Emacs Lisp
This is edition 3.0 of the GNU Emacs Lisp Reference Manual,
corresponding to Emacs version 23.3.
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Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with the Invariant Sections being “GNU General Public License,” with the Front-Cover texts being “A GNU Manual,” and with the Back-Cover Texts as in (a) below. A copy of the license is included in the section entitled “GNU Free Documentation License.”(a) The FSF's Back-Cover Text is: “You have the freedom to copy and modify this GNU manual. Buying copies from the FSF supports it in developing GNU and promoting software freedom.”
| Introduction | Introduction and conventions used. |
| Lisp Data Types | Data types of objects in Emacs Lisp. |
| Numbers | Numbers and arithmetic functions. |
| Strings and Characters | Strings, and functions that work on them. |
| Lists | Lists, cons cells, and related functions. |
| Sequences Arrays Vectors | Lists, strings and vectors are called sequences. Certain functions act on any kind of sequence. The description of vectors is here as well. |
| Hash Tables | Very fast lookup-tables. |
| Symbols | Symbols represent names, uniquely. |
| Evaluation | How Lisp expressions are evaluated. |
| Control Structures | Conditionals, loops, nonlocal exits. |
| Variables | Using symbols in programs to stand for values. |
| Functions | A function is a Lisp program that can be invoked from other functions. |
| Macros | Macros are a way to extend the Lisp language. |
| Customization | Writing customization declarations. |
| Loading | Reading files of Lisp code into Lisp. |
| Byte Compilation | Compilation makes programs run faster. |
| Advising Functions | Adding to the definition of a function. |
| Debugging | Tools and tips for debugging Lisp programs. |
| Read and Print | Converting Lisp objects to text and back. |
| Minibuffers | Using the minibuffer to read input. |
| Command Loop | How the editor command loop works, and how you can call its subroutines. |
| Keymaps | Defining the bindings from keys to commands. |
| Modes | Defining major and minor modes. |
| Documentation | Writing and using documentation strings. |
| Files | Accessing files. |
| Backups and Auto-Saving | Controlling how backups and auto-save files are made. |
| Buffers | Creating and using buffer objects. |
| Windows | Manipulating windows and displaying buffers. |
| Frames | Making multiple system-level windows. |
| Positions | Buffer positions and motion functions. |
| Markers | Markers represent positions and update automatically when the text is changed. |
| Text | Examining and changing text in buffers. |
| Non-ASCII Characters | Non-ASCII text in buffers and strings. |
| Searching and Matching | Searching buffers for strings or regexps. |
| Syntax Tables | The syntax table controls word and list parsing. |
| Abbrevs | How Abbrev mode works, and its data structures. |
| Processes | Running and communicating with subprocesses. |
| Display | Features for controlling the screen display. |
| System Interface | Getting the user id, system type, environment variables, and other such things. |
| Appendices | |
|---|---|
| Antinews | Info for users downgrading to Emacs 22. |
| GNU Free Documentation License | The license for this documentation. |
| GPL | Conditions for copying and changing GNU Emacs. |
| Tips | Advice and coding conventions for Emacs Lisp. |
| GNU Emacs Internals | Building and dumping Emacs; internal data structures. |
| Standard Errors | List of all error symbols. |
| Standard Buffer-Local Variables | List of variables buffer-local in all buffers. |
| Standard Keymaps | List of standard keymaps. |
| Standard Hooks | List of standard hook variables. |
| Index | Index including concepts, functions, variables, and other terms. |
Detailed Node Listing
Here are other nodes that are inferiors of those already listed, mentioned here so you can get to them in one step:
| Introduction | |
|---|---|
| Caveats | Flaws and a request for help. |
| Lisp History | Emacs Lisp is descended from Maclisp. |
| Conventions | How the manual is formatted. |
| Version Info | Which Emacs version is running? |
| Acknowledgements | The authors, editors, and sponsors of this manual. |
| Conventions | |
| Some Terms | Explanation of terms we use in this manual. |
| nil and t | How the symbols nil and t are used. |
| Evaluation Notation | The format we use for examples of evaluation. |
| Printing Notation | The format we use when examples print text. |
| Error Messages | The format we use for examples of errors. |
| Buffer Text Notation | The format we use for buffer contents in examples. |
| Format of Descriptions | Notation for describing functions, variables, etc. |
| Format of Descriptions | |
| A Sample Function Description | A description of an imaginary
function, foo. |
| A Sample Variable Description | A description of an imaginary
variable, electric-future-map.
|
| Lisp Data Types | |
| Printed Representation | How Lisp objects are represented as text. |
| Comments | Comments and their formatting conventions. |
| Programming Types | Types found in all Lisp systems. |
| Editing Types | Types specific to Emacs. |
| Circular Objects | Read syntax for circular structure. |
| Type Predicates | Tests related to types. |
| Equality Predicates | Tests of equality between any two objects. |
| Programming Types | |
| Integer Type | Numbers without fractional parts. |
| Floating Point Type | Numbers with fractional parts and with a large range. |
| Character Type | The representation of letters, numbers and control characters. |
| Symbol Type | A multi-use object that refers to a function, variable, or property list, and has a unique identity. |
| Sequence Type | Both lists and arrays are classified as sequences. |
| Cons Cell Type | Cons cells, and lists (which are made from cons cells). |
| Array Type | Arrays include strings and vectors. |
| String Type | An (efficient) array of characters. |
| Vector Type | One-dimensional arrays. |
| Char-Table Type | One-dimensional sparse arrays indexed by characters. |
| Bool-Vector Type | One-dimensional arrays of t or nil. |
| Hash Table Type | Super-fast lookup tables. |
| Function Type | A piece of executable code you can call from elsewhere. |
| Macro Type | A method of expanding an expression into another expression, more fundamental but less pretty. |
| Primitive Function Type | A function written in C, callable from Lisp. |
| Byte-Code Type | A function written in Lisp, then compiled. |
| Autoload Type | A type used for automatically loading seldom-used functions. |
| Character Type | |
| Basic Char Syntax | Syntax for regular characters. |
| General Escape Syntax | How to specify characters by their codes. |
| Ctl-Char Syntax | Syntax for control characters. |
| Meta-Char Syntax | Syntax for meta-characters. |
| Other Char Bits | Syntax for hyper-, super-, and alt-characters. |
| Cons Cell and List Types | |
| Box Diagrams | Drawing pictures of lists. |
| Dotted Pair Notation | A general syntax for cons cells. |
| Association List Type | A specially constructed list. |
| String Type | |
| Syntax for Strings | How to specify Lisp strings. |
| Non-ASCII in Strings | International characters in strings. |
| Nonprinting Characters | Literal unprintable characters in strings. |
| Text Props and Strings | Strings with text properties. |
| Editing Types | |
| Buffer Type | The basic object of editing. |
| Marker Type | A position in a buffer. |
| Window Type | Buffers are displayed in windows. |
| Frame Type | Windows subdivide frames. |
| Terminal Type | A terminal device displays frames. |
| Window Configuration Type | Recording the way a frame is subdivided. |
| Frame Configuration Type | Recording the status of all frames. |
| Process Type | A subprocess of Emacs running on the underlying OS. |
| Stream Type | Receive or send characters. |
| Keymap Type | What function a keystroke invokes. |
| Overlay Type | How an overlay is represented. |
| Font Type | Fonts for displaying text. |
| Numbers | |
| Integer Basics | Representation and range of integers. |
| Float Basics | Representation and range of floating point. |
| Predicates on Numbers | Testing for numbers. |
| Comparison of Numbers | Equality and inequality predicates. |
| Numeric Conversions | Converting float to integer and vice versa. |
| Arithmetic Operations | How to add, subtract, multiply and divide. |
| Rounding Operations | Explicitly rounding floating point numbers. |
| Bitwise Operations | Logical and, or, not, shifting. |
| Math Functions | Trig, exponential and logarithmic functions. |
| Random Numbers | Obtaining random integers, predictable or not. |
| Strings and Characters | |
| String Basics | Basic properties of strings and characters. |
| Predicates for Strings | Testing whether an object is a string or char. |
| Creating Strings | Functions to allocate new strings. |
| Modifying Strings | Altering the contents of an existing string. |
| Text Comparison | Comparing characters or strings. |
| String Conversion | Converting to and from characters and strings. |
| Formatting Strings | format: Emacs's analogue of printf. |
| Case Conversion | Case conversion functions. |
| Case Tables | Customizing case conversion. |
| Lists | |
| Cons Cells | How lists are made out of cons cells. |
| List-related Predicates | Is this object a list? Comparing two lists. |
| List Elements | Extracting the pieces of a list. |
| Building Lists | Creating list structure. |
| List Variables | Modifying lists stored in variables. |
| Modifying Lists | Storing new pieces into an existing list. |
| Sets And Lists | A list can represent a finite mathematical set. |
| Association Lists | A list can represent a finite relation or mapping. |
| Rings | Managing a fixed-size ring of objects. |
| Modifying Existing List Structure | |
| Setcar | Replacing an element in a list. |
| Setcdr | Replacing part of the list backbone. This can be used to remove or add elements. |
| Rearrangement | Reordering the elements in a list; combining lists. |
| Sequences, Arrays, and Vectors | |
| Sequence Functions | Functions that accept any kind of sequence. |
| Arrays | Characteristics of arrays in Emacs Lisp. |
| Array Functions | Functions specifically for arrays. |
| Vectors | Special characteristics of Emacs Lisp vectors. |
| Vector Functions | Functions specifically for vectors. |
| Char-Tables | How to work with char-tables. |
| Bool-Vectors | How to work with bool-vectors. |
| Hash Tables | |
| Creating Hash | Functions to create hash tables. |
| Hash Access | Reading and writing the hash table contents. |
| Defining Hash | Defining new comparison methods. |
| Other Hash | Miscellaneous. |
| Symbols | |
| Symbol Components | Symbols have names, values, function definitions and property lists. |
| Definitions | A definition says how a symbol will be used. |
| Creating Symbols | How symbols are kept unique. |
| Property Lists | Each symbol has a property list for recording miscellaneous information. |
| Property Lists | |
| Plists and Alists | Comparison of the advantages of property lists and association lists. |
| Symbol Plists | Functions to access symbols' property lists. |
| Other Plists | Accessing property lists stored elsewhere. |
| Evaluation | |
| Intro Eval | Evaluation in the scheme of things. |
| Forms | How various sorts of objects are evaluated. |
| Quoting | Avoiding evaluation (to put constants in the program). |
| Eval | How to invoke the Lisp interpreter explicitly. |
| Kinds of Forms | |
| Self-Evaluating Forms | Forms that evaluate to themselves. |
| Symbol Forms | Symbols evaluate as variables. |
| Classifying Lists | How to distinguish various sorts of list forms. |
| Function Indirection | When a symbol appears as the car of a list, we find the real function via the symbol. |
| Function Forms | Forms that call functions. |
| Macro Forms | Forms that call macros. |
| Special Forms | "Special forms" are idiosyncratic primitives, most of them extremely important. |
| Autoloading | Functions set up to load files containing their real definitions. |
| Control Structures | |
| Sequencing | Evaluation in textual order. |
| Conditionals | if, cond, when, unless. |
| Combining Conditions | and, or, not. |
| Iteration | while loops. |
| Nonlocal Exits | Jumping out of a sequence. |
| Nonlocal Exits | |
| Catch and Throw | Nonlocal exits for the program's own purposes. |
| Examples of Catch | Showing how such nonlocal exits can be written. |
| Errors | How errors are signaled and handled. |
| Cleanups | Arranging to run a cleanup form if an error happens. |
| Errors | |
| Signaling Errors | How to report an error. |
| Processing of Errors | What Emacs does when you report an error. |
| Handling Errors | How you can trap errors and continue execution. |
| Error Symbols | How errors are classified for trapping them. |
| Variables | |
| Global Variables | Variable values that exist permanently, everywhere. |
| Constant Variables | Certain "variables" have values that never change. |
| Local Variables | Variable values that exist only temporarily. |
| Void Variables | Symbols that lack values. |
| Defining Variables | A definition says a symbol is used as a variable. |
| Tips for Defining | Things you should think about when you define a variable. |
| Accessing Variables | Examining values of variables whose names are known only at run time. |
| Setting Variables | Storing new values in variables. |
| Variable Scoping | How Lisp chooses among local and global values. |
| Buffer-Local Variables | Variable values in effect only in one buffer. |
| File Local Variables | Handling local variable lists in files. |
| Directory Local Variables | Local variables common to all files in a directory. |
| Frame-Local Variables | Frame-local bindings for variables. |
| Variable Aliases | Variables that are aliases for other variables. |
| Variables with Restricted Values | Non-constant variables whose value can not be an arbitrary Lisp object. |
| Scoping Rules for Variable Bindings | |
| Scope | Scope means where in the program a value is visible. Comparison with other languages. |
| Extent | Extent means how long in time a value exists. |
| Impl of Scope | Two ways to implement dynamic scoping. |
| Using Scoping | How to use dynamic scoping carefully and avoid problems. |
| Buffer-Local Variables | |
| Intro to Buffer-Local | Introduction and concepts. |
| Creating Buffer-Local | Creating and destroying buffer-local bindings. |
| Default Value | The default value is seen in buffers that don't have their own buffer-local values. |
| Functions | |
| What Is a Function | Lisp functions vs. primitives; terminology. |
| Lambda Expressions | How functions are expressed as Lisp objects. |
| Function Names | A symbol can serve as the name of a function. |
| Defining Functions | Lisp expressions for defining functions. |
| Calling Functions | How to use an existing function. |
| Mapping Functions | Applying a function to each element of a list, etc. |
| Anonymous Functions | Lambda expressions are functions with no names. |
| Function Cells | Accessing or setting the function definition of a symbol. |
| Obsolete Functions | Declaring functions obsolete. |
| Inline Functions | Defining functions that the compiler will open code. |
| Declaring Functions | Telling the compiler that a function is defined. |
| Function Safety | Determining whether a function is safe to call. |
| Related Topics | Cross-references to specific Lisp primitives that have a special bearing on how functions work. |
| Lambda Expressions | |
| Lambda Components | The parts of a lambda expression. |
| Simple Lambda | A simple example. |
| Argument List | Details and special features of argument lists. |
| Function Documentation | How to put documentation in a function. |
| Macros | |
| Simple Macro | A basic example. |
| Expansion | How, when and why macros are expanded. |
| Compiling Macros | How macros are expanded by the compiler. |
| Defining Macros | How to write a macro definition. |
| Backquote | Easier construction of list structure. |
| Problems with Macros | Don't evaluate the macro arguments too many times. Don't hide the user's variables. |
| Indenting Macros | Specifying how to indent macro calls. |
| Common Problems Using Macros | |
| Wrong Time | Do the work in the expansion, not in the macro. |
| Argument Evaluation | The expansion should evaluate each macro arg once. |
| Surprising Local Vars | Local variable bindings in the expansion require special care. |
| Eval During Expansion | Don't evaluate them; put them in the expansion. |
| Repeated Expansion | Avoid depending on how many times expansion is done. |
| Writing Customization Definitions | |
| Common Keywords | Common keyword arguments for all kinds of customization declarations. |
| Group Definitions | Writing customization group definitions. |
| Variable Definitions | Declaring user options. |
| Customization Types | Specifying the type of a user option. |
| Customization Types | |
| Simple Types | Simple customization types: sexp, integer, number, string, file, directory, alist. |
| Composite Types | Build new types from other types or data. |
| Splicing into Lists | Splice elements into list with :inline. |
| Type Keywords | Keyword-argument pairs in a customization type. |
| Defining New Types | Give your type a name. |
| Loading | |
| How Programs Do Loading | The load function and others. |
| Load Suffixes | Details about the suffixes that load tries. |
| Library Search | Finding a library to load. |
| Loading Non-ASCII | Non-ASCII characters in Emacs Lisp files. |
| Autoload | Setting up a function to autoload. |
| Repeated Loading | Precautions about loading a file twice. |
| Named Features | Loading a library if it isn't already loaded. |
| Where Defined | Finding which file defined a certain symbol. |
| Unloading | How to "unload" a library that was loaded. |
| Hooks for Loading | Providing code to be run when particular libraries are loaded. |
| Byte Compilation | |
| Speed of Byte-Code | An example of speedup from byte compilation. |
| Compilation Functions | Byte compilation functions. |
| Docs and Compilation | Dynamic loading of documentation strings. |
| Dynamic Loading | Dynamic loading of individual functions. |
| Eval During Compile | Code to be evaluated when you compile. |
| Compiler Errors | Handling compiler error messages. |
| Byte-Code Objects | The data type used for byte-compiled functions. |
| Disassembly | Disassembling byte-code; how to read byte-code. |
| Advising Emacs Lisp Functions | |
| Simple Advice | A simple example to explain the basics of advice. |
| Defining Advice | Detailed description of defadvice. |
| Around-Advice | Wrapping advice around a function's definition. |
| Computed Advice | ...is to defadvice as fset is to defun. |
| Activation of Advice | Advice doesn't do anything until you activate it. |
| Enabling Advice | You can enable or disable each piece of advice. |
| Preactivation | Preactivation is a way of speeding up the loading of compiled advice. |
| Argument Access in Advice | How advice can access the function's arguments. |
| Advising Primitives | Accessing arguments when advising a primitive. |
| Combined Definition | How advice is implemented. |
| Debugging Lisp Programs | |
| Debugger | How the Emacs Lisp debugger is implemented. |
| Edebug | A source-level Emacs Lisp debugger. |
| Syntax Errors | How to find syntax errors. |
| Test Coverage | Ensuring you have tested all branches in your code. |
| Compilation Errors | How to find errors that show up in byte compilation. |
| The Lisp Debugger | |
| Error Debugging | Entering the debugger when an error happens. |
| Infinite Loops | Stopping and debugging a program that doesn't exit. |
| Function Debugging | Entering it when a certain function is called. |
| Explicit Debug | Entering it at a certain point in the program. |
| Using Debugger | What the debugger does; what you see while in it. |
| Debugger Commands | Commands used while in the debugger. |
| Invoking the Debugger | How to call the function debug. |
| Internals of Debugger | Subroutines of the debugger, and global variables. |
| Edebug | |
| Using Edebug | Introduction to use of Edebug. |
| Instrumenting | You must instrument your code in order to debug it with Edebug. |
| Edebug Execution Modes | Execution modes, stopping more or less often. |
| Jumping | Commands to jump to a specified place. |
| Edebug Misc | Miscellaneous commands. |
| Breaks | Setting breakpoints to make the program stop. |
| Trapping Errors | Trapping errors with Edebug. |
| Edebug Views | Views inside and outside of Edebug. |
| Edebug Eval | Evaluating expressions within Edebug. |
| Eval List | Expressions whose values are displayed each time you enter Edebug. |
| Printing in Edebug | Customization of printing. |
| Trace Buffer | How to produce trace output in a buffer. |
| Coverage Testing | How to test evaluation coverage. |
| The Outside Context | Data that Edebug saves and restores. |
| Edebug and Macros | Specifying how to handle macro calls. |
| Edebug Options | Option variables for customizing Edebug. |
| Breaks | |
| Breakpoints | Breakpoints at stop points. |
| Global Break Condition | Breaking on an event. |
| Source Breakpoints | Embedding breakpoints in source code. |
| The Outside Context | |
| Checking Whether to Stop | When Edebug decides what to do. |
| Edebug Display Update | When Edebug updates the display. |
| Edebug Recursive Edit | When Edebug stops execution. |
| Edebug and Macros | |
| Instrumenting Macro Calls | The basic problem. |
| Specification List | How to specify complex patterns of evaluation. |
| Backtracking | What Edebug does when matching fails. |
| Specification Examples | To help understand specifications. |
| Debugging Invalid Lisp Syntax | |
| Excess Open | How to find a spurious open paren or missing close. |
| Excess Close | How to find a spurious close paren or missing open. |
| Reading and Printing Lisp Objects | |
| Streams Intro | Overview of streams, reading and printing. |
| Input Streams | Various data types that can be used as input streams. |
| Input Functions | Functions to read Lisp objects from text. |
| Output Streams | Various data types that can be used as output streams. |
| Output Functions | Functions to print Lisp objects as text. |
| Output Variables | Variables that control what the printing functions do. |
| Minibuffers | |
| Intro to Minibuffers | Basic information about minibuffers. |
| Text from Minibuffer | How to read a straight text string. |
| Object from Minibuffer | How to read a Lisp object or expression. |
| Minibuffer History | Recording previous minibuffer inputs so the user can reuse them. |
| Initial Input | Specifying initial contents for the minibuffer. |
| Completion | How to invoke and customize completion. |
| Yes-or-No Queries | Asking a question with a simple answer. |
| Multiple Queries | Asking a series of similar questions. |
| Reading a Password | Reading a password from the terminal. |
| Minibuffer Commands | Commands used as key bindings in minibuffers. |
| Minibuffer Contents | How such commands access the minibuffer text. |
| Minibuffer Windows | Operating on the special minibuffer windows. |
| Recursive Mini | Whether recursive entry to minibuffer is allowed. |
| Minibuffer Misc | Various customization hooks and variables. |
| Completion | |
| Basic Completion | Low-level functions for completing strings. |
| Minibuffer Completion | Invoking the minibuffer with completion. |
| Completion Commands | Minibuffer commands that do completion. |
| High-Level Completion | Convenient special cases of completion (reading buffer name, file name, etc.). |
| Reading File Names | Using completion to read file names and shell commands. |
| Completion Styles | Specifying rules for performing completion. |
| Programmed Completion | Writing your own completion-function. |
| Command Loop | |
| Command Overview | How the command loop reads commands. |
| Defining Commands | Specifying how a function should read arguments. |
| Interactive Call | Calling a command, so that it will read arguments. |
| Distinguish Interactive | Making a command distinguish interactive calls. |
| Command Loop Info | Variables set by the command loop for you to examine. |
| Adjusting Point | Adjustment of point after a command. |
| Input Events | What input looks like when you read it. |
| Reading Input | How to read input events from the keyboard or mouse. |
| Special Events | Events processed immediately and individually. |
| Waiting | Waiting for user input or elapsed time. |
| Quitting | How C-g works. How to catch or defer quitting. |
| Prefix Command Arguments | How the commands to set prefix args work. |
| Recursive Editing | Entering a recursive edit, and why you usually shouldn't. |
| Disabling Commands | How the command loop handles disabled commands. |
| Command History | How the command history is set up, and how accessed. |
| Keyboard Macros | How keyboard macros are implemented. |
| Defining Commands | |
| Using Interactive | General rules for interactive. |
| Interactive Codes | The standard letter-codes for reading arguments in various ways. |
| Interactive Examples | Examples of how to read interactive arguments. |
| Input Events | |
| Keyboard Events | Ordinary characters--keys with symbols on them. |
| Function Keys | Function keys--keys with names, not symbols. |
| Mouse Events | Overview of mouse events. |
| Click Events | Pushing and releasing a mouse button. |
| Drag Events | Moving the mouse before releasing the button. |
| Button-Down Events | A button was pushed and not yet released. |
| Repeat Events | Double and triple click (or drag, or down). |
| Motion Events | Just moving the mouse, not pushing a button. |
| Focus Events | Moving the mouse between frames. |
| Misc Events | Other events the system can generate. |
| Event Examples | Examples of the lists for mouse events. |
| Classifying Events | Finding the modifier keys in an event symbol. Event types. |
| Accessing Mouse | Functions to extract info from mouse events. |
| Accessing Scroll | Functions to get info from scroll bar events. |
| Strings of Events | Special considerations for putting keyboard character events in a string. |
| Reading Input | |
| Key Sequence Input | How to read one key sequence. |
| Reading One Event | How to read just one event. |
| Event Mod | How Emacs modifies events as they are read. |
| Invoking the Input Method | How reading an event uses the input method. |
| Quoted Character Input | Asking the user to specify a character. |
| Event Input Misc | How to reread or throw away input events. |
| Keymaps | |
| Key Sequences | Key sequences as Lisp objects. |
| Keymap Basics | Basic concepts of keymaps. |
| Format of Keymaps | What a keymap looks like as a Lisp object. |
| Creating Keymaps | Functions to create and copy keymaps. |
| Inheritance and Keymaps | How one keymap can inherit the bindings of another keymap. |
| Prefix Keys | Defining a key with a keymap as its definition. |
| Active Keymaps | How Emacs searches the active keymaps for a key binding. |
| Searching Keymaps | A pseudo-Lisp summary of searching active maps. |
| Controlling Active Maps | Each buffer has a local keymap to override the standard (global) bindings. A minor mode can also override them. |
| Key Lookup | Finding a key's binding in one keymap. |
| Functions for Key Lookup | How to request key lookup. |
| Changing Key Bindings | Redefining a key in a keymap. |
| Remapping Commands | A keymap can translate one command to another. |
| Translation Keymaps | Keymaps for translating sequences of events. |
| Key Binding Commands | Interactive interfaces for redefining keys. |
| Scanning Keymaps | Looking through all keymaps, for printing help. |
| Menu Keymaps | Defining a menu as a keymap. |
| Menu Keymaps | |
| Defining Menus | How to make a keymap that defines a menu. |
| Mouse Menus | How users actuate the menu with the mouse. |
| Keyboard Menus | How users actuate the menu with the keyboard. |
| Menu Example | Making a simple menu. |
| Menu Bar | How to customize the menu bar. |
| Tool Bar | A tool bar is a row of images. |
| Modifying Menus | How to add new items to a menu. |
| Defining Menus | |
| Simple Menu Items | A simple kind of menu key binding, limited in capabilities. |
| Extended Menu Items | More powerful menu item definitions let you specify keywords to enable various features. |
| Menu Separators | Drawing a horizontal line through a menu. |
| Alias Menu Items | Using command aliases in menu items. |
| Major and Minor Modes | |
| Hooks | How to use hooks; how to write code that provides hooks. |
| Major Modes | Defining major modes. |
| Minor Modes | Defining minor modes. |
| Mode Line Format | Customizing the text that appears in the mode line. |
| Imenu | How a mode can provide a menu of definitions in the buffer. |
| Font Lock Mode | How modes can highlight text according to syntax. |
| Desktop Save Mode | How modes can have buffer state saved between Emacs sessions. |
| Hooks | |
| Running Hooks | How to run a hook. |
| Setting Hooks | How to put functions on a hook, or remove them. |
| Major Modes | |
| Major Mode Basics | |
| Major Mode Conventions | Coding conventions for keymaps, etc. |
| Auto Major Mode | How Emacs chooses the major mode automatically. |
| Mode Help | Finding out how to use a mode. |
| Derived Modes | Defining a new major mode based on another major mode. |
| Generic Modes | Defining a simple major mode that supports comment syntax and Font Lock mode. |
| Mode Hooks | Hooks run at the end of major mode functions. |
| Example Major Modes | Text mode and Lisp modes. |
| Minor Modes | |
| Minor Mode Conventions | Tips for writing a minor mode. |
| Keymaps and Minor Modes | How a minor mode can have its own keymap. |
| Defining Minor Modes | A convenient facility for defining minor modes. |
| Mode Line Format | |
| Mode Line Basics | Basic ideas of mode line control. |
| Mode Line Data | The data structure that controls the mode line. |
| Mode Line Top | The top level variable, mode-line-format. |
| Mode Line Variables | Variables used in that data structure. |
| %-Constructs | Putting information into a mode line. |
| Properties in Mode | Using text properties in the mode line. |
| Header Lines | Like a mode line, but at the top. |
| Emulating Mode Line | Formatting text as the mode line would. |
| Font Lock Mode | |
| Font Lock Basics | Overview of customizing Font Lock. |
| Search-based Fontification | Fontification based on regexps. |
| Customizing Keywords | Customizing search-based fontification. |
| Other Font Lock Variables | Additional customization facilities. |
| Levels of Font Lock | Each mode can define alternative levels so that the user can select more or less. |
| Precalculated Fontification | How Lisp programs that produce the buffer contents can also specify how to fontify it. |
| Faces for Font Lock | Special faces specifically for Font Lock. |
| Syntactic Font Lock | Fontification based on syntax tables. |
| Setting Syntax Properties | Defining character syntax based on context using the Font Lock mechanism. |
| Multiline Font Lock | How to coerce Font Lock into properly highlighting multiline constructs. |
| Multiline Font Lock Constructs | |
| Font Lock Multiline | Marking multiline chunks with a text property. |
| Region to Fontify | Controlling which region gets refontified after a buffer change. |
| Documentation | |
| Documentation Basics | Good style for doc strings. Where to put them. How Emacs stores them. |
| Accessing Documentation | How Lisp programs can access doc strings. |
| Keys in Documentation | Substituting current key bindings. |
| Describing Characters | Making printable descriptions of non-printing characters and key sequences. |
| Help Functions | Subroutines used by Emacs help facilities. |
| Files | |
| Visiting Files | Reading files into Emacs buffers for editing. |
| Saving Buffers | Writing changed buffers back into files. |
| Reading from Files | Reading files into buffers without visiting. |
| Writing to Files | Writing new files from parts of buffers. |
| File Locks | Locking and unlocking files, to prevent simultaneous editing by two people. |
| Information about Files | Testing existence, accessibility, size of files. |
| Changing Files | Renaming files, changing protection, etc. |
| File Names | Decomposing and expanding file names. |
| Contents of Directories | Getting a list of the files in a directory. |
| Create/Delete Dirs | Creating and Deleting Directories. |
| Magic File Names | Defining "magic" special handling for certain file names. |
| Format Conversion | Conversion to and from various file formats. |
| Visiting Files | |
| Visiting Functions | The usual interface functions for visiting. |
| Subroutines of Visiting | Lower-level subroutines that they use. |
| Information about Files | |
| Testing Accessibility | Is a given file readable? Writable? |
| Kinds of Files | Is it a directory? A symbolic link? |
| Truenames | Eliminating symbolic links from a file name. |
| File Attributes | How large is it? Any other names? Etc. |
| Locating Files | How to find a file in standard places. |
| File Names | |
| File Name Components | The directory part of a file name, and the rest. |
| Relative File Names | Some file names are relative to a current directory. |
| Directory Names | A directory's name as a directory is different from its name as a file. |
| File Name Expansion | Converting relative file names to absolute ones. |
| Unique File Names | Generating names for temporary files. |
| File Name Completion | Finding the completions for a given file name. |
| Standard File Names | If your package uses a fixed file name, how to handle various operating systems simply. |
| File Format Conversion | |
| Format Conversion Overview | insert-file-contents and write-region. |
| Format Conversion Round-Trip | Using format-alist. |
| Format Conversion Piecemeal | Specifying non-paired conversion. |
| Backups and Auto-Saving | |
| Backup Files | How backup files are made; how their names are chosen. |
| Auto-Saving | How auto-save files are made; how their names are chosen. |
| Reverting | revert-buffer, and how to customize
what it does.
|
| Backup Files | |
| Making Backups | How Emacs makes backup files, and when. |
| Rename or Copy | Two alternatives: renaming the old file or copying it. |
| Numbered Backups | Keeping multiple backups for each source file. |
| Backup Names | How backup file names are computed; customization. |
| Buffers | |
| Buffer Basics | What is a buffer? |
| Current Buffer | Designating a buffer as current so that primitives will access its contents. |
| Buffer Names | Accessing and changing buffer names. |
| Buffer File Name | The buffer file name indicates which file is visited. |
| Buffer Modification | A buffer is modified if it needs to be saved. |
| Modification Time | Determining whether the visited file was changed ``behind Emacs's back''. |
| Read Only Buffers | Modifying text is not allowed in a read-only buffer. |
| The Buffer List | How to look at all the existing buffers. |
| Creating Buffers | Functions that create buffers. |
| Killing Buffers | Buffers exist until explicitly killed. |
| Indirect Buffers | An indirect buffer shares text with some other buffer. |
| Swapping Text | Swapping text between two buffers. |
| Buffer Gap | The gap in the buffer. |
| Windows | |
| Basic Windows | Basic information on using windows. |
| Splitting Windows | Splitting one window into two windows. |
| Deleting Windows | Deleting a window gives its space to other windows. |
| Selecting Windows | The selected window is the one that you edit in. |
| Cyclic Window Ordering | Moving around the existing windows. |
| Buffers and Windows | Each window displays the contents of a buffer. |
| Displaying Buffers | Higher-level functions for displaying a buffer and choosing a window for it. |
| Choosing Window | How to choose a window for displaying a buffer. |
| Dedicated Windows | How to avoid displaying another buffer in a specific window. |
| Window Point | Each window has its own location of point. |
| Window Start and End | Buffer positions indicating which text is on-screen in a window. |
| Textual Scrolling | Moving text up and down through the window. |
| Vertical Scrolling | Moving the contents up and down on the window. |
| Horizontal Scrolling | Moving the contents sideways on the window. |
| Size of Window | Accessing the size of a window. |
| Resizing Windows | Changing the size of a window. |
| Coordinates and Windows | Converting coordinates to windows. |
| Window Tree | The layout and sizes of all windows in a frame. |
| Window Configurations | Saving and restoring the state of the screen. |
| Window Parameters | Associating additional information with windows. |
| Window Hooks | Hooks for scrolling, window size changes, redisplay going past a certain point, or window configuration changes. |
| Frames | |
| Creating Frames | Creating additional frames. |
| Multiple Terminals | Displaying on several different devices. |
| Frame Parameters | Controlling frame size, position, font, etc. |
| Terminal Parameters | Parameters common for all frames on terminal. |
| Frame Titles | Automatic updating of frame titles. |
| Deleting Frames | Frames last until explicitly deleted. |
| Finding All Frames | How to examine all existing frames. |
| Frames and Windows | A frame contains windows; display of text always works through windows. |
| Minibuffers and Frames | How a frame finds the minibuffer to use. |
| Input Focus | Specifying the selected frame. |
| Visibility of Frames | Frames may be visible or invisible, or icons. |
| Raising and Lowering | Raising a frame makes it hide other windows; lowering it makes the others hide it. |
| Frame Configurations | Saving the state of all frames. |
| Mouse Tracking | Getting events that say when the mouse moves. |
| Mouse Position | Asking where the mouse is, or moving it. |
| Pop-Up Menus | Displaying a menu for the user to select from. |
| Dialog Boxes | Displaying a box to ask yes or no. |
| Pointer Shape | Specifying the shape of the mouse pointer. |
| Window System Selections | Transferring text to and from other X clients. |
| Drag and Drop | Internals of Drag-and-Drop implementation. |
| Color Names | Getting the definitions of color names. |
| Text Terminal Colors | Defining colors for text-only terminals. |
| Resources | Getting resource values from the server. |
| Display Feature Testing | Determining the features of a terminal. |
| Frame Parameters | |
| Parameter Access | How to change a frame's parameters. |
| Initial Parameters | Specifying frame parameters when you make a frame. |
| Window Frame Parameters | List of frame parameters for window systems. |
| Size and Position | Changing the size and position of a frame. |
| Geometry | Parsing geometry specifications. |
| Window Frame Parameters | |
| Basic Parameters | Parameters that are fundamental. |
| Position Parameters | The position of the frame on the screen. |
| Size Parameters | Frame's size. |
| Layout Parameters | Size of parts of the frame, and enabling or disabling some parts. |
| Buffer Parameters | Which buffers have been or should be shown. |
| Management Parameters | Communicating with the window manager. |
| Cursor Parameters | Controlling the cursor appearance. |
| Font and Color Parameters | Fonts and colors for the frame text. |
| Positions | |
| Point | The special position where editing takes place. |
| Motion | Changing point. |
| Excursions | Temporary motion and buffer changes. |
| Narrowing | Restricting editing to a portion of the buffer. |
| Motion | |
| Character Motion | Moving in terms of characters. |
| Word Motion | Moving in terms of words. |
| Buffer End Motion | Moving to the beginning or end of the buffer. |
| Text Lines | Moving in terms of lines of text. |
| Screen Lines | Moving in terms of lines as displayed. |
| List Motion | Moving by parsing lists and sexps. |
| Skipping Characters | Skipping characters belonging to a certain set. |
| Markers | |
| Overview of Markers | The components of a marker, and how it relocates. |
| Predicates on Markers | Testing whether an object is a marker. |
| Creating Markers | Making empty markers or markers at certain places. |
| Information from Markers | Finding the marker's buffer or character position. |
| Marker Insertion Types | Two ways a marker can relocate when you insert where it points. |
| Moving Markers | Moving the marker to a new buffer or position. |
| The Mark | How "the mark" is implemented with a marker. |
| The Region | How to access "the region". |
| Text | |
| Near Point | Examining text in the vicinity of point. |
| Buffer Contents | Examining text in a general fashion. |
| Comparing Text | Comparing substrings of buffers. |
| Insertion | Adding new text to a buffer. |
| Commands for Insertion | User-level commands to insert text. |
| Deletion | Removing text from a buffer. |
| User-Level Deletion | User-level commands to delete text. |
| The Kill Ring | Where removed text sometimes is saved for later use. |
| Undo | Undoing changes to the text of a buffer. |
| Maintaining Undo | How to enable and disable undo information. How to control how much information is kept. |
| Filling | Functions for explicit filling. |
| Margins | How to specify margins for filling commands. |
| Adaptive Fill | Adaptive Fill mode chooses a fill prefix from context. |
| Auto Filling | How auto-fill mode is implemented to break lines. |
| Sorting | Functions for sorting parts of the buffer. |
| Columns | Computing horizontal positions, and using them. |
| Indentation | Functions to insert or adjust indentation. |
| Case Changes | Case conversion of parts of the buffer. |
| Text Properties | Assigning Lisp property lists to text characters. |
| Substitution | Replacing a given character wherever it appears. |
| Transposition | Swapping two portions of a buffer. |
| Registers | How registers are implemented. Accessing the text or position stored in a register. |
| Base 64 | Conversion to or from base 64 encoding. |
| MD5 Checksum | Compute the MD5 "message digest"/"checksum". |
| Atomic Changes | Installing several buffer changes "atomically". |
| Change Hooks | Supplying functions to be run when text is changed. |
| The Kill Ring | |
| Kill Ring Concepts | What text looks like in the kill ring. |
| Kill Functions | Functions that kill text. |
| Yanking | How yanking is done. |
| Yank Commands | Commands that access the kill ring. |
| Low-Level Kill Ring | Functions and variables for kill ring access. |
| Internals of Kill Ring | Variables that hold kill ring data. |
| Indentation | |
| Primitive Indent | Functions used to count and insert indentation. |
| Mode-Specific Indent | Customize indentation for different modes. |
| Region Indent | Indent all the lines in a region. |
| Relative Indent | Indent the current line based on previous lines. |
| Indent Tabs | Adjustable, typewriter-like tab stops. |
| Motion by Indent | Move to first non-blank character. |
| Text Properties | |
| Examining Properties | Looking at the properties of one character. |
| Changing Properties | Setting the properties of a range of text. |
| Property Search | Searching for where a property changes value. |
| Special Properties | Particular properties with special meanings. |
| Format Properties | Properties for representing formatting of text. |
| Sticky Properties | How inserted text gets properties from neighboring text. |
| Lazy Properties | Computing text properties in a lazy fashion only when text is examined. |
| Clickable Text | Using text properties to make regions of text do something when you click on them. |
| Fields | The field property defines
fields within the buffer. |
| Not Intervals | Why text properties do not use Lisp-visible text intervals. |
| Non-ASCII Characters | |
| Text Representations | How Emacs represents text. |
| Converting Representations | Converting unibyte to multibyte and vice versa. |
| Selecting a Representation | Treating a byte sequence as unibyte or multi. |
| Character Codes | How unibyte and multibyte relate to codes of individual characters. |
| Character Properties | Character attributes that define their behavior and handling. |
| Character Sets | The space of possible character codes is divided into various character sets. |
| Scanning Charsets | Which character sets are used in a buffer? |
| Translation of Characters | Translation tables are used for conversion. |
| Coding Systems | Coding systems are conversions for saving files. |
| Input Methods | Input methods allow users to enter various non-ASCII characters without special keyboards. |
| Locales | Interacting with the POSIX locale. |
| Coding Systems | |
| Coding System Basics | Basic concepts. |
| Encoding and I/O | How file I/O functions handle coding systems. |
| Lisp and Coding Systems | Functions to operate on coding system names. |
| User-Chosen Coding Systems | Asking the user to choose a coding system. |
| Default Coding Systems | Controlling the default choices. |
| Specifying Coding Systems | Requesting a particular coding system for a single file operation. |
| Explicit Encoding | Encoding or decoding text without doing I/O. |
| Terminal I/O Encoding | Use of encoding for terminal I/O. |
| MS-DOS File Types | How DOS "text" and "binary" files relate to coding systems. |
| Searching and Matching | |
| String Search | Search for an exact match. |
| Searching and Case | Case-independent or case-significant searching. |
| Regular Expressions | Describing classes of strings. |
| Regexp Search | Searching for a match for a regexp. |
| POSIX Regexps | Searching POSIX-style for the longest match. |
| Match Data | Finding out which part of the text matched, after a string or regexp search. |
| Search and Replace | Commands that loop, searching and replacing. |
| Standard Regexps | Useful regexps for finding sentences, pages,... |
| Regular Expressions | |
| Syntax of Regexps | Rules for writing regular expressions. |
| Regexp Example | Illustrates regular expression syntax. |
| Regexp Functions | Functions for operating on regular expressions. |
| Syntax of Regular Expressions | |
| Regexp Special | Special characters in regular expressions. |
| Char Classes | Character classes used in regular expressions. |
| Regexp Backslash | Backslash-sequences in regular expressions. |
| The Match Data | |
| Replacing Match | Replacing a substring that was matched. |
| Simple Match Data | Accessing single items of match data, such as where a particular subexpression started. |
| Entire Match Data | Accessing the entire match data at once, as a list. |
| Saving Match Data | Saving and restoring the match data. |
| Syntax Tables | |
| Syntax Basics | Basic concepts of syntax tables. |
| Syntax Descriptors | How characters are classified. |
| Syntax Table Functions | How to create, examine and alter syntax tables. |
| Syntax Properties | Overriding syntax with text properties. |
| Motion and Syntax | Moving over characters with certain syntaxes. |
| Parsing Expressions | Parsing balanced expressions using the syntax table. |
| Standard Syntax Tables | Syntax tables used by various major modes. |
| Syntax Table Internals | How syntax table information is stored. |
| Categories | Another way of classifying character syntax. |
| Syntax Descriptors | |
| Syntax Class Table | Table of syntax classes. |
| Syntax Flags | Additional flags each character can have. |
| Parsing Expressions | |
| Motion via Parsing | Motion functions that work by parsing. |
| Position Parse | Determining the syntactic state of a position. |
| Parser State | How Emacs represents a syntactic state. |
| Low-Level Parsing | Parsing across a specified region. |
| Control Parsing | Parameters that affect parsing. |
| Abbrevs and Abbrev Expansion | |
| Abbrev Mode | Setting up Emacs for abbreviation. |
| Abbrev Tables | Creating and working with abbrev tables. |
| Defining Abbrevs | Specifying abbreviations and their expansions. |
| Abbrev Files | Saving abbrevs in files. |
| Abbrev Expansion | Controlling expansion; expansion subroutines. |
| Standard Abbrev Tables | Abbrev tables used by various major modes. |
| Abbrev Properties | How to read and set abbrev properties. Which properties have which effect. |
| Abbrev Table Properties | How to read and set abbrev table properties. Which properties have which effect. |
| Processes | |
| Subprocess Creation | Functions that start subprocesses. |
| Shell Arguments | Quoting an argument to pass it to a shell. |
| Synchronous Processes | Details of using synchronous subprocesses. |
| Asynchronous Processes | Starting up an asynchronous subprocess. |
| Deleting Processes | Eliminating an asynchronous subprocess. |
| Process Information | Accessing run-status and other attributes. |
| Input to Processes | Sending input to an asynchronous subprocess. |
| Signals to Processes | Stopping, continuing or interrupting an asynchronous subprocess. |
| Output from Processes | Collecting output from an asynchronous subprocess. |
| Sentinels | Sentinels run when process run-status changes. |
| Query Before Exit | Whether to query if exiting will kill a process. |
| System Processes | Accessing other processes running on your system. |
| Transaction Queues | Transaction-based communication with subprocesses. |
| Network | Opening network connections. |
| Network Servers | Network servers let Emacs accept net connections. |
| Datagrams | UDP network connections. |
| Low-Level Network | Lower-level but more general function to create connections and servers. |
| Misc Network | Additional relevant functions for network connections. |
| Serial Ports | Communicating with serial ports. |
| Byte Packing | Using bindat to pack and unpack binary data. |
| Receiving Output from Processes | |
| Process Buffers | If no filter, output is put in a buffer. |
| Filter Functions | Filter functions accept output from the process. |
| Decoding Output | Filters can get unibyte or multibyte strings. |
| Accepting Output | How to wait until process output arrives. |
| Low-Level Network Access | |
| Network Processes | Using make-network-process. |
| Network Options | Further control over network connections. |
| Network Feature Testing | Determining which network features work on the machine you are using. |
| Packing and Unpacking Byte Arrays | |
| Bindat Spec | Describing data layout. |
| Bindat Functions | Doing the unpacking and packing. |
| Bindat Examples | Samples of what bindat.el can do for you! |
| Emacs Display | |
| Refresh Screen | Clearing the screen and redrawing everything on it. |
| Forcing Redisplay | Forcing redisplay. |
| Truncation | Folding or wrapping long text lines. |
| The Echo Area | Displaying messages at the bottom of the screen. |
| Warnings | Displaying warning messages for the user. |
| Invisible Text | Hiding part of the buffer text. |
| Selective Display | Hiding part of the buffer text (the old way). |
| Temporary Displays | Displays that go away automatically. |
| Overlays | Use overlays to highlight parts of the buffer. |
| Width | How wide a character or string is on the screen. |
| Line Height | Controlling the height of lines. |
| Faces | A face defines a graphics style for text characters: font, colors, etc. |
| Fringes | Controlling window fringes. |
| Scroll Bars | Controlling vertical scroll bars. |
| Display Property | Enabling special display features. |
| Images | Displaying images in Emacs buffers. |
| Buttons | Adding clickable buttons to Emacs buffers. |
| Abstract Display | Emacs' Widget for Object Collections. |
| Blinking | How Emacs shows the matching open parenthesis. |
| Usual Display | The usual conventions for displaying nonprinting chars. |
| Display Tables | How to specify other conventions. |
| Beeping | Audible signal to the user. |
| Window Systems | Which window system is being used. |
| The Echo Area | |
| Displaying Messages | Explicitly displaying text in the echo area. |
| Progress | Informing user about progress of a long operation. |
| Logging Messages | Echo area messages are logged for the user. |
| Echo Area Customization | Controlling the echo area. |
| Reporting Warnings | |
| Warning Basics | Warnings concepts and functions to report them. |
| Warning Variables | Variables programs bind to customize their warnings. |
| Warning Options | Variables users set to control display of warnings. |
| Overlays | |
| Managing Overlays | Creating and moving overlays. |
| Overlay Properties | How to read and set properties. What properties do to the screen display. |
| Finding Overlays | Searching for overlays. |
| Faces | |
| Defining Faces | How to define a face with defface. |
| Face Attributes | What is in a face? |
| Attribute Functions | Functions to examine and set face attributes. |
| Displaying Faces | How Emacs combines the faces specified for a character. |
| Face Remapping | Remapping faces to alternative definitions. |
| Face Functions | How to define and examine faces. |
| Auto Faces | Hook for automatic face assignment. |
| Font Selection | Finding the best available font for a face. |
| Font Lookup | Looking up the names of available fonts and information about them. |
| Fontsets | A fontset is a collection of fonts that handle a range of character sets. |
| Low-Level Font | Lisp representation for character display fonts. |
| Fringes | |
| Fringe Size/Pos | Specifying where to put the window fringes. |
| Fringe Indicators | Displaying indicator icons in the window fringes. |
| Fringe Cursors | Displaying cursors in the right fringe. |
| Fringe Bitmaps | Specifying bitmaps for fringe indicators. |
| Customizing Bitmaps | Specifying your own bitmaps to use in the fringes. |
| Overlay Arrow | Display of an arrow to indicate position. |
The display Property
| |
| Replacing Specs | Display specs that replace the text. |
| Specified Space | Displaying one space with a specified width. |
| Pixel Specification | Specifying space width or height in pixels. |
| Other Display Specs | Displaying an image; adjusting the height, spacing, and other properties of text. |
| Display Margins | Displaying text or images to the side of the main text. |
| Images | |
| Image Formats | Supported image formats. |
| Image Descriptors | How to specify an image for use in :display. |
| XBM Images | Special features for XBM format. |
| XPM Images | Special features for XPM format. |
| GIF Images | Special features for GIF format. |
| TIFF Images | Special features for TIFF format. |
| PostScript Images | Special features for PostScript format. |
| Other Image Types | Various other formats are supported. |
| Defining Images | Convenient ways to define an image for later use. |
| Showing Images | Convenient ways to display an image once it is defined. |
| Image Cache | Internal mechanisms of image display. |
| Buttons | |
| Button Properties | Button properties with special meanings. |
| Button Types | Defining common properties for classes of buttons. |
| Making Buttons | Adding buttons to Emacs buffers. |
| Manipulating Buttons | Getting and setting properties of buttons. |
| Button Buffer Commands | Buffer-wide commands and bindings for buttons. |
| Abstract Display | |
| Abstract Display Functions | Functions in the Ewoc package. |
| Abstract Display Example | Example of using Ewoc. |
| Display Tables | |
| Display Table Format | What a display table consists of. |
| Active Display Table | How Emacs selects a display table to use. |
| Glyphs | How to define a glyph, and what glyphs mean. |
| Operating System Interface | |
| Starting Up | Customizing Emacs startup processing. |
| Getting Out | How exiting works (permanent or temporary). |
| System Environment | Distinguish the name and kind of system. |
| User Identification | Finding the name and user id of the user. |
| Time of Day | Getting the current time. |
| Time Conversion | Converting a time from numeric form to calendrical data and vice versa. |
| Time Parsing | Converting a time from numeric form to text and vice versa. |
| Processor Run Time | Getting the run time used by Emacs. |
| Time Calculations | Adding, subtracting, comparing times, etc. |
| Timers | Setting a timer to call a function at a certain time. |
| Idle Timers | Setting a timer to call a function when Emacs has been idle for a certain length of time. |
| Terminal Input | Accessing and recording terminal input. |
| Terminal Output | Controlling and recording terminal output. |
| Sound Output | Playing sounds on the computer's speaker. |
| X11 Keysyms | Operating on key symbols for X Windows. |
| Batch Mode | Running Emacs without terminal interaction. |
| Session Management | Saving and restoring state with X Session Management. |
| Starting Up Emacs | |
| Startup Summary | Sequence of actions Emacs performs at startup. |
| Init File | Details on reading the init file. |
| Terminal-Specific | How the terminal-specific Lisp file is read. |
| Command-Line Arguments | How command-line arguments are processed, and how you can customize them. |
| Getting Out of Emacs | |
| Killing Emacs | Exiting Emacs irreversibly. |
| Suspending Emacs | Exiting Emacs reversibly. |
| Terminal Input | |
| Input Modes | Options for how input is processed. |
| Recording Input | Saving histories of recent or all input events. |
| Tips and Conventions | |
| Coding Conventions | Conventions for clean and robust programs. |
| Key Binding Conventions | Which keys should be bound by which programs. |
| Programming Tips | Making Emacs code fit smoothly in Emacs. |
| Compilation Tips | Making compiled code run fast. |
| Warning Tips | Turning off compiler warnings. |
| Documentation Tips | Writing readable documentation strings. |
| Comment Tips | Conventions for writing comments. |
| Library Headers | Standard headers for library packages. |
| GNU Emacs Internals | |
| Building Emacs | How the dumped Emacs is made. |
| Pure Storage | A kludge to make preloaded Lisp functions sharable. |
| Garbage Collection | Reclaiming space for Lisp objects no longer used. |
| Memory Usage | Info about total size of Lisp objects made so far. |
| Writing Emacs Primitives | Writing C code for Emacs. |
| Object Internals | Data formats of buffers, windows, processes. |
| Object Internals | |
| Buffer Internals | Components of a buffer structure. |
| Window Internals | Components of a window structure. |
| Process Internals | Components of a process structure. |
Next: Lisp Data Types, Previous: Top, Up: Top
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 edition 3.0 of the GNU Emacs Lisp Reference Manual, corresponding to Emacs version 23.3.
Next: Lisp History, Up: Introduction
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.
Next: Conventions, Previous: Caveats, Up: Introduction
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 implementors 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 library. see Overview.
Emacs Lisp is not at all influenced by Scheme; but the GNU project has an implementation of Scheme, called Guile. We use Guile in all new GNU software that calls for extensibility.
Next: Version Info, Previous: Lisp History, Up: Introduction
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.
Next: nil and t, Up: Conventions
1.3.1 Some Terms
Throughout this manual, the phrases “the Lisp reader” and “the Lisp printer” refer to those routines in Lisp that convert textual representations of Lisp objects into actual Lisp objects, and vice versa. See Printed Representation, for more details. You, the person reading this manual, are thought of as “the programmer” and are addressed as “you.” “The user” is the person who uses Lisp programs, including those you write.
Examples of Lisp code are formatted like this: (list 1 2 3).
Names that represent metasyntactic variables, or arguments to a function
being described, are formatted like this: first-number.
Next: Evaluation Notation, Previous: Some Terms, Up: Conventions
1.3.2 nil and t
In Lisp, the symbol nil has three separate meanings: it
is a symbol with the name ‘nil’; it is the logical truth value
false; and it is the empty list—the list of zero elements.
When used as a variable, nil always has the value nil.
As far as the Lisp reader is concerned, ‘()’ and ‘nil’ are
identical: they stand for the same object, the symbol nil. The
different ways of writing the symbol are intended entirely for human
readers. After the Lisp reader has read either ‘()’ or ‘nil’,
there is no way to determine which representation was actually written
by the programmer.
In this manual, we write () when we wish to emphasize that it
means the empty list, and we write nil when we wish to emphasize
that it means the truth value false. That is a good convention to use
in Lisp programs also.
(cons 'foo ()) ; Emphasize the empty list (setq foo-flag nil) ; Emphasize the truth value false
In contexts where a truth value is expected, any non-nil value
is considered to be true. However, t is the preferred way
to represent the truth value true. When you need to choose a
value which represents true, and there is no other basis for
choosing, use t. The symbol t always has the value
t.
In Emacs Lisp, nil and t are special symbols that always
evaluate to themselves. This is so that you do not need to quote them
to use them as constants in a program. An attempt to change their
values results in a setting-constant error. See Constant Variables.
Return non-
nilif object is one of the two canonical boolean values:tornil.
Next: Printing Notation, Previous: nil and t, Up: Conventions
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
Sometimes to help describe one form we show another form that produces identical results. The exact equivalence of two forms is indicated with ‘==’.
(make-sparse-keymap) == (list 'keymap)
Next: Error Messages, Previous: Evaluation Notation, Up: Conventions
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 (here bar) follows on a separate line with
‘⇒’.
(progn (prin1 'foo) (princ "\n") (prin1 'bar))
-| foo
-| bar
⇒ bar
Next: Buffer Text Notation, Previous: Printing Notation, Up: Conventions
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
Next: Format of Descriptions, Previous: Error Messages, Up: Conventions
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 ----------
Previous: Buffer Text Notation, Up: Conventions
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 will receive, 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:
The function
foosubtracts 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) ⇒ 14More generally,
(foo w x y...) == (+ (- x w) y...)
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. Arguments named object may be of any type. (See Lisp Data Types, for a list of Emacs object types.) Arguments with other sorts of names (e.g., new-file) are discussed specifically in the description of the function. In some sections, features common to the arguments of several functions are described at the beginning.
See Lambda Expressions, for a more complete description of optional and rest arguments.
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.
Special form descriptions 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:
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
nilbefore the loop begins, and the loop exits if var is non-nilat 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.
Previous: A Sample Function Description, Up: Format of Descriptions
1.3.7.2 A Sample Variable Description
A variable is a name that can hold a 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.
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'.
Next: Acknowledgements, Previous: Conventions, Up: Introduction
1.4 Version Information
These facilities provide information about which version of Emacs is in use.
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 returnsnil. When this function is called interactively, it prints the same information in the echo area, but giving a prefix argument makes here non-nil.
The value of this variable indicates the time at which Emacs was built at the local site. It is a list of three integers, like the value of
current-time(see Time of Day).emacs-build-time ⇒ (18846 52016 156039)
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 you build Emacs in any given directory. A value with four numeric components, such as"22.0.91.1", indicates an unreleased test version.
The following two variables have existed since Emacs version 19.23:
The major version number of Emacs, as an integer. For Emacs version 23.1, the value is 23.
The minor version number of Emacs, as an integer. For Emacs version 23.1, the value is 1.
Previous: Version Info, Up: Introduction
1.5 Acknowledgements
This manual was 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 were 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.
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.
Next: Numbers, Previous: Introduction, Up: Top
2 Lisp Data Types
A Lisp object is a piece of data used and manipulated by Lisp programs. For our purposes, a type or data type is a set of possible objects.
Every object belongs to at least one type. Objects of the same type have similar structures and may usually be used in the same contexts. Types can overlap, and objects can belong to two or more types. Consequently, we can ask whether an object belongs to a particular type, but not for “the” type of an object.
A few fundamental object types are built into Emacs. These, from which all other types are constructed, are called primitive types. Each object belongs to one and only one primitive type. These types include integer, float, cons, symbol, string, vector, hash-table, subr, and byte-code function, plus several special types, such as buffer, that are related to editing. (See Editing Types.)
Each primitive type has a corresponding Lisp function that checks whether an object is a member of that type.
Lisp is unlike many other languages in that its objects are self-typing: the primitive type of each object is implicit in the object itself. For example, if an object is a vector, nothing can treat it as a number; Lisp knows it is a vector, not a number.
In most languages, the programmer must declare the data type of each variable, and the type is known by the compiler but not represented in the data. Such type declarations do not exist in Emacs Lisp. A Lisp variable can have any type of value, and it remembers whatever value you store in it, type and all. (Actually, a small number of Emacs Lisp variables can only take on values of a certain type. See Variables with Restricted Values.)
This chapter describes the purpose, printed representation, and read syntax of each of the standard types in GNU Emacs Lisp. Details on how to use these types can be found in later chapters.
Next: Comments, Up: Lisp Data Types
2.1 Printed Representation and Read Syntax
The printed representation of an object is the format of the
output generated by the Lisp printer (the function prin1) for
that object. Every data type has a unique printed representation.
The read syntax of an object is the format of the input accepted
by the Lisp reader (the function read) for that object. This
is not necessarily unique; many kinds of object have more than one
syntax. See Read and Print.
In most cases, an object's printed representation is also a read syntax for the object. However, some types have no read syntax, since it does not make sense to enter objects of these types as constants in a Lisp program. These objects are printed in hash notation, which consists of the characters ‘#<’, a descriptive string (typically the type name followed by the name of the object), and a closing ‘>’. For example:
(current-buffer)
⇒ #<buffer objects.texi>
Hash notation cannot be read at all, so the Lisp reader signals the
error invalid-read-syntax whenever it encounters ‘#<’.
In other languages, an expression is text; it has no other form. In
Lisp, an expression is primarily a Lisp object and only secondarily the
text that is the object's read syntax. Often there is no need to
emphasize this distinction, but you must keep it in the back of your
mind, or you will occasionally be very confused.
When you evaluate an expression interactively, the Lisp interpreter
first reads the textual representation of it, producing a Lisp object,
and then evaluates that object (see Evaluation). However,
evaluation and reading are separate activities. Reading returns the
Lisp object represented by the text that is read; the object may or may
not be evaluated later. See Input Functions, for a description of
read, the basic function for reading objects.
Next: Programming Types, Previous: Printed Representation, Up: Lisp Data Types
2.2 Comments
A comment is text that is written in a program only for the sake of humans that read the program, and that has no effect on the meaning of the program. In Lisp, a semicolon (‘;’) starts a comment if it is not within a string or character constant. The comment continues to the end of line. The Lisp reader discards comments; they do not become part of the Lisp objects which represent the program within the Lisp system.
The ‘#@count’ construct, which skips the next count characters, is useful for program-generated comments containing binary data. The Emacs Lisp byte compiler uses this in its output files (see Byte Compilation). It isn't meant for source files, however.
See Comment Tips, for conventions for formatting comments.
Next: Editing Types, Previous: Comments, Up: Lisp Data Types
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.
Next: Floating Point Type, Up: Programming Types
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 most machines. (Some machines may provide a wider range.) It is
important to note that the Emacs Lisp arithmetic functions do not check
for overflow. Thus (1+ 536870911) is −536870912 on most
machines.
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. 1073741825 ; Also the integer 1 on a 30-bit implementation.
As a special exception, if a sequence of digits specifies an integer
too large or too small to be a valid integer object, the Lisp reader
reads it as a floating-point number (see Floating Point Type).
For instance, on most machines 536870912 is read as the
floating-point number 536870912.0.
See Numbers, for more information.
Next: Character Type, Previous: Integer Type, Up: Programming Types
2.3.2 Floating Point Type
Floating point numbers are the computer equivalent of scientific
notation; you can think of a floating point number as a fraction
together with a power of ten. The precise number of significant
figures and the range of possible exponents is machine-specific; Emacs
uses the C data type double to store the value, and internally
this records a power of 2 rather than a power of 10.
The printed representation for floating point numbers requires either a decimal point (with at least one digit following), an exponent, or both. For example, ‘1500.0’, ‘15e2’, ‘15.0e2’, ‘1.5e3’, and ‘.15e4’ are five ways of writing a floating point number whose value is 1500. They are all equivalent.
See Numbers, for more information.
Next: Symbol Type, Previous: Floating Point Type, Up: Programming Types
2.3.3 Character Type
A character in Emacs Lisp is nothing more than an integer. In other words, characters are represented by their character codes. For example, the character A is represented as the integer 65.
Individual characters are used occasionally in programs, but it is more common to work with strings, which are sequences composed of characters. See String Type.
Characters in strings and buffers are currently limited to the range of 0 to 4194303—twenty two bits (see Character Codes). Codes 0 through 127 are ASCII codes; the rest are non-ASCII (see Non-ASCII Characters). Characters that represent keyboard input have a much wider range, to encode modifier keys such as Control, Meta and Shift.
There are special functions for producing a human-readable textual description of a character for the sake of messages. See Describing Characters.
Next: General Escape Syntax, Up: Character Type
2.3.3.1 Basic Char Syntax
Since characters are really integers, the printed representation of a character is a decimal number. This is also a possible read syntax for a character, but writing characters that way in Lisp programs is not clear programming. You should always use the special read syntax formats that Emacs Lisp provides for characters. These syntax formats start with a question mark.
The usual read syntax for alphanumeric characters is a question mark followed by the character; thus, ‘?A’ for the character A, ‘?B’ for the character B, and ‘?a’ for the character a.
For example:
?Q ⇒ 81 ?q ⇒ 113
You can use the same syntax for punctuation characters, but it is often a good idea to add a ‘\’ so that the Emacs commands for editing Lisp code don't get confused. For example, ‘?\(’ is the way to write the open-paren character. If the character is ‘\’, you must use a second ‘\’ to quote it: ‘?\\’.
