares that a function is a command, and that it may therefore be called interactively (via `M-x' or by entering a key sequence bound to it). The argument ARG-DESCRIPTOR declares how to compute the arguments to the command when the command is called interactively. A command may be called from Lisp programs like any other function, but then the caller supplies the arguments and ARG-DESCRIPTOR has no effect. The `interactive' form must be located at top-level in the function body, or in the function symbol's `interactive-form' property (*note Symbol Properties::). It has its effect because the command loop looks for it before calling the function (*note Interactive Call::). Once the function is called, all its body forms are executed; at this time, if the `interactive' form occurs within the body, the form simply returns `nil' without even evaluating its argument. By convention, you should put the `interactive' form in the function body, as the first top-level form. If there is an `interactive' form in both the `interactive-form' symbol property and the function body, the former takes precedence. The `interactive-form' symbol property can be used to add an interactive form to an existing function, or change how its arguments are processed interactively, without redefining the function. There are three possibilities for the argument ARG-DESCRIPTOR: * It may be omitted or `nil'; then the command is called with no arguments. This leads quickly to an error if the command requires one or more arguments. * It may be a string; its contents are a sequence of elements separated by newlines, one for each argument(1). Each element consists of a code character (*note Interactive Codes::) optionally followed by a prompt (which some code characters use and some ignore). Here is an example: (interactive "P\nbFrobnicate buffer: ") The code letter `P' sets the command's first argument to the raw command prefix (*note Prefix Command Arguments::). `bFrobnicate buffer: ' prompts the user with `Frobnicate buffer: ' to enter the name of an existing buffer, which becomes the second and final argument. The prompt string can use `%' to include previous argument values (starting with the first argument) in the prompt. This is done using `format' (*note Formatting Strings::). For example, here is how you could read the name of an existing buffer followed by a new name to give to that buffer: (interactive "bBuffer to rename: \nsRename buffer %s to: ") If `*' appears at the beginning of the string, then an error is signaled if the buffer is read-only. If `@' appears at the beginning of the string, and if the key sequence used to invoke the command includes any mouse events, then the window associated with the first of those events is selected before the command is run. If `^' appears at the beginning of the string, and if the command was invoked through "shift-translation", set the mark and activate the region temporarily, or extend an already active region, before the command is run. If the command was invoked without shift-translation, and the region is temporarily active, deactivate the region before the command is run. Shift-translation is controlled on the user level by `shift-select-mode'; see *note Shift Selection: (emacs)Shift Selection. You can use `*', `@', and `^' together; the order does not matter. Actual reading of arguments is controlled by the rest of the prompt string (starting with the first character that is not `*', `@', or `^'). * It may be a Lisp expression that is not a string; then it should be a form that is evaluated to get a list of arguments to pass to the command. Usually this form will call various functions to read input from the user, most often through the minibuffer (*note Minibuffers::) or directly from the keyboard (*note Reading Input::). Providing point or the mark as an argument value is also common, but if you do this _and_ read input (whether using the minibuffer or not), be sure to get the integer values of point or the mark after reading. The current buffer may be receiving subprocess output; if subprocess output arrives while the command is waiting for input, it could relocate point and the mark. Here's an example of what _not_ to do: (interactive (list (region-beginning) (region-end) (read-string "Foo: " nil 'my-history))) Here's how to avoid the problem, by examining point and the mark after reading the keyboard input: (interactive (let ((string (read-string "Foo: " nil 'my-history))) (list (region-beginning) (region-end) string))) *Warning:* the argument values should not include any data types that can't be printed and then read. Some facilities save `command-history' in a file to be read in the subsequent sessions; if a command's arguments contain a data type that prints using `#<...>' syntax, those facilities won't work. There are, however, a few exceptions: it is ok to use a limited set of expressions such as `(point)', `(mark)', `(region-beginning)', and `(region-end)', because Emacs recognizes them specially and puts the expression (rather than its value) into the command history. To see whether the expression you wrote is one of these exceptions, run the command, then examine `(car command-history)'. -- Function: interactive-form function This function returns the `interactive' form of FUNCTION. If FUNCTION is an interactively callable function (*note Interactive Call::), the value is the command's `interactive' form `(interactive SPEC)', which specifies how to compute its arguments. Otherwise, the value is `nil'. If FUNCTION is a symbol, its function definition is used. ---------- Footnotes ---------- (1) Some elements actually supply two arguments. 21.2.2 Code Characters for `interactive' ---------------------------------------- The code character descriptions below contain a number of key words, defined here as follows: Completion Provide completion. , , and perform name completion because the argument is read using `completing-read' (*note Completion::). `?' displays a list of possible completions. Existing Require the name of an existing object. An invalid name is not accepted; the commands to exit the minibuffer do not exit if the current input is not valid. Default A default value of some sort is used if the user enters no text in the minibuffer. The default depends on the code character. No I/O This code letter computes an argument without reading any input. Therefore, it does not use a prompt string, and any prompt string you supply is ignored. Even though the code letter doesn't use a prompt string, you must follow it with a newline if it is not the last code character in the string. Prompt A prompt immediately follows the code character. The prompt ends either with the end of the string or with a newline. Special This code character is meaningful only at the beginning of the interactive string, and it does not look for a prompt or a newline. It is a single, isolated character. Here are the code character descriptions for use with `interactive': `*' Signal an error if the current buffer is read-only. Special. `@' Select the window mentioned in the first mouse event in the key sequence that invoked this command. Special. `^' If the command was invoked through shift-translation, set the mark and activate the region temporarily, or extend an already active region, before the command is run. If the command was invoked without shift-translation, and the region is temporarily active, deactivate the region before the command is run. Special. `a' A function name (i.e., a symbol satisfying `fboundp'). Existing, Completion, Prompt. `b' The name of an existing buffer. By default, uses the name of the current buffer (*note Buffers::). Existing, Completion, Default, Prompt. `B' A buffer name. The buffer need not exist. By default, uses the name of a recently used buffer other than the current buffer. Completion, Default, Prompt. `c' A character. The cursor does not move into the echo area. Prompt. `C' A command name (i.e., a symbol satisfying `commandp'). Existing, Completion, Prompt. `d' The position of point, as an integer (*note Point::). No I/O. `D' A directory name. The default is the current default directory of the current buffer, `default-directory' (*note File Name Expansion::). Existing, Completion, Default, Prompt. `e' The first or next non-keyboard event in the key sequence that invoked the command. More precisely, `e' gets events that are lists, so you can look at the data in the lists. *Note Input Events::. No I/O. You use `e' for mouse events and for special system events (*note Misc Events::). The event list that the command receives depends on the event. *Note Input Events::, which describes the forms of the list for each event in the corresponding subsections. You can use `e' more than once in a single command's interactive specification. If the key sequence that invoked the command has N events that are lists, the Nth `e' provides the Nth such event. Events that are not lists, such as function keys and ASCII characters, do not count where `e' is concerned. `f' A file name of an existing file (*note File Names::). The default directory is `default-directory'. Existing, Completion, Default, Prompt. `F' A file name. The file need not exist. Completion, Default, Prompt. `G' A file name. The file need not exist. If the user enters just a directory name, then the value is just that directory name, with no file name within the directory added. Completion, Default, Prompt. `i' An irrelevant argument. This code always supplies `nil' as the argument's value. No I/O. `k' A key sequence (*note Key Sequences::). This keeps reading events until a command (or undefined command) is found in the current key maps. The key sequence argument is represented as a string or vector. The cursor does not move into the echo area. Prompt. If `k' reads a key sequence that ends with a down-event, it also reads and discards the following up-event. You can get access to that up-event with the `U' code character. This kind of input is used by commands such as `describe-key' and `global-set-key'. `K' A key sequence, whose definition you intend to change. This works like `k', except that it suppresses, for the last input event in the key sequence, the conversions that are normally used (when necessary) to convert an undefined key into a defined one. `m' The position of the mark, as an integer. No I/O. `M' Arbitrary text, read in the minibuffer using the current buffer's input method, and returned as a string (*note Input Methods: (emacs)Input Methods.). Prompt. `n' A number, read with the minibuffer. If the input is not a number, the user has to try again. `n' never uses the prefix argument. Prompt. `N' The numeric prefix argument; but if there is no prefix argument, read a number as with `n'. The value is always a number. *Note Prefix Command Arguments::. Prompt. `p' The numeric prefix argument. (Note that this `p' is lower case.) No I/O. `P' The raw prefix argument. (Note that this `P' is upper case.) No I/O. `r' Point and the mark, as two numeric arguments, smallest first. This is the only code letter that specifies two successive arguments rather than one. No I/O. `s' Arbitrary text, read in the minibuffer and returned as a string (*note Text from Minibuffer::). Terminate the input with either `C-j' or . (`C-q' may be used to include either of these characters in the input.) Prompt. `S' An interned symbol whose name is read in the minibuffer. Terminate the input with either `C-j' or . Other characters that normally terminate a symbol (e.g., whitespace, parentheses and brackets) do not do so here. Prompt. `U' A key sequence or `nil'. Can be used after a `k' or `K' argument to get the up-event that was discarded (if any) after `k' or `K' read a down-event. If no up-event has been discarded, `U' provides `nil' as the argument. No I/O. `v' A variable declared to be a user option (i.e., satisfying the predicate `custom-variable-p'). This reads the variable using `read-variable'. *Note Definition of read-variable::. Existing, Completion, Prompt. `x' A Lisp object, specified with its read syntax, terminated with a `C-j' or . The object is not evaluated. *Note Object from Minibuffer::. Prompt. `X' A Lisp form's value. `X' reads as `x' does, then evaluates the form so that its value becomes the argument for the command. Prompt. `z' A coding system name (a symbol). If the user enters null input, the argument value is `nil'. *Note Coding Systems::. Completion, Existing, Prompt. `Z' A coding system name (a symbol)--but only if this command has a prefix argument. With no prefix argument, `Z' provides `nil' as the argument value. Completion, Existing, Prompt. 21.2.3 Examples of Using `interactive' -------------------------------------- Here are some examples of `interactive': (defun foo1 () ; `foo1' takes no arguments, (interactive) ; just moves forward two words. (forward-word 2)) => foo1 (defun foo2 (n) ; `foo2' takes one argument, (interactive "^p") ; which is the numeric prefix. ; under `shift-select-mode', ; will activate or extend region. (forward-word (* 2 n))) => foo2 (defun foo3 (n) ; `foo3' takes one argument, (interactive "nCount:") ; which is read with the Minibuffer. (forward-word (* 2 n))) => foo3 (defun three-b (b1 b2 b3) "Select three existing buffers. Put them into three windows, selecting the last one." (interactive "bBuffer1:\nbBuffer2:\nbBuffer3:") (delete-other-windows) (split-window (selected-window) 8) (switch-to-buffer b1) (other-window 1) (split-window (selected-window) 8) (switch-to-buffer b2) (other-window 1) (switch-to-buffer b3)) => three-b (three-b "*scratch*" "declarations.texi" "*mail*") => nil 21.3 Interactive Call ===================== After the command loop has translated a key sequence into a command, it invokes that command using the function `command-execute'. If the command is a function, `command-execute' calls `call-interactively', which reads the arguments and calls the command. You can also call these functions yourself. Note that the term "command", in this context, refers to an interactively callable function (or function-like object), or a keyboard macro. It does not refer to the key sequence used to invoke a command (*note Keymaps::). -- Function: commandp object &optional for-call-interactively This function returns `t' if OBJECT is a command. Otherwise, it returns `nil'. Commands include strings and vectors (which are treated as keyboard macros), lambda expressions that contain a top-level `interactive' form (*note Using Interactive::), byte-code function objects made from such lambda expressions, autoload objects that are declared as interactive (non-`nil' fourth argument to `autoload'), and some primitive functions. Also, a symbol is considered a command if it has a non-`nil' `interactive-form' property, or if its function definition satisfies `commandp'. If FOR-CALL-INTERACTIVELY is non-`nil', then `commandp' returns `t' only for objects that `call-interactively' could call--thus, not for keyboard macros. See `documentation' in *note Accessing Documentation::, for a realistic example of using `commandp'. -- Function: call-interactively command &optional record-flag keys This function calls the interactively callable function COMMAND, providing arguments according to its interactive calling specifications. It returns whatever COMMAND returns. If, for instance, you have a function with the following signature: (defun foo (begin end) (interactive "r") ...) then saying (call-interactively 'foo) will call `foo' with the region (`point' and `mark') as the arguments. An error is signaled if COMMAND is not a function or if it cannot be called interactively (i.e., is not a command). Note that keyboard macros (strings and vectors) are not accepted, even though they are considered commands, because they are not functions. If COMMAND is a symbol, then `call-interactively' uses its function definition. If RECORD-FLAG is non-`nil', then this command and its arguments are unconditionally added to the list `command-history'. Otherwise, the command is added only if it uses the minibuffer to read an argument. *Note Command History::. The argument KEYS, if given, should be a vector which specifies the sequence of events to supply if the command inquires which events were used to invoke it. If KEYS is omitted or `nil', the default is the return value of `this-command-keys-vector'. *Note Definition of this-command-keys-vector::. -- Function: command-execute command &optional record-flag keys special This function executes COMMAND. The argument COMMAND must satisfy the `commandp' predicate; i.e., it must be an interactively callable function or a keyboard macro. A string or vector as COMMAND is executed with `execute-kbd-macro'. A function is passed to `call-interactively' (see above), along with the RECORD-FLAG and KEYS arguments. If COMMAND is a symbol, its function definition is used in its place. A symbol with an `autoload' definition counts as a command if it was declared to stand for an interactively callable function. Such a definition is handled by loading the specified library and then rechecking the definition of the symbol. The argument SPECIAL, if given, means to ignore the prefix argument and not clear it. This is used for executing special events (*note Special Events::). -- Command: execute-extended-command prefix-argument This function reads a command name from the minibuffer using `completing-read' (*note Completion::). Then it uses `command-execute' to call the specified command. Whatever that command returns becomes the value of `execute-extended-command'. If the command asks for a prefix argument, it receives the value PREFIX-ARGUMENT. If `execute-extended-command' is called interactively, the current raw prefix argument is used for PREFIX-ARGUMENT, and thus passed on to whatever command is run. `execute-extended-command' is the normal definition of `M-x', so it uses the string `M-x ' as a prompt. (It would be better to take the prompt from the events used to invoke `execute-extended-command', but that is painful to implement.) A description of the value of the prefix argument, if any, also becomes part of the prompt. (execute-extended-command 3) ---------- Buffer: Minibuffer ---------- 3 M-x forward-word RET ---------- Buffer: Minibuffer ---------- => t 21.4 Distinguish Interactive Calls ================================== Sometimes a command should display additional visual feedback (such as an informative message in the echo area) for interactive calls only. There are three ways to do this. The recommended way to test whether the function was called using `call-interactively' is to give it an optional argument `print-message' and use the `interactive' spec to make it non-`nil' in interactive calls. Here's an example: (defun foo (&optional print-message) (interactive "p") (when print-message (message "foo"))) We use `"p"' because the numeric prefix argument is never `nil'. Defined in this way, the function does display the message when called from a keyboard macro. The above method with the additional argument is usually best, because it allows callers to say "treat this call as interactive". But you can also do the job by testing `called-interactively-p'. -- Function: called-interactively-p kind This function returns `t' when the calling function was called using `call-interactively'. The argument KIND should be either the symbol `interactive' or the symbol `any'. If it is `interactive', then `called-interactively-p' returns `t' only if the call was made directly by the user--e.g., if the user typed a key sequence bound to the calling function, but _not_ if the user ran a keyboard macro that called the function (*note Keyboard Macros::). If KIND is `any', `called-interactively-p' returns `t' for any kind of interactive call, including keyboard macros. If in doubt, use `any'; the only known proper use of `interactive' is if you need to decide whether to display a helpful message while a function is running. A function is never considered to be called interactively if it was called via Lisp evaluation (or with `apply' or `funcall'). Here is an example of using `called-interactively-p': (defun foo () (interactive) (when (called-interactively-p 'any) (message "Interactive!") 'foo-called-interactively)) ;; Type `M-x foo'. -| Interactive! (foo) => nil Here is another example that contrasts direct and indirect calls to `called-interactively-p'. (defun bar () (interactive) (message "%s" (list (foo) (called-interactively-p 'any)))) ;; Type `M-x bar'. -| (nil t) 21.5 Information from the Command Loop ====================================== The editor command loop sets several Lisp variables to keep status records for itself and for commands that are run. With the exception of `this-command' and `last-command' it's generally a bad idea to change any of these variables in a Lisp program. -- Variable: last-command This variable records the name of the previous command executed by the command loop (the one before the current command). Normally the value is a symbol with a function definition, but this is not guaranteed. The value is copied from `this-command' when a command returns to the command loop, except when the command has specified a prefix argument for the following command. This variable is always local to the current terminal and cannot be buffer-local. *Note Multiple Terminals::. -- Variable: real-last-command This variable is set up by Emacs just like `last-command', but never altered by Lisp programs. -- Variable: last-repeatable-command This variable stores the most recently executed command that was not part of an input event. This is the command `repeat' will try to repeat, *Note Repeating: (emacs)Repeating. -- Variable: this-command This variable records the name of the command now being executed by the editor command loop. Like `last-command', it is normally a symbol with a function definition. The command loop sets this variable just before running a command, and copies its value into `last-command' when the command finishes (unless the command specified a prefix argument for the following command). Some commands set this variable during their execution, as a flag for whatever command runs next. In particular, the functions for killing text set `this-command' to `kill-region' so that any kill commands immediately following will know to append the killed text to the previous kill. If you do not want a particular command to be recognized as the previous command in the case where it got an error, you must code that command to prevent this. One way is to set `this-command' to `t' at the beginning of the command, and set `this-command' back to its proper value at the end, like this: (defun foo (args...) (interactive ...) (let ((old-this-command this-command)) (setq this-command t) ...do the work... (setq this-command old-this-command))) We do not bind `this-command' with `let' because that would restore the old value in case of error--a feature of `let' which in this case does precisely what we want to avoid. -- Variable: this-original-command This has the same value as `this-command' except when command remapping occurs (*note Remapping Commands::). In that case, `this-command' gives the command actually run (the result of remapping), and `this-original-command' gives the command that was specified to run but remapped into another command. -- Function: this-command-keys This function returns a string or vector containing the key sequence that invoked the present command, plus any previous commands that generated the prefix argument for this command. Any events read by the command using `read-event' without a timeout get tacked on to the end. However, if the command has called `read-key-sequence', it returns the last read key sequence. *Note Key Sequence Input::. The value is a string if all events in the sequence were characters that fit in a string. *Note Input Events::. (this-command-keys) ;; Now use `C-u C-x C-e' to evaluate that. => "^U^X^E" -- Function: this-command-keys-vector Like `this-command-keys', except that it always returns the events in a vector, so you don't need to deal with the complexities of storing input events in a string (*note Strings of Events::). -- Function: clear-this-command-keys &optional keep-record This function empties out the table of events for `this-command-keys' to return. Unless KEEP-RECORD is non-`nil', it also empties the records that the function `recent-keys' (*note Recording Input::) will subsequently return. This is useful after reading a password, to prevent the password from echoing inadvertently as part of the next command in certain cases. -- Variable: last-nonmenu-event This variable holds the last input event read as part of a key sequence, not counting events resulting from mouse menus. One use of this variable is for telling `x-popup-menu' where to pop up a menu. It is also used internally by `y-or-n-p' (*note Yes-or-No Queries::). -- Variable: last-command-event This variable is set to the last input event that was read by the command loop as part of a command. The principal use of this variable is in `self-insert-command', which uses it to decide which character to insert. last-command-event ;; Now use `C-u C-x C-e' to evaluate that. => 5 The value is 5 because that is the ASCII code for `C-e'. -- Variable: last-event-frame This variable records which frame the last input event was directed to. Usually this is the frame that was selected when the event was generated, but if that frame has redirected input focus to another frame, the value is the frame to which the event was redirected. *Note Input Focus::. If the last event came from a keyboard macro, the value is `macro'. 21.6 Adjusting Point After Commands =================================== It is not easy to display a value of point in the middle of a sequence of text that has the `display', `composition' or is invisible. Therefore, after a command finishes and returns to the command loop, if point is within such a sequence, the command loop normally moves point to the edge of the sequence. A command can inhibit this feature by setting the variable `disable-point-adjustment': -- Variable: disable-point-adjustment If this variable is non-`nil' when a command returns to the command loop, then the command loop does not check for those text properties, and does not move point out of sequences that have them. The command loop sets this variable to `nil' before each command, so if a command sets it, the effect applies only to that command. -- Variable: global-disable-point-adjustment If you set this variable to a non-`nil' value, the feature of moving point out of these sequences is completely turned off. 21.7 Input Events ================= The Emacs command loop reads a sequence of "input events" that represent keyboard or mouse activity, or system events sent to Emacs. The events for keyboard activity are characters or symbols; other events are always lists. This section describes the representation and meaning of input events in detail. -- Function: eventp object This function returns non-`nil' if OBJECT is an input event or event type. Note that any symbol might be used as an event or an event type. `eventp' cannot distinguish whether a symbol is intended by Lisp code to be used as an event. Instead, it distinguishes whether the symbol has actually been used in an event that has been read as input in the current Emacs session. If a symbol has not yet been so used, `eventp' returns `nil'. 21.7.1 Keyboard Events ---------------------- There are two kinds of input you can get from the keyboard: ordinary keys, and function keys. Ordinary keys correspond to characters; the events they generate are represented in Lisp as characters. The event type of a character event is the character itself (an integer); see *note Classifying Events::. An input character event consists of a "basic code" between 0 and 524287, plus any or all of these "modifier bits": meta The 2**27 bit in the character code indicates a character typed with the meta key held down. control The 2**26 bit in the character code indicates a non-ASCII control character. ASCII control characters such as `C-a' have special basic codes of their own, so Emacs needs no special bit to indicate them. Thus, the code for `C-a' is just 1. But if you type a control combination not in ASCII, such as `%' with the control key, the numeric value you get is the code for `%' plus 2**26 (assuming the terminal supports non-ASCII control characters). shift The 2**25 bit in the character code indicates an ASCII control character typed with the shift key held down. For letters, the basic code itself indicates upper versus lower case; for digits and punctuation, the shift key selects an entirely different character with a different basic code. In order to keep within the ASCII character set whenever possible, Emacs avoids using the 2**25 bit for those characters. However, ASCII provides no way to distinguish `C-A' from `C-a', so Emacs uses the 2**25 bit in `C-A' and not in `C-a'. hyper The 2**24 bit in the character code indicates a character typed with the hyper key held down. super The 2**23 bit in the character code indicates a character typed with the super key held down. alt The 2**22 bit in the character code indicates a character typed with the alt key held down. (The key labeled on most keyboards is actually treated as the meta key, not this.) It is best to avoid mentioning specific bit numbers in your program. To test the modifier bits of a character, use the function `event-modifiers' (*note Classifying Events::). When making key bindings, you can use the read syntax for characters with modifier bits (`\C-', `\M-', and so on). For making key bindings with `define-key', you can use lists such as `(control hyper ?x)' to specify the characters (*note Changing Key Bindings::). The function `event-convert-list' converts such a list into an event type (*note Classifying Events::). 21.7.2 Function Keys -------------------- Most keyboards also have "function keys"--keys that have names or symbols that are not characters. Function keys are represented in Emacs Lisp as symbols; the symbol's name is the function key's label, in lower case. For example, pressing a key labeled generates an input event represented by the symbol `f1'. The event type of a function key event is the event symbol itself. *Note Classifying Events::. Here are a few special cases in the symbol-naming convention for function keys: `backspace', `tab', `newline', `return', `delete' These keys correspond to common ASCII control characters that have special keys on most keyboards. In ASCII, `C-i' and are the same character. If the terminal can distinguish between them, Emacs conveys the distinction to Lisp programs by representing the former as the integer 9, and the latter as the symbol `tab'. Most of the time, it's not useful to distinguish the two. So normally `local-function-key-map' (*note Translation Keymaps::) is set up to map `tab' into 9. Thus, a key binding for character code 9 (the character `C-i') also applies to `tab'. Likewise for the other symbols in this group. The function `read-char' likewise converts these events into characters. In ASCII, is really `C-h'. But `backspace' converts into the character code 127 (), not into code 8 (). This is what most users prefer. `left', `up', `right', `down' Cursor arrow keys `kp-add', `kp-decimal', `kp-divide', ... Keypad keys (to the right of the regular keyboard). `kp-0', `kp-1', ... Keypad keys with digits. `kp-f1', `kp-f2', `kp-f3', `kp-f4' Keypad PF keys. `kp-home', `kp-left', `kp-up', `kp-right', `kp-down' Keypad arrow keys. Emacs normally translates these into the corresponding non-keypad keys `home', `left', ... `kp-prior', `kp-next', `kp-end', `kp-begin', `kp-insert', `kp-delete' Additional keypad duplicates of keys ordinarily found elsewhere. Emacs normally translates these into the like-named non-keypad keys. You can use the modifier keys , , , , , and with function keys. The way to represent them is with prefixes in the symbol name: `A-' The alt modifier. `C-' The control modifier. `H-' The hyper modifier. `M-' The meta modifier. `S-' The shift modifier. `s-' The super modifier. Thus, the symbol for the key with held down is `M-f3'. When you use more than one prefix, we recommend you write them in alphabetical order; but the order does not matter in arguments to the key-binding lookup and modification functions. 21.7.3 Mouse Events ------------------- Emacs supports four kinds of mouse events: click events, drag events, button-down events, and motion events. All mouse events are represented as lists. The CAR of the list is the event type; this says which mouse button was involved, and which modifier keys were used with it. The event type can also distinguish double or triple button presses (*note Repeat Events::). The rest of the list elements give position and time information. For key lookup, only the event type matters: two events of the same type necessarily run the same command. The command can access the full values of these events using the `e' interactive code. *Note Interactive Codes::. A key sequence that starts with a mouse event is read using the keymaps of the buffer in the window that the mouse was in, not the current buffer. This does not imply that clicking in a window selects that window or its buffer--that is entirely under the control of the command binding of the key sequence. 21.7.4 Click Events ------------------- When the user presses a mouse button and releases it at the same location, that generates a "click" event. All mouse click event share the same format: (EVENT-TYPE POSITION CLICK-COUNT) EVENT-TYPE This is a symbol that indicates which mouse button was used. It is one of the symbols `mouse-1', `mouse-2', ..., where the buttons are numbered left to right. You can also use prefixes `A-', `C-', `H-', `M-', `S-' and `s-' for modifiers alt, control, hyper, meta, shift and super, just as you would with function keys. This symbol also serves as the event type of the event. Key bindings describe events by their types; thus, if there is a key binding for `mouse-1', that binding would apply to all events whose EVENT-TYPE is `mouse-1'. POSITION This is a "mouse position list" specifying where the mouse click occurred; see below for details. CLICK-COUNT This is the number of rapid repeated presses so far of the same mouse button. *Note Repeat Events::. To access the contents of a mouse position list in the POSITION slot of a click event, you should typically use the functions documented in *note Accessing Mouse::. The explicit format of the list depends on where the click occurred. For clicks in the text area, mode line, header line, or in the fringe or marginal areas, the mouse position list has the form (WINDOW POS-OR-AREA (X . Y) TIMESTAMP OBJECT TEXT-POS (COL . ROW) IMAGE (DX . DY) (WIDTH . HEIGHT)) The meanings of these list elements are as follows: WINDOW The window in which the click occurred. POS-OR-AREA The buffer position of the character clicked on in the text area; or, if the click was outside the text area, the window area where it occurred. It is one of the symbols `mode-line', `header-line', `vertical-line', `left-margin', `right-margin', `left-fringe', or `right-fringe'. In one special case, POS-OR-AREA is a list containing a symbol (one of the symbols listed above) instead of just the symbol. This happens after the imaginary prefix keys for the event are registered by Emacs. *Note Key Sequence Input::. X, Y The relative pixel coordinates of the click. For clicks in the text area of a window, the coordinate origin `(0 . 0)' is taken to be the top left corner of the text area. *Note Window Sizes::. For clicks in a mode line or header line, the coordinate origin is the top left corner of the window itself. For fringes, margins, and the vertical border, X does not have meaningful data. For fringes and margins, Y is relative to the bottom edge of the header line. In all cases, the X and Y coordinates increase rightward and downward respectively. TIMESTAMP The time at which the event occurred, as an integer number of milliseconds since a system-dependent initial time. OBJECT Either `nil' if there is no string-type text property at the click position, or a cons cell of the form (STRING . STRING-POS) if there is one: STRING The string which was clicked on, including any properties. STRING-POS The position in the string where the click occurred. TEXT-POS For clicks on a marginal area or on a fringe, this is the buffer position of the first visible character in the corresponding line in the window. For other events, it is the current buffer position in the window. COL, ROW These are the actual column and row coordinate numbers of the glyph under the X, Y position. If X lies beyond the last column of actual text on its line, COL is reported by adding fictional extra columns that have the default character width. Row 0 is taken to be the header line if the window has one, or the topmost row of the text area otherwise. Column 0 is taken to be the leftmost column of the text area for clicks on a window text area, or the leftmost mode line or header line column for clicks there. For clicks on fringes or vertical borders, these have no meaningful data. For clicks on margins, COL is measured from the left edge of the margin area and ROW is measured from the top of the margin area. IMAGE This is the image object on which the click occurred. It is either `nil' if there is no image at the position clicked on, or it is an image object as returned by `find-image' if click was in an image. DX, DY These are the pixel coordinates of the click, relative to the top left corner of OBJECT, which is `(0 . 0)'. If OBJECT is `nil', the coordinates are relative to the top left corner of the character glyph clicked on. WIDTH, HEIGHT These are the pixel width and height of OBJECT or, if this is `nil', those of the character glyph clicked on. For clicks on a scroll bar, POSITION has this form: (WINDOW AREA (PORTION . WHOLE) TIMESTAMP PART) WINDOW The window whose scroll bar was clicked on. AREA This is the symbol `vertical-scroll-bar'. PORTION The number of pixels from the top of the scroll bar to the click position. On some toolkits, including GTK+, Emacs cannot extract this data, so the value is always `0'. WHOLE The total length, in pixels, of the scroll bar. On some toolkits, including GTK+, Emacs cannot extract this data, so the value is always `0'. TIMESTAMP The time at which the event occurred, in milliseconds. On some toolkits, including GTK+, Emacs cannot extract this data, so the value is always `0'. PART The part of the scroll bar on which the click occurred. It is one of the symbols `handle' (the scroll bar handle), `above-handle' (the area above the handle), `below-handle' (the area below the handle), `up' (the up arrow at one end of the scroll bar), or `down' (the down arrow at one end of the scroll bar). 