You can express the characters control-g, backspace, tab, newline, vertical tab, formfeed, space, return, del, and escape as ‘?\a’, ‘?\b’, ‘?\t’, ‘?\n’, ‘?\v’, ‘?\f’, ‘?\s’, ‘?\r’, ‘?\d’, and ‘?\e’, respectively. (‘?\s’ followed by a dash has a different meaning—it applies the “super” modifier to the following character.) Thus,
?\a ⇒ 7 ; control-g, C-g ?\b ⇒ 8 ; backspace, <BS>, C-h ?\t ⇒ 9 ; tab, <TAB>, C-i ?\n ⇒ 10 ; newline, C-j ?\v ⇒ 11 ; vertical tab, C-k ?\f ⇒ 12 ; formfeed character, C-l ?\r ⇒ 13 ; carriage return, <RET>, C-m ?\e ⇒ 27 ; escape character, <ESC>, C-[ ?\s ⇒ 32 ; space character, <SPC> ?\\ ⇒ 92 ; backslash character, \ ?\d ⇒ 127 ; delete character, <DEL>
These sequences which start with backslash are also known as escape sequences, because backslash plays the role of an “escape character”; this terminology has nothing to do with the character <ESC>. ‘\s’ is meant for use in character constants; in string constants, just write the space.
A backslash is allowed, and harmless, preceding any character without a special escape meaning; thus, ‘?\+’ is equivalent to ‘?+’. There is no reason to add a backslash before most characters. However, you should add a backslash before any of the characters ‘()\|;'`"#.,’ to avoid confusing the Emacs commands for editing Lisp code. You can also add a backslash before whitespace characters such as space, tab, newline and formfeed. However, it is cleaner to use one of the easily readable escape sequences, such as ‘\t’ or ‘\s’, instead of an actual whitespace character such as a tab or a space. (If you do write backslash followed by a space, you should write an extra space after the character constant to separate it from the following text.)
Next: Ctl-Char Syntax, Previous: Basic Char Syntax, Up: Character Type
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.
You can specify characters by their Unicode values.
?\unnnn represents a character that maps to the Unicode
code point ‘U+nnnn’ (by convention, Unicode code points are
given in hexadecimal). There is a slightly different syntax for
specifying characters with code points higher than
U+ffff: \U00nnnnnn represents the character
whose code point is ‘U+nnnnnn’. 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.
This peculiar and inconvenient syntax was adopted for compatibility with other programming languages. Unlike some other languages, Emacs Lisp supports this syntax only in character literals and strings.
The most general read syntax for a character represents the
character code in either octal or hex. To use octal, write a question
mark followed by a backslash and the octal character code (up to three
octal digits); thus, ‘?\101’ for the character A,
‘?\001’ for the character C-a, and ?\002 for the
character C-b. Although this syntax can represent any
ASCII character, it is preferred only when the precise octal
value is more important than the ASCII representation.
?\012 ⇒ 10 ?\n ⇒ 10 ?\C-j ⇒ 10
?\101 ⇒ 65 ?A ⇒ 65
To use hex, write a question mark followed by a backslash, ‘x’,
and the hexadecimal character code. You can use any number of hex
digits, so you can represent any character code in this way.
Thus, ‘?\x41’ for the character A, ‘?\x1’ for the
character C-a, and ?\x8e0 for the Latin-1 character
‘a’ with grave accent.
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 terminals have no way of generating non-ASCII control characters, but you can generate them straightforwardly using X and other window systems.
For historical reasons, Emacs treats the <DEL> character as the control equivalent of ?:
?\^? ⇒ 127 ?\C-? ⇒ 127
As a result, it is currently not possible to represent the character Control-?, which is a meaningful input character under X, using ‘\C-’. It is not easy to change this, as various Lisp files refer to <DEL> in this way.
For representing control characters to be found in files or strings, we recommend the ‘^’ syntax; for control characters in keyboard input, we prefer the ‘C-’ syntax. Which one you use does not affect the meaning of the program, but may guide the understanding of people who read it.
2.3.3.4 Meta-Character Syntax
A meta character is a character typed with the <META> modifier key. The integer that represents such a character has the 2**27 bit set. We use high bits for this and other modifiers to make possible a wide range of basic character codes.
In a string, the 2**7 bit attached to an ASCII character indicates a meta character; thus, the meta characters that can fit in a string have codes in the range from 128 to 255, and are the meta versions of the ordinary ASCII characters. See Strings of Events, for details about <META>-handling in strings.
The read syntax for meta characters uses ‘\M-’. For example, ‘?\M-A’ stands for M-A. You can use ‘\M-’ together with octal character codes (see below), with ‘\C-’, or with any other syntax for a character. Thus, you can write M-A as ‘?\M-A’, or as ‘?\M-\101’. Likewise, you can write C-M-b as ‘?\M-\C-b’, ‘?\C-\M-b’, or ‘?\M-\002’.
Previous: Meta-Char Syntax, Up: Character Type
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 terminals do not report the distinction to the computer in any way. 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.
Next: Sequence Type, Previous: Character Type, Up: Programming Types
2.3.4 Symbol Type
A symbol in GNU Emacs Lisp is an object with a name. The symbol name serves as the printed representation of the symbol. In ordinary Lisp use, with one single obarray (see Creating Symbols), a symbol's name is unique—no two symbols have the same name.
A symbol can serve as a variable, as a function name, or to hold a property list. Or it may serve only to be distinct from all other Lisp objects, so that its presence in a data structure may be recognized reliably. In a given context, usually only one of these uses is intended. But you can use one symbol in all of these ways, independently.
A symbol whose name starts with a colon (‘:’) is called a keyword symbol. These symbols automatically act as constants, and are normally used only by comparing an unknown symbol with a few specific alternatives.
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 fifth example is escaped to prevent it from being read as a number. This is not necessary in the fourth 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.
Normally the Lisp reader interns all symbols (see Creating Symbols). To prevent interning, you can write ‘#:’ before the name of the symbol.
Next: Cons Cell Type, Previous: Symbol Type, Up: Programming Types
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. Thus, an object of type list or of type array is also considered a sequence.
Arrays are further subdivided into strings, vectors, char-tables and
bool-vectors. Vectors can hold elements of any type, but string
elements must be characters, and bool-vector elements must be t
or nil. Char-tables are like vectors except that they are
indexed by any valid character code. The characters in a string can
have text properties like characters in a buffer (see Text Properties), but vectors do not support text properties, even when
their elements happen to be characters.
Lists, strings and the other array types are different, but they have
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 elt can be used to extract
an element of a sequence, given its index. See Sequences Arrays Vectors.
It is generally impossible to read the same sequence twice, since
sequences are always created anew upon reading. If you read the read
syntax for a sequence twice, you get two sequences with equal contents.
There is one exception: the empty list () always stands for the
same object, nil.
Next: Array Type, Previous: Sequence Type, Up: Programming Types
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 or refer to 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 note to C programmers: in Lisp, we do not distinguish between “holding” a value and “pointing to” the value, because pointers in Lisp are implicit.
A list is a series of cons cells, linked together so that the
cdr slot of each cons cell holds either the next cons cell or the
empty list. The empty list is actually the symbol nil.
See Lists, for functions that work on lists. Because most cons
cells are used as part of lists, the phrase list structure has
come to refer to any structure made out of cons cells.
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:Aand 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.
Next: Dotted Pair Notation, Up: Cons Cell Type
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 |
| | | | | |
-------------- ----------------
Next: Association List Type, Previous: Box Diagrams, Up: Cons Cell Type
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
Previous: Dotted Pair Notation, Up: Cons Cell Type
2.3.6.3 Association List Type
An association list or alist is a specially-constructed list whose elements are cons cells. In each element, the car is considered a key, and the cdr is considered an associated value. (In some cases, the associated value is stored in the car of the cdr.) Association lists are often used as stacks, since it is easy to add or remove associations at the front of the list.
For example,
(setq alist-of-colors
'((rose . red) (lily . white) (buttercup . yellow)))
sets the variable alist-of-colors to an alist of three elements. In the
first element, rose is the key and red is the value.
See Association Lists, for a further explanation of alists and for functions that work on alists. See Hash Tables, for another kind of lookup table, which is much faster for handling a large number of keys.
Next: String Type, Previous: Cons Cell Type, Up: Programming Types
2.3.7 Array Type
An array is composed of an arbitrary number of slots for holding or referring to other Lisp objects, arranged in a contiguous block of memory. Accessing any element of an array takes approximately the same amount of time. In contrast, accessing an element of a list requires time proportional to the position of the element in the list. (Elements at the end of a list take longer to access than elements at the beginning of a list.)
Emacs defines four types of array: strings, vectors, bool-vectors, and char-tables.
A string is an array of characters and a vector is an array of
arbitrary objects. A bool-vector can hold only t or nil.
These kinds of array may have any length up to the largest integer.
Char-tables are sparse arrays indexed by any valid character code; they
can hold arbitrary objects.
The first element of an array has index zero, the second element has index 1, and so on. This is called zero-origin indexing. For example, an array of four elements has indices 0, 1, 2, and 3. The largest possible index value is one less than the length of the array. Once an array is created, its length is fixed.
All Emacs Lisp arrays are one-dimensional. (Most other programming languages support multidimensional arrays, but they are not essential; you can get the same effect with nested one-dimensional arrays.) Each type of array has its own read syntax; see the following sections for details.
The array type is a subset of the sequence type, and contains the string type, the vector type, the bool-vector type, and the char-table type.
Next: Vector Type, Previous: Array Type, Up: Programming Types
2.3.8 String Type
A string is an array of characters. Strings are used for many purposes in Emacs, as can be expected in a text editor; for example, as the names of Lisp symbols, as messages for the user, and to represent text extracted from buffers. Strings in Lisp are constants: evaluation of a string returns the same string.
See Strings and Characters, for functions that operate on strings.
Next: Non-ASCII in Strings, Up: String Type
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
You can include a non-ASCII international character in a string constant by writing it literally. There are two text representations for non-ASCII characters in Emacs strings (and in buffers): unibyte and multibyte. 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 the character is read as a multibyte character, and that makes the string multibyte. If the string constant is read from a unibyte source, then the character is read as unibyte and that makes the string unibyte.
You can also represent a multibyte non-ASCII character with its character code: use a hex escape, ‘\xnnnnnnn’, with as many digits as necessary. (Multibyte non-ASCII character codes are all greater than 256.) Any character which is not a valid hex digit terminates this construct. If the next character in the string could be interpreted as a hex digit, write ‘\ ’ (backslash and space) to terminate the hex escape—for example, ‘\x8e0\ ’ 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 the preceding hex escape.
You can represent a unibyte non-ASCII character with its character code, which must be in the range from 128 (0200 octal) to 255 (0377 octal). If you write all such character codes in octal and the string contains no other characters forcing it to be multibyte, this produces a unibyte string. However, using any hex escape in a string (even for an ASCII character) forces the string to be multibyte.
You can also specify characters in a string by their numeric values in Unicode, using ‘\u’ and ‘\U’ (see Character Type).
See Text Representations, for more information about the two text representations.
Next: Text Props and Strings, Previous: Non-ASCII in Strings, Up: String Type
2.3.8.3 Nonprinting Characters in Strings
You can use the same backslash escape-sequences in a string constant
as in character literals (but do not use the question mark that begins a
character constant). For example, you can write a string containing the
nonprinting characters tab and C-a, with commas and spaces between
them, like this: "\t, \C-a". See Character Type, for a
description of the read syntax for characters.
However, not all of the characters you can write with backslash escape-sequences are valid in strings. The only control characters that a string can hold are the ASCII control characters. Strings do not distinguish case in ASCII control characters.
Properly speaking, strings cannot hold meta characters; but when a
string is to be used as a key sequence, there is a special convention
that provides a way to represent meta versions of ASCII
characters in a string. If you use the ‘\M-’ syntax to indicate
a meta character in a string constant, this sets the
2**7
bit of the character in the string. If the string is used in
define-key or lookup-key, this numeric code is translated
into the equivalent meta character. See Character Type.
Strings cannot hold characters that have the hyper, super, or alt modifiers.
Previous: Nonprinting Characters, Up: String Type
2.3.8.4 Text Properties in Strings
A string can hold properties for the characters it contains, in addition to the characters themselves. This enables programs that copy text between strings and buffers to copy the text's properties with no special effort. See Text Properties, for an explanation of what text properties mean. Strings with text properties use a special read and print syntax:
#("characters" property-data...)
where property-data consists of zero or more elements, in groups of three as follows:
beg end plist
The elements beg and end are integers, and together specify a range of indices in the string; plist is the property list for that range. For example,
#("foo bar" 0 3 (face bold) 3 4 nil 4 7 (face italic))
represents a string whose textual contents are ‘foo bar’, in which
the first three characters have a face property with value
bold, and the last three have a face property with value
italic. (The fourth character has no text properties, so its
property list is nil. It is not actually necessary to mention
ranges with nil as the property list, since any characters not
mentioned in any range will default to having no properties.)
Next: Char-Table Type, Previous: String Type, Up: Programming Types
2.3.9 Vector Type
A vector is a one-dimensional array of elements of any type. It takes a constant amount of time to access any element of a vector. (In a list, the access time of an element is proportional to the distance of the element from the beginning of the list.)
The printed representation of a vector consists of a left square bracket, the elements, and a right square bracket. This is also the read syntax. Like numbers and strings, vectors are considered constants for evaluation.
[1 "two" (three)] ; A vector of three elements.
⇒ [1 "two" (three)]
See Vectors, for functions that work with vectors.
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.
See Char-Tables, for special functions to operate on char-tables. Uses of char-tables include:
- Case tables (see Case Tables).
- Character category tables (see Categories).
- Display tables (see Display Tables).
- Syntax tables (see Syntax Tables).
2.3.11 Bool-Vector Type
A bool-vector is a one-dimensional array whose elements must
be t or nil.
The printed representation of a bool-vector is like a string, except
that it begins with ‘#&’ followed by the length. The string
constant that follows actually specifies the contents of the bool-vector
as a bitmap—each “character” in the string contains 8 bits, which
specify the next 8 elements of the bool-vector (1 stands for t,
and 0 for nil). The least significant bits of the character
correspond to the lowest indices in the bool-vector.
(make-bool-vector 3 t)
⇒ #&3"^G"
(make-bool-vector 3 nil)
⇒ #&3"^@"
These results make sense, because the binary code for ‘C-g’ is 111 and ‘C-@’ is the character with code 0.
If the length is not a multiple of 8, the printed representation shows extra elements, but these extras really make no difference. For instance, in the next example, the two bool-vectors are equal, because only the first 3 bits are used:
(equal #&3"\377" #&3"\007")
⇒ t
Next: Function Type, Previous: Bool-Vector Type, Up: Programming Types
2.3.12 Hash Table Type
A hash table is a very fast kind of lookup table, somewhat like an alist in that it maps keys to corresponding values, but much faster. The printed representation of a hash table specifies its properties and contents, like this:
(make-hash-table)
⇒ #s(hash-table size 65 test eql rehash-size 1.5
rehash-threshold 0.8 data ())
See Hash Tables, for more information about hash tables.
Next: Macro Type, Previous: Hash Table Type, Up: Programming Types
2.3.13 Function Type
Lisp functions are executable code, just like functions in other
programming languages. In Lisp, unlike most languages, functions are
also Lisp objects. A non-compiled function in Lisp is a lambda
expression: that is, a list whose first element is the symbol
lambda (see Lambda Expressions).
In most programming languages, it is impossible to have a function without a name. In Lisp, a function has no intrinsic name. A lambda expression can be called as a function even though it has no name; to emphasize this, we also call it an anonymous function (see Anonymous Functions). A named function in Lisp is just a symbol with a valid function in its function cell (see Defining Functions).
Most of the time, functions are called when their names are written in
Lisp expressions in Lisp programs. However, you can construct or obtain
a function object at run time and then call it with the primitive
functions funcall and apply. See Calling Functions.
Next: Primitive Function Type, Previous: Function Type, Up: Programming Types
2.3.14 Macro Type
A Lisp macro is a user-defined construct that extends the Lisp
language. It is represented as an object much like a function, but with
different argument-passing semantics. A Lisp macro has the form of a
list whose first element is the symbol macro and whose cdr
is a Lisp function object, including the lambda symbol.
Lisp macro objects are usually defined with the built-in
defmacro function, but any list that begins with macro is
a macro as far as Emacs is concerned. See Macros, for an explanation
of how to write a macro.
Warning: Lisp macros and keyboard macros (see Keyboard Macros) are entirely different things. When we use the word “macro” without qualification, we mean a Lisp macro, not a keyboard macro.
Next: Byte-Code Type, Previous: Macro Type, Up: Programming Types
2.3.15 Primitive Function Type
A primitive function is a function callable from Lisp but written in the C programming language. Primitive functions are also called subrs or built-in functions. (The word “subr” is derived from “subroutine.”) Most primitive functions evaluate all their arguments when they are called. A primitive function that does not evaluate all its arguments is called a special form (see Special Forms).
It does not matter to the caller of a function whether the function is primitive. However, this does matter if you try to redefine a primitive with a function written in Lisp. The reason is that the primitive function may be called directly from C code. Calls to the redefined function from Lisp will use the new definition, but calls from C code may still use the built-in definition. Therefore, we discourage redefinition of primitive functions.
The term function refers to all Emacs functions, whether written in Lisp or C. See Function Type, for information about the functions written in Lisp.
Primitive functions have no read syntax and print in hash notation with the name of the subroutine.
(symbol-function 'car) ; Access the function cell ; of the symbol. ⇒ #<subr car> (subrp (symbol-function 'car)) ; Is this a primitive function? ⇒ t ; Yes.
2.3.16 Byte-Code Function Type
The byte compiler produces byte-code function objects. Internally, a byte-code function object is much like a vector; however, the evaluator handles this data type specially when it appears as a function to be called. See Byte Compilation, for information about the byte compiler.
The printed representation and read syntax for a byte-code function object is like that for a vector, with an additional ‘#’ before the opening ‘[’.
Previous: Byte-Code Type, Up: Programming Types
2.3.17 Autoload Type
An autoload object is a list whose first element is the symbol
autoload. It is stored as the function definition of a symbol,
where it serves as a placeholder for the real definition. The autoload
object says that the real definition is found in a file of Lisp code
that should be loaded when necessary. It contains the name of the file,
plus some other information about the real definition.
After the file has been loaded, the symbol should have a new function definition that is not an autoload object. The new definition is then called as if it had been there to begin with. From the user's point of view, the function call works as expected, using the function definition in the loaded file.
An autoload object is usually created with the function
autoload, which stores the object in the function cell of a
symbol. See Autoload, for more details.
Next: Circular Objects, Previous: Programming Types, Up: Lisp Data Types
2.4 Editing Types
The types in the previous section are used for general programming purposes, and most of them are common to most Lisp dialects. Emacs Lisp provides several additional data types for purposes connected with editing.
Next: Marker Type, Up: Editing Types
2.4.1 Buffer Type
A buffer is an object that holds text that can be edited (see Buffers). Most buffers hold the contents of a disk file (see Files) so they can be edited, but some are used for other purposes. Most buffers are also meant to be seen by the user, and therefore displayed, at some time, in a window (see Windows). But a buffer need not be displayed in any window. Each buffer has a designated position called point (see Positions); most editing commands act on the contents of the current buffer in the neighborhood of point. At any time, one buffer is the current buffer.
The contents of a buffer are much like a string, but buffers are not used like strings in Emacs Lisp, and the available operations are different. For example, you can insert text efficiently into an existing buffer, altering the buffer's contents, whereas “inserting” text into a string requires concatenating substrings, and the result is an entirely new string object.
Many of the standard Emacs functions manipulate or test the characters in the current buffer; a whole chapter in this manual is devoted to describing these functions (see Text).
Several other data structures are associated with each buffer:
- a local syntax table (see Syntax Tables);
- a local keymap (see Keymaps); and,
- a list of buffer-local variable bindings (see Buffer-Local Variables).
- overlays (see Overlays).
- text properties for the text in the buffer (see Text Properties).
The local keymap and variable list contain entries that individually override global bindings or values. These are used to customize the behavior of programs in different buffers, without actually changing the programs.
A buffer may be indirect, which means it shares the text of another buffer, but presents it differently. See Indirect Buffers.
Buffers have no read syntax. They print in hash notation, showing the buffer name.
(current-buffer)
⇒ #<buffer objects.texi>
Next: Window Type, Previous: Buffer Type, Up: Editing Types
2.4.2 Marker Type
A marker denotes a position in a specific buffer. Markers therefore have two components: one for the buffer, and one for the position. Changes in the buffer's text automatically relocate the position value as necessary to ensure that the marker always points between the same two characters in the buffer.
Markers have no read syntax. They print in hash notation, giving the current character position and the name of the buffer.
(point-marker)
⇒ #<marker at 10779 in objects.texi>
See Markers, for information on how to test, create, copy, and move markers.
Next: Frame Type, Previous: Marker Type, Up: Editing Types
2.4.3 Window Type
A window describes the portion of the terminal screen that Emacs uses to display a buffer. Every window has one associated buffer, whose contents appear in the window. By contrast, a given buffer may appear in one window, no window, or several windows.
Though many windows may exist simultaneously, at any time one window is designated the selected window. This is the window where the cursor is (usually) displayed when Emacs is ready for a command. The selected window usually displays the current buffer, but this is not necessarily the case.
Windows are grouped on the screen into frames; each window belongs to one and only one frame. See Frame Type.
Windows have no read syntax. They print in hash notation, giving the window number and the name of the buffer being displayed. The window numbers exist to identify windows uniquely, since the buffer displayed in any given window can change frequently.
(selected-window)
⇒ #<window 1 on objects.texi>
See Windows, for a description of the functions that work on windows.
Next: Terminal Type, Previous: Window Type, Up: Editing Types
2.4.4 Frame Type
A frame is a screen area that contains one or more Emacs windows; we also use the term “frame” to refer to the Lisp object that Emacs uses to refer to the screen area.
Frames have no read syntax. They print in hash notation, giving the frame's title, plus its address in core (useful to identify the frame uniquely).
(selected-frame)
⇒ #<frame emacs@psilocin.gnu.org 0xdac80>
See Frames, for a description of the functions that work on frames.
Next: Window Configuration Type, Previous: Frame Type, Up: Editing Types
2.4.5 Terminal Type
A terminal is a device capable of displaying one or more Emacs frames (see Frame Type).
Terminals have no read syntax. They print in hash notation giving the terminal's ordinal number and its TTY device file name.
(get-device-terminal nil)
⇒ #<terminal 1 on /dev/tty>
Next: Frame Configuration Type, Previous: Terminal Type, Up: Editing Types
2.4.6 Window Configuration Type
A window configuration stores information about the positions, sizes, and contents of the windows in a frame, so you can recreate the same arrangement of windows later.
Window configurations do not have a read syntax; their print syntax looks like ‘#<window-configuration>’. See Window Configurations, for a description of several functions related to window configurations.
Next: Process Type, Previous: Window Configuration Type, Up: Editing Types
2.4.7 Frame Configuration Type
A frame configuration stores information about the positions,
sizes, and contents of the windows in all frames. It is not a
primitive type—it is actually a list whose car is
frame-configuration and whose cdr is an alist. Each alist
element describes one frame, which appears as the car of that
element.
See Frame Configurations, for a description of several functions related to frame configurations.
Next: Stream Type, Previous: Frame Configuration Type, Up: Editing Types
2.4.8 Process Type
The word process usually means a running program. Emacs itself runs in a process of this sort. However, in Emacs Lisp, a process is a Lisp object that designates a subprocess created by the Emacs process. Programs such as shells, GDB, ftp, and compilers, running in subprocesses of Emacs, extend the capabilities of Emacs.
An Emacs subprocess takes textual input from Emacs and returns textual output to Emacs for further manipulation. Emacs can also send signals to the subprocess.
Process objects have no read syntax. They print in hash notation, giving the name of the process:
(process-list)
⇒ (#<process shell>)
See Processes, for information about functions that create, delete, return information about, send input or signals to, and receive output from processes.
Next: Keymap Type, Previous: Process Type, Up: Editing Types
2.4.9 Stream Type
A stream is an object that can be used as a source or sink for characters—either to supply characters for input or to accept them as output. Many different types can be used this way: markers, buffers, strings, and functions. Most often, input streams (character sources) obtain characters from the keyboard, a buffer, or a file, and output streams (character sinks) send characters to a buffer, such as a *Help* buffer, or to the echo area.
The object nil, in addition to its other meanings, may be used
as a stream. It stands for the value of the variable
standard-input or standard-output. Also, the object
t as a stream specifies input using the minibuffer
(see Minibuffers) or output in the echo area (see The Echo Area).
Streams have no special printed representation or read syntax, and print as whatever primitive type they are.
See Read and Print, for a description of functions related to streams, including parsing and printing functions.
Next: Overlay Type, Previous: Stream Type, Up: Editing Types
2.4.10 Keymap Type
A keymap maps keys typed by the user to commands. This mapping
controls how the user's command input is executed. A keymap is actually
a list whose car is the symbol keymap.
See Keymaps, for information about creating keymaps, handling prefix keys, local as well as global keymaps, and changing key bindings.
Next: Font Type, Previous: Keymap Type, Up: Editing Types
2.4.11 Overlay Type
An overlay specifies properties that apply to a part of a buffer. Each overlay applies to a specified range of the buffer, and contains a property list (a list whose elements are alternating property names and values). Overlay properties are used to present parts of the buffer temporarily in a different display style. Overlays have no read syntax, and print in hash notation, giving the buffer name and range of positions.
See Overlays, for how to create and use overlays.
Previous: Overlay Type, Up: Editing Types
2.4.12 Font Type
A font specifies how to display text on a graphical terminal. There are actually three separate font types—font objects, font specs, and font entities—each of which has slightly different properties. None of them have a read syntax; their print syntax looks like ‘#<font-object>’, ‘#<font-spec>’, and ‘#<font-entity>’ respectively. See Low-Level Font, for a description of these Lisp objects.
Next: Type Predicates, Previous: Editing Types, Up: Lisp Data Types
2.5 Read Syntax for Circular Objects
To represent shared or circular structures within a complex of Lisp objects, you can use the reader constructs ‘#n=’ and ‘#n#’.
Use #n= before an object to label it for later reference;
subsequently, you can use #n# to refer the same object in
another place. Here, n is some integer. For example, here is how
to make a list in which the first element recurs as the third element:
(#1=(a) b #1#)
This differs from ordinary syntax such as this
((a) b (a))
which would result in a list whose first and third elements look alike but are not the same Lisp object. This shows the difference:
(prog1 nil
(setq x '(#1=(a) b #1#)))
(eq (nth 0 x) (nth 2 x))
⇒ t
(setq x '((a) b (a)))
(eq (nth 0 x) (nth 2 x))
⇒ nil
You can also use the same syntax to make a circular structure, which appears as an “element” within itself. Here is an example:
#1=(a #1#)
This makes a list whose second element is the list itself. Here's how you can see that it really works:
(prog1 nil
(setq x '#1=(a #1#)))
(eq x (cadr x))
⇒ t
The Lisp printer can produce this syntax to record circular and shared
structure in a Lisp object, if you bind the variable print-circle
to a non-nil value. See Output Variables.
Next: Equality Predicates, Previous: Circular Objects, Up: Lisp Data Types
2.6 Type Predicates
The Emacs Lisp interpreter itself does not perform type checking on the actual arguments passed to functions when they are called. It could not do so, since function arguments in Lisp do not have declared data types, as they do in other programming languages. It is therefore up to the individual function to test whether each actual argument belongs to a type that the function can use.
All built-in functions do check the types of their actual arguments
when appropriate, and signal a wrong-type-argument error if an
argument is of the wrong type. For example, here is what happens if you
pass an argument to + that it cannot handle:
(+ 2 'a)
error--> Wrong type argument: number-or-marker-p, a
If you want your program to handle different types differently, you must do explicit type checking. The most common way to check the type of an object is to call a type predicate function. Emacs has a type predicate for each type, as well as some predicates for combinations of types.
A type predicate function takes one argument; it returns t if
the argument belongs to the appropriate type, and nil otherwise.
Following a general Lisp convention for predicate functions, most type
predicates' names end with ‘p’.
Here is an example which uses the predicates listp to check for
a list and symbolp to check for a symbol.
(defun add-on (x)
(cond ((symbolp x)
;; If X is a symbol, put it on LIST.
(setq list (cons x list)))
((listp x)
;; If X is a list, add its elements to LIST.
(setq list (append x list)))
(t
;; We handle only symbols and lists.
(error "Invalid argument %s in add-on" x))))
Here is a table of predefined type predicates, in alphabetical order, with references to further information.
atom- See atom.
arrayp- See arrayp.
bool-vector-p- See bool-vector-p.
bufferp- See bufferp.
byte-code-function-p- See byte-code-function-p.
case-table-p- See case-table-p.
char-or-string-p- See char-or-string-p.
char-table-p- See char-table-p.
commandp- See commandp.
consp- See consp.
display-table-p- See display-table-p.
floatp- See floatp.
fontp- See Low-Level Font.
frame-configuration-p- See frame-configuration-p.
frame-live-p- See frame-live-p.
framep- See framep.
functionp- See functionp.
hash-table-p- See hash-table-p.
integer-or-marker-p- See integer-or-marker-p.
integerp- See integerp.
keymapp- See keymapp.
keywordp- See Constant Variables.
listp- See listp.
markerp- See markerp.
wholenump- See wholenump.
nlistp- See nlistp.
numberp- See numberp.
number-or-marker-p- See number-or-marker-p.
overlayp- See overlayp.
processp- See processp.
sequencep- See sequencep.
stringp- See stringp.
subrp- See subrp.
symbolp- See symbolp.
syntax-table-p- See syntax-table-p.
user-variable-p- See user-variable-p.
vectorp- See vectorp.
window-configuration-p- See window-configuration-p.
window-live-p- See window-live-p.
windowp- See windowp.
booleanp- See booleanp.
string-or-null-p- See string-or-null-p.
The most general way to check the type of an object is to call the
function type-of. Recall that each object belongs to one and
only one primitive type; type-of tells you which one (see Lisp Data Types). But type-of knows nothing about non-primitive
types. In most cases, it is more convenient to use type predicates than
type-of.
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, orwindow-configuration.(type-of 1) ⇒ integer (type-of 'nil) ⇒ symbol (type-of '()) ;()isnil. ⇒ symbol (type-of '(x)) ⇒ cons
Previous: Type Predicates, Up: Lisp Data Types
2.7 Equality Predicates
Here we describe functions that test for equality between any 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.
This function returns
tif object1 and object2 are the same object,nilotherwise.
eqreturnstif object1 and object2 are integers with the same value. Also, since symbol names are normally unique, if the arguments are symbols with the same name, they areeq. For other types (e.g., lists, vectors, strings), two arguments with the same contents or elements are not necessarilyeqto each other: they areeqonly 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)) ⇒ nilThe
make-symbolfunction returns an uninterned symbol, distinct from the symbol that is used if you write the name in a Lisp expression. Distinct symbols with the same name are noteq. See Creating Symbols.(eq (make-symbol "foo") 'foo) ⇒ nil
This function returns
tif object1 and object2 have equal components,nilotherwise. Whereaseqtests if its arguments are the same object,equallooks inside nonidentical arguments to see if their elements or contents are the same. So, if two objects areeq, they areequal, but the converse is not always true.(equal 'foo 'foo) ⇒ t (equal 456 456) ⇒ t (equal "asdf" "asdf") ⇒ t (eq "asdf" "asdf") ⇒ nil (equal '(1 (2 (3))) '(1 (2 (3)))) ⇒ t (eq '(1 (2 (3))) '(1 (2 (3)))) ⇒ nil (equal [(1 2) 3] [(1 2) 3]) ⇒ t (eq [(1 2) 3] [(1 2) 3]) ⇒ nil (equal (point-marker) (point-marker)) ⇒ t (eq (point-marker) (point-marker)) ⇒ nilComparison of strings is case-sensitive, but does not take account of text properties—it compares only the characters in the strings. Use
equal-including-propertiesto also compare text properties. For technical reasons, a unibyte string and a multibyte string areequalif 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). (see Text Representations).(equal "asdf" "ASDF") ⇒ nilHowever, 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).
This function behaves like
equalin all cases but also requires that for two strings to be equal, they have the same text properties.(equal "asdf" (propertize "asdf" '(asdf t))) ⇒ t (equal-including-properties "asdf" (propertize "asdf" '(asdf t))) ⇒ nil
Next: Strings and Characters, Previous: Lisp Data Types, Up: Top
3 Numbers
GNU Emacs supports two numeric data types: integers and floating point numbers. Integers are whole numbers such as −3, 0, 7, 13, and 511. 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.
Next: Float Basics, Up: Numbers
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 some machines may provide a wider range. Many examples in this chapter assume an integer has 30 bits. The Lisp reader reads an integer as a sequence of digits with optional initial sign and optional final period.
1 ; The integer 1. 1. ; The integer 1. +1 ; Also the integer 1. -1 ; The integer −1. 1073741825 ; Also the integer 1, due to overflow. 0 ; The integer 0. -0 ; The integer 0.
The syntax for integers in bases other than 10 uses ‘#’ followed by a letter that specifies the radix: ‘b’ for binary, ‘o’ for octal, ‘x’ for hex, or ‘radixr’ to specify radix radix. Case is not significant for the letter that specifies the radix. Thus, ‘#binteger’ reads integer in binary, and ‘#radixrinteger’ reads integer in radix radix. Allowed values of radix run from 2 to 36. For example:
#b101100 ⇒ 44
#o54 ⇒ 44
#x2c ⇒ 44
#24r1k ⇒ 44
To understand how various functions work on integers, especially the bitwise operators (see Bitwise Operations), it is often helpful to view the numbers in their binary form.
In 30-bit binary, the decimal integer 5 looks like this:
00 0000 0000 0000 0000 0000 0000 0101
(We have inserted spaces between groups of 4 bits, and two spaces between groups of 8 bits, to make the binary integer easier to read.)
The integer −1 looks like this:
11 1111 1111 1111 1111 1111 1111 1111
−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:
11 1111 1111 1111 1111 1111 1111 1011
In this implementation, the largest 30-bit binary integer value is 536,870,911 in decimal. In binary, it looks like this:
01 1111 1111 1111 1111 1111 1111 1111
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
⇒ 10 0000 0000 0000 0000 0000 0000 0000
Many of the functions described in this chapter accept markers for arguments in place of numbers. (See Markers.) Since the actual arguments to such functions may be either numbers or markers, we often give these arguments the name number-or-marker. When the argument value is a marker, its position value is used and its buffer is ignored.
The value of this variable is the largest integer that Emacs Lisp can handle.
The value of this variable is the smallest integer that Emacs Lisp can handle. It is negative.
See max-char, for the maximum value of a valid character codepoint.
Next: Predicates on Numbers, Previous: Integer Basics, Up: Numbers
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.
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’.
Most modern computers support the IEEE floating point standard,
which provides for 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. For practical purposes, there's no significant difference between
different NaN values in Emacs Lisp, and there's no rule for precisely
which NaN value should be used in a particular case, so Emacs Lisp
doesn't try to distinguish them (but it does report the sign, if you
print it). 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’.
To test whether a floating point value is a NaN, compare it with
itself using =. That returns nil for a NaN, and
t for any other floating point value.
The value -0.0 is distinguishable from ordinary zero in
IEEE floating point, but Emacs Lisp equal and
= consider them equal values.
You can use logb to extract the binary exponent of a floating
point number (or estimate the logarithm of an integer):
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
Next: Comparison of Numbers, Previous: Float Basics, Up: Numbers
3.3 Type Predicates for Numbers
The functions in this section test for numbers, or for a specific
type of number. The functions integerp and floatp can
take any type of Lisp object as argument (they would not be of much
use otherwise), but the zerop predicate requires a number as
its argument. See also integer-or-marker-p and
number-or-marker-p, in Predicates on Markers.
This predicate tests whether its argument is a floating point number and returns
tif so,nilotherwise.
floatpdoes not exist in Emacs versions 18 and earlier.
This predicate tests whether its argument is an integer, and returns
tif so,nilotherwise.
This predicate tests whether its argument is a number (either integer or floating point), and returns
tif so,nilotherwise.
The
wholenumppredicate (whose name comes from the phrase “whole-number-p”) tests to see whether its argument is a nonnegative integer, and returnstif so,nilotherwise. 0 is considered non-negative.
This predicate tests whether its argument is zero, and returns
tif so,nilotherwise. The argument must be a number.
(zerop x)is equivalent to(= x 0).
Next: Numeric Conversions, Previous: Predicates on Numbers, Up: Numbers
3.4 Comparison of Numbers
To test numbers for numerical equality, you should normally use
=, not eq. There can be many distinct floating point
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.
At present, each integer value has a unique Lisp object in Emacs Lisp.
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 a good idea to use = if you
can, even for comparing integers, just in case we change the
representation of integers in a future Emacs version.
Sometimes it is useful to compare numbers with equal; it
treats two numbers as equal if they have the same data type (both
integers, or both floating point) and the same value. By contrast,
= can treat an integer and a floating point number as equal.
See Equality Predicates.
There is another wrinkle: because floating point arithmetic is not exact, it is often a bad idea to check for equality of two floating point values. Usually it is better to test for approximate equality. Here's a function to do this:
(defvar fuzz-factor 1.0e-6)
(defun approx-equal (x y)
(or (and (= x 0) (= y 0))
(< (/ (abs (- x y))
(max (abs x) (abs y)))
fuzz-factor)))
Common Lisp note: Comparing numbers in Common Lisp always requires
= because Common Lisp implements multi-word integers, and two
distinct integer objects can have the same numeric value. Emacs Lisp
can have just one integer object for any given value because it has a
limited range of integer values.
This function tests whether its arguments are numerically equal, and returns
tif so,nilotherwise.
This function acts like
eqexcept when both arguments are numbers. It compares numbers by type and numeric value, so that(eql 1.0 1)returnsnil, but(eql 1.0 1.0)and(eql 1 1)both returnt.
This function tests whether its arguments are numerically equal, and returns
tif they are not, andnilif they are.
This function tests whether its first argument is strictly less than its second argument. It returns
tif so,nilotherwise.
This function tests whether its first argument is less than or equal to its second argument. It returns
tif so,nilotherwise.
This function tests whether its first argument is strictly greater than its second argument. It returns
tif so,nilotherwise.
This function tests whether its first argument is greater than or equal to its second argument. It returns
tif so,nilotherwise.
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
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
Next: Arithmetic Operations, Previous: Comparison of Numbers, Up: Numbers
3.5 Numeric Conversions
To convert an integer to floating point, use the function float.
This returns number converted to floating point. If number is already a floating point number,
floatreturns 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. An arith-error results if divisor is 0.
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
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
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
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
Next: Rounding Operations, Previous: Numeric Conversions, Up: Numbers
3.6 Arithmetic Operations
Emacs Lisp provides the traditional four arithmetic operations: addition, subtraction, multiplication, and division. Remainder and modulus functions supplement the division functions. The functions to add or subtract 1 are provided because they are traditional in Lisp and commonly used.
All of these functions except % return a floating point value
if any argument is floating.
It is important to note that in Emacs Lisp, arithmetic functions
do not check for overflow. Thus (1+ 268435455) may evaluate to
−268435456, depending on your hardware.
This function returns number-or-marker plus 1. For example,
(setq foo 4) ⇒ 4 (1+ foo) ⇒ 5This function is not analogous to the C operator
++—it does not increment a variable. It just computes a sum. Thus, if we continue,foo ⇒ 4If you want to increment the variable, you must use
setq, like this:(setq foo (1+ foo)) ⇒ 5
This function adds its arguments together. When given no arguments,
+returns 0.(+) ⇒ 0 (+ 1) ⇒ 1 (+ 1 2 3 4) ⇒ 10
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
This function multiplies its arguments together, and returns the product. When given no arguments,
*returns 1.(*) ⇒ 1 (* 1) ⇒ 1 (* 1 2 3 4) ⇒ 24
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, then the result is an integer too. This means the result has to be rounded. On most machines, the result is rounded towards zero after each division, but some machines may round differently with negative arguments. This is because the Lisp function
/is implemented using the C division operator, which also permits machine-dependent rounding. As a practical matter, all known machines round in the standard fashion.If you divide an integer by 0, an
arith-errorerror is signaled. (See Errors.) Floating point division by zero returns either infinity or a NaN if your machine supports IEEE floating point; otherwise, it signals anarith-errorerror.(/ 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 (could in theory be −3 on some machines)
This function returns the integer remainder after division of dividend by divisor. The arguments must be integers or markers.
For negative arguments, the remainder is in principle machine-dependent since the quotient is; but in practice, all known machines behave alike.
An
arith-errorresults if divisor is 0.(% 9 4) ⇒ 1 (% -9 4) ⇒ -1 (% 9 -4) ⇒ 1 (% -9 -4) ⇒ -1For any two integers dividend and divisor,
(+ (% dividend divisor) (* (/ dividend divisor) divisor))always equals dividend.
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
%,modreturns a well-defined result for negative arguments. It also permits floating point arguments; it rounds the quotient downward (towards minus infinity) to an integer, and uses that quotient to compute the remainder.An
arith-errorresults if divisor is 0.(mod 9 4) ⇒ 1 (mod -9 4) ⇒ 3 (mod 9 -4) ⇒ -3 (mod -9 -4) ⇒ -1 (mod 5.5 2.5) ⇒ .5For 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 Numeric Conversions.
Next: Bitwise Operations, Previous: Arithmetic Operations, Up: Numbers
3.7 Rounding Operations
The functions ffloor, fceiling, fround, and
ftruncate take a floating point argument and return a floating
point result whose value is a nearby integer. ffloor returns the
nearest integer below; fceiling, the nearest integer above;
ftruncate, the nearest integer in the direction towards zero;
fround, the nearest integer.
This function rounds float to the next lower integral value, and returns that value as a floating point number.
This function rounds float to the next higher integral value, and returns that value as a floating point number.
This function rounds float towards zero to an integral value, and returns that value as a floating point number.
This function rounds float to the nearest integral value, and returns that value as a floating point number.
Next: Math Functions, Previous: Rounding Operations, Up: Numbers
3.8 Bitwise Operations on Integers
In a computer, an integer is represented as a binary number, a sequence of bits (digits which are either zero or one). A bitwise operation acts on the individual bits of such a sequence. For example, shifting moves the whole sequence left or right one or more places, reproducing the same pattern “moved over.”
The bitwise operations in Emacs Lisp apply only to integers.
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,lshshifts zeros into the leftmost (most-significant) bit, producing a positive result even if integer1 is negative. Contrast this withash, 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 ⇒ 00001110As 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 ⇒ 00001100On 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 ⇒ 00000010As 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 on a 30-bit machine:(lsh 536870911 1) ; left shift ⇒ -2In binary, in the 30-bit implementation, the argument looks like this:
;; Decimal 536,870,911 01 1111 1111 1111 1111 1111 1111 1111which becomes the following when left shifted:
;; Decimal −2 11 1111 1111 1111 1111 1111 1111 1110
ash(arithmetic shift) shifts the bits in integer1 to the left count places, or to the right if count is negative.
ashgives the same results aslshexcept when integer1 and count are both negative. In that case,ashputs ones in the empty bit positions on the left, whilelshputs 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. 11 1111 1111 1111 1111 1111 1111 1010 ⇒ 11 1111 1111 1111 1111 1111 1111 1101In contrast, shifting the pattern of bits one place to the right with
lshlooks like this:(lsh -6 -1) ⇒ 536870909 ;; Decimal −6 becomes decimal 536,870,909. 11 1111 1111 1111 1111 1111 1111 1010 ⇒ 01 1111 1111 1111 1111 1111 1111 1101Here are other examples:
; 30-bit binary values (lsh 5 2) ; 5 = 00 0000 0000 0000 0000 0000 0000 0101 ⇒ 20 ; = 00 0000 0000 0000 0000 0000 0001 0100 (ash 5 2) ⇒ 20 (lsh -5 2) ; -5 = 11 1111 1111 1111 1111 1111 1111 1011 ⇒ -20 ; = 11 1111 1111 1111 1111 1111 1110 1100 (ash -5 2) ⇒ -20 (lsh 5 -2) ; 5 = 00 0000 0000 0000 0000 0000 0000 0101 ⇒ 1 ; = 00 0000 0000 0000 0000 0000 0000 0001 (ash 5 -2) ⇒ 1 (lsh -5 -2) ; -5 = 11 1111 1111 1111 1111 1111 1111 1011 ⇒ 268435454 ; = 00 0111 1111 1111 1111 1111 1111 1110 (ash -5 -2) ; -5 = 11 1111 1111 1111 1111 1111 1111 1011 ⇒ -2 ; = 11 1111 1111 1111 1111 1111 1111 1110
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) ⇒ 12If
logandis not passed any argument, it returns a value of −1. This number is an identity element forlogandbecause its binary representation consists entirely of ones. Iflogandis passed just one argument, it returns that argument.; 30-bit binary values (logand 14 13) ; 14 = 00 0000 0000 0000 0000 0000 0000 1110 ; 13 = 00 0000 0000 0000 0000 0000 0000 1101 ⇒ 12 ; 12 = 00 0000 0000 0000 0000 0000 0000 1100 (logand 14 13 4) ; 14 = 00 0000 0000 0000 0000 0000 0000 1110 ; 13 = 00 0000 0000 0000 0000 0000 0000 1101 ; 4 = 00 0000 0000 0000 0000 0000 0000 0100 ⇒ 4 ; 4 = 00 0000 0000 0000 0000 0000 0000 0100 (logand) ⇒ -1 ; -1 = 11 1111 1111 1111 1111 1111 1111 1111
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
logioris passed just one argument, it returns that argument.; 30-bit binary values (logior 12 5) ; 12 = 00 0000 0000 0000 0000 0000 0000 1100 ; 5 = 00 0000 0000 0000 0000 0000 0000 0101 ⇒ 13 ; 13 = 00 0000 0000 0000 0000 0000 0000 1101 (logior 12 5 7) ; 12 = 00 0000 0000 0000 0000 0000 0000 1100 ; 5 = 00 0000 0000 0000 0000 0000 0000 0101 ; 7 = 00 0000 0000 0000 0000 0000 0000 0111 ⇒ 15 ; 15 = 00 0000 0000 0000 0000 0000 0000 1111
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
logxoris passed just one argument, it returns that argument.; 30-bit binary values (logxor 12 5) ; 12 = 00 0000 0000 0000 0000 0000 0000 1100 ; 5 = 00 0000 0000 0000 0000 0000 0000 0101 ⇒ 9 ; 9 = 00 0000 0000 0000 0000 0000 0000 1001 (logxor 12 5 7) ; 12 = 00 0000 0000 0000 0000 0000 0000 1100 ; 5 = 00 0000 0000 0000 0000 0000 0000 0101 ; 7 = 00 0000 0000 0000 0000 0000 0000 0111 ⇒ 14 ; 14 = 00 0000 0000 0000 0000 0000 0000 1110
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 = 00 0000 0000 0000 0000 0000 0000 0101 ;; becomes ;; -6 = 11 1111 1111 1111 1111 1111 1111 1010
Next: Random Numbers, Previous: Bitwise Operations, Up: Numbers
3.9 Standard Mathematical Functions
These mathematical functions allow integers as well as floating point numbers as arguments.
— Function: cos arg
— Function: tan arg
These are the ordinary trigonometric functions, with argument measured in radians.
The value of
(asinarg)is a number between −pi/2 and pi/2 (inclusive) whose sine is arg; if, however, arg is out of range (outside [−1, 1]), it signals adomain-errorerror.
The value of
(acosarg)is a number between 0 and pi (inclusive) whose cosine is arg; if, however, arg is out of range (outside [−1, 1]), it signals adomain-errorerror.
The value of
(atany)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 theXaxis.
This is the exponential function; it returns e to the power arg. e is a fundamental mathematical constant also called the base of natural logarithms.
This function returns the logarithm of arg, with base base. If you don't specify base, the base e is used. If arg is negative, it signals a
domain-errorerror.
This function returns the logarithm of arg, with base 10. If arg is negative, it signals a
domain-errorerror.(log10x)==(logx10), at least approximately.
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.
This returns the square root of arg. If arg is negative, it signals a
domain-errorerror.
Previous: Math Functions, Up: Numbers
3.10 Random Numbers
A deterministic computer program cannot generate true random numbers. For most purposes, pseudo-random numbers suffice. A series of pseudo-random numbers is generated in a deterministic fashion. The numbers are not truly random, but they have certain properties that mimic a random series. For example, all possible values occur equally often in a pseudo-random series.
In Emacs, pseudo-random numbers are generated from a “seed” number.
Starting from any given seed, the random function always
generates the same sequence of numbers. Emacs always starts with the
same seed value, so the sequence of values of random is actually
the same in each Emacs run! For example, in one operating system, the
first call to (random) after you start Emacs always returns
−1457731, and the second one always returns −7692030. This
repeatability is helpful for debugging.
If you want random numbers that don't always come out the same, execute
(random t). This chooses a new seed based on the current time of
day and on Emacs's process ID number.
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.
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.On some machines, any integer representable in Lisp may be the result of
random. On other machines, the result can never be larger than a certain maximum or less than a certain (negative) minimum.
4 Strings and Characters
A string in Emacs Lisp is an array that contains an ordered sequence of characters. Strings are used as names of symbols, buffers, and files; to send messages to users; to hold text being copied between buffers; and for many other purposes. Because strings are so important, Emacs Lisp has many functions expressly for manipulating them. Emacs Lisp programs use strings more often than individual characters.
See Strings of Events, for special considerations for strings of keyboard character events.
Next: Predicates for Strings, Up: Strings and Characters
4.1 String and Character Basics
Characters are represented in Emacs Lisp as integers; whether an integer is a character or not is determined only by how it is used. Thus, strings really contain integers. See Character Codes, for details about character representation in Emacs.
The length of a string (like any array) is fixed, and cannot be altered once the string exists. Strings in Lisp are not terminated by a distinguished character code. (By contrast, strings in C are terminated by a character with ASCII code 0.)
Since strings are arrays, and therefore sequences as well, you can
operate on them with the general array and sequence functions.
(See Sequences Arrays Vectors.) For example, you can access or
change individual characters in a string using the functions aref
and aset (see Array Functions).
There are two text representations for non-ASCII characters in Emacs strings (and in buffers): unibyte and multibyte (see Text Representations). For most Lisp programming, you don't need to be concerned with these two representations.
Sometimes key sequences are represented as unibyte strings. When a unibyte string is a key sequence, string elements in the range 128 to 255 represent meta characters (which are large integers) rather than character codes in the range 128 to 255. Strings cannot hold characters that have the hyper, super or alt modifiers; they can hold ASCII control characters, but no other control characters. They do not distinguish case in ASCII control characters. If you want to store such characters in a sequence, such as a key sequence, you must use a vector instead of a string. See Character Type, for more information about keyboard input characters.
Strings are useful for holding regular expressions. You can also
match regular expressions against strings with string-match
(see Regexp Search). The functions match-string
(see Simple Match Data) and replace-match (see Replacing Match) are useful for decomposing and modifying strings after
matching regular expressions against them.
Like a buffer, a string can contain text properties for the characters in it, as well as the characters themselves. See Text Properties. All the Lisp primitives that copy text from strings to buffers or other strings also copy the properties of the characters being copied.
See Text, for information about functions that display strings or copy them into buffers. See Character Type, and String Type, for information about the syntax of characters and strings. See Non-ASCII Characters, for functions to convert between text representations and to encode and decode character codes.
Next: Creating Strings, Previous: String Basics, Up: Strings and Characters
4.2 The Predicates for Strings
For more information about general sequence and array predicates, see Sequences Arrays Vectors, and Arrays.
This function returns
tif object is a string ornil. It returnsnilotherwise.
This function returns
tif object is a string or a character (i.e., an integer),nilotherwise.
Next: Modifying Strings, Previous: Predicates for Strings, Up: Strings and Characters
4.3 Creating Strings
The following functions create strings, either from scratch, or by putting strings together, or by taking them apart.
This function returns a string made up of count repetitions of character. If count is negative, an error is signaled.
(make-string 5 ?x) ⇒ "xxxxx" (make-string 0 ?x) ⇒ ""Other functions to compare with this one include
make-vector(see Vectors) andmake-list(see Building Lists).
This returns a string containing the characters characters.
(string ?a ?b ?c) ⇒ "abc"
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
nilis 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(substringstring0)returns a copy of all of string.(substring "abcdefg" 0) ⇒ "abcdefg"But we recommend
copy-sequencefor this purpose (see Sequence Functions).If the characters copied from string have text properties, the properties are copied into the new string also. See Text Properties.
substringalso accepts a vector for the first argument. For example:(substring [a b (c) "d"] 1 3) ⇒ [b (c)]A
wrong-type-argumenterror is signaled if start is not an integer or if end is neither an integer nornil. Anargs-out-of-rangeerror is signaled if start indicates a character following end, or if either integer is out of range for string.Contrast this function with
buffer-substring(see Buffer Contents), which returns a string containing a portion of the text in the current buffer. The beginning of a string is at index 0, but the beginning of a buffer is at index 1.
This works like
substringbut discards all text properties from the value. Also, start may be omitted ornil, which is equivalent to 0. Thus,(substring-no-propertiesstring)returns a copy of string, with all text properties removed.
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
concatreceives no arguments, it returns an empty string.(concat "abc" "-def") ⇒ "abc-def" (concat "abc" (list 120 121) [122]) ⇒ "abcxyz" ;;nilis 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
eqto 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
mapconcatin Mapping Functions,vconcatin Vector Functions, andappendin Building Lists. For concatenating individual command-line arguments into a string to be used as a shell command, see combine-and-quote-strings.
This function splits string into substrings based on the regular expression separators (see Regular Expressions). Each match for separators defines a splitting point; the substrings between splitting points are made into a list, which is returned.
If omit-nulls is
nil(or omitted), the result contains null strings whenever there are two consecutive matches for separators, or a match is adjacent to the beginning or end of string. If omit-nulls ist, these null strings are omitted from the result.If separators is
nil(or omitted), the default is the value ofsplit-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-stringwill 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) ⇒ nilSomewhat 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-processorstart-process, see split-string-and-unquote.
The default value of separators for
split-string. Its usual value is"[ \f\t\n\r\v]+".
Next: Text Comparison, Previous: Creating Strings, Up: Strings and Characters
4.4 Modifying Strings
The most basic way to alter the contents of an existing string is with
aset (see Array Functions). (aset string
idx char) stores char into string at index
idx. Each character occupies one or more bytes, and if char
needs a different number of bytes from the character already present at
that index, aset signals an error.
A more powerful function is store-substring:
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:
This makes string a unibyte string and clears its contents to zeros. It may also change string's length.
Next: String Conversion, Previous: Modifying Strings, Up: Strings and Characters
4.5 Comparison of Characters and Strings
This function returns
tif the arguments represent the same character,nilotherwise. This function ignores differences in case ifcase-fold-searchis non-nil.(char-equal ?x ?x) ⇒ t (let ((case-fold-search nil)) (char-equal ?x ?X)) ⇒ nil
This function returns
tif the characters of the two strings match exactly. Symbols are also allowed as arguments, in which case their print names are used. Case is always significant, regardless ofcase-fold-search.(string= "abc" "abc") ⇒ t (string= "abc" "ABC") ⇒ nil (string= "ab" "ABC") ⇒ nilThe function
string=ignores the text properties of the two strings. Whenequal(see Equality Predicates) compares two strings, it usesstring=.For technical reasons, a unibyte and a multibyte string are
equalif 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 onlyequalif 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 areequalwithout both being all ASCII is a technical oddity that very few Emacs Lisp programmers ever get confronted with. See Text Representations.
This function compares two strings a character at a time. It scans both the strings at the same time to find the first pair of corresponding characters that do not match. If the lesser character of these two is the character from string1, then string1 is less, and this function returns
t. If the lesser character is the one from string2, then string1 is greater, and this function returnsnil. If the two strings match entirely, the value isnil.Pairs of characters are compared according to their character codes. Keep in mind that lower case letters have higher numeric values in the ASCII character set than their upper case counterparts; digits and many punctuation characters have a lower numeric value than upper case letters. An ASCII character is less than any non-ASCII character; a unibyte non-ASCII character is always less than any multibyte non-ASCII character (see Text Representations).
(string< "abc" "abd") ⇒ t (string< "abd" "abc") ⇒ nil (string< "123" "abc") ⇒ tWhen the strings have different lengths, and they match up to the length of string1, then the result is
t. If they match up to the length of string2, the result isnil. A string of no characters is less than any other string.(string< "" "abc") ⇒ t (string< "ab" "abc") ⇒ t (string< "abc" "") ⇒ nil (string< "abc" "ab") ⇒ nil (string< "" "") ⇒ nilSymbols are also allowed as arguments, in which case their print names are used.
This function compares the specified part of string1 with the specified part of string2. The specified part of string1 runs from index start1 up to index end1 (
nilmeans the end of the string). The specified part of string2 runs from index start2 up to index end2 (nilmeans the end of the string).The strings are both converted to multibyte for the comparison (see Text Representations) so that a unibyte string and its conversion to multibyte are always regarded as equal. If ignore-case is non-
nil, then case is ignored, so that upper case letters can be equal to lower case letters.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.
This function works like
assoc, except that key must be a string or symbol, and comparison is done usingcompare-strings. Symbols are converted to strings before testing. If case-fold is non-nil, it ignores case differences. Unlikeassoc, this function can also match elements of the alist that are strings or symbols rather than conses. In particular, alist can be a list of strings or symbols rather than an actual alist. See Association Lists.
See also the function compare-buffer-substrings in
Comparing Text, for a way to compare text in buffers. The
function string-match, which matches a regular expression
against a string, can be used for a kind of string comparison; see
Regexp Search.
Next: Formatting Strings, Previous: Text Comparison, Up: Strings and Characters
4.6 Conversion of Characters and Strings
This section describes functions for converting between characters,
strings and integers. format (see Formatting Strings) and
prin1-to-string (see Output Functions) can also convert
Lisp objects into strings. read-from-string (see Input Functions) can “convert” a string representation of a Lisp object
into an object. The functions string-make-multibyte and
string-make-unibyte convert the text representation of a string
(see Converting Representations).
See Documentation, for functions that produce textual descriptions
of text characters and general input events
(single-key-description and text-char-description). These
are used primarily for making help messages.
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-stringis a semi-obsolete alias for this function.See also the function
formatin Formatting Strings.
This function returns the numeric value of the characters in string. If base is non-
nil, it must be an integer between 2 and 16 (inclusive), and integers are converted in that base. If base isnil, then base ten is used. Floating point conversion only works in base ten; we have not implemented other radices for floating point numbers, because that would be much more work and does not seem useful. If string looks like an integer but its value is too large to fit into a Lisp integer,string-to-numberreturns 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
This function returns a new string containing one character, character. This function is semi-obsolete because the function
stringis more general. See Creating Strings.
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.
See Creating Strings.
vconcat- This function converts a string into a vector. See Vector Functions.
append- This function converts a string into a list. See Building Lists.
byte-to-string- This function converts a byte of character data into a unibyte string. See Converting Representations.
Next: Case Conversion, Previous: String Conversion, Up: Strings and Characters
4.7 Formatting Strings
Formatting means constructing a string by substituting computed values at various places in a constant string. This constant string controls how the other values are printed, as well as where they appear; it is called a format string.
Formatting is often useful for computing messages to be displayed. In
fact, the functions message and error provide the same
formatting feature described here; they differ from format only
in how they use the result of formatting.
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, notprin1—see Output Functions). Thus, strings are represented by their contents alone, with no ‘"’ characters, and symbols appear without ‘\’ characters.If the object is a string, its text properties are copied into the output. The text properties of the ‘%s’ itself are also copied, but those of the object take priority.
- ‘%S’
- Replace the specification with the printed representation of the object,
made with quoting (that is, using
prin1—see Output Functions). Thus, strings are enclosed in ‘"’ characters, and ‘\’ characters appear where necessary before special characters. - ‘%o’
- Replace the specification with the base-eight representation of an
integer.
- ‘%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' actually has %d letters in it."
"foo" (length "foo"))
⇒ "The word ` foo' actually has 3 letters in it."
(format "The word `%7s' actually has %d letters in it."
"specification" (length "specification"))
⇒ "The word `specification' actually 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 ‘-’ causes the padding inserted by the width specifier, if any, to be inserted on the right rather than the left. The flag ‘0’ ensures that the padding consists of ‘0’ characters instead of spaces, inserted on the left. These flags are ignored for specification characters for which they do not make sense: ‘%s’, ‘%S’ and ‘%c’ accept the ‘0’ flag, but still pad with spaces on the left. If both ‘-’ and ‘0’ are present and valid, ‘-’ takes precedence.
(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.
Next: Case Tables, Previous: Formatting Strings, Up: Strings and Characters
4.8 Case Conversion in Lisp
The character case functions change the case of single characters or of the contents of strings. The functions normally convert only alphabetic characters (the letters ‘A’ through ‘Z’ and ‘a’ through ‘z’, as well as non-ASCII letters); other characters are not altered. You can specify a different case conversion mapping by specifying a case table (see Case Tables).
These functions do not modify the strings that are passed to them as arguments.
The examples below use the characters ‘X’ and ‘x’ which have ASCII codes 88 and 120 respectively.
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
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
This function capitalizes strings or characters. If string-or-char is a string, the function returns a new string whose contents are a copy of string-or-char in which each word has been capitalized. This means that the first character of each word is converted to upper case, and the rest are converted to lower case.
The definition of a word is any sequence of consecutive characters that are assigned to the word constituent syntax class in the current syntax table (see Syntax Class Table).
When string-or-char is a character, this function does the same thing as
upcase.(capitalize "The cat in the hat") ⇒ "The Cat In The Hat" (capitalize "THE 77TH-HATTED CAT") ⇒ "The 77th-Hatted Cat" (capitalize ?x) ⇒ 88
If string-or-char is a string, this function capitalizes the initials of the words in string-or-char, without altering any letters other than the initials. It returns a new string whose contents are a copy of string-or-char, in which each word has had its initial letter converted to upper case.
The definition of a word is any sequence of consecutive characters that are assigned to the word constituent syntax class in the current syntax table (see Syntax Class Table).
When the argument to
upcase-initialsis a character,upcase-initialshas the same result asupcase.(upcase-initials "The CAT in the hAt") ⇒ "The CAT In The HAt"
See Text Comparison, for functions that compare strings; some of them ignore case differences, or can optionally ignore case differences.
Previous: Case Conversion, Up: Strings and Characters
4.9 The Case Table
You can customize case conversion by installing a special case table. A case table specifies the mapping between upper case and lower case letters. It affects both the case conversion functions for Lisp objects (see the previous section) and those that apply to text in the buffer (see Case Changes). Each buffer has a case table; there is also a standard case table which is used to initialize the case table of new buffers.
A case table is a char-table (see Char-Tables) whose subtype is
case-table. This char-table maps each character into the
corresponding lower case character. It has three extra slots, which
hold related tables:
- 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:
This function makes table the standard case table, so that it will be used in any buffers created subsequently.
The
with-case-tablemacro 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 viathrowor error (see 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.
The case table for the ASCII character set. This should not be modified by any language environment settings.
The following three functions are convenient subroutines for packages that define non-ASCII character sets. They modify the specified case table case-table; they also modify the standard syntax table. See Syntax Tables. Normally you would use these functions to change the standard case table.
This function specifies a pair of corresponding letters, one upper case and one lower case.
This function makes characters l and r a matching pair of case-invariant delimiters.
This function makes char case-invariant, with syntax syntax.
This command displays a description of the contents of the current buffer's case table.
Next: Sequences Arrays Vectors, Previous: Strings and Characters, Up: Top
5 Lists
A list represents a sequence of zero or more elements (which may be any Lisp objects). The important difference between lists and vectors is that two or more lists can share part of their structure; in addition, you can insert or delete elements in a list without copying the whole list.
Next: List-related Predicates, Up: Lists
5.1 Lists and Cons Cells
Lists in Lisp are not a primitive data type; they are built up from cons cells. A cons cell is a data object that represents an ordered pair. That is, it has two slots, and each slot holds, or refers to, some Lisp object. One slot is known as the car, and the other is known as the cdr. (These names are traditional; see Cons Cell Type.) cdr is pronounced “could-er.”
We say that “the car of this cons cell is” whatever object its car slot currently holds, and likewise for the cdr.
A list is a series of cons cells “chained together,” so that each
cell refers to the next one. There is one cons cell for each element of
the list. By convention, the cars of the cons cells hold the
elements of the list, and the cdrs are used to chain the list: the
cdr slot of each cons cell refers to the following cons cell. The
cdr of the last cons cell is nil. This asymmetry between
the car and the cdr is entirely a matter of convention; at the
level of cons cells, the car and cdr slots have the same
characteristics.
Since nil is the conventional value to put in the cdr of
the last cons cell in the list, we call that case a true list.
In Lisp, we consider the symbol nil a list as well as a
symbol; it is the list with no elements. For convenience, the symbol
nil is considered to have nil as its cdr (and also
as its car). Therefore, the cdr of a true list is always a
true list.
If the cdr of a list's last cons cell is some other value,
neither nil nor another cons cell, we call the structure a
dotted list, since its printed representation would use
‘.’. 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 the 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, the phrase list structure has come to mean any structure made out of cons cells.
The cdr of any nonempty true list l is a list containing all the elements of l except the first.
See Cons Cell Type, for the read and print syntax of cons cells and lists, and for “box and arrow” illustrations of lists.
5.2 Predicates on Lists
The following predicates test whether a Lisp object is an atom,
whether it is a cons cell or is a list, or whether it is the
distinguished object nil. (Many of these predicates can be
defined in terms of the others, but they are used so often that it is
worth having all of them.)
This function returns
tif object is a cons cell,nilotherwise.nilis not a cons cell, although it is a list.
This function returns
tif object is an atom,nilotherwise. All objects except cons cells are atoms. The symbolnilis an atom and is also a list; it is the only Lisp object that is both.(atom object) == (not (consp object))
This function returns
tif object is a cons cell ornil. Otherwise, it returnsnil.(listp '(1)) ⇒ t (listp '()) ⇒ t
This function is the opposite of
listp: it returnstif object is not a list. Otherwise, it returnsnil.(listp object) == (not (nlistp object))
This function returns
tif object isnil, and returnsnilotherwise. This function is identical tonot, but as a matter of clarity we usenullwhen object is considered a list andnotwhen it is considered a truth value (seenotin Combining Conditions).(null '(1)) ⇒ nil (null '()) ⇒ t
Next: Building Lists, Previous: List-related Predicates, Up: Lists
5.3 Accessing Elements of Lists
This function returns the value referred to by the first slot of the cons cell cons-cell. In other words, it returns the car of cons-cell.
As a special case, if cons-cell is
nil, this function returnsnil. Therefore, any list is a valid argument. An error is signaled if the argument is not a cons cell ornil.(car '(a b c)) ⇒ a (car '()) ⇒ nil
This function returns the value referred to by the second slot of the cons cell cons-cell. In other words, it returns the cdr of cons-cell.
As a special case, if cons-cell is
nil, this function returnsnil; therefore, any list is a valid argument. An error is signaled if the argument is not a cons cell ornil.(cdr '(a b c)) ⇒ (b c) (cdr '()) ⇒ nil
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,
nilotherwise. This is in contrast tocar, which signals an error if object is not a list.(car-safe object) == (let ((x object)) (if (consp x) (car x) nil))
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,
nilotherwise. This is in contrast tocdr, which signals an error if object is not a list.(cdr-safe object) == (let ((x object)) (if (consp x) (cdr x) nil))
This macro is a way of examining the car of a list, and taking it off the list, all at once.
It operates on the list which is stored in the symbol listname. It removes this element from the list by setting listname to the cdr of its old value—but it also returns the car of that list, which is the element being removed.
x ⇒ (a b c) (pop x) ⇒ a x ⇒ (b c)
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,
nthreturns 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
eltis similar, but applies to any kind of sequence. For historical reasons, it takes its arguments in the opposite order. See Sequence Functions.
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,
nthcdrreturns all of list. If the length of list is n or less,nthcdrreturnsnil.(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)
This function returns the last link of list. The
carof this link is the list's last element. If list is null,nilis 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.
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
nilor a cons cell,safe-lengthreturns 0.
The most common way to compute the length of a list, when you are not
worried that it may be circular, is with length. See Sequence Functions.
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 (butlastx n) (lastx n))will return a list equal to x.
This is a version of
butlastthat works by destructively modifying thecdrof the appropriate element, rather than making a copy of the list.
Next: List Variables, Previous: List Elements, Up: Lists
5.4 Building Cons Cells and Lists
Many functions build lists, as lists reside at the very heart of Lisp.
cons is the fundamental list-building function; however, it is
interesting to note that list is used more times in the source
code for Emacs than cons.
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)
consis 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
listused in this example and the function namedlistdescribed below; any symbol can serve both purposes.
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
This function creates a list of length elements, in which each element is object. Compare
make-listwithmake-string(see Creating Strings).(make-list 3 'pigs) ⇒ (pigs pigs pigs) (make-list 0 'pigs) ⇒ nil (setq l (make-list 3 '(a b)) ⇒ ((a b) (a b) (a b)) (eq (car l) (cadr l)) ⇒ t
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
nconcin Rearrangement, for a way to join lists with no copying.)More generally, the final argument to
appendmay 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 notnilas 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. See Sequences Arrays Vectors.
Here we show the use of vectors and strings as arguments to append:
(append [a b] "cd" nil)
⇒ (a b 99 100)
With the help of apply (see Calling Functions), we can append
all the lists in a list of lists:
(apply 'append '((a b c) nil (x y z) nil))
⇒ (a b c x y z)
If no sequences are given, nil is returned:
(append)
⇒ nil
Here are some examples where the final argument is not a list:
(append '(x y) 'z)
⇒ (x y . z)
(append '(x y) [z])
⇒ (x y . [z])
The second example shows that when the final argument is a sequence but not a list, the sequence's elements do not become elements of the resulting list. Instead, the sequence becomes the final cdr, like any other non-list final argument.
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)
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-treesimply returns tree. However, if vecp is non-nil, it copies vectors too (and operates recursively on their elements).
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
nilor 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 isnilbecause those arguments specify an empty sequence.If separation is 0 and to is neither
nilnor numerically equal to from,number-sequencesignals 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)
Next: Modifying Lists, Previous: Building Lists, Up: Lists
5.5 Modifying List Variables
These functions, and one macro, provide convenient ways to modify a list which is stored in a variable.
This macro provides an alternative way to write
(setqlistname(consnewelt listname)).(setq l '(a b)) ⇒ (a b) (push 'c l) ⇒ (c a b) l ⇒ (c a b)
Two functions modify lists that are the values of variables.
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-listuses compare-fn to compare element against existing list members; if compare-fn isnil, it usesequal.Normally, if element is added, it is added to the front of symbol, but if the optional argument append is non-
nil, it is added at the end.The argument symbol is not implicitly quoted;
add-to-listis an ordinary function, likesetand unlikesetq. 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)))
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-listis an ordinary function, likesetand unlikesetq. Quote the argument yourself if that is what you want.The ordering information is stored in a hash table on symbol's
list-orderproperty.
Here's a scenario showing how to use add-to-ordered-list:
(setq foo '())
⇒ nil
(add-to-ordered-list 'foo 'a 1) ;; Add a.
⇒ (a)
(add-to-ordered-list 'foo 'c 3) ;; Add c.
⇒ (a c)
(add-to-ordered-list 'foo 'b 2) ;; Add b.
⇒ (a b c)
(add-to-ordered-list 'foo 'b 4) ;; Move b.
⇒ (a c b)
(add-to-ordered-list 'foo 'd) ;; Append d.
⇒ (a c b d)
(add-to-ordered-list 'foo 'e) ;; Add e.
⇒ (a c b e d)
foo ;; foo was changed.
⇒ (a c b e d)
Next: Sets And Lists, Previous: List Variables, Up: Lists
5.6 Modifying Existing List Structure
You can modify the car and cdr contents of a cons cell with the
primitives setcar and setcdr. We call these “destructive”
operations because they change existing list structure.
Common Lisp note: Common Lisp uses functionsrplacaandrplacdto alter list structure; they change structure the same way assetcarandsetcdr, but the Common Lisp functions return the cons cell whilesetcarandsetcdrreturn the new car or cdr.
Next: Setcdr, Up: Modifying Lists
5.6.1 Altering List Elements with setcar
Changing the car of a cons cell is done with setcar. When
used on a list, setcar replaces one element of a list with a
different element.
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----
| | |
--------------
Next: Rearrangement, Previous: Setcar, Up: Modifying Lists
5.6.2 Altering the CDR of a List
The lowest-level primitive for modifying a cdr is setcdr:
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------
| | |
---------------
Previous: Setcdr, Up: Modifying Lists
5.6.3 Functions that Rearrange Lists
Here are some functions that rearrange lists “destructively” by modifying the cdrs of their component cons cells. We call these functions “destructive” because they chew up the original lists passed to them as arguments, relinking their cons cells to form a new list that is the returned value.
See delq, in Sets And Lists, for another function
that modifies cons cells.
This function returns a list containing all the elements of lists. Unlike
append(see Building Lists), the lists are not copied. Instead, the last cdr of each of the lists is changed to refer to the following list. The last of the lists is not altered. For example:(setq x '(1 2 3)) ⇒ (1 2 3) (nconc x '(4 5)) ⇒ (1 2 3 4 5) x ⇒ (1 2 3 4 5)Since the last argument of
nconcis 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)) ;footo 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)))
This function reverses the order of the elements of list. Unlike
reverse,nreversealters 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
nreverseback in the same variable which held the original list:(setq x (nreverse x))Here is the
nreverseof 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 | | | | | | | | | | | | | | ------------- | --------- | - | -------- | - | | | | ------------- ------------
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-
nilif the first element is “less than” the second, ornilif 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 ofsortis unpredictable.The destructive aspect of
sortis 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 withcopy-sequenceand then sort.Sorting does not change the cars of the cons cells in list; the cons cell that originally contained the element
ain list still hasain 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
numsno 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 ofsortand use that. Most often we store the result back into the variable that held the original list:(setq nums (sort nums '<))See Sorting, for more functions that perform sorting. See
documentationin Accessing Documentation, for a useful example ofsort.
Next: Association Lists, Previous: Modifying Lists, Up: Lists
5.7 Using Lists as Sets
A list can represent an unordered mathematical set—simply consider a
value an element of a set if it appears in the list, and ignore the
order of the list. To form the union of two sets, use append (as
long as you don't mind having duplicate elements). You can remove
equal duplicates using delete-dups. Other useful
functions for sets include memq and delq, and their
equal versions, member and delete.
Common Lisp note: Common Lisp has functionsunion(which avoids duplicate elements) andintersectionfor set operations, but GNU Emacs Lisp does not have them. You can write them in Lisp if you wish.
This function tests to see whether object is a member of list. If it is,
memqreturns a list starting with the first occurrence of object. Otherwise, it returnsnil. The letter ‘q’ inmemqsays that it useseqto 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 noteq. ⇒ nil
This function destructively removes all elements
eqto object from list. The letter ‘q’ indelqsays that it useseqto compare object against the elements of the list, likememqandremq.
When delq deletes elements from the front of the list, it does so
simply by advancing down the list and returning a sublist that starts
after those elements:
(delq 'a '(a b c)) == (cdr '(a b c))
When an element to be deleted appears in the middle of the list, removing it involves changing the cdrs (see Setcdr).
(setq sample-list '(a b c (4)))
⇒ (a b c (4))
(delq 'a sample-list)
⇒ (b c (4))
sample-list
⇒ (a b c (4))
(delq 'c sample-list)
⇒ (a b (4))
sample-list
⇒ (a b (4))
Note that (delq 'c sample-list) modifies sample-list to
splice out the third element, but (delq 'a sample-list) does not
splice anything—it just returns a shorter list. Don't assume that a
variable which formerly held the argument list now has fewer
elements, or that it still holds the original list! Instead, save the
result of delq and use that. Most often we store the result back
into the variable that held the original list:
(setq flowers (delq 'rose flowers))
In the following example, the (4) that delq attempts to match
and the (4) in the sample-list are not eq:
(delq '(4) sample-list)
⇒ (a c (4))
If you want to delete elements that are equal to a given value,
use delete (see below).
This function returns a copy of list, with all elements removed which are
eqto object. The letter ‘q’ inremqsays that it useseqto compare object against the elements oflist.(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)
The function
memqltests to see whether object is a member of list, comparing members with object usingeql, so floating point elements are compared by value. If object is a member,memqlreturns a list starting with its first occurrence in list. Otherwise, it returnsnil.Compare this with
memq:(memql 1.2 '(1.1 1.2 1.3)) ;1.2and1.2areeql. ⇒ (1.2 1.3) (memq 1.2 '(1.1 1.2 1.3)) ;1.2and1.2are noteq. ⇒ nil
The following three functions are like memq, delq and
remq, but use equal rather than eq to compare
elements. See Equality Predicates.
The function
membertests to see whether object is a member of list, comparing members with object usingequal. If object is a member,memberreturns a list starting with its first occurrence in list. Otherwise, it returnsnil.Compare this with
memq:(member '(2) '((1) (2))) ;(2)and(2)areequal. ⇒ ((2)) (memq '(2) '((1) (2))) ;(2)and(2)are noteq. ⇒ nil ;; Two strings with the same contents areequal. (member "foo" '("foo" "bar")) ⇒ ("foo" "bar")
If
sequenceis a list, this function destructively removes all elementsequalto object from sequence. For lists,deleteis todelqasmemberis tomemq: it usesequalto compare elements with object, likemember; when it finds an element that matches, it cuts the element out just asdelqwould.If
sequenceis a vector or string,deletereturns a copy ofsequencewith all elementsequaltoobjectremoved.For example:
(setq l '((2) (1) (2))) (delete '(2) l) ⇒ ((1)) l ⇒ ((2) (1)) ;; If you want to changelreliably, ;; write(setq l (delete elt l)). (setq l '((2) (1) (2))) (delete '(1) l) ⇒ ((2) (2)) l ⇒ ((2) (2)) ;; In this case, it makes no difference whether you setl, ;; but you should do so for the sake of the other case. (delete '(2) [(2) (1) (2)]) ⇒ [(1)]
This function is the non-destructive counterpart of
delete. It returns a copy ofsequence, a list, vector, or string, with elementsequaltoobjectremoved. For example:(remove '(2) '((2) (1) (2))) ⇒ ((1)) (remove '(2) [(2) (1) (2)]) ⇒ [(1)]
Common Lisp note: The functionsmember,deleteandremovein GNU Emacs Lisp are derived from Maclisp, not Common Lisp. The Common Lisp versions do not useequalto compare elements.
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.
This function destructively removes all
equalduplicates from list, stores the result in list and returns it. Of severalequaloccurrences of an element in list,delete-dupskeeps the first one.
See also the function add-to-list, in List Variables,
for a way to add an element to a list stored in a variable and used as a
set.
Next: Rings, Previous: Sets And Lists, Up: Lists
5.8 Association Lists
An association list, or alist for short, records a mapping from keys to values. It is a list of cons cells called associations: the car of each cons cell is the key, and the cdr is the associated value.2
Here is an example of an alist. The key pine is associated with
the value cones; the key oak is associated with
acorns; and the key maple is associated with seeds.
((pine . cones)
(oak . acorns)
(maple . seeds))
Both the values and the keys in an alist may be any Lisp objects.
For example, in the following alist, the symbol a is
associated with the number 1, and the string "b" is
associated with the list (2 3), which is the cdr of
the alist element:
((a . 1) ("b" 2 3))
Sometimes it is better to design an alist to store the associated value in the car of the cdr of the element. Here is an example of such an alist:
((rose red) (lily white) (buttercup yellow))
Here we regard red as the value associated with rose. One
advantage of this kind of alist is that you can store other related
information—even a list of other items—in the cdr of the
cdr. One disadvantage is that you cannot use rassq (see
below) to find the element containing a given value. When neither of
these considerations is important, the choice is a matter of taste, as
long as you are consistent about it for any given alist.
The same alist shown above could be regarded as having the
associated value in the cdr of the element; the value associated
with rose would be the list (red).
Association lists are often used to record information that you might otherwise keep on a stack, since new associations may be added easily to the front of the list. When searching an association list for an association with a given key, the first one found is returned, if there is more than one.
In Emacs Lisp, it is not an error if an element of an association list is not a cons cell. The alist search functions simply ignore such elements. Many other versions of Lisp signal errors in such cases.
Note that property lists are similar to association lists in several respects. A property list behaves like an association list in which each key can occur only once. See Property Lists, for a comparison of property lists and association lists.
This function returns the first association for key in alist, comparing key against the alist elements using
equal(see Equality Predicates). It returnsnilif no association in alist has a carequalto 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) ⇒ nilHere is another example, in which the keys and values are not symbols:
(setq needles-per-cluster '((2 "Austrian Pine" "Red Pine") (3 "Pitch Pine") (5 "White Pine"))) (cdr (assoc 3 needles-per-cluster)) ⇒ ("Pitch Pine") (cdr (assoc 2 needles-per-cluster)) ⇒ ("Austrian Pine" "Red Pine")
The function assoc-string is much like assoc except
that it ignores certain differences between strings. See Text Comparison.
This function returns the first association with value value in alist. It returns
nilif no association in alist has a cdrequalto value.
rassocis likeassocexcept that it compares the cdr of each alist association instead of the car. You can think of this as “reverseassoc,” finding the key for a given value.
This function is like
associn that it returns the first association for key in alist, but it makes the comparison usingeqinstead ofequal.assqreturnsnilif no association in alist has a careqto key. This function is used more often thanassoc, sinceeqis faster thanequaland most alists use symbols as keys. See Equality Predicates.(setq trees '((pine . cones) (oak . acorns) (maple . seeds))) ⇒ ((pine . cones) (oak . acorns) (maple . seeds)) (assq 'pine trees) ⇒ (pine . cones)On the other hand,
assqis 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)
This function returns the first association with value value in alist. It returns
nilif no association in alist has a cdreqto value.
rassqis likeassqexcept that it compares the cdr of each alist association instead of the car. You can think of this as “reverseassq,” finding the key for a given value.For example:
(setq trees '((pine . cones) (oak . acorns) (maple . seeds))) (rassq 'acorns trees) ⇒ (oak . acorns) (rassq 'spores trees) ⇒ nil
rassqcannot 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) ⇒ nilIn this case, the cdr of the association
(lily white)is not the symbolwhite, but rather the list(white). This becomes clearer if the association is written in dotted pair notation:(lily white) == (lily . (white))
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-matchwith an alist that contains regular expressions (see Regexp Search). If test is omitted ornil,equalis used for comparison.If an alist element matches key by this criterion, then
assoc-defaultreturns 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-defaultreturnsnil.
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)))) ⇒ tThis example shows how
copy-alistmakes 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")
This function deletes from alist all the elements whose car is
eqto key, much as if you useddelqto delete each such element one by one. It returns the shortened alist, and often modifies the original list structure of alist. For correct results, use the return value ofassq-delete-allrather 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))
This function deletes from alist all the elements whose cdr is
eqto value. It returns the shortened alist, and often modifies the original list structure of alist.rassq-delete-allis likeassq-delete-allexcept that it compares the cdr of each alist association instead of the car.
Previous: Association Lists, Up: Lists
5.9 Managing a Fixed-Size Ring of Objects
This section describes functions for operating on rings. A ring is a fixed-size data structure that supports insertion, deletion, rotation, and modulo-indexed reference and traversal.
This returns a new ring capable of holding size objects. size should be an integer.
This returns the number of objects that ring currently contains. The value will never exceed that returned by
ring-size.
This returns a new ring which is a copy of ring. The new ring contains the same (
eq) objects as ring.
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.
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-refsignals an error.
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.
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-removesignals an error.
This inserts object into ring, treating it as the oldest element. The return value is not significant.
If the ring is full, this function removes the newest element to make room for the inserted element.
If you are careful not to exceed the ring size, you can use the ring as a first-in-first-out queue. For example:
(let ((fifo (make-ring 5)))
(mapc (lambda (obj) (ring-insert fifo obj))
'(0 one "two"))
(list (ring-remove fifo) t
(ring-remove fifo) t
(ring-remove fifo)))
⇒ (0 t one t "two")
Next: Hash Tables, Previous: Lists, Up: Top
6 Sequences, Arrays, and Vectors
Recall that the sequence type is the union of two other Lisp types: lists and arrays. In other words, any list is a sequence, and any array is a sequence. The common property that all sequences have is that each is an ordered collection of elements.
An array is a fixed-length object with a slot for each of its elements. All the elements are accessible in constant time. The four types of arrays are strings, vectors, char-tables and bool-vectors.
A list is a sequence of elements, but it is not a single primitive object; it is made of cons cells, one cell per element. Finding the nth element requires looking through n cons cells, so elements farther from the beginning of the list take longer to access. But it is possible to add elements to the list, or remove elements.
The following diagram shows the relationship between these types:
_____________________________________________
| |
| Sequence |
| ______ ________________________________ |
| | | | | |
| | List | | Array | |
| | | | ________ ________ | |
| |______| | | | | | | |
| | | Vector | | String | | |
| | |________| |________| | |
| | ____________ _____________ | |
| | | | | | | |
| | | Char-table | | Bool-vector | | |
| | |____________| |_____________| | |
| |________________________________| |
|_____________________________________________|
Next: Arrays, Up: Sequences Arrays Vectors
6.1 Sequences
In Emacs Lisp, a sequence is either a list or an array. The common property of all sequences is that they are ordered collections of elements. This section describes functions that accept any kind of sequence.
Returns
tif object is a list, vector, string, bool-vector, or char-table,nilotherwise.
This function returns the number of elements in sequence. If sequence is a dotted list, a
wrong-type-argumenterror 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.See Definition of safe-length, for the related function
safe-length.(length '(1 2 3)) ⇒ 3 (length ()) ⇒ 0 (length "foobar") ⇒ 6 (length [1 2 3]) ⇒ 3 (length (make-bool-vector 5 nil)) ⇒ 5
See also string-bytes, in Text Representations.
This function returns the element of sequence indexed by index. Legitimate values of index are integers ranging from 0 up to one less than the length of sequence. If sequence is a list, out-of-range values behave as for
nth. See Definition of nth. Otherwise, out-of-range values trigger anargs-out-of-rangeerror.(elt [1 2 3 4] 2) ⇒ 3 (elt '(1 2 3 4) 2) ⇒ 3 ;; We usestringto show clearly which charactereltreturns. (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], -1This function generalizes
aref(see Array Functions) andnth(see Definition of nth).
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. See Text Properties.
This function does not work for dotted lists. Trying to copy a circular list may cause an infinite loop.
See also
appendin Building Lists,concatin Creating Strings, andvconcatin 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)]
Next: Array Functions, Previous: Sequence Functions, Up: Sequences Arrays Vectors
6.2 Arrays
An array object has slots that hold a number of other Lisp objects, called the elements of the array. Any element of an array may be accessed in constant time. In contrast, the time to access an element of a list is proportional to the position of that element in the list.
Emacs defines four types of array, all one-dimensional:
strings (see String Type), vectors (see Vector Type), bool-vectors (see Bool-Vector Type), and
char-tables (see Char-Table Type). Vectors and char-tables
can hold elements of any type, but strings can only hold characters,
and bool-vectors can only hold t and nil.
All four kinds of array share these characteristics:
- 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—in other words, it evaluates to itself.
- The elements of an array may be referenced or changed with the functions
arefandaset, respectively (see Array Functions).
When you create an array, other than a char-table, you must specify its length. You cannot specify the length of a char-table, because that is determined by the range of character codes.
In principle, if you want an array of text characters, you could use either a string or a vector. In practice, we always choose strings for such applications, for four reasons:
- They occupy one-fourth the space of a vector of the same elements.
- Strings are printed in a way that shows the contents more clearly as text.
- Strings can hold text properties. See Text Properties.
- Many of the specialized editing and I/O facilities of Emacs accept only strings. For example, you cannot insert a vector of characters into a buffer the way you can insert a string. See Strings and Characters.
By contrast, for an array of keyboard input characters (such as a key sequence), a vector may be necessary, because many keyboard input characters are outside the range that will fit in a string. See Key Sequence Input.
Next: Vectors, Previous: Arrays, Up: Sequences Arrays Vectors
6.3 Functions that Operate on Arrays
In this section, we describe the functions that accept all types of arrays.
This function returns
tif 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
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 Sequence Functions.
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-argumenterror results. The function converts a unibyte string to multibyte if necessary to insert a character.
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-argumenterror results.
The general sequence functions copy-sequence and length
are often useful for objects known to be arrays. See Sequence Functions.
Next: Vector Functions, Previous: Array Functions, Up: Sequences Arrays Vectors
6.4 Vectors
A vector is a general-purpose array whose elements can be any Lisp objects. (By contrast, the elements of a string can only be characters. See Strings and Characters.) Vectors are used in Emacs for many purposes: as key sequences (see Key Sequences), as symbol-lookup tables (see Creating Symbols), as part of the representation of a byte-compiled function (see Byte Compilation), and more.
In Emacs Lisp, the indices of the elements of a vector start from zero and count up from there.
Vectors are printed with square brackets surrounding the elements.
Thus, a vector whose elements are the symbols a, b and
a is printed as [a b a]. You can write vectors in the
same way in Lisp input.
A vector, like a string or a number, is considered a constant for evaluation: the result of evaluating it is the same vector. This does not evaluate or even examine the elements of the vector. See Self-Evaluating Forms.
Here are examples illustrating these principles:
(setq avector [1 two '(three) "four" [five]])
⇒ [1 two (quote (three)) "four" [five]]
(eval avector)
⇒ [1 two (quote (three)) "four" [five]]
(eq avector (eval avector))
⇒ t
Next: Char-Tables, Previous: Vectors, Up: Sequences Arrays Vectors
6.5 Functions for Vectors
Here are some functions that relate to vectors:
This function returns
tif object is a vector.(vectorp [a]) ⇒ t (vectorp "asdf") ⇒ nil
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) ⇒ []
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]
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
eqto 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
vconcatfunction also allows byte-code function objects as arguments. This is a special feature to make it easy to access the entire contents of a byte-code function object. See Byte-Code Objects.For other concatenation functions, see
mapconcatin Mapping Functions,concatin Creating Strings, andappendin 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-tableas the subtype, and syntax tables are char-tables withsyntax-tableas the subtype. The subtype can be queried using the functionchar-table-subtype, described below. - The subtype controls the number of extra slots in the
char-table. This number is specified by the subtype's
char-table-extra-slotssymbol property, which should be an integer between 0 and 10. If the subtype has no such symbol property, the char-table has no extra slots. See Property Lists, for information about symbol properties.
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.
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-slotssymbol property, that specifies the number of extra slots in the char-table. This should be an integer between 0 and 10; otherwise,make-char-tableraises an error. If subtype has nochar-table-extra-slotssymbol property (see Property Lists), the char-table has no extra slots.
This function returns
tif object is a char-table, andnilotherwise.
There is no special function to access default values in a char-table.
To do that, use char-table-range (see below).
This function returns the parent of char-table. The parent is always either
nilor another char-table.
This function sets the parent of char-table to new-parent.
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.
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.
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]’.
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]’.
This function calls its argument function for each element of char-table that has a non-
nilvalue. The call to function is with two arguments, a key and a value. The key is a possible range argument forchar-table-range—either a valid character or a cons cell(from.to), specifying a range of characters that share the same value. The value is what(char-table-rangechar-table key)returns.Overall, the key-value pairs passed to function describe all the values stored in char-table.
The return value is always
nil; to make calls tomap-char-tableuseful, 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.
Return a new bool-vector of length elements, each one initialized to initial.
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.
Next: Symbols, Previous: Sequences Arrays Vectors, Up: Top
7 Hash Tables
A hash table is a very fast kind of lookup table, somewhat like an alist (see Association Lists) in that it maps keys to corresponding values. It differs from an alist in these ways:
- Lookup in a hash table is extremely fast for large tables—in fact, the time required is essentially independent of how many elements are stored in the table. For smaller tables (a few tens of elements) alists may still be faster because hash tables have a more-or-less constant overhead.
- The correspondences in a hash table are in no particular order.
- There is no way to share structure between two hash tables, the way two alists can share a common tail.
Emacs Lisp provides a general-purpose hash table data type, along with a series of functions for operating on them. Hash tables have a special printed representation, which consists of ‘#s’ followed by a list specifying the hash table properties and contents. See Creating Hash. (Note that the term “hash notation”, which refers to the initial ‘#’ character used in the printed representations of objects with no read representation, has nothing to do with the term “hash table”. See Printed Representation.)
Obarrays are also a kind of hash table, but they are a different type of object and are used only for recording interned symbols (see Creating Symbols).
Next: Hash Access, Up: Hash Tables
7.1 Creating Hash Tables
The principal function for creating a hash table is
make-hash-table.
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:testand:weakness.
:testtest- This specifies the method of key lookup for this hash table. The default is
eql;eqandequalare 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(see Defining Hash) to define additional possibilities for test.:weaknessweak- 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, ortwhich is an alias forkey-and-value. If weak iskeythen 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-valueort, 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.:sizesize- 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-sizerehash-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-thresholdthreshold- 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.
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-tableinstead.
You can also create a new hash table using the printed representation
for hash tables. The Lisp reader can read this printed
representation, provided each element in the specified hash table has
a valid read syntax (see Printed Representation). For instance,
the following specifies a new hash table containing the keys
key1 and key2 (both symbols) associated with val1
(a symbol) and 300 (a number) respectively.
#s(hash-table size 30 data (key1 val1 key2 300))
The printed representation for a hash table consists of ‘#s’
followed by a list beginning with ‘hash-table’. The rest of the
list should consist of zero or more property-value pairs specifying
the hash table's properties and initial contents. The properties and
values are read literally. Valid property names are size,
test, weakness, rehash-size,
rehash-threshold, and data. The data property
should be a list of key-value pairs for the initial contents; the
other properties have the same meanings as the matching
make-hash-table keywords (:size, :test, etc.),
described above.
Note that you cannot specify a hash table whose initial contents include objects that have no read syntax, such as buffers and frames. Such objects may be added to the hash table after it is created.
Next: Defining Hash, Previous: Creating Hash, Up: Hash Tables
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.
This function looks up key in table, and returns its associated value—or default, if key has no association in table.
This function enters an association for key in table, with value value. If key already has an association in table, value replaces the old associated value.
This function removes the association for key from table, if there is one. If key has no association,
remhashdoes nothing.Common Lisp note: In Common Lisp,
remhashreturns non-nilif it actually removed an association andnilotherwise. In Emacs Lisp,remhashalways returnsnil.
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,
clrhashreturns the empty table. In Emacs Lisp, it returnsnil.
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.
maphashreturnsnil.
Next: Other Hash, Previous: Hash Access, Up: Hash Tables
7.3 Defining Hash Comparisons
You can define new methods of key lookup by means of
define-hash-table-test. In order to use this feature, you need
to understand how hash tables work, and what a hash code means.
You can think of a hash table conceptually as a large array of many
slots, each capable of holding one association. To look up a key,
gethash first computes an integer, the hash code, from the key.
It reduces this integer modulo the length of the array, to produce an
index in the array. Then it looks in that slot, and if necessary in
other nearby slots, to see if it has found the key being sought.
Thus, to define a new method of key lookup, you need to specify both a function to compute the hash code from a key, and a function to compare two keys directly.
This 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-
nilif 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).
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
(sxhashobj1)and(sxhashobj2)are the same integer.If the two objects are not equal, the values returned by
sxhashare usually different, but not always; once in a rare while, by luck, you will encounter two distinct-looking objects that give the same result fromsxhash.
This example creates a hash table whose keys are strings that are compared case-insensitively.
(defun case-fold-string= (a b)
(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)
Previous: Defining Hash, Up: Hash Tables
7.4 Other Hash Table Functions
Here are some other functions for working with hash tables.
This function creates and returns a copy of table. Only the table itself is copied—the keys and values are shared.
This returns the test value that was given when table was created, to specify how to hash and compare keys. See
make-hash-table(see Creating Hash).
This function returns the weak value that was specified for hash table table.
Next: Evaluation, Previous: Hash Tables, Up: Top
8 Symbols
A symbol is an object with a unique name. This chapter describes symbols, their components, their property lists, and how they are created and interned. Separate chapters describe the use of symbols as variables and as function names; see Variables, and Functions. For the precise read syntax for symbols, see Symbol Type.
You can test whether an arbitrary Lisp object is a symbol
with symbolp:
Next: Definitions, Previous: Symbols, Up: Symbols
8.1 Symbol Components
Each symbol has four components (or “cells”), each of which references another object:
- Print name
- The print name cell holds a string that names the symbol for
reading and printing. See
symbol-namein Creating Symbols. - Value
- The value cell holds the current value of the symbol as a
variable. When a symbol is used as a form, the value of the form is the
contents of the symbol's value cell. See
symbol-valuein Accessing Variables. - Function
- The function cell holds the function definition of the symbol.
When a symbol is used as a function, its function definition is used in
its place. This cell is also used to make a symbol stand for a keymap
or a keyboard macro, for editor command execution. Because each symbol
has separate value and function cells, variables names and function names do
not conflict. See
symbol-functionin Function Cells. - Property list
- The property list cell holds the property list of the symbol. See
symbol-plistin Property Lists.
The print name cell always holds a string, and cannot be changed. The other three cells can be set individually to any specified Lisp object.
The print name cell holds the string that is the name of the 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. (In GNU Emacs Lisp, this lookup uses a hashing algorithm and an obarray; see Creating Symbols.)
The value cell holds the symbol's value as a variable
(see Variables). That is what you get if you evaluate the symbol as
a Lisp expression (see Evaluation). Any Lisp object is a legitimate
value. Certain symbols have values that cannot be changed; these
include nil and t, and any symbol whose name starts with
‘:’ (those are called keywords). See Constant Variables.
We often refer to “the function foo” when we really mean
the function stored in the function cell of the symbol foo. We
make the distinction explicit only when necessary. In normal
usage, the function cell usually contains a function
(see Functions) or a macro (see Macros), as that is what the
Lisp interpreter expects to see there (see Evaluation). Keyboard
macros (see Keyboard Macros), keymaps (see Keymaps) and
autoload objects (see Autoloading) are also sometimes stored in
the function cells of symbols.
The property list cell normally should hold a correctly formatted property list (see Property Lists), as a number of functions expect to see a property list there.
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’.
The four functions symbol-name, symbol-value,
symbol-plist, and symbol-function return the contents of
the four cells of a symbol. Here as an example we show the contents of
the four cells of the symbol buffer-file-name:
(symbol-name 'buffer-file-name)
⇒ "buffer-file-name"
(symbol-value 'buffer-file-name)
⇒ "/gnu/elisp/symbols.texi"
(symbol-function 'buffer-file-name)
⇒ #<subr buffer-file-name>
(symbol-plist 'buffer-file-name)
⇒ (variable-documentation 29529)
Because this symbol is the variable which holds the name of the file
being visited in the current buffer, the value cell contents we see are
the name of the source file of this chapter of the Emacs Lisp Manual.
The property list cell contains the list (variable-documentation
29529) which tells the documentation functions where to find the
documentation string for the variable buffer-file-name in the
DOC-version file. (29529 is the offset from the beginning
of the DOC-version file to where that documentation string
begins—see Documentation Basics.) The function cell contains
the function for returning the name of the file.
buffer-file-name names a primitive function, which has no read
syntax and prints in hash notation (see Primitive Function Type). A
symbol naming a function written in Lisp would have a lambda expression
(or a byte-code object) in this cell.
Next: Creating Symbols, Previous: Symbol Components, Up: Symbols
8.2 Defining Symbols
A definition in Lisp is a special form that announces your intention to use a certain symbol in a particular way. In Emacs Lisp, you can define a symbol as a variable, or define it as a function (or macro), or both independently.
A definition construct 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. They are documented in detail in
Defining Variables. For defining user option variables that can
be customized, use defcustom (see Customization).
defun defines a symbol as a function, creating a lambda
expression and storing it in the function cell of the symbol. This
lambda expression thus becomes the function definition of the symbol.
(The term “function definition,” meaning the contents of the function
cell, is derived from the idea that defun gives the symbol its
definition as a function.) defsubst and defalias are two
other ways of defining a function. See Functions.
defmacro defines a symbol as a macro. It creates a macro
object and stores it in the function cell of the symbol. Note that a
given symbol can be a macro or a function, but not both at once, because
both macro and function definitions are kept in the function cell, and
that cell can hold only one Lisp object at any given time.
See Macros.
In Emacs Lisp, a definition is not required in order to use a symbol
as a variable or function. Thus, you can make a symbol a global
variable with setq, whether you define it first or not. The real
purpose of definitions is to guide programmers and programming tools.
They inform programmers who read the code that certain symbols are
intended to be used as variables, or as functions. In addition,
utilities such as etags and make-docfile recognize
definitions, and add appropriate information to tag tables and the
DOC-version file. See Accessing Documentation.
Next: Property Lists, Previous: Definitions, Up: Symbols
8.3 Creating and Interning Symbols
To understand how symbols are created in GNU Emacs Lisp, you must know how Lisp reads them. Lisp must ensure that it finds the same symbol every time it reads the same set of characters. Failure to do so would cause complete confusion.
When the Lisp reader encounters a symbol, it reads all the characters of the name. Then it “hashes” those characters to find an index in a table called an obarray. Hashing is an efficient method of looking something up. For example, instead of searching a telephone book cover to cover when looking up Jan Jones, you start with the J's and go from there. That is a simple version of hashing. Each element of the obarray is a bucket which holds all the symbols with a given hash code; to look for a given name, it is sufficient to look through all the symbols in the bucket for that name's hash code. (The same idea is used for general Emacs hash tables, but they are a different data type; see Hash Tables.)
If a symbol with the desired name is found, the reader uses that symbol. If the obarray does not contain a symbol with that name, the reader makes a new symbol and adds it to the obarray. Finding or adding a symbol with a certain name is called interning it, and the symbol is then called an interned symbol.
Interning ensures that each obarray has just one symbol with any particular name. Other like-named symbols may exist, but not in the same obarray. Thus, the reader gets the same symbols for the same names, as long as you keep reading with the same obarray.
Interning usually happens automatically in the reader, but sometimes other programs need to do it. For example, after the M-x command obtains the command name as a string using the minibuffer, it then interns the string, to get the interned symbol with that name.
No obarray contains all symbols; in fact, some symbols are not in any obarray. They are called uninterned symbols. An uninterned symbol has the same four cells as other symbols; however, the only way to gain access to it is by finding it in some other object or as the value of a variable.
Creating an uninterned symbol is useful in generating Lisp code, because an uninterned symbol used as a variable in the code you generate cannot clash with any variables used in other Lisp programs.
In Emacs Lisp, an obarray is actually a vector. Each element of the
vector is a bucket; its value is either an interned symbol whose name
hashes to that bucket, or 0 if the bucket is empty. Each interned
symbol has an internal link (invisible to the user) to the next symbol
in the bucket. Because these links are invisible, there is no way to
find all the symbols in an obarray except using mapatoms (below).
The order of symbols in a bucket is not significant.
In an empty obarray, every element is 0, so you can create an obarray
with (make-vector length 0). This is the only
valid way to create an obarray. Prime numbers as lengths tend
to result in good hashing; lengths one less than a power of two are also
good.
Do not try to put symbols in an obarray yourself. This does
not work—only intern can enter a symbol in an obarray properly.
Common Lisp note: In Common Lisp, a single symbol may be interned 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.
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!
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 ofsymis noteqtofoobecause it is a distinct uninterned symbol whose name is also ‘foo’.(setq sym (make-symbol "foo")) ⇒ foo (eq sym 'foo) ⇒ nil
This function returns the interned symbol whose name is name. If there is no such symbol in the obarray obarray,
interncreates a new one, adds it to the obarray, and returns it. If obarray is omitted, the value of the global variableobarrayis 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.
This function returns the symbol in obarray whose name is name, or
nilif obarray has no symbol with that name. Therefore, you can useintern-softto test whether a symbol with a given name is already interned. If obarray is omitted, the value of the global variableobarrayis 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
This function calls function once with each symbol in the obarray obarray. Then it returns
nil. If obarray is omitted, it defaults to the value ofobarray, the standard obarray for ordinary symbols.(setq count 0) ⇒ 0 (defun count-syms (s) (setq count (1+ count))) ⇒ count-syms (mapatoms 'count-syms) ⇒ nil count ⇒ 1871See
documentationin Accessing Documentation, for another example usingmapatoms.
This function deletes symbol from the obarray obarray. If
symbolis not actually in the obarray,uninterndoes nothing. If obarray isnil, the current obarray is used.If you provide a string instead of a symbol as symbol, it stands for a symbol name. Then
uninterndeletes the symbol (if any) in the obarray which has that name. If there is no such symbol,uninterndoes nothing.If
uninterndoes delete a symbol, it returnst. Otherwise it returnsnil.
Previous: Creating Symbols, Up: Symbols
8.4 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.
Every symbol has a cell that stores a property list (see Symbol Components). This property list is used to record information about the symbol, such as its variable documentation and the name of the file where it was defined.
Property lists can also be used in other contexts. For instance, you can assign property lists to character positions in a string or buffer. See Text Properties.
The property names and values in a property list can be any Lisp
objects, but the names are usually symbols. Property list functions
compare the property names using eq. Here is an example of a
property list, found on the symbol progn when the compiler is
loaded:
(lisp-indent-function 0 byte-compile byte-compile-progn)
Here lisp-indent-function and byte-compile are property
names, and the other two elements are the corresponding values.
Next: Symbol Plists, Up: Property Lists
8.4.1 Property Lists and Association Lists
Association lists (see Association Lists) are very similar to property lists. In contrast to association lists, the order of the pairs in the property list is not significant since the property names must be distinct.
Property lists are better than association lists for attaching
information to various Lisp function names or variables. If your
program keeps all of its associations in one association list, it will
typically need to search that entire list each time it checks for an
association. This 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.
Next: Other Plists, Previous: Plists and Alists, Up: Property Lists
8.4.2 Property List Functions for Symbols
This function sets symbol's property list to plist. Normally, plist should be a well-formed property list, but this is not enforced. The return value is plist.
(setplist 'foo '(a 1 b (2 3) c nil)) ⇒ (a 1 b (2 3) c nil) (symbol-plist 'foo) ⇒ (a 1 b (2 3) c nil)For symbols in special obarrays, which are not used for ordinary purposes, it may make sense to use the property list cell in a nonstandard fashion; in fact, the abbrev mechanism does so (see Abbrevs).
This function finds the value of the property named property in symbol's property list. If there is no such property,
nilis returned. Thus, there is no distinction between a value ofniland 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
putfor an example.
This function puts value onto symbol's property list under the property name property, replacing any previous property value. The
putfunction 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))
Previous: Symbol Plists, Up: Property Lists
8.4.3 Property Lists Outside Symbols
These functions are useful for manipulating property lists that are stored in places other than symbols:
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
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))
You could define put in terms of plist-put as follows:
(defun put (symbol prop value)
(setplist symbol
(plist-put (symbol-plist symbol) prop value)))
Like
plist-getexcept that it compares properties usingequalinstead ofeq.
Like
plist-putexcept that it compares properties usingequalinstead ofeq.
This returns non-
nilif plist contains the given property. Unlikeplist-get, this allows you to distinguish between a missing property and a property with the valuenil. The value is actually the tail of plist whosecaris property.
Next: Control Structures, Previous: Symbols, Up: Top
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.
Next: Forms, Up: Evaluation
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 an expression or a form. The fact that forms are data objects and not merely text is one of the fundamental differences between Lisp-like languages and typical programming languages. Any object can be evaluated, but in practice only numbers, symbols, lists and strings are evaluated very often.
In subsequent sections, we will describe the details of what evaluation means for each kind of form.
It is very common to read a Lisp form and then evaluate the form,
but reading and evaluation are separate activities, and either can be
performed alone. Reading per se does not evaluate anything; it
converts the printed representation of a Lisp object to the object
itself. It is up to the caller of read to specify whether this
object is a form to be evaluated, or serves some entirely different
purpose. See Input Functions.
Evaluation is a recursive process, and evaluating a form often
involves evaluating parts within that form. For instance, when you
evaluate a function call form such as (car x), Emacs
first evaluates the argument (the subform x). After evaluating
the argument, Emacs executes the function (car), and if
the function is written in Lisp, execution works by evaluating the
body of the function. (In this example, however, car is
not a Lisp function; it is a primitive function implemented in C.)
See Functions, for more information about functions and function
calls.
Evaluation takes place in a context called the environment, which consists of the current values and bindings of all Lisp variables (see Variables).3 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 create a new environment for recursive evaluation, by binding variables (see Local Variables). Such environments are temporary, and vanish when the evaluation of the form is complete.
Evaluating a form may also make changes that persist; these changes
are called side effects. An example of a form that produces a
side effect is (setq foo 1).
Do not confuse evaluation with command key interpretation. The
editor command loop translates keyboard input into a command (an
interactively callable function) using the active keymaps, and then
uses call-interactively to execute that command. Executing the
command usually involves evaluation, if the command is written in
Lisp; however, this step is not considered a part of command key
interpretation. See Command Loop.
Next: Quoting, Previous: Intro Eval, Up: Evaluation
9.2 Kinds of Forms
A Lisp object that is intended to be evaluated is called a form. 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 #<buffer eval.texi>) ;; Evaluate it. (eval print-exp) -| #<buffer eval.texi> ⇒ #<buffer eval.texi>
Next: Classifying Lists, Previous: Self-Evaluating Forms, Up: Forms
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 it has none (if its value cell is void), an error is signaled. For more information on the use of variables, see Variables.
In the following example, we set the value of a symbol with
setq. Then we evaluate the symbol, and get back the value that
setq stored.
(setq a 123)
⇒ 123
(eval 'a)
⇒ 123
a
⇒ 123
The symbols nil and t are treated specially, so that the
value of nil is always nil, and the value of t is
always t; you cannot set or bind them to any other values. Thus,
these two symbols act like self-evaluating forms, even though
eval treats them like any other symbol. A symbol whose name
starts with ‘:’ also self-evaluates in the same way; likewise,
its value ordinarily cannot be changed. See Constant Variables.
Next: Function Indirection, Previous: Symbol Forms, Up: Forms
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.
Next: Function Forms, Previous: Classifying Lists, Up: Forms
9.2.4 Symbol Function Indirection
If the first element of the list is a symbol then evaluation examines the symbol's function cell, and uses its contents instead of the original symbol. If the contents are another symbol, this process, called symbol function indirection, is repeated until it obtains a non-symbol. See Function Names, for more information about symbol function indirection.
One possible consequence of this process is an infinite loop, in the
event that a symbol's function cell refers to the same symbol. Or a
symbol may have a void function cell, in which case the subroutine
symbol-function signals a void-function error. But if
neither of these things happens, we eventually obtain a non-symbol,
which ought to be a function or other suitable object.
More precisely, we should now have a Lisp function (a lambda
expression), a byte-code function, a primitive function, a Lisp macro,
a special form, or an autoload object. Each of these types is a case
described in one of the following sections. If the object is not one
of these types, Emacs signals an invalid-function error.
The following example illustrates the symbol indirection process. We
use fset to set the function cell of a symbol and
symbol-function to get the function cell contents
(see Function Cells). Specifically, we store the symbol car
into the function cell of first, and the symbol first into
the function cell of erste.
;; Build this function cell linkage:
;; ------------- ----- ------- -------
;; | #<subr car> | <-- | car | <-- | first | <-- | erste |
;; ------------- ----- ------- -------
(symbol-function 'car)
⇒ #<subr car>
(fset 'first 'car)
⇒ car
(fset 'erste 'first)
⇒ first
(erste '(1 2 3)) ; Call the function referenced by erste.
⇒ 1
By contrast, the following example calls a function without any symbol function indirection, because the first element is an anonymous Lisp function, not a symbol.
((lambda (arg) (erste arg))
'(1 2 3))
⇒ 1
Executing the function itself evaluates its body; this does involve
symbol function indirection when calling erste.
The built-in function indirect-function provides an easy way to
perform symbol function indirection explicitly.
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-functionerror if the final symbol is unbound and optional argument noerror isnilor omitted. Otherwise, if noerror is non-nil, it returnsnilif the final symbol is unbound.It signals a
cyclic-function-indirectionerror if there is a loop in the chain of symbols.Here is how you could define
indirect-functionin Lisp:(defun indirect-function (function) (if (symbolp function) (indirect-function (symbol-function function)) function))
Next: Macro Forms, Previous: Function Indirection, Up: Forms
9.2.5 Evaluation of Function Forms
If the first element of a list being evaluated is a Lisp function
object, byte-code object or primitive function object, then that list is
a function call. For example, here is a call to the function
+:
(+ 1 x)
The first step in evaluating a function call is to evaluate the
remaining elements of the list from left to right. The results are the
actual argument values, one value for each list element. The next step
is to call the function with this list of arguments, effectively using
the function apply (see Calling Functions). If the function
is written in Lisp, the arguments are used to bind the argument
variables of the function (see Lambda Expressions); then the forms
in the function body are evaluated in order, and the value of the last
body form becomes the value of the function call.
Next: Special Forms, Previous: Function Forms, Up: Forms
9.2.6 Lisp Macro Evaluation
If the first element of a list being evaluated is a macro object, then the list is a macro call. When a macro call is evaluated, the elements of the rest of the list are not initially evaluated. Instead, these elements themselves are used as the arguments of the macro. The macro definition computes a replacement form, called the expansion of the macro, to be evaluated in place of the original form. The expansion may be any sort of form: a self-evaluating constant, a symbol, or a list. If the expansion is itself a macro call, this process of expansion repeats until some other sort of form results.
Ordinary evaluation of a macro call finishes by evaluating the expansion. However, the macro expansion is not necessarily evaluated right away, or at all, because other programs also expand macro calls, and they may or may not evaluate the expansions.
Normally, the argument expressions are not evaluated as part of computing the macro expansion, but instead appear as part of the expansion, so they are computed when the expansion is evaluated.
For example, given a macro defined as follows:
(defmacro cadr (x)
(list 'car (list 'cdr x)))
an expression such as (cadr (assq 'handler list)) is a macro
call, and its expansion is:
(car (cdr (assq 'handler list)))
Note that the argument (assq 'handler list) appears in the
expansion.
See Macros, for a complete description of Emacs Lisp macros.
Next: Autoloading, Previous: Macro Forms, Up: Forms
9.2.7 Special Forms
A special form is a primitive function specially marked so that its arguments are not all evaluated. Most special forms define control structures or perform variable bindings—things which functions cannot do.
Each special form has its own rules for which arguments are evaluated and which are used without evaluation. Whether a particular argument is evaluated may depend on the results of evaluating other arguments.
Here is a list, in alphabetical order, of all of the special forms in Emacs Lisp with a reference to where each is described.
and- see Combining Conditions
catch- see Catch and Throw
cond- see Conditionals
condition-case- see Handling Errors
defconst- see Defining Variables
defmacro- see Defining Macros
defun- see Defining Functions
defvar- see Defining Variables
function- see Anonymous Functions
if- see Conditionals
interactive- see Interactive Call
letlet*- see Local Variables
or- see Combining Conditions
prog1prog2progn- see Sequencing
quote- see Quoting
save-current-buffer- see Current Buffer
save-excursion- see Excursions
save-restriction- see Narrowing
save-window-excursion- see Window Configurations
setq- see Setting Variables
setq-default- see Creating Buffer-Local
track-mouse- see Mouse Tracking
unwind-protect- see Nonlocal Exits
while- see Iteration
with-output-to-temp-buffer- see Temporary Displays
Common Lisp note: Here are some comparisons of special forms in GNU Emacs Lisp and Common Lisp.setq,if, andcatchare special forms in both Emacs Lisp and Common Lisp.defunis a special form in Emacs Lisp, but a macro in Common Lisp.save-excursionis a special form in Emacs Lisp, but doesn't exist in Common Lisp.throwis a special form in Common Lisp (because it must be able to throw multiple values), but it is a function in Emacs Lisp (which doesn't have multiple values).
Previous: Special Forms, Up: Forms
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. See Autoload.
Next: Eval, Previous: Forms, Up: Evaluation
9.3 Quoting
The special form quote returns its single argument, as written,
without evaluating it. This provides a way to include constant symbols
and lists, which are not self-evaluating objects, in a program. (It is
not necessary to quote self-evaluating objects such as numbers, strings,
and vectors.)
Because quote is used so often in programs, Lisp provides a
convenient read syntax for it. An apostrophe character (‘'’)
followed by a Lisp object (in read syntax) expands to a list whose first
element is quote, and whose second element is the object. Thus,
the read syntax 'x is an abbreviation for (quote x).
Here are some examples of expressions that use quote:
(quote (+ 1 2))
⇒ (+ 1 2)
(quote foo)
⇒ foo
'foo
⇒ foo
''foo
⇒ (quote foo)
'(quote foo)
⇒ (quote foo)
['foo]
⇒ [(quote foo)]
Other quoting constructs include function (see Anonymous Functions), which causes an anonymous lambda expression written in Lisp
to be compiled, and ‘`’ (see Backquote), which is used to quote
only part of a list, while computing and substituting other parts.
Previous: Quoting, Up: Evaluation
9.4 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.
The functions and variables described in this section evaluate forms, specify limits to the evaluation process, or record recently returned values. Loading a file also does evaluation (see Loading).
It is generally cleaner and more flexible to store a function in a
data structure, and call it with funcall or apply, than
to store an expression in the data structure and evaluate it. Using
functions provides the ability to pass information to them as
arguments.
This is the basic function 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 (see Forms).
Since
evalis a function, the argument expression that appears in a call toevalis evaluated twice: once as preparation beforeevalis called, and again by theevalfunction itself. Here is an example:(setq foo 'bar) ⇒ bar (setq bar 'baz) ⇒ baz ;; Hereevalreceives argumentfoo(eval 'foo) ⇒ bar ;; Hereevalreceives argumentbar, which is the value offoo(eval foo) ⇒ bazThe number of currently active calls to
evalis limited tomax-lisp-eval-depth(see below).
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
evalon them until the end of the region is reached, or until an error is signaled and not handled.By default,
eval-regiondoes not produce any output. However, if stream is non-nil, any output produced by output functions (see Output Functions), as well as the values that result from evaluating the expressions in the region are printed using stream. See Output Streams.If read-function is non-
nil, it should be a function, which is used instead ofreadto read expressions one by one. This function is called with one argument, the stream for reading input. You can also use the variableload-read-function(see How Programs Do Loading) to specify this function, but it is more robust to use the read-function argument.
eval-regiondoes not move point. It always returnsnil.
This is similar to
eval-region, but the arguments provide different optional features.eval-bufferoperates on the entire accessible portion of buffer buffer-or-name. buffer-or-name can be a buffer, a buffer name (a string), ornil(or omitted), which means to use the current buffer. stream is used as ineval-region, unless stream isniland print non-nil. In that case, values that result from evaluating the expressions are still discarded, but the output of the output functions is printed in the echo area. filename is the file name to use forload-history(see Unloading), and defaults tobuffer-file-name(see Buffer File Name). If unibyte is non-nil,readconverts strings to unibyte whenever possible.
This variable defines the maximum depth allowed in calls to
eval,apply, andfuncallbefore 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-depthtoo much, such code can cause stack overflow instead. The depth limit counts internal uses ofeval,apply, andfuncall, such as for calling the functions mentioned in Lisp expressions, and recursive evaluation of function call arguments and function body forms, as well as explicit calls in Lisp code.The default value of this variable is 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-sizeprovides another limit on nesting. See Local Variables.
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
valuesitself, 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
Next: Variables, Previous: Evaluation, Up: Top
10 Control Structures
A Lisp program consists of expressions or forms (see Forms). We control the order of execution of these forms by enclosing them in control structures. Control structures are special forms which control when, whether, or how many times to execute the forms they contain.
The simplest order of execution is sequential execution: first form a, then form b, and so on. This is what happens when you write several forms in succession in the body of a function, or at top level in a file of Lisp code—the forms are executed in the order written. We call this textual order. For example, if a function body consists of two forms a and b, evaluation of the function evaluates first a and then b. The result of evaluating b becomes the value of the function.
Explicit control structures make possible an order of execution other than sequential.
Emacs Lisp provides several kinds of control structure, including other varieties of sequencing, conditionals, iteration, and (controlled) jumps—all discussed below. The built-in control structures are special forms since their subforms are not necessarily evaluated or not evaluated sequentially. You can use macros to define your own control structure constructs (see Macros).
Next: Conditionals, Up: Control Structures
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.
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 control constructs likewise evaluate a series of forms but return a different value:
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)))
This special form evaluates form1, form2, and all of the following forms, in textual order, returning the result of form2.
(prog2 (print "The first form") (print "The second form") (print "The third form")) -| "The first form" -| "The second form" -| "The third form" ⇒ "The second form"
Next: Combining Conditions, Previous: Sequencing, Up: Control Structures
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.
ifchooses 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 ofifis an example of an implicitprogn. See Sequencing.)If condition has the value
nil, and no else-forms are given,ifreturnsnil.
ifis a special form because the branch that is not selected is never evaluated—it is ignored. Thus, in the example below,trueis not printed because(if nil (print 'true) 'very-false) ⇒ very-false
This is a variant of
ifwhere 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)
This is a variant of
ifwhere there is no then-form:(unless condition a b c)is entirely equivalent to
(if condition nil a b c)
condchooses among an arbitrary number of alternatives. Each clause in thecondmust 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...)
condtries the clauses in textual order, by evaluating the condition of each clause. If the value of condition is non-nil, the clause “succeeds”; thencondevaluates its body-forms, and the value of the last of body-forms becomes the value of thecond. The remaining clauses are ignored.If the value of condition is
nil, the clause “fails,” so thecondmoves on to the following clause, trying its condition.If every condition evaluates to
nil, so that every clause fails,condreturnsnil.A clause may also look like this:
(condition)Then, if condition is non-
nilwhen tested, the value of condition becomes the value of thecondform.The following example has four clauses, which test for the cases where the value of
xis 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
tas the condition of the last clause, like this:(tbody-forms). The formtevaluates tot, which is nevernil, so this clause never fails, provided thecondgets to it at all.For example,
(setq a 5) (cond ((eq a 'hack) 'foo) (t "default")) ⇒ "default"This
condexpression returnsfooif the value ofaishack, and returns the string"default"otherwise.
Any conditional construct can be expressed with cond or with
if. Therefore, the choice between them is a matter of style.
For example:
(if a b c)
==
(cond (a b) (t c))
Next: Iteration, Previous: Conditionals, Up: Control Structures
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.
This function tests for the falsehood of condition. It returns
tif condition isnil, andnilotherwise. The functionnotis identical tonull, and we recommend using the namenullif you are testing for an empty list.
The
andspecial 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 theandmust benilregardless of the remaining conditions; soandreturnsnilright 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 theandform. Just(and), with no conditions, returnst, appropriate because all the conditions turned out non-nil. (Think about it; which one did not?)Here is an example. The first condition returns the integer 1, which is not
nil. Similarly, the second condition returns the integer 2, which is notnil. The third condition isnil, so the remaining condition is never evaluated.(and (print 1) (print 2) nil (print 3)) -| 1 -| 2 ⇒ nilHere is a more realistic example of using
and:(if (and (consp foo) (eq (car foo) 'x)) (message "foo is a list starting with x"))Note that
(car foo)is not executed if(consp foo)returnsnil, thus avoiding an error.
andexpressions can also be written using eitheriforcond. Here's how:(and arg1 arg2 arg3) == (if arg1 (if arg2 arg3)) == (cond (arg1 (cond (arg2 arg3))))
The
orspecial 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-
nilvalue, then the result of theormust be non-nil; soorreturns right away, ignoring the remaining conditions. The value it returns is the non-nilvalue of the condition just evaluated.If all the conditions turn out
nil, then theorexpression returnsnil. Just(or), with no conditions, returnsnil, appropriate because all the conditions turned outnil. (Think about it; which one did not?)For example, this expression tests whether
xis eithernilor the integer zero:(or (eq x nil) (eq x 0))Like the
andconstruct,orcan be written in terms ofcond. For example:(or arg1 arg2 arg3) == (cond (arg1) (arg2) (arg3))You could almost write
orin terms ofif, but not quite:(if arg1 arg1 (if arg2 arg2 arg3))This is not completely equivalent because it can evaluate arg1 or arg2 twice. By contrast,
(orarg1 arg2 arg3)never evaluates any argument more than once.
Next: Nonlocal Exits, Previous: Combining Conditions, Up: Control Structures
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:
whilefirst evaluates condition. If the result is non-nil, it evaluates forms in textual order. Then it reevaluates condition, and if the result is non-nil, it evaluates forms again. This process repeats until condition evaluates tonil.There is no limit on the number of iterations that may occur. The loop will continue until either condition evaluates to
nilor until an error orthrowjumps out of it (see Nonlocal Exits).The value of a
whileform is alwaysnil.(setq num 0) ⇒ 0 (while (< num 4) (princ (format "Iteration %d." num)) (setq num (1+ num))) -| Iteration 0. -| Iteration 1. -| Iteration 2. -| Iteration 3. ⇒ nilTo 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
prognas the first argument ofwhile, as shown here:(while (progn (forward-line 1) (not (looking-at "^$"))))This moves forward one line and continues moving by lines until it reaches an empty line. It is peculiar in that the
whilehas 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.
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
nilif result is omitted. For example, here is how you could usedolistto define thereversefunction:(defun reverse (list) (let (value) (dolist (elt list value) (setq value (cons elt value)))))
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
nilif result is omitted. Here is an example of usingdotimesto do something 100 times:(dotimes (i 100) (insert "I will not obey absurd orders\n"))
Previous: Iteration, Up: Control Structures
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.
Next: Examples of Catch, Up: Nonlocal Exits
10.5.1 Explicit Nonlocal Exits: catch and throw
Most control constructs affect only the flow of control within the
construct itself. The function throw is the exception to this
rule of normal program execution: it performs a nonlocal exit on
request. (There are other exceptions, but they are for error handling
only.) throw is used inside a catch, and jumps back to
that catch. For example:
(defun foo-outer ()
(catch 'foo
(foo-inner)))
(defun foo-inner ()
...
(if x
(throw 'foo t))
...)
The throw form, if executed, transfers control straight back to
the corresponding catch, which returns immediately. The code
following the throw is not executed. The second argument of
throw is used as the return value of the catch.
The function throw finds the matching catch based on the
first argument: it searches for a catch whose first argument is
eq to the one specified in the throw. If there is more
than one applicable catch, the innermost one takes precedence.
Thus, in the above example, the throw specifies foo, and
the catch in foo-outer specifies the same symbol, so that
catch is the applicable one (assuming there is no other matching
catch in between).
Executing throw exits all Lisp constructs up to the matching
catch, including function calls. When binding constructs such as
let or function calls are exited in this way, the bindings are
unbound, just as they are when these constructs exit normally
(see Local Variables). Likewise, throw restores the buffer
and position saved by save-excursion (see Excursions), and
the narrowing status saved by save-restriction and the window
selection saved by save-window-excursion (see Window Configurations). It also runs any cleanups established with the
unwind-protect special form when it exits that form
(see Cleanups).