21.7.5 Drag Events ------------------ With Emacs, you can have a drag event without even changing your clothes. A "drag event" happens every time the user presses a mouse button and then moves the mouse to a different character position before releasing the button. Like all mouse events, drag events are represented in Lisp as lists. The lists record both the starting mouse position and the final position, like this: (EVENT-TYPE (WINDOW1 START-POSITION) (WINDOW2 END-POSITION)) For a drag event, the name of the symbol EVENT-TYPE contains the prefix `drag-'. For example, dragging the mouse with button 2 held down generates a `drag-mouse-2' event. The second and third elements of the event give the starting and ending position of the drag, as mouse position lists (*note Click Events::). You can access the second element of any mouse event in the same way, with no need to distinguish drag events from others. The `drag-' prefix follows the modifier key prefixes such as `C-' and `M-'. If `read-key-sequence' receives a drag event that has no key binding, and the corresponding click event does have a binding, it changes the drag event into a click event at the drag's starting position. This means that you don't have to distinguish between click and drag events unless you want to. 21.7.6 Button-Down Events ------------------------- Click and drag events happen when the user releases a mouse button. They cannot happen earlier, because there is no way to distinguish a click from a drag until the button is released. If you want to take action as soon as a button is pressed, you need to handle "button-down" events.(1) These occur as soon as a button is pressed. They are represented by lists that look exactly like click events (*note Click Events::), except that the EVENT-TYPE symbol name contains the prefix `down-'. The `down-' prefix follows modifier key prefixes such as `C-' and `M-'. The function `read-key-sequence' ignores any button-down events that don't have command bindings; therefore, the Emacs command loop ignores them too. This means that you need not worry about defining button-down events unless you want them to do something. The usual reason to define a button-down event is so that you can track mouse motion (by reading motion events) until the button is released. *Note Motion Events::. ---------- Footnotes ---------- (1) Button-down is the conservative antithesis of drag. 21.7.7 Repeat Events -------------------- If you press the same mouse button more than once in quick succession without moving the mouse, Emacs generates special "repeat" mouse events for the second and subsequent presses. The most common repeat events are "double-click" events. Emacs generates a double-click event when you click a button twice; the event happens when you release the button (as is normal for all click events). The event type of a double-click event contains the prefix `double-'. Thus, a double click on the second mouse button with held down comes to the Lisp program as `M-double-mouse-2'. If a double-click event has no binding, the binding of the corresponding ordinary click event is used to execute it. Thus, you need not pay attention to the double click feature unless you really want to. When the user performs a double click, Emacs generates first an ordinary click event, and then a double-click event. Therefore, you must design the command binding of the double click event to assume that the single-click command has already run. It must produce the desired results of a double click, starting from the results of a single click. This is convenient, if the meaning of a double click somehow "builds on" the meaning of a single click--which is recommended user interface design practice for double clicks. If you click a button, then press it down again and start moving the mouse with the button held down, then you get a "double-drag" event when you ultimately release the button. Its event type contains `double-drag' instead of just `drag'. If a double-drag event has no binding, Emacs looks for an alternate binding as if the event were an ordinary drag. Before the double-click or double-drag event, Emacs generates a "double-down" event when the user presses the button down for the second time. Its event type contains `double-down' instead of just `down'. If a double-down event has no binding, Emacs looks for an alternate binding as if the event were an ordinary button-down event. If it finds no binding that way either, the double-down event is ignored. To summarize, when you click a button and then press it again right away, Emacs generates a down event and a click event for the first click, a double-down event when you press the button again, and finally either a double-click or a double-drag event. If you click a button twice and then press it again, all in quick succession, Emacs generates a "triple-down" event, followed by either a "triple-click" or a "triple-drag". The event types of these events contain `triple' instead of `double'. If any triple event has no binding, Emacs uses the binding that it would use for the corresponding double event. If you click a button three or more times and then press it again, the events for the presses beyond the third are all triple events. Emacs does not have separate event types for quadruple, quintuple, etc. events. However, you can look at the event list to find out precisely how many times the button was pressed. -- Function: event-click-count event This function returns the number of consecutive button presses that led up to EVENT. If EVENT is a double-down, double-click or double-drag event, the value is 2. If EVENT is a triple event, the value is 3 or greater. If EVENT is an ordinary mouse event (not a repeat event), the value is 1. -- User Option: double-click-fuzz To generate repeat events, successive mouse button presses must be at approximately the same screen position. The value of `double-click-fuzz' specifies the maximum number of pixels the mouse may be moved (horizontally or vertically) between two successive clicks to make a double-click. This variable is also the threshold for motion of the mouse to count as a drag. -- User Option: double-click-time To generate repeat events, the number of milliseconds between successive button presses must be less than the value of `double-click-time'. Setting `double-click-time' to `nil' disables multi-click detection entirely. Setting it to `t' removes the time limit; Emacs then detects multi-clicks by position only. 21.7.8 Motion Events -------------------- Emacs sometimes generates "mouse motion" events to describe motion of the mouse without any button activity. Mouse motion events are represented by lists that look like this: (mouse-movement POSITION) POSITION is a mouse position list (*note Click Events::), specifying the current position of the mouse cursor. The special form `track-mouse' enables generation of motion events within its body. Outside of `track-mouse' forms, Emacs does not generate events for mere motion of the mouse, and these events do not appear. *Note Mouse Tracking::. 21.7.9 Focus Events ------------------- Window systems provide general ways for the user to control which window gets keyboard input. This choice of window is called the "focus". When the user does something to switch between Emacs frames, that generates a "focus event". The normal definition of a focus event, in the global keymap, is to select a new frame within Emacs, as the user would expect. *Note Input Focus::. Focus events are represented in Lisp as lists that look like this: (switch-frame NEW-FRAME) where NEW-FRAME is the frame switched to. Some X window managers are set up so that just moving the mouse into a window is enough to set the focus there. Usually, there is no need for a Lisp program to know about the focus change until some other kind of input arrives. Emacs generates a focus event only when the user actually types a keyboard key or presses a mouse button in the new frame; just moving the mouse between frames does not generate a focus event. A focus event in the middle of a key sequence would garble the sequence. So Emacs never generates a focus event in the middle of a key sequence. If the user changes focus in the middle of a key sequence--that is, after a prefix key--then Emacs reorders the events so that the focus event comes either before or after the multi-event key sequence, and not within it. 21.7.10 Miscellaneous System Events ----------------------------------- A few other event types represent occurrences within the system. `(delete-frame (FRAME))' This kind of event indicates that the user gave the window manager a command to delete a particular window, which happens to be an Emacs frame. The standard definition of the `delete-frame' event is to delete FRAME. `(iconify-frame (FRAME))' This kind of event indicates that the user iconified FRAME using the window manager. Its standard definition is `ignore'; since the frame has already been iconified, Emacs has no work to do. The purpose of this event type is so that you can keep track of such events if you want to. `(make-frame-visible (FRAME))' This kind of event indicates that the user deiconified FRAME using the window manager. Its standard definition is `ignore'; since the frame has already been made visible, Emacs has no work to do. `(wheel-up POSITION)' `(wheel-down POSITION)' These kinds of event are generated by moving a mouse wheel. The POSITION element is a mouse position list (*note Click Events::), specifying the position of the mouse cursor when the event occurred. This kind of event is generated only on some kinds of systems. On some systems, `mouse-4' and `mouse-5' are used instead. For portable code, use the variables `mouse-wheel-up-event' and `mouse-wheel-down-event' defined in `mwheel.el' to determine what event types to expect for the mouse wheel. `(drag-n-drop POSITION FILES)' This kind of event is generated when a group of files is selected in an application outside of Emacs, and then dragged and dropped onto an Emacs frame. The element POSITION is a list describing the position of the event, in the same format as used in a mouse-click event (*note Click Events::), and FILES is the list of file names that were dragged and dropped. The usual way to handle this event is by visiting these files. This kind of event is generated, at present, only on some kinds of systems. `help-echo' This kind of event is generated when a mouse pointer moves onto a portion of buffer text which has a `help-echo' text property. The generated event has this form: (help-echo FRAME HELP WINDOW OBJECT POS) The precise meaning of the event parameters and the way these parameters are used to display the help-echo text are described in *note Text help-echo::. `sigusr1' `sigusr2' These events are generated when the Emacs process receives the signals `SIGUSR1' and `SIGUSR2'. They contain no additional data because signals do not carry additional information. They can be useful for debugging (*note Error Debugging::). To catch a user signal, bind the corresponding event to an interactive command in the `special-event-map' (*note Active Keymaps::). The command is called with no arguments, and the specific signal event is available in `last-input-event'. For example: (defun sigusr-handler () (interactive) (message "Caught signal %S" last-input-event)) (define-key special-event-map [sigusr1] 'sigusr-handler) To test the signal handler, you can make Emacs send a signal to itself: (signal-process (emacs-pid) 'sigusr1) `language-change' This kind of event is generated on MS-Windows when the input language has changed. This typically means that the keyboard keys will send to Emacs characters from a different language. The generated event has this form: (language-change FRAME CODEPAGE LANGUAGE-ID) Here FRAME is the frame which was current when the input language changed; CODEPAGE is the new codepage number; and LANGUAGE-ID is the numerical ID of the new input language. The coding-system (*note Coding Systems::) that corresponds to CODEPAGE is `cpCODEPAGE' or `windows-CODEPAGE'. To convert LANGUAGE-ID to a string (e.g., to use it for various language-dependent features, such as `set-language-environment'), use the `w32-get-locale-info' function, like this: ;; Get the abbreviated language name, such as "ENU" for English (w32-get-locale-info language-id) ;; Get the full English name of the language, ;; such as "English (United States)" (w32-get-locale-info language-id 4097) ;; Get the full localized name of the language (w32-get-locale-info language-id t) If one of these events arrives in the middle of a key sequence--that is, after a prefix key--then Emacs reorders the events so that this event comes either before or after the multi-event key sequence, not within it. 21.7.11 Event Examples ---------------------- If the user presses and releases the left mouse button over the same location, that generates a sequence of events like this: (down-mouse-1 (# 2613 (0 . 38) -864320)) (mouse-1 (# 2613 (0 . 38) -864180)) While holding the control key down, the user might hold down the second mouse button, and drag the mouse from one line to the next. That produces two events, as shown here: (C-down-mouse-2 (# 3440 (0 . 27) -731219)) (C-drag-mouse-2 (# 3440 (0 . 27) -731219) (# 3510 (0 . 28) -729648)) While holding down the meta and shift keys, the user might press the second mouse button on the window's mode line, and then drag the mouse into another window. That produces a pair of events like these: (M-S-down-mouse-2 (# mode-line (33 . 31) -457844)) (M-S-drag-mouse-2 (# mode-line (33 . 31) -457844) (# 161 (33 . 3) -453816)) To handle a SIGUSR1 signal, define an interactive function, and bind it to the `signal usr1' event sequence: (defun usr1-handler () (interactive) (message "Got USR1 signal")) (global-set-key [signal usr1] 'usr1-handler) 21.7.12 Classifying Events -------------------------- Every event has an "event type", which classifies the event for key binding purposes. For a keyboard event, the event type equals the event value; thus, the event type for a character is the character, and the event type for a function key symbol is the symbol itself. For events that are lists, the event type is the symbol in the CAR of the list. Thus, the event type is always a symbol or a character. Two events of the same type are equivalent where key bindings are concerned; thus, they always run the same command. That does not necessarily mean they do the same things, however, as some commands look at the whole event to decide what to do. For example, some commands use the location of a mouse event to decide where in the buffer to act. Sometimes broader classifications of events are useful. For example, you might want to ask whether an event involved the key, regardless of which other key or mouse button was used. The functions `event-modifiers' and `event-basic-type' are provided to get such information conveniently. -- Function: event-modifiers event This function returns a list of the modifiers that EVENT has. The modifiers are symbols; they include `shift', `control', `meta', `alt', `hyper' and `super'. In addition, the modifiers list of a mouse event symbol always contains one of `click', `drag', and `down'. For double or triple events, it also contains `double' or `triple'. The argument EVENT may be an entire event object, or just an event type. If EVENT is a symbol that has never been used in an event that has been read as input in the current Emacs session, then `event-modifiers' can return `nil', even when EVENT actually has modifiers. Here are some examples: (event-modifiers ?a) => nil (event-modifiers ?A) => (shift) (event-modifiers ?\C-a) => (control) (event-modifiers ?\C-%) => (control) (event-modifiers ?\C-\S-a) => (control shift) (event-modifiers 'f5) => nil (event-modifiers 's-f5) => (super) (event-modifiers 'M-S-f5) => (meta shift) (event-modifiers 'mouse-1) => (click) (event-modifiers 'down-mouse-1) => (down) The modifiers list for a click event explicitly contains `click', but the event symbol name itself does not contain `click'. -- Function: event-basic-type event This function returns the key or mouse button that EVENT describes, with all modifiers removed. The EVENT argument is as in `event-modifiers'. For example: (event-basic-type ?a) => 97 (event-basic-type ?A) => 97 (event-basic-type ?\C-a) => 97 (event-basic-type ?\C-\S-a) => 97 (event-basic-type 'f5) => f5 (event-basic-type 's-f5) => f5 (event-basic-type 'M-S-f5) => f5 (event-basic-type 'down-mouse-1) => mouse-1 -- Function: mouse-movement-p object This function returns non-`nil' if OBJECT is a mouse movement event. -- Function: event-convert-list list This function converts a list of modifier names and a basic event type to an event type which specifies all of them. The basic event type must be the last element of the list. For example, (event-convert-list '(control ?a)) => 1 (event-convert-list '(control meta ?a)) => -134217727 (event-convert-list '(control super f1)) => C-s-f1 21.7.13 Accessing Mouse Events ------------------------------ This section describes convenient functions for accessing the data in a mouse button or motion event. The following two functions return a mouse position list (*note Click Events::), specifying the position of a mouse event. -- Function: event-start event This returns the starting position of EVENT. If EVENT is a click or button-down event, this returns the location of the event. If EVENT is a drag event, this returns the drag's starting position. -- Function: event-end event This returns the ending position of EVENT. If EVENT is a drag event, this returns the position where the user released the mouse button. If EVENT is a click or button-down event, the value is actually the starting position, which is the only position such events have. -- Function: posnp object This function returns non-`nil' if OBJECT is a mouse position list, in either of the formats documented in *note Click Events::); and `nil' otherwise. These functions take a mouse position list as argument, and return various parts of it: -- Function: posn-window position Return the window that POSITION is in. -- Function: posn-area position Return the window area recorded in POSITION. It returns `nil' when the event occurred in the text area of the window; otherwise, it is a symbol identifying the area in which the event occurred. -- Function: posn-point position Return the buffer position in POSITION. When the event occurred in the text area of the window, in a marginal area, or on a fringe, this is an integer specifying a buffer position. Otherwise, the value is undefined. -- Function: posn-x-y position Return the pixel-based x and y coordinates in POSITION, as a cons cell `(X . Y)'. These coordinates are relative to the window given by `posn-window'. This example shows how to convert the window-relative coordinates in the text area of a window into frame-relative coordinates: (defun frame-relative-coordinates (position) "Return frame-relative coordinates from POSITION. POSITION is assumed to lie in a window text area." (let* ((x-y (posn-x-y position)) (window (posn-window position)) (edges (window-inside-pixel-edges window))) (cons (+ (car x-y) (car edges)) (+ (cdr x-y) (cadr edges))))) -- Function: posn-col-row position This function returns a cons cell `(COL . ROW)', containing the estimated column and row corresponding to buffer position POSITION. The return value is given in units of the frame's default character width and height, as computed from the X and Y values corresponding to POSITION. (So, if the actual characters have non-default sizes, the actual row and column may differ from these computed values.) Note that ROW is counted from the top of the text area. If the window possesses a header line (*note Header Lines::), it is _not_ counted as the first line. -- Function: posn-actual-col-row position Return the actual row and column in POSITION, as a cons cell `(COL . ROW)'. The values are the actual row and column numbers in the window. *Note Click Events::, for details. It returns `nil' if POSITION does not include actual positions values. -- Function: posn-string position Return the string object in POSITION, either `nil', or a cons cell `(STRING . STRING-POS)'. -- Function: posn-image position Return the image object in POSITION, either `nil', or an image `(image ...)'