The throw need not appear lexically within the catch
that it jumps to. It can equally well be called from another function
called within the catch. As long as the throw takes place
chronologically after entry to the catch, and chronologically
before exit from it, it has access to that catch. This is why
throw can be used in commands such as exit-recursive-edit
that throw back to the editor command loop (see Recursive Editing).
Common Lisp note: Most other versions of Lisp, including Common Lisp, have several ways of transferring control nonsequentially:return,return-from, andgo, for example. Emacs Lisp has onlythrow.
catchestablishes a return point for thethrowfunction. The return point is distinguished from other such return points by tag, which may be any Lisp object exceptnil. The argument tag is evaluated normally before the return point is established.With the return point in effect,
catchevaluates the forms of the body in textual order. If the forms execute normally (without error or nonlocal exit) the value of the last body form is returned from thecatch.If a
throwis executed during the execution of body, specifying the same value tag, thecatchform exits immediately; the value it returns is whatever was specified as the second argument ofthrow.
The purpose of
throwis to return from a return point previously established withcatch. The argument tag is used to choose among the various existing return points; it must beeqto the value specified in thecatch. If multiple return points match tag, the innermost one is used.The argument value is used as the value to return from that
catch.If no return point is in effect with tag tag, then a
no-catcherror is signaled with data(tag value).
Next: Errors, Previous: Catch and Throw, Up: Nonlocal Exits
10.5.2 Examples of catch and throw
One way to use catch and throw is to exit from a doubly
nested loop. (In most languages, this would be done with a “go to.”)
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.
Next: Cleanups, Previous: Examples of Catch, Up: Nonlocal Exits
10.5.3 Errors
When Emacs Lisp attempts to evaluate a form that, for some reason, cannot be evaluated, it signals an error.
When an error is signaled, Emacs's default reaction is to print an error message and terminate execution of the current command. This is the right thing to do in most cases, such as if you type C-f at the end of the buffer.
In complicated programs, simple termination may not be what you want.
For example, the program may have made temporary changes in data
structures, or created temporary buffers that should be deleted before
the program is finished. In such cases, you would use
unwind-protect to establish cleanup expressions to be
evaluated in case of error. (See Cleanups.) Occasionally, you may
wish the program to continue execution despite an error in a subroutine.
In these cases, you would use condition-case to establish
error handlers to recover control in case of error.
Resist the temptation to use error handling to transfer control from
one part of the program to another; use catch and throw
instead. See Catch and Throw.
Next: Processing of Errors, Up: Errors
10.5.3.1 How to Signal an Error
Signaling an error means beginning error processing. Error processing normally aborts all or part of the running program and returns to a point that is set up to handle the error (see Processing of Errors). Here we describe how to signal an error.
Most errors are signaled “automatically” within Lisp primitives
which you call for other purposes, such as if you try to take the
car of an integer or move forward a character at the end of the
buffer. You can also signal errors explicitly with the functions
error and signal.
Quitting, which happens when the user types C-g, is not considered an error, but it is handled almost like an error. See Quitting.
Every error specifies an error message, one way or another. The message should state what is wrong (“File does not exist”), not how things ought to be (“File must exist”). The convention in Emacs Lisp is that error messages should start with a capital letter, but should not end with any sort of punctuation.
This function signals an error with an error message constructed by applying
format(see Formatting Strings) to format-string and args.These examples show typical uses of
error:(error "That is an error -- try something else") error--> That is an error -- try something else (error "You have committed %d errors" 10) error--> You have committed 10 errors
errorworks by callingsignalwith two arguments: the error symbolerror, and a list containing the string returned byformat.Warning: If you want to use your own string as an error message verbatim, don't just write
(errorstring). If string contains ‘%’, it will be interpreted as a format specifier, with undesirable results. Instead, use(error "%s"string).
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-conditionswhose value is a list of condition names. This is how Emacs Lisp classifies different sorts of errors. See Error Symbols, for a description of error symbols, error conditions and condition names.If the error is not handled, the two arguments are used in printing the error message. Normally, this error message is provided by the
error-messageproperty of error-symbol. If data is non-nil, this is followed by a colon and a comma separated list of the unevaluated elements of data. Forerror, the error message is the car of data (that must be a string). Subcategories offile-errorare handled specially.The number and significance of the objects in data depends on error-symbol. For example, with a
wrong-type-argumenterror, 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-casebinds a local variable to a list of the form(error-symbol.data)(see Handling Errors).The function
signalnever returns (though in older Emacs versions it could sometimes return).(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"
Common Lisp note: Emacs Lisp has nothing like the Common Lisp concept of continuable errors.
Next: Handling Errors, Previous: Signaling Errors, Up: 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:
This variable, if non-
nil, specifies a function to use to handle errors that return control to the Emacs command loop. The function should take three arguments: data, a list of the same form thatcondition-casewould bind to its variable; context, a string describing the situation in which the error occurred, or (more often)nil; and caller, the Lisp function which called the primitive that signaled the error.
An error that has no explicit handler may call the Lisp debugger. The
debugger is enabled if the variable debug-on-error (see Error Debugging) is non-nil. Unlike error handlers, the debugger runs
in the environment of the error, so that you can examine values of
variables precisely as they were at the time of the error.
Next: Error Symbols, Previous: Processing of Errors, Up: Errors
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 occurs4.
The condition-case construct is often used to trap errors that
are predictable, such as failure to open a file in a call to
insert-file-contents. It is also used to trap errors that are
totally unpredictable, such as when the program evaluates an expression
read from the user.
The second argument of condition-case is called the
protected form. (In the example above, the protected form is a
call to delete-file.) The error handlers go into effect when
this form begins execution and are deactivated when this form returns.
They remain in effect for all the intervening time. In particular, they
are in effect during the execution of functions called by this form, in
their subroutines, and so on. This is a good thing, since, strictly
speaking, errors can be signaled only by Lisp primitives (including
signal and error) called by the protected form, not by the
protected form itself.
The arguments after the protected form are handlers. Each handler
lists one or more condition names (which are symbols) to specify
which errors it will handle. The error symbol specified when an error
is signaled also defines a list of condition names. A handler applies
to an error if they have any condition names in common. In the example
above, there is one handler, and it specifies one condition name,
error, which covers all errors.
The search for an applicable handler checks all the established handlers
starting with the most recently established one. Thus, if two nested
condition-case forms offer to handle the same error, the inner of
the two gets to handle it.
If an error is handled by some condition-case form, this
ordinarily prevents the debugger from being run, even if
debug-on-error says this error should invoke the debugger.
If you want to be able to debug errors that are caught by a
condition-case, set the variable debug-on-signal to a
non-nil value. You can also specify that a particular handler
should let the debugger run first, by writing debug among the
conditions, like this:
(condition-case nil
(delete-file filename)
((debug error) nil))
The effect of debug here is only to prevent
condition-case from suppressing the call to the debugger. Any
given error will invoke the debugger only if debug-on-error and
the other usual filtering mechanisms say it should. See Error Debugging.
Once Emacs decides that a certain handler handles the error, it
returns control to that handler. To do so, Emacs unbinds all variable
bindings made by binding constructs that are being exited, and
executes the cleanups of all unwind-protect forms that are
being exited. Once control arrives at the handler, the body of the
handler executes normally.
After execution of the handler body, execution returns from the
condition-case form. Because the protected form is exited
completely before execution of the handler, the handler cannot resume
execution at the point of the error, nor can it examine variable
bindings that were made within the protected form. All it can do is
clean up and proceed.
Error signaling and handling have some resemblance to throw and
catch (see Catch and Throw), but they are entirely separate
facilities. An error cannot be caught by a catch, and a
throw cannot be handled by an error handler (though using
throw when there is no suitable catch signals an error
that can be handled).
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-caseform; in this case, thecondition-casehas no effect. Thecondition-caseform 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 includedebugto 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-conditionsproperty of this symbol is a list of condition names (see Error Symbols). Emacs searches all the activecondition-caseforms for a handler that specifies one or more of these condition names; the innermost matchingcondition-casehandles the error. Within thiscondition-case, the first applicable handler handles the error.After executing the body of the handler, the
condition-casereturns normally, using the value of the last form in the handler body as the overall value.The argument var is a variable.
condition-casedoes 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
erris the error description variable, the first argument tocondition-casewhose error condition you want to re-throw. See Definition of signal.
This function returns the error message string for a given error descriptor. It is useful if you want to handle an error by printing the usual error message for that error. See Definition of signal.
Here is an example of using condition-case to handle the error
that results from dividing by zero. The handler displays the error
message (but without a beep), then returns a very large number.
(defun safe-divide (dividend divisor)
(condition-case err
;; Protected form.
(/ dividend divisor)
;; The handler.
(arith-error ; Condition.
;; Display the usual message for this error.
(message "%s" (error-message-string err))
1000000)))
⇒ safe-divide
(safe-divide 5 0)
-| Arithmetic error: (arith-error)
⇒ 1000000
The handler specifies condition name arith-error so that it will handle only division-by-zero errors. Other kinds of errors will not be handled, at least not 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 signaled with 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
This construct executes body, ignoring any errors that occur during its execution. If the execution is without error,
ignore-errorsreturns the value of the last form in body; otherwise, it returnsnil.Here's the example at the beginning of this subsection rewritten using
ignore-errors:(ignore-errors (delete-file filename))
Previous: Handling Errors, Up: Errors
10.5.3.4 Error Symbols and Condition Names
When you signal an error, you specify an error symbol to specify the kind of error you have in mind. Each error has one and only one error symbol to categorize it. This is the finest classification of errors defined by the Emacs Lisp language.
These narrow classifications are grouped into a hierarchy of wider
classes called error conditions, identified by condition
names. The narrowest such classes belong to the error symbols
themselves: each error symbol is also a condition name. There are also
condition names for more extensive classes, up to the condition name
error which takes in all kinds of errors (but not quit).
Thus, each error has one or more condition names: error, the
error symbol if that is distinct from error, and perhaps some
intermediate classifications.
In order for a symbol to be an error symbol, it must have an
error-conditions property which gives a list of condition names.
This list defines the conditions that this kind of error belongs to.
(The error symbol itself, and the symbol error, should always be
members of this list.) Thus, the hierarchy of condition names is
defined by the error-conditions properties of the error symbols.
Because quitting is not considered an error, the value of the
error-conditions property of quit is just (quit).
In addition to the error-conditions list, the error symbol
should have an error-message property whose value is a string to
be printed when that error is signaled but not handled. If the
error symbol has no error-message property or if the
error-message property exists, but is not a string, the error
message ‘peculiar error’ is used. See Definition of signal.
Here is how we define a new error symbol, new-error:
(put 'new-error
'error-conditions
'(error my-own-errors new-error))
⇒ (error my-own-errors new-error)
(put 'new-error 'error-message "A new error")
⇒ "A new error"
This error has three condition names: new-error, the narrowest
classification; my-own-errors, which we imagine is a wider
classification; and error, which is the widest of all.
The error string should start with a capital letter but it should not end with a period. This is for consistency with the rest of Emacs.
Naturally, Emacs will never signal new-error on its own; only
an explicit call to signal (see Definition of signal) in
your code can do this:
(signal 'new-error '(x y))
error--> A new error: x, y
This error can be handled through any of the three condition names.
This example handles new-error and any other errors in the class
my-own-errors:
(condition-case foo
(bar nil t)
(my-own-errors nil))
The significant way that errors are classified is by their condition
names—the names used to match errors with handlers. An error symbol
serves only as a convenient way to specify the intended error message
and list of condition names. It would be cumbersome to give
signal a list of condition names rather than one error symbol.
By contrast, using only error symbols without condition names would
seriously decrease the power of condition-case. Condition names
make it possible to categorize errors at various levels of generality
when you write an error handler. Using error symbols alone would
eliminate all but the narrowest level of classification.
See Standard Errors, for a list of all the standard error symbols and their conditions.
Previous: Errors, Up: Nonlocal Exits
10.5.4 Cleaning Up from Nonlocal Exits
The unwind-protect construct is essential whenever you
temporarily put a data structure in an inconsistent state; it permits
you to make the data consistent again in the event of an error or
throw. (Another more specific cleanup construct that is used only for
changes in buffer contents is the atomic change group; Atomic Changes.)
unwind-protectexecutes 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 athrowout of theunwind-protect, or cause an error; in all cases, the cleanup-forms will be evaluated.If body-form finishes normally,
unwind-protectreturns the value of body-form, after it evaluates the cleanup-forms. If body-form does not finish,unwind-protectdoes 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 athrowor an error),unwind-protectis not guaranteed to evaluate the rest of them. If the failure of one of the cleanup-forms has the potential to cause trouble, then protect it with anotherunwind-protectaround that form.The number of currently active
unwind-protectforms counts, together with the number of local variable bindings, against the limitmax-specpdl-size(see Local Variables).
For example, here we make an invisible buffer for temporary use, and make sure to kill it before finishing:
(let ((buffer (get-buffer-create " *temp*")))
(with-current-buffer buffer
(unwind-protect
body-form
(kill-buffer buffer))))
You might think that we could just as well write (kill-buffer
(current-buffer)) and dispense with the variable buffer.
However, the way shown above is safer, if body-form happens to
get an error after switching to a different buffer! (Alternatively,
you could write a save-current-buffer around body-form,
to ensure that the temporary buffer becomes current again in time to
kill it.)
Emacs includes a standard macro called with-temp-buffer which
expands into more or less the code shown above (see Current Buffer). Several of the macros defined in
this manual use unwind-protect in this way.
Here is an actual example derived from an FTP package. It creates a
process (see Processes) to try to establish a connection to a remote
machine. As the function ftp-login is highly susceptible to
numerous problems that the writer of the function cannot anticipate, it
is protected with a form that guarantees deletion of the process in the
event of failure. Otherwise, Emacs might fill up with useless
subprocesses.
(let ((win nil))
(unwind-protect
(progn
(setq process (ftp-setup-buffer host file))
(if (setq win (ftp-login process host user password))
(message "Logged in")
(error "Ftp login failed")))
(or win (and process (delete-process process)))))
This example has a small bug: if the user types C-g to
quit, and the quit happens immediately after the function
ftp-setup-buffer returns but before the variable process is
set, the process will not be killed. There is no easy way to fix this bug,
but at least it is very unlikely.
Next: Functions, Previous: Control Structures, Up: Top
11 Variables
A variable is a name used in a program to stand for a value. Nearly all programming languages have variables of some sort. In the text of a Lisp program, variables are written using the syntax for symbols.
In Lisp, unlike most programming languages, programs are represented primarily as Lisp objects and only secondarily as text. The Lisp objects used for variables are symbols: the symbol name is the variable name, and the variable's value is stored in the value cell of the symbol. The use of a symbol as a variable is independent of its use as a function name. See Symbol Components.
The textual form of a Lisp program is given by the read syntax of the Lisp objects that constitute the program. Hence, a variable in a textual Lisp program is written using the read syntax for the symbol representing the variable.
Next: Constant Variables, Up: Variables
11.1 Global Variables
The simplest way to use a variable is globally. This means that the variable has just one value at a time, and this value is in effect (at least for the moment) throughout the Lisp system. The value remains in effect until you specify a new one. When a new value replaces the old one, no trace of the old value remains in the variable.
You specify a value for a symbol with setq. For example,
(setq x '(a b))
gives the variable x the value (a b). Note that
setq is a special form (see Special Forms); it does not
evaluate its first argument, the name of the variable, but it does
evaluate the second argument, the new value.
Once the variable has a value, you can refer to it by using the symbol itself as an expression. Thus,
x ⇒ (a b)
assuming the setq form shown above has already been executed.
If you do set the same variable again, the new value replaces the old one:
x
⇒ (a b)
(setq x 4)
⇒ 4
x
⇒ 4
Next: Local Variables, Previous: Global Variables, Up: Variables
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 returns
tif object is a symbol whose name starts with ‘:’, interned in the standard obarray, and returnsnilotherwise.
These constants are fundamentally different from the “constants”
defined using the defconst special form (see Defining Variables). A defconst form serves to inform human readers
that you do not intend to change the value of a variable, but Emacs
does not raise an error if you actually change it.
Next: Void Variables, Previous: Constant Variables, Up: Variables
11.3 Local Variables
Global variables have values that last until explicitly superseded with new values. Sometimes it is useful to create variable values that exist temporarily—only until a certain part of the program finishes. These values are called local, and the variables so used are called local variables.
For example, when a function is called, its argument variables receive
new local values that last until the function exits. The let
special form explicitly establishes new local values for specified
variables; these last until exit from the let form.
Establishing a local value saves away the variable's previous value (or lack of one). We say that the previous value is shadowed and not visible. Both global and local values may be shadowed (see Scope). After the life span of the local value is over, the previous value (or lack of one) is restored.
If you set a variable (such as with setq) while it is local,
this replaces the local value; it does not alter the global value, or
previous local values, that are shadowed. To model this behavior, we
speak of a local binding of the variable as well as a local value.
The local binding is a conceptual place that holds a local value.
Entering a function, or a special form such as let, creates the
local binding; exiting the function or the let removes the
local binding. While the local binding lasts, the variable's value is
stored within it. Using setq or set while there is a
local binding stores a different value into the local binding; it does
not create a new binding.
We also speak of the global binding, which is where (conceptually) the global value is kept.
A variable can have more than one local binding at a time (for
example, if there are nested let forms that bind it). In such a
case, the most recently created local binding that still exists is the
current binding of the variable. (This rule is called
dynamic scoping; see Variable Scoping.) If there are no
local bindings, the variable's global binding is its current binding.
We sometimes call the current binding the most-local existing
binding, for emphasis. Ordinary evaluation of a symbol always returns
the value of its current binding.
The special forms let and let* exist to create
local bindings.
This special form binds variables according to bindings and then evaluates all of the forms in textual order. The
let-form returns the value of the last form in forms.Each of the bindings is either (i) a symbol, in which case that symbol is bound to
nil; or (ii) a list of the form(symbol value-form), in which case symbol is bound to the result of evaluating value-form. If value-form is omitted,nilis 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:
zis bound to the old value ofy, which is 2, not the new value ofy, which is 1.(setq y 2) ⇒ 2 (let ((y 1) (z y)) (list y z)) ⇒ (1 2)
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 reasonably refer to the preceding symbols bound in thislet*form. Compare the following example with the example above forlet.(setq y 2) ⇒ 2 (let* ((y 1) (z y)) ; Use the just-established value ofy. (list y z)) ⇒ (1 1)
Here is a complete list of the other facilities that create local bindings:
Variables can also have buffer-local bindings (see Buffer-Local Variables); a few variables have terminal-local bindings (see Multiple Terminals). These kinds of bindings work somewhat like ordinary local bindings, but they are localized depending on “where” you are in Emacs, rather than localized in time.
This variable defines the limit on the total number of local variable bindings and
unwind-protectcleanups (see Cleaning Up from Nonlocal Exits) that are allowed before Emacs signals an error (with data"Variable binding depth exceeds max-specpdl-size").This limit, with the associated error when it is exceeded, is one way that Lisp avoids infinite recursion on an ill-defined function.
max-lisp-eval-depthprovides another limit on depth of nesting. See Eval.The default value is 1000. Entry to the Lisp debugger increases the value, if there is little room left, to make sure the debugger itself has room to execute.
Next: Defining Variables, Previous: Local Variables, Up: Variables
11.4 When a Variable is “Void”
If you have never given a symbol any value as a global variable, we
say that that symbol's global value is void. In other words, the
symbol's value cell does not have any Lisp object in it. If you try to
evaluate the symbol, you get a void-variable error rather than
a value.
Note that a value of nil is not the same as void. 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 a value. A void variable does not
have any value.
After you have given a variable a value, you can make it void once more
using makunbound.
This function makes the current variable binding of symbol void. Subsequent attempts to use this symbol's value as a variable will signal the error
void-variable, unless and until you set it again.
makunboundreturns symbol.(makunbound 'x) ; Make the global value ofxvoid. ⇒ x x error--> Symbol's value as variable is void: xIf symbol is locally bound,
makunboundaffects the most local existing binding. This is the only way a symbol can have a void local binding, since all the constructs that create local bindings create them with values. In this case, the voidness lasts at most as long as the binding does; when the binding is removed due to exit from the construct that made it, the previous local or global binding is reexposed as usual, and the variable is no longer void unless the newly reexposed binding was void all along.(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 outerletbinding is visible. ⇒ 2
A variable that has been made void with makunbound is
indistinguishable from one that has never received a value and has
always been void.
You can use the function boundp to test whether a variable is
currently void.
boundpreturnstif variable (a symbol) is not void; more precisely, if its current binding is not void. It returnsnilotherwise.(boundp 'abracadabra) ; Starts out void. ⇒ nil (let ((abracadabra 5)) ; Locally bind it. (boundp 'abracadabra)) ⇒ t (boundp 'abracadabra) ; Still globally void. ⇒ nil (setq abracadabra 5) ; Make it globally nonvoid. ⇒ 5 (boundp 'abracadabra) ⇒ t
Next: Tips for Defining, Previous: Void Variables, Up: Variables
11.5 Defining Global Variables
You may announce your intention to use a symbol as a global variable
with a variable definition: a special form, either defconst
or defvar.
In Emacs Lisp, definitions serve three purposes. First, they inform
people who read the code that certain symbols are intended to be
used a certain way (as variables). Second, they inform the Lisp system
of these things, supplying a value and documentation. Third, they
provide information to utilities such as etags and
make-docfile, which create data bases of the functions and
variables in a program.
The difference between defconst and defvar is primarily
a matter of intent, serving to inform human readers of whether the value
should ever change. Emacs Lisp does not restrict the ways in which a
variable can be used based on defconst or defvar
declarations. However, it does make a difference for initialization:
defconst unconditionally initializes the variable, while
defvar initializes it only if it is void.
This special form defines symbol as a variable and can also initialize and document it. The definition informs a person reading your code that symbol is used as a variable that might be set or changed. Note that symbol is not evaluated; the symbol to be defined must appear explicitly in the
defvar.If symbol is void and value is specified,
defvarevaluates it 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,
defvaroperates 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. See Buffer-Local Variables.When you evaluate a top-level
defvarform with C-M-x in Emacs Lisp mode (eval-defun), a special feature ofeval-defunarranges to set the variable unconditionally, without testing whether its value is void.If the doc-string argument appears, it specifies the documentation for the variable. (This opportunity to specify documentation is one of the main benefits of defining the variable.) The documentation is stored in the symbol's
variable-documentationproperty. The Emacs help functions (see Documentation) look for this property.If the documentation string begins with the character ‘*’, Emacs allows users to set it interactively using the
set-variablecommand. However, you should nearly always usedefcustominstead ofdefvarto define such variables, so that users can use M-x customize and related commands to set them. In that case, it is not necessary to begin the documentation string with ‘*’. See Customization.Here are some examples. This form defines
foobut does not initialize it:(defvar foo) ⇒ fooThis example initializes the value of
barto23, and gives it a documentation string:(defvar bar 23 "The normal weight of a bar.") ⇒ barThe following form changes the documentation string for
bar, making it a user option, but does not change the value, sincebaralready has a value. (The addition(1+ nil)would get an error if it were evaluated, but since it is not evaluated, there is no error.)(defvar bar (1+ nil) "*The normal weight of a bar.") ⇒ bar bar ⇒ 23Here is an equivalent expression for the
defvarspecial form:(defvar symbol value doc-string) == (progn (if (not (boundp 'symbol)) (setq symbol value)) (if 'doc-string (put 'symbol 'variable-documentation 'doc-string)) 'symbol)The
defvarform returns symbol, but it is normally used at top level in a file where its value does not matter.
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.
defconstalways evaluates value, and sets the value of symbol to the result. If symbol does have a buffer-local binding in the current buffer,defconstsets the default value, not the buffer-local value. (But you should not be making buffer-local bindings for a symbol that is defined withdefconst.)An example of the use of
defconstis Emacs' definition offloat-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,defconstis only advisory.(defconst float-pi 3.141592653589793 "The value of Pi.") ⇒ float-pi (setq float-pi 3) ⇒ float-pi float-pi ⇒ 3
This function returns
tif variable is a user option—a variable intended to be set by the user for customization—andnilotherwise. (Variables other than user options exist for the internal purposes of Lisp programs, and users need not know about them.)User option variables are distinguished from other variables either though being declared using
defcustom5 or by the first character of theirvariable-documentationproperty. If the property exists and is a string, and its first character is ‘*’, then the variable is a user option. Aliases of user options are also user options.
If a user option variable has a variable-interactive property,
the set-variable command uses that value to control reading the
new value for the variable. The property's value is used as if it were
specified in interactive (see Using Interactive). However,
this feature is largely obsoleted by defcustom
(see Customization).
Warning: If the defconst and defvar special
forms are used while the variable has a local binding (made with
let, or a function argument), they set the local-binding's
value; the top-level binding is not changed. This is not what you
usually want. To prevent it, use these special forms at top level in
a file, where normally no local binding is in effect, and make sure to
load the file before making a local binding for the variable.
Next: Accessing Variables, Previous: Defining Variables, Up: Variables
11.6 Tips for Defining Variables Robustly
When you define a variable whose value is a function, or a list of functions, use a name that ends in ‘-function’ or ‘-functions’, respectively.
There are several other variable name conventions; here is a complete list:
- ‘...-hook’
- The variable is a normal hook (see Hooks).
- ‘...-function’
- The value is a function.
- ‘...-functions’
- The value is a list of functions.
- ‘...-form’
- The value is a form (an expression).
- ‘...-forms’
- The value is a list of forms (expressions).
- ‘...-predicate’
- The value is a predicate—a function of one argument that returns
non-
nilfor “good” arguments andnilfor “bad” arguments. - ‘...-flag’
- The value is significant only as to whether it is
nilor not. Since such variables often end up acquiring more values over time, this convention is not strongly recommended. - ‘...-program’
- The value is a program name.
- ‘...-command’
- The value is a whole shell command.
- ‘...-switches’
- The value specifies options for a command.
When you define a variable, always consider whether you should mark it as “safe” or “risky”; see File Local Variables.
When defining and initializing a variable that holds a complicated
value (such as a keymap with bindings in it), it's best to put the
entire computation of the value into the defvar, like this:
(defvar my-mode-map
(let ((map (make-sparse-keymap)))
(define-key map "\C-c\C-a" 'my-command)
...
map)
docstring)
This method has several benefits. First, if the user quits while
loading the file, the variable is either still uninitialized or
initialized properly, never in-between. If it is still uninitialized,
reloading the file will initialize it properly. Second, reloading the
file once the variable is initialized will not alter it; that is
important if the user has run hooks to alter part of the contents (such
as, to rebind keys). Third, evaluating the defvar form with
C-M-x will reinitialize the map completely.
Putting so much code in the defvar form has one disadvantage:
it puts the documentation string far away from the line which names the
variable. Here's a safe way to avoid that:
(defvar my-mode-map nil
docstring)
(unless my-mode-map
(let ((map (make-sparse-keymap)))
(define-key map "\C-c\C-a" 'my-command)
...
(setq my-mode-map map)))
This has all the same advantages as putting the initialization inside
the defvar, except that you must type C-M-x twice, once on
each form, if you do want to reinitialize the variable.
But be careful not to write the code like this:
(defvar my-mode-map nil
docstring)
(unless my-mode-map
(setq my-mode-map (make-sparse-keymap))
(define-key my-mode-map "\C-c\C-a" 'my-command)
...)
This code sets the variable, then alters it, but it does so in more than
one step. If the user quits just after the setq, that leaves the
variable neither correctly initialized nor void nor nil. Once
that happens, reloading the file will not initialize the variable; it
will remain incomplete.
Next: Setting Variables, Previous: Tips for Defining, Up: Variables
11.7 Accessing Variable Values
The usual way to reference a variable is to write the symbol which
names it (see Symbol Forms). This requires you to specify the
variable name when you write the program. Usually that is exactly what
you want to do. Occasionally you need to choose at run time which
variable to reference; then you can use symbol-value.
This function returns the value of symbol. This is the value in the innermost local binding of the symbol, or its global value if it has no local bindings.
(setq abracadabra 5) ⇒ 5 (setq foo 9) ⇒ 9 ;; Here the symbolabracadabra;; is the symbol whose value is examined. (let ((abracadabra 'foo)) (symbol-value 'abracadabra)) ⇒ foo ;; Here, the value ofabracadabra, ;; which isfoo, ;; is the symbol whose value is examined. (let ((abracadabra 'foo)) (symbol-value abracadabra)) ⇒ 9 (symbol-value 'abracadabra) ⇒ 5A
void-variableerror is signaled if the current binding of symbol is void.
Next: Variable Scoping, Previous: Accessing Variables, Up: Variables
11.8 How to Alter a Variable Value
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.
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 most-local existing binding of the symbol is changed.
setqdoes not evaluate symbol; it sets the symbol that you write. We say that this argument is automatically quoted. The ‘q’ insetqstands for “quoted.”The value of the
setqform is the value of the last form.(setq x (1+ 2)) ⇒ 3 x ;xnow has a global value. ⇒ 3 (let ((x 5)) (setq x 6) ; The local binding ofxis set. x) ⇒ 6 x ; The global value is unchanged. ⇒ 3Note 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 thatxis set before y (1+ x)) ; the value ofyis computed. ⇒ 11
This function sets symbol's value to value, then returns value. Since
setis a function, the expression written for symbol is evaluated to obtain the symbol to set.The most-local existing binding of the variable is the binding that is set; shadowed bindings are not affected.
(set one 1) error--> Symbol's value as variable is void: one (set 'one 1) ⇒ 1 (set 'two 'one) ⇒ one (set two 2) ;twoevaluates to symbolone. ⇒ 2 one ; So it isonethat was set. ⇒ 2 (let ((one 1)) ; This binding ofoneis set, (set 'one 3) ; not the global value. one) ⇒ 3 one ⇒ 2If symbol is not actually a symbol, a
wrong-type-argumenterror is signaled.(set '(x y) 'z) error--> Wrong type argument: symbolp, (x y)Logically speaking,
setis a more fundamental primitive thansetq. Any use ofsetqcan be trivially rewritten to useset;setqcould even be defined as a macro, given the availability ofset. However,setitself is rarely used; beginners hardly need to know about it. It is useful only for choosing at run time which variable to set. For example, the commandset-variable, which reads a variable name from the user and then sets the variable, needs to useset.Common Lisp note: In Common Lisp,setalways changes the symbol's “special” or dynamic value, ignoring any lexical bindings. In Emacs Lisp, all variables and all bindings are dynamic, sosetalways affects the most local existing binding.
Next: Buffer-Local Variables, Previous: Setting Variables, Up: Variables
11.9 Scoping Rules for Variable Bindings
A given symbol foo can have several local variable bindings,
established at different places in the Lisp program, as well as a global
binding. The most recently established binding takes precedence over
the others.
Local bindings in Emacs Lisp have indefinite scope and dynamic extent. Scope refers to where textually in the source code the binding can be accessed. “Indefinite scope” means that any part of the program can potentially access the variable binding. Extent refers to when, as the program is executing, the binding exists. “Dynamic extent” means that the binding lasts as long as the activation of the construct that established it.
The combination of dynamic extent and indefinite scope is called dynamic scoping. By contrast, most programming languages use lexical scoping, in which references to a local variable must be located textually within the function or block that binds the variable.
Common Lisp note: Variables declared “special” in Common Lisp are dynamically scoped, like all variables in Emacs Lisp.
Next: Extent, Up: Variable Scoping
11.9.1 Scope
Emacs Lisp uses indefinite scope for local variable bindings. This means that any function anywhere in the program text might access a given binding of a variable. Consider the following function definitions:
(defun binder (x) ;xis bound inbinder. (foo 5)) ;foois some other function. (defun user () ;xis used ``free'' inuser. (list x))
In a lexically scoped language, the binding of x in
binder would never be accessible in user, because
user is not textually contained within the function
binder. However, in dynamically-scoped Emacs Lisp, user
may or may not refer to the binding of x established in
binder, depending on the circumstances:
- If we call
userdirectly without callingbinderat all, then whatever binding ofxis found, it cannot come frombinder. - If we define
fooas follows and then callbinder, then the binding made inbinderwill be seen inuser:(defun foo (lose) (user)) - However, if we define
fooas follows and then callbinder, then the binding made inbinderwill not be seen inuser:(defun foo (x) (user))Here, when
foois called bybinder, it bindsx. (The binding infoois said to shadow the one made inbinder.) Therefore,userwill access thexbound byfooinstead of the one bound bybinder.
Emacs Lisp uses dynamic scoping because simple implementations of lexical scoping are slow. In addition, every Lisp system needs to offer dynamic scoping at least as an option; if lexical scoping is the norm, there must be a way to specify dynamic scoping instead for a particular variable. It might not be a bad thing for Emacs to offer both, but implementing it with dynamic scoping only was much easier.
Next: Impl of Scope, Previous: Scope, Up: Variable Scoping
11.9.2 Extent
Extent refers to the time during program execution that a variable name is valid. In Emacs Lisp, a variable is valid only while the form that bound it is executing. This is called dynamic extent. “Local” or “automatic” variables in most languages, including C and Pascal, have dynamic extent.
One alternative to dynamic extent is indefinite extent. This means that a variable binding can live on past the exit from the form that made the binding. Common Lisp and Scheme, for example, support this, but Emacs Lisp does not.
To illustrate this, the function below, make-add, returns a
function that purports to add n to its own argument m. This
would work in Common Lisp, but it does not do the job in Emacs Lisp,
because after the call to make-add exits, the variable n
is no longer bound to the actual argument 2.
(defun make-add (n)
(function (lambda (m) (+ n m)))) ; Return a function.
⇒ make-add
(fset 'add2 (make-add 2)) ; Define function add2
; with (make-add 2).
⇒ (lambda (m) (+ n m))
(add2 4) ; Try to add 2 to 4.
error--> Symbol's value as variable is void: n
Some Lisp dialects have “closures,” objects that are like functions but record additional variable bindings. Emacs Lisp does not have closures.
Next: Using Scoping, Previous: Extent, Up: Variable Scoping
11.9.3 Implementation of Dynamic Scoping
A simple sample implementation (which is not how Emacs Lisp actually works) may help you understand dynamic binding. This technique is called deep binding and was used in early Lisp systems.
Suppose there is a stack of bindings, which are variable-value pairs.
At entry to a function or to a let form, we can push bindings
onto the stack for the arguments or local variables created there. We
can pop those bindings from the stack at exit from the binding
construct.
We can find the value of a variable by searching the stack from top to bottom for a binding for that variable; the value from that binding is the value of the variable. To set the variable, we search for the current binding, then store the new value into that binding.
As you can see, a function's bindings remain in effect as long as it continues execution, even during its calls to other functions. That is why we say the extent of the binding is dynamic. And any other function can refer to the bindings, if it uses the same variables while the bindings are in effect. That is why we say the scope is indefinite.
The actual implementation of variable scoping in GNU Emacs Lisp uses a technique called shallow binding. Each variable has a standard place in which its current value is always found—the value cell of the symbol.
In shallow binding, setting the variable works by storing a value in the value cell. Creating a new binding works by pushing the old value (belonging to a previous binding) onto a stack, and storing the new local value in the value cell. Eliminating a binding works by popping the old value off the stack, into the value cell.
We use shallow binding because it has the same results as deep binding, but runs faster, since there is never a need to search for a binding.
Previous: Impl of Scope, Up: Variable Scoping
11.9.4 Proper Use of Dynamic Scoping
Binding a variable in one function and using it in another is a powerful technique, but if used without restraint, it can make programs hard to understand. There are two clean ways to use this technique:
- Use or bind the variable only in a few related functions, written close
together in one file. Such a variable is used for communication within
one program.
You should write comments to inform other programmers that they can see all uses of the variable before them, and to advise them not to add uses elsewhere.
- Give the variable a well-defined, documented meaning, and make all
appropriate functions refer to it (but not bind it or set it) wherever
that meaning is relevant. For example, the variable
case-fold-searchis defined as “non-nilmeans ignore case when searching”; various search and replace functions refer to it directly or through their subroutines, but do not bind or set it.Then you can bind the variable in other programs, knowing reliably what the effect will be.
In either case, you should define the variable with defvar.
This helps other people understand your program by telling them to look
for inter-function usage. It also avoids a warning from the byte
compiler. Choose the variable's name to avoid name conflicts—don't
use short names like x.
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, or to each frame. See Multiple Terminals, and See Frame-Local Variables.)
11.10.1 Introduction to Buffer-Local Variables
A buffer-local variable has a buffer-local binding associated with a particular buffer. The binding is in effect when that buffer is current; otherwise, it is not in effect. If you set the variable while a buffer-local binding is in effect, the new value goes in that binding, so its other bindings are unchanged. This means that the change is visible only in the buffer where you made it.
The variable's ordinary binding, which is not associated with any specific buffer, is called the default binding. In most cases, this is the global binding.
A variable can have buffer-local bindings in some buffers but not in other buffers. The default binding is shared by all the buffers that don't have their own bindings for the variable. (This includes all newly-created buffers.) If you set the variable in a buffer that does not have a buffer-local binding for it, this sets the default binding, so the new value is visible in all the buffers that see the default binding.
The most common use of buffer-local bindings is for major modes to change
variables that control the behavior of commands. For example, C mode and
Lisp mode both set the variable paragraph-start to specify that only
blank lines separate paragraphs. They do this by making the variable
buffer-local in the buffer that is being put into C mode or Lisp mode, and
then setting it to the new value for that mode. See Major Modes.
The usual way to make a buffer-local binding is with
make-local-variable, which is what major mode commands typically
use. This affects just the current buffer; all other buffers (including
those yet to be created) will continue to share the default value unless
they are explicitly given their own buffer-local bindings.
A more powerful operation is to mark the variable as
automatically buffer-local by calling
make-variable-buffer-local. You can think of this as making the
variable local in all buffers, even those yet to be created. More
precisely, the effect is that setting the variable automatically makes
the variable local to the current buffer if it is not already so. All
buffers start out by sharing the default value of the variable as usual,
but setting the variable creates a buffer-local binding for the current
buffer. The new value is stored in the buffer-local binding, leaving
the default binding untouched. This means that the default value cannot
be changed with setq in any buffer; the only way to change it is
with setq-default.
Warning: When a variable has buffer-local
bindings in one or more buffers, let rebinds the binding that's
currently in effect. For instance, if the current buffer has a
buffer-local value, let temporarily rebinds that. If no
buffer-local bindings are in effect, let rebinds
the default value. If inside the let you then change to a
different current buffer in which a different binding is in effect,
you won't see the let binding any more. And if you exit the
let while still in the other buffer, you won't see the
unbinding occur (though it will occur properly). Here is an example
to illustrate:
(setq foo 'g)
(set-buffer "a")
(make-local-variable 'foo)
(setq foo 'a)
(let ((foo 'temp))
;; foo ⇒ 'temp ; let binding in buffer ‘a’
(set-buffer "b")
;; foo ⇒ 'g ; the global value since foo is not local in ‘b’
body...)
foo ⇒ 'g ; exiting restored the local value in buffer ‘a’,
; but we don't see that in buffer ‘b’
(set-buffer "a") ; verify the local value was restored
foo ⇒ 'a
Note that references to foo in body access the
buffer-local binding of buffer ‘b’.
When a file specifies local variable values, these become buffer-local values when you visit the file. See File Variables.
A buffer-local variable cannot be made frame-local (see Frame-Local Variables) or terminal-local (see Multiple Terminals).
11.10.2 Creating and Deleting Buffer-Local Bindings
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) ⇒ 5Making 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 thelet. This is becauseletdoes not distinguish between different kinds of bindings; it knows only which variable the binding was made for.If the variable is terminal-local (see Multiple Terminals), or frame-local (see Frame-Local Variables), this function signals an error. Such variables cannot have buffer-local bindings as well.
Warning: do not use
make-local-variablefor a hook variable. The hook variables are automatically made buffer-local as needed if you use the local argument toadd-hookorremove-hook.
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.
A peculiar wrinkle of this feature is that binding the variable (with
letor other binding constructs) does not create a buffer-local binding for it. Only setting the variable (withsetorsetq), while the variable does not have alet-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 callingmakunboundon 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-localfor 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-localis 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 usingmake-variable-buffer-localcan be the best solution.
This returns
tif variable is buffer-local in buffer buffer (which defaults to the current buffer); otherwise,nil.
This returns
tif variable will become buffer-local in buffer buffer (which defaults to the current buffer) if it is set there.
This function returns the buffer-local binding of variable (a symbol) in buffer buffer. If variable does not have a buffer-local binding in buffer buffer, it returns the default value (see Default Value) of variable instead.
This function returns a list describing the buffer-local variables in buffer buffer. (If buffer is omitted, the current buffer is used.) It returns an association list (see Association Lists) in which each element contains one buffer-local variable and its value. However, when a variable's buffer-local binding in buffer is void, then the variable appears directly in the resulting list.
(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.
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-variablereturns 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.
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-
nilpermanent-local-hookproperty (see Setting Hooks). As a result, the buffer will see the default values of most variables.This function also resets certain other information pertaining to the buffer: it sets the local keymap to
nil, the syntax table to the value of(standard-syntax-table), the case table to(standard-case-table), and the abbrev table to the value offundamental-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-variablesreturnsnil.
The function
kill-all-local-variablesruns this normal hook before it does anything else. This gives major modes a way to arrange for something special to be done if the user switches to a different major mode. It is also useful for buffer-specific minor modes that should be forgotten if the user changes the major mode.For best results, make this variable buffer-local, so that it will disappear after doing its job and will not interfere with the subsequent major mode. See Hooks.
A buffer-local variable is permanent if the variable name (a
symbol) has a permanent-local property that is non-nil.
Such variables are unaffected by kill-all-local-variables, and
their local bindings are therefore not cleared by changing major modes.
Permanent locals are appropriate for data pertaining to where the file
came from or how to save it, rather than with how to edit the contents.
Previous: Creating Buffer-Local, Up: Buffer-Local Variables
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.
This function returns symbol's default value. This is the value that is seen in buffers and frames that do not have their own values for this variable. If symbol is not buffer-local, this is equivalent to
symbol-value(see Accessing Variables).
The function
default-boundptells you whether symbol's default value is nonvoid. If(default-boundp 'foo)returnsnil, then(default-value 'foo)would get an error.
default-boundpis todefault-valueasboundpis tosymbol-value.
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-defaultform 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-defaulthas the same effect assetq. 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
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
Next: Directory Local Variables, Previous: Buffer-Local Variables, Up: Variables
11.11 File Local Variables
A file can specify local variable values; Emacs uses these to create buffer-local bindings for those variables in the buffer visiting that file. See Local Variables in Files, for basic information about file-local variables. This section describes the functions and variables that affect how file-local variables are processed.
If a file-local variable could specify an arbitrary function or Lisp expression that would be called later, visiting a file could take over your Emacs. Emacs protects against this by automatically setting only those file-local variables whose specified values are known to be safe. Other file-local variables are set only if the user agrees.
For additional safety, read-circle is temporarily bound to
nil when Emacs reads file-local variables (see Input Functions). This prevents the Lisp reader from recognizing circular
and shared Lisp structures (see Circular Objects).
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.
This function parses, and binds or evaluates as appropriate, any local variables specified by the contents of the current buffer. The variable
enable-local-variableshas its effect here. However, this function does not look for the ‘mode:’ local variable in the ‘-*-’ line.set-auto-modedoes that, also takingenable-local-variablesinto account (see Auto Major Mode).This function works by walking the alist stored in
file-local-variables-alistand applying each local variable in turn. It callsbefore-hack-local-variables-hookandhack-local-variables-hookbefore and after applying the variables, respectively.If the optional argument mode-only is non-
nil, then all this function does is returntif the ‘-*-’ line or the local variables list specifies a mode andnilotherwise. It does not set the mode nor any other file-local variable.
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 thehack-local-variablesfunction applies them one by one.
Emacs calls this hook immediately before applying file-local variables stored in
file-local-variables-alist.
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.
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.
This function returns non-
nilif it is safe to give sym the value val, based on the above criteria.
Some variables are considered risky. A 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 considered risky. The
variables ‘font-lock-keywords’, ‘font-lock-keywords’
followed by a digit, and ‘font-lock-syntactic-keywords’ are also
considered risky. Finally, any variable whose name has a
non-nil risky-local-variable property is considered
risky.
This function returns non-
nilif sym is a risky variable, based on the above criteria.
If a variable is risky, it will not be entered automatically into
safe-local-variable-values as described above. Therefore,
Emacs will always query before setting a risky variable, unless the
user explicitly allows the setting by customizing
safe-local-variable-values directly.
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.
This variable controls processing of ‘Eval:’ in ‘-*-’ lines or local variables lists in files being visited. A value of
tmeans process them unconditionally;nilmeans ignore them; anything else means ask the user what to do for each file. The default value ismaybe.
This variable holds a list of expressions that are safe to evaluate when found in the ‘Eval:’ “variable” in a file local variables list.
If the expression is a function call and the function has a
safe-local-eval-function property, the property value
determines whether the expression is safe to evaluate. The property
value can be a predicate to call to test the expression, a list of
such predicates (it's safe if any predicate succeeds), or t
(always safe provided the arguments are constant).
Text properties are also potential loopholes, since their values could include functions to call. So Emacs discards all text properties from string values specified for file-local variables.
Next: Frame-Local Variables, Previous: File Local Variables, Up: 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.
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.el6. 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. 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 Per-directory Local Variables, for more details.
This function reads the
.dir-locals.elfile and stores the directory-local variables infile-local-variables-alistthat is local to the buffer visiting any file in the directory, without applying them. It also stores the directory-local settings indir-locals-class-alist, where it defines a special class for the directory in which .dir-locals.el file was found. This function works by callingdir-locals-set-class-variablesanddir-locals-set-directory-class, described below.
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 valuenilfor major-mode means the settings are applicable to any mode.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.
This function assigns class to all the files in
directoryand 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 bydir-locals-set-class-variables.Emacs uses this function internally when it loads directory variables from a
.dir-locals.elfile. In that case, the optional argument mtime holds the file modification time (as returned byfile-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 benil.
This alist holds the class symbols and the associated variable settings. It is updated by
dir-locals-set-class-variables.
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-classupdates this list.
11.13 Frame-Local Values for Variables
In addition to buffer-local variable bindings (see Buffer-Local Variables), Emacs supports frame-local bindings. A frame-local binding for a variable is in effect in a frame for which it was defined.
In practice, frame-local variables have not proven very useful.
Ordinary frame parameters are generally used instead (see Frame Parameters). The function make-variable-frame-local, which
was used to define frame-local variables, has been deprecated since
Emacs 22.2. However, you can still define a frame-specific binding
for a variable var in frame frame, by setting the
var frame parameter for that frame:
(modify-frame-parameters frame '((var . value)))
This causes the variable var to be bound to the specified
value in the named frame. To check the frame-specific
values of such variables, use frame-parameter. See Parameter Access.
Note that you cannot have a frame-local binding for a variable that has a buffer-local binding.
Next: Variables with Restricted Values, Previous: Frame-Local Variables, Up: Variables
11.14 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.
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.
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.
If provided, when should be a string indicating when the variable was first made obsolete—for example, a date or a release number.
You can make two variables synonyms and declare one obsolete at the
same time using the macro define-obsolete-variable-alias.
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)
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-indirectionerror 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
Previous: Variable Aliases, Up: Variables
11.15 Variables with Restricted Values
Ordinary Lisp variables can be assigned any value that is a valid
Lisp object. However, certain Lisp variables are not defined in Lisp,
but in C. Most of these variables are defined in the C code using
DEFVAR_LISP. Like variables defined in Lisp, these can take on
any value. However, some variables are defined using
DEFVAR_INT or DEFVAR_BOOL. See Writing Emacs Primitives, in particular the
description of functions of the type syms_of_filename,
for a brief discussion of the C implementation.
Variables of type DEFVAR_BOOL can only take on the values
nil or t. Attempting to assign them any other value
will set them to t:
(let ((display-hourglass 5))
display-hourglass)
⇒ t
Variables of type DEFVAR_INT can only take on integer values.
Attempting to assign them any other value will result in an error:
(setq window-min-height 5.0)
error--> Wrong type argument: integerp, 5.0
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.
Next: Lambda Expressions, Up: Functions
12.1 What Is a Function?
In a general sense, a function is a rule for carrying on a computation given several values called arguments. The result of the computation is called the value of the function. The computation can also have side effects: lasting changes in the values of variables or the contents of data structures.
Here are important terms for functions in Emacs Lisp and for other function-like objects.
- function
- In Emacs Lisp, a function is anything that can be applied to
arguments in a Lisp program. In some cases, we use it more
specifically to mean a function written in Lisp. Special forms and
macros are not functions.
- primitive
- A primitive is a function callable from Lisp that is written in C,
such as
carorappend. These functions are also called built-in functions, or subrs. (Special forms are also considered primitives.)Usually the reason we implement a function as a primitive is either because it is fundamental, because it provides a low-level interface to operating system services, or because it needs to run fast. Primitives can be modified or added only by changing the C sources and recompiling the editor. See Writing Emacs Primitives.
- lambda expression
- A lambda expression is a function written in Lisp.
These are described in the following section.
See Lambda Expressions.
- special form
- A special form is 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. Many special forms are described in Control Structures.
- macro
- A macro is a construct defined in Lisp by the programmer. It
differs from a function in that it translates a Lisp expression that you
write into an equivalent expression to be evaluated instead of the
original expression. Macros enable Lisp programmers to do the sorts of
things that special forms can do. See Macros, for how to define and
use macros.
- command
- A command is an object that
command-executecan invoke; it is a possible definition for a key sequence. Some functions are commands; a function written in Lisp is a command if it contains an interactive declaration (see Defining Commands). Such a function can be called from Lisp expressions like other functions; in this case, the fact that the function is a command makes no difference.Keyboard macros (strings and vectors) are commands also, even though they are not functions. A symbol is a command if its function definition is a command; such symbols can be invoked with M-x. The symbol is a function as well if the definition is a function. See Interactive Call.
- keystroke command
- A keystroke command is a command that is bound to a key sequence
(typically one to three keystrokes). The distinction is made here
merely to avoid confusion with the meaning of “command” in non-Emacs
editors; for Lisp programs, the distinction is normally unimportant.
- byte-code function
- A byte-code function is a function that has been compiled by the byte compiler. See Byte-Code Type.
This function returns
tif object is any kind of function, i.e. can be passed tofuncall. Note thatfunctionpreturnsnilfor special forms (see Special Forms).
Unlike functionp, the next three functions do not
treat a symbol as its function definition.
This function returns
tif object is a built-in function (i.e., a Lisp primitive).(subrp 'message) ;messageis a symbol, ⇒ nil ; not a subr object. (subrp (symbol-function 'message)) ⇒ t
This function returns
tif object is a byte-code function. For example:(byte-code-function-p (symbol-function 'next-line)) ⇒ t
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 symbolmany, for a function with&restarguments, or the symbolunevalledif subr is a special form.
Next: Function Names, Previous: What Is a Function, Up: Functions
12.2 Lambda Expressions
A function written in Lisp is a list that looks like this:
(lambda (arg-variables...)
[documentation-string]
[interactive-declaration]
body-forms...)
Such a list is called a lambda expression. In Emacs Lisp, it actually is valid as an expression—it evaluates to itself. In some other Lisp dialects, a lambda expression is not a valid expression at all. In either case, its main use is not to be evaluated as an expression, but to be called as a function.
Next: Simple Lambda, Up: Lambda Expressions
12.2.1 Components of a Lambda Expression
A function written in Lisp (a “lambda expression”) is a list that looks like this:
(lambda (arg-variables...)
[documentation-string]
[interactive-declaration]
body-forms...)
The first element of a lambda expression is always the symbol
lambda. This indicates that the list represents a function. The
reason functions are defined to start with lambda is so that
other lists, intended for other uses, will not accidentally be valid as
functions.
The second element is a list of symbols—the argument variable names. This is called the lambda list. When a Lisp function is called, the argument values are matched up against the variables in the lambda list, which are given local bindings with the values provided. See Local Variables.
The documentation string is a Lisp string object placed within the function definition to describe the function for the Emacs help facilities. See Function Documentation.
The interactive declaration is a list of the form (interactive
code-string). This declares how to provide arguments if the
function is used interactively. Functions with this declaration are called
commands; they can be called using M-x or bound to a key.
Functions not intended to be called in this way should not have interactive
declarations. See Defining Commands, for how to write an interactive
declaration.
The rest of the elements are the body of the function: the Lisp code to do the work of the function (or, as a Lisp programmer would say, “a list of Lisp forms to evaluate”). The value returned by the function is the value returned by the last element of the body.
Next: Argument List, Previous: Lambda Components, Up: Lambda Expressions
12.2.2 A Simple Lambda-Expression Example
Consider for example the following function:
(lambda (a b c) (+ a b c))
We can call this function by writing it as the car of an expression, like this:
((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:
((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.
It is not often useful to write a lambda expression as the car of
a form in this way. You can get the same result, of making local
variables and giving them values, using the special form let
(see Local Variables). And let is clearer and easier to use.
In practice, lambda expressions are either stored as the function
definitions of symbols, to produce named functions, or passed as
arguments to other functions (see Anonymous Functions).
However, calls to explicit lambda expressions were very useful in the
old days of Lisp, before the special form let was invented. At
that time, they were the only way to bind and initialize local
variables.
Next: Function Documentation, Previous: Simple Lambda, Up: Lambda Expressions
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:
((lambda (n) (1+ n)) ; One required: 1) ; requires exactly one argument. ⇒ 2 ((lambda (n &optional n1) ; One required and one optional: (if n1 (+ n n1) (1+ n))) ; 1 or 2 arguments. 1 2) ⇒ 3 ((lambda (n &rest ns) ; One required and one rest: (+ n (apply '+ ns))) ; 1 or more arguments. 1 2 3 4 5) ⇒ 15
Previous: Argument List, Up: Lambda Expressions
12.2.4 Documentation Strings of Functions
A lambda expression may optionally have a documentation string just after the lambda list. This string does not affect execution of the function; it is a kind of comment, but a systematized comment which actually appears inside the Lisp world and can be used by the Emacs help facilities. See Documentation, for how the documentation-string is accessed.
It is a good idea to provide documentation strings for all the functions in your program, even those that are called only from within your program. Documentation strings are like comments, except that they are easier to access.
The first line of the documentation string should stand on its own,
because apropos displays just this first line. It should consist
of one or two complete sentences that summarize the function's purpose.
The start of the documentation string is usually indented in the source file, but since these spaces come before the starting double-quote, they are not part of the string. Some people make a practice of indenting any additional lines of the string so that the text lines up in the program source. That is a mistake. The indentation of the following lines is inside the string; what looks nice in the source code will look ugly when displayed by the help commands.
You may wonder how the documentation string could be optional, since there are required components of the function that follow it (the body). Since evaluation of a string returns that string, without any side effects, it has no effect if it is not the last form in the body. Thus, in practice, there is no confusion between the first form of the body and the documentation string; if the only body form is a string then it serves both as the return value and as the documentation.
The last line of the documentation string can specify calling conventions different from the actual function arguments. Write text like this:
\(fn arglist)
following a blank line, at the beginning of the line, with no newline following it inside the documentation string. (The ‘\’ is used to avoid confusing the Emacs motion commands.) The calling convention specified in this way appears in help messages in place of the one derived from the actual arguments of the function.
This feature is particularly useful for macro definitions, since the arguments written in a macro definition often do not correspond to the way users think of the parts of the macro call.
Next: Defining Functions, Previous: Lambda Expressions, Up: Functions
12.3 Naming a Function
In most computer languages, every function has a name; the idea of a
function without a name is nonsensical. In Lisp, a function in the
strictest sense has no name. It is simply a list whose first element is
lambda, a byte-code function object, or a primitive subr-object.
However, a symbol can serve as the name of a function. This happens when you put the function in the symbol's function cell (see Symbol Components). Then the symbol itself becomes a valid, callable function, equivalent to the list or subr-object that its function cell refers to. The contents of the function cell are also called the symbol's function definition. The procedure of using a symbol's function definition in place of the symbol is called symbol function indirection; see Function Indirection.
In practice, nearly all functions are given names in this way and
referred to through their names. For example, the symbol car works
as a function and does what it does because the primitive subr-object
#<subr car> is stored in its function cell.
We give functions names because it is convenient to refer to them by
their names in Lisp expressions. For primitive subr-objects such as
#<subr car>, names are the only way you can refer to them: there
is no read syntax for such objects. For functions written in Lisp, the
name is more convenient to use in a call than an explicit lambda
expression. Also, a function with a name can refer to itself—it can
be recursive. Writing the function's name in its own definition is much
more convenient than making the function definition point to itself
(something that is not impossible but that has various disadvantages in
practice).
We often identify functions with the symbols used to name them. For
example, we often speak of “the function car,” not
distinguishing between the symbol car and the primitive
subr-object that is its function definition. For most purposes, the
distinction is not important.
Even so, keep in mind that a function need not have a unique name. While
a given function object usually appears in the function cell of only
one symbol, this is just a matter of convenience. It is easy to store
it in several symbols using fset; then each of the symbols is
equally well a name for the same function.
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. (Some Lisp dialects, such as Scheme, do not distinguish between a symbol's value and its function definition; a symbol's value as a variable is also its function definition.) If you have not given a symbol a function definition, you cannot use it as a function; whether the symbol has a value as a variable makes no difference to this.
Next: Calling Functions, Previous: Function Names, Up: Functions
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 special form.
defunis the usual way to define new Lisp functions. It defines the symbol name as a function that looks like this:(lambda argument-list . body-forms)
defunstores this lambda expression in the function cell of name. It returns the value name, but usually we ignore this value.As described previously, argument-list is a list of argument names and may include the keywords
&optionaland&rest(see Lambda Expressions). Also, the first two of the body-forms may be a documentation string and an interactive declaration.There is no conflict if the same symbol name is also used as a variable, since the symbol's value cell is independent of the function cell. See Symbol Components.
Here are some examples:
(defun foo () 5) ⇒ foo (foo) ⇒ 5 (defun bar (a &optional b &rest c) (list a b c)) ⇒ bar (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 a word." (interactive) (backward-word 1) (forward-word 1) (backward-char 1) (capitalize-word 1)) ⇒ capitalize-backwardsBe careful not to redefine existing functions unintentionally.
defunredefines even primitive functions such ascarwithout any hesitation or notification. Redefining a function already defined is often done deliberately, and there is no way to distinguish deliberate redefinition from unintentional redefinition.
This special form defines the symbol name as a function, with definition definition (which can be any valid Lisp function). It returns definition.
If docstring is non-
nil, it becomes the function documentation of name. Otherwise, any documentation provided by definition is used.The proper place to use
defaliasis where a specific function name is being defined—especially where that name appears explicitly in the source file being loaded. This is becausedefaliasrecords which file defined the function, just likedefun(see Unloading).By contrast, in programs that manipulate function definitions for other purposes, it is better to use
fset, which does not keep such records. See Function Cells.
You cannot create a new primitive function with defun or
defalias, but you can use them to change the function definition of
any symbol, even one such as car or x-popup-menu whose
normal definition is a primitive. However, this is risky: for
instance, it is next to impossible to redefine car without
breaking Lisp completely. Redefining an obscure function such as
x-popup-menu is less dangerous, but it still may not work as
you expect. If there are calls to the primitive from C code, they
call the primitive's C definition directly, so changing the symbol's
definition will have no effect on them.
See also defsubst, which defines a function like defun
and tells the Lisp compiler to open-code it. See Inline Functions.
Next: Mapping Functions, Previous: Defining Functions, Up: Functions
12.5 Calling Functions
Defining functions is only half the battle. Functions don't do anything until you call them, i.e., tell them to run. Calling a function is also known as invocation.
The most common way of invoking a function is by evaluating a list.
For example, evaluating the list (concat "a" "b") calls the
function concat with arguments "a" and "b".
See Evaluation, for a description of evaluation.
When you write a list as an expression in your program, you specify
which function to call, and how many arguments to give it, in the text
of the program. Usually that's just what you want. Occasionally you
need to compute at run time which function to call. To do that, use
the function funcall. When you also need to determine at run
time how many arguments to pass, use apply.
funcallcalls function with arguments, and returns whatever function returns.Since
funcallis a function, all of its arguments, including function, are evaluated beforefuncallis called. This means that you can use any expression to obtain the function to be called. It also means thatfuncalldoes 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 offuncallis 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.
funcallcannot 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: #<subr and>Compare these examples with the examples of
apply.
applycalls function with arguments, just likefuncallbut 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 thatapplyspreads this list so that each individual element becomes an argument.
applyreturns the result of calling function. As withfuncall, function must either be a Lisp function or a primitive function; special forms and macros do not make sense inapply.(setq f 'list) ⇒ list (apply f 'x 'y 'z) error--> Wrong type argument: listp, z (apply '+ 1 2 '(3 4)) ⇒ 10 (apply '+ '(1 2 3 4)) ⇒ 10 (apply 'append '((a b c) nil (x y z) nil)) ⇒ (a b c x y z)For an interesting example of using
apply, see Definition of mapcar.
Sometimes it is useful to fix some of the function's arguments at certain values, and leave the rest of arguments for when the function is actually called. The act of fixing some of the function's arguments is called partial application of the function7. 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:
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-partiallywith 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, usingapply-partiallyand+, 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:
Next: Anonymous Functions, Previous: Calling Functions, Up: Functions
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; mapcar and
mapconcat, which scan a list, are described here.
See Definition of mapatoms, for the function mapatoms which
maps over the symbols in an obarray. See Definition of maphash,
for the function maphash which maps over key/value associations
in a hash table.
These mapping functions do not allow char-tables because a char-table
is a sparse array whose nominal range of indices is very large. To map
over a char-table in a way that deals properly with its sparse nature,
use the function map-char-table (see Char-Tables).
mapcarapplies 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 inmy-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))
mapcis likemapcarexcept that function is used for side-effects only—the values it returns are ignored, not collected into a list.mapcalways returns sequence.
mapconcatapplies function to each element of sequence: the results, which must be strings, are concatenated. Between each pair of result strings,mapconcatinserts 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"
Next: Function Cells, Previous: Mapping Functions, Up: Functions
12.7 Anonymous Functions
In Lisp, a function is a list that starts with lambda, a
byte-code function compiled from such a list, or alternatively a
primitive subr-object; names are “extra.” Although functions are
usually defined with defun and given names at the same time, it
is occasionally more concise to use an explicit lambda expression—an
anonymous function. Such a list is valid wherever a function name is.
Any method of creating such a list makes a valid function. Even this:
(setq silly (append '(lambda (x)) (list (list '+ (* 3 4) 'x))))
⇒ (lambda (x) (+ 12 x))
This computes a list that looks like (lambda (x) (+ 12 x)) and
makes it the value (not the function definition!) of
silly.
Here is how we might call this function:
(funcall silly 1)
⇒ 13
It does not work to write (silly 1), because this
function is not the function definition of silly. We
have not given silly any function definition, just a value as a
variable.
Most of the time, anonymous functions are constants that appear in
your program. For instance, you might want to pass one as an argument
to the function mapcar, which applies any given function to
each element of a list (see Mapping Functions).
See describe-symbols example, for a realistic example of this.
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))))
In the double-property function, we did not quote the
lambda form. This is permissible, because a lambda form
is self-quoting: evaluating the form yields the form itself.