. -- Function: posn-object position Return the image or string object in POSITION, either `nil', an image `(image ...)', or a cons cell `(STRING . STRING-POS)'. -- Function: posn-object-x-y position Return the pixel-based x and y coordinates relative to the upper left corner of the object in POSITION as a cons cell `(DX . DY)'. If the POSITION is a buffer position, return the relative position in the character at that position. -- Function: posn-object-width-height position Return the pixel width and height of the object in POSITION as a cons cell `(WIDTH . HEIGHT)'. If the POSITION is a buffer position, return the size of the character at that position. -- Function: posn-timestamp position Return the timestamp in POSITION. This is the time at which the event occurred, in milliseconds. These functions compute a position list given particular buffer position or screen position. You can access the data in this position list with the functions described above. -- Function: posn-at-point &optional pos window This function returns a position list for position POS in WINDOW. POS defaults to point in WINDOW; WINDOW defaults to the selected window. `posn-at-point' returns `nil' if POS is not visible in WINDOW. -- Function: posn-at-x-y x y &optional frame-or-window whole This function returns position information corresponding to pixel coordinates X and Y in a specified frame or window, FRAME-OR-WINDOW, which defaults to the selected window. The coordinates X and Y are relative to the frame or window used. If WHOLE is `nil', the coordinates are relative to the window text area, otherwise they are relative to the entire window area including scroll bars, margins and fringes. 21.7.14 Accessing Scroll Bar Events ----------------------------------- These functions are useful for decoding scroll bar events. -- Function: scroll-bar-event-ratio event This function returns the fractional vertical position of a scroll bar event within the scroll bar. The value is a cons cell `(PORTION . WHOLE)' containing two integers whose ratio is the fractional position. -- Function: scroll-bar-scale ratio total This function multiplies (in effect) RATIO by TOTAL, rounding the result to an integer. The argument RATIO is not a number, but rather a pair `(NUM . DENOM)'--typically a value returned by `scroll-bar-event-ratio'. This function is handy for scaling a position on a scroll bar into a buffer position. Here's how to do that: (+ (point-min) (scroll-bar-scale (posn-x-y (event-start event)) (- (point-max) (point-min)))) Recall that scroll bar events have two integers forming a ratio, in place of a pair of x and y coordinates. 21.7.15 Putting Keyboard Events in Strings ------------------------------------------ In most of the places where strings are used, we conceptualize the string as containing text characters--the same kind of characters found in buffers or files. Occasionally Lisp programs use strings that conceptually contain keyboard characters; for example, they may be key sequences or keyboard macro definitions. However, storing keyboard characters in a string is a complex matter, for reasons of historical compatibility, and it is not always possible. We recommend that new programs avoid dealing with these complexities by not storing keyboard events in strings. Here is how to do that: * Use vectors instead of strings for key sequences, when you plan to use them for anything other than as arguments to `lookup-key' and `define-key'. For example, you can use `read-key-sequence-vector' instead of `read-key-sequence', and `this-command-keys-vector' instead of `this-command-keys'. * Use vectors to write key sequence constants containing meta characters, even when passing them directly to `define-key'. * When you have to look at the contents of a key sequence that might be a string, use `listify-key-sequence' (*note Event Input Misc::) first, to convert it to a list. The complexities stem from the modifier bits that keyboard input characters can include. Aside from the Meta modifier, none of these modifier bits can be included in a string, and the Meta modifier is allowed only in special cases. The earliest GNU Emacs versions represented meta characters as codes in the range of 128 to 255. At that time, the basic character codes ranged from 0 to 127, so all keyboard character codes did fit in a string. Many Lisp programs used `\M-' in string constants to stand for meta characters, especially in arguments to `define-key' and similar functions, and key sequences and sequences of events were always represented as strings. When we added support for larger basic character codes beyond 127, and additional modifier bits, we had to change the representation of meta characters. Now the flag that represents the Meta modifier in a character is 2**27 and such numbers cannot be included in a string. To support programs with `\M-' in string constants, there are special rules for including certain meta characters in a string. Here are the rules for interpreting a string as a sequence of input characters: * If the keyboard character value is in the range of 0 to 127, it can go in the string unchanged. * The meta variants of those characters, with codes in the range of 2**27 to 2**27+127, can also go in the string, but you must change their numeric values. You must set the 2**7 bit instead of the 2**27 bit, resulting in a value between 128 and 255. Only a unibyte string can include these codes. * Non-ASCII characters above 256 can be included in a multibyte string. * Other keyboard character events cannot fit in a string. This includes keyboard events in the range of 128 to 255. Functions such as `read-key-sequence' that construct strings of keyboard input characters follow these rules: they construct vectors instead of strings, when the events won't fit in a string. When you use the read syntax `\M-' in a string, it produces a code in the range of 128 to 255--the same code that you get if you modify the corresponding keyboard event to put it in the string. Thus, meta events in strings work consistently regardless of how they get into the strings. However, most programs would do well to avoid these issues by following the recommendations at the beginning of this section. 21.8 Reading Input ================== The editor command loop reads key sequences using the function `read-key-sequence', which uses `read-event'. These and other functions for event input are also available for use in Lisp programs. See also `momentary-string-display' in *note Temporary Displays::, and `sit-for' in *note Waiting::. *Note Terminal Input::, for functions and variables for controlling terminal input modes and debugging terminal input. For higher-level input facilities, see *note Minibuffers::. 21.8.1 Key Sequence Input ------------------------- The command loop reads input a key sequence at a time, by calling `read-key-sequence'. Lisp programs can also call this function; for example, `describe-key' uses it to read the key to describe. -- Function: read-key-sequence prompt &optional continue-echo dont-downcase-last switch-frame-ok command-loop This function reads a key sequence and returns it as a string or vector. It keeps reading events until it has accumulated a complete key sequence; that is, enough to specify a non-prefix command using the currently active keymaps. (Remember that a key sequence that starts with a mouse event is read using the keymaps of the buffer in the window that the mouse was in, not the current buffer.) If the events are all characters and all can fit in a string, then `read-key-sequence' returns a string (*note Strings of Events::). Otherwise, it returns a vector, since a vector can hold all kinds of events--characters, symbols, and lists. The elements of the string or vector are the events in the key sequence. Reading a key sequence includes translating the events in various ways. *Note Translation Keymaps::. The argument PROMPT is either a string to be displayed in the echo area as a prompt, or `nil', meaning not to display a prompt. The argument CONTINUE-ECHO, if non-`nil', means to echo this key as a continuation of the previous key. Normally any upper case event is converted to lower case if the original event is undefined and the lower case equivalent is defined. The argument DONT-DOWNCASE-LAST, if non-`nil', means do not convert the last event to lower case. This is appropriate for reading a key sequence to be defined. The argument SWITCH-FRAME-OK, if non-`nil', means that this function should process a `switch-frame' event if the user switches frames before typing anything. If the user switches frames in the middle of a key sequence, or at the start of the sequence but SWITCH-FRAME-OK is `nil', then the event will be put off until after the current key sequence. The argument COMMAND-LOOP, if non-`nil', means that this key sequence is being read by something that will read commands one after another. It should be `nil' if the caller will read just one key sequence. In the following example, Emacs displays the prompt `?' in the echo area, and then the user types `C-x C-f'. (read-key-sequence "?") ---------- Echo Area ---------- ?C-x C-f ---------- Echo Area ---------- => "^X^F" The function `read-key-sequence' suppresses quitting: `C-g' typed while reading with this function works like any other character, and does not set `quit-flag'. *Note Quitting::. -- Function: read-key-sequence-vector prompt &optional continue-echo dont-downcase-last switch-frame-ok command-loop This is like `read-key-sequence' except that it always returns the key sequence as a vector, never as a string. *Note Strings of Events::. If an input character is upper-case (or has the shift modifier) and has no key binding, but its lower-case equivalent has one, then `read-key-sequence' converts the character to lower case. Note that `lookup-key' does not perform case conversion in this way. When reading input results in such a "shift-translation", Emacs sets the variable `this-command-keys-shift-translated' to a non-`nil' value. Lisp programs can examine this variable if they need to modify their behavior when invoked by shift-translated keys. For example, the function `handle-shift-selection' examines the value of this variable to determine how to activate or deactivate the region (*note handle-shift-selection: The Mark.). The function `read-key-sequence' also transforms some mouse events. It converts unbound drag events into click events, and discards unbound button-down events entirely. It also reshuffles focus events and miscellaneous window events so that they never appear in a key sequence with any other events. When mouse events occur in special parts of a window, such as a mode line or a scroll bar, the event type shows nothing special--it is the same symbol that would normally represent that combination of mouse button and modifier keys. The information about the window part is kept elsewhere in the event--in the coordinates. But `read-key-sequence' translates this information into imaginary "prefix keys", all of which are symbols: `header-line', `horizontal-scroll-bar', `menu-bar', `mode-line', `vertical-line', and `vertical-scroll-bar'. You can define meanings for mouse clicks in special window parts by defining key sequences using these imaginary prefix keys. For example, if you call `read-key-sequence' and then click the mouse on the window's mode line, you get two events, like this: (read-key-sequence "Click on the mode line: ") => [mode-line (mouse-1 (# mode-line (40 . 63) 5959987))] -- Variable: num-input-keys This variable's value is the number of key sequences processed so far in this Emacs session. This includes key sequences read from the terminal and key sequences read from keyboard macros being executed. 21.8.2 Reading One Event ------------------------ The lowest level functions for command input are `read-event', `read-char', and `read-char-exclusive'. -- Function: read-event &optional prompt inherit-input-method seconds This function reads and returns the next event of command input, waiting if necessary until an event is available. Events can come directly from the user or from a keyboard macro. If the optional argument PROMPT is non-`nil', it should be a string to display in the echo area as a prompt. Otherwise, `read-event' does not display any message to indicate it is waiting for input; instead, it prompts by echoing: it displays descriptions of the events that led to or were read by the current command. *Note The Echo Area::. If INHERIT-INPUT-METHOD is non-`nil', then the current input method (if any) is employed to make it possible to enter a non-ASCII character. Otherwise, input method handling is disabled for reading this event. If `cursor-in-echo-area' is non-`nil', then `read-event' moves the cursor temporarily to the echo area, to the end of any message displayed there. Otherwise `read-event' does not move the cursor. If SECONDS is non-`nil', it should be a number specifying the maximum time to wait for input, in seconds. If no input arrives within that time, `read-event' stops waiting and returns `nil'. A floating-point value for SECONDS means to wait for a fractional number of seconds. Some systems support only a whole number of seconds; on these systems, SECONDS is rounded down. If SECONDS is `nil', `read-event' waits as long as necessary for input to arrive. If SECONDS is `nil', Emacs is considered idle while waiting for user input to arrive. Idle timers--those created with `run-with-idle-timer' (*note Idle Timers::)--can run during this period. However, if SECONDS is non-`nil', the state of idleness remains unchanged. If Emacs is non-idle when `read-event' is called, it remains non-idle throughout the operation of `read-event'; if Emacs is idle (which can happen if the call happens inside an idle timer), it remains idle. If `read-event' gets an event that is defined as a help character, then in some cases `read-event' processes the event directly without returning. *Note Help Functions::. Certain other events, called "special events", are also processed directly within `read-event' (*note Special Events::). Here is what happens if you call `read-event' and then press the right-arrow function key: (read-event) => right -- Function: read-char &optional prompt inherit-input-method seconds This function reads and returns a character of command input. If the user generates an event which is not a character (i.e., a mouse click or function key event), `read-char' signals an error. The arguments work as in `read-event'. In the first example, the user types the character `1' (ASCII code 49). The second example shows a keyboard macro definition that calls `read-char' from the minibuffer using `eval-expression'. `read-char' reads the keyboard macro's very next character, which is `1'. Then `eval-expression' displays its return value in the echo area. (read-char) => 49 ;; We assume here you use `M-:' to evaluate this. (symbol-function 'foo) => "^[:(read-char)^M1" (execute-kbd-macro 'foo) -| 49 => nil -- Function: read-char-exclusive &optional prompt inherit-input-method seconds This function reads and returns a character of command input. If the user generates an event which is not a character, `read-char-exclusive' ignores it and reads another event, until it gets a character. The arguments work as in `read-event'. None of the above functions suppress quitting. -- Variable: num-nonmacro-input-events This variable holds the total number of input events received so far from the terminal--not counting those generated by keyboard macros. We emphasize that, unlike `read-key-sequence', the functions `read-event', `read-char', and `read-char-exclusive' do not perform the translations described in *note Translation Keymaps::. If you wish to read a single key taking these translations into account, use the function `read-key': -- Function: read-key &optional prompt This function reads a single key. It is "intermediate" between `read-key-sequence' and `read-event'. Unlike the former, it reads a single key, not a key sequence. Unlike the latter, it does not return a raw event, but decodes and translates the user input according to `input-decode-map', `local-function-key-map', and `key-translation-map' (*note Translation Keymaps::). The argument PROMPT is either a string to be displayed in the echo area as a prompt, or `nil', meaning not to display a prompt. -- Function: read-char-choice prompt chars &optional inhibit-quit This function uses `read-key' to read and return a single character. It ignores any input that is not a member of CHARS, a list of accepted characters. Optionally, it will also ignore keyboard-quit events while it is waiting for valid input. If you bind `help-form' (*note Help Functions::) to a non-`nil' value while calling `read-char-choice', then pressing `help-char' causes it to evaluate `help-form' and display the result. It then continues to wait for a valid input character, or keyboard-quit. 21.8.3 Modifying and Translating Input Events --------------------------------------------- Emacs modifies every event it reads according to `extra-keyboard-modifiers', then translates it through `keyboard-translate-table' (if applicable), before returning it from `read-event'. -- Variable: extra-keyboard-modifiers This variable lets Lisp programs "press" the modifier keys on the keyboard. The value is a character. Only the modifiers of the character matter. Each time the user types a keyboard key, it is altered as if those modifier keys were held down. For instance, if you bind `extra-keyboard-modifiers' to `?\C-\M-a', then all keyboard input characters typed during the scope of the binding will have the control and meta modifiers applied to them. The character `?\C-@', equivalent to the integer 0, does not count as a control character for this purpose, but as a character with no modifiers. Thus, setting `extra-keyboard-modifiers' to zero cancels any modification. When using a window system, the program can "press" any of the modifier keys in this way. Otherwise, only the and keys can be virtually pressed. Note that this variable applies only to events that really come from the keyboard, and has no effect on mouse events or any other events. -- Variable: keyboard-translate-table This terminal-local variable is the translate table for keyboard characters. It lets you reshuffle the keys on the keyboard without changing any command bindings. Its value is normally a char-table, or else `nil'. (It can also be a string or vector, but this is considered obsolete.) If `keyboard-translate-table' is a char-table (*note Char-Tables::), then each character read from the keyboard is looked up in this char-table. If the value found there is non-`nil', then it is used instead of the actual input character. Note that this translation is the first thing that happens to a character after it is read from the terminal. Record-keeping features such as `recent-keys' and dribble files record the characters after translation. Note also that this translation is done before the characters are supplied to input methods (*note Input Methods::). Use `translation-table-for-input' (*note Translation of Characters::), if you want to translate characters after input methods operate. -- Function: keyboard-translate from to This function modifies `keyboard-translate-table' to translate character code FROM into character code TO. It creates the keyboard translate table if necessary. Here's an example of using the `keyboard-translate-table' to make `C-x', `C-c' and `C-v' perform the cut, copy and paste operations: (keyboard-translate ?