Whether or not you quote a lambda form makes a difference if
you compile the code (see Byte Compilation). If the lambda
form is unquoted, as in the above example, the anonymous function is
also compiled. Suppose, however, that we quoted the lambda
form:
(defun double-property (symbol prop)
(change-property symbol prop '(lambda (x) (* 2 x))))
If you compile this, the argument passed to change-property is
the precise list shown:
(lambda (x) (* x 2))
The Lisp compiler cannot assume this list is a function, even though
it looks like one, since it does not know what change-property
will do with the list. Perhaps it will check whether the car of
the third element is the symbol *!
The function special form explicitly tells the byte-compiler
that its argument is a function:
This special form returns function-object without evaluating it. In this, it is equivalent to
quote. However, it serves as a note to the Emacs Lisp compiler that function-object is intended to be used only as a function, and therefore can safely be compiled. Contrast this withquote, in Quoting.
The read syntax #' is a short-hand for using function.
Generally, it is not necessary to use either #' or
function; just use an unquoted lambda form instead.
(Actually, lambda is a macro defined using function.)
The following forms are all equivalent:
#'(lambda (x) (* x x))
(function (lambda (x) (* x x)))
(lambda (x) (* x x))
We sometimes write function instead of quote when
quoting the name of a function, but this usage is just a sort of
comment:
(function symbol) == (quote symbol) == 'symbol
Next: Obsolete Functions, Previous: Anonymous Functions, Up: Functions
12.8 Accessing Function Cell Contents
The function definition of a symbol is the object stored in the function cell of the symbol. The functions described here access, test, and set the function cell of symbols.
See also the function indirect-function. See Definition of indirect-function.
This returns the object in the function cell of symbol. If the symbol's function cell is void, a
void-functionerror is signaled.This function does not check that the returned object is a legitimate function.
(defun bar (n) (+ n 2)) ⇒ bar (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.
This function returns
tif the symbol has an object in its function cell,nilotherwise. It does not check that the object is a legitimate function.
This function makes symbol's function cell void, so that a subsequent attempt to access this cell will cause a
void-functionerror. It returns symbol. (See alsomakunbound, in Void Variables.)(defun foo (x) x) ⇒ foo (foo 1) ⇒1 (fmakunbound 'foo) ⇒ foo (foo 1) error--> Symbol's function definition is void: foo
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.
There are three normal uses of this function:
- Copying one symbol's function definition to another—in other words, making an alternate name for a function. (If you think of this as the definition of the new name, you should use
defaliasinstead offset; see Definition of defalias.)- Giving a symbol a function definition that is not a list and therefore cannot be made with
defun. For example, you can usefsetto give a symbols1a function definition which is another symbols2; thens1serves as an alias for whatever definitions2presently has. (Once again usedefaliasinstead offsetif you think of this as the definition ofs1.)- In constructs for defining or altering functions. If
defunwere not a primitive, it could be written in Lisp (as a macro) usingfset.Here are examples of these uses:
;; Savefoo's definition inold-foo. (fset 'old-foo (symbol-function 'foo)) ;; Make the symbolcarthe function definition ofxfirst. ;; (Most likely,defaliaswould be better thanfsethere.) (fset 'xfirst 'car) ⇒ car (xfirst '(1 2 3)) ⇒ 1 (symbol-function 'xfirst) ⇒ car (symbol-function (symbol-function 'xfirst)) ⇒ #<subr car> ;; Define a named keyboard macro. (fset 'kill-two-lines "\^u2\^k") ⇒ "\^u2\^k" ;; Here is a function that alters other functions. (defun copy-function-definition (new old) "Define NEW with the same function definition as OLD." (fset new (symbol-function old)))
fset is sometimes used to save the old definition of a
function before redefining it. That permits the new definition to
invoke the old definition. But it is unmodular and unclean for a Lisp
file to redefine a function defined elsewhere. If you want to modify
a function defined by another package, it is cleaner to use
defadvice (see Advising Functions).
Next: Inline Functions, Previous: Function Cells, Up: Functions
12.9 Declaring Functions Obsolete
You can use make-obsolete to declare a function obsolete. This
indicates that the function may be removed at some stage in the future.
This function makes the byte compiler warn that the function obsolete-name is obsolete. 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. If current-name is a string, it is the warning message.
If provided, when should be a string indicating when the function was first made obsolete—for example, a date or a release number.
You can define a function as an alias and declare it obsolete at the
same time using the macro define-obsolete-function-alias:
This 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 docstring) (make-obsolete obsolete-name current-name when)
In addition, you can mark a certain a particular calling convention for a function as obsolete:
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).
For instance, in old versions of Emacs the
sit-forfunction accepted three arguments, like this(sit-for seconds milliseconds nodisp)However, calling
sit-forthis way is considered obsolete (see Waiting). The old calling convention is deprecated like this:(set-advertised-calling-convention 'sit-for '(seconds &optional nodisp))
Next: Declaring Functions, Previous: Obsolete Functions, Up: Functions
12.10 Inline Functions
You can define an inline function by using defsubst instead
of defun. An inline function works just like an ordinary
function except for one thing: when you compile a call to the function,
the function's definition is open-coded into the caller.
Making a function inline makes explicit calls run faster. But it also has disadvantages. For one thing, it reduces flexibility; if you change the definition of the function, calls already inlined still use the old definition until you recompile them.
Another disadvantage is that making a large function inline can increase the size of compiled code both in files and in memory. Since the speed advantage of inline functions is greatest for small functions, you generally should not make large functions inline.
Also, inline functions do not behave well with respect to debugging,
tracing, and advising (see Advising Functions). Since ease of
debugging and the flexibility of redefining functions are important
features of Emacs, you should not make a function inline, even if it's
small, unless its speed is really crucial, and you've timed the code
to verify that using defun actually has performance problems.
It's possible to define a macro to expand into the same code that an
inline function would execute. (See Macros.) But the macro would be
limited to direct use in expressions—a macro cannot be called with
apply, mapcar and so on. Also, it takes some work to
convert an ordinary function into a macro. To convert it into an inline
function is very easy; simply 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. (See Argument Evaluation.)
Inline functions can be used and open-coded later on in the same file, following the definition, just like macros.
Next: Function Safety, Previous: Inline Functions, Up: Functions
12.11 Telling the Compiler that a Function is Defined
Byte-compiling a file often produces warnings about functions that the compiler doesn't know about (see Compiler Errors). Sometimes this indicates a real problem, but usually the functions in question are defined in other files which would be loaded if that code is run. For example, byte-compiling fortran.el used to warn:
In end of data:
fortran.el:2152:1:Warning: the function `gud-find-c-expr' is not known
to be defined.
In fact, gud-find-c-expr is only used in the function that
Fortran mode uses for the local value of
gud-find-expr-function, which is a callback from GUD; if it is
called, the GUD functions will be loaded. When you know that such a
warning does not indicate a real problem, it is good to suppress the
warning. That makes new warnings which might mean real problems more
visible. You do that with declare-function.
All you need to do is add a declare-function statement before the
first use of the function in question:
(declare-function gud-find-c-expr "gud.el" nil)
This says that gud-find-c-expr is defined in gud.el (the
‘.el’ can be omitted). The compiler takes for granted that that file
really defines the function, and does not check.
The optional third argument specifies the argument list of
gud-find-c-expr. In this case, it takes no arguments
(nil is different from not specifying a value). In other
cases, this might be something like (file &optional overwrite).
You don't have to specify the argument list, but if you do the
byte compiler can check that the calls match the declaration.
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-
nilmeans 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 defined by C code by specifying a
file name ending in ‘.c’ or ‘.m’. check-declare-file
looks for these files in the C source code directory. This is useful
only when you call a function that is defined only on certain systems.
Most of the primitive functions of Emacs 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).
Next: Related Topics, Previous: Declaring Functions, Up: Functions
12.12 Determining whether a Function is Safe to Call
Some major modes such as SES call functions that are stored in user files. (see Top, for more information on SES.) User files sometimes have poor pedigrees—you can get a spreadsheet from someone you've just met, or you can get one through email from someone you've never met. So it is risky to call a function whose source code is stored in a user file until you have determined that it is safe.
Returns
nilif 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.
Previous: Function Safety, Up: Functions
12.13 Other Topics Related to Functions
Here is a table of several functions that do things related to function calling and function definitions. They are documented elsewhere, but we provide cross references here.
apply- See Calling Functions.
autoload- See Autoload.
call-interactively- See Interactive Call.
called-interactively-p- See Distinguish Interactive.
commandp- See Interactive Call.
documentation- See Accessing Documentation.
eval- See Eval.
funcall- See Calling Functions.
function- See Anonymous Functions.
ignore- See Calling Functions.
indirect-function- See Function Indirection.
interactive- See Using Interactive.
interactive-p- See Distinguish Interactive.
mapatoms- See Creating Symbols.
mapcar- See Mapping Functions.
map-char-table- See Char-Tables.
mapconcat- See Mapping Functions.
undefined- See Functions for Key Lookup.
Next: Customization, Previous: Functions, Up: Top
13 Macros
Macros enable you to define new control constructs and other language features. A macro is defined much like a function, but instead of telling how to compute a value, it tells how to compute another Lisp expression which will in turn compute the value. We call this expression the expansion of the macro.
Macros can do this because they operate on the unevaluated expressions for the arguments, not on the argument values as functions do. They can therefore construct an expansion containing these argument expressions or parts of them.
If you are using a macro to do something an ordinary function could do, just for the sake of speed, consider using an inline function instead. See Inline Functions.
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.
Next: Compiling Macros, Previous: Simple Macro, Up: Macros
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 not the value of the macro call. Instead, it is an alternate expression for computing that value, 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.
You can see the expansion of a given macro call by calling
macroexpand.
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
macroexpanddoes not look at the subexpressions of form (although some macro definitions may do so). Even if they are macro calls themselves,macroexpanddoes not expand them.The function
macroexpanddoes 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))) ⇒ inc (macroexpand '(inc r)) ⇒ (setq r (1+ r)) (defmacro inc2 (var1 var2) (list 'progn (list 'inc var1) (list 'inc var2))) ⇒ inc2 (macroexpand '(inc2 r s)) ⇒ (progn (inc r) (inc s)) ;incnot expanded here.
macroexpand-allexpands macros likemacroexpand, but will look for and expand all macros in form, not just at the top-level. If no macros are expanded, the return value iseqto form.Repeating the example used for
macroexpandabove withmacroexpand-all, we see thatmacroexpand-alldoes expand the embedded calls toinc:(macroexpand-all '(inc2 r s)) ⇒ (progn (setq r (1+ r)) (setq s (1+ s)))
Next: Defining Macros, Previous: Expansion, Up: Macros
13.3 Macros and Byte Compilation
You might ask why we take the trouble to compute an expansion for a macro and then evaluate the expansion. Why not have the macro body produce the desired results directly? The reason has to do with compilation.
When a macro call appears in a Lisp program being compiled, the Lisp compiler calls the macro definition just as the interpreter would, and receives an expansion. But instead of evaluating this expansion, it compiles the expansion as if it had appeared directly in the program. As a result, the compiled code produces the value and side effects intended for the macro, but executes at full compiled speed. This would not work if the macro body computed the value and side effects itself—they would be computed at compile time, which is not useful.
In order for compilation of macro calls to work, the macros must
already be defined in Lisp when the calls to them are compiled. The
compiler has a special feature to help you do this: if a file being
compiled contains a defmacro form, the macro is defined
temporarily for the rest of the compilation of that file.
Byte-compiling a file also executes any require calls at
top-level in the file, so you can ensure that necessary macro
definitions are available during compilation by requiring the files
that define them (see Named Features). To avoid loading the macro
definition files when someone runs the compiled program, write
eval-when-compile around the require calls (see Eval During Compile).
Next: Backquote, Previous: Compiling Macros, Up: Macros
13.4 Defining Macros
A Lisp macro is a list whose car is macro. Its cdr should
be a function; expansion of the macro works by applying the function
(with apply) to the list of unevaluated argument-expressions
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 usually defined with the
special form defmacro.
defmacrodefines the symbol name as a macro that looks like this:(macro lambda argument-list . body-forms)(Note that the cdr of this list is a function—a lambda expression.) This macro object is stored in the function cell of name. The value returned by evaluating the
defmacroform is name, but usually we ignore this value.The shape and meaning of argument-list is the same as in a function, and the keywords
&restand&optionalmay be used (see Argument List). Macros may have a documentation string, but anyinteractivedeclaration is ignored since macros cannot be called interactively.
The body of the macro definition can include a declare form,
which can specify how <TAB> should indent macro calls, and how to
step through them for Edebug.
A
declareform is used in a macro definition to specify various additional information about it. Two kinds of specification are currently supported:
(debugedebug-form-spec)- Specify how to step through macro calls for Edebug. See Instrumenting Macro Calls.
(indentindent-spec)- Specify how to indent calls to this macro. See Indenting Macros, for more details.
A
declareform only has its special effect in the body of adefmacroform if it immediately follows the documentation string, if present, or the argument list otherwise. (Strictly speaking, severaldeclareforms can follow the documentation string or argument list, but since adeclareform can have several specs, they can always be combined into a single form.) When used at other places in adefmacroform, or outside adefmacroform,declarejust returnsnilwithout evaluating any specs.
No macro absolutely needs a declare form, because that form
has no effect on how the macro expands, on what the macro means in the
program. It only affects secondary features: indentation and Edebug.
Next: Problems with Macros, Previous: Defining Macros, Up: Macros
13.5 Backquote
Macros often need to construct large list structures from a mixture of constants and nonconstant parts. To make this easier, use the ‘`’ syntax (usually called backquote).
Backquote allows you to quote a list, but selectively evaluate
elements of that list. In the simplest case, it is identical to the
special form quote (see Quoting). For example, these
two forms yield identical results:
`(a list of (+ 2 3) elements)
⇒ (a list of (+ 2 3) elements)
'(a list of (+ 2 3) elements)
⇒ (a list of (+ 2 3) elements)
The special marker ‘,’ inside of the argument to backquote indicates a value that isn't constant. Backquote evaluates the argument of ‘,’ and puts the value in the list structure:
(list 'a 'list 'of (+ 2 3) 'elements)
⇒ (a list of 5 elements)
`(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:
(defmacro t-becomes-nil (variable)
`(if (eq ,variable t)
(setq ,variable nil)))
(t-becomes-nil foo)
== (if (eq foo t) (setq foo nil))
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)
Next: Indenting Macros, Previous: Backquote, Up: Macros
13.6 Common Problems Using Macros
The basic facts of macro expansion have counterintuitive consequences. This section describes some important consequences that can lead to trouble, and rules to follow to avoid trouble.
Next: Argument Evaluation, Up: Problems with Macros
13.6.1 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.
Next: Surprising Local Vars, Previous: Wrong Time, Up: Problems with Macros
13.6.2 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 simple “for” loop such as one might find in Pascal.
(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
(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.
Next: Eval During Expansion, Previous: Argument Evaluation, Up: Problems with Macros
13.6.3 Local Variables in Macro Expansions
In the previous section, the definition of for was fixed as
follows to make the expansion evaluate the macro arguments the proper
number of times:
(defmacro for (var from init to final do &rest body)
"Execute a simple for loop: (for i from 1 to 10 do (print i))."
`(let ((,var ,init)
(max ,final))
(while (<= ,var max)
,@body
(inc ,var))))
The new definition of for has a new problem: it introduces a
local variable named max which the user does not expect. This
causes trouble in examples such as the following:
(let ((max 0))
(for x from 0 to 10 do
(let ((this (frob x)))
(if (< max this)
(setq max this)))))
The references to max inside the body of the for, which
are supposed to refer to the user's binding of max, really access
the binding made by for.
The way to correct this is to use an uninterned symbol instead of
max (see Creating Symbols). The uninterned symbol can be
bound and referred to just like any other symbol, but since it is
created by for, we know that it cannot already appear in the
user's program. Since it is not interned, there is no way the user can
put it into the program later. It will never appear anywhere except
where put by for. Here is a definition of for that works
this way:
(defmacro for (var from init to final do &rest body)
"Execute a simple for loop: (for i from 1 to 10 do (print i))."
(let ((tempvar (make-symbol "max")))
`(let ((,var ,init)
(,tempvar ,final))
(while (<= ,var ,tempvar)
,@body
(inc ,var)))))
This creates an uninterned symbol named max and puts it in the
expansion instead of the usual interned symbol max that appears
in expressions ordinarily.
Next: Repeated Expansion, Previous: Surprising Local Vars, Up: Problems with Macros
13.6.4 Evaluating Macro Arguments in Expansion
Another problem can happen if the macro definition itself
evaluates any of the macro argument expressions, such as by calling
eval (see Eval). If the argument is supposed to refer to the
user's variables, you may have trouble if the user happens to use a
variable with the same name as one of the macro arguments. Inside the
macro body, the macro argument binding is the most local binding of this
variable, so any references inside the form being evaluated do refer to
it. Here is an example:
(defmacro foo (a)
(list 'setq (eval a) t))
⇒ foo
(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.
Previous: Eval During Expansion, Up: Problems with Macros
13.6.5 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.
Previous: Problems with Macros, Up: Macros
13.7 Indenting Macros
You can use the declare form in the macro definition to
specify how to <TAB> should indent calls to the macro. You
write it 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-indentmore 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:
- 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. - pos
- The position at which the line being indented begins.
14 Writing Customization Definitions
This chapter describes how to declare user options for customization, and also customization groups for classifying them. We use the term customization item to include both kinds of customization definitions—as well as face definitions (see Defining Faces).
Next: Group Definitions, Up: Customization
14.1 Common Item Keywords
All kinds of customization declarations (for variables and groups, and for faces) accept keyword arguments for specifying various information. This section describes some keywords that apply to all kinds.
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.
:taglabel- 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.
:groupgroup- Put this customization item in group group. When you use
:groupin adefgroup, 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.
:linklink-data- Include an external link after the documentation string for this item.
This is a sentence containing an active field which references some
other documentation.
There are several alternatives you can use for link-data:
(custom-manualinfo-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-linkinfo-node)- Like
custom-manualexcept that the link appears in the customization buffer with the Info node name. (url-linkurl)- 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-linklibrary)- Link to the commentary section of a library; library is a string
which specifies the library name.
(emacs-library-linklibrary)- Link to an Emacs Lisp library file; library is a string which
specifies the library name.
(file-linkfile)- Link to a file; file is a string which specifies the name of the
file to visit with
find-filewhen the user invokes this link. (function-linkfunction)- Link to the documentation of a function; function is a string
which specifies the name of the function to describe with
describe-functionwhen the user invokes this link. (variable-linkvariable)- Link to the documentation of a variable; variable is a string
which specifies the name of the variable to describe with
describe-variablewhen the user invokes this link. (custom-group-linkgroup)- 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
:tagname 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’.An item can have more than one external link; however, most items have none at all.
:loadfile- Load file file (a string) before displaying this customization
item (see Loading). Loading is done with
load, and only if the file is not already loaded. :requirefeature- Execute
(require 'feature)when your saved customizations set the value of this item. feature should be a symbol.The most common reason to use
:requireis 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. :versionversion- 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. The value of package is a
symbol and version is a string.
This keyword takes priority over
:version.package should be the official name of the package, such as MH-E or Gnus. 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.
This alist provides a mapping for the versions of Emacs that are associated with versions of a package listed in the
:package-versionkeyword. Its elements look like this:(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-versionkeyword. Since the user might see the value in a error message, a good choice is the official name of the package, such as MH-E or Gnus.
Next: Variable Definitions, Previous: Common Keywords, Up: Customization
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 twelve 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.
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:groupkeyword 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 arecustom-variablefor a variable,custom-facefor a face, andcustom-groupfor a group.When you introduce a new group into Emacs, use the
:versionkeyword in thedefgroup; then you need not use it for the individual members of the group.In addition to the common keywords (see Common Keywords), you can also use this keyword in
defgroup:
The prefix-discarding feature is currently turned off, which means
that :prefix currently has no effect. We did this because we
found that discarding the specified prefixes often led to confusing
names for options. This happened because the people who wrote the
defgroup definitions for various groups added :prefix
keywords whenever they make logical sense—that is, whenever the
variables in the library have a common prefix.
In order to obtain good results with :prefix, it would be
necessary to check the specific effects of discarding a particular
prefix, given the specific items in a group and their names and
documentation. If the resulting text is not clear, then :prefix
should not be used in that case.
It should be possible to recheck all the customization groups, delete
the :prefix specifications which give unclear results, and then
turn this feature back on, if someone would like to do the work.
Next: Customization Types, Previous: Group Definitions, Up: Customization
14.3 Defining Customization Variables
Use defcustom to declare user-customizable variables.
This macro declares option as a customizable user option. You should not quote option.
This causes the function
user-variable-pto returntwhen given option as an argument. See Defining Variables. The argument doc specifies the documentation string for the variable. (Note that there is no need to start doc with a ‘*’.)The argument standard is an expression that specifies the standard value for option. Evaluating the
defcustomform evaluates standard, but does not necessarily install the standard value. If option already has a default value,defcustomdoes not change it. If the user has saved a customization for option,defcustominstalls the user's customized value as option's default value. If neither of those cases applies,defcustominstalls 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. We recommend avoiding backquotes in standard, because they are not expanded when editing the value, so list values will appear to have the wrong structure.
Every
defcustomshould specify:groupat least once.If you specify the
:setkeyword, to make the variable take other special actions when set through the customization buffer, the variable's documentation string should tell the user specifically how to do the same job in hand-written Lisp code.When you evaluate a
defcustomform with C-M-x in Emacs Lisp mode (eval-defun), a special feature ofeval-defunarranges to set the variable unconditionally, without testing whether its value is void. (The same feature applies todefvar.) See Defining Variables.If you put a
defcustomin a file that is preloaded at dump time (see Building Emacs), and the standard value installed for the variable at that time might not be correct, usecustom-reevaluate-setting, described below, to re-evaluate the standard value during or after Emacs startup.
defcustom accepts the following additional keywords:
:typetype- Use type as the data type for this option. It specifies which
values are legitimate, and how to display the value.
See Customization Types, for more information.
:optionsvalue-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,plistandalist. See the definition of the individual types for a description of how to use:options. :setsetfunction- Specify setfunction as the way to change the value of this
option. 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. :getgetfunction- 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
:getcorrectly. 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 specifygetfunctionfor a value that really is stored in a Lisp variable. :initializefunction- function should be a function used to initialize the variable
when the
defcustomis 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
:setfunction 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 functionset-defaultto set the variable, instead of the variable's:setfunction. This is the usual choice for a variable whose:setfunction 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
:setfunction to initialize the variable. If the variable is already non-void, reset it by calling the:setfunction using the current value (returned by the:getmethod). This is the default:initializefunction. custom-initialize-changed- Use the
:setfunction to initialize the variable, if it is already set or has been customized; otherwise, just useset-default. custom-initialize-safe-setcustom-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 tonilusingset-default, and throw no error.These two functions are only meant for options defined in pre-loaded files, where some variables or functions used to compute the option's value may not yet be defined. The option normally gets updated in startup.el, ignoring the previously computed value. Because of this typical usage, the value which these two functions compute normally only matters when, after startup, one unsets the option's value and then reevaluates the defcustom. By that time, the necessary variables and functions will be defined, so there will not be an error.
:riskyvalue- Set the variable's
risky-local-variableproperty to value (see File Local Variables). :safefunction- Set the variable's
safe-local-variableproperty to function (see File Local Variables). :set-aftervariables- When setting variables according to saved customizations, make sure to
set the variables variables before this one; in other words, delay
setting this variable until after those others have been handled. Use
:set-afterif setting this variable won't work properly unless those other variables already have their intended values.
It is useful to specify the :require keyword for an option
that “turns on” a certain feature. This causes Emacs to load the
feature, if it is not already loaded, whenever the option is set.
See Common Keywords. Here is an example, from the library
saveplace.el:
(defcustom save-place nil
"Non-nil means automatically save place in each file..."
:type 'boolean
:require 'saveplace
:group 'save-place)
If a customization item has a type such as hook or
alist, which supports :options, you can add additional
values to the list from outside the defcustom declaration by
calling custom-add-frequent-value. For example, if you define a
function my-lisp-mode-initialization intended to be called from
emacs-lisp-mode-hook, you might want to add that to the list of
reasonable values for emacs-lisp-mode-hook, but not by editing
its definition. You can do it thus:
(custom-add-frequent-value 'emacs-lisp-mode-hook
'my-lisp-mode-initialization)
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.
See Property Lists. These properties are lists, the car of which
is an expression that evaluates to the value.
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.) This is useful for customizable options that are defined before their value could be computed correctly, such as variables defined in packages that are loaded at dump time, but depend on the run-time information. For example, the value could be a file whose precise name depends on the hierarchy of files when Emacs runs, or a name of a program that needs to be searched at run time.A good place to put calls to this function is in the function
command-linethat is run during startup (see Startup Summary) or in the various hooks it calls.
Previous: Variable Definitions, Up: Customization
14.4 Customization Types
When you define a user option with defcustom, you must specify
its customization type. That is a Lisp object which describes (1)
which values are legitimate and (2) how to display the value in the
customization buffer for editing.
You specify the customization type in defcustom with the
:type keyword. The argument of :type is evaluated, but
only once when the defcustom is executed, so it isn't useful
for the value to vary. Normally we use a quoted constant. For
example:
(defcustom diff-command "diff"
"The command to use to run diff."
:type '(string)
:group 'diff)
In general, a customization type is a list whose first element is a symbol, one of the customization type names defined in the following sections. After this symbol come a number of arguments, depending on the symbol. Between the type symbol and its arguments, you can optionally write keyword-value pairs (see Type Keywords).
Some type symbols do not use any arguments; those are called
simple types. For a simple type, if you do not use any
keyword-value pairs, you can omit the parentheses around the type
symbol. For example just string as a customization type is
equivalent to (string).
All customization types are implemented as widgets; see Introduction, for details.
Next: Composite Types, Up: Customization Types
14.4.1 Simple Types
This section describes all the simple customization types.
sexp- The value may be any Lisp object that can be printed and read back. You
can use
sexpas 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, and is represented textually
in the customization buffer.
number- The value must be a number (floating point or integer), and is
represented textually in the customization buffer.
float- The value must be a floating point number, and is represented
textually in the customization buffer.
string- The value must be a string, and the customization buffer shows just the
contents, with no delimiting ‘"’ characters and no quoting with
‘\’.
regexp- Like
stringexcept 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, and you can do completion with
M-<TAB>.
(file :must-match t)- The value must be a file name for an existing file, and you can do
completion with M-<TAB>.
directory- The value must be a directory name, and you can do completion with
M-<TAB>.
hook- The value must be a list of functions (or a single function, but that is
obsolete usage). This customization type is used for hook variables.
You can use the
:optionskeyword in a hook variable'sdefcustomto specify a list of functions recommended for use in the hook; see Variable Definitions. alist- The value must be a list of cons-cells, the car of each cell
representing a key, and the cdr of the same cell representing an
associated value. The user can add and delete key/value pairs, and
edit both the key and the value of each pair.
You can specify the key and value types like this:
(alist :key-type key-type :value-type value-type)
where key-type and value-type are customization type specifications. The default key type is
sexp, and the default value type issexp.The user can add any key matching the specified key type, but you can give some keys a preferential treatment by specifying them with the
:options(see Variable Definitions). The specified keys will always be shown in the customize buffer (together with a suitable value), with a checkbox to include or exclude or disable the key/value pair from the alist. The user will not be able to edit the keys specified by the:optionskeyword argument.The argument to the
:optionskeywords should be a list of specifications for reasonable keys in the alist. Ordinarily, they are simply atoms, which stand for themselves as. 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:optionskeyword. However, you may want to use a more specialized type for presenting the key, likefunction-itemif 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-alistin 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
groupwidget is used here instead oflistonly 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))) (defcustom pets '(("brian") ("dorith" "dog" "guppy") ("ken" "cat")) "Alist of people's pets. In an element (KEY . VALUE), KEY is the person's name, and the VALUE is a list of that person's pets." :type '(alist :value-type (repeat string))) plist- The
plistcustom type is similar to thealist(see above), except that the information is stored as a property list, i.e. a list of this form:(key value key value key value ...)
The default
:key-typeforplistissymbol, rather thansexp. symbol- The value must be a symbol. It appears in the customization buffer as
the name of the symbol.
function- The value must be either a lambda expression or a function name. When
it is a function name, you can do completion with M-<TAB>.
variable- The value must be a variable name, and you can do completion with
M-<TAB>.
face- The value must be a symbol which is a face name, and you can do
completion with M-<TAB>.
boolean- The value is boolean—either
nilort. Note that by usingchoiceandconsttogether (see the next section), you can specify that the value must benilort, but also specify the text to describe each value in a way that fits the specific meaning of the alternative. coding-system- The value must be a coding-system name, and you can do completion with
M-<TAB>.
color- The value must be a valid color name, and you can do completion with M-<TAB>. A sample is provided.
Next: Splicing into Lists, Previous: Simple Types, Up: Customization Types
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:
(conscar-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 the cdr are displayed and edited separately, each according to the type that you specify for it.
(listelement-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.
(groupelement-types...)- This works like
listexcept for the formatting of text in the Custom buffer.listlabels each element value with its tag;groupdoes not. (vectorelement-types...)- Like
listexcept that the value must be a vector instead of a list. The elements work the same as inlist. (choicealternative-types...)- The value must fit at least 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
:tagkeyword 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
nilis not a valid value, other than aconst, you should specify a valid default for that alternative using the:valuekeyword. See Type Keywords.If some values are covered by more than one of the alternatives, customize will choose the first alternative that the value fits. This means you should always list the most specific types first, and the most general last. Here's an example of proper usage:
(choice (const :tag "Off" nil) symbol (sexp :tag "Other"))This way, the special value
nilis not treated like other symbols, and symbols are not treated like other Lisp expressions. (radioelement-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-itemcustomization types). (constvalue)- The value must be value—nothing else is allowed.
The main use of
constis inside ofchoice. For example,(choice integer (const nil))allows either an integer ornil.:tagis often used withconst, inside ofchoice. For example,(choice (const :tag "Yes" t) (const :tag "No" nil) (const :tag "Ask" foo))describes a variable for which
tmeans yes,nilmeans no, andfoomeans “ask.” (othervalue)- This alternative can match any Lisp value, but if the user chooses this
alternative, that selects the value value.
The main use of
otheris as the last element ofchoice. For example,(choice (const :tag "Yes" t) (const :tag "No" nil) (other :tag "Ask" foo))describes a variable for which
tmeans yes,nilmeans no, and anything else means “ask.” If the user chooses ‘Ask’ from the menu of alternatives, that specifies the valuefoo; but any other value (nott,nilorfoo) displays as ‘Ask’, just likefoo. (function-itemfunction)- 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-itemvariable)- 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. (settypes...)- 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 asintegerin aset.Most often, the types in a
setareconsttypes, 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.
(repeatelement-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-alternativescriteria)- 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
nilor non-nilaccording to the argument. Using a predicate in the list says that objects for which the predicate returns non-nilare 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,
tandnilas legitimate values.The customization buffer shows all legitimate values using their read syntax, and the user edits them textually.
- A predicate—that is, a function of one argument that has no side
effects, and returns either
Here is a table of the keywords you can use in keyword-value pairs in a composite type:
:tagtag- 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-alternativescriteria- Use criteria to match possible values. This is used only in
restricted-sexp. :argsargument-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:argsexplicitly, because normally the arguments are recognized automatically as whatever follows the last keyword-value pair.
Next: Type Keywords, Previous: Composite Types, Up: Customization Types
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. You use it in a
set, choice or repeat type which appears among the
element-types of a list or vector.
Normally, each of the element-types in a list or vector
describes one and only one element of the list or vector. Thus, if an
element-type is a repeat, that specifies a list of unspecified
length which appears as one element.
But when the element-type uses :inline, the value it matches is
merged directly into the containing sequence. For example, if it
matches a list with three elements, those become three elements of the
overall sequence. This is analogous to using ‘,@’ in the backquote
construct.
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.
Next: Defining New Types, Previous: Splicing into Lists, Up: Customization Types
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:
:valuedefault- This is used 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.
If
nilis not a valid value for the alternative, then it is essential to specify a valid default with:value. :formatformat-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
:actionattribute 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 an active field to control whether to show all of it or just the
first line.
- ‘%t’
- Substitute the tag here. You specify the tag with the
:tagkeyword. - ‘%%’
- Display a literal ‘%’.
:actionaction- Perform action if the user clicks on a button.
:button-faceface- Use the face face (a face name or a list of face names) for button
text displayed with ‘%[...%]’.
:button-prefixprefix:button-suffixsuffix- 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.
:tagtag- Use tag (a string) as the tag for the value (or part of the value)
that corresponds to this type.
:docdoc- 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
:choicetype or the parts of some other composite type. :help-echomotion-doc- When you move to this item with
widget-forwardorwidget-backward, it will display the string motion-doc in the echo area. In addition, motion-doc is used as the mousehelp-echostring 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. :matchfunction- 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-
nilif the value is acceptable. :validatefunction- Specify a validation function for input. function takes a
widget as an argument, and should return
nilif 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:errorproperty to a string explaining the error.
Previous: Type Keywords, Up: Customization Types
14.4.5 Defining New Types
In the previous sections we have described how to construct elaborate
type specifications for defcustom. In some cases you may want
to give such a type specification a name. The obvious case is when
you are using the same type for many user options: rather than repeat
the specification for each option, you can give the type specification
a name, and use that name each defcustom. The other case is
when a user option's value is a recursive data structure. To make it
possible for a datatype to refer to itself, it needs to have a name.
Since custom types are implemented as widgets, the way to define a new customize type is to define a new widget. We are not going to describe the widget interface here in details, see Introduction, for that. Instead we are going to demonstrate the minimal functionality needed for defining new customize types by a simple example.
(define-widget 'binary-tree-of-string 'lazy
"A binary tree made of cons-cells and strings."
:offset 4
:tag "Node"
:type '(choice (string :tag "Leaf" :value "")
(cons :tag "Interior"
:value ("" . "")
binary-tree-of-string
binary-tree-of-string)))
(defcustom foo-bar ""
"Sample variable holding a binary tree of strings."
:type 'binary-tree-of-string)
The function to define a new widget is called define-widget. The
first argument is the symbol we want to make a new widget type. The
second argument is a symbol representing an existing widget, the new
widget is going to be defined in terms of difference from the existing
widget. For the purpose of defining new customization types, the
lazy widget is perfect, because it accepts a :type keyword
argument with the same syntax as the keyword argument to
defcustom with the same name. The third argument is a
documentation string for the new widget. You will be able to see that
string with the M-x widget-browse <RET> binary-tree-of-string
<RET> command.
After these mandatory arguments follow the keyword arguments. The most
important is :type, which describes the data type we want to match
with this widget. Here a binary-tree-of-string is described as
being either a string, or a cons-cell whose car and cdr are themselves
both binary-tree-of-string. Note the reference to the widget
type we are currently in the process of defining. The :tag
attribute is a string to name the widget in the user interface, and the
:offset argument is there to ensure that child nodes are
indented four spaces relative to the parent node, making the tree
structure apparent in the customization buffer.
The defcustom shows how the new widget can be used as an ordinary
customization type.
The reason for the name lazy is that the other composite
widgets convert their inferior widgets to internal form when the
widget is instantiated in a buffer. This conversion is recursive, so
the inferior widgets will convert their inferior widgets. If
the data structure is itself recursive, this conversion is an infinite
recursion. The lazy widget prevents the recursion: it convert
its :type argument only when needed.
Next: Byte Compilation, Previous: Customization, Up: Top
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.
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.
A file containing Lisp code is often called a library. Thus, the “Rmail library” is a file containing code for Rmail mode. Similarly, a “Lisp library directory” is a directory of files containing Lisp code.
Next: Load Suffixes, Up: Loading
15.1 How Programs Do Loading
Emacs Lisp has several interfaces for loading. For example,
autoload creates a placeholder object for a function defined in a
file; trying to call the autoloading function loads the file to get the
function's real definition (see Autoload). require loads a
file if it isn't already loaded (see Named Features). Ultimately,
all these facilities call the load function to do the work.
This function finds and opens a file of Lisp code, evaluates all the forms in it, and closes the file.
To find the file,
loadfirst 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, thenloadlooks for a file named filename.el. If that file exists, it is loaded. Finally, if neither of those names is found,loadlooks for a file named filename with nothing appended, and loads it if it exists. (Theloadfunction 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
loadcan 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 injka-compr-load-suffixesto 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, thenloaddoes 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,loadwill still usejka-compr-load-suffixesto find compressed versions. By specifying the precise file name and usingtfor nosuffix, you can prevent perverse file names such as foo.el.el from being tried.If the optional argument must-suffix is non-
nil, thenloadinsists 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,
loadsearches for the file using the variableload-path. It appends filename to each of the directories listed inload-path, and loads the first file it finds whose name matches. The current default directory is tried only if it is specified inload-path, wherenilstands for the default directory.loadtries all three possible suffixes in the first directory inload-path, then all three suffixes in the second directory, and so on. See 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-nameto 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. See Byte Compilation.
When loading a source file (not compiled),
loadperforms character set translation just as Emacs would do when visiting the file. See Coding Systems.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
loadcan't find the file to load, then normally it signals the errorfile-error(with ‘Cannot open load file filename’). But if missing-ok is non-nil, thenloadjust returnsnil.You can use the variable
load-read-functionto specify a function forloadto use instead ofreadfor reading expressions. See below.
loadreturnstif the file loads successfully.
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.
This command loads the library named library. It is equivalent to
load, except for the way it reads its argument interactively. See Lisp Libraries.
This variable is non-
nilif Emacs is in the process of loading a file, and it isnilotherwise.
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.
This variable specifies an alternate expression-reading function for
loadandeval-regionto use instead ofread. The function should accept one argument, just asreaddoes.Normally, the variable's value is
nil, which means those functions should useread.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. See Eval.
For information about how load is used in building Emacs, see
Building Emacs.
Next: Library Search, Previous: How Programs Do Loading, Up: Loading
15.2 Load Suffixes
We now describe some technical details about the exact suffixes that
load tries.
This is a list of suffixes indicating (compiled or source) Emacs Lisp files. It should not include the empty string.
loaduses 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.
This is a list of suffixes that indicate representations of the same file. This list should normally start with the empty string. When
loadsearches 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-suffixesto this list and disabling Auto Compression mode removes them again. The standard value ofload-file-rep-suffixesif Auto Compression mode is disabled is(""). Given that the standard value ofjka-compr-load-suffixesis(".gz"), the standard value ofload-file-rep-suffixesif Auto Compression mode is enabled is("" ".gz").
This function returns the list of all suffixes that
loadshould try, in order, when its must-suffix argument is non-nil. This takes bothload-suffixesandload-file-rep-suffixesinto account. Ifload-suffixes,jka-compr-load-suffixesandload-file-rep-suffixesall 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.
Next: Loading Non-ASCII, Previous: Load Suffixes, Up: Loading
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.
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) ornil(which stands for the current working directory).
The value of load-path is initialized from the environment
variable EMACSLOADPATH, if that exists; otherwise its default
value is specified in emacs/src/epaths.h when Emacs is built.
Then the list is expanded by adding subdirectories of the directories
in the list.
The syntax of EMACSLOADPATH is the same as used for PATH;
‘:’ (or ‘;’, according to the operating system) separates
directory names, and ‘.’ is used for the current default directory.
Here is an example of how to set your EMACSLOADPATH variable from
a csh .login file:
setenv EMACSLOADPATH .:/user/bil/emacs:/usr/local/share/emacs/20.3/lisp
Here is how to set it using sh:
export EMACSLOADPATH
EMACSLOADPATH=.:/user/bil/emacs:/usr/local/share/emacs/20.3/lisp
Here is an example of code you can place in your init file (see Init File) to add several directories to the front of your default
load-path:
(setq load-path
(append (list nil "/user/bil/emacs"
"/usr/local/lisplib"
"~/emacs")
load-path))
In this example, the path searches the current working directory first, followed then by the /user/bil/emacs directory, the /usr/local/lisplib directory, and the ~/emacs directory, which are then followed by the standard directories for Lisp code.
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.
Therefore, if you want to change load-path temporarily for
loading a few libraries in site-init.el or site-load.el,
you should bind load-path locally with let around the
calls to load.
The default value of load-path, when running an Emacs which has
been installed on the system, includes two special directories (and
their subdirectories as well):
"/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.
There are several reasons why a Lisp package that works well in one Emacs version can cause trouble in another. Sometimes packages need updating for incompatible changes in Emacs; sometimes they depend on undocumented internal Emacs data that can change without notice; sometimes a newer Emacs version incorporates a version of the package, and should be used only with that version.
Emacs finds these directories' subdirectories and adds them to
load-path when it starts up. Both immediate subdirectories and
subdirectories multiple levels down are added to load-path.
Not all subdirectories are included, though. Subdirectories whose names do not start with a letter or digit are excluded. Subdirectories named RCS or CVS are excluded. Also, a subdirectory which contains a file named .nosearch is excluded. You can use these methods to prevent certain subdirectories of the site-lisp directories from being searched.
If you run Emacs from the directory where it was built—that is, an
executable that has not been formally installed—then load-path
normally contains two additional directories. These are the lisp
and site-lisp subdirectories of the main build directory. (Both
are represented as absolute file names.)
This command finds the precise file name for library library. It searches for the library in the same way
loaddoes, and the argument nosuffix has the same meaning as inload: 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 ofload-path.When
locate-libraryis called from a program, it returns the file name as a string. When the user runslocate-libraryinteractively, the argument interactive-call ist, and this tellslocate-libraryto display the file name in the echo area.
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 onload-path.For instance, suppose
load-pathis 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
stringpis 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 (see Text Representations). Which representation is used depends on how the file is read into Emacs. If it is read with decoding into multibyte representation, the text of the Lisp program will be multibyte text, and its string constants will be multibyte strings. If a file containing Latin-1 characters (for example) is read without decoding, the text of the program will be unibyte text, and its string constants will be unibyte strings. See Coding Systems.
To make the results more predictable, Emacs always performs decoding into the multibyte representation when loading Lisp files, even if it was started with the ‘--unibyte’ option. This means that string constants with non-ASCII characters translate into multibyte strings. The only exception is when a particular file specifies no decoding.
The reason Emacs is designed this way is so that Lisp programs give predictable results, regardless of how Emacs was started. In addition, this enables programs that depend on using multibyte text to work even in a unibyte Emacs.
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 ‘-*-unibyte: t;-*-’ in a
comment on the file's first line. With that designator, the file will
unconditionally be interpreted as unibyte, even in an ordinary
multibyte Emacs session. This can matter when making keybindings to
non-ASCII characters written as ?vliteral.
Next: Repeated Loading, Previous: Loading Non-ASCII, Up: Loading
15.5 Autoload
The autoload facility allows you to make a function or macro known in Lisp, but put off loading the file that defines it. The first call to the function automatically reads the proper file to install the real definition and other associated code, then runs the real definition as if it had been loaded all along.
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.
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
.elor.elc, thenautoloadinsists on adding one of these suffixes, and it will not load from a file whose name is just filename with no added suffix. (The variableload-suffixesspecifies the exact required suffixes.)The argument docstring is the documentation string for the function. Specifying the documentation string in the call to
autoloadmakes 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
macroif function is really a macro. Specify type askeymapif 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 a non-void function definition that is not an autoload object,
autoloaddoes nothing and returnsnil. If the function cell of function is void, or is already an autoload object, then it is defined as an autoload object like this:(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 (see Documentation Basics),tmeans the function is interactive, andnilthat it is not a macro or a keymap.
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. If the form following the magic comment is not a
function-defining form or a defcustom form, it is copied
verbatim. “Function-defining forms” include define-skeleton,
define-derived-mode, define-generic-mode and
define-minor-mode as well as defun and
defmacro. To save space, a defcustom form is converted to
a defvar in loaddefs.el, with some additional information
if it uses :require.
You can also use a magic comment to execute a form at build time without executing it when the file itself is loaded. To do this, write the form on the same line as the magic comment. Since it is in a comment, it does nothing when you load the source file; but M-x update-file-autoloads copies it to loaddefs.el, where it is executed while building Emacs.
The following example shows how doctor is prepared for
autoloading with a magic comment:
;;;###autoload
(defun doctor ()
"Switch to *doctor* buffer and start giving psychotherapy."
(interactive)
(switch-to-buffer "*doctor*")
(doctor-mode))
Here's what that produces in loaddefs.el:
(autoload (quote doctor) "doctor" "\
Switch to *doctor* buffer and start giving psychotherapy.
\(fn)" t nil)
The backslash and newline immediately following the double-quote are a
convention used only in the preloaded uncompiled Lisp files such as
loaddefs.el; they tell make-docfile to put the
documentation string in the etc/DOC file. See Building Emacs.
See also the commentary in lib-src/make-docfile.c. ‘(fn)’
in the usage part of the documentation string is replaced with the
function's name when the various help functions (see Help Functions) display it.
If you write a function definition with an unusual macro that is not
one of the known and recognized function definition methods, use of an
ordinary magic autoload comment would copy the whole definition into
loaddefs.el. That is not desirable. You can put the desired
autoload call into loaddefs.el instead by writing this:
;;;###autoload (autoload 'foo "myfile")
(mydefunmacro foo
...)
You can use a non-default string as the autoload cookie and have the corresponding autoload calls written into a file whose name is different from the default loaddefs.el. Emacs provides two variables to control this:
The 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".
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 (see File Local Variables). The autoload file is assumed to contain a trailer starting with a formfeed character.
Next: Named Features, Previous: Autoload, Up: Loading
15.6 Repeated Loading
You can load a given file more than once in an Emacs session. For example, after you have rewritten and reinstalled a function definition by editing it in a buffer, you may wish to return to the original version; you can do this by reloading the file it came from.
When you load or reload files, bear in mind that the load and
load-library functions automatically load a byte-compiled file
rather than a non-compiled file of similar name. If you rewrite a file
that you intend to save and reinstall, you need to byte-compile the new
version; otherwise Emacs will load the older, byte-compiled file instead
of your newer, non-compiled file! If that happens, the message
displayed when loading the file includes, ‘(compiled; note, source is
newer)’, to remind you to recompile it.
When writing the forms in a Lisp library file, keep in mind that the
file might be loaded more than once. For example, think about whether
each variable should be reinitialized when you reload the library;
defvar does not change the value if the variable is already
initialized. (See Defining Variables.)
The simplest way to add an element to an alist is like this:
(push '(leif-mode " Leif") minor-mode-alist)
But this would add multiple elements if the library is reloaded. To
avoid the problem, use add-to-list (see List Variables):
(add-to-list 'minor-mode-alist '(leif-mode " Leif"))
Occasionally you will want to test explicitly whether a library has
already been loaded. If the library uses provide to provide a
named feature, you can use featurep earlier in the file to test
whether the provide call has been executed before (see Named Features). Alternatively, you could use something like this:
(defvar foo-was-loaded nil)
(unless foo-was-loaded
execute-first-time-only
(setq foo-was-loaded t))
Next: Where Defined, Previous: Repeated Loading, Up: Loading
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 emacs/lisp/prolog.el,
the definition for run-prolog includes the following code:
(defun run-prolog ()
"Run an inferior Prolog process, with I/O via buffer *prolog*."
(interactive)
(require 'comint)
(switch-to-buffer (make-comint "prolog" prolog-program-name))
(inferior-prolog-mode))
The expression (require 'comint) loads the file comint.el
if it has not yet been loaded. This ensures that make-comint is
defined. Features are normally named after the files that provide them,
so that require need not be given the file name.
The comint.el file contains the following top-level expression:
(provide 'comint)
This adds comint to the global features list, so that
(require 'comint) will henceforth know that nothing needs to be
done.
When require is used at top level in a file, it takes effect
when you byte-compile that file (see Byte Compilation) as well as
when you load it. This is in case the required package contains macros
that the byte compiler must know about. It also avoids byte compiler
warnings for functions and variables defined in the file loaded with
require.
Although top-level calls to require are evaluated during
byte compilation, provide calls are not. Therefore, you can
ensure that a file of definitions is loaded before it is byte-compiled
by including a provide followed by a require for the same
feature, as in the following example.
(provide 'my-feature) ; Ignored by byte compiler,
; evaluated by load.
(require 'my-feature) ; Evaluated by byte compiler.
The compiler ignores the provide, then processes the
require by loading the file in question. Loading the file does
execute the provide call, so the subsequent require call
does nothing when the file is loaded.
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
provideis if not already in features then to add feature to the front of that list and call anyeval-after-loadcode waiting for it (see Hooks for Loading). The argument feature must be a symbol.providereturns 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. See 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
providecalls that occurred during the load are undone. See Autoload.
This function checks whether feature is present in the current Emacs session (using
(featurepfeature); see below). The argument feature must be a symbol.If the feature is not present, then
requireloads filename withload. 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,requireinsists 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 variableload-suffixesspecifies the exact required Lisp suffixes.)If noerror is non-
nil, that suppresses errors from actual loading of the file. In that case,requirereturnsnilif loading the file fails. Normally,requirereturns feature.If loading the file succeeds but does not provide feature,
requiresignals an error, ‘Required feature feature was not provided’.
This function returns
tif feature has been provided in the current Emacs session (i.e., if feature is a member offeatures.) If subfeature is non-nil, then the function returnstonly if that subfeature is provided as well (i.e. if subfeature is a member of thesubfeatureproperty of the feature symbol.)
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 thefeatureslist is not significant.
Next: Unloading, Previous: Named Features, Up: Loading
15.8 Which File Defined a Certain Symbol
This function returns the name of the file that defined symbol. If type is
nil, then any kind of definition is acceptable. If type isdefun,defvar, ordefface, 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.
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-historymay have one element whose car isnil. This element describes definitions made witheval-bufferon 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. See Eval.
Next: Hooks for Loading, Previous: Where Defined, Up: Loading
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:
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, anddefcustom. It then restores any autoloads formerly associated with those symbols. (Loading saves these in theautoloadproperty of the symbol.)Before restoring the previous definitions,
unload-featurerunsremove-hookto remove functions in the library from certain hooks. These hooks include variables whose names end in ‘hook’ or ‘-hooks’, plus those listed inunload-feature-special-hooks, as well asauto-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-featurecalls it with no arguments before doing anything else. It can do whatever is appropriate to unload the library. If it returnsnil,unload-featureproceeds to take the normal unload actions. Otherwise it considers the job to be done.Ordinarily,
unload-featurerefuses to unload a library on which other loaded libraries depend. (A library a depends on library b if a contains arequirefor 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.
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:
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:
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/barwill 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 when
(providelibrary)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 (see Named Features).
But it is OK to use eval-after-load in your personal
customizations if you don't feel that they must meet the design
standards for programs meant for wider use.
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.
Next: Advising Functions, Previous: Loading, Up: Top
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.
Compiling a Lisp file with the Emacs byte compiler always reads the file as multibyte text, even if Emacs was started with ‘--unibyte’, unless the file specifies otherwise. This is so that compilation gives results compatible with running the same file without compilation. See Loading Non-ASCII.
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; -*-
See Compilation Errors, for how to investigate errors occurring in byte compilation.
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 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
(silly-loop 50000000)
⇒ ("Wed Mar 11 21:10:19 2009"
"Wed Mar 11 21:10:41 2009") ; 22 seconds
(byte-compile 'silly-loop)
⇒ [Compiled code not shown]
(silly-loop 50000000)
⇒ ("Wed Mar 11 21:12:26 2009"
"Wed Mar 11 21:12:32 2009") ; 6 seconds
In this example, the interpreted code required 22 seconds to run, whereas the byte-compiled code required 6 seconds. These results are representative, but actual results will vary greatly.
Next: Docs and Compilation, Previous: Speed of Byte-Code, Up: Byte Compilation
16.2 The 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.
The byte compiler produces error messages and warnings about each file in a buffer called ‘*Compile-Log*’. These report things in your program that suggest a problem but are not necessarily erroneous.
Be careful when writing macro calls in files that you may someday
byte-compile. Macro calls are expanded when they are compiled, so the
macros must already be defined for proper compilation. For more
details, see Compiling Macros. If a program does not work the
same way when compiled as it does when interpreted, erroneous macro
definitions are one likely cause (see Problems with 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.
Normally, compiling a file does not evaluate the file's contents or
load the file. But it does execute any require calls at top
level in the file. One way to ensure that necessary macro definitions
are available during compilation is to require the file that defines
them (see Named Features). To avoid loading the macro definition files
when someone runs the compiled program, write
eval-when-compile around the require calls (see Eval During Compile).
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; i.e., the compiler does not follow indirection to another symbol.
byte-compilereturns the new, compiled definition of symbol.If symbol's definition is a byte-code function object,
byte-compiledoes nothing and returnsnil. Lisp records only one function definition for any symbol, and if that is already compiled, non-compiled code is not available anywhere. So there is no way to “compile the same definition again.”(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."]The result is a byte-code function object. The string it contains is the actual byte-code; each character in it is an instruction or an operand of an instruction. The vector contains all the constants, variable names and function names used by the function, except for certain primitives that are coded as special instructions.
If the argument to
byte-compileis alambdaexpression, it returns the corresponding compiled code, but does not store it anywhere.
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-defunnormally 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.
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
tif there were no errors andnilotherwise. 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
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 neithernilnor 0, it asks the user whether to compile each such file, and asks about each subdirectory as well.Interactively,
byte-recompile-directoryprompts 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.
This function runs
byte-compile-fileon 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
This function actually interprets byte-code. A byte-compiled function is actually defined with a body that calls
byte-code. Don't call this function yourself—only the byte compiler knows how to generate valid calls to this function.In Emacs version 18, byte-code was always executed by way of a call to the function
byte-code. Nowadays, byte-code is usually executed as part of a byte-code function object, and only rarely through an explicit call tobyte-code.
Next: Dynamic Loading, Previous: Compilation Functions, Up: Byte Compilation
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.
If your site installs Emacs following the usual procedures, these problems will never normally occur. Installing a new version uses a new directory with a different name; as long as the old version remains installed, its files will remain unmodified in the places where they are expected to be.
However, if you have built Emacs yourself and use it from the directory where you built it, you will experience this problem occasionally if you edit and recompile Lisp files. When it happens, you can cure the problem by reloading the file after recompiling it.
You can turn off this 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;-*-
If this is non-
nil, the byte compiler generates compiled files that are set up for dynamic loading of documentation strings.
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.
Next: Eval During Compile, Previous: Docs and Compilation, Up: Byte Compilation
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;-*-
If this is non-
nil, the byte compiler generates compiled files that are set up for dynamic function loading.
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.
Next: Compiler Errors, Previous: Dynamic Loading, Up: Byte Compilation
16.5 Evaluation During Compilation
These features permit you to write code to be evaluated during compilation of a program.
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. Effectivelyrequireis automaticallyeval-and-compile, the package is loaded both when compiling and executing.
autoloadis also effectivelyeval-and-compiletoo. 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-compileare 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-compileshould be used to get the helper both when compiling and then later when running.If functions are defined programmatically (with
fsetsay), theneval-and-compilecan 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).
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-compilecan 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-compilecan be used to load it for compiling, but not executing. For example,(eval-when-compile (require 'my-macro-package)) ;; only macros needed from thisThe same sort of thing goes for macros and
defsubstfunctions 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-compileis analogous to the Common Lisp idiom(eval-when (compile eval) ...). Elsewhere, the Common Lisp ‘#.’ reader macro (but not when interpreting) is closer to whateval-when-compiledoes.
Next: Byte-Code Objects, Previous: Eval During Compile, Up: Byte Compilation
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.
However, the warnings about functions that were used but not defined are always “located” at the end of the file, so these commands won't find the places they are really used. To do that, 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 (see 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:
In execution, this is equivalent to
(prognbody...), 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 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.
Internally, a byte-code function object is much like a vector; however, the evaluator handles this data type specially when it appears as a function to be called. The printed representation for a byte-code function object is like that for a vector, with an additional ‘#’ before the opening ‘[’.
A byte-code function object 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 functiondocumentationto get the real documentation string (see Accessing Documentation). - interactive
- The interactive spec (if any). This can be a string or a Lisp
expression. It is
nilfor 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:
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).
You can access the elements of a byte-code object using aref;
you can also use vconcat to create a vector with the same
elements.
Previous: Byte-Code Objects, Up: Byte Compilation
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.
This command displays the disassembled code for object. In interactive use, or if buffer-or-name is
nilor 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,
disassemblecompiles 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
; the first (i.e., the top) element
; of the stack as the 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 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,
; pushing 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 onto the stack.
4 sub1 ; Subtract 1 from top of stack.
5 dup ; Duplicate the top of the stack;
; i.e., copy the top of
; the stack and push the
; copy onto the 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 top of 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 list onto stack.
17 return ; Return value of the top of stack.
Next: Debugging, Previous: Byte Compilation, Up: Top
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, separately defined. Each defined piece of advice can be enabled or disabled explicitly. 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.
Usage Note: Advice is useful for altering the behavior of existing calls to an existing function. If you want the new behavior for new calls, or for key bindings, you should define a new function (or a new command) which uses the existing function.
Usage note: 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 (or ask someone to do so) 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, this means that a file in Emacs should not put advice on a function in Emacs. There are currently a few exceptions to this convention, but we aim to correct them.
Next: Defining Advice, Up: Advising Functions
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.
Next: Around-Advice, Previous: Simple Advice, Up: Advising Functions
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.
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 (see 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.
See 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-protectform, so that it will execute even if the previous code gets an error or usesthrow. See Cleanups. compile- Compile the combined definition that is used to run the advice. This
flag is ignored unless
activateis also specified. See Combined Definition. disable- Initially disable this piece of advice, so that it will not be used
unless subsequently explicitly enabled. See Enabling Advice.
preactivate- Activate advice for function when this
defadviceis compiled or macroexpanded. This generates a compiled advised definition according to the current advice state, which will be used during activation if appropriate. See Preactivation.This is useful only if this
defadviceis 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.
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.
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.
Next: Activation of Advice, Previous: Around-Advice, Up: Advising Functions
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.
Calling
ad-add-adviceadds 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 (see Defining Advice), and if enabled isnilthe advice is initially disabled (see 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. See Lambda Expressions.
If the function argument to
ad-add-advicealready 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 befirst,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.
Next: Enabling Advice, Previous: Computed Advice, Up: Advising Functions
17.5 Activation of Advice
By default, advice does not take effect when you define it—only when
you activate advice for the function that was advised. 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. But normally you activate the advice
for a function 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 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.
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.
This command activates the advice for function if its advice is already activated. This is useful if you change the advice.
This command activates the advice for all functions whose advice is already activated. This is useful if you change the advice of some functions.
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.
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.
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; see Enabling Advice) since the last time it was activated.
Turn on automatic advice activation when a function is defined or redefined. This is the default mode.
Turn off automatic advice activation when a function is defined or redefined.
This variable controls whether to compile the combined definition that results from activating advice for a function.
A value of
alwaysspecifies to compile unconditionally. A value ofneverspecifies never compile the advice.A value of
maybespecifies to compile if the byte compiler is already loaded. A value oflike-originalspecifies 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”
(see 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.
Next: Preactivation, Previous: Activation of Advice, Up: Advising Functions
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)
This command disables the piece of advice named name in class class on function.
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.
This command disables all pieces of advice whose names match regexp, in all classes, on all functions.
This command enables all pieces of advice whose names match regexp, in all classes, on all functions.
Next: Argument Access in Advice, Previous: Enabling Advice, Up: Advising 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-compileis included in the value offeatureseven 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.
Next: Advising Primitives, Previous: Preactivation, Up: Advising Functions
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.
This returns the list of actual arguments supplied starting at position.
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.
Next: Combined Definition, Previous: Argument Access in Advice, Up: Advising Functions
17.9 Advising Primitives
Advising a primitive function (see What Is a Function) is risky. Some primitive functions are used by the advice mechanism; advising them could cause an infinite recursion. Also, many primitive functions 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.
When the advice facility constructs the combined definition, it needs
to know the argument list of the original function. This is not
always possible for primitive functions. When advice cannot determine
the argument list, it uses (&rest ad-subr-args), which always
works but is inefficient because it constructs a list of the argument
values. You can use ad-define-subr-args to declare the proper
argument names for a primitive function:
This function specifies that arglist should be used as the argument list for function function.
For example,
(ad-define-subr-args 'fset '(sym newdef))
specifies the argument list for the function fset.
Previous: Advising Primitives, Up: Advising Functions
17.10 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.
Next: Read and Print, Previous: Advising Functions, Up: Top
18 Debugging Lisp Programs
There are three ways to investigate a problem in an Emacs Lisp program, depending on what you are doing with the program when the problem appears.
- If the problem occurs when you run the program, you can use a Lisp debugger to investigate what is happening during execution. In addition to the ordinary debugger, Emacs comes with a source-level debugger, Edebug. This chapter describes both of them.
- If the problem is syntactic, so that Lisp cannot even read the program, you can use the Emacs facilities for editing Lisp to localize it.
- If the problem occurs when trying to compile the program with the byte compiler, you need to know how to examine the compiler's input buffer.
Another useful debugging tool is the dribble file. When a dribble file is open, Emacs copies all keyboard input characters to that file. Afterward, you can examine the file to find out what input was used. See Terminal Input.
For debugging problems in terminal descriptions, the
open-termscript function can be useful. See 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. See Recursive Editing.
Next: Infinite Loops, Up: Debugger
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 frequently cause 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.)
This variable determines whether the debugger is called when an error is signaled and not handled. If
debug-on-errorist, all kinds of errors call the debugger, except those listed indebug-ignored-errors(see below). If it isnil, none call the debugger. (Note thateval-expression-debug-on-erroraffects the setting of this variable in some cases; see below.)The value can also be a list of error conditions that should call the debugger. For example, if you set it to the list
(void-variable), then only errors about a variable that has no value invoke the debugger.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. See Processes.
This variable specifies certain kinds of errors that should not enter the debugger. 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, regardless of the value of
debug-on-error.The normal value of this variable lists 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 will need to change this list in order to debug the error. The easiest way is usually to set
debug-ignored-errorstonil.
If this variable has a non-
nilvalue, thendebug-on-erroris set totwhen evaluating with the commandeval-expression. Ifeval-expression-debug-on-errorisnil, then the value ofdebug-on-erroris not changed. See Evaluating Emacs-Lisp Expressions.
Normally, errors that are caught by
condition-casenever run the debugger, even ifdebug-on-erroris non-nil. In other words,condition-casegets a chance to handle the error before the debugger gets a chance.If you set
debug-on-signalto a non-nilvalue, then the debugger gets the first chance at every error; an error will invoke the debugger regardless of anycondition-case, if it fits the criteria specified by the values ofdebug-on-erroranddebug-ignored-errors.Warning: This variable is strong medicine! Various parts of Emacs handle errors in the normal course of affairs, and you may not even realize that errors happen there. If you set
debug-on-signalto a non-nilvalue, those errors will enter the debugger.Warning:
debug-on-signalhas no effect whendebug-on-errorisnil.
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.
Next: Function Debugging, Previous: Error Debugging, Up: Debugger
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.
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. 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.
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 will probably get enough information to solve the problem.
This variable determines whether the debugger is called when
quitis signaled and not handled. Ifdebug-on-quitis non-nil, then the debugger is called whenever you quit (that is, type C-g). Ifdebug-on-quitisnil, then the debugger is not called when you quit. See Quitting.
Next: Explicit Debug, Previous: Infinite Loops, Up: Debugger
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.
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-entryis 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-entrydoes nothing.debug-on-entryalways returns function-name.Warning: if you redefine a function after using
debug-on-entryon 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)))))
This function undoes the effect of
debug-on-entryon function-name. When called interactively, it prompts for function-name in the minibuffer. If function-name is omitted ornil, it cancels break-on-entry for all functions. Callingcancel-debug-on-entrydoes nothing to a function which is not currently set up to break on entry.