\C-x 'control-x) (keyboard-translate ?\C-c 'control-c) (keyboard-translate ?\C-v 'control-v) (global-set-key [control-x] 'kill-region) (global-set-key [control-c] 'kill-ring-save) (global-set-key [control-v] 'yank) On a graphical terminal that supports extended ASCII input, you can still get the standard Emacs meanings of one of those characters by typing it with the shift key. That makes it a different character as far as keyboard translation is concerned, but it has the same usual meaning. *Note Translation Keymaps::, for mechanisms that translate event sequences at the level of `read-key-sequence'. 21.8.4 Invoking the Input Method -------------------------------- The event-reading functions invoke the current input method, if any (*note Input Methods::). If the value of `input-method-function' is non-`nil', it should be a function; when `read-event' reads a printing character (including ) with no modifier bits, it calls that function, passing the character as an argument. -- Variable: input-method-function If this is non-`nil', its value specifies the current input method function. *Warning:* don't bind this variable with `let'. It is often buffer-local, and if you bind it around reading input (which is exactly when you _would_ bind it), switching buffers asynchronously while Emacs is waiting will cause the value to be restored in the wrong buffer. The input method function should return a list of events which should be used as input. (If the list is `nil', that means there is no input, so `read-event' waits for another event.) These events are processed before the events in `unread-command-events' (*note Event Input Misc::). Events returned by the input method function are not passed to the input method function again, even if they are printing characters with no modifier bits. If the input method function calls `read-event' or `read-key-sequence', it should bind `input-method-function' to `nil' first, to prevent recursion. The input method function is not called when reading the second and subsequent events of a key sequence. Thus, these characters are not subject to input method processing. The input method function should test the values of `overriding-local-map' and `overriding-terminal-local-map'; if either of these variables is non-`nil', the input method should put its argument into a list and return that list with no further processing. 21.8.5 Quoted Character Input ----------------------------- You can use the function `read-quoted-char' to ask the user to specify a character, and allow the user to specify a control or meta character conveniently, either literally or as an octal character code. The command `quoted-insert' uses this function. -- Function: read-quoted-char &optional prompt This function is like `read-char', except that if the first character read is an octal digit (0-7), it reads any number of octal digits (but stopping if a non-octal digit is found), and returns the character represented by that numeric character code. If the character that terminates the sequence of octal digits is , it is discarded. Any other terminating character is used as input after this function returns. Quitting is suppressed when the first character is read, so that the user can enter a `C-g'. *Note Quitting::. If PROMPT is supplied, it specifies a string for prompting the user. The prompt string is always displayed in the echo area, followed by a single `-'. In the following example, the user types in the octal number 177 (which is 127 in decimal). (read-quoted-char "What character") ---------- Echo Area ---------- What character 1 7 7- ---------- Echo Area ---------- => 127 21.8.6 Miscellaneous Event Input Features ----------------------------------------- This section describes how to "peek ahead" at events without using them up, how to check for pending input, and how to discard pending input. See also the function `read-passwd' (*note Reading a Password::). -- Variable: unread-command-events This variable holds a list of events waiting to be read as command input. The events are used in the order they appear in the list, and removed one by one as they are used. The variable is needed because in some cases a function reads an event and then decides not to use it. Storing the event in this variable causes it to be processed normally, by the command loop or by the functions to read command input. For example, the function that implements numeric prefix arguments reads any number of digits. When it finds a non-digit event, it must unread the event so that it can be read normally by the command loop. Likewise, incremental search uses this feature to unread events with no special meaning in a search, because these events should exit the search and then execute normally. The reliable and easy way to extract events from a key sequence so as to put them in `unread-command-events' is to use `listify-key-sequence' (see below). Normally you add events to the front of this list, so that the events most recently unread will be reread first. Events read from this list are not normally added to the current command's key sequence (as returned by, e.g., `this-command-keys'), as the events will already have been added once as they were read for the first time. An element of the form `(`t' . EVENT)' forces EVENT to be added to the current command's key sequence. -- Function: listify-key-sequence key This function converts the string or vector KEY to a list of individual events, which you can put in `unread-command-events'. -- Function: input-pending-p This function determines whether any command input is currently available to be read. It returns immediately, with value `t' if there is available input, `nil' otherwise. On rare occasions it may return `t' when no input is available. -- Variable: last-input-event This variable records the last terminal input event read, whether as part of a command or explicitly by a Lisp program. In the example below, the Lisp program reads the character `1', ASCII code 49. It becomes the value of `last-input-event', while `C-e' (we assume `C-x C-e' command is used to evaluate this expression) remains the value of `last-command-event'. (progn (print (read-char)) (print last-command-event) last-input-event) -| 49 -| 5 => 49 -- Macro: while-no-input body... This construct runs the BODY forms and returns the value of the last one--but only if no input arrives. If any input arrives during the execution of the BODY forms, it aborts them (working much like a quit). The `while-no-input' form returns `nil' if aborted by a real quit, and returns `t' if aborted by arrival of other input. If a part of BODY binds `inhibit-quit' to non-`nil', arrival of input during those parts won't cause an abort until the end of that part. If you want to be able to distinguish all possible values computed by BODY from both kinds of abort conditions, write the code like this: (while-no-input (list (progn . BODY))) -- Function: discard-input This function discards the contents of the terminal input buffer and cancels any keyboard macro that might be in the process of definition. It returns `nil'. In the following example, the user may type a number of characters right after starting the evaluation of the form. After the `sleep-for' finishes sleeping, `discard-input' discards any characters typed during the sleep. (progn (sleep-for 2) (discard-input)) => nil 21.9 Special Events =================== Certain "special events" are handled at a very low level--as soon as they are read. The `read-event' function processes these events itself, and never returns them. Instead, it keeps waiting for the first event that is not special and returns that one. Special events do not echo, they are never grouped into key sequences, and they never appear in the value of `last-command-event' or `(this-command-keys)'. They do not discard a numeric argument, they cannot be unread with `unread-command-events', they may not appear in a keyboard macro, and they are not recorded in a keyboard macro while you are defining one. Special events do, however, appear in `last-input-event' immediately after they are read, and this is the way for the event's definition to find the actual event. The events types `iconify-frame', `make-frame-visible', `delete-frame', `drag-n-drop', `language-change', and user signals like `sigusr1' are normally handled in this way. The keymap which defines how to handle special events--and which events are special--is in the variable `special-event-map' (*note Active Keymaps::). 21.10 Waiting for Elapsed Time or Input ======================================= The wait functions are designed to wait for a certain amount of time to pass or until there is input. For example, you may wish to pause in the middle of a computation to allow the user time to view the display. `sit-for' pauses and updates the screen, and returns immediately if input comes in, while `sleep-for' pauses without updating the screen. -- Function: sit-for seconds &optional nodisp This function performs redisplay (provided there is no pending input from the user), then waits SECONDS seconds, or until input is available. The usual purpose of `sit-for' is to give the user time to read text that you display. The value is `t' if `sit-for' waited the full time with no input arriving (*note Event Input Misc::). Otherwise, the value is `nil'. The argument SECONDS need not be an integer. If it is a floating point number, `sit-for' waits for a fractional number of seconds. Some systems support only a whole number of seconds; on these systems, SECONDS is rounded down. The expression `(sit-for 0)' is equivalent to `(redisplay)', i.e., it requests a redisplay, without any delay, if there is no pending input. *Note Forcing Redisplay::. If NODISP is non-`nil', then `sit-for' does not redisplay, but it still returns as soon as input is available (or when the timeout elapses). In batch mode (*note Batch Mode::), `sit-for' cannot be interrupted, even by input from the standard input descriptor. It is thus equivalent to `sleep-for', which is described below. It is also possible to call `sit-for' with three arguments, as `(sit-for SECONDS MILLISEC NODISP)', but that is considered obsolete. -- Function: sleep-for seconds &optional millisec This function simply pauses for SECONDS seconds without updating the display. It pays no attention to available input. It returns `nil'. The argument SECONDS need not be an integer. If it is a floating point number, `sleep-for' waits for a fractional number of seconds. Some systems support only a whole number of seconds; on these systems, SECONDS is rounded down. The optional argument MILLISEC specifies an additional waiting period measured in milliseconds. This adds to the period specified by SECONDS. If the system doesn't support waiting fractions of a second, you get an error if you specify nonzero MILLISEC. Use `sleep-for' when you wish to guarantee a delay. *Note Time of Day::, for functions to get the current time. 21.11 Quitting ============== Typing `C-g' while a Lisp function is running causes Emacs to "quit" whatever it is doing. This means that control returns to the innermost active command loop. Typing `C-g' while the command loop is waiting for keyboard input does not cause a quit; it acts as an ordinary input character. In the simplest case, you cannot tell the difference, because `C-g' normally runs the command `keyboard-quit', whose effect is to quit. However, when `C-g' follows a prefix key, they combine to form an undefined key. The effect is to cancel the prefix key as well as any prefix argument. In the minibuffer, `C-g' has a different definition: it aborts out of the minibuffer. This means, in effect, that it exits the minibuffer and then quits. (Simply quitting would return to the command loop _within_ the minibuffer.) The reason why `C-g' does not quit directly when the command reader is reading input is so that its meaning can be redefined in the minibuffer in this way. `C-g' following a prefix key is not redefined in the minibuffer, and it has its normal effect of canceling the prefix key and prefix argument. This too would not be possible if `C-g' always quit directly. When `C-g' does directly quit, it does so by setting the variable `quit-flag' to `t'. Emacs checks this variable at appropriate times and quits if it is not `nil'. Setting `quit-flag' non-`nil' in any way thus causes a quit. At the level of C code, quitting cannot happen just anywhere; only at the special places that check `quit-flag'. The reason for this is that quitting at other places might leave an inconsistency in Emacs's internal state. Because quitting is delayed until a safe place, quitting cannot make Emacs crash. Certain functions such as `read-key-sequence' or `read-quoted-char' prevent quitting entirely even though they wait for input. Instead of quitting, `C-g' serves as the requested input. In the case of `read-key-sequence', this serves to bring about the special behavior of `C-g' in the command loop. In the case of `read-quoted-char', this is so that `C-q' can be used to quote a `C-g'. You can prevent quitting for a portion of a Lisp function by binding the variable `inhibit-quit' to a non-`nil' value. Then, although `C-g' still sets `quit-flag' to `t' as usual, the usual result of this--a quit--is prevented. Eventually, `inhibit-quit' will become `nil' again, such as when its binding is unwound at the end of a `let' form. At that time, if `quit-flag' is still non-`nil', the requested quit happens immediately. This behavior is ideal when you wish to make sure that quitting does not happen within a "critical section" of the program. In some functions (such as `read-quoted-char'), `C-g' is handled in a special way that does not involve quitting. This is done by reading the input with `inhibit-quit' bound to `t', and setting `quit-flag' to `nil' before `inhibit-quit' becomes `nil' again. This excerpt from the definition of `read-quoted-char' shows how this is done; it also shows that normal quitting is permitted after the first character of input. (defun read-quoted-char (&optional prompt) "...DOCUMENTATION..." (let ((message-log-max nil) done (first t) (code 0) char) (while (not done) (let ((inhibit-quit first) ...) (and prompt (message "%s-" prompt)) (setq char (read-event)) (if inhibit-quit (setq quit-flag nil))) ...set the variable `code'...) code)) -- Variable: quit-flag If this variable is non-`nil', then Emacs quits immediately, unless `inhibit-quit' is non-`nil'. Typing `C-g' ordinarily sets `quit-flag' non-`nil', regardless of `inhibit-quit'. -- Variable: inhibit-quit This variable determines whether Emacs should quit when `quit-flag' is set to a value other than `nil'. If `inhibit-quit' is non-`nil', then `quit-flag' has no special effect. -- Macro: with-local-quit body... This macro executes BODY forms in sequence, but allows quitting, at least locally, within BODY even if `inhibit-quit' was non-`nil' outside this construct. It returns the value of the last form in BODY, unless exited by quitting, in which case it returns `nil'. If `inhibit-quit' is `nil' on entry to `with-local-quit', it only executes the BODY, and setting `quit-flag' causes a normal quit. However, if `inhibit-quit' is non-`nil' so that ordinary quitting is delayed, a non-`nil' `quit-flag' triggers a special kind of local quit. This ends the execution of BODY and exits the `with-local-quit' body with `quit-flag' still non-`nil', so that another (ordinary) quit will happen as soon as that is allowed. If `quit-flag' is already non-`nil' at the beginning of BODY, the local quit happens immediately and the body doesn't execute at all. This macro is mainly useful in functions that can be called from timers, process filters, process sentinels, `pre-command-hook', `post-command-hook', and other places where `inhibit-quit' is normally bound to `t'. -- Command: keyboard-quit This function signals the `quit' condition with `(signal 'quit nil)'. This is the same thing that quitting does. (See `signal' in *note Errors::.) You can specify a character other than `C-g' to use for quitting. See the function `set-input-mode' in *note Terminal Input::. 21.12 Prefix Command Arguments ============================== Most Emacs commands can use a "prefix argument", a number specified before the command itself. (Don't confuse prefix arguments with prefix keys.) The prefix argument is at all times represented by a value, which may be `nil', meaning there is currently no prefix argument. Each command may use the prefix argument or ignore it. There are two representations of the prefix argument: "raw" and "numeric". The editor command loop uses the raw representation internally, and so do the Lisp variables that store the information, but commands can request either representation. Here are the possible values of a raw prefix argument: * `nil', meaning there is no prefix argument. Its numeric value is 1, but numerous commands make a distinction between `nil' and the integer 1. * An integer, which stands for itself. * A list of one element, which is an integer. This form of prefix argument results from one or a succession of `C-u's with no digits. The numeric value is the integer in the list, but some commands make a distinction between such a list and an integer alone. * The symbol `-'. This indicates that `M--' or `C-u -' was typed, without following digits. The equivalent numeric value is -1, but some commands make a distinction between the integer -1 and the symbol `-'. We illustrate these possibilities by calling the following function with various prefixes: (defun display-prefix (arg) "Display the value of the raw prefix arg." (interactive "P") (message "%s" arg)) Here are the results of calling `display-prefix' with various raw prefix arguments: M-x display-prefix -| nil C-u M-x display-prefix -| (4) C-u C-u M-x display-prefix -| (16) C-u 3 M-x display-prefix -| 3 M-3 M-x display-prefix -| 3 ; (Same as `C-u 3'.) C-u - M-x display-prefix -| - M-- M-x display-prefix -| - ; (Same as `C-u -'.) C-u - 7 M-x display-prefix -| -7 M-- 7 M-x display-prefix -| -7 ; (Same as `C-u -7'.) Emacs uses two variables to store the prefix argument: `prefix-arg' and `current-prefix-arg'. Commands such as `universal-argument' that set up prefix arguments for other commands store them in `prefix-arg'. In contrast, `current-prefix-arg' conveys the prefix argument to the current command, so setting it has no effect on the prefix arguments for future commands. Normally, commands specify which representation to use for the prefix argument, either numeric or raw, in the `interactive' specification. (*Note Using Interactive::.) Alternatively, functions may look at the value of the prefix argument directly in the variable `current-prefix-arg', but this is less clean. -- Function: prefix-numeric-value arg This function returns the numeric meaning of a valid raw prefix argument value, ARG. The argument may be a symbol, a number, or a list. If it is `nil', the value 1 is returned; if it is `-', the value -1 is returned; if it is a number, that number is returned; if it is a list, the CAR of that list (which should be a number) is returned. -- Variable: current-prefix-arg This variable holds the raw prefix argument for the _current_ command. Commands may examine it directly, but the usual method for accessing it is with `(interactive "P")'. -- Variable: prefix-arg The value of this variable is the raw prefix argument for the _next_ editing command. Commands such as `universal-argument' that specify prefix arguments for the following command work by setting this variable. -- Variable: last-prefix-arg The raw prefix argument value used by the previous command. The following commands exist to set up prefix arguments for the following command. Do not call them for any other reason. -- Command: universal-argument This command reads input and specifies a prefix argument for the following command. Don't call this command yourself unless you know what you are doing. -- Command: digit-argument arg This command adds to the prefix argument for the following command. The argument ARG is the raw prefix argument as it was before this command; it is used to compute the updated prefix argument. Don't call this command yourself unless you know what you are doing. -- Command: negative-argument arg This command adds to the numeric argument for the next command. The argument ARG is the raw prefix argument as it was before this command; its value is negated to form the new prefix argument. Don't call this command yourself unless you know what you are doing. 21.13 Recursive Editing ======================= The Emacs command loop is entered automatically when Emacs starts up. This top-level invocation of the command loop never exits; it keeps running as long as Emacs does. Lisp programs can also invoke the command loop. Since this makes more than one activation of the command loop, we call it "recursive editing". A recursive editing level has the effect of suspending whatever command invoked it and permitting the user to do arbitrary editing before resuming that command. The commands available during recursive editing are the same ones available in the top-level editing loop and defined in the keymaps. Only a few special commands exit the recursive editing level; the others return to the recursive editing level when they finish. (The special commands for exiting are always available, but they do nothing when recursive editing is not in progress.) All command loops, including recursive ones, set up all-purpose error handlers so that an error in a command run from the command loop will not exit the loop. Minibuffer input is a special kind of recursive editing. It has a few special wrinkles, such as enabling display of the minibuffer and the minibuffer window, but fewer than you might suppose. Certain keys behave differently in the minibuffer, but that is only because of the minibuffer's local map; if you switch windows, you get the usual Emacs commands. To invoke a recursive editing level, call the function `recursive-edit'. This function contains the command loop; it also contains a call to `catch' with tag `exit', which makes it possible to exit the recursive editing level by throwing to `exit' (*note Catch and Throw::). If you throw a value other than `t', then `recursive-edit' returns normally to the function that called it. The command `C-M-c' (`exit-recursive-edit') does this. Throwing a `t' value causes `recursive-edit' to quit, so that control returns to the command loop one level up. This is called "aborting", and is done by `C-]' (`abort-recursive-edit'). Most applications should not use recursive editing, except as part of using the minibuffer. Usually it is more convenient for the user if you change the major mode of the current buffer temporarily to a special major mode, which should have a command to go back to the previous mode. (The `e' command in Rmail uses this technique.) Or, if you wish to give the user different text to edit "recursively", create and select a new buffer in a special mode. In this mode, define a command to complete the processing and go back to the previous buffer. (The `m' command in Rmail does this.) Recursive edits are useful in debugging. You can insert a call to `debug' into a function definition as a sort of breakpoint, so that you can look around when the function gets there. `debug' invokes a recursive edit but also provides the other features of the debugger. Recursive editing levels are also used when you type `C-r' in `query-replace' or use `C-x q' (`kbd-macro-query'). -- Command: recursive-edit This function invokes the editor command loop. It is called automatically by the initialization of Emacs, to let the user begin editing. When called from a Lisp program, it enters a recursive editing level. If the current buffer is not the same as the selected window's buffer, `recursive-edit' saves and restores the current buffer. Otherwise, if you switch buffers, the buffer you switched to is current after `recursive-edit' returns. In the following example, the function `simple-rec' first advances point one word, then enters a recursive edit, printing out a message in the echo area. The user can then do any editing desired, and then type `C-M-c' to exit and continue executing `simple-rec'. (defun simple-rec () (forward-word 1) (message "Recursive edit in progress") (recursive-edit) (forward-word 1)) => simple-rec (simple-rec) => nil -- Command: exit-recursive-edit This function exits from the innermost recursive edit (including minibuffer input). Its definition is effectively `(throw 'exit nil)'. -- Command: abort-recursive-edit This function aborts the command that requested the innermost recursive edit (including minibuffer input), by signaling `quit' after exiting the recursive edit. Its definition is effectively `(throw 'exit t)'. *Note Quitting::. -- Command: top-level This function exits all recursive editing levels; it does not return a value, as it jumps completely out of any computation directly back to the main command loop. -- Function: recursion-depth This function returns the current depth of recursive edits. When no recursive edit is active, it returns 0. 21.14 Disabling Commands ======================== "Disabling a command" marks the command as requiring user confirmation before it can be executed. Disabling is used for commands which might be confusing to beginning users, to prevent them from using the commands by accident. The low-level mechanism for disabling a command is to put a non-`nil' `disabled' property on the Lisp symbol for the command. These properties are normally set up by the user's init file (*note Init File::) with Lisp expressions such as this: (put 'upcase-region 'disabled t) For a few commands, these properties are present by default (you can remove them in your init file if you wish). If the value of the `disabled' property is a string, the message saying the command is disabled includes that string. For example: (put 'delete-region 'disabled "Text deleted this way cannot be yanked back!\n") *Note Disabling: (emacs)Disabling, for the details on what happens when a disabled command is invoked interactively. Disabling a command has no effect on calling it as a function from Lisp programs. -- Command: enable-command command Allow COMMAND (a symbol) to be executed without special confirmation from now on, and alter the user's init file (*note Init File::) so that this will apply to future sessions. -- Command: disable-command command Require special confirmation to execute COMMAND from now on, and alter the user's init file so that this will apply to future sessions. -- Variable: disabled-command-function The value of this variable should be a function. When the user invokes a disabled command interactively, this function is called instead of the disabled command. It can use `this-command-keys' to determine what the user typed to run the command, and thus find the command itself. The value may also be `nil'. Then all commands work normally, even disabled ones. By default, the value is a function that asks the user whether to proceed. 21.15 Command History ===================== The command loop keeps a history of the complex commands that have been executed, to make it convenient to repeat these commands. A "complex command" is one for which the interactive argument reading uses the minibuffer. This includes any `M-x' command, any `M-:' command, and any command whose `interactive' specification reads an argument from the minibuffer. Explicit use of the minibuffer during the execution of the command itself does not cause the command to be considered complex. -- Variable: command-history This variable's value is a list of recent complex commands, each represented as a form to evaluate. It continues to accumulate all complex commands for the duration of the editing session, but when it reaches the maximum size (*note Minibuffer History::), the oldest elements are deleted as new ones are added. command-history => ((switch-to-buffer "chistory.texi") (describe-key "^X^[") (visit-tags-table "~/emacs/src/") (find-tag "repeat-complex-command")) This history list is actually a special case of minibuffer history (*note Minibuffer History::), with one special twist: the elements are expressions rather than strings. There are a number of commands devoted to the editing and recall of previous commands. The commands `repeat-complex-command', and `list-command-history' are described in the user manual (*note Repetition: (emacs)Repetition.). Within the minibuffer, the usual minibuffer history commands are available. 21.16 Keyboard Macros ===================== A "keyboard macro" is a canned sequence of input events that can be considered a command and made the definition of a key. The Lisp representation of a keyboard macro is a string or vector containing the events. Don't confuse keyboard macros with Lisp macros (*note Macros::). -- Function: execute-kbd-macro kbdmacro &optional count loopfunc This function executes KBDMACRO as a sequence of events. If KBDMACRO is a string or vector, then the events in it are executed exactly as if they had been input by the user. The sequence is _not_ expected to be a single key sequence; normally a keyboard macro definition consists of several key sequences concatenated. If KBDMACRO is a symbol, then its function definition is used in place of KBDMACRO. If that is another symbol, this process repeats. Eventually the result should be a string or vector. If the result is not a symbol, string, or vector, an error is signaled. The argument COUNT is a repeat count; KBDMACRO is executed that many times. If COUNT is omitted or `nil', KBDMACRO is executed once. If it is 0, KBDMACRO is executed over and over until it encounters an error or a failing search. If LOOPFUNC is non-`nil', it is a function that is called, without arguments, prior to each iteration of the macro. If LOOPFUNC returns `nil', then this stops execution of the macro. *Note Reading One Event::, for an example of using `execute-kbd-macro'. -- Variable: executing-kbd-macro This variable contains the string or vector that defines the keyboard macro that is currently executing. It is `nil' if no macro is currently executing. A command can test this variable so as to behave differently when run from an executing macro. Do not set this variable yourself. -- Variable: defining-kbd-macro This variable is non-`nil' if and only if a keyboard macro is being defined. A command can test this variable so as to behave differently while a macro is being defined. The value is `append' while appending to the definition of an existing macro. The commands `start-kbd-macro', `kmacro-start-macro' and `end-kbd-macro' set this variable--do not set it yourself. The variable is always local to the current terminal and cannot be buffer-local. *Note Multiple Terminals::. -- Variable: last-kbd-macro This variable is the definition of the most recently defined keyboard macro. Its value is a string or vector, or `nil'. The variable is always local to the current terminal and cannot be buffer-local. *Note Multiple Terminals::. -- Variable: kbd-macro-termination-hook This normal hook is run when a keyboard macro terminates, regardless of what caused it to terminate (reaching the macro end or an error which ended the macro prematurely). 22 Keymaps ********** The command bindings of input events are recorded in data structures called "keymaps". Each entry in a keymap associates (or "binds") an individual event type, either to another keymap or to a command. When an event type is bound to a keymap, that keymap is used to look up the next input event; this continues until a command is found. The whole process is called "key lookup". 22.1 Key Sequences ================== A "key sequence", or "key" for short, is a sequence of one or more input events that form a unit. Input events include characters, function keys, mouse actions, or system events external to Emacs, such as `iconify-frame' (*note Input Events::). The Emacs Lisp representation for a key sequence is a string or vector. Unless otherwise stated, any Emacs Lisp function that accepts a key sequence as an argument can handle both representations. In the string representation, alphanumeric characters ordinarily stand for themselves; for example, `"a"' represents `a' and `"2"' represents `2'. Control character events are prefixed by the substring `"\C-"', and meta characters by `"\M-"'; for example, `"\C-x"' represents the key `C-x'. In addition, the , , , and events are represented by `"\t"', `"\r"', `"\e"', and `"\d"' respectively. The string representation of a complete key sequence is the concatenation of the string representations of the constituent events; thus, `"\C-xl"' represents the key sequence `C-x l'. Key sequences containing function keys, mouse button events, system events, or non-ASCII characters such as `C-=' or `H-a' cannot be represented as strings; they have to be represented as vectors. In the vector representation, each element of the vector represents an input event, in its Lisp form. *Note Input Events::. For example, the vector `[?\C-x ?l]' represents the key sequence `C-x l'. For examples of key sequences written in string and vector representations, *note Init Rebinding: (emacs)Init Rebinding. -- Function: kbd keyseq-text This function converts the text KEYSEQ-TEXT (a string constant) into a key sequence (a string or vector constant). The contents of KEYSEQ-TEXT should use the same syntax as in the buffer invoked by the `C-x C-k ' (`kmacro-edit-macro') command; in particular, you must surround function key names with `<...>'. *Note Edit Keyboard Macro: (emacs)Edit Keyboard Macro. (kbd "C-x") => "\C-x" (kbd "C-x C-f") => "\C-x\C-f" (kbd "C-x 4 C-f") => "\C-x4\C-f" (kbd "X") => "X" (kbd "RET") => "\^M" (kbd "C-c SPC") => "\C-c " (kbd " SPC") => [f1 32] (kbd "C-M-") => [C-M-down] 22.2 Keymap Basics ================== A keymap is a Lisp data structure that specifies "key bindings" for various key sequences. A single keymap directly specifies definitions for individual events. When a key sequence consists of a single event, its binding in a keymap is the keymap's definition for that event. The binding of a longer key sequence is found by an iterative process: first find the definition of the first event (which must itself be a keymap); then find the second event's definition in that keymap, and so on until all the events in the key sequence have been processed. If the binding of a key sequence is a keymap, we call the key sequence a "prefix key". Otherwise, we call it a "complete key" (because no more events can be added to it). If the binding is `nil', we call the key "undefined". Examples of prefix keys are `C-c', `C-x', and `C-x 4'. Examples of defined complete keys are `X', , and `C-x 4 C-f'. Examples of undefined complete keys are `C-x C-g', and `C-c 3'. *Note Prefix Keys::, for more details. The rule for finding the binding of a key sequence assumes that the intermediate bindings (found for the events before the last) are all keymaps; if this is not so, the sequence of events does not form a unit--it is not really one key sequence. In other words, removing one or more events from the end of any valid key sequence must always yield a prefix key. For example, `C-f C-n' is not a key sequence; `C-f' is not a prefix key, so a longer sequence starting with `C-f' cannot be a key sequence. The set of possible multi-event key sequences depends on the bindings for prefix keys; therefore, it can be different for different keymaps, and can change when bindings are changed. However, a one-event sequence is always a key sequence, because it does not depend on any prefix keys for its well-formedness. At any time, several primary keymaps are "active"--that is, in use for finding key bindings. These are the "global map", which is shared by all buffers; the "local keymap", which is usually associated with a specific major mode; and zero or more "minor mode keymaps", which belong to currently enabled minor modes. (Not all minor modes have keymaps.) The local keymap bindings shadow (i.e., take precedence over) the corresponding global bindings. The minor mode keymaps shadow both local and global keymaps. *Note Active Keymaps::, for details. 