Next: Using Debugger, Previous: Function Debugging, Up: Debugger
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 (see Sequencing).
Next: Debugger Commands, Previous: Explicit Debug, Up: Debugger
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 (see 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 kills the backtrace buffer.
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 Mouse-2 on that name, or move to it and type <RET>, 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.
Next: Invoking the Debugger, Previous: Using Debugger, Up: Debugger
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. When continuing is possible,
it 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).
Continuing is possible after entry to the debugger due to function entry or exit, explicit invocation, or quitting. You cannot continue if the debugger was entered because of an error.
- 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
debugand 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 ofdebug-on-entry, it may erroneously show up in this list.
Next: Internals of Debugger, Previous: Debugger Commands, Up: Debugger
18.1.7 Invoking the Debugger
Here we describe in full detail the function debug that is used
to invoke the debugger.
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
debugswitches back to the previous buffer and returns to whatever calleddebug. This is the only way the functiondebugcan return to its caller.The use of the debugger-args is that
debugdisplays 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
debughave a special significance. (Normally, these values are used only by the internals of Emacs, and not by programmers callingdebug.) Here is a table of these special values:
lambda- A first argument of
lambdameansdebugwas called because of entry to a function whendebug-on-next-callwas non-nil. The debugger displays ‘Debugger entered--entering a function:’ as a line of text at the top of the buffer.debugdebugas first argument meansdebugwas 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 thelambdacase. 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 todebugdue to evaluation of a function call form whendebug-on-next-callis 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 todebugin 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 orquitwas signaled and not handled, by displaying ‘Debugger entered--Lisp error:’ followed by the error signaled and any arguments tosignal. 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-erroris non-nil. Ifquitwas signaled, then presumably the variabledebug-on-quitis non-nil.nil- Use
nilas 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 whichdebugis called.
Previous: Invoking the Debugger, Up: Debugger
18.1.8 Internals of the Debugger
This section describes functions and variables used internally by the 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(see Invoking the Debugger).
This function prints a trace of Lisp function calls currently active. This is the function used by
debugto 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 alwaysnil.In the following example, a Lisp expression calls
backtraceexplicitly. This prints the backtrace to the streamstandard-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 ------------
If this variable is non-
nil, it says to call the debugger before the nexteval,applyorfuncall. Entering the debugger setsdebug-on-next-calltonil.The d command in the debugger works by setting this variable.
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.
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.
The function
backtrace-frameis 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
(nilfunction arg-forms...).If that frame has evaluated its arguments and called its function already, the return value is
(tfunction arg-values...).In the return value, function is whatever was supplied as the car of the evaluated list, or a
lambdaexpression in the case of a macro call. If the function has a&restargument, that is represented as the tail of the list arg-values.If frame-number is out of range,
backtrace-framereturnsnil.
Next: Syntax Errors, Previous: Debugger, Up: Debugging
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.
Next: Instrumenting, Up: Edebug
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
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. See 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 <SPC> to execute until the next stop point.
If you type <SPC> 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.
Next: Edebug Execution Modes, Previous: Using Edebug, Up: Edebug
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.
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
(see 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, see 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.
See Edebug Eval, for other evaluation functions available inside of Edebug.
Next: Jumping, Previous: Instrumenting, Up: 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). - <SPC>
- 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 seeedebug-forward-sexpin 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). See 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 Edebug Options.
When you enter a new Edebug level, the initial execution mode comes
from the value of the variable edebug-initial-mode
(see 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.
This option specifies how many seconds to wait between execution steps in trace mode or continue mode. The default is 1 second.
Next: Edebug Misc, Previous: Edebug Execution Modes, Up: Edebug
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. See 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.
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 withunwind-protectorcondition-caseforms 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.
Next: Trapping Errors, Previous: Edebug Misc, Up: Edebug
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.
Next: Global Break Condition, Up: Breaks
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 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 <RET>
- Set a conditional breakpoint which stops the program only if
evaluating condition produces a non-
nilvalue (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.
Next: Source Breakpoints, Previous: Breakpoints, Up: Breaks
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.
Previous: Global Break Condition, Up: Breaks
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.
Next: Edebug Views, Previous: Breaks, Up: Edebug
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 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 (see 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.
Next: Edebug Eval, Previous: Trapping Errors, Up: 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,
Wonly 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.
Next: Eval List, Previous: Edebug Views, Up: Edebug
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. See The Outside Context, for details on this process.
- e exp <RET>
- 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 <RET>
- 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.
Next: Printing in Edebug, Previous: Edebug Eval, Up: Edebug
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 (see 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)
#<buffer *scratch*>
;---------------------------------------------------------------
(selected-window)
#<window 16 on *scratch*>
;---------------------------------------------------------------
(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.
Next: Trace Buffer, Previous: Eval List, Up: Edebug
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). See Output Variables.
If non-
nil, Edebug bindsprint-lengthto this value while printing results. The default value is50.
If non-
nil, Edebug bindsprint-levelto this value while printing results. The default value is50.
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.
If non-
nil, Edebug bindsprint-circleto this value while printing results. The default value ist.
Other programs can also use custom printing; see cust-print.el for details.
Next: Coverage Testing, Previous: Printing in Edebug, Up: Edebug
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 (see 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.
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-tracingreturns the value of the last form in body.
This function inserts text in the trace buffer. It computes the text with
(apply 'formatformat-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.
Next: The Outside Context, Previous: Trace Buffer, Up: Edebug
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.
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
undocommand. 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.
Next: Edebug and Macros, Previous: Coverage Testing, Up: Edebug
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.
Next: Edebug Display Update, Up: The Outside Context
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-depthandmax-specpdl-sizeare 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-macrois bound tonilunlessedebug-continue-kbd-macrois non-nil.
Next: Edebug Recursive Edit, Previous: Checking Whether to Stop, Up: The Outside Context
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 (see Window Configurations). When you exit Edebug (by continuing the program), 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-windowsis non-nil(see 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-excursionis active. If the value ofedebug-save-windowsis 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-pointsis non-nil. - The variables
overlay-arrow-positionandoverlay-arrow-stringare saved and restored, so you can safely invoke Edebug from the recursive edit elsewhere in the same buffer. cursor-in-echo-areais locally bound tonilso that the cursor shows up in the window.
Previous: Edebug Display Update, Up: The Outside Context
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. See Match Data.
- The variables
last-command,this-command,last-input-event,last-command-event,last-event-frame,last-nonmenu-event, andtrack-mouse. Commands used within 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-outputandstandard-inputare bound tonilby therecursive-edit, but Edebug temporarily restores them during evaluations.- The state of keyboard macro definition is saved and restored. While
Edebug is active,
defining-kbd-macrois bound toedebug-continue-kbd-macro.
Next: Edebug Options, Previous: The Outside Context, Up: Edebug
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.
Next: Specification List, Up: Edebug and Macros
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 (see 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. See Defining Macros, for more explanation of
the declare form.
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.
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.
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.
Next: Backtracking, Previous: Instrumenting Macro Calls, Up: Edebug and Macros
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, see 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 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 place to store a value, as in the Common Lisp
setfconstruct. body- Short for
&rest form. See&restbelow. 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
quoterather thanfunction, 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
[&optionalspecs...]. To specify that several elements must all match or none, use&optional [specs...]. See thedefunexample. &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
[&restspecs...]. 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
&orspecification fails.Each list element following
&oris 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
&definekeyword 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
Backtracking, for more details. Also see the
letexample. - 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-specjust as for macros. See thedefunexample.Otherwise, the symbol should be a predicate. The predicate is called with the argument and the specification fails if the predicate returns
nil, and the argument is not instrumented.Some suitable predicates include
symbolp,integerp,stringp,vectorp, andatom. [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. (vectorelements...)- 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 anilspecification 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
:nameshould 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. Usedef-bodyfor 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-formalso means that tracing information is not output when the form is executed. See theinteractiveexample.
Next: Specification Examples, Previous: Specification List, Up: Edebug and Macros
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.
Previous: Backtracking, Up: Edebug and Macros
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))
Previous: Edebug and Macros, Up: Edebug
18.2.16 Edebug Options
These options affect the behavior of Edebug:
Functions to call before Edebug is used. Each time it is set to a new value, Edebug will call those functions once and then
edebug-setup-hookis reset tonil. You could use this to load up Edebug specifications associated with a package you are using but only when you also use Edebug. See Instrumenting.
If this is non-
nil, normal evaluation of defining forms such asdefunanddefmacroinstruments them for Edebug. This applies toeval-defun,eval-region,eval-buffer, andeval-current-buffer.Use the command M-x edebug-all-defs to toggle the value of this option. See Instrumenting.
If this is non-
nil, the commandseval-defun,eval-region,eval-buffer, andeval-current-bufferinstrument 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. See Instrumenting.
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 tonil.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. See Edebug Display Update.
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. See Edebug Display Update.
If this variable is non-
nil, it specifies the initial execution mode for Edebug when it is first activated. Possible values arestep,next,go,Go-nonstop,trace,Trace-fast,continue, andContinue-fast.The default value is
step. See Edebug Execution Modes.
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 Trace Buffer.
If non-
nil, Edebug tests coverage of all expressions debugged. See Coverage Testing.
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. See Edebug Execution Modes.
Edebug binds
debug-on-errorto this value, ifdebug-on-errorwas previouslynil. See Trapping Errors.
Edebug binds
debug-on-quitto this value, ifdebug-on-quitwas previouslynil. See 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.
If non-
nil, an expression to test for at every stop point. If the result is non-nil, then break. Errors are ignored. See Global Break Condition.
Next: Test Coverage, Previous: Edebug, Up: Debugging
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.)
Next: Excess Close, Up: Syntax Errors
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.
Previous: Excess Open, Up: Syntax Errors
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.
Next: Compilation Errors, Previous: Syntax Errors, Up: Debugging
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 <RET> file <RET> 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.
Evaluate form and return its value, but inform coverage testing that form's value should always be the same.
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 (see Coverage Testing). These features partly duplicate each other, and it would be cleaner to combine them.
Previous: Test Coverage, Up: Debugging
18.5 Debugging Problems in Compilation
When an error happens during byte compilation, it is normally due to invalid syntax in the program you are compiling. The compiler prints a suitable error message in the ‘*Compile-Log*’ buffer, and then stops. The message may state a function name in which the error was found, or it may not. Either way, here is how to find out where in the file the error occurred.
What you should do is switch to the buffer ‘ *Compiler Input*’. (Note that the 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.
If the error was due to invalid Lisp syntax, point shows exactly where the invalid syntax was detected. The cause of the error is not necessarily near by! Use the techniques in the previous section to find the error.
If the error was detected while compiling a form that had been read successfully, then point is located at the end of the form. In this case, this technique can't localize the error precisely, but can still show you which function to check.
Next: Minibuffers, Previous: Debugging, Up: Top
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 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).
Next: Input Streams, Up: Read and Print
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 (see 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.
Next: Input Functions, Previous: Streams Intro, Up: Read and Print
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.
ttused 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.
nilnilsupplied as an input stream means to use the value ofstandard-inputinstead; that value is the default input stream, and must be a non-nilinput 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")))
⇒ #<marker at 1 in foo>
(read m)
⇒ This
m
⇒ #<marker at 5 in foo> ;; 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 <RET>
---------- 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.
This function is used internally as an input stream to read from the input file opened by the function
load. Don't use this function yourself.
Next: Output Streams, Previous: Input Streams, Up: Read and Print
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.
This function reads one textual Lisp expression from stream, returning it as a Lisp object. This is the basic Lisp input function.
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)
This variable holds the default input stream—the stream that
readuses when the stream argument isnil. The default ist, meaning use the minibuffer.
If non-
nil, this variable enables the reading of circular and shared structures. See Circular Objects. Its default value ist.
Next: Output Functions, Previous: Input Functions, Up: Read and Print
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.
nilnilspecified as an output stream means to use the value ofstandard-outputinstead; that value is the default output stream, and must not benil.- 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))
⇒ #<marker at 10 in foo>
(print "More output for foo." m)
⇒ "More output for foo."
---------- Buffer: foo ----------
This is t
"More output for foo."
he -!-output
---------- Buffer: foo ----------
m
⇒ #<marker at 34 in foo>
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 (see 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.
Next: Output Variables, Previous: Output Streams, Up: Read and Print
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. See 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.
The
(progn (print 'The\ cat\ in) (print "the hat") (print " came back")) -| -| The\ cat\ in -| -| "the hat" -| -| " came back" ⇒ " came back"
This function outputs the printed representation of object to stream. It does not print newlines to separate output as
(progn (prin1 'The\ cat\ in) (prin1 "the hat") (prin1 " came back")) -| The\ cat\ in"the hat"" came back" ⇒ " came back"
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\""
This function outputs a newline to stream. The name stands for “terminate print.”
This function outputs character to stream. It returns character.
This function returns a string containing the text that
prin1would have printed for the same argument.(prin1-to-string 'foo) ⇒ "foo" (prin1-to-string (mark-marker)) ⇒ "#<marker at 2773 in strings.texi>"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 Formatting Strings, for other ways to obtain the printed representation of a Lisp object as a string.
This macro executes the body forms with
standard-outputset 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".
Previous: Output Functions, Up: Read and Print
19.6 Variables Affecting 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 ist, meaning display in the echo area.
If this is non-
nil, that means to print quoted forms using abbreviated reader syntax.(quote foo)prints as'foo,(function foo)as#'foo, and backquoted forms print using modern backquote syntax.
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
prin1andprinc. Here is an example usingprin1:(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-newlinesis in effect during the call toprin1, but not during the printing of the result.
If this variable is non-
nil, then unibyte non-ASCII characters in strings are unconditionally printed as backslash sequences by the print functionsprin1andThose 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.
If this variable is non-
nil, then multibyte non-ASCII characters in strings are unconditionally printed as backslash sequences by the print functionsprin1andThose 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.
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 ...)
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-level
These are the values for
print-lengthandprint-levelused byeval-expression, and thus, indirectly, by many interactive evaluation commands (see Evaluating Emacs-Lisp Expressions).
These variables are used for detecting and reporting circular and shared structure:
If non-
nil, this variable enables detection of circular and shared structure in printing. See Circular Objects.
If non-
nil, this variable enables detection of uninterned symbols (see Creating Symbols) in printing. When this is enabled, uninterned symbols print with the prefix ‘#:’, which tells the Lisp reader to produce an uninterned symbol.
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 totwithlet. When you do that, you should also bindprint-number-tabletonil.
This variable holds a vector used internally by printing to implement the
print-circlefeature. You should not use it except to bind it tonilwhen you bindprint-continuous-numbering.
This variable specifies how to print floating point numbers. Its default value 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.
Next: Command Loop, Previous: Read and Print, Up: Top
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 (see The Echo Area), but only while it is in use for reading an argument.
Next: Text from Minibuffer, Up: Minibuffers
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 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 (see 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. You can explicitly resize it temporarily with the window sizing commands; it reverts to its normal size when the minibuffer is exited. You can resize it permanently by using the window sizing commands in the frame's other window, when the minibuffer is not active. 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
(see 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 minibuffers are called a
recursive minibuffer. The first minibuffer is named
‘ *Minibuf-0*’. 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 (See Recursive Mini.)
Like other buffers, a minibuffer uses a local keymap (see 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. See Text from Minibuffer, for the non-completion minibuffer local maps. See Completion Commands, for the minibuffer local maps for completion.
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.
Next: Object from Minibuffer, Previous: Intro to Minibuffers, Up: Minibuffers
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 (see Regular Expressions),
which are a special kind of string. There are also specialized
functions for reading commands, variables, file names, etc.
(see 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. See Defining Commands.
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 usesreadto convert the text into a Lisp object (see Input Functions).The first thing this function does is to activate a minibuffer and display it with prompt-string as the prompt. This value must be a string. Then the user can edit text in the minibuffer.
When the user types a command to exit the minibuffer,
read-from-minibufferconstructs 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-minibufferreads the text and returns the resulting Lisp object, unevaluated. (See 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 toread, if the user enters empty input. If default is a list of strings, the first string is used as the input. If default isnil, empty input results in anend-of-fileerror. However, in the usual case (where read isnil),read-from-minibufferignores 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 ornil, the value ofminibuffer-local-mapis used as the keymap. Specifying a keymap is the most important way to customize the minibuffer for various applications such as completion.The argument hist specifies which history list variable to use for saving the input and for history commands used in the minibuffer. It defaults to
minibuffer-history. See Minibuffer History.If the variable
minibuffer-allow-text-propertiesis non-nil, then the string which 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 (see Input Methods) and the setting ofenable-multibyte-characters(see Text Representations) from whichever buffer was current before entering the minibuffer.Use of initial-contents is mostly deprecated; we recommend using a non-
nilvalue only in conjunction with specifying a cons cell for hist. See Initial Input.
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 isminibuffer-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 inread-from-minibufferit should be a string, a list of strings, ornilwhich 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-minibufferfunction:(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))
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 keymap used isminibuffer-local-map, andregexp-historyis used as the history list (see regexp-history).The optional argument default-value specifies a default value to return if the user enters null input; it should be a string, or
nilwhich is equivalent to an empty string.In addition,
read-regexpcollects a few useful candidates for input and passes them toread-from-minibuffer, to make them available to the user as the “future minibuffer history list” (see future list). 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-minibufferfunction, after computing the list of defaults as described above.
If this variable is
nil, thenread-from-minibufferstrips all text properties from the minibuffer input before returning it. This variable also affectsread-string. However,read-no-blanks-input(see below), as well asread-minibufferand related functions (see Reading Lisp Objects With the Minibuffer), and all functions that do minibuffer input with completion, discard text properties unconditionally, regardless of the value of this variable.
This is the default local keymap for reading from the minibuffer. By default, it makes the following bindings:
- C-j
exit-minibuffer- <RET>
exit-minibuffer- C-g
abort-recursive-edit- M-n
- <DOWN>
next-history-element- M-p
- <UP>
previous-history-element- M-s
next-matching-history-element- M-r
previous-matching-history-element
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-minibufferfunction, and passes the value of theminibuffer-local-ns-mapkeymap as the keymap argument for that function. Since the keymapminibuffer-local-ns-mapdoes 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))
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 ofminibuffer-local-map:
Next: Minibuffer History, Previous: Text from Minibuffer, Up: Minibuffers
20.3 Reading Lisp Objects with the Minibuffer
This section describes functions for reading Lisp objects with the minibuffer.
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-minibufferfunction:(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 <RET> immediately to use the initial input as a default, or can edit the input.
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))
This function reads a Lisp expression in the minibuffer, and then evaluates it. The difference between this command and
eval-minibufferis 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 withprin1, so if it is a string, double-quote characters (‘"’) appear in the initial text. See Output Functions.The first thing
edit-and-eval-commanddoes is to activate the minibuffer with prompt as the prompt. Then it inserts the printed representation of form in the minibuffer, and lets the user edit it. When the user exits the minibuffer, the edited text is read withreadand then evaluated. The resulting value becomes the value ofedit-and-eval-command.In the following example, we offer the user an expression with initial text which is a valid form already:
(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 <RET> right away would exit the minibuffer and evaluate the expression, thus moving point forward one word.
edit-and-eval-commandreturnsnilin this example.
Next: Initial Input, Previous: Object from Minibuffer, Up: Minibuffers
20.4 Minibuffer History
A minibuffer history list records previous minibuffer inputs so the user can reuse them conveniently. A history list is actually a symbol, not a list; it is a variable whose value is a list of strings (previous inputs), most recent first.
There are many separate 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 the history list with the optional hist argument
to either 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-elementwill 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(eltvariable(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 (see Initial Input).
If you don't specify hist, 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.
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) orhistory-length(described below). The possible values of maxelt have the same meaning as the values ofhistory-length.Normally,
add-to-historyremoves duplicate members from the history list ifhistory-delete-duplicatesis 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.
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 usingadd-to-history. By default,history-add-new-inputis set to a non-nilvalue.
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). The value ofhistory-lengthproperty of the history list variable's symbol, if set, overrides this variable for that particular history list.
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:
A history list for arguments to
query-replace(and similar arguments to other commands).
A history list for arguments that are names of extended commands.
A history list for arguments that are Lisp expressions to evaluate.
Next: Completion, Previous: Minibuffer History, Up: Minibuffers
20.5 Initial Input
Several of the functions for minibuffer input have an argument called initial or initial-contents. 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 <RET> 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 hist or history argument. See 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 in user code.
Next: Yes-or-No Queries, Previous: Initial Input, Up: Minibuffers
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 <TAB> (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.
Next: Minibuffer Completion, Up: 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.
This function returns the longest common substring of all possible completions of string in collection. The value of collection must be a list of strings or symbols, an alist, an obarray, a hash table, or a completion function (see Programmed Completion).
Completion compares string against each of the permissible completions specified by collection. If no permissible completions match,
try-completionreturnsnil. If there is just one matching completion, and the match is exact, it returnst. Otherwise, it returns the longest initial sequence common to all possible matching completions.If collection is an alist (see Association Lists), the permissible completions are the elements of the alist that are either strings, symbols, or conses whose car is a string or symbol. Symbols are converted to strings using
symbol-name. Other elements of the alist are ignored. (Remember that in Emacs Lisp, the elements of alists do not have to be conses.) In particular, a list of strings or symbols is allowed, even though we usually do not think of such lists as alists.If collection is an obarray (see Creating Symbols), the names of all symbols in the obarray form the set of permissible completions. The global variable
obarrayholds an obarray containing the names of all interned Lisp symbols.Note that the only valid way to make a new obarray is to create it empty and then add symbols to it one by one using
intern. Also, you cannot intern a given symbol in more than one obarray.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 symbol that is a function as collection. Then the function is solely responsible for performing completion;
try-completionreturns whatever this function returns. The function is called with three arguments: string, predicate andnil(the reason for the third argument is so that the same function can be used inall-completionsand do the appropriate thing in either case). See 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 handlecompletion-regexp-listitself.)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 value is
t.(try-completion "foo" '(("foobar1" 1) ("barfoo" 2) ("foobaz" 3) ("foobar2" 4))) ⇒ "fooba" (try-completion "foo" '(("barfoo" 2) ("foo" 3))) ⇒ tIn 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"
This function returns a list of all possible completions of string. The arguments to this function (aside from nospace) are the same as those of
try-completion. Also, this function usescompletion-regexp-listin the same way thattry-completiondoes.The optional argument nospace is obsolete. If it is non-
nil, completions that start with a space are ignored unless string starts with a space.If collection is a function, it is called with three arguments: string, predicate and
t; thenall-completionsreturns whatever the function returns. See Programmed Completion.Here is an example, using the function
testshown in the example fortry-completion:(defun test (s) (> (length (car s)) 6)) ⇒ test (all-completions "foo" '(("foobar1" 1) ("barfoo" 2) ("foobaz" 3) ("foobar2" 4)) 'test) ⇒ ("foobar1" "foobar2")
This function returns non-
nilif string is a valid completion possibility specified by collection and predicate. The arguments are the same as intry-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-listin the same way thattry-completiondoes.If predicate is non-
niland if collection contains several strings that are equal to each other, as determined bycompare-stringsaccording tocompletion-ignore-case, then predicate should accept either all or none of them. Otherwise, the return value oftest-completionis essentially unpredictable.If collection is a function, it is called with three arguments, the values string, predicate and
lambda; whatever it returns,test-completionreturns in turn.
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 . (lengthsuffix)). 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. Alsoall-completionson"/usr/sh"will not include"/usr/share/"but only"share/". So if string is"/usr/sh"and suffix is"e/doc",completion-boundarieswill 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” with a non-nil
risky-local-variable property. See File Local Variables.
If the value of this variable is non-
nil, Emacs does not consider case significant in completion. Note, however, that this variable is overridden byread-file-name-completion-ignore-casewithinread-file-name(see Reading File Names), and byread-buffer-completion-ignore-casewithinread-buffer(see High-Level Completion).
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(see Searching and Case) bound to the value ofcompletion-ignore-case.
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 of use:
(defvar foo (lazy-completion-table foo make-my-alist))
The function completion-in-region provides a convenient way to
perform completion on an arbitrary stretch of text in an Emacs buffer:
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(see Basic Completion).This function inserts the completion text directly into the current buffer. Unlike
completing-read(see Minibuffer Completion), it does not activate the minibuffer.For this function to work, point must be somewhere between start and end.
Next: Completion Commands, Previous: Basic Completion, Up: Completion
20.6.2 Completion and the Minibuffer
This section describes the basic interface for reading from the minibuffer with completion.
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 collection and predicate to the function
try-completion(see Basic Completion). This happens in certain commands bound in the local keymaps used for completion. Some of these commands also calltest-completion. Thus, if predicate is non-nil, it should be compatible with collection andcompletion-ignore-case. See 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 inminibuffer-confirm-exit-commands) and the resulting input is not an element of collection. See 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-readreturns the first element of default, if it is a list;"", if default isnil; or default. The string or strings in default are also available to the user through the history commands.The function
completing-readusesminibuffer-local-completion-mapas the keymap if require-match isnil, and usesminibuffer-local-must-match-mapif require-match is non-nil. See Completion Commands.The argument hist specifies which history list variable to use for saving the input and for minibuffer history commands. It defaults to
minibuffer-history. See Minibuffer History.The argument initial is mostly deprecated; we recommend using a non-
nilvalue only in conjunction with specifying a cons cell for hist. See 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 (see Input Methods) and the setting ofenable-multibyte-characters(see Text Representations) from whichever buffer was current before entering the minibuffer.If the built-in variable
completion-ignore-caseis non-nil, completion ignores case when comparing the input against the possible matches. See 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 <DEL> <DEL> b <RET>,
completing-readreturnsbarfoo.The
completing-readfunction binds variables to pass information to the commands that actually do completion. They are described in the following section.
Next: High-Level Completion, Previous: Minibuffer Completion, Up: Completion
20.6.3 Minibuffer Commands that Do Completion
This section describes the keymaps, commands and user options used in the minibuffer to do completion.
The value of this variable is the collection used for completion in the minibuffer. This is the global variable that contains what
completing-readpasses totry-completion. It is used by minibuffer completion commands such asminibuffer-complete-word.
This variable's value is the predicate that
completing-readpasses totry-completion. The variable is also used by the other minibuffer completion functions.
This variable determines whether Emacs asks for confirmation before exiting the minibuffer;
completing-readbinds this variable, and the functionminibuffer-complete-and-exitchecks the value before exiting. If the value isnil, confirmation is not required. If the value isconfirm, the user may exit with an input that is not a valid completion alternative, but Emacs asks for confirmation. If the value isconfirm-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 inminibuffer-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-readisconfirm-after-completion. The confirmation is requested if the user attempts to exit the minibuffer immediately after calling any command in this list.
This function completes the minibuffer contents by at most a single word. Even if the minibuffer contents have only one completion,
minibuffer-complete-worddoes not add any characters beyond the first character that is not a word constituent. See Syntax Tables.
This function completes the minibuffer contents, and exits if confirmation is not required, i.e., if
minibuffer-completion-confirmisnil. 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.
This function creates a list of the possible completions of the current minibuffer contents. It works by calling
all-completionsusing the value of the variableminibuffer-completion-tableas the collection argument, and the value ofminibuffer-completion-predicateas the predicate argument. The list of completions is displayed as text in a buffer named ‘*Completions*’.
This function displays completions to the stream in
standard-output, usually a buffer. (See Read and Print, for more information about streams.) The argument completions is normally a list of completions just returned byall-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-listuses this to highlight text in the completion list for better visual feedback. This is not needed in the minibuffer; for minibuffer completion, you can passnil.This function is called by
minibuffer-completion-help. The most common way to use it is together withwith-output-to-temp-buffer, like this:(with-output-to-temp-buffer "*Completions*" (display-completion-list (all-completions (buffer-string) my-alist) (buffer-string)))
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.
completing-readuses 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- <SPC>
minibuffer-complete-word- <TAB>
minibuffer-completewith other characters bound as in
minibuffer-local-map(see Definition of minibuffer-local-map).
completing-readuses this value as the local keymap when an exact match of one of the completions is required. Therefore, no keys are bound toexit-minibuffer, the command that exits the minibuffer unconditionally. By default, this keymap makes the following bindings:
- ?
minibuffer-completion-help- <SPC>
minibuffer-complete-word- <TAB>
minibuffer-complete- C-j
minibuffer-complete-and-exit- <RET>
minibuffer-complete-and-exitwith other characters bound as in
minibuffer-local-map.
This is like
minibuffer-local-completion-mapexcept that it does not bind <SPC>. This keymap is used by the functionread-file-name.
This is like
minibuffer-local-must-match-mapexcept that it does not bind <SPC>. This keymap is used by the functionread-file-name.
20.6.4 High-Level Completion Functions
This section describes the higher-level convenient 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. See Defining Commands.
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 (see Programming Tips).The optional argument require-match has the same meaning as in
completing-read. See Minibuffer Completion.In the following example, the user enters ‘minibuffer.t’, and then types <RET>. 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 <RET>. ⇒ "minibuffer.texi"
This variable specifies how to read buffer names. The function is called with the arguments passed to
read-buffer. For example, if you set this variable toiswitchb-read-buffer, all Emacs commands that callread-bufferto read a buffer name will actually use theiswitchbpackage to read it.
If this variable is non-
nil,read-bufferignores case when performing completion.
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 whichcommandpreturnst, and a command name is a symbol for whichcommandpreturnst. See 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-commandinterns it before returning it. If it is a list,read-commandreturns the first element of this list. If default isnil, 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 <RET>, then this function returns
forward-char.The
read-commandfunction is a simplified interface tocompleting-read. It uses the variableobarrayso as to complete in the set of extant Lisp symbols, and it uses thecommandppredicate so as to accept only command names:(read-command prompt) == (intern (completing-read prompt obarray 'commandp t nil))
This function reads the name of a user variable and returns it as a symbol.
The argument default specifies the default value 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-variableinterns it to make the default value. If it is a list,read-variableinterns the first element. If default isnil, that means no default has been specified; then if the user enters null input, the return value is(intern "").(read-variable "Variable name? ") ;; After evaluation of the preceding expression, ;; the following prompt appears, ;; with an empty minibuffer: ---------- Buffer: Minibuffer ---------- Variable name? -!- ---------- Buffer: Minibuffer ----------If the user then types fill-p <RET>,
read-variablereturnsfill-prefix.In general,
read-variableis similar toread-command, but uses the predicateuser-variable-pinstead ofcommandp:(read-variable prompt) == (intern (completing-read prompt obarray 'user-variable-p t nil))
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 #R+G+B+):") 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 Color Names.
The function's return value is the color name typed by the user in the minibuffer. However, when called interactively or if the optional argument convert is non-
nil, it converts the name into the color's RGB value and returns that value as a string. If an invalid color name was specified, this function signals an error, except that empty color names are allowed whenallow-emptyis non-niland 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 User-Chosen Coding Systems,
and read-input-method-name, in Input Methods.
Next: Completion Styles, Previous: High-Level Completion, Up: Completion
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.
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 (i) it is invoked via a mouse command, and (ii) the selected frame is on a graphical display supporting such dialogs, and (iii) the variable
use-dialog-boxis non-nil(see Dialog Boxes), and (iv) the directory argument, described below, does not specify a remote file (see Remote Files). The exact behavior when using a graphical file dialog is platform-dependent. Here, we simply document the behavior when using the minibuffer.The optional argument require-match has the same meaning as in
completing-read. See Minibuffer Completion.
read-file-nameusesminibuffer-local-filename-completion-mapas the keymap if require-match isnil, and usesminibuffer-local-filename-must-match-mapif require-match is non-nil. See Completion Commands.The argument directory specifies the directory to use for completion of relative file names. It should be an absolute directory name. If
insert-default-directoryis non-nil, directory is also inserted in the minibuffer as initial input. It defaults to the current buffer's value ofdefault-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. 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 thatread-file-nameinserted initially. The initial minibuffer contents are always non-empty ifinsert-default-directoryis 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. Otherwiseread-file-nameattempts 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-nametries 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 isnil, but initial is non-nil, then the default is the absolute file name obtained from directory and initial. If both default and initial areniland the buffer is visiting a file,read-file-nameuses 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 <RET> without any editing,read-file-namesimply returns the pre-inserted contents of the minibuffer.If the user types <RET> 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 possibilities. A file name is an acceptable value if predicate returns non-nilfor it.
read-file-namedoes not automatically expand file names. You must callexpand-file-nameyourself if an absolute file name is required.Here is an example:
(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 <TAB> results in the following:
---------- Buffer: Minibuffer ---------- The file is /gp/gnu/elisp/manual.texi-!- ---------- Buffer: Minibuffer ----------If the user types <RET>,
read-file-namereturns the file name as the string"/gp/gnu/elisp/manual.texi".
If non-
nil, this should be a function that accepts the same arguments asread-file-name. Whenread-file-nameis called, it calls this function with the supplied arguments instead of doing its usual work.
If this variable is non-
nil,read-file-nameignores case when performing completion.
This function is like
read-file-namebut allows only directory names as completion possibilities.If default is
niland initial is non-nil,read-directory-nameconstructs a substitute default by combining directory (or the current buffer's default directory if directory isnil) and initial. If both default and initial arenil, this function uses directory as substitute default, or the current buffer's default directory if directory isnil.
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. See Code Characters for interactive.) Its value controls whetherread-file-namestarts by placing the name of the default directory in the minibuffer, plus the initial file name if any. If the value of this variable isnil, thenread-file-namedoes 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
niland 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 <RET> 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 ----------
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-mapas the keymap for minibuffer input. The hist argument specifies the history list to use; if is omitted ornil, it defaults toshell-command-history(see shell-command-history). The optional argument initial-contents specifies the initial content of the minibuffer (see Initial Input). The rest of args, if present, are used as the default and inherit-input-method arguments inread-from-minibuffer(see Text from Minibuffer).
This keymap is used by
read-shell-commandfor completing command and file names that are part of a shell command.
Next: Programmed Completion, Previous: Reading File Names, Up: Completion
20.6.6 Completion Styles
A completion style is a set of rules for generating
completions. The user option completion-styles stores a list
of completion styles, which are represented by symbols.
This is a list of completion style symbols to use for performing completion. Each completion style in this list must be defined in
completion-styles-alist.
This variable stores a list of available completion styles. Each element in the list must have the form ‘(name try-completion all-completions)’. Here, name is the name of the completion style (a symbol), which may be used in
completion-styles-alistto refer to this style.try-completion is the function that does the completion, and all-completions is the function that lists the completions. These functions should accept four arguments: string, collection, predicate, and point. The string, collection, and predicate arguments have the same meanings as in
try-completion(see Basic Completion), and the point argument is the position of point within string. Each function should return a non-nilvalue if it performed its job, andnilif 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, such as
minibuffer-complete(see Completion Commands), Emacs looks for the first style listed incompletion-stylesand calls its try-completion function. If this function returnsnil, Emacs moves to the next completion style listed incompletion-stylesand calls its try-completion function, and so on until one of the try-completion functions successfully performs completion and returns a non-nilvalue. A similar procedure is used for listing completions, via the all-completions functions.
By default, completion-styles-alist contains five pre-defined
completion styles: basic, a basic completion style;
partial-completion, which does partial completion (completing
each word in the input separately); emacs22, which performs
completion according to the rules used in Emacs 22; emacs21,
which performs completion according to the rules used in Emacs 21; and
initials, which completes acronyms and initialisms.
Previous: Completion Styles, Up: Completion
20.6.7 Programmed Completion
Sometimes it is not possible to create an alist or an obarray containing all the intended possible completions. 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 (see 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.
- The predicate function to filter possible matches, or
nilif none. Your function should call the predicate for each possible match, and ignore the possible match if the predicate returnsnil. - A flag specifying the type of operation. The best way to think about it is that the function stands for an object (in the “object-oriented” sense of the word), and this third argument specifies which method to run.
There are currently four methods, i.e. four flag values, one for each of the four different basic operations:
nilspecifiestry-completion. The completion function should return the completion of the specified string, ortif the string is a unique and exact match already, ornilif the string matches no possibility.If the string is an exact match for one possibility, but also matches other longer possibilities, the function should return the string, not
t.tspecifiesall-completions. The completion function should return a list of all possible completions of the specified string.lambdaspecifiestest-completion. The completion function should returntif the specified string is an exact match for some possibility;nilotherwise.(boundaries . SUFFIX)specifiescompletion-boundaries. The function should return a value of the form(boundaries START . END)where START is the position of the beginning boundary in in the string to complete, and END is the position of the end boundary in SUFFIX.
It would be consistent and clean for completion functions to allow lambda expressions (lists that are functions) as well as function symbols as collection, but this is impossible. Lists as completion tables already have other meanings, and it would be unreliable to treat one differently just because it is also a possible function. So you must arrange for any function you wish to use for completion to be encapsulated in a symbol.
This function is a convenient way to write a function that can act as 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-dynamicas a transducer between that interface and the interface for programmed completion functions.
The value of this variable, if non-
nil, should be a function for “annotating” the entries in the ‘*Completions*’ buffer. The function should accept a single argument, the completion string for an entry. It should return an additional string to display next to that entry in the ‘*Completions*’ buffer, ornilif no additional string is to be displayed.The function can determine the collection used for the current completion via the variable
minibuffer-completion-table(see Completion Commands).
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
(see 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 of the mouse or use 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.
This function asks the user a question, expecting input in the echo area. It returns
tif the user types y,nilif the user types n. This function also accepts <SPC> to mean yes and <DEL> 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 <RET> 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, <SPC>, <DEL>, 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 (see 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. The keymap
query-replace-mapspecifies them. See Search and Replace.In the following example, the user first types q, which is invalid. At the next prompt the user types y.
(y-or-n-p "Do you need a lift? ") ;; After evaluation of the preceding expression, ;; the following prompt appears in the echo area: ---------- Echo area ---------- Do you need a lift? (y or n) ---------- Echo area ---------- ;; If the user then types q, the following appears: ---------- Echo area ---------- Please answer y or n. Do you need a lift? (y or n) ---------- Echo area ---------- ;; When the user types a valid answer, ;; it is displayed after the question: ---------- Echo area ---------- Do you need a lift? (y or n) y ---------- Echo area ----------We show successive lines of echo area messages, but only one actually appears on the screen at a time.
Like
y-or-n-p, except that if the user fails to answer within seconds seconds, this function stops waiting and returns default-value. It works by setting up a timer; see Timers. The argument seconds may be an integer or a floating point number.
This function asks the user a question, expecting input in the minibuffer. It returns
tif the user enters ‘yes’,nilif the user types ‘no’. The user must type <RET> to finalize the response. Upper and lower case are equivalent.
yes-or-no-pstarts 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-prequires more work from the user thany-or-n-pand 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 <RET>, 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 ----------
Next: Reading a Password, Previous: Yes-or-No Queries, Up: Minibuffers
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.
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
nilmeaning 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
tmeaning do act on this object (and don't ask the user), ornilmeaning 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 <SPC> to act on that object; n, N, or <DEL> to skip that object; ! to act on all following objects; <ESC> 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-replaceaccepts. The keymapquery-replace-mapdefines their meaning formap-y-or-n-pas well as forquery-replace; see 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-pcalls function. If it returns non-nil, the object is considered “acted upon,” andmap-y-or-n-padvances to the next object in list. If it returnsnil, the prompt is repeated for the same object.Normally,
map-y-or-n-pbindscursor-in-echo-areawhile prompting. But if no-cursor-in-echo-area is non-nil, it does not do that.If
map-y-or-n-pis called in a command that was invoked using the mouse—more precisely, iflast-nonmenu-event(see Command Loop Info) is eithernilor 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 of the mouse or use of keyboard input by bindinglast-nonmenu-eventto a suitable value around the call.The return value of
map-y-or-n-pis the number of objects acted on.
Next: Minibuffer Commands, Previous: Multiple Queries, Up: Minibuffers
20.9 Reading a Password
To read a password to pass to another program, you can use the
function read-passwd.
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, thenread-passwdreturns the null string in that case.
Next: Minibuffer Contents, Previous: Reading a Password, Up: Minibuffers
20.10 Minibuffer Commands
This section describes some commands meant for use in the minibuffer.
This command exits the active minibuffer. It is normally bound to keys in minibuffer local keymaps.
This command exits the active minibuffer after inserting the last character typed on the keyboard (found in
last-command-event; see Command Loop Info).
This command replaces the minibuffer contents with the value of the nth previous (older) history element.
This command replaces the minibuffer contents with the value of the nth more recent history element.
This command replaces the minibuffer contents with the value of the nth previous (older) history element that matches pattern (a regular expression).
This command replaces the minibuffer contents with the value of the nth next (newer) history element that matches pattern (a regular expression).
Next: Recursive Mini, Previous: Minibuffer Contents, Up: Minibuffers
20.11 Minibuffer Windows
These functions access and select minibuffer windows and test whether they are active.
This function returns the currently active minibuffer window, or
nilif none is currently active.
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.
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.
This function returns non-
nilif 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.
This function returns non-
nilif window, assumed to be a minibuffer window, is currently active.
Next: Minibuffer Windows, Previous: Minibuffer Commands, Up: Minibuffers
20.12 Minibuffer Contents
These functions access the minibuffer prompt and contents.
This function returns the prompt string of the currently active minibuffer. If no minibuffer is active, it returns
nil.
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.
This function returns the current display-width of the minibuffer prompt, if a minibuffer is current. Otherwise, it returns zero.
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.
This is like
minibuffer-contents, except that it does not copy text properties, just the characters themselves. See Text Properties.
This is like
minibuffer-contents, except that it returns only the contents before point. That is the part that completion commands operate on. See Minibuffer Completion.
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.
Next: Minibuffer Misc, Previous: Minibuffer Windows, Up: Minibuffers
20.13 Recursive Minibuffers
These functions and variables deal with recursive minibuffers (see Recursive Editing):
This function returns the current depth of activations of the minibuffer, a nonnegative integer. If no minibuffers are active, it returns zero.
If this variable is non-
nil, you can invoke commands (such asfind-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 (see Using Interactive).
The minibuffer command next-matching-history-element (normally
M-s in the minibuffer) does the latter.
Previous: Recursive Mini, Up: Minibuffers
20.14 Minibuffer Miscellany
This function returns non-
nilif buffer-or-name is a minibuffer. If buffer-or-name is omitted, it tests the current buffer.
This is a normal hook that is run whenever the minibuffer is entered. See Hooks.
This is a normal hook that is run whenever the minibuffer is exited. See Hooks.
The current value of this variable is used to rebind
help-formlocally inside the minibuffer (see Help Functions).
If the value of this variable is non-
nil, it should be a window object. When the functionscroll-other-windowis called in the minibuffer, it scrolls this window.
This function returns the window which was selected when the minibuffer was entered. If selected window is not a minibuffer window, it returns
nil.
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.
This function displays string temporarily at the end of the minibuffer text, for two seconds, or until the next input event arrives, whichever comes first. If args is non-
nil, the actual message is obtained by passing string and args throughformat. See Formatting Strings.
Next: Keymaps, Previous: Minibuffers, Up: Top
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.
Next: Defining Commands, Up: Command Loop
21.1 Command Loop Overview
The first thing the command loop must do is read a key sequence, which
is a sequence of events that translates into a command. It does this by
calling the function read-key-sequence. Your Lisp code can also
call this function (see Key Sequence Input). Lisp programs can also
do input at a lower level with read-event (see Reading One Event) or discard pending input with discard-input
(see Event Input Misc).
The key sequence is translated into a command through the currently
active keymaps. See 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 (see Interactive Call).
Prior to executing the command, Emacs runs undo-boundary to
create an undo boundary. See Maintaining Undo.
To execute a command, Emacs first reads its arguments by calling
command-execute (see Interactive Call). For commands
written in Lisp, the interactive specification says how to read
the arguments. This may use the prefix argument (see Prefix Command Arguments) or may read with prompting in the minibuffer
(see 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 string or vector (i.e., a keyboard macro) then
execute-kbd-macro is used to execute it. You can call this
function yourself (see Keyboard Macros).
To terminate the execution of a running command, type C-g. This character causes quitting (see Quitting).
The editor command loop runs this normal hook before each command. At that time,
this-commandcontains the command that is about to run, andlast-commanddescribes the previous command. See Command Loop Info.
The editor command loop runs this normal hook after each command (including commands terminated prematurely by quitting or by errors), and also when the command loop is first entered. At that time,
this-commandrefers to the command that just ran, andlast-commandrefers to the command before that.
Quitting is suppressed while running pre-command-hook and
post-command-hook. If an error happens while executing one of
these hooks, it terminates execution of the hook, and clears the hook
variable to nil so as to prevent an infinite loop of errors.
A request coming into the Emacs server (see Emacs Server) runs these two hooks just as a keyboard command does.
Next: Interactive Call, Previous: Command Overview, Up: Command Loop
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.
Next: Interactive Codes, Up: Defining Commands
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.
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
interactiveform must be located at top-level in the function body, or in the function symbol'sinteractive-formproperty (see Symbol Plists). It has its effect because the command loop looks for it before calling the function (see Interactive Call). Once the function is called, all its body forms are executed; at this time, if theinteractiveform occurs within the body, the form simply returnsnilwithout even evaluating its argument.By convention, you should put the
interactiveform in the function body, as the first top-level form. If there is aninteractiveform in both theinteractive-formsymbol property and the function body, the former takes precedence. Theinteractive-formsymbol 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 parameter8. Each element consists of a code character
(see 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 (see 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(see 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 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 (see Minibuffers)
or directly from the keyboard (see 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-historyin 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).
This function returns the
interactiveform of function. If function is an interactively callable function (see Interactive Call), the value is the command'sinteractiveform(interactivespec), which specifies how to compute its arguments. Otherwise, the value isnil. If function is a symbol, its function definition is used.
Next: Interactive Examples, Previous: Using Interactive, Up: Defining Commands
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. <TAB>, <SPC>, and <RET> perform name
completion because the argument is read using
completing-read(see 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 (see 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 (see Point). No I/O.
- ‘D’
- A directory name. The default is the current default directory of the
current buffer,
default-directory(see File Name Expansion). Existing, Completion, Default, Prompt. - ‘e’
- The first or next mouse 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. See Input Events. No I/O.
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 (see 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
nilas the argument's value. No I/O. - ‘k’
- A key sequence (see 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-keyandglobal-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 (see 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. See 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
(see Text from Minibuffer). Terminate the input with either
C-j or <RET>. (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. Any whitespace
character terminates the input. (Use C-q to include whitespace in
the string.) Other characters that normally terminate a symbol (e.g.,
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’ providesnilas the argument. No I/O. - ‘v’
- A variable declared to be a user option (i.e., satisfying the
predicate
user-variable-p). This reads the variable usingread-variable. See Definition of read-variable. Existing, Completion, Prompt. - ‘x’
- A Lisp object, specified with its read syntax, terminated with a
C-j or <RET>. The object is not evaluated. See 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. See 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
nilas the argument value. Completion, Existing, Prompt.
Previous: Interactive Codes, Up: Defining Commands
21.2.3 Examples of Using interactive
Here are some examples of interactive:
(defun foo1 () ;foo1takes no arguments, (interactive) ; just moves forward two words. (forward-word 2)) ⇒ foo1 (defun foo2 (n) ;foo2takes one argument, (interactive "^p") ; which is the numeric prefix. ; undershift-select-mode, ; will activate or extend region. (forward-word (* 2 n))) ⇒ foo2 (defun foo3 (n) ;foo3takes 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
Next: Distinguish Interactive, Previous: Defining Commands, Up: Command Loop
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.
Returns
tif object is suitable for calling interactively; that is, if object is a command. Otherwise, returnsnil.Interactively-callable objects include strings and vectors (which are treated as keyboard macros), lambda expressions that contain a top-level
interactiveform (see Using Interactive), byte-code function objects made from such lambda expressions, autoload objects that are declared as interactive (non-nilfourth argument toautoload), and some primitive functions.A symbol satisfies
commandpif it has a non-nilinteractive-formproperty, or if its function definition satisfiescommandp. Keys and keymaps are not commands. Rather, they are used to look up commands (see Keymaps).If for-call-interactively is non-
nil, thencommandpreturnstonly for objects thatcall-interactivelycould call—thus, not for keyboard macros.See
documentationin Accessing Documentation, for a realistic example of usingcommandp.
This function calls the interactively callable function command, reading arguments according to its interactive calling specifications. It returns whatever command returns. 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-interactivelyuses its function definition.If record-flag is non-
nil, then this command and its arguments are unconditionally added to the listcommand-history. Otherwise, the command is added only if it uses the minibuffer to read an argument. See 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 ofthis-command-keys-vector. See Definition of this-command-keys-vector.
This function executes command. The argument command must satisfy the
commandppredicate; 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 tocall-interactively, along with the optional record-flag and keys.A symbol is handled by using its function definition in its place. A symbol with an
autoloaddefinition 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 (see Special Events).
This function reads a command name from the minibuffer using
completing-read(see Completion). Then it usescommand-executeto call the specified command. Whatever that command returns becomes the value ofexecute-extended-command.If the command asks for a prefix argument, it receives the value prefix-argument. If
execute-extended-commandis called interactively, the current raw prefix argument is used for prefix-argument, and thus passed on to whatever command is run.
execute-extended-commandis 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 invokeexecute-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
Next: Command Loop Info, Previous: Interactive Call, Up: Command Loop
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.
This function returns
twhen the calling function was called usingcall-interactively.The argument kind should be either the symbol
interactiveor the symbolany. If it isinteractive, thencalled-interactively-preturnstonly 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 (see Keyboard Macros). If kind isany,called-interactively-preturnstfor any kind of interactive call, including keyboard macros.If in doubt, use
any; the only known proper use ofinteractiveis 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
applyorfuncall).
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)
Next: Adjusting Point, Previous: Distinguish Interactive, Up: Command Loop
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.
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-commandwhen 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. See Multiple Terminals.
This variable is set up by Emacs just like
last-command, but never altered by Lisp programs.
This variable stores the most recently executed command that was not part of an input event. This is the command
repeatwill try to repeat, See Repeating.
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-commandwhen 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-commandtokill-regionso 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.
This has the same value as
this-commandexcept when command remapping occurs (see Remapping Commands). In that case,this-commandgives the command actually run (the result of remapping), andthis-original-commandgives the command that was specified to run but remapped into another command.
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-eventwithout a timeout get tacked on to the end.However, if the command has called
read-key-sequence, it returns the last read key sequence. See Key Sequence Input. The value is a string if all events in the sequence were characters that fit in a string. See Input Events.(this-command-keys) ;; Now use C-u C-x C-e to evaluate that. ⇒ "^U^X^E"
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 (see Strings of Events).
This function empties out the table of events for
this-command-keysto return. Unless keep-record is non-nil, it also empties the records that the functionrecent-keys(see 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.
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-menuwhere to pop up a menu. It is also used internally byy-or-n-p(see Yes-or-No Queries).
— Variable: last-command-char
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. ⇒ 5The value is 5 because that is the ASCII code for C-e.
The alias
last-command-charis obsolete.
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. See Input Focus.
If the last event came from a keyboard macro, the value is
macro.
Next: Input Events, Previous: Command Loop Info, Up: Command Loop
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
intangible property, 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:
If this variable is non-
nilwhen 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
nilbefore each command, so if a command sets it, the effect applies only to that command.
If you set this variable to a non-
nilvalue, the feature of moving point out of these sequences is completely turned off.
Next: Reading Input, Previous: Adjusting Point, Up: Command Loop
21.7 Input Events
The Emacs command loop reads a sequence of input events that represent keyboard or mouse activity. The events for keyboard activity are characters or symbols; mouse events are always lists. This section describes the representation and meaning of input events in detail.
This function returns non-
nilif object is an input event or event type.Note that any symbol might be used as an event or an event type.
eventpcannot 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,eventpreturnsnil.
Next: Function Keys, Up: Input Events
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 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. (On some terminals, the key labeled <ALT> is actually the meta key.)
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 (see 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 (see Changing Key Bindings). The function
event-convert-list converts such a list into an event type
(see Classifying Events).
Next: Mouse Events, Previous: Keyboard Events, Up: Input 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 <F1> places the symbol
f1 in the input stream.
The event type of a function key event is the event symbol itself. See 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 <TAB> 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(see Translation Keymaps) is set up to maptabinto 9. Thus, a key binding for character code 9 (the character C-i) also applies totab. Likewise for the other symbols in this group. The functionread-charlikewise converts these events into characters.In ASCII, <BS> is really C-h. But
backspaceconverts into the character code 127 (<DEL>), not into code 8 (<BS>). 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 <ALT>, <CTRL>, <HYPER>, <META>, <SHIFT>, and <SUPER> 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 <F3> with <META> 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.
Next: Click Events, Previous: Function Keys, Up: Input Events
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 (see 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. See 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.
Next: Drag Events, Previous: Mouse Events, Up: Input Events
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 ismouse-1. - position
- This is the position where the mouse click occurred. The actual
format of position depends on what part of a window was clicked
on.
For mouse click events in the text area, mode line, header line, or in the marginal areas, position has this form:
(window pos-or-area (x . y) timestamp object text-pos (col . row) image (dx . dy) (width . height))- window
- This is the window in which the click occurred.
- pos-or-area
- This is the buffer position of the character clicked on in the text
area, or if clicked outside the text area, it is the window area in
which the click occurred. It is one of the symbols
mode-line,header-line,vertical-line,left-margin,right-margin,left-fringe, orright-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 inserted into the input stream. See Key Sequence Input.
- x, y
- These are the pixel coordinates of the click, relative to
the top left corner of window, which is
(0 . 0). For the mode or header line, y does not have meaningful data. For the vertical line, x does not have meaningful data. - timestamp
- This is the time at which the event occurred, in milliseconds.
- object
- This is the object on which the click occurred. It is either
nilif there is no string property, or it has the form (string . string-pos) when there is a string-type text property at the click position.- string
- This is the string on which the click occurred, including any
properties.
- string-pos
- This is the position in the string on which the click occurred, relevant if properties at the click need to be looked up.
- 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 coordinates of the glyph under the x,
y position, possibly padded with default character width
glyphs if x is beyond the last glyph on the line.
- image
- This is the image object on which the click occurred. It is either
nilif there is no image at the position clicked on, or it is an image object as returned byfind-imageif 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 isnil, 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 mouse clicks on a scroll-bar, position has this form:(window area (portion . whole) timestamp part)
- window
- This is the window whose scroll-bar was clicked on.
- area
- This is the scroll bar where the click occurred. It is one of the
symbols
vertical-scroll-barorhorizontal-scroll-bar. - portion
- This is the distance of the click from the top or left end of
the scroll bar.
- whole
- This is the length of the entire scroll bar.
- timestamp
- This is the time at which the event occurred, in milliseconds.
- part
- This is the part of the scroll-bar which was clicked on. It is one
of the symbols
above-handle,handle,below-handle,up,down,top,bottom, andend-scroll.
- click-count
- This is the number of rapid repeated presses so far of the same mouse button. See Repeat Events.
Next: Button-Down Events, Previous: Click Events, Up: Input Events
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. They have the same form as position in a click event
(see Click Events) that is not on the scroll bar part of the
window. 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.9 These occur as soon as a button is pressed. They are represented by lists that look exactly like click events (see 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.
See Motion Events.
Next: Motion Events, Previous: Button-Down Events, Up: Input Events
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
<meta> 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.
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.
To generate repeat events, successive mouse button presses must be at approximately the same screen position. The value of
double-click-fuzzspecifies 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.
To generate repeat events, the number of milliseconds between successive button presses must be less than the value of
double-click-time. Settingdouble-click-timetonildisables multi-click detection entirely. Setting it totremoves the time limit; Emacs then detects multi-clicks by position only.
Next: Focus Events, Previous: Repeat Events, Up: Input Events
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)
The second element of the list describes the current position of the mouse, just as in a click event (see Click Events).
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. See Mouse Tracking.
Next: Misc Events, Previous: Motion Events, Up: Input Events
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. See 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.
Next: Event Examples, Previous: Focus Events, Up: Input Events
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-frameevent 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-upposition)(wheel-downposition)- These kinds of event are generated by moving a mouse wheel. Their
usual meaning is a kind of scroll or zoom.
The element position is a list describing the position of the event, in the same format as used in a mouse-click event.
This kind of event is generated only on some kinds of systems. On some systems,
mouse-4andmouse-5are used instead. For portable code, use the variablesmouse-wheel-up-eventandmouse-wheel-down-eventdefined in mwheel.el to determine what event types to expect for the mouse wheel. (drag-n-dropposition 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, 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-echotext 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 Text help-echo.
sigusr1sigusr2- These events are generated when the Emacs process receives
the signals
SIGUSR1andSIGUSR2. They contain no additional data because signals do not carry additional information.To catch a user signal, bind the corresponding event to an interactive command in the
special-event-map(see Active Keymaps). The command is called with no arguments, and the specific signal event is available inlast-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)
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.
Next: Classifying Events, Previous: Misc Events, Up: Input Events
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 (#<window 18 on NEWS> 2613 (0 . 38) -864320))
(mouse-1 (#<window 18 on NEWS> 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 (#<window 18 on NEWS> 3440 (0 . 27) -731219))
(C-drag-mouse-2 (#<window 18 on NEWS> 3440 (0 . 27) -731219)
(#<window 18 on NEWS> 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 (#<window 18 on NEWS> mode-line (33 . 31) -457844))
(M-S-drag-mouse-2 (#<window 18 on NEWS> mode-line (33 . 31) -457844)
(#<window 20 on carlton-sanskrit.tex> 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)
Next: Accessing Mouse, Previous: Event Examples, Up: Input Events
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 <META> 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.
This function returns a list of the modifiers that event has. The modifiers are symbols; they include
shift,control,meta,alt,hyperandsuper. In addition, the modifiers list of a mouse event symbol always contains one ofclick,drag, anddown. For double or triple events, it also containsdoubleortriple.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-modifierscan returnnil, 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’.
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
This function returns non-
nilif object is a mouse movement event.
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
Next: Accessing Scroll, Previous: Classifying Events, Up: Input Events
21.7.13 Accessing Mouse Events
This section describes convenient functions for accessing the data in a mouse button or motion event.
These two functions return the starting or ending position of a mouse-button event, as a list of this form:
(window pos-or-area (x . y) timestamp
object text-pos (col . row)
image (dx . dy) (width . height))
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.
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.
These functions take a position list as described above, and return various parts of it.
Return the window area recorded in position. It returns
nilwhen the event occurred in the text area of the window; otherwise, it is a symbol identifying the area in which the event occurred.
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.
Return the pixel-based x and y coordinates in position, as a cons cell
(x.y). These coordinates are relative to the window given byposn-window.This example shows how to convert these window-relative coordinates into frame-relative coordinates:
(defun frame-relative-coordinates (position) "Return frame-relative coordinates from POSITION." (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)))))
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 (see Header Lines), it is not counted as the first line.
Return the actual row and column in position, as a cons cell
(col.row). The values are the actual row number in the window, and the actual character number in that row. It returnsnilif position does not include actual positions values. You can useposn-col-rowto get approximate values.
Return the string object in position, either
nil, or a cons cell(string.string-pos).
Return the image object in position, either
nil, or an image(image ...).
Return the image or string object in position, either
nil, an image(image ...), or a cons cell(string.string-pos).
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.
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.
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.
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-pointreturnsnilif pos is not visible in window.
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.
Next: Strings of Events, Previous: Accessing Mouse, Up: Input Events
21.7.14 Accessing Scroll Bar Events
These functions are useful for decoding scroll bar events.
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.
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 byscroll-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.
Previous: Accessing Scroll, Up: Input Events
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-keyanddefine-key. For example, you can useread-key-sequence-vectorinstead ofread-key-sequence, andthis-command-keys-vectorinstead ofthis-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(see 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.
Next: Special Events, Previous: Input Events, Up: Command Loop
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 Temporary Displays,
and sit-for in Waiting. See Terminal Input, for
functions and variables for controlling terminal input modes and
debugging terminal input.
For higher-level input facilities, see Minibuffers.
Next: Reading One Event, Up: Reading Input
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.
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-sequencereturns a string (see 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. See 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 aswitch-frameevent 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 isnil, 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 benilif 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-sequencesuppresses quitting: C-g typed while reading with this function works like any other character, and does not setquit-flag. See Quitting.
This is like
read-key-sequenceexcept that it always returns the key sequence as a vector, never as a string. See 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 (see handle-shift-selection).
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
(#<window 6 on NEWS> mode-line
(40 . 63) 5959987))]
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.
Next: Event Mod, Previous: Key Sequence Input, Up: Reading Input
21.8.2 Reading One Event
The lowest level functions for command input are read-event,
read-char, and read-char-exclusive.
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-eventdoes 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. See 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-areais non-nil, thenread-eventmoves the cursor temporarily to the echo area, to the end of any message displayed there. Otherwiseread-eventdoes 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-eventstops waiting and returnsnil. 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 isnil,read-eventwaits 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 withrun-with-idle-timer(see Idle Timers)—can run during this period. However, if seconds is non-nil, the state of idleness remains unchanged. If Emacs is non-idle whenread-eventis called, it remains non-idle throughout the operation ofread-event; if Emacs is idle (which can happen if the call happens inside an idle timer), it remains idle.If
read-eventgets an event that is defined as a help character, then in some casesread-eventprocesses the event directly without returning. See Help Functions. Certain other events, called special events, are also processed directly withinread-event(see Special Events).Here is what happens if you call
read-eventand then press the right-arrow function key:(read-event) ⇒ right
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-charsignals an error. The arguments work as inread-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-charfrom the minibuffer usingeval-expression.read-charreads the keyboard macro's very next character, which is 1. Theneval-expressiondisplays 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
This function reads and returns a character of command input. If the user generates an event which is not a character,
read-char-exclusiveignores it and reads another event, until it gets a character. The arguments work as inread-event.
None of the above functions suppress quitting.
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 Translation Keymaps.
If you wish to read a single key taking these translations into
account, use the function read-key:
This function reads a single key. It is “intermediate” between
read-key-sequenceandread-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 toinput-decode-map,local-function-key-map, andkey-translation-map(see 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.
Next: Invoking the Input Method, Previous: Reading One Event, Up: Reading Input
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.
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-modifiersto?\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, settingextra-keyboard-modifiersto zero cancels any modification.When using a window system, the program can “press” any of the modifier keys in this way. Otherwise, only the <CTL> and <META> 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.
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-tableis a char-table (see 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-keysand dribble files record the characters after translation.Note also that this translation is done before the characters are supplied to input methods (see Input Methods). Use
translation-table-for-input(see Translation of Characters), if you want to translate characters after input methods operate.
This function modifies
keyboard-translate-tableto 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.
See Translation Keymaps, for mechanisms that translate event sequences
at the level of read-key-sequence.
Next: Quoted Character Input, Previous: Event Mod, Up: Reading Input
21.8.4 Invoking the Input Method
The event-reading functions invoke the current input method, if any
(see 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 <SPC>) with no modifier bits, it
calls that function, passing the character as an argument.
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
(see 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.
Next: Event Input Misc, Previous: Invoking the Input Method, Up: Reading Input
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.
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 <RET>, 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. See 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
Previous: Quoted Character Input, Up: Reading Input
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 (see Reading a Password).
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-eventsis to uselistify-key-sequence(see Strings of Events).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.
This function converts the string or vector key to a list of individual events, which you can put in
unread-command-events.
This variable holds a character to be read as command input. A value of -1 means “empty.”
This variable is mostly obsolete now that you can use
unread-command-eventsinstead; it exists only to support programs written for Emacs versions 18 and earlier.
This function determines whether any command input is currently available to be read. It returns immediately, with value
tif there is available input,nilotherwise. On rare occasions it may returntwhen no input is available.