22.3 Format of Keymaps ====================== Each keymap is a list whose CAR is the symbol `keymap'. The remaining elements of the list define the key bindings of the keymap. A symbol whose function definition is a keymap is also a keymap. Use the function `keymapp' (see below) to test whether an object is a keymap. Several kinds of elements may appear in a keymap, after the symbol `keymap' that begins it: `(TYPE . BINDING)' This specifies one binding, for events of type TYPE. Each ordinary binding applies to events of a particular "event type", which is always a character or a symbol. *Note Classifying Events::. In this kind of binding, BINDING is a command. `(TYPE ITEM-NAME . BINDING)' This specifies a binding which is also a simple menu item that displays as ITEM-NAME in the menu. *Note Simple Menu Items::. `(TYPE ITEM-NAME HELP-STRING . BINDING)' This is a simple menu item with help string HELP-STRING. `(TYPE menu-item . DETAILS)' This specifies a binding which is also an extended menu item. This allows use of other features. *Note Extended Menu Items::. `(t . BINDING)' This specifies a "default key binding"; any event not bound by other elements of the keymap is given BINDING as its binding. Default bindings allow a keymap to bind all possible event types without having to enumerate all of them. A keymap that has a default binding completely masks any lower-precedence keymap, except for events explicitly bound to `nil' (see below). `CHAR-TABLE' If an element of a keymap is a char-table, it counts as holding bindings for all character events with no modifier bits (*note modifier bits::): element N is the binding for the character with code N. This is a compact way to record lots of bindings. A keymap with such a char-table is called a "full keymap". Other keymaps are called "sparse keymaps". `STRING' Aside from elements that specify bindings for keys, a keymap can also have a string as an element. This is called the "overall prompt string" and makes it possible to use the keymap as a menu. *Note Defining Menus::. `(keymap ...)' If an element of a keymap is itself a keymap, it counts as if this inner keymap were inlined in the outer keymap. This is used for multiple-inheritance, such as in `make-composed-keymap'. When the binding is `nil', it doesn't constitute a definition but it does take precedence over a default binding or a binding in the parent keymap. On the other hand, a binding of `nil' does _not_ override lower-precedence keymaps; thus, if the local map gives a binding of `nil', Emacs uses the binding from the global map. Keymaps do not directly record bindings for the meta characters. Instead, meta characters are regarded for purposes of key lookup as sequences of two characters, the first of which is (or whatever is currently the value of `meta-prefix-char'). Thus, the key `M-a' is internally represented as ` a', and its global binding is found at the slot for `a' in `esc-map' (*note Prefix Keys::). This conversion applies only to characters, not to function keys or other input events; thus, `M-' has nothing to do with ` '. Here as an example is the local keymap for Lisp mode, a sparse keymap. It defines bindings for , `C-c C-z', `C-M-q', and `C-M-x' (the actual value also contains a menu binding, which is omitted here for the sake of brevity). lisp-mode-map => (keymap (3 keymap ;; C-c C-z (26 . run-lisp)) (27 keymap ;; `C-M-x', treated as ` C-x' (24 . lisp-send-defun)) ;; This part is inherited from `lisp-mode-shared-map'. keymap ;; (127 . backward-delete-char-untabify) (27 keymap ;; `C-M-q', treated as ` C-q' (17 . indent-sexp))) -- Function: keymapp object This function returns `t' if OBJECT is a keymap, `nil' otherwise. More precisely, this function tests for a list whose CAR is `keymap', or for a symbol whose function definition satisfies `keymapp'. (keymapp '(keymap)) => t (fset 'foo '(keymap)) (keymapp 'foo) => t (keymapp (current-global-map)) => t 22.4 Creating Keymaps ===================== Here we describe the functions for creating keymaps. -- Function: make-sparse-keymap &optional prompt This function creates and returns a new sparse keymap with no entries. (A sparse keymap is the kind of keymap you usually want.) The new keymap does not contain a char-table, unlike `make-keymap', and does not bind any events. (make-sparse-keymap) => (keymap) If you specify PROMPT, that becomes the overall prompt string for the keymap. You should specify this only for menu keymaps (*note Defining Menus::). A keymap with an overall prompt string will always present a mouse menu or a keyboard menu if it is active for looking up the next input event. Don't specify an overall prompt string for the main map of a major or minor mode, because that would cause the command loop to present a keyboard menu every time. -- Function: make-keymap &optional prompt This function creates and returns a new full keymap. That keymap contains a char-table (*note Char-Tables::) with slots for all characters without modifiers. The new keymap initially binds all these characters to `nil', and does not bind any other kind of event. The argument PROMPT specifies a prompt string, as in `make-sparse-keymap'. (make-keymap) => (keymap #^[nil nil keymap nil nil nil ...]) A full keymap is more efficient than a sparse keymap when it holds lots of bindings; for just a few, the sparse keymap is better. -- Function: copy-keymap keymap This function returns a copy of KEYMAP. Any keymaps that appear directly as bindings in KEYMAP are also copied recursively, and so on to any number of levels. However, recursive copying does not take place when the definition of a character is a symbol whose function definition is a keymap; the same symbol appears in the new copy. (setq map (copy-keymap (current-local-map))) => (keymap ;; (This implements meta characters.) (27 keymap (83 . center-paragraph) (115 . center-line)) (9 . tab-to-tab-stop)) (eq map (current-local-map)) => nil (equal map (current-local-map)) => t 22.5 Inheritance and Keymaps ============================ A keymap can inherit the bindings of another keymap, which we call the "parent keymap". Such a keymap looks like this: (keymap ELEMENTS... . PARENT-KEYMAP) The effect is that this keymap inherits all the bindings of PARENT-KEYMAP, whatever they may be at the time a key is looked up, but can add to them or override them with ELEMENTS. If you change the bindings in PARENT-KEYMAP using `define-key' or other key-binding functions, these changed bindings are visible in the inheriting keymap, unless shadowed by the bindings made by ELEMENTS. The converse is not true: if you use `define-key' to change bindings in the inheriting keymap, these changes are recorded in ELEMENTS, but have no effect on PARENT-KEYMAP. The proper way to construct a keymap with a parent is to use `set-keymap-parent'; if you have code that directly constructs a keymap with a parent, please convert the program to use `set-keymap-parent' instead. -- Function: keymap-parent keymap This returns the parent keymap of KEYMAP. If KEYMAP has no parent, `keymap-parent' returns `nil'. -- Function: set-keymap-parent keymap parent This sets the parent keymap of KEYMAP to PARENT, and returns PARENT. If PARENT is `nil', this function gives KEYMAP no parent at all. If KEYMAP has submaps (bindings for prefix keys), they too receive new parent keymaps that reflect what PARENT specifies for those prefix keys. Here is an example showing how to make a keymap that inherits from `text-mode-map': (let ((map (make-sparse-keymap))) (set-keymap-parent map text-mode-map) map) A non-sparse keymap can have a parent too, but this is not very useful. A non-sparse keymap always specifies something as the binding for every numeric character code without modifier bits, even if it is `nil', so these character's bindings are never inherited from the parent keymap. Sometimes you want to make a keymap that inherits from more than one map. You can use the function `make-composed-keymap' for this. -- Function: make-composed-keymap maps &optional parent This function returns a new keymap composed of the existing keymap(s) MAPS, and optionally inheriting from a parent keymap PARENT. MAPS can be a single keymap or a list of more than one. When looking up a key in the resulting new map, Emacs searches in each of the MAPS in turn, and then in PARENT, stopping at the first match. A `nil' binding in any one of MAPS overrides any binding in PARENT, but it does not override any non-`nil' binding in any other of the MAPS. For example, here is how Emacs sets the parent of `help-mode-map', such that it inherits from both `button-buffer-map' and `special-mode-map': (defvar help-mode-map (let ((map (make-sparse-keymap))) (set-keymap-parent map (make-composed-keymap button-buffer-map special-mode-map)) ... map) ... ) 22.6 Prefix Keys ================ A "prefix key" is a key sequence whose binding is a keymap. The keymap defines what to do with key sequences that extend the prefix key. For example, `C-x' is a prefix key, and it uses a keymap that is also stored in the variable `ctl-x-map'. This keymap defines bindings for key sequences starting with `C-x'. Some of the standard Emacs prefix keys use keymaps that are also found in Lisp variables: * `esc-map' is the global keymap for the prefix key. Thus, the global definitions of all meta characters are actually found here. This map is also the function definition of `ESC-prefix'. * `help-map' is the global keymap for the `C-h' prefix key. * `mode-specific-map' is the global keymap for the prefix key `C-c'. This map is actually global, not mode-specific, but its name provides useful information about `C-c' in the output of `C-h b' (`display-bindings'), since the main use of this prefix key is for mode-specific bindings. * `ctl-x-map' is the global keymap used for the `C-x' prefix key. This map is found via the function cell of the symbol `Control-X-prefix'. * `mule-keymap' is the global keymap used for the `C-x ' prefix key. * `ctl-x-4-map' is the global keymap used for the `C-x 4' prefix key. * `ctl-x-5-map' is the global keymap used for the `C-x 5' prefix key. * `2C-mode-map' is the global keymap used for the `C-x 6' prefix key. * `vc-prefix-map' is the global keymap used for the `C-x v' prefix key. * `goto-map' is the global keymap used for the `M-g' prefix key. * `search-map' is the global keymap used for the `M-s' prefix key. * `facemenu-keymap' is the global keymap used for the `M-o' prefix key. * The other Emacs prefix keys are `C-x @', `C-x a i', `C-x ' and ` '. They use keymaps that have no special names. The keymap binding of a prefix key is used for looking up the event that follows the prefix key. (It may instead be a symbol whose function definition is a keymap. The effect is the same, but the symbol serves as a name for the prefix key.) Thus, the binding of `C-x' is the symbol `Control-X-prefix', whose function cell holds the keymap for `C-x' commands. (The same keymap is also the value of `ctl-x-map'.) Prefix key definitions can appear in any active keymap. The definitions of `C-c', `C-x', `C-h' and as prefix keys appear in the global map, so these prefix keys are always available. Major and minor modes can redefine a key as a prefix by putting a prefix key definition for it in the local map or the minor mode's map. *Note Active Keymaps::. If a key is defined as a prefix in more than one active map, then its various definitions are in effect merged: the commands defined in the minor mode keymaps come first, followed by those in the local map's prefix definition, and then by those from the global map. In the following example, we make `C-p' a prefix key in the local keymap, in such a way that `C-p' is identical to `C-x'. Then the binding for `C-p C-f' is the function `find-file', just like `C-x C-f'. The key sequence `C-p 6' is not found in any active keymap. (use-local-map (make-sparse-keymap)) => nil (local-set-key "\C-p" ctl-x-map) => nil (key-binding "\C-p\C-f") => find-file (key-binding "\C-p6") => nil -- Function: define-prefix-command symbol &optional mapvar prompt This function prepares SYMBOL for use as a prefix key's binding: it creates a sparse keymap and stores it as SYMBOL's function definition. Subsequently binding a key sequence to SYMBOL will make that key sequence into a prefix key. The return value is `symbol'. This function also sets SYMBOL as a variable, with the keymap as its value. But if MAPVAR is non-`nil', it sets MAPVAR as a variable instead. If PROMPT is non-`nil', that becomes the overall prompt string for the keymap. The prompt string should be given for menu keymaps (*note Defining Menus::). 22.7 Active Keymaps =================== Emacs normally contains many keymaps; at any given time, just a few of them are "active", meaning that they participate in the interpretation of user input. All the active keymaps are used together to determine what command to execute when a key is entered. Normally the active keymaps are the `keymap' property keymap, the keymaps of any enabled minor modes, the current buffer's local keymap, and the global keymap, in that order. Emacs searches for each input key sequence in all these keymaps. *Note Searching Keymaps::, for more details of this procedure. When the key sequence starts with a mouse event, the active keymaps are determined based on the position in that event. If the event happened on a string embedded with a `display', `before-string', or `after-string' property (*note Special Properties::), the non-`nil' map properties of the string override those of the buffer (if the underlying buffer text contains map properties in its text properties or overlays, they are ignored). The "global keymap" holds the bindings of keys that are defined regardless of the current buffer, such as `C-f'. The variable `global-map' holds this keymap, which is always active. Each buffer may have another keymap, its "local keymap", which may contain new or overriding definitions for keys. The current buffer's local keymap is always active except when `overriding-local-map' overrides it. The `local-map' text or overlay property can specify an alternative local keymap for certain parts of the buffer; see *note Special Properties::. Each minor mode can have a keymap; if it does, the keymap is active when the minor mode is enabled. Modes for emulation can specify additional active keymaps through the variable `emulation-mode-map-alists'. The highest precedence normal keymap comes from the `keymap' text or overlay property. If that is non-`nil', it is the first keymap to be processed, in normal circumstances. Next comes any keymap added by the function `set-temporary-overlay-map'. *Note Controlling Active Maps::. However, there are also special ways for programs to substitute other keymaps for some of those. The variable `overriding-local-map', if non-`nil', specifies a keymap that replaces all the usual active keymaps except the global keymap. Another way to do this is with `overriding-terminal-local-map'; it operates on a per-terminal basis. These variables are documented below. Since every buffer that uses the same major mode normally uses the same local keymap, you can think of the keymap as local to the mode. A change to the local keymap of a buffer (using `local-set-key', for example) is seen also in the other buffers that share that keymap. The local keymaps that are used for Lisp mode and some other major modes exist even if they have not yet been used. These local keymaps are the values of variables such as `lisp-mode-map'. For most major modes, which are less frequently used, the local keymap is constructed only when the mode is used for the first time in a session. The minibuffer has local keymaps, too; they contain various completion and exit commands. *Note Intro to Minibuffers::. Emacs has other keymaps that are used in a different way--translating events within `read-key-sequence'. *Note Translation Keymaps::. *Note Standard Keymaps::, for a list of some standard keymaps. -- Function: current-active-maps &optional olp position This returns the list of active keymaps that would be used by the command loop in the current circumstances to look up a key sequence. Normally it ignores `overriding-local-map' and `overriding-terminal-local-map', but if OLP is non-`nil' then it pays attention to them. POSITION can optionally be either an event position as returned by `event-start' or a buffer position, and may change the keymaps as described for `key-binding'. -- Function: key-binding key &optional accept-defaults no-remap position This function returns the binding for KEY according to the current active keymaps. The result is `nil' if KEY is undefined in the keymaps. The argument ACCEPT-DEFAULTS controls checking for default bindings, as in `lookup-key' (*note Functions for Key Lookup::). When commands are remapped (*note Remapping Commands::), `key-binding' normally processes command remappings so as to return the remapped command that will actually be executed. However, if NO-REMAP is non-`nil', `key-binding' ignores remappings and returns the binding directly specified for KEY. If KEY starts with a mouse event (perhaps following a prefix event), the maps to be consulted are determined based on the event's position. Otherwise, they are determined based on the value of point. However, you can override either of them by specifying POSITION. If POSITION is non-`nil', it should be either a buffer position or an event position like the value of `event-start'. Then the maps consulted are determined based on POSITION. An error is signaled if KEY is not a string or a vector. (key-binding "\C-x\C-f") => find-file 22.8 Searching the Active Keymaps ================================= After translation of event subsequences (*note Translation Keymaps::) Emacs looks for them in the active keymaps. Here is a pseudo-Lisp description of the order and conditions for searching them: (or (cond (overriding-terminal-local-map (FIND-IN overriding-terminal-local-map)) (overriding-local-map (FIND-IN overriding-local-map)) ((or (FIND-IN (get-char-property (point) 'keymap)) (FIND-IN TEMP-MAP) (FIND-IN-ANY emulation-mode-map-alists) (FIND-IN-ANY minor-mode-overriding-map-alist) (FIND-IN-ANY minor-mode-map-alist) (if (get-text-property (point) 'local-map) (FIND-IN (get-char-property (point) 'local-map)) (FIND-IN (current-local-map)))))) (FIND-IN (current-global-map))) FIND-IN and FIND-IN-ANY are pseudo functions that search in one keymap and in an alist of keymaps, respectively. (Searching a single keymap for a binding is called "key lookup"; see *note Key Lookup::.) If the key sequence starts with a mouse event, that event's position is used instead of point and the current buffer. Mouse events on an embedded string use non-`nil' text properties from that string instead of the buffer. TEMP-MAP is a pseudo variable that represents the effect of a `set-temporary-overlay-map' call. When a match is found (*note Key Lookup::), if the binding in the keymap is a function, the search is over. However if the keymap entry is a symbol with a value or a string, Emacs replaces the input key sequences with the variable's value or the string, and restarts the search of the active keymaps. The function finally found might also be remapped. *Note Remapping Commands::. 