— Variable: last-input-char
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 oflast-command-event.(progn (print (read-char)) (print last-command-event) last-input-event) -| 49 -| 5 ⇒ 49The alias
last-input-charis obsolete.
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-inputform returnsnilif aborted by a real quit, and returnstif aborted by arrival of other input.If a part of body binds
inhibit-quitto 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)))
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-forfinishes sleeping,discard-inputdiscards any characters typed during the sleep.(progn (sleep-for 2) (discard-input)) ⇒ nil
Next: Waiting, Previous: Reading Input, Up: Command Loop
21.9 Special Events
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.
Events that are handled in this way 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.
These 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, 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 (see Active Keymaps).
Next: Quitting, Previous: Special Events, Up: Command Loop
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.
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-foris to give the user time to read text that you display. The value istifsit-forwaited the full time with no input arriving (see Event Input Misc). Otherwise, the value isnil.The argument seconds need not be an integer. If it is a floating point number,
sit-forwaits 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. See Forcing Redisplay.If nodisp is non-
nil, thensit-fordoes not redisplay, but it still returns as soon as input is available (or when the timeout elapses).In batch mode (see Batch Mode),
sit-forcannot be interrupted, even by input from the standard input descriptor. It is thus equivalent tosleep-for, which is described below.It is also possible to call
sit-forwith three arguments, as(sit-forseconds millisec nodisp), but that is considered obsolete.
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-forwaits 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-forwhen you wish to guarantee a delay.
See Time of Day, for functions to get the current time.
Next: Prefix Command Arguments, Previous: Waiting, Up: Command Loop
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))
If this variable is non-
nil, then Emacs quits immediately, unlessinhibit-quitis non-nil. Typing C-g ordinarily setsquit-flagnon-nil, regardless ofinhibit-quit.
This variable determines whether Emacs should quit when
quit-flagis set to a value other thannil. Ifinhibit-quitis non-nil, thenquit-flaghas no special effect.
This macro executes body forms in sequence, but allows quitting, at least locally, within body even if
inhibit-quitwas non-niloutside this construct. It returns the value of the last form in body, unless exited by quitting, in which case it returnsnil.If
inhibit-quitisnilon entry towith-local-quit, it only executes the body, and settingquit-flagcauses a normal quit. However, ifinhibit-quitis non-nilso that ordinary quitting is delayed, a non-nilquit-flagtriggers a special kind of local quit. This ends the execution of body and exits thewith-local-quitbody withquit-flagstill non-nil, so that another (ordinary) quit will happen as soon as that is allowed. Ifquit-flagis already non-nilat 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 whereinhibit-quitis normally bound tot.
This function signals the
quitcondition with(signal 'quit nil). This is the same thing that quitting does. (Seesignalin Errors.)
You can specify a character other than C-g to use for quitting.
See the function set-input-mode in Terminal Input.
Next: Recursive Editing, Previous: Quitting, Up: Command Loop
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 betweenniland 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.
(See 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.
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.
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").
The value of this variable is the raw prefix argument for the next editing command. Commands such as
universal-argumentthat specify prefix arguments for the following command work by setting this variable.
The following commands exist to set up prefix arguments for the following command. Do not call them for any other reason.
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.
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.
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.
Next: Disabling Commands, Previous: Prefix Command Arguments, Up: Command Loop
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
(see 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).
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-editsaves and restores the current buffer. Otherwise, if you switch buffers, the buffer you switched to is current afterrecursive-editreturns.In the following example, the function
simple-recfirst 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 executingsimple-rec.(defun simple-rec () (forward-word 1) (message "Recursive edit in progress") (recursive-edit) (forward-word 1)) ⇒ simple-rec (simple-rec) ⇒ nil
This function exits from the innermost recursive edit (including minibuffer input). Its definition is effectively
(throw 'exit nil).
This function aborts the command that requested the innermost recursive edit (including minibuffer input), by signaling
quitafter exiting the recursive edit. Its definition is effectively(throw 'exit t). See Quitting.
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.
This function returns the current depth of recursive edits. When no recursive edit is active, it returns 0.
Next: Command History, Previous: Recursive Editing, Up: Command Loop
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 (see 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")
See 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.
Allow command (a symbol) to be executed without special confirmation from now on, and alter the user's init file (see Init File) so that this will apply to future sessions.
Require special confirmation to execute command from now on, and alter the user's init file so that this will apply to future sessions.
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-keysto 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.
Next: Keyboard Macros, Previous: Disabling Commands, Up: Command Loop
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.
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 (see 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 (see 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
(see Repetition). Within the
minibuffer, the usual minibuffer history commands are available.
Previous: Command History, Up: Command Loop
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 (see Macros).
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 returnsnil, then this stops execution of the macro.See Reading One Event, for an example of using
execute-kbd-macro.
This variable contains the string or vector that defines the keyboard macro that is currently executing. It is
nilif 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.
This variable is non-
nilif 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 isappendwhile appending to the definition of an existing macro. The commandsstart-kbd-macro,kmacro-start-macroandend-kbd-macroset this variable—do not set it yourself.The variable is always local to the current terminal and cannot be buffer-local. See Multiple Terminals.
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. See Multiple Terminals.
This normal hook (see Standard Hooks) 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).
Next: Modes, Previous: Command Loop, Up: Top
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.
Next: Keymap Basics, Up: Keymaps
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, and mouse actions (see 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 <TAB>, <RET>, <ESC>, and <DEL> 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, 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. See 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, Init Rebinding.
This macro converts the text keyseq-text (a string constant) into a key sequence (a string or vector constant). The contents of keyseq-text should describe the key sequence using almost the same syntax used in this manual. More precisely, it uses the same syntax that Edit Macro mode uses for editing keyboard macros (see Edit Keyboard Macro); you must surround function key names with ‘<...>’.
(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 "<f1> SPC") ⇒ [f1 32] (kbd "C-M-<down>") ⇒ [C-M-down]This macro is not meant for use with arguments that vary—only with string constants.
Next: Format of Keymaps, Previous: Key Sequences, Up: Keymaps
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, <RET>, and C-x 4 C-f. Examples of undefined complete
keys are C-x C-g, and C-c 3. See 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. See Active Keymaps, for details.
Next: Creating Keymaps, Previous: Keymap Basics, Up: Keymaps
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. See Classifying Events.
In this kind of binding, binding is a command.
(type item-name [cache].binding)- This specifies a binding which is also a simple menu item that
displays as item-name in the menu. cache, if present,
caches certain information for display in the menu. See Simple Menu Items.
(type item-name help-string [cache].binding)- This is a simple menu item with help string help-string.
(typemenu-item .details)- This specifies a binding which is also an extended menu item. This
allows use of other features. See 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
(see 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. See Defining Menus.
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 <ESC> (or
whatever is currently the value of meta-prefix-char). Thus, the
key M-a is internally represented as <ESC> a, and its
global binding is found at the slot for a in esc-map
(see Prefix Keys).
This conversion applies only to characters, not to function keys or other input events; thus, M-<end> has nothing to do with <ESC> <end>.
Here as an example is the local keymap for Lisp mode, a sparse keymap. It defines bindings for <DEL> and <TAB>, plus C-c C-l, M-C-q, and M-C-x.
lisp-mode-map
⇒
(keymap
(3 keymap
;; C-c C-z
(26 . run-lisp))
(27 keymap
;; M-C-x, treated as <ESC> C-x
(24 . lisp-send-defun)
keymap
;; M-C-q, treated as <ESC> C-q
(17 . indent-sexp))
;; This part is inherited from lisp-mode-shared-map.
keymap
;; <DEL>
(127 . backward-delete-char-untabify)
(27 keymap
;; M-C-q, treated as <ESC> C-q
(17 . indent-sexp))
(9 . lisp-indent-line))
This function returns
tif object is a keymap,nilotherwise. More precisely, this function tests for a list whose car iskeymap, or for a symbol whose function definition satisfieskeymapp.(keymapp '(keymap)) ⇒ t (fset 'foo '(keymap)) (keymapp 'foo) ⇒ t (keymapp (current-global-map)) ⇒ t
Next: Inheritance and Keymaps, Previous: Format of Keymaps, Up: Keymaps
22.4 Creating Keymaps
Here we describe the functions for creating keymaps.
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 (see 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.
This function creates and returns a new full keymap. That keymap contains a char-table (see 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 inmake-sparse-keymap.(make-keymap) ⇒ (keymap #^[t nil nil nil ... nil nil keymap])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.
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
Next: Prefix Keys, Previous: Creating Keymaps, Up: Keymaps
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.
This returns the parent keymap of keymap. If keymap has no parent,
keymap-parentreturnsnil.
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.
Next: Active Keymaps, Previous: Inheritance and Keymaps, Up: Keymaps
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-mapis the global keymap for the <ESC> prefix key. Thus, the global definitions of all meta characters are actually found here. This map is also the function definition ofESC-prefix.help-mapis the global keymap for the C-h prefix key.mode-specific-mapis 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-mapis the global keymap used for the C-x prefix key. This map is found via the function cell of the symbolControl-X-prefix.mule-keymapis the global keymap used for the C-x <RET> prefix key.ctl-x-4-mapis the global keymap used for the C-x 4 prefix key.ctl-x-5-mapis the global keymap used for the C-x 5 prefix key.2C-mode-mapis the global keymap used for the C-x 6 prefix key.vc-prefix-mapis the global keymap used for the C-x v prefix key.goto-mapis the global keymap used for the M-g prefix key.search-mapis the global keymap used for the M-s prefix key.facemenu-keymapis the global keymap used for the M-o prefix key.- The other Emacs prefix keys are C-x @, C-x a i, C-x <ESC> and <ESC> <ESC>. 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 <ESC> 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. See 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
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 (see Defining Menus).
Next: Searching Keymaps, Previous: Prefix Keys, Up: Keymaps
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. See Searching Keymaps,
for more details of this procedure.
When the key sequence starts with a mouse event (optionally preceded
by a symbolic prefix), 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 (see 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 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.
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. See Intro to Minibuffers.
Emacs has other keymaps that are used in a different way—translating
events within read-key-sequence. See Translation Keymaps.
See Standard Keymaps, for a list of standard keymaps.
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-mapandoverriding-terminal-local-map, but if olp is non-nilthen it pays attention to them. position can optionally be either an event position as returned byevent-startor a buffer position, and may change the keymaps as described forkey-binding.
This function returns the binding for key according to the current active keymaps. The result is
nilif key is undefined in the keymaps.The argument accept-defaults controls checking for default bindings, as in
lookup-key(see Functions for Key Lookup).When commands are remapped (see Remapping Commands),
key-bindingnormally processes command remappings so as to returns the remapped command that will actually be executed. However, if no-remap is non-nil,key-bindingignores 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 ofevent-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
Next: Controlling Active Maps, Previous: Active Keymaps, Up: Keymaps
22.8 Searching the Active Keymaps
After translation of event subsequences (see 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 (if overriding-terminal-local-map
(find-in overriding-terminal-local-map)
(if overriding-local-map
(find-in overriding-local-map)
(or (find-in (get-char-property (point) 'keymap))
(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)))
The 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 Key Lookup.) If the key sequence starts with a mouse event,
or a symbolic prefix event followed by 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.
- The function finally found may be remapped (see Remapping Commands).
- Characters that are bound to
self-insert-commandare translated according totranslation-table-for-inputbefore insertion. current-active-mapsreturns a list of the currently active keymaps at point.- When a match is found (see 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.
Next: Key Lookup, Previous: Searching Keymaps, Up: Keymaps
22.9 Controlling the Active Keymaps
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-commandto 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.
This function returns the current global keymap. This is the same as the value of
global-mapunless you change one or the other. The return value is a reference, not a copy; if you usedefine-keyor other functions on it you will alter global bindings.(current-global-map) ⇒ (keymap [set-mark-command beginning-of-line ... delete-backward-char])
This function returns the current buffer's local keymap, or
nilif 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 <ESC>, 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.
This function returns a list of the keymaps of currently enabled minor modes.
This function makes keymap the new current global keymap. It returns
nil.It is very unusual to change the global 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-mapreturnsnil. Most major mode commands use this function.
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-
nilvalue. Typically variable is the variable that enables or disables a minor mode. See Keymaps and Minor Modes.Note that elements of
minor-mode-map-alistdo not have the same structure as elements ofminor-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-alisttakes 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 Keymaps and Minor Modes, for more information about minor modes. See also
minor-mode-key-binding(see Functions for Key Lookup).
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 inminor-mode-map-alist.
minor-mode-overriding-map-alistis automatically buffer-local in all buffers.
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.
If non-
nil, this variable holds a keymap to use instead ofoverriding-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. See Multiple Terminals. It is used to implement incremental search mode.
If this variable is non-
nil, the value ofoverriding-local-maporoverriding-terminal-local-mapcan affect the display of the menu bar. The default value isnil, 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.
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. See Special Events.
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 beforeminor-mode-map-alistandminor-mode-overriding-map-alist.
Next: Functions for Key Lookup, Previous: Controlling Active Maps, Up: 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). See 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:
nilnilmeans 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 ofnilfor that event type.- command
- The events used so far in the lookup form a complete key,
and command is its binding. See 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 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 aninteractivespecification. See 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-mapand whose cdr is 32 (the code for <SPC>) means, “Use the global binding of Meta-<SPC>, whatever that may be.”
- If the car of list is the symbol
- 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(see Interactive Call).The symbol
undefinedis worth special mention: it means to treat the key as undefined. Strictly speaking, the key is defined, and its binding is the commandundefined; but that command does the same thing that is done automatically for an undefined key: it rings the bell (by callingding) but does not signal an error.undefinedis used in local keymaps to override a global key binding and make the key “undefined” locally. A local binding ofnilwould 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.
Here is an example of a sparse keymap with two characters bound to
commands and one bound to another keymap. This map is the normal value
of emacs-lisp-mode-map. Note that 9 is the code for <TAB>,
127 for <DEL>, 27 for <ESC>, 17 for C-q and 24 for
C-x.
(keymap (9 . lisp-indent-line)
(127 . backward-delete-char-untabify)
(27 keymap (17 . indent-sexp) (24 . eval-defun)))
Next: Changing Key Bindings, Previous: Key Lookup, Up: Keymaps
22.11 Functions for Key Lookup
Here are the functions and variables pertaining to key lookup.
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") ⇒ 2If 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, thenlookup-keyconsiders default bindings as well as bindings for the specific events in key. Otherwise,lookup-keyreports only bindings for the specific sequence key, ignoring default bindings except when you explicitly ask about them. (To do this, supplytas an element of key; see 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-wordUnlike
read-key-sequence, this function does not modify the specified events in ways that discard information (see Key Sequence Input). In particular, it does not convert letters to lower case and it does not change drag events to clicks.
This function returns the binding for key in the current local keymap, or
nilif it is undefined there.The argument accept-defaults controls checking for default bindings, as in
lookup-key(above).
This function returns the binding for command key in the current global keymap, or
nilif it is undefined there.The argument accept-defaults controls checking for default bindings, as in
lookup-key(above).
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 isnil.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).
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 (see Prefix Keys). The default value is 27, which is the ASCII code for <ESC>.
As long as the value of
meta-prefix-charremains 27, key lookup translates M-b into <ESC> b, which is normally defined as thebackward-wordcommand. However, if you were to setmeta-prefix-charto 24, the code for C-x, then Emacs will translate M-b into C-x b, whose standard binding is theswitch-to-buffercommand. (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-<F1>, a function key, is not converted into <ESC> <F1>.
Next: Remapping Commands, Previous: Functions for Key Lookup, Up: Keymaps
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 (see Key Binding Commands). You can also use define-key, a more general
function; then you must specify explicitly the map to change.
When choosing the key sequences for Lisp programs to rebind, please follow the Emacs conventions for use of various keys (see Key Binding Conventions).
In writing the key sequence to rebind, it is good to use the special
escape sequences for control and meta characters (see 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]’. See 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 macro (see Key Sequences) is a convenient
way to specify the key sequence.
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 Key Lookup.) The value returned by
define-keyis 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-keydefines 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 (see 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 (see Remapping Commands).
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 ofsubstitute-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-mapfor 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))
This function changes the contents of the full keymap keymap by remapping
self-insert-commandto the commandundefined(see Remapping Commands). This has the effect of undefining all printing characters, thus making ordinary insertion of text impossible.suppress-keymapreturnsnil.If nodigits is
nil, thensuppress-keymapdefines digits to rundigit-argument, and - to runnegative-argument. Otherwise it makes them undefined like the rest of the printing characters.The
suppress-keymapfunction does not make it impossible to modify a buffer, as it does not suppress commands such asyankandquoted-insert. To prevent any modification of a buffer, make it read-only (see 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-mapwould make it impossible to use most of Emacs.Most often,
suppress-keymapis used to initialize local keymaps of modes such as Rmail and Dired where insertion of text is not desirable and the buffer is read-only. Here is an example taken from the file emacs/lisp/dired.el, showing how the local keymap for Dired mode is set up:(setq dired-mode-map (make-keymap)) (suppress-keymap dired-mode-map) (define-key dired-mode-map "r" 'dired-rename-file) (define-key dired-mode-map "\C-d" 'dired-flag-file-deleted) (define-key dired-mode-map "d" 'dired-flag-file-deleted) (define-key dired-mode-map "v" 'dired-view-file) (define-key dired-mode-map "e" 'dired-find-file) (define-key dired-mode-map "f" 'dired-find-file) ...
Next: Translation Keymaps, Previous: Changing Key Bindings, Up: Keymaps
22.13 Remapping Commands
A special kind of key binding, using a special “key sequence”
which includes a command name, has the effect of remapping that
command into another. Here's how it works. You make a key binding
for a key sequence that starts with the dummy event remap,
followed by the command name you want to remap. Specify the remapped
definition as the definition in this binding. The remapped definition
is usually a command name, but it can be any valid definition for
a key binding.
Here's an example. Suppose that My mode uses special commands
my-kill-line and my-kill-word, which should be invoked
instead of kill-line and kill-word. It can establish
this by making these two command-remapping bindings in its keymap:
(define-key my-mode-map [remap kill-line] 'my-kill-line)
(define-key my-mode-map [remap kill-word] 'my-kill-word)
Whenever my-mode-map is an active keymap, if the user types
C-k, Emacs will find the standard global binding of
kill-line (assuming nobody has changed it). But
my-mode-map remaps kill-line to my-kill-line,
so instead of running kill-line, Emacs runs
my-kill-line.
Remapping only works through a single level. In other words,
(define-key my-mode-map [remap kill-line] 'my-kill-line)
(define-key my-mode-map [remap my-kill-line] 'my-other-kill-line)
does not have the effect of remapping kill-line into
my-other-kill-line. If an ordinary key binding specifies
kill-line, this keymap will remap it to my-kill-line;
if an ordinary binding specifies my-kill-line, this keymap will
remap it to my-other-kill-line.
To undo the remapping of a command, remap it to nil; e.g.
(define-key my-mode-map [remap kill-line] nil)
This function returns the remapping for command (a symbol), given the current active keymaps. If command is not remapped (which is the usual situation), or not a symbol, the function returns
nil.positioncan optionally specify a buffer position or an event position to determine the keymaps to use, as inkey-binding.If the optional argument
keymapsis non-nil, it specifies a list of keymaps to search in. This argument is ignored ifpositionis non-nil.
Next: Key Binding Commands, Previous: Remapping Commands, Up: Keymaps
22.14 Keymaps for Translating Sequences of Events
This section describes keymaps that are used during reading a key
sequence, to translate certain event sequences into others.
read-key-sequence checks every subsequence of the key sequence
being read, as it is read, against input-decode-map, then
local-function-key-map, and then against key-translation-map.
This variable holds a keymap that describes the character sequences sent by function keys on an ordinary character terminal. This keymap has the same structure as other keymaps, but is used differently: it specifies translations to make while reading key sequences, rather than bindings for key sequences.
If
input-decode-map“binds” a key sequence k to a vector v, then when k appears as a subsequence anywhere in a key sequence, it is replaced with the events in v.For example, VT100 terminals send <ESC> O P when the keypad <PF1> key is pressed. Therefore, we want Emacs to translate that sequence of events into the single event
pf1. We accomplish this by “binding” <ESC> O P to[pf1]ininput-decode-map, when using a VT100.Thus, typing C-c <PF1> sends the character sequence C-c <ESC> O P; later the function
read-key-sequencetranslates this back into C-c <PF1>, which it returns as the vector[?\C-c pf1].The value of
input-decode-mapis usually set up automatically according to the terminal's Terminfo or Termcap entry, but sometimes those need help from terminal-specific Lisp files. Emacs comes with terminal-specific files for many common terminals; their main purpose is to make entries ininput-decode-mapbeyond those that can be deduced from Termcap and Terminfo. See Terminal-Specific.
This variable holds a keymap similar to
input-decode-mapexcept that it describes key sequences which should be translated to alternative interpretations that are usually preferred. It applies afterinput-decode-mapand beforekey-translation-map.Entries in
local-function-key-mapare ignored if they conflict with bindings made in the minor mode, local, or global keymaps. I.e. the remapping only applies if the original key sequence would otherwise not have any binding.
local-function-key-mapinherits fromfunction-key-map, but the latter should not be used directly.
This variable is another keymap used just like
input-decode-mapto translate input events into other events. It differs frominput-decode-mapin that it goes to work afterlocal-function-key-mapis finished rather than before; it receives the results of translation bylocal-function-key-map.Just like
input-decode-map, but unlikelocal-function-key-map, this keymap is applied regardless of whether the input key-sequence has a normal binding. Note however that actual key bindings can have an effect onkey-translation-map, even though they are overridden by it. Indeed, actual key bindings overridelocal-function-key-mapand thus may alter the key sequence thatkey-translation-mapreceives. Clearly, it is better to avoid this type of situation.The intent of
key-translation-mapis for users to map one character set to another, including ordinary characters normally bound toself-insert-command.
You can use input-decode-map, local-function-key-map, or
key-translation-map for more than simple aliases, by using a
function, instead of a key sequence, as the “translation” of a key.
Then this function is called to compute the translation of that key.
The key translation function receives one argument, which is the prompt
that was specified in read-key-sequence—or nil if the
key sequence is being read by the editor command loop. In most cases
you can ignore the prompt value.
If the function reads input itself, it can have the effect of altering the event that follows. For example, here's how to define C-c h to turn the character that follows into a Hyper character:
(defun hyperify (prompt)
(let ((e (read-event)))
(vector (if (numberp e)
(logior (lsh 1 24) e)
(if (memq 'hyper (event-modifiers e))
e
(add-event-modifier "H-" e))))))
(defun add-event-modifier (string e)
(let ((symbol (if (symbolp e) e (car e))))
(setq symbol (intern (concat string
(symbol-name symbol))))
(if (symbolp e)
symbol
(cons symbol (cdr e)))))
(define-key local-function-key-map "\C-ch" 'hyperify)
If you have enabled keyboard character set decoding using
set-keyboard-coding-system, decoding is done after the
translations listed above. See Terminal I/O Encoding. However, in
future Emacs versions, character set decoding may be done at an
earlier stage.
Next: Scanning Keymaps, Previous: Translation Keymaps, Up: Keymaps
22.15 Commands for Binding Keys
This section describes some convenient interactive interfaces for
changing key bindings. They work by calling define-key.
People often use global-set-key in their init files
(see Init File) for simple customization. For example,
(global-set-key (kbd "C-x C-\\") 'next-line)
or
(global-set-key [?\C-x ?\C-\\] 'next-line)
or
(global-set-key [(control ?x) (control ?\\)] 'next-line)
redefines C-x C-\ to move down a line.
(global-set-key [M-mouse-1] 'mouse-set-point)
redefines the first (leftmost) mouse button, entered with the Meta key, to set point where you click.
Be careful when using non-ASCII text characters in Lisp specifications of keys to bind. If these are read as multibyte text, as they usually will be in a Lisp file (see Loading Non-ASCII), you must type the keys as multibyte too. For instance, if you use this:
(global-set-key "ö" 'my-function) ; bind o-umlaut
or
(global-set-key ?ö 'my-function) ; bind o-umlaut
and your language environment is multibyte Latin-1, these commands actually bind the multibyte character with code 2294, not the unibyte Latin-1 character with code 246 (M-v). In order to use this binding, you need to enter the multibyte Latin-1 character as keyboard input. One way to do this is by using an appropriate input method (see Input Methods).
If you want to use a unibyte character in the key binding, you can
construct the key sequence string using multibyte-char-to-unibyte
or string-make-unibyte (see Converting Representations).
This function sets the binding of key in the current global map to binding.
(global-set-key key binding) == (define-key (current-global-map) key binding)
This function removes the binding of key from the current global map.
One use of this function is in preparation for defining a longer key that uses key as a prefix—which would not be allowed if key has a non-prefix binding. For example:
(global-unset-key "\C-l") ⇒ nil (global-set-key "\C-l\C-l" 'redraw-display) ⇒ nilThis function is implemented simply using
define-key:(global-unset-key key) == (define-key (current-global-map) key nil)
This function sets the binding of key in the current local keymap to binding.
(local-set-key key binding) == (define-key (current-local-map) key binding)
This function removes the binding of key from the current local map.
(local-unset-key key) == (define-key (current-local-map) key nil)
Next: Menu Keymaps, Previous: Key Binding Commands, Up: Keymaps
22.16 Scanning Keymaps
This section describes functions used to scan all the current keymaps for the sake of printing help information.
This function returns a list of all the keymaps that can be reached (via zero or more prefix keys) from keymap. The value is an association list with elements of the form
(key.map), where key is a prefix key whose definition in keymap is map.The elements of the alist are ordered so that the key increases in length. The first element is always
([] .keymap), because the specified keymap is accessible from itself with a prefix of no events.If prefix is given, it should be a prefix key sequence; then
accessible-keymapsincludes only the submaps whose prefixes start with prefix. These elements look just as they do in the value of(accessible-keymaps); the only difference is that some elements are omitted.In the example below, the returned alist indicates that the key <ESC>, which is displayed as ‘^[’, is a prefix key whose definition is the sparse keymap
(keymap (83 . center-paragraph) (115 . foo)).(accessible-keymaps (current-local-map)) ⇒(([] keymap (27 keymap ; Note this keymap for <ESC> is repeated below. (83 . center-paragraph) (115 . center-line)) (9 . tab-to-tab-stop)) ("^[" keymap (83 . center-paragraph) (115 . foo)))In the following example, C-h is a prefix key that uses a sparse keymap starting with
(keymap (118 . describe-variable)...). Another prefix, C-x 4, uses a keymap which is also the value of the variablectl-x-4-map. The eventmode-lineis one of several dummy events used as prefixes for mouse actions in special parts of a window.(accessible-keymaps (current-global-map)) ⇒ (([] keymap [set-mark-command beginning-of-line ... delete-backward-char]) ("^H" keymap (118 . describe-variable) ... (8 . help-for-help)) ("^X" keymap [x-flush-mouse-queue ... backward-kill-sentence]) ("^[" keymap [mark-sexp backward-sexp ... backward-kill-word]) ("^X4" keymap (15 . display-buffer) ...) ([mode-line] keymap (S-mouse-2 . mouse-split-window-horizontally) ...))These are not all the keymaps you would see in actuality.
The function
map-keymapcalls function once for each binding in keymap. It passes two arguments, the event type and the value of the binding. If keymap has a parent, the parent's bindings are included as well. This works recursively: if the parent has itself a parent, then the grandparent's bindings are also included and so on.This function is the cleanest way to examine all the bindings in a keymap.
This function is a subroutine used by the
where-iscommand (see Help). It returns a list of all key sequences (of any length) that are bound to command in a set of keymaps.The argument command can be any object; it is compared with all keymap entries using
eq.If keymap is
nil, then the maps used are the current active keymaps, disregardingoverriding-local-map(that is, pretending its value isnil). If keymap is a keymap, then the maps searched are keymap and the global keymap. If keymap is a list of keymaps, only those keymaps are searched.Usually it's best to use
overriding-local-mapas the expression for keymap. Thenwhere-is-internalsearches precisely the keymaps that are active. To search only the global map, pass(keymap)(an empty keymap) as keymap.If firstonly is
non-ascii, then the value is a single vector representing the first key sequence found, rather than a list of all possible key sequences. If firstonly ist, then the value is the first key sequence, except that key sequences consisting entirely of ASCII characters (or meta variants of ASCII characters) are preferred to all other key sequences and that the return value can never be a menu binding.If noindirect is non-
nil,where-is-internaldoesn't follow indirect keymap bindings. This makes it possible to search for an indirect definition itself.When command remapping is in effect (see Remapping Commands),
where-is-internalfigures out when a command will be run due to remapping and reports keys accordingly. It also returnsnilif command won't really be run because it has been remapped to some other command. However, if no-remap is non-nil.where-is-internalignores remappings.(where-is-internal 'describe-function) ⇒ ([8 102] [f1 102] [help 102] [menu-bar help-menu describe describe-function])
This function creates a listing of all current key bindings, and displays it in a buffer named ‘*Help*’. The text is grouped by modes—minor modes first, then the major mode, then global bindings.
If prefix is non-
nil, it should be a prefix key; then the listing includes only keys that start with prefix.The listing describes meta characters as <ESC> followed by the corresponding non-meta character.
When several characters with consecutive ASCII codes have the same definition, they are shown together, as ‘firstchar..lastchar’. In this instance, you need to know the ASCII codes to understand which characters this means. For example, in the default global map, the characters ‘<SPC> .. ~’ are described by a single line. <SPC> is ASCII 32, ~ is ASCII 126, and the characters between them include all the normal printing characters, (e.g., letters, digits, punctuation, etc.); all these characters are bound to
self-insert-command.If buffer-or-name is non-
nil, it should be a buffer or a buffer name. Thendescribe-bindingslists that buffer's bindings, instead of the current buffer's.
Previous: Scanning Keymaps, Up: Keymaps
22.17 Menu Keymaps
A keymap can operate as a menu as well as defining bindings for keyboard keys and mouse buttons. Menus are usually actuated with the mouse, but they can function with the keyboard also. If a menu keymap is active for the next input event, that activates the keyboard menu feature.
Next: Mouse Menus, Up: Menu Keymaps
22.17.1 Defining Menus
A keymap acts as a menu if it has an overall prompt string, which is a string that appears as an element of the keymap. (See Format of Keymaps.) The string should describe the purpose of the menu's commands. Emacs displays the overall prompt string as the menu title in some cases, depending on the toolkit (if any) used for displaying menus.10 Keyboard menus also display the overall prompt string.
The easiest way to construct a keymap with a prompt string is to
specify the string as an argument when you call make-keymap,
make-sparse-keymap (see Creating Keymaps), or
define-prefix-command (see Definition of define-prefix-command). If you do not want the keymap to operate as
a menu, don't specify a prompt string for it.
This function returns the overall prompt string of keymap, or
nilif it has none.
The menu's items are the bindings in the keymap. Each binding associates an event type to a definition, but the event types have no significance for the menu appearance. (Usually we use pseudo-events, symbols that the keyboard cannot generate, as the event types for menu item bindings.) The menu is generated entirely from the bindings that correspond in the keymap to these events.
The order of items in the menu is the same as the order of bindings in
the keymap. Since define-key puts new bindings at the front, you
should define the menu items starting at the bottom of the menu and
moving to the top, if you care about the order. When you add an item to
an existing menu, you can specify its position in the menu using
define-key-after (see Modifying Menus).
Next: Extended Menu Items, Up: Defining Menus
22.17.1.1 Simple Menu Items
The simpler (and original) way to define a menu item is to bind some event type (it doesn't matter what event type) to a binding like this:
(item-string . real-binding)
The car, item-string, is the string to be displayed in the menu. It should be short—preferably one to three words. It should describe the action of the command it corresponds to. Note that it is not generally possible to display non-ASCII text in menus. It will work for keyboard menus and will work to a large extent when Emacs is built with the Gtk+ toolkit.11
You can also supply a second string, called the help string, as follows:
(item-string help . real-binding)
help specifies a “help-echo” string to display while the mouse
is on that item in the same way as help-echo text properties
(see Help display).
As far as define-key is concerned, item-string and
help-string are part of the event's binding. However,
lookup-key returns just real-binding, and only
real-binding is used for executing the key.
If real-binding is nil, then item-string appears in
the menu but cannot be selected.
If real-binding is a symbol and has a non-nil
menu-enable property, that property is an expression that
controls whether the menu item is enabled. Every time the keymap is
used to display a menu, Emacs evaluates the expression, and it enables
the menu item only if the expression's value is non-nil. When a
menu item is disabled, it is displayed in a “fuzzy” fashion, and
cannot be selected.
The menu bar does not recalculate which items are enabled every time you
look at a menu. This is because the X toolkit requires the whole tree
of menus in advance. To force recalculation of the menu bar, call
force-mode-line-update (see Mode Line Format).
You've probably noticed that menu items show the equivalent keyboard key sequence (if any) to invoke the same command. To save time on recalculation, menu display caches this information in a sublist in the binding, like this:
(item-string [help] (key-binding-data) . real-binding)
Don't put these sublists in the menu item yourself; menu display calculates them automatically. Don't mention keyboard equivalents in the item strings themselves, since that is redundant.
Next: Menu Separators, Previous: Simple Menu Items, Up: Defining Menus
22.17.1.2 Extended Menu Items
An extended-format menu item is a more flexible and also cleaner
alternative to the simple format. You define an event type with a
binding that's a list starting with the symbol menu-item.
For a non-selectable string, the binding looks like this:
(menu-item item-name)
A string starting with two or more dashes specifies a separator line; see Menu Separators.
To define a real menu item which can be selected, the extended format binding looks like this:
(menu-item item-name real-binding
. item-property-list)
Here, item-name is an expression which evaluates to the menu item string. Thus, the string need not be a constant. The third element, real-binding, is the command to execute. The tail of the list, item-property-list, has the form of a property list which contains other information.
When an equivalent keyboard key binding is cached, the extended menu item binding looks like this:
(menu-item item-name real-binding (key-binding-data)
. item-property-list)
Here is a table of the properties that are supported:
:enableform- The result of evaluating form determines whether the item is
enabled (non-
nilmeans yes). If the item is not enabled, you can't really click on it. :visibleform- The result of evaluating form determines whether the item should
actually appear in the menu (non-
nilmeans yes). If the item does not appear, then the menu is displayed as if this item were not defined at all. :helphelp- The value of this property, help, specifies a “help-echo” string
to display while the mouse is on that item. This is displayed in the
same way as
help-echotext properties (see Help display). Note that this must be a constant string, unlike thehelp-echoproperty for text and overlays. :button (type.selected)- This property provides a way to define radio buttons and toggle buttons.
The car, type, says which: it should be
:toggleor:radio. The cdr, selected, should be a form; the result of evaluating it says whether this button is currently selected.A toggle is a menu item which is labeled as either “on” or “off” according to the value of selected. The command itself should toggle selected, setting it to
tif it isnil, and tonilif it ist. Here is how the menu item to toggle thedebug-on-errorflag is defined:(menu-item "Debug on Error" toggle-debug-on-error :button (:toggle . (and (boundp 'debug-on-error) debug-on-error)))This works because
toggle-debug-on-erroris defined as a command which toggles the variabledebug-on-error.Radio buttons are a group of menu items, in which at any time one and only one is “selected.” There should be a variable whose value says which one is selected at any time. The selected form for each radio button in the group should check whether the variable has the right value for selecting that button. Clicking on the button should set the variable so that the button you clicked on becomes selected.
:key-sequencekey-sequence- This property specifies which key sequence is likely to be bound to the
same command invoked by this menu item. If you specify the right key
sequence, that makes preparing the menu for display run much faster.
If you specify the wrong key sequence, it has no effect; before Emacs displays key-sequence in the menu, it verifies that key-sequence is really equivalent to this menu item.
:key-sequence nil- This property indicates that there is normally no key binding which is
equivalent to this menu item. Using this property saves time in
preparing the menu for display, because Emacs does not need to search
the keymaps for a keyboard equivalent for this menu item.
However, if the user has rebound this item's definition to a key sequence, Emacs ignores the
:keysproperty and finds the keyboard equivalent anyway. :keysstring- This property specifies that string is the string to display
as the keyboard equivalent for this menu item. You can use
the ‘\\[...]’ documentation construct in string.
:filterfilter-fn- This property provides a way to compute the menu item dynamically.
The property value filter-fn should be a function of one argument;
when it is called, its argument will be real-binding. The
function should return the binding to use instead.
Emacs can call this function at any time that it does redisplay or operates on menu data structures, so you should write it so it can safely be called at any time.
Next: Alias Menu Items, Previous: Extended Menu Items, Up: Defining Menus
22.17.1.3 Menu Separators
A menu separator is a kind of menu item that doesn't display any text—instead, it divides the menu into subparts with a horizontal line. A separator looks like this in the menu keymap:
(menu-item separator-type)
where separator-type is a string starting with two or more dashes.
In the simplest case, separator-type consists of only dashes.
That specifies the default kind of separator. (For compatibility,
"" and - also count as separators.)
Certain other values of separator-type specify a different style of separator. Here is a table of them:
"--no-line""--space"- An extra vertical space, with no actual line.
"--single-line"- A single line in the menu's foreground color.
"--double-line"- A double line in the menu's foreground color.
"--single-dashed-line"- A single dashed line in the menu's foreground color.
"--double-dashed-line"- A double dashed line in the menu's foreground color.
"--shadow-etched-in"- A single line with a 3D sunken appearance. This is the default,
used separators consisting of dashes only.
"--shadow-etched-out"- A single line with a 3D raised appearance.
"--shadow-etched-in-dash"- A single dashed line with a 3D sunken appearance.
"--shadow-etched-out-dash"- A single dashed line with a 3D raised appearance.
"--shadow-double-etched-in"- Two lines with a 3D sunken appearance.
"--shadow-double-etched-out"- Two lines with a 3D raised appearance.
"--shadow-double-etched-in-dash"- Two dashed lines with a 3D sunken appearance.
"--shadow-double-etched-out-dash"- Two dashed lines with a 3D raised appearance.
You can also give these names in another style, adding a colon after
the double-dash and replacing each single dash with capitalization of
the following word. Thus, "--:singleLine", is equivalent to
"--single-line".
Some systems and display toolkits don't really handle all of these separator types. If you use a type that isn't supported, the menu displays a similar kind of separator that is supported.
Previous: Menu Separators, Up: Defining Menus
22.17.1.4 Alias Menu Items
Sometimes it is useful to make menu items that use the “same”
command but with different enable conditions. The best way to do this
in Emacs now is with extended menu items; before that feature existed,
it could be done by defining alias commands and using them in menu
items. Here's an example that makes two aliases for
toggle-read-only and gives them different enable conditions:
(defalias 'make-read-only 'toggle-read-only)
(put 'make-read-only 'menu-enable '(not buffer-read-only))
(defalias 'make-writable 'toggle-read-only)
(put 'make-writable 'menu-enable 'buffer-read-only)
When using aliases in menus, often it is useful to display the
equivalent key bindings for the “real” command name, not the aliases
(which typically don't have any key bindings except for the menu
itself). To request this, give the alias symbol a non-nil
menu-alias property. Thus,
(put 'make-read-only 'menu-alias t)
(put 'make-writable 'menu-alias t)
causes menu items for make-read-only and make-writable to
show the keyboard bindings for toggle-read-only.
Next: Keyboard Menus, Previous: Defining Menus, Up: Menu Keymaps
22.17.2 Menus and the Mouse
The usual way to make a menu keymap produce a menu is to make it the definition of a prefix key. (A Lisp program can explicitly pop up a menu and receive the user's choice—see Pop-Up Menus.)
If the prefix key ends with a mouse event, Emacs handles the menu keymap by popping up a visible menu, so that the user can select a choice with the mouse. When the user clicks on a menu item, the event generated is whatever character or symbol has the binding that brought about that menu item. (A menu item may generate a series of events if the menu has multiple levels or comes from the menu bar.)
It's often best to use a button-down event to trigger the menu. Then the user can select a menu item by releasing the button.
A single keymap can appear as multiple menu panes, if you explicitly arrange for this. The way to do this is to make a keymap for each pane, then create a binding for each of those maps in the main keymap of the menu. Give each of these bindings an item string that starts with ‘@’. The rest of the item string becomes the name of the pane. See the file lisp/mouse.el for an example of this. Any ordinary bindings with ‘@’-less item strings are grouped into one pane, which appears along with the other panes explicitly created for the submaps.
X toolkit menus don't have panes; instead, they can have submenus. Every nested keymap becomes a submenu, whether the item string starts with ‘@’ or not. In a toolkit version of Emacs, the only thing special about ‘@’ at the beginning of an item string is that the ‘@’ doesn't appear in the menu item.
Multiple keymaps that define the same menu prefix key produce separate panes or separate submenus.
Next: Menu Example, Previous: Mouse Menus, Up: Menu Keymaps
22.17.3 Menus and the Keyboard
When a prefix key ending with a keyboard event (a character or function key) has a definition that is a menu keymap, the keymap operates as a keyboard menu; the user specifies the next event by choosing a menu item with the keyboard.
Emacs displays the keyboard menu with the map's overall prompt
string, followed by the alternatives (the item strings of the map's
bindings), in the echo area. If the bindings don't all fit at once,
the user can type <SPC> to see the next line of alternatives.
Successive uses of <SPC> eventually get to the end of the menu and
then cycle around to the beginning. (The variable
menu-prompt-more-char specifies which character is used for
this; <SPC> is the default.)
When the user has found the desired alternative from the menu, he or she should type the corresponding character—the one whose binding is that alternative.
This way of using menus in an Emacs-like editor was inspired by the Hierarkey system.
This variable specifies the character to use to ask to see the next line of a menu. Its initial value is 32, the code for <SPC>.
Next: Menu Bar, Previous: Keyboard Menus, Up: Menu Keymaps
22.17.4 Menu Example
Here is a complete example of defining a menu keymap. It is the definition of the ‘Replace’ submenu in the ‘Edit’ menu in the menu bar, and it uses the extended menu item format (see Extended Menu Items). First we create the keymap, and give it a name:
(defvar menu-bar-replace-menu (make-sparse-keymap "Replace"))
Next we define the menu items:
(define-key menu-bar-replace-menu [tags-repl-continue]
'(menu-item "Continue Replace" tags-loop-continue
:help "Continue last tags replace operation"))
(define-key menu-bar-replace-menu [tags-repl]
'(menu-item "Replace in tagged files" tags-query-replace
:help "Interactively replace a regexp in all tagged files"))
(define-key menu-bar-replace-menu [separator-replace-tags]
'(menu-item "--"))
;; ...
Note the symbols which the bindings are “made for”; these appear
inside square brackets, in the key sequence being defined. In some
cases, this symbol is the same as the command name; sometimes it is
different. These symbols are treated as “function keys,” but they are
not real function keys on the keyboard. They do not affect the
functioning of the menu itself, but they are “echoed” in the echo area
when the user selects from the menu, and they appear in the output of
where-is and apropos.
The menu in this example is intended for use with the mouse. If a menu is intended for use with the keyboard, that is, if it is bound to a key sequence ending with a keyboard event, then the menu items should be bound to characters or “real” function keys, that can be typed with the keyboard.
The binding whose definition is ("--") is a separator line.
Like a real menu item, the separator has a key symbol, in this case
separator-replace-tags. If one menu has two separators, they
must have two different key symbols.
Here is how we make this menu appear as an item in the parent menu:
(define-key menu-bar-edit-menu [replace]
(list 'menu-item "Replace" menu-bar-replace-menu))
Note that this incorporates the submenu keymap, which is the value of
the variable menu-bar-replace-menu, rather than the symbol
menu-bar-replace-menu itself. Using that symbol in the parent
menu item would be meaningless because menu-bar-replace-menu is
not a command.
If you wanted to attach the same replace menu to a mouse click, you can do it this way:
(define-key global-map [C-S-down-mouse-1]
menu-bar-replace-menu)
Next: Tool Bar, Previous: Menu Example, Up: Menu Keymaps
22.17.5 The Menu Bar
Most window systems allow each frame to have a menu bar—a
permanently displayed menu stretching horizontally across the top of
the frame. (In order for a frame to display a menu bar, its
menu-bar-lines parameter must be greater than zero.
See Layout Parameters.)
The items of the menu bar are the subcommands of the fake “function
key” menu-bar, as defined in the active keymaps.
To add an item to the menu bar, invent a fake “function key” of your
own (let's call it key), and make a binding for the key sequence
[menu-bar key]. Most often, the binding is a menu keymap,
so that pressing a button on the menu bar item leads to another menu.
When more than one active keymap defines the same fake function key for the menu bar, the item appears just once. If the user clicks on that menu bar item, it brings up a single, combined menu containing all the subcommands of that item—the global subcommands, the local subcommands, and the minor mode subcommands.
The variable overriding-local-map is normally ignored when
determining the menu bar contents. That is, the menu bar is computed
from the keymaps that would be active if overriding-local-map
were nil. See Active Keymaps.
Here's an example of setting up a menu bar item:
(modify-frame-parameters (selected-frame)
'((menu-bar-lines . 2)))
;; Make a menu keymap (with a prompt string)
;; and make it the menu bar item's definition.
(define-key global-map [menu-bar words]
(cons "Words" (make-sparse-keymap "Words")))
;; Define specific subcommands in this menu.
(define-key global-map
[menu-bar words forward]
'("Forward word" . forward-word))
(define-key global-map
[menu-bar words backward]
'("Backward word" . backward-word))
A local keymap can cancel a menu bar item made by the global keymap by
rebinding the same fake function key with undefined as the
binding. For example, this is how Dired suppresses the ‘Edit’ menu
bar item:
(define-key dired-mode-map [menu-bar edit] 'undefined)
Here, edit is the fake function key used by the global map for
the ‘Edit’ menu bar item. The main reason to suppress a global
menu bar item is to regain space for mode-specific items.
Normally the menu bar shows global items followed by items defined by the local maps.
This variable holds a list of fake function keys for items to display at the end of the menu bar rather than in normal sequence. The default value is
(help-menu); thus, the ‘Help’ menu item normally appears at the end of the menu bar, following local menu items.
This normal hook is run by redisplay to update the menu bar contents, before redisplaying the menu bar. You can use it to update submenus whose contents should vary. Since this hook is run frequently, we advise you to ensure that the functions it calls do not take much time in the usual case.
Next to every menu bar item, Emacs displays a key binding that runs
the same command (if such a key binding exists). This serves as a
convenient hint for users who do not know the key binding. If a
command has multiple bindings, Emacs normally displays the first one
it finds. You can specify one particular key binding by assigning an
:advertised-binding symbol property to the command. For
instance, the following tells Emacs to show C-/ for the
undo menu item:
(put 'undo :advertised-binding [?\C-/])
If the :advertised-binding property specifies a key binding
that the command does not actually have, it is ignored.
Next: Modifying Menus, Previous: Menu Bar, Up: Menu Keymaps
22.17.6 Tool bars
A tool bar is a row of icons at the top of a frame, that execute commands when you click on them—in effect, a kind of graphical menu bar.
The frame parameter tool-bar-lines (X resource ‘toolBar’)
controls how many lines' worth of height to reserve for the tool bar. A
zero value suppresses the tool bar. If the value is nonzero, and
auto-resize-tool-bars is non-nil, the tool bar expands and
contracts automatically as needed to hold the specified contents.
If the value of auto-resize-tool-bars is grow-only,
the tool bar expands automatically, but does not contract automatically.
To contract the tool bar, the user has to redraw the frame by entering
C-l.
The tool bar contents are controlled by a menu keymap attached to a
fake “function key” called tool-bar (much like the way the menu
bar is controlled). So you define a tool bar item using
define-key, like this:
(define-key global-map [tool-bar key] item)
where key is a fake “function key” to distinguish this item from other items, and item is a menu item key binding (see Extended Menu Items), which says how to display this item and how it behaves.
The usual menu keymap item properties, :visible,
:enable, :button, and :filter, are useful in
tool bar bindings and have their normal meanings. The real-binding
in the item must be a command, not a keymap; in other words, it does not
work to define a tool bar icon as a prefix key.
The :help property specifies a “help-echo” string to display
while the mouse is on that item. This is displayed in the same way as
help-echo text properties (see Help display).
In addition, you should use the :image property;
this is how you specify the image to display in the tool bar:
:imageimage- images is either a single image specification or a vector of four
image specifications. If you use a vector of four,
one of them is used, depending on circumstances:
- item 0
- Used when the item is enabled and selected.
- item 1
- Used when the item is enabled and deselected.
- item 2
- Used when the item is disabled and selected.
- item 3
- Used when the item is disabled and deselected.
If image is a single image specification, Emacs draws the tool bar button in disabled state by applying an edge-detection algorithm to the image.
The :rtl property specifies an alternative image to use for
right-to-left languages. Only the Gtk+ version of Emacs supports this
at present.
The default tool bar is defined so that items specific to editing do not
appear for major modes whose command symbol has a mode-class
property of special (see Major Mode Conventions). Major
modes may add items to the global bar by binding [tool-bar
foo] in their local map. It makes sense for some major modes to
replace the default tool bar items completely, since not many can be
accommodated conveniently, and the default bindings make this easy by
using an indirection through tool-bar-map.
By default, the global map binds
[tool-bar]as follows:(global-set-key [tool-bar] '(menu-item "tool bar" ignore :filter (lambda (ignore) tool-bar-map)))Thus the tool bar map is derived dynamically from the value of variable
tool-bar-mapand you should normally adjust the default (global) tool bar by changing that map. Major modes may replace the global bar completely by makingtool-bar-mapbuffer-local and set to a keymap containing only the desired items. Info mode provides an example.
There are two convenience functions for defining tool bar items, as follows.
This function adds an item to the tool bar by modifying
tool-bar-map. The image to use is defined by icon, which is the base name of an XPM, XBM or PBM image file to be located byfind-image. Given a value ‘"exit"’, say, exit.xpm, exit.pbm and exit.xbm would be searched for in that order on a color display. On a monochrome display, the search order is ‘.pbm’, ‘.xbm’ and ‘.xpm’. The binding to use is the command def, and key is the fake function key symbol in the prefix keymap. The remaining arguments props are additional property list elements to add to the menu item specification.To define items in some local map, bind
tool-bar-mapwithletaround calls of this function:(defvar foo-tool-bar-map (let ((tool-bar-map (make-sparse-keymap))) (tool-bar-add-item ...) ... tool-bar-map))
This function is a convenience for defining tool bar items which are consistent with existing menu bar bindings. The binding of command is looked up in the menu bar in map (default
global-map) and modified to add an image specification for icon, which is found in the same way as bytool-bar-add-item. The resulting binding is then placed intool-bar-map, so use this function only for global tool bar items.map must contain an appropriate keymap bound to
[menu-bar]. The remaining arguments props are additional property list elements to add to the menu item specification.
This function is used for making non-global tool bar items. Use it like
tool-bar-add-item-from-menuexcept that in-map specifies the local map to make the definition in. The argument from-map is like the map argument oftool-bar-add-item-from-menu.
If this variable is non-
nil, the tool bar automatically resizes to show all defined tool bar items—but not larger than a quarter of the frame's height.If the value is
grow-only, the tool bar expands automatically, but does not contract automatically. To contract the tool bar, the user has to redraw the frame by entering C-l.If Emacs is built with GTK or Nextstep, the tool bar can only show one line, so this variable has no effect.
If this variable is non-
nil, tool bar items display in raised form when the mouse moves over them.
This variable specifies an extra margin to add around tool bar items. The value is an integer, a number of pixels. The default is 4.
This variable specifies the shadow width for tool bar items. The value is an integer, a number of pixels. The default is 1.
This variable specifies the height of the border drawn below the tool bar area. An integer value specifies height as a number of pixels. If the value is one of
internal-border-width(the default) orborder-width, the tool bar border height corresponds to the corresponding frame parameter.
You can define a special meaning for clicking on a tool bar item with the shift, control, meta, etc., modifiers. You do this by setting up additional items that relate to the original item through the fake function keys. Specifically, the additional items should use the modified versions of the same fake function key used to name the original item.
Thus, if the original item was defined this way,
(define-key global-map [tool-bar shell]
'(menu-item "Shell" shell
:image (image :type xpm :file "shell.xpm")))
then here is how you can define clicking on the same tool bar image with the shift modifier:
(define-key global-map [tool-bar S-shell] 'some-command)
See Function Keys, for more information about how to add modifiers to function keys.
Previous: Tool Bar, Up: Menu Keymaps
22.17.7 Modifying Menus
When you insert a new item in an existing menu, you probably want to
put it in a particular place among the menu's existing items. If you
use define-key to add the item, it normally goes at the front of
the menu. To put it elsewhere in the menu, use define-key-after:
Define a binding in map for key, with value binding, just like
define-key, but position the binding in map after the binding for the event after. The argument key should be of length one—a vector or string with just one element. But after should be a single event type—a symbol or a character, not a sequence. The new binding goes after the binding for after. If after istor is omitted, then the new binding goes last, at the end of the keymap. However, new bindings are added before any inherited keymap.Here is an example:
(define-key-after my-menu [drink] '("Drink" . drink-command) 'eat)makes a binding for the fake function key <DRINK> and puts it right after the binding for <EAT>.
Here is how to insert an item called ‘Work’ in the ‘Signals’ menu of Shell mode, after the item
break:(define-key-after (lookup-key shell-mode-map [menu-bar signals]) [work] '("Work" . work-command) 'break)
Next: Documentation, Previous: Keymaps, Up: Top
23 Major and Minor Modes
A mode is a set of definitions that customize Emacs and can be turned on and off while you edit. There are two varieties of modes: major modes, which are mutually exclusive and used for editing particular kinds of text, and minor modes, which provide features that users can enable individually.
This chapter describes how to write both major and minor modes, how to indicate them in the mode line, and how they run hooks supplied by the user. For related topics such as keymaps and syntax tables, see Keymaps, and Syntax Tables.
Next: Major Modes, Up: Modes
23.1 Hooks
A hook is a variable where you can store a function or functions to be called on a particular occasion by an existing program. Emacs provides hooks for the sake of customization. Most often, hooks are set up in the init file (see Init File), but Lisp programs can set them also. See Standard Hooks, for a list of standard hook variables.
Most of the hooks in Emacs are normal hooks. These variables contain lists of functions to be called with no arguments. By convention, whenever the hook name ends in ‘-hook’, that tells you it is normal. We try to make all hooks normal, as much as possible, so that you can use them in a uniform way.
Every major mode function is supposed to run a normal hook called
the mode hook as the one of the last steps of initialization.
This makes it easy for a user to customize the behavior of the mode,
by overriding the buffer-local variable assignments already made by
the mode. Most minor mode functions also run a mode hook at the end.
But hooks are used in other contexts too. For example, the hook
suspend-hook runs just before Emacs suspends itself
(see Suspending Emacs).
The recommended way to add a hook function to a normal hook is by
calling add-hook (see below). The hook functions may be any of
the valid kinds of functions that funcall accepts (see What Is a Function). Most normal hook variables are initially void;
add-hook knows how to deal with this. You can add hooks either
globally or buffer-locally with add-hook.
If the hook variable's name does not end with ‘-hook’, that
indicates it is probably an abnormal hook. That means the hook
functions are called with arguments, or their return values are used
in some way. The hook's documentation says how the functions are
called. You can use add-hook to add a function to an abnormal
hook, but you must write the function to follow the hook's calling
convention.
By convention, abnormal hook names end in ‘-functions’ or ‘-hooks’. If the variable's name ends in ‘-function’, then its value is just a single function, not a list of functions.
Next: Setting Hooks, Up: Hooks
23.1.1 Running Hooks
At the appropriate times, Emacs uses the run-hooks function
and the other functions below to run particular hooks.
This function takes one or more normal hook variable names as arguments, and runs each hook in turn. Each argument should be a symbol that is a normal hook variable. These arguments are processed in the order specified.
If a hook variable has a non-
nilvalue, that value should be a list of functions.run-hookscalls all the functions, one by one, with no arguments.The hook variable's value can also be a single function—either a lambda expression or a symbol with a function definition—which
run-hookscalls. But this usage is obsolete.
This function is the way to run an abnormal hook and always call all of the hook functions. It calls each of the hook functions one by one, passing each of them the arguments args.
This function is the way to run an abnormal hook until one of the hook functions fails. It calls each of the hook functions, passing each of them the arguments args, until some hook function returns
nil. It then stops and returnsnil. If none of the hook functions returnnil, it returns a non-nilvalue.
This function is the way to run an abnormal hook until a hook function succeeds. It calls each of the hook functions, passing each of them the arguments args, until some hook function returns non-
nil. Then it stops, and returns whatever was returned by the last hook function that was called. If all hook functions returnnil, it returnsnilas well.
Previous: Running Hooks, Up: Hooks
23.1.2 Setting Hooks
Here's an example that uses a mode hook to turn on Auto Fill mode when in Lisp Interaction mode:
(add-hook 'lisp-interaction-mode-hook 'turn-on-auto-fill)
This function is the handy way to add function function to hook variable hook. You can use it for abnormal hooks as well as for normal hooks. function can be any Lisp function that can accept the proper number of arguments for hook. For example,
(add-hook 'text-mode-hook 'my-text-hook-function)adds
my-text-hook-functionto the hook calledtext-mode-hook.If function is already present in hook (comparing using
equal), thenadd-hookdoes not add it a second time.If function has a non-
nilpropertypermanent-local-hook, thenkill-all-local-variables(or changing major modes) won't delete it from the hook variable's local value.It is best to design your hook functions so that the order in which they are executed does not matter. Any dependence on the order is asking for trouble. However, the order is predictable: normally, function goes at the front of the hook list, so it will be executed first (barring another
add-hookcall). If the optional argument append is non-nil, the new hook function goes at the end of the hook list and will be executed last.
add-hookcan handle the cases where hook is void or its value is a single function; it sets or changes the value to a list of functions.If local is non-
nil, that says to add function to the buffer-local hook list instead of to the global hook list. If needed, this makes the hook buffer-local and addstto the buffer-local value. The latter acts as a flag to run the hook functions in the default value as well as in the local value.
This function removes function from the hook variable hook. It compares function with elements of hook using
equal, so it works for both symbols and lambda expressions.If local is non-
nil, that says to remove function from the buffer-local hook list instead of from the global hook list.
Next: Minor Modes, Previous: Hooks, Up: Modes
23.2 Major Modes
Major modes specialize Emacs for editing particular kinds of text. Each buffer has only one major mode at a time. For each major mode there is a function to switch to that mode in the current buffer; its name should end in ‘-mode’. These functions work by setting buffer-local variable bindings and other data associated with the buffer, such as a local keymap. The effect lasts until you switch to another major mode in the same buffer.
Next: Major Mode Conventions, Up: Major Modes
23.2.1 Major Mode Basics
The least specialized major mode is called Fundamental mode.
This mode has no mode-specific definitions or variable settings, so each
Emacs command behaves in its default manner, and each option is in its
default state. All other major modes redefine various keys and options.
For example, Lisp Interaction mode provides special key bindings for
C-j (eval-print-last-sexp), <TAB>
(lisp-indent-line), and other keys.
When you need to write several editing commands to help you perform a specialized editing task, creating a new major mode is usually a good idea. In practice, writing a major mode is easy (in contrast to writing a minor mode, which is often difficult).
If the new mode is similar to an old one, it is often unwise to
modify the old one to serve two purposes, since it may become harder
to use and maintain. Instead, copy and rename an existing major mode
definition and alter the copy—or use the define-derived-mode
macro to define a derived mode (see Derived Modes). For
example, Rmail Edit mode is a major mode that is very similar to Text
mode except that it provides two additional commands. Its definition
is distinct from that of Text mode, but uses that of Text mode.
Even if the new mode is not an obvious derivative of any other mode,
we recommend to use define-derived-mode, since it automatically
enforces the most important coding conventions for you.
For a very simple programming language major mode that handles
comments and fontification, you can use define-generic-mode.
See Generic Modes.
Rmail Edit mode offers an example of changing the major mode temporarily for a buffer, so it can be edited in a different way (with ordinary Emacs commands rather than Rmail commands). In such cases, the temporary major mode usually provides a command to switch back to the buffer's usual mode (Rmail mode, in this case). You might be tempted to present the temporary redefinitions inside a recursive edit and restore the usual ones when the user exits; but this is a bad idea because it constrains the user's options when it is done in more than one buffer: recursive edits must be exited most-recently-entered first. Using an alternative major mode avoids this limitation. See Recursive Editing.
The standard GNU Emacs Lisp library directory tree contains the code for several major modes, in files such as text-mode.el, texinfo.el, lisp-mode.el, c-mode.el, and rmail.el. They are found in various subdirectories of the lisp directory. You can study these libraries to see how modes are written. Text mode is perhaps the simplest major mode aside from Fundamental mode. Rmail mode is a complicated and specialized mode.
Next: Auto Major Mode, Previous: Major Mode Basics, Up: Major Modes
23.2.2 Major Mode Conventions
The code for existing major modes follows various coding conventions, including conventions for local keymap and syntax table initialization, global names, and hooks. Please follow these conventions when you define a new major mode. (Fundamental mode is an exception to many of these conventions, because its definition is to present the global state of Emacs.)
This list of conventions is only partial, because each major mode should aim for consistency in general with other Emacs major modes. This makes Emacs as a whole more coherent. It is impossible to list here all the possible points where this issue might come up; if the Emacs developers point out an area where your major mode deviates from the usual conventions, please make it compatible.
- Define a command whose name ends in ‘-mode’, with no arguments, that switches to the new mode in the current buffer. This command should set up the keymap, syntax table, and buffer-local variables in an existing buffer, without changing the buffer's contents.
- Write a documentation string for this command that describes the
special commands available in this mode. C-h m
(
describe-mode) in your mode will display this string.The documentation string may include the special documentation substrings, ‘\[command]’, ‘\{keymap}’, and ‘\<keymap>’, which enable the documentation to adapt automatically to the user's own key bindings. See Keys in Documentation.
- The major mode command should start by calling
kill-all-local-variables. This runs the normal hookchange-major-mode-hook, then gets rid of the buffer-local variables of the major mode previously in effect. See Creating Buffer-Local. - The major mode command should set the variable
major-modeto the major mode command symbol. This is howdescribe-modediscovers which documentation to print. - The major mode command should set the variable
mode-nameto the “pretty” name of the mode, usually a string (but see Mode Line Data, for other possible forms). The name of the mode appears in the mode line. - Since all global names are in the same name space, all the global variables, constants, and functions that are part of the mode should have names that start with the major mode name (or with an abbreviation of it if the name is long). See Coding Conventions.
- In a major mode for editing some kind of structured text, such as a
programming language, indentation of text according to structure is
probably useful. So the mode should set
indent-line-functionto a suitable function, and probably customize other variables for indentation. See Auto-Indentation. - The major mode should usually have its own keymap, which is used as the
local keymap in all buffers in that mode. The major mode command should
call
use-local-mapto install this local map. See Active Keymaps, for more information.This keymap should be stored permanently in a global variable named modename
-mode-map. Normally the library that defines the mode sets this variable.See Tips for Defining, for advice about how to write the code to set up the mode's keymap variable.
- The key sequences bound in a major mode keymap should usually start with
C-c, followed by a control character, a digit, or {,
}, <, >, : or ;. The other punctuation
characters are reserved for minor modes, and ordinary letters are
reserved for users.
A major mode can also rebind the keys M-n, M-p and M-s. The bindings for M-n and M-p should normally be some kind of “moving forward and backward,” but this does not necessarily mean cursor motion.