22.9 Controlling the Active Keymaps =================================== -- Variable: global-map This variable contains the default global keymap that maps Emacs keyboard input to commands. The global keymap is normally this keymap. The default global keymap is a full keymap that binds `self-insert-command' to all of the printing characters. It is normal practice to change the bindings in the global keymap, but you should not assign this variable any value other than the keymap it starts out with. -- Function: current-global-map This function returns the current global keymap. This is the same as the value of `global-map' unless you change one or the other. The return value is a reference, not a copy; if you use `define-key' or other functions on it you will alter global bindings. (current-global-map) => (keymap [set-mark-command beginning-of-line ... delete-backward-char]) -- Function: current-local-map This function returns the current buffer's local keymap, or `nil' if it has none. In the following example, the keymap for the `*scratch*' buffer (using Lisp Interaction mode) is a sparse keymap in which the entry for , ASCII code 27, is another sparse keymap. (current-local-map) => (keymap (10 . eval-print-last-sexp) (9 . lisp-indent-line) (127 . backward-delete-char-untabify) (27 keymap (24 . eval-defun) (17 . indent-sexp))) `current-local-map' returns a reference to the local keymap, not a copy of it; if you use `define-key' or other functions on it you will alter local bindings. -- Function: current-minor-mode-maps This function returns a list of the keymaps of currently enabled minor modes. -- Function: use-global-map keymap This function makes KEYMAP the new current global keymap. It returns `nil'. It is very unusual to change the global keymap. -- Function: use-local-map keymap This function makes KEYMAP the new local keymap of the current buffer. If KEYMAP is `nil', then the buffer has no local keymap. `use-local-map' returns `nil'. Most major mode commands use this function. -- Variable: minor-mode-map-alist This variable is an alist describing keymaps that may or may not be active according to the values of certain variables. Its elements look like this: (VARIABLE . KEYMAP) The keymap KEYMAP is active whenever VARIABLE has a non-`nil' value. Typically VARIABLE is the variable that enables or disables a minor mode. *Note Keymaps and Minor Modes::. Note that elements of `minor-mode-map-alist' do not have the same structure as elements of `minor-mode-alist'. The map must be the CDR of the element; a list with the map as the second element will not do. The CDR can be either a keymap (a list) or a symbol whose function definition is a keymap. When more than one minor mode keymap is active, the earlier one in `minor-mode-map-alist' takes priority. But you should design minor modes so that they don't interfere with each other. If you do this properly, the order will not matter. See *note Keymaps and Minor Modes::, for more information about minor modes. See also `minor-mode-key-binding' (*note Functions for Key Lookup::). -- Variable: minor-mode-overriding-map-alist This variable allows major modes to override the key bindings for particular minor modes. The elements of this alist look like the elements of `minor-mode-map-alist': `(VARIABLE . KEYMAP)'. If a variable appears as an element of `minor-mode-overriding-map-alist', the map specified by that element totally replaces any map specified for the same variable in `minor-mode-map-alist'. `minor-mode-overriding-map-alist' is automatically buffer-local in all buffers. -- Variable: overriding-local-map If non-`nil', this variable holds a keymap to use instead of the buffer's local keymap, any text property or overlay keymaps, and any minor mode keymaps. This keymap, if specified, overrides all other maps that would have been active, except for the current global map. -- Variable: overriding-terminal-local-map If non-`nil', this variable holds a keymap to use instead of `overriding-local-map', the buffer's local keymap, text property or overlay keymaps, and all the minor mode keymaps. This variable is always local to the current terminal and cannot be buffer-local. *Note Multiple Terminals::. It is used to implement incremental search mode. -- Variable: overriding-local-map-menu-flag If this variable is non-`nil', the value of `overriding-local-map' or `overriding-terminal-local-map' can affect the display of the menu bar. The default value is `nil', so those map variables have no effect on the menu bar. Note that these two map variables do affect the execution of key sequences entered using the menu bar, even if they do not affect the menu bar display. So if a menu bar key sequence comes in, you should clear the variables before looking up and executing that key sequence. Modes that use the variables would typically do this anyway; normally they respond to events that they do not handle by "unreading" them and exiting. -- Variable: special-event-map This variable holds a keymap for special events. If an event type has a binding in this keymap, then it is special, and the binding for the event is run directly by `read-event'. *Note Special Events::. -- Variable: emulation-mode-map-alists This variable holds a list of keymap alists to use for emulations modes. It is intended for modes or packages using multiple minor-mode keymaps. Each element is a keymap alist which has the same format and meaning as `minor-mode-map-alist', or a symbol with a variable binding which is such an alist. The "active" keymaps in each alist are used before `minor-mode-map-alist' and `minor-mode-overriding-map-alist'. -- Function: set-temporary-overlay-map keymap &optional keep This function adds KEYMAP as a temporary keymap that takes precedence over most other keymaps. It does not take precedence over the "overriding" maps (see above); and unlike them, if no match for a key is found in KEYMAP, the search continues. Normally, KEYMAP is used only once. If the optional argument PRED is `t', the map stays active if a key from KEYMAP is used. PRED can also be a function of no arguments: if it returns non-`nil' then KEYMAP stays active. For a pseudo-Lisp description of exactly how and when this keymap applies, *note Searching Keymaps::. 22.10 Key Lookup ================ "Key lookup" is the process of finding the binding of a key sequence from a given keymap. The execution or use of the binding is not part of key lookup. Key lookup uses just the event type of each event in the key sequence; the rest of the event is ignored. In fact, a key sequence used for key lookup may designate a mouse event with just its types (a symbol) instead of the entire event (a list). *Note Input Events::. Such a "key sequence" is insufficient for `command-execute' to run, but it is sufficient for looking up or rebinding a key. When the key sequence consists of multiple events, key lookup processes the events sequentially: the binding of the first event is found, and must be a keymap; then the second event's binding is found in that keymap, and so on until all the events in the key sequence are used up. (The binding thus found for the last event may or may not be a keymap.) Thus, the process of key lookup is defined in terms of a simpler process for looking up a single event in a keymap. How that is done depends on the type of object associated with the event in that keymap. Let's use the term "keymap entry" to describe the value found by looking up an event type in a keymap. (This doesn't include the item string and other extra elements in a keymap element for a menu item, because `lookup-key' and other key lookup functions don't include them in the returned value.) While any Lisp object may be stored in a keymap as a keymap entry, not all make sense for key lookup. Here is a table of the meaningful types of keymap entries: `nil' `nil' means that the events used so far in the lookup form an undefined key. When a keymap fails to mention an event type at all, and has no default binding, that is equivalent to a binding of `nil' for that event type. COMMAND The events used so far in the lookup form a complete key, and COMMAND is its binding. *Note What Is a Function::. ARRAY The array (either a string or a vector) is a keyboard macro. The events used so far in the lookup form a complete key, and the array is its binding. See *note Keyboard Macros::, for more information. KEYMAP The events used so far in the lookup form a prefix key. The next event of the key sequence is looked up in KEYMAP. LIST The meaning of a list depends on what it contains: * If the CAR of LIST is the symbol `keymap', then the list is a keymap, and is treated as a keymap (see above). * If the CAR of LIST is `lambda', then the list is a lambda expression. This is presumed to be a function, and is treated as such (see above). In order to execute properly as a key binding, this function must be a command--it must have an `interactive' specification. *Note Defining Commands::. * If the CAR of LIST is a keymap and the CDR is an event type, then this is an "indirect entry": (OTHERMAP . OTHERTYPE) When key lookup encounters an indirect entry, it looks up instead the binding of OTHERTYPE in OTHERMAP and uses that. This feature permits you to define one key as an alias for another key. For example, an entry whose CAR is the keymap called `esc-map' and whose CDR is 32 (the code for ) means, "Use the global binding of `Meta-', whatever that may be". SYMBOL The function definition of SYMBOL is used in place of SYMBOL. If that too is a symbol, then this process is repeated, any number of times. Ultimately this should lead to an object that is a keymap, a command, or a keyboard macro. A list is allowed if it is a keymap or a command, but indirect entries are not understood when found via symbols. Note that keymaps and keyboard macros (strings and vectors) are not valid functions, so a symbol with a keymap, string, or vector as its function definition is invalid as a function. It is, however, valid as a key binding. If the definition is a keyboard macro, then the symbol is also valid as an argument to `command-execute' (*note Interactive Call::). The symbol `undefined' is worth special mention: it means to treat the key as undefined. Strictly speaking, the key is defined, and its binding is the command `undefined'; but that command does the same thing that is done automatically for an undefined key: it rings the bell (by calling `ding') but does not signal an error. `undefined' is used in local keymaps to override a global key binding and make the key "undefined" locally. A local binding of `nil' would fail to do this because it would not override the global binding. ANYTHING ELSE If any other type of object is found, the events used so far in the lookup form a complete key, and the object is its binding, but the binding is not executable as a command. In short, a keymap entry may be a keymap, a command, a keyboard macro, a symbol that leads to one of them, or an indirection or `nil'. 22.11 Functions for Key Lookup ============================== Here are the functions and variables pertaining to key lookup. -- Function: lookup-key keymap key &optional accept-defaults This function returns the definition of KEY in KEYMAP. All the other functions described in this chapter that look up keys use `lookup-key'. Here are examples: (lookup-key (current-global-map) "\C-x\C-f") => find-file (lookup-key (current-global-map) (kbd "C-x C-f")) => find-file (lookup-key (current-global-map) "\C-x\C-f12345") => 2 If the string or vector KEY is not a valid key sequence according to the prefix keys specified in KEYMAP, it must be "too long" and have extra events at the end that do not fit into a single key sequence. Then the value is a number, the number of events at the front of KEY that compose a complete key. If ACCEPT-DEFAULTS is non-`nil', then `lookup-key' considers default bindings as well as bindings for the specific events in KEY. Otherwise, `lookup-key' reports only bindings for the specific sequence KEY, ignoring default bindings except when you explicitly ask about them. (To do this, supply `t' as an element of KEY; see *note Format of Keymaps::.) If KEY contains a meta character (not a function key), that character is implicitly replaced by a two-character sequence: the value of `meta-prefix-char', followed by the corresponding non-meta character. Thus, the first example below is handled by conversion into the second example. (lookup-key (current-global-map) "\M-f") => forward-word (lookup-key (current-global-map) "\ef") => forward-word Unlike `read-key-sequence', this function does not modify the specified events in ways that discard information (*note Key Sequence Input::). In particular, it does not convert letters to lower case and it does not change drag events to clicks. -- Command: undefined Used in keymaps to undefine keys. It calls `ding', but does not cause an error. -- Function: local-key-binding key &optional accept-defaults This function returns the binding for KEY in the current local keymap, or `nil' if it is undefined there. The argument ACCEPT-DEFAULTS controls checking for default bindings, as in `lookup-key' (above). -- Function: global-key-binding key &optional accept-defaults This function returns the binding for command KEY in the current global keymap, or `nil' if it is undefined there. The argument ACCEPT-DEFAULTS controls checking for default bindings, as in `lookup-key' (above). -- Function: minor-mode-key-binding key &optional accept-defaults This function returns a list of all the active minor mode bindings of KEY. More precisely, it returns an alist of pairs `(MODENAME . BINDING)', where MODENAME is the variable that enables the minor mode, and BINDING is KEY's binding in that mode. If KEY has no minor-mode bindings, the value is `nil'. If the first binding found is not a prefix definition (a keymap or a symbol defined as a keymap), all subsequent bindings from other minor modes are omitted, since they would be completely shadowed. Similarly, the list omits non-prefix bindings that follow prefix bindings. The argument ACCEPT-DEFAULTS controls checking for default bindings, as in `lookup-key' (above). -- User Option: meta-prefix-char This variable is the meta-prefix character code. It is used for translating a meta character to a two-character sequence so it can be looked up in a keymap. For useful results, the value should be a prefix event (*note Prefix Keys::). The default value is 27, which is the ASCII code for . As long as the value of `meta-prefix-char' remains 27, key lookup translates `M-b' into ` b', which is normally defined as the `backward-word' command. However, if you were to set `meta-prefix-char' to 24, the code for `C-x', then Emacs will translate `M-b' into `C-x b', whose standard binding is the `switch-to-buffer' command. (Don't actually do this!) Here is an illustration of what would happen: meta-prefix-char ; The default value. => 27 (key-binding "\M-b") => backward-word ?\C-x ; The print representation => 24 ; of a character. (setq meta-prefix-char 24) => 24 (key-binding "\M-b") => switch-to-buffer ; Now, typing `M-b' is ; like typing `C-x b'. (setq meta-prefix-char 27) ; Avoid confusion! => 27 ; Restore the default value! This translation of one event into two happens only for characters, not for other kinds of input events. Thus, `M-', a function key, is not converted into ` '. 22.12 Changing Key Bindings =========================== The way to rebind a key is to change its entry in a keymap. If you change a binding in the global keymap, the change is effective in all buffers (though it has no direct effect in buffers that shadow the global binding with a local one). If you change the current buffer's local map, that usually affects all buffers using the same major mode. The `global-set-key' and `local-set-key' functions are convenient interfaces for these operations (*note Key Binding Commands::). You can also use `define-key', a more general function; then you must explicitly specify the map to change. When choosing the key sequences for Lisp programs to rebind, please follow the Emacs conventions for use of various keys (*note Key Binding Conventions::). In writing the key sequence to rebind, it is good to use the special escape sequences for control and meta characters (*note String Type::). The syntax `\C-' means that the following character is a control character and `\M-' means that the following character is a meta character. Thus, the string `"\M-x"' is read as containing a single `M-x', `"\C-f"' is read as containing a single `C-f', and `"\M-\C-x"' and `"\C-\M-x"' are both read as containing a single `C-M-x'. You can also use this escape syntax in vectors, as well as others that aren't allowed in strings; one example is `[?\C-\H-x home]'. *Note Character Type::. The key definition and lookup functions accept an alternate syntax for event types in a key sequence that is a vector: you can use a list containing modifier names plus one base event (a character or function key name). For example, `(control ?a)' is equivalent to `?\C-a' and `(hyper control left)' is equivalent to `C-H-left'. One advantage of such lists is that the precise numeric codes for the modifier bits don't appear in compiled files. The functions below signal an error if KEYMAP is not a keymap, or if KEY is not a string or vector representing a key sequence. You can use event types (symbols) as shorthand for events that are lists. The `kbd' function (*note Key Sequences::) is a convenient way to specify the key sequence. -- Function: define-key keymap key binding This function sets the binding for KEY in KEYMAP. (If KEY is more than one event long, the change is actually made in another keymap reached from KEYMAP.) The argument BINDING can be any Lisp object, but only certain types are meaningful. (For a list of meaningful types, see *note Key Lookup::.) The value returned by `define-key' is BINDING. If KEY is `[t]', this sets the default binding in KEYMAP. When an event has no binding of its own, the Emacs command loop uses the keymap's default binding, if there is one. Every prefix of KEY must be a prefix key (i.e., bound to a keymap) or undefined; otherwise an error is signaled. If some prefix of KEY is undefined, then `define-key' defines it as a prefix key so that the rest of KEY can be defined as specified. If there was previously no binding for KEY in KEYMAP, the new binding is added at the beginning of KEYMAP. The order of bindings in a keymap makes no difference for keyboard input, but it does matter for menu keymaps (*note Menu Keymaps::). This example creates a sparse keymap and makes a number of bindings in it: (setq map (make-sparse-keymap)) => (keymap) (define-key map "\C-f" 'forward-char) => forward-char map => (keymap (6 . forward-char)) ;; Build sparse submap for `C-x' and bind `f' in that. (define-key map (kbd "C-x f") 'forward-word) => forward-word map => (keymap (24 keymap ; C-x (102 . forward-word)) ; f (6 . forward-char)) ; C-f ;; Bind `C-p' to the `ctl-x-map'. (define-key map (kbd "C-p") ctl-x-map) ;; `ctl-x-map' => [nil ... find-file ... backward-kill-sentence] ;; Bind `C-f' to `foo' in the `ctl-x-map'. (define-key map (kbd "C-p C-f") 'foo) => 'foo map => (keymap ; Note `foo' in `ctl-x-map'. (16 keymap [nil ... foo ... backward-kill-sentence]) (24 keymap (102 . forward-word)) (6 . forward-char)) Note that storing a new binding for `C-p C-f' actually works by changing an entry in `ctl-x-map', and this has the effect of changing the bindings of both `C-p C-f' and `C-x C-f' in the default global map. The function `substitute-key-definition' scans a keymap for keys that have a certain binding and rebinds them with a different binding. Another feature which is cleaner and can often produce the same results to remap one command into another (*note Remapping Commands::). -- Function: substitute-key-definition olddef newdef keymap &optional oldmap This function replaces OLDDEF with NEWDEF for any keys in KEYMAP that were bound to OLDDEF. In other words, OLDDEF is replaced with NEWDEF wherever it appears. The function returns `nil'. For example, this redefines `C-x C-f', if you do it in an Emacs with standard bindings: (substitute-key-definition 'find-file 'find-file-read-only (current-global-map)) If OLDMAP is non-`nil', that changes the behavior of `substitute-key-definition': the bindings in OLDMAP determine which keys to rebind. The rebindings still happen in KEYMAP, not in OLDMAP. Thus, you can change one map under the control of the bindings in another. For example, (substitute-key-definition 'delete-backward-char 'my-funny-delete my-map global-map) puts the special deletion command in `my-map' for whichever keys are globally bound to the standard deletion command. Here is an example showing a keymap before and after substitution: (setq map '(keymap (?1 . olddef-1) (?2 . olddef-2) (?3 . olddef-1))) => (keymap (49 . olddef-1) (50 . olddef-2) (51 . olddef-1)) (substitute-key-definition 'olddef-1 'newdef map) => nil map => (keymap (49 . newdef) (50 . olddef-2) (51 . newdef)) -- Function: suppress-keymap keymap &optional nodigits This function changes the contents of the full keymap KEYMAP by remapping `self-insert-command' to the command `undefined' (*note Remapping Commands::). This has the effect of undefining all printing characters, thus making ordinary insertion of text impossible. `suppress-keymap' returns `nil'. If NODIGITS is `nil', then `suppress-keymap' defines digits to run `digit-argument', and `-' to run `negative-argument'. Otherwise it makes them undefined like the rest of the printing characters. The `suppress-keymap' function does not make it impossible to modify a buffer, as it does not suppress commands such as `yank' and `quoted-insert'. To prevent any modification of a buffer, make it read-only (*note Read Only Buffers::). Since this function modifies KEYMAP, you would normally use it on a newly created keymap. Operating on an existing keymap that is used for some other purpose is likely to cause trouble; for example, suppressing `global-map' would make it impossible to use most of Emacs. This function can be used to initialize the local keymap of a major mode for which insertion of text is not desirable. But usually such a mode should be derived from `special-mode' (*note Basic Major Modes::); then its keymap will automatically inh