It is legitimate for a major mode to rebind a standard key sequence if it provides a command that does “the same job” in a way better suited to the text this mode is used for. For example, a major mode for editing a programming language might redefine C-M-a to “move to the beginning of a function” in a way that works better for that language.
It is also legitimate for a major mode to rebind a standard key sequence whose standard meaning is rarely useful in that mode. For instance, minibuffer modes rebind M-r, whose standard meaning is rarely of any use in the minibuffer. Major modes such as Dired or Rmail that do not allow self-insertion of text can reasonably redefine letters and other printing characters as special commands.
- Major modes for editing text should not define <RET> to do anything other than insert a newline. However, it is ok for specialized modes for text that users don't directly edit, such as Dired and Info modes, to redefine <RET> to do something entirely different.
- Major modes should not alter options that are primarily a matter of user preference, such as whether Auto-Fill mode is enabled. Leave this to each user to decide. However, a major mode should customize other variables so that Auto-Fill mode will work usefully if the user decides to use it.
- The mode may have its own syntax table or may share one with other
related modes. If it has its own syntax table, it should store this in
a variable named modename
-mode-syntax-table. See Syntax Tables. - If the mode handles a language that has a syntax for comments, it should set the variables that define the comment syntax. See Options Controlling Comments.
- The mode may have its own abbrev table or may share one with other
related modes. If it has its own abbrev table, it should store this
in a variable named modename
-mode-abbrev-table. If the major mode command defines any abbrevs itself, it should passtfor the system-flag argument todefine-abbrev. See Defining Abbrevs. - The mode should specify how to do highlighting for Font Lock mode, by
setting up a buffer-local value for the variable
font-lock-defaults(see Font Lock Mode). - The mode should specify how Imenu should find the definitions or
sections of a buffer, by setting up a buffer-local value for the
variable
imenu-generic-expression, for the two variablesimenu-prev-index-position-functionandimenu-extract-index-name-function, or for the variableimenu-create-index-function(see Imenu). - The mode can specify a local value for
eldoc-documentation-functionto tell ElDoc mode how to handle this mode. - The mode can specify how to complete various keywords by adding
to the special hook
completion-at-point-functions. - Use
defvarordefcustomto set mode-related variables, so that they are not reinitialized if they already have a value. (Such reinitialization could discard customizations made by the user.) - To make a buffer-local binding for an Emacs customization variable, use
make-local-variablein the major mode command, notmake-variable-buffer-local. The latter function would make the variable local to every buffer in which it is subsequently set, which would affect buffers that do not use this mode. It is undesirable for a mode to have such global effects. See Buffer-Local Variables.With rare exceptions, the only reasonable way to use
make-variable-buffer-localin a Lisp package is for a variable which is used only within that package. Using it on a variable used by other packages would interfere with them. - Each major mode should have a normal mode hook named
modename
-mode-hook. The very last thing the major mode command should do is to callrun-mode-hooks. This runs the mode hook, and then runs the normal hookafter-change-major-mode-hook. See Mode Hooks. - The major mode command may start by calling some other major mode
command (called the parent mode) and then alter some of its
settings. A mode that does this is called a derived mode. The
recommended way to define one is to use the
define-derived-modemacro, but this is not required. Such a mode should call the parent mode command inside adelay-mode-hooksform. (Usingdefine-derived-modedoes this automatically.) See Derived Modes, and Mode Hooks. - If something special should be done if the user switches a buffer from
this mode to any other major mode, this mode can set up a buffer-local
value for
change-major-mode-hook(see Creating Buffer-Local). - If this mode is appropriate only for specially-prepared text, then the
major mode command symbol should have a property named
mode-classwith valuespecial, put on as follows:(put 'funny-mode 'mode-class 'special)
This tells Emacs that new buffers created while the current buffer is in Funny mode should not inherit Funny mode, in case the default value of
major-modeisnil. Modes such as Dired, Rmail, and Buffer List use this feature.The
define-derived-modemacro automatically marks the derived mode as special if the parent mode is special. The special modespecial-modeprovides a convenient parent for other special modes to inherit from; it setsbuffer-read-onlytot, and does little else. - If you want to make the new mode the default for files with certain
recognizable names, add an element to
auto-mode-alistto select the mode for those file names (see Auto Major Mode). If you define the mode command to autoload, you should add this element in the same file that callsautoload. If you use an autoload cookie for the mode command, you can also use an autoload cookie for the form that adds the element (see autoload cookie). If you do not autoload the mode command, it is sufficient to add the element in the file that contains the mode definition. - In the comments that document the file, you should provide a sample
autoloadform and an example of how to add toauto-mode-alist, that users can include in their init files (see Init File). - The top-level forms in the file defining the mode should be written so that they may be evaluated more than once without adverse consequences. Even if you never load the file more than once, someone else will.
Next: Mode Help, Previous: Major Mode Conventions, Up: Major Modes
23.2.3 How Emacs Chooses a Major Mode
Based on information in the file name or in the file itself, Emacs automatically selects a major mode for the new buffer when a file is visited. It also processes local variables specified in the file text.
Fundamental mode is a major mode that is not specialized for anything in particular. Other major modes are defined in effect by comparison with this one—their definitions say what to change, starting from Fundamental mode. The
fundamental-modefunction does not run any mode hooks; you're not supposed to customize it. (If you want Emacs to behave differently in Fundamental mode, change the global state of Emacs.)
This function establishes the proper major mode and buffer-local variable bindings for the current buffer. First it calls
set-auto-mode(see below), then it runshack-local-variablesto parse, and bind or evaluate as appropriate, the file's local variables (see File Local Variables).If the find-file argument to
normal-modeis non-nil,normal-modeassumes that thefind-filefunction is calling it. In this case, it may process local variables in the ‘-*-’ line or at the end of the file. The variableenable-local-variablescontrols whether to do so. See Local Variables in Files, for the syntax of the local variables section of a file.If you run
normal-modeinteractively, the argument find-file is normallynil. In this case,normal-modeunconditionally processes any file local variables.If
normal-modeprocesses the local variables list and this list specifies a major mode, that mode overrides any mode chosen byset-auto-mode. If neitherset-auto-modenorhack-local-variablesspecify a major mode, the buffer stays in the major mode determined by the default value ofmajor-mode(see below).
normal-modeusescondition-casearound the call to the major mode function, so errors are caught and reported as a ‘File mode specification error’, followed by the original error message.
This function selects the major mode that is appropriate for the current buffer. It bases its decision (in order of precedence) on the ‘-*-’ line, on the ‘#!’ line (using
interpreter-mode-alist), on the text at the beginning of the buffer (usingmagic-mode-alist), and finally on the visited file name (usingauto-mode-alist). See How Major Modes are Chosen. However, this function does not look for the ‘mode:’ local variable near the end of a file; thehack-local-variablesfunction does that. Ifenable-local-variablesisnil,set-auto-modedoes not check the ‘-*-’ line for a mode tag either.If keep-mode-if-same is non-
nil, this function does not call the mode command if the buffer is already in the proper major mode. For instance,set-visited-file-namesets this totto avoid killing buffer local variables that the user may have set.
The buffer-local value of this variable holds the major mode currently active. The default value of this variable holds the default major mode for new buffers. The standard default value is
fundamental-mode.If the default value of
major-modeisnil, Emacs uses the (previously) current buffer's major mode as the default major mode of a new buffer. However, if that major mode symbol has amode-classproperty with valuespecial, then it is not used for new buffers; Fundamental mode is used instead. The modes that have this property are those such as Dired and Rmail that are useful only with text that has been specially prepared.
This function sets the major mode of buffer to the default value of
major-mode; if that isnil, it uses the current buffer's major mode (if that is suitable). As an exception, if buffer's name is ‘*scratch*’, it sets the mode toinitial-major-mode.The low-level primitives for creating buffers do not use this function, but medium-level commands such as
switch-to-bufferandfind-file-noselectuse it whenever they create buffers.
The value of this variable determines the major mode of the initial ‘*scratch*’ buffer. The value should be a symbol that is a major mode command. The default value is
lisp-interaction-mode.
This variable specifies major modes to use for scripts that specify a command interpreter in a ‘#!’ line. Its value is an alist with elements of the form
(interpreter.mode); for example,("perl" . perl-mode)is one element present by default. The element says to use mode mode if the file specifies an interpreter which matches interpreter.
This variable's value is an alist with elements of the form
(regexp.function), where regexp is a regular expression and function is a function ornil. After visiting a file,set-auto-modecalls function if the text at the beginning of the buffer matches regexp and function is non-nil; if function isnil,auto-mode-alistgets to decide the mode.
This works like
magic-mode-alist, except that it is handled only ifauto-mode-alistdoes not specify a mode for this file.
This variable contains an association list of file name patterns (regular expressions) and corresponding major mode commands. Usually, the file name patterns test for suffixes, such as ‘.el’ and ‘.c’, but this need not be the case. An ordinary element of the alist looks like
(regexp.mode-function).For example,
(("\\`/tmp/fol/" . text-mode) ("\\.texinfo\\'" . texinfo-mode) ("\\.texi\\'" . texinfo-mode) ("\\.el\\'" . emacs-lisp-mode) ("\\.c\\'" . c-mode) ("\\.h\\'" . c-mode) ...)When you visit a file whose expanded file name (see File Name Expansion), with version numbers and backup suffixes removed using
file-name-sans-versions(see File Name Components), matches a regexp,set-auto-modecalls the corresponding mode-function. This feature enables Emacs to select the proper major mode for most files.If an element of
auto-mode-alisthas the form(regexp functiont), then after calling function, Emacs searchesauto-mode-alistagain for a match against the portion of the file name that did not match before. This feature is useful for uncompression packages: an entry of the form("\\.gz\\'"functiont)can uncompress the file and then put the uncompressed file in the proper mode according to the name sans ‘.gz’.Here is an example of how to prepend several pattern pairs to
auto-mode-alist. (You might use this sort of expression in your init file.)(setq auto-mode-alist (append ;; File name (within directory) starts with a dot. '(("/\\.[^/]*\\'" . fundamental-mode) ;; File name has no dot. ("/[^\\./]*\\'" . fundamental-mode) ;; File name ends in ‘.C’. ("\\.C\\'" . c++-mode)) auto-mode-alist))
Next: Derived Modes, Previous: Auto Major Mode, Up: Major Modes
23.2.4 Getting Help about a Major Mode
The describe-mode function is used to provide information
about major modes. It is normally called with C-h m. The
describe-mode function uses the value of major-mode,
which is why every major mode function needs to set the
major-mode variable.
This function displays the documentation of the current major mode.
The
describe-modefunction calls thedocumentationfunction using the value ofmajor-modeas an argument. Thus, it displays the documentation string of the major mode function. (See Accessing Documentation.)
This buffer-local variable holds the symbol for the current buffer's major mode. This symbol should have a function definition that is the command to switch to that major mode. The
describe-modefunction uses the documentation string of the function as the documentation of the major mode.
Next: Generic Modes, Previous: Mode Help, Up: Major Modes
23.2.5 Defining Derived Modes
The recommended way to define a new major mode is to derive it
from an existing one using define-derived-mode. If there is no
closely related mode, you can inherit from text-mode,
special-mode, or in the worst case fundamental-mode.
This macro defines variant as a major mode command, using name as the string form of the mode name. variant and parent should be unquoted symbols.
The new command variant is defined to call the function parent, then override certain aspects of that parent mode:
- The new mode has its own sparse keymap, named variant
-map.define-derived-modemakes the parent mode's keymap the parent of the new map, unless variant-mapis already set and already has a parent.- The new mode has its own syntax table, kept in the variable variant
-syntax-table, unless you override this using the:syntax-tablekeyword (see below).define-derived-modemakes the parent mode's syntax-table the parent of variant-syntax-table, unless the latter is already set and already has a parent different from the standard syntax table.- The new mode has its own abbrev table, kept in the variable variant
-abbrev-table, unless you override this using the:abbrev-tablekeyword (see below).- The new mode has its own mode hook, variant
-hook. It runs this hook, after running the hooks of its ancestor modes, withrun-mode-hooks, as the last thing it does. See Mode Hooks.In addition, you can specify how to override other aspects of parent with body. The command variant evaluates the forms in body after setting up all its usual overrides, just before running the mode hooks.
If parent has a non-
nilmode-classsymbol property, thendefine-derived-modesets themode-classproperty of variant to the same value. This ensures, for example, that if parent is a special mode, then variant is also a special mode (see Major Mode Conventions).You can also specify
nilfor parent. This gives the new mode no parent. Thendefine-derived-modebehaves as described above, but, of course, omits all actions connected with parent.The argument docstring specifies the documentation string for the new mode.
define-derived-modeadds some general information about the mode's hook, followed by the mode's keymap, at the end of this docstring. If you omit docstring,define-derived-modegenerates a documentation string.The keyword-args are pairs of keywords and values. The values are evaluated. The following keywords are currently supported:
:syntax-table- You can use this to explicitly specify a syntax table for the new mode. If you specify a
nilvalue, the new mode uses the same syntax table as parent, or the standard syntax table if parent isnil. (Note that this does not follow the convention used for non-keyword arguments that anilvalue is equivalent with not specifying the argument.):abbrev-table- You can use this to explicitly specify an abbrev table for the new mode. If you specify a
nilvalue, the new mode uses the same abbrev table as parent, orfundamental-mode-abbrev-tableif parent isnil. (Again, anilvalue is not equivalent to not specifying this keyword.):group- If this is specified, the value should be the customization group for this mode. (Not all major modes have one.) Only the (still experimental and unadvertised) command
customize-modecurrently uses this.define-derived-modedoes not automatically define the specified customization group.Here is a hypothetical example:
(define-derived-mode hypertext-mode text-mode "Hypertext" "Major mode for hypertext. \\{hypertext-mode-map}" (setq case-fold-search nil)) (define-key hypertext-mode-map [down-mouse-3] 'do-hyper-link)Do not write an
interactivespec in the definition;define-derived-modedoes that automatically.
Next: Mode Hooks, Previous: Derived Modes, Up: Major Modes
23.2.6 Generic Modes
Generic modes are simple major modes with basic support for
comment syntax and Font Lock mode. To define a generic mode, use the
macro define-generic-mode. See the file generic-x.el
for some examples of the use of define-generic-mode.
This macro defines a generic mode command named mode (a symbol, not quoted). The optional argument docstring is the documentation for the mode command. If you do not supply it,
define-generic-modegenerates one by default.The argument comment-list is a list in which each element is either a character, a string of one or two characters, or a cons cell. A character or a string is set up in the mode's syntax table as a “comment starter.” If the entry is a cons cell, the car is set up as a “comment starter” and the cdr as a “comment ender.” (Use
nilfor the latter if you want comments to end at the end of the line.) Note that the syntax table mechanism has limitations about what comment starters and enders are actually possible. See Syntax Tables.The argument keyword-list is a list of keywords to highlight with
font-lock-keyword-face. Each keyword should be a string. Meanwhile, font-lock-list is a list of additional expressions to highlight. Each element of this list should have the same form as an element offont-lock-keywords. See Search-based Fontification.The argument auto-mode-list is a list of regular expressions to add to the variable
auto-mode-alist. They are added by the execution of thedefine-generic-modeform, not by expanding the macro call.Finally, function-list is a list of functions for the mode command to call for additional setup. It calls these functions just before it runs the mode hook variable mode
-hook.
Next: Example Major Modes, Previous: Generic Modes, Up: Major Modes
23.2.7 Mode Hooks
Every major mode function should finish by running its mode hook and
the mode-independent normal hook after-change-major-mode-hook.
It does this by calling run-mode-hooks. If the major mode is a
derived mode, that is if it calls another major mode (the parent mode)
in its body, it should do this inside delay-mode-hooks so that
the parent won't run these hooks itself. Instead, the derived mode's
call to run-mode-hooks runs the parent's mode hook too.
See Major Mode Conventions.
Emacs versions before Emacs 22 did not have delay-mode-hooks.
When user-implemented major modes have not been updated to use it,
they won't entirely follow these conventions: they may run the
parent's mode hook too early, or fail to run
after-change-major-mode-hook. If you encounter such a major
mode, please correct it to follow these conventions.
When you defined a major mode using define-derived-mode, it
automatically makes sure these conventions are followed. If you
define a major mode “by hand,” not using define-derived-mode,
use the following functions to handle these conventions automatically.
Major modes should run their mode hook using this function. It is similar to
run-hooks(see Hooks), but it also runsafter-change-major-mode-hook.When this function is called during the execution of a
delay-mode-hooksform, it does not run the hooks immediately. Instead, it arranges for the next call torun-mode-hooksto run them.
When one major mode command calls another, it should do so inside of
delay-mode-hooks.This macro executes body, but tells all
run-mode-hookscalls during the execution of body to delay running their hooks. The hooks will actually run during the next call torun-mode-hooksafter the end of thedelay-mode-hooksconstruct.
This is a normal hook run by
run-mode-hooks. It is run at the very end of every properly-written major mode function.
Previous: Mode Hooks, Up: Major Modes
23.2.8 Major Mode Examples
Text mode is perhaps the simplest mode besides Fundamental mode. Here are excerpts from text-mode.el that illustrate many of the conventions listed above:
;; Create the syntax table for this mode. (defvar text-mode-syntax-table (let ((st (make-syntax-table))) (modify-syntax-entry ?\" ". " st) (modify-syntax-entry ?\\ ". " st) ;; Add `p' so M-c on `hello' leads to `Hello', not `hello'. (modify-syntax-entry ?' "w p" st) st) "Syntax table used while in `text-mode'.") ;; Create the keymap for this mode. (defvar text-mode-map (let ((map (make-sparse-keymap))) (define-key map "\e\t" 'ispell-complete-word) (define-key map "\es" 'center-line) (define-key map "\eS" 'center-paragraph) map) "Keymap for `text-mode'. Many other modes, such as Mail mode, Outline mode and Indented Text mode, inherit all the commands defined in this map.")
Here is how the actual mode command is defined now:
(define-derived-mode text-mode nil "Text"
"Major mode for editing text written for humans to read.
In this mode, paragraphs are delimited only by blank or white lines.
You can thus get the full benefit of adaptive filling
(see the variable `adaptive-fill-mode').
\\{text-mode-map}
Turning on Text mode runs the normal hook `text-mode-hook'."
(set (make-local-variable 'text-mode-variant) t)
;; These two lines are a feature added recently.
(set (make-local-variable 'require-final-newline)
mode-require-final-newline)
(set (make-local-variable 'indent-line-function) 'indent-relative))
(The last line is redundant nowadays, since indent-relative is
the default value, and we'll delete it in a future version.)
Here is how it was defined formerly, before
define-derived-mode existed:
;; This isn't needed nowadays, since define-derived-mode does it.
(define-abbrev-table 'text-mode-abbrev-table ()
"Abbrev table used while in text mode.")
(defun text-mode ()
"Major mode for editing text intended for humans to read...
Special commands: \\{text-mode-map}
Turning on text-mode runs the hook `text-mode-hook'."
(interactive)
(kill-all-local-variables)
(use-local-map text-mode-map)
(setq local-abbrev-table text-mode-abbrev-table)
(set-syntax-table text-mode-syntax-table)
;; These four lines are absent from the current version
;; not because this is done some other way, but rather
;; because nowadays Text mode uses the normal definition of paragraphs.
(set (make-local-variable 'paragraph-start)
(concat "[ \t]*$\\|" page-delimiter))
(set (make-local-variable 'paragraph-separate) paragraph-start)
(set (make-local-variable 'indent-line-function) 'indent-relative-maybe)
(setq mode-name "Text")
(setq major-mode 'text-mode)
(run-mode-hooks 'text-mode-hook)) ; Finally, this permits the user to
; customize the mode with a hook.
The three Lisp modes (Lisp mode, Emacs Lisp mode, and Lisp Interaction mode) have more features than Text mode and the code is correspondingly more complicated. Here are excerpts from lisp-mode.el that illustrate how these modes are written.
;; Create mode-specific table variables.
(defvar lisp-mode-syntax-table nil "")
(defvar lisp-mode-abbrev-table nil "")
(defvar emacs-lisp-mode-syntax-table
(let ((table (make-syntax-table)))
(let ((i 0))
;; Set syntax of chars up to ‘0’ to say they are
;; part of symbol names but not words.
;; (The digit ‘0’ is 48 in the ASCII character set.)
(while (< i ?0)
(modify-syntax-entry i "_ " table)
(setq i (1+ i)))
;; ... similar code follows for other character ranges.
;; Then set the syntax codes for characters that are special in Lisp.
(modify-syntax-entry ? " " table)
(modify-syntax-entry ?\t " " table)
(modify-syntax-entry ?\f " " table)
(modify-syntax-entry ?\n "> " table)
;; Give CR the same syntax as newline, for selective-display.
(modify-syntax-entry ?\^m "> " table)
(modify-syntax-entry ?\; "< " table)
(modify-syntax-entry ?` "' " table)
(modify-syntax-entry ?' "' " table)
(modify-syntax-entry ?, "' " table)
;; ...likewise for many other characters...
(modify-syntax-entry ?\( "() " table)
(modify-syntax-entry ?\) ")( " table)
(modify-syntax-entry ?\[ "(] " table)
(modify-syntax-entry ?\] ")[ " table))
table))
;; Create an abbrev table for lisp-mode.
(define-abbrev-table 'lisp-mode-abbrev-table ())
The three modes for Lisp share much of their code. For instance, each calls the following function to set various variables:
(defun lisp-mode-variables (lisp-syntax)
(when lisp-syntax
(set-syntax-table lisp-mode-syntax-table))
(setq local-abbrev-table lisp-mode-abbrev-table)
...
In Lisp and most programming languages, we want the paragraph
commands to treat only blank lines as paragraph separators. And the
modes should understand the Lisp conventions for comments. The rest of
lisp-mode-variables sets this up:
(set (make-local-variable 'paragraph-start) (concat page-delimiter "\\|$" ))
(set (make-local-variable 'paragraph-separate) paragraph-start)
...
(set (make-local-variable 'comment-indent-function) 'lisp-comment-indent))
...
Each of the different Lisp modes has a slightly different keymap. For
example, Lisp mode binds C-c C-z to run-lisp, but the other
Lisp modes do not. However, all Lisp modes have some commands in
common. The following code sets up the common commands:
(defvar shared-lisp-mode-map
(let ((map (make-sparse-keymap)))
(define-key shared-lisp-mode-map "\e\C-q" 'indent-sexp)
(define-key shared-lisp-mode-map "\177"
'backward-delete-char-untabify)
map)
"Keymap for commands shared by all sorts of Lisp modes.")
And here is the code to set up the keymap for Lisp mode:
(defvar lisp-mode-map
(let ((map (make-sparse-keymap)))
(set-keymap-parent map shared-lisp-mode-map)
(define-key map "\e\C-x" 'lisp-eval-defun)
(define-key map "\C-c\C-z" 'run-lisp)
map)
"Keymap for ordinary Lisp mode...")
Finally, here is the complete major mode function definition for Lisp mode.
(defun lisp-mode ()
"Major mode for editing Lisp code for Lisps other than GNU Emacs Lisp.
Commands:
Delete converts tabs to spaces as it moves back.
Blank lines separate paragraphs. Semicolons start comments.
\\{lisp-mode-map}
Note that `run-lisp' may be used either to start an inferior Lisp job
or to switch back to an existing one.
Entry to this mode calls the value of `lisp-mode-hook'
if that value is non-nil."
(interactive)
(kill-all-local-variables)
(use-local-map lisp-mode-map) ; Select the mode's keymap.
(setq major-mode 'lisp-mode) ; This is how describe-mode
; finds out what to describe.
(setq mode-name "Lisp") ; This goes into the mode line.
(lisp-mode-variables t) ; This defines various variables.
(set (make-local-variable 'comment-start-skip)
"\\(\\(^\\|[^\\\\\n]\\)\\(\\\\\\\\\\)*\\)\\(;+\\|#|\\) *")
(set (make-local-variable 'font-lock-keywords-case-fold-search) t)
(setq imenu-case-fold-search t)
(set-syntax-table lisp-mode-syntax-table)
(run-mode-hooks 'lisp-mode-hook)) ; This permits the user to use a
; hook to customize the mode.
Next: Mode Line Format, Previous: Major Modes, Up: Modes
23.3 Minor Modes
A minor mode provides features that users may enable or disable independently of the choice of major mode. Minor modes can be enabled individually or in combination. Minor modes would be better named “generally available, optional feature modes,” except that such a name would be unwieldy.
A minor mode is not usually meant as a variation of a single major mode. Usually they are general and can apply to many major modes. For example, Auto Fill mode works with any major mode that permits text insertion. To be general, a minor mode must be effectively independent of the things major modes do.
A minor mode is often much more difficult to implement than a major mode. One reason is that you should be able to activate and deactivate minor modes in any order. A minor mode should be able to have its desired effect regardless of the major mode and regardless of the other minor modes in effect.
Often the biggest problem in implementing a minor mode is finding a way to insert the necessary hook into the rest of Emacs. Minor mode keymaps make this easier than it used to be.
Next: Keymaps and Minor Modes, Up: Minor Modes
23.3.1 Conventions for Writing Minor Modes
There are conventions for writing minor modes just as there are for major modes. Several of the major mode conventions apply to minor modes as well: those regarding the name of the mode initialization function, the names of global symbols, the use of a hook at the end of the initialization function, and the use of keymaps and other tables.
In addition, there are several conventions that are specific to
minor modes. (The easiest way to follow all the conventions is to use
the macro define-minor-mode; Defining Minor Modes.)
- Make a variable whose name ends in ‘-mode’ to control the minor
mode. We call this the mode variable. The minor mode command
should set this variable (
nilto disable; anything else to enable).If possible, implement the mode so that setting the variable automatically enables or disables the mode. Then the minor mode command does not need to do anything except set the variable.
This variable is used in conjunction with the
minor-mode-alistto display the minor mode name in the mode line. It can also enable or disable a minor mode keymap. Individual commands or hooks can also check the variable's value.If you want the minor mode to be enabled separately in each buffer, make the variable buffer-local.
- Define a command whose name is the same as the mode variable.
Its job is to enable and disable the mode by setting the variable.
The command should accept one optional argument. If the argument is
nil, it should toggle the mode (turn it on if it is off, and off if it is on). It should turn the mode on if the argument is a positive integer, the symbolt, or a list whose car is one of those. It should turn the mode off if the argument is a negative integer or zero, the symbol-, or a list whose car is a negative integer or zero. The meaning of other arguments is not specified.Here is an example taken from the definition of
transient-mark-mode. It shows the use oftransient-mark-modeas a variable that enables or disables the mode's behavior, and also shows the proper way to toggle, enable or disable the minor mode based on the raw prefix argument value.(setq transient-mark-mode (if (null arg) (not transient-mark-mode) (> (prefix-numeric-value arg) 0))) - Add an element to
minor-mode-alistfor each minor mode (see Definition of minor-mode-alist), if you want to indicate the minor mode in the mode line. This element should be a list of the following form:(mode-variable string)
Here mode-variable is the variable that controls enabling of the minor mode, and string is a short string, starting with a space, to represent the mode in the mode line. These strings must be short so that there is room for several of them at once.
When you add an element to
minor-mode-alist, useassqto check for an existing element, to avoid duplication. For example:(unless (assq 'leif-mode minor-mode-alist) (setq minor-mode-alist (cons '(leif-mode " Leif") minor-mode-alist)))or like this, using
add-to-list(see List Variables):(add-to-list 'minor-mode-alist '(leif-mode " Leif"))
Global minor modes distributed with Emacs should if possible support
enabling and disabling via Custom (see Customization). To do this,
the first step is to define the mode variable with defcustom, and
specify :type 'boolean.
If just setting the variable is not sufficient to enable the mode, you
should also specify a :set method which enables the mode by
invoking the mode command. Note in the variable's documentation string that
setting the variable other than via Custom may not take effect.
Also mark the definition with an autoload cookie (see autoload cookie),
and specify a :require so that customizing the variable will load
the library that defines the mode. This will copy suitable definitions
into loaddefs.el so that users can use customize-option to
enable the mode. For example:
;;;###autoload
(defcustom msb-mode nil
"Toggle msb-mode.
Setting this variable directly does not take effect;
use either \\[customize] or the function `msb-mode'."
:set 'custom-set-minor-mode
:initialize 'custom-initialize-default
:version "20.4"
:type 'boolean
:group 'msb
:require 'msb)
Next: Defining Minor Modes, Previous: Minor Mode Conventions, Up: Minor Modes
23.3.2 Keymaps and Minor Modes
Each minor mode can have its own keymap, which is active when the mode
is enabled. To set up a keymap for a minor mode, add an element to the
alist minor-mode-map-alist. See Definition of minor-mode-map-alist.
One use of minor mode keymaps is to modify the behavior of certain
self-inserting characters so that they do something else as well as
self-insert. In general, this is the only way to do that, since the
facilities for customizing self-insert-command are limited to
special cases (designed for abbrevs and Auto Fill mode). (Do not try
substituting your own definition of self-insert-command for the
standard one. The editor command loop handles this function specially.)
The key sequences bound in a minor mode should consist of C-c followed by one of .,/?`'"[]\|~!#$%^&*()-_+=. (The other punctuation characters are reserved for major modes.)
Previous: Keymaps and Minor Modes, Up: Minor Modes
23.3.3 Defining Minor Modes
The macro define-minor-mode offers a convenient way of
implementing a mode in one self-contained definition.
This macro defines a new minor mode whose name is mode (a symbol). It defines a command named mode to toggle the minor mode, with doc as its documentation string. It also defines a variable named mode, which is set to
tornilby enabling or disabling the mode. The variable is initialized to init-value. Except in unusual circumstances (see below), this value must benil.The string lighter says what to display in the mode line when the mode is enabled; if it is
nil, the mode is not displayed in the mode line.The optional argument keymap specifies the keymap for the minor mode. If non-
nil, it should be a variable name (whose value is a keymap), a keymap, or an alist of the form(key-sequence . definition)where each key-sequence and definition are arguments suitable for passing to
define-key(see Changing Key Bindings). If keymap is a keymap or an alist, this also defines the variable mode-map.The above three arguments init-value, lighter, and keymap can be (partially) omitted when keyword-args are used. The keyword-args consist of keywords followed by corresponding values. A few keywords have special meanings:
:groupgroup- Custom group name to use in all generated
defcustomforms. Defaults to mode without the possible trailing ‘-mode’. Warning: don't use this default group name unless you have written adefgroupto define that group properly. See Group Definitions.:globalglobal- If non-
nil, this specifies that the minor mode should be global rather than buffer-local. It defaults tonil.One of the effects of making a minor mode global is that the mode variable becomes a customization variable. Toggling it through the Custom interface turns the mode on and off, and its value can be saved for future Emacs sessions (see Saving Customizations. For the saved variable to work, you should ensure that the
define-minor-modeform is evaluated each time Emacs starts; for packages that are not part of Emacs, the easiest way to do this is to specify a:requirekeyword.:init-valueinit-value- This is equivalent to specifying init-value positionally.
:lighterlighter- This is equivalent to specifying lighter positionally.
:keymapkeymap- This is equivalent to specifying keymap positionally.
Any other keyword arguments are passed directly to the
defcustomgenerated for the variable mode.The command named mode first performs the standard actions such as setting the variable named mode and then executes the body forms, if any. It finishes by running the mode hook variable mode
-hook.
The initial value must be nil except in cases where (1) the
mode is preloaded in Emacs, or (2) it is painless for loading to
enable the mode even though the user did not request it. For
instance, if the mode has no effect unless something else is enabled,
and will always be loaded by that time, enabling it by default is
harmless. But these are unusual circumstances. Normally, the
initial value must be nil.
The name easy-mmode-define-minor-mode is an alias
for this macro.
Here is an example of using define-minor-mode:
(define-minor-mode hungry-mode
"Toggle Hungry mode.
With no argument, this command toggles the mode.
Non-null prefix argument turns on the mode.
Null prefix argument turns off the mode.
When Hungry mode is enabled, the control delete key
gobbles all preceding whitespace except the last.
See the command \\[hungry-electric-delete]."
;; The initial value.
nil
;; The indicator for the mode line.
" Hungry"
;; The minor mode bindings.
'(([C-backspace] . hungry-electric-delete))
:group 'hunger)
This defines a minor mode named “Hungry mode,” a command named
hungry-mode to toggle it, a variable named hungry-mode
which indicates whether the mode is enabled, and a variable named
hungry-mode-map which holds the keymap that is active when the
mode is enabled. It initializes the keymap with a key binding for
C-<DEL>. It puts the variable hungry-mode into
custom group hunger. There are no body forms—many
minor modes don't need any.
Here's an equivalent way to write it:
(define-minor-mode hungry-mode
"Toggle Hungry mode.
With no argument, this command toggles the mode.
Non-null prefix argument turns on the mode.
Null prefix argument turns off the mode.
When Hungry mode is enabled, the control delete key
gobbles all preceding whitespace except the last.
See the command \\[hungry-electric-delete]."
;; The initial value.
:init-value nil
;; The indicator for the mode line.
:lighter " Hungry"
;; The minor mode bindings.
:keymap
'(([C-backspace] . hungry-electric-delete)
([C-M-backspace]
. (lambda ()
(interactive)
(hungry-electric-delete t))))
:group 'hunger)
This defines a global toggle named global-mode whose meaning is to enable or disable the buffer-local minor mode mode in all buffers. To turn on the minor mode in a buffer, it uses the function turn-on; to turn off the minor mode, it calls
modewith −1 as argument.Globally enabling the mode also affects buffers subsequently created by visiting files, and buffers that use a major mode other than Fundamental mode; but it does not detect the creation of a new buffer in Fundamental mode.
This defines the customization option global-mode (see Customization), which can be toggled in the Custom interface to turn the minor mode on and off. As with
define-minor-mode, you should ensure that thedefine-globalized-minor-modeform is evaluated each time Emacs starts, for example by providing a:requirekeyword.Use
:groupgroup in keyword-args to specify the custom group for the mode variable of the global minor mode.
Next: Imenu, Previous: Minor Modes, Up: Modes
23.4 Mode-Line Format
Each Emacs window (aside from minibuffer windows) typically has a mode line at the bottom, which displays status information about the buffer displayed in the window. The mode line contains information about the buffer, such as its name, associated file, depth of recursive editing, and major and minor modes. A window can also have a header line, which is much like the mode line but appears at the top of the window.
This section describes how to control the contents of the mode line and header line. We include it in this chapter because much of the information displayed in the mode line relates to the enabled major and minor modes.
Next: Mode Line Data, Up: Mode Line Format
23.4.1 Mode Line Basics
mode-line-format is a buffer-local variable that holds a
mode line construct, a kind of template, which controls what is
displayed on the mode line of the current buffer. The value of
header-line-format specifies the buffer's header line in the
same way. All windows for the same buffer use the same
mode-line-format and header-line-format.
For efficiency, Emacs does not continuously recompute the mode
line and header line of a window. It does so when circumstances
appear to call for it—for instance, if you change the window
configuration, switch buffers, narrow or widen the buffer, scroll, or
change the buffer's modification status. If you modify any of the
variables referenced by mode-line-format (see Mode Line Variables), or any other variables and data structures that affect
how text is displayed (see Display), you may want to force an
update of the mode line so as to display the new information or
display it in the new way.
Force redisplay of the current buffer's mode line and header line. The next redisplay will update the mode line and header line based on the latest values of all relevant variables. With optional non-
nilall, force redisplay of all mode lines and header lines.This function also forces recomputation of the menu bar menus and the frame title.
The selected window's mode line is usually displayed in a different
color using the face mode-line. Other windows' mode lines
appear in the face mode-line-inactive instead. See Faces.
Next: Mode Line Top, Previous: Mode Line Basics, Up: Mode Line Format
23.4.2 The Data Structure of the Mode Line
The mode-line contents are controlled by a data structure called a mode-line construct, made up of lists, strings, symbols, and numbers kept in buffer-local variables. Each data type has a specific meaning for the mode-line appearance, as described below. The same data structure is used for constructing frame titles (see Frame Titles) and header lines (see Header Lines).
A mode-line construct may be as simple as a fixed string of text, but it usually specifies how to combine fixed strings with variables' values to construct the text. Many of these variables are themselves defined to have mode-line constructs as their values.
Here are the meanings of various data types as mode-line constructs:
- string
- A string as a mode-line construct appears verbatim except for
%-constructs in it. These stand for substitution of other data; see %-Constructs.If parts of the string have
faceproperties, they control display of the text just as they would text in the buffer. Any characters which have nofaceproperties are displayed, by default, in the facemode-lineormode-line-inactive(see Standard Faces). Thehelp-echoandlocal-mapproperties in string have special meanings. See Properties in Mode. - symbol
- A symbol as a mode-line construct stands for its value. The value of
symbol is used as a mode-line construct, in place of symbol.
However, the symbols
tandnilare ignored, as is any symbol whose value is void.There is one exception: if the value of symbol is a string, it is displayed verbatim: the
%-constructs are not recognized.Unless symbol is marked as “risky” (i.e., it has a non-
nilrisky-local-variableproperty), all text properties specified in symbol's value are ignored. This includes the text properties of strings in symbol's value, as well as all:evaland:propertizeforms in it. (The reason for this is security: non-risky variables could be set automatically from file variables without prompting the user.) (string rest...)(list rest...)- A list whose first element is a string or list means to process all the
elements recursively and concatenate the results. This is the most
common form of mode-line construct.
(:evalform)- A list whose first element is the symbol
:evalsays to evaluate form, and use the result as a string to display. Make sure this evaluation cannot load any files, as doing so could cause infinite recursion. (:propertizeelt props...)- A list whose first element is the symbol
:propertizesays to process the mode-line construct elt recursively, then add the text properties specified by props to the result. The argument props should consist of zero or more pairs text-property value. (This feature is new as of Emacs 22.1.) (symbol then else)- A list whose first element is a symbol that is not a keyword specifies
a conditional. Its meaning depends on the value of symbol. If
symbol has a non-
nilvalue, the second element, then, is processed recursively as a mode-line element. Otherwise, the third element, else, is processed recursively. You may omit else; then the mode-line element displays nothing if the value of symbol isnilor void. (width rest...)- A list whose first element is an integer specifies truncation or
padding of the results of rest. The remaining elements
rest are processed recursively as mode-line constructs and
concatenated together. When width is positive, the result is
space filled on the right if its width is less than width. When
width is negative, the result is truncated on the right to
−width columns if its width exceeds −width.
For example, the usual way to show what percentage of a buffer is above the top of the window is to use a list like this:
(-3 "%p").
Next: Mode Line Variables, Previous: Mode Line Data, Up: Mode Line Format
23.4.3 The Top Level of Mode Line Control
The variable in overall control of the mode line is
mode-line-format.
The value of this variable is a mode-line construct that controls the contents of the mode-line. It is always buffer-local in all buffers.
If you set this variable to
nilin a buffer, that buffer does not have a mode line. (A window that is just one line tall never displays a mode line.)
The default value of mode-line-format is designed to use the
values of other variables such as mode-line-position and
mode-line-modes (which in turn incorporates the values of the
variables mode-name and minor-mode-alist). Very few
modes need to alter mode-line-format itself. For most
purposes, it is sufficient to alter some of the variables that
mode-line-format either directly or indirectly refers to.
If you do alter mode-line-format itself, the new value should
use the same variables that appear in the default value (see Mode Line Variables), rather than duplicating their contents or displaying
the information in another fashion. This way, customizations made by
the user or by Lisp programs (such as display-time and major
modes) via changes to those variables remain effective.
Here is an example of a mode-line-format that might be
useful for shell-mode, since it contains the host name and default
directory.
(setq mode-line-format
(list "-"
'mode-line-mule-info
'mode-line-modified
'mode-line-frame-identification
"%b--"
;; Note that this is evaluated while making the list.
;; It makes a mode-line construct which is just a string.
(getenv "HOST")
":"
'default-directory
" "
'global-mode-string
" %[("
'(:eval (mode-line-mode-name))
'mode-line-process
'minor-mode-alist
"%n"
")%]--"
'(which-func-mode ("" which-func-format "--"))
'(line-number-mode "L%l--")
'(column-number-mode "C%c--")
'(-3 "%p")
"-%-"))
(The variables line-number-mode, column-number-mode
and which-func-mode enable particular minor modes; as usual,
these variable names are also the minor mode command names.)
Next: %-Constructs, Previous: Mode Line Top, Up: Mode Line Format
23.4.4 Variables Used in the Mode Line
This section describes variables incorporated by the standard value
of mode-line-format into the text of the mode line. There is
nothing inherently special about these variables; any other variables
could have the same effects on the mode line if
mode-line-format's value were changed to use them. However,
various parts of Emacs set these variables on the understanding that
they will control parts of the mode line; therefore, practically
speaking, it is essential for the mode line to use them.
This variable holds the value of the mode-line construct that displays information about the language environment, buffer coding system, and current input method. See Non-ASCII Characters.
This variable holds the value of the mode-line construct that displays whether the current buffer is modified. Its default value displays ‘**’ if the buffer is modified, ‘--’ if the buffer is not modified, ‘%%’ if the buffer is read only, and ‘%*’ if the buffer is read only and modified.
Changing this variable does not force an update of the mode line.
This variable identifies the current frame. Its default value displays
" "if you are using a window system which can show multiple frames, or"-%F "on an ordinary terminal which shows only one frame at a time.
This variable identifies the buffer being displayed in the window. Its default value displays the buffer name, padded with spaces to at least 12 columns.
This variable indicates the position in the buffer. Its default value displays the buffer percentage and, optionally, the buffer size, the line number and the column number.
The variable
vc-mode, buffer-local in each buffer, records whether the buffer's visited file is maintained with version control, and, if so, which kind. Its value is a string that appears in the mode line, ornilfor no version control.
This variable displays the buffer's major and minor modes. Its default value also displays the recursive editing level, information on the process status, and whether narrowing is in effect.
The following three variables are used in mode-line-modes:
This buffer-local variable holds the “pretty” name of the current buffer's major mode. Each major mode should set this variable so that the mode name will appear in the mode line. The value does not have to be a string, but can use any of the data types valid in a mode-line construct (see Mode Line Data). To compute the string that will identify the mode name in the mode line, use
format-mode-line(see Emulating Mode Line).
This buffer-local variable contains the mode-line information on process status in modes used for communicating with subprocesses. It is displayed immediately following the major mode name, with no intervening space. For example, its value in the ‘*shell*’ buffer is
(":%s"), which allows the shell to display its status along with the major mode as: ‘(Shell:run)’. Normally this variable isnil.
This variable holds an association list whose elements specify how the mode line should indicate that a minor mode is active. Each element of the
minor-mode-alistshould be a two-element list:(minor-mode-variable mode-line-string)More generally, mode-line-string can be any mode-line spec. It appears in the mode line when the value of minor-mode-variable is non-
nil, and not otherwise. These strings should begin with spaces so that they don't run together. Conventionally, the minor-mode-variable for a specific mode is set to a non-nilvalue when that minor mode is activated.
minor-mode-alistitself is not buffer-local. Each variable mentioned in the alist should be buffer-local if its minor mode can be enabled separately in each buffer.
This variable holds a mode-line spec that, by default, appears in the mode line just after the
which-func-modeminor mode if set, else aftermode-line-modes. The commanddisplay-timesetsglobal-mode-stringto refer to the variabledisplay-time-string, which holds a string containing the time and load information.The ‘%M’ construct substitutes the value of
global-mode-string, but that is obsolete, since the variable is included in the mode line frommode-line-format.
Here is a simplified version of the default value of
mode-line-format. The real default value also
specifies addition of text properties.
("-"
mode-line-mule-info
mode-line-modified
mode-line-frame-identification
mode-line-buffer-identification
" "
mode-line-position
(vc-mode vc-mode)
" "
mode-line-modes
(which-func-mode ("" which-func-format "--"))
(global-mode-string ("--" global-mode-string))
"-%-")
23.4.5 %-Constructs in the Mode Line
Strings used as mode-line constructs can use certain
%-constructs to substitute various kinds of data. Here is a
list of the defined %-constructs, and what they mean. In any
construct except ‘%%’, you can add a decimal integer after the
‘%’ to specify a minimum field width. If the width is less, the
field is padded with spaces to the right.
%b- The current buffer name, obtained with the
buffer-namefunction. See Buffer Names. %c- The current column number of point.
%e- When Emacs is nearly out of memory for Lisp objects, a brief message
saying so. Otherwise, this is empty.
%f- The visited file name, obtained with the
buffer-file-namefunction. See Buffer File Name. %F- The title (only on a window system) or the name of the selected frame.
See Basic Parameters.
%i- The size of the accessible part of the current buffer; basically
(- (point-max) (point-min)). %I- Like ‘%i’, but the size is printed in a more readable way by using
‘k’ for 10^3, ‘M’ for 10^6, ‘G’ for 10^9, etc., to
abbreviate.
%l- The current line number of point, counting within the accessible portion
of the buffer.
%n- ‘Narrow’ when narrowing is in effect; nothing otherwise (see
narrow-to-regionin Narrowing). %p- The percentage of the buffer text above the top of window, or
‘Top’, ‘Bottom’ or ‘All’. Note that the default
mode-line specification truncates this to three characters.
%P- The percentage of the buffer text that is above the bottom of
the window (which includes the text visible in the window, as well as
the text above the top), plus ‘Top’ if the top of the buffer is
visible on screen; or ‘Bottom’ or ‘All’.
%s- The status of the subprocess belonging to the current buffer, obtained with
process-status. See Process Information. %t- Whether the visited file is a text file or a binary file. This is a
meaningful distinction only on certain operating systems (see MS-DOS File Types).
%z- The mnemonics of keyboard, terminal, and buffer coding systems.
%Z- Like ‘%z’, but including the end-of-line format.
%*- ‘%’ if the buffer is read only (see
buffer-read-only);
‘*’ if the buffer is modified (seebuffer-modified-p);
‘-’ otherwise. See Buffer Modification. %+- ‘*’ if the buffer is modified (see
buffer-modified-p);
‘%’ if the buffer is read only (seebuffer-read-only);
‘-’ otherwise. This differs from ‘%*’ only for a modified read-only buffer. See Buffer Modification. %&- ‘*’ if the buffer is modified, and ‘-’ otherwise.
%[- An indication of the depth of recursive editing levels (not counting
minibuffer levels): one ‘[’ for each editing level.
See Recursive Editing.
%]- One ‘]’ for each recursive editing level (not counting minibuffer
levels).
%-- Dashes sufficient to fill the remainder of the mode line.
%%- The character ‘%’—this is how to include a literal ‘%’ in a
string in which
%-constructs are allowed.
The following two %-constructs are still supported, but they are
obsolete, since you can get the same results with the variables
mode-name and global-mode-string.
%m- The value of
mode-name. %M- The value of
global-mode-string.
Next: Header Lines, Previous: %-Constructs, Up: Mode Line Format
23.4.6 Properties in the Mode Line
Certain text properties are meaningful in the
mode line. The face property affects the appearance of text; the
help-echo property associates help strings with the text, and
local-map can make the text mouse-sensitive.
There are four ways to specify text properties for text in the mode line:
- Put a string with a text property directly into the mode-line data structure.
- Put a text property on a mode-line %-construct such as ‘%12b’; then the expansion of the %-construct will have that same text property.
- Use a
(:propertizeelt props...)construct to give elt a text property specified by props. - Use a list containing
:evalform in the mode-line data structure, and make form evaluate to a string that has a text property.
You can use the local-map property to specify a keymap. This
keymap only takes real effect for mouse clicks; binding character keys
and function keys to it has no effect, since it is impossible to move
point into the mode line.
When the mode line refers to a variable which does not have a
non-nil risky-local-variable property, any text
properties given or specified within that variable's values are
ignored. This is because such properties could otherwise specify
functions to be called, and those functions could come from file
local variables.
Next: Emulating Mode Line, Previous: Properties in Mode, Up: Mode Line Format
23.4.7 Window Header Lines
A window can have a header line at the top, just as it can have a mode line at the bottom. The header line feature works just like the mode-line feature, except that it's controlled by different variables.
This variable, local in every buffer, specifies how to display the header line, for windows displaying the buffer. The format of the value is the same as for
mode-line-format(see Mode Line Data). It is normallynil, so that ordinary buffers have no header line.
A window that is just one line tall never displays a header line. A window that is two lines tall cannot display both a mode line and a header line at once; if it has a mode line, then it does not display a header line.
Previous: Header Lines, Up: Mode Line Format
23.4.8 Emulating Mode-Line Formatting
You can use the function format-mode-line to compute
the text that would appear in a mode line or header line
based on a certain mode-line specification.
This function formats a line of text according to format as if it were generating the mode line for window, but it also returns the text as a string. The argument window defaults to the selected window. If buffer is non-
nil, all the information used is taken from buffer; by default, it comes from window's buffer.The value string normally has text properties that correspond to the faces, keymaps, etc., that the mode line would have. Any character for which no
faceproperty is specified by format gets a default value determined by face. If face ist, that stands for eithermode-lineif window is selected, otherwisemode-line-inactive. If face isnilor omitted, that stands for the default face. If face is an integer, the value returned by this function will have no text properties.You can also specify other valid faces as the value of face. If specified, that face provides the
faceproperty for characters whose face is not specified by format.Note that using
mode-line,mode-line-inactive, orheader-lineas face will actually redisplay the mode line or the header line, respectively, using the current definitions of the corresponding face, in addition to returning the formatted string. (Other faces do not cause redisplay.)For example,
(format-mode-line header-line-format)returns the text that would appear in the selected window's header line (""if it has no header line).(format-mode-line header-line-format 'header-line)returns the same text, with each character carrying the face that it will have in the header line itself, and also redraws the header line.
Next: Font Lock Mode, Previous: Mode Line Format, Up: Modes
23.5 Imenu
Imenu is a feature that lets users select a definition or
section in the buffer, from a menu which lists all of them, to go
directly to that location in the buffer. Imenu works by constructing
a buffer index which lists the names and buffer positions of the
definitions, or other named portions of the buffer; then the user can
choose one of them and move point to it. Major modes can add a menu
bar item to use Imenu using imenu-add-to-menubar.
This function defines a local menu bar item named name to run Imenu.
The user-level commands for using Imenu are described in the Emacs Manual (see Imenu). This section explains how to customize Imenu's method of finding definitions or buffer portions for a particular major mode.
The usual and simplest way is to set the variable
imenu-generic-expression:
This variable, if non-
nil, is a list that specifies regular expressions for finding definitions for Imenu. Simple elements ofimenu-generic-expressionlook like this:(menu-title regexp index)Here, if menu-title is non-
nil, it says that the matches for this element should go in a submenu of the buffer index; menu-title itself specifies the name for the submenu. If menu-title isnil, the matches for this element go directly in the top level of the buffer index.The second item in the list, regexp, is a regular expression (see Regular Expressions); anything in the buffer that it matches is considered a definition, something to mention in the buffer index. The third item, index, is a non-negative integer that indicates which subexpression in regexp matches the definition's name.
An element can also look like this:
(menu-title regexp index function arguments...)Each match for this element creates an index item, and when the index item is selected by the user, it calls function with arguments consisting of the item name, the buffer position, and arguments.
For Emacs Lisp mode,
imenu-generic-expressioncould look like this:((nil "^\\s-*(def\\(un\\|subst\\|macro\\|advice\\)\ \\s-+\\([-A-Za-z0-9+]+\\)" 2) ("*Vars*" "^\\s-*(def\\(var\\|const\\)\ \\s-+\\([-A-Za-z0-9+]+\\)" 2) ("*Types*" "^\\s-*\ (def\\(type\\|struct\\|class\\|ine-condition\\)\ \\s-+\\([-A-Za-z0-9+]+\\)" 2))Setting this variable makes it buffer-local in the current buffer.
This variable controls whether matching against the regular expressions in the value of
imenu-generic-expressionis case-sensitive:t, the default, means matching should ignore case.Setting this variable makes it buffer-local in the current buffer.
This variable is an alist of syntax table modifiers to use while processing
imenu-generic-expression, to override the syntax table of the current buffer. Each element should have this form:(characters . syntax-description)The car, characters, can be either a character or a string. The element says to give that character or characters the syntax specified by syntax-description, which is passed to
modify-syntax-entry(see Syntax Table Functions).This feature is typically used to give word syntax to characters which normally have symbol syntax, and thus to simplify
imenu-generic-expressionand speed up matching. For example, Fortran mode uses it this way:(setq imenu-syntax-alist '(("_$" . "w")))The
imenu-generic-expressionregular expressions can then use ‘\\sw+’ instead of ‘\\(\\sw\\|\\s_\\)+’. Note that this technique may be inconvenient when the mode needs to limit the initial character of a name to a smaller set of characters than are allowed in the rest of a name.Setting this variable makes it buffer-local in the current buffer.
Another way to customize Imenu for a major mode is to set the
variables imenu-prev-index-position-function and
imenu-extract-index-name-function:
If this variable is non-
nil, its value should be a function that finds the next “definition” to put in the buffer index, scanning backward in the buffer from point. It should returnnilif it doesn't find another “definition” before point. Otherwise it should leave point at the place it finds a “definition” and return any non-nilvalue.Setting this variable makes it buffer-local in the current buffer.
If this variable is non-
nil, its value should be a function to return the name for a definition, assuming point is in that definition as theimenu-prev-index-position-functionfunction would leave it.Setting this variable makes it buffer-local in the current buffer.
The last way to customize Imenu for a major mode is to set the
variable imenu-create-index-function:
This variable specifies the function to use for creating a buffer index. The function should take no arguments, and return an index alist for the current buffer. It is called within
save-excursion, so where it leaves point makes no difference.The index alist can have three types of elements. Simple elements look like this:
(index-name . index-position)Selecting a simple element has the effect of moving to position index-position in the buffer. Special elements look like this:
(index-name index-position function arguments...)Selecting a special element performs:
(funcall function index-name index-position arguments...)A nested sub-alist element looks like this:
(menu-title sub-alist)It creates the submenu menu-title specified by sub-alist.
The default value of
imenu-create-index-functionisimenu-default-create-index-function. This function calls the value ofimenu-prev-index-position-functionand the value ofimenu-extract-index-name-functionto produce the index alist. However, if either of these two variables isnil, the default function usesimenu-generic-expressioninstead.Setting this variable makes it buffer-local in the current buffer.
Next: Auto-Indentation, Previous: Imenu, Up: Modes
23.6 Font Lock Mode
Font Lock mode is a feature that automatically attaches
face properties to certain parts of the buffer based on their
syntactic role. How it parses the buffer depends on the major mode;
most major modes define syntactic criteria for which faces to use in
which contexts. This section explains how to customize Font Lock for a
particular major mode.
Font Lock mode finds text to highlight in two ways: through syntactic parsing based on the syntax table, and through searching (usually for regular expressions). Syntactic fontification happens first; it finds comments and string constants and highlights them. Search-based fontification happens second.
Next: Search-based Fontification, Up: Font Lock Mode
23.6.1 Font Lock Basics
There are several variables that control how Font Lock mode highlights
text. But major modes should not set any of these variables directly.
Instead, they should set font-lock-defaults as a buffer-local
variable. The value assigned to this variable is used, if and when Font
Lock mode is enabled, to set all the other variables.
This variable is set by major modes, as a buffer-local variable, to specify how to fontify text in that mode. It automatically becomes buffer-local when you set it. If its value is
nil, Font-Lock mode does no highlighting, and you can use the ‘Faces’ menu (under ‘Edit’ and then ‘Text Properties’ in the menu bar) to assign faces explicitly to text in the buffer.If non-
nil, the value should look like this:(keywords [keywords-only [case-fold [syntax-alist [syntax-begin other-vars...]]]])The first element, keywords, indirectly specifies the value of
font-lock-keywordswhich directs search-based fontification. It can be a symbol, a variable or a function whose value is the list to use forfont-lock-keywords. It can also be a list of several such symbols, one for each possible level of fontification. The first symbol specifies the ‘mode default’ level of fontification, the next symbol level 1 fontification, the next level 2, and so on. The ‘mode default’ level is normally the same as level 1. It is used whenfont-lock-maximum-decorationhas anilvalue. See Levels of Font Lock.The second element, keywords-only, specifies the value of the variable
font-lock-keywords-only. If this is omitted ornil, syntactic fontification (of strings and comments) is also performed. If this is non-nil, such fontification is not performed. See Syntactic Font Lock.The third element, case-fold, specifies the value of
font-lock-keywords-case-fold-search. If it is non-nil, Font Lock mode ignores case when searching as directed byfont-lock-keywords.If the fourth element, syntax-alist, is non-
nil, it should be a list of cons cells of the form(char-or-string.string). These are used to set up a syntax table for syntactic fontification (see Syntax Table Functions). The resulting syntax table is stored infont-lock-syntax-table.The fifth element, syntax-begin, specifies the value of
font-lock-beginning-of-syntax-function. We recommend setting this variable toniland usingsyntax-begin-functioninstead.All the remaining elements (if any) are collectively called other-vars. Each of these elements should have the form
(variable.value)—which means, make variable buffer-local and then set it to value. You can use these other-vars to set other variables that affect fontification, aside from those you can control with the first five elements. See Other Font Lock Variables.
If your mode fontifies text explicitly by adding
font-lock-face properties, it can specify (nil t) for
font-lock-defaults to turn off all automatic fontification.
However, this is not required; it is possible to fontify some things
using font-lock-face properties and set up automatic
fontification for other parts of the text.
23.6.2 Search-based Fontification
The most important variable for customizing Font Lock mode is
font-lock-keywords. It specifies the search criteria for
search-based fontification. You should specify the value of this
variable with keywords in font-lock-defaults.
This variable's value is a list of the keywords to highlight. Be careful when composing regular expressions for this list; a poorly written pattern can dramatically slow things down!
Each element of font-lock-keywords specifies how to find
certain cases of text, and how to highlight those cases. Font Lock mode
processes the elements of font-lock-keywords one by one, and for
each element, it finds and handles all matches. Ordinarily, once
part of the text has been fontified already, this cannot be overridden
by a subsequent match in the same text; but you can specify different
behavior using the override element of a subexp-highlighter.
Each element of font-lock-keywords should have one of these
forms:
- regexp
- Highlight all matches for regexp using
font-lock-keyword-face. For example,;; Highlight occurrences of the word ‘foo’ ;; using
font-lock-keyword-face. "\\<foo\\>"The function
regexp-opt(see Regexp Functions) is useful for calculating optimal regular expressions to match a number of different keywords. - function
- Find text by calling function, and highlight the matches
it finds using
font-lock-keyword-face.When function is called, it receives one argument, the limit of the search; it should begin searching at point, and not search beyond the limit. It should return non-
nilif it succeeds, and set the match data to describe the match that was found. Returningnilindicates failure of the search.Fontification will call function repeatedly with the same limit, and with point where the previous invocation left it, until function fails. On failure, function need not reset point in any particular way.
(matcher.subexp)- In this kind of element, matcher is either a regular
expression or a function, as described above. The cdr,
subexp, specifies which subexpression of matcher should be
highlighted (instead of the entire text that matcher matched).
;; Highlight the ‘bar’ in each occurrence of ‘fubar’, ;; using
font-lock-keyword-face. ("fu\\(bar\\)" . 1)If you use
regexp-optto produce the regular expression matcher, you can useregexp-opt-depth(see Regexp Functions) to calculate the value for subexp. (matcher.facespec)- In this kind of element, facespec is an expression whose value
specifies the face to use for highlighting. In the simplest case,
facespec is a Lisp variable (a symbol) whose value is a face
name.
;; Highlight occurrences of ‘fubar’, ;; using the face which is the value of
fubar-face. ("fubar" . fubar-face)However, facespec can also evaluate to a list of this form:
(face face prop1 val1 prop2 val2...)
to specify the face face and various additional text properties to put on the text that matches. If you do this, be sure to add the other text property names that you set in this way to the value of
font-lock-extra-managed-propsso that the properties will also be cleared out when they are no longer appropriate. Alternatively, you can set the variablefont-lock-unfontify-region-functionto a function that clears these properties. See Other Font Lock Variables. (matcher.subexp-highlighter)- In this kind of element, subexp-highlighter is a list
which specifies how to highlight matches found by matcher.
It has the form:
(subexp facespec [override [laxmatch]])
The car, subexp, is an integer specifying which subexpression of the match to fontify (0 means the entire matching text). The second subelement, facespec, is an expression whose value specifies the face, as described above.
The last two values in subexp-highlighter, override and laxmatch, are optional flags. If override is
t, this element can override existing fontification made by previous elements offont-lock-keywords. If it iskeep, then each character is fontified if it has not been fontified already by some other element. If it isprepend, the face specified by facespec is added to the beginning of thefont-lock-faceproperty. If it isappend, the face is added to the end of thefont-lock-faceproperty.If laxmatch is non-
nil, it means there should be no error if there is no subexpression numbered subexp in matcher. Obviously, fontification of the subexpression numbered subexp will not occur. However, fontification of other subexpressions (and other regexps) will continue. If laxmatch isnil, and the specified subexpression is missing, then an error is signaled which terminates search-based fontification.Here are some examples of elements of this kind, and what they do:
;; Highlight occurrences of either ‘foo’ or ‘bar’, using ;;
foo-bar-face, even if they have already been highlighted. ;;foo-bar-faceshould be a variable whose value is a face. ("foo\\|bar" 0 foo-bar-face t) ;; Highlight the first subexpression within each occurrence ;; that the functionfubar-matchfinds, ;; using the face which is the value offubar-face. (fubar-match 1 fubar-face) (matcher.anchored-highlighter)- In this kind of element, anchored-highlighter specifies how to
highlight text that follows a match found by matcher. So a
match found by matcher acts as the anchor for further searches
specified by anchored-highlighter. anchored-highlighter
is a list of the following form:
(anchored-matcher pre-form post-form subexp-highlighters...)Here, anchored-matcher, like matcher, is either a regular expression or a function. After a match of matcher is found, point is at the end of the match. Now, Font Lock evaluates the form pre-form. Then it searches for matches of anchored-matcher and uses subexp-highlighters to highlight these. A subexp-highlighter is as described above. Finally, Font Lock evaluates post-form.
The forms pre-form and post-form can be used to initialize before, and cleanup after, anchored-matcher is used. Typically, pre-form is used to move point to some position relative to the match of matcher, before starting with anchored-matcher. post-form might be used to move back, before resuming with matcher.
After Font Lock evaluates pre-form, it does not search for anchored-matcher beyond the end of the line. However, if pre-form returns a buffer position that is greater than the position of point after pre-form is evaluated, then the position returned by pre-form is used as the limit of the search instead. It is generally a bad idea to return a position greater than the end of the line; in other words, the anchored-matcher search should not span lines.
For example,
;; Highlight occurrences of the word ‘item’ following ;; an occurrence of the word ‘anchor’ (on the same line) ;; in the value of
item-face. ("\\<anchor\\>" "\\<item\\>" nil nil (0 item-face))Here, pre-form and post-form are
nil. Therefore searching for ‘item’ starts at the end of the match of ‘anchor’, and searching for subsequent instances of ‘anchor’ resumes from where searching for ‘item’ concluded. (matcher highlighters...)- This sort of element specifies several highlighter lists for a
single matcher. A highlighter list can be of the type
subexp-highlighter or anchored-highlighter as described
above.
For example,
;; Highlight occurrences of the word ‘anchor’ in the value ;; of
anchor-face, and subsequent occurrences of the word ;; ‘item’ (on the same line) in the value ofitem-face. ("\\<anchor\\>" (0 anchor-face) ("\\<item\\>" nil nil (0 item-face))) (eval .form)- Here form is an expression to be evaluated the first time
this value of
font-lock-keywordsis used in a buffer. Its value should have one of the forms described in this table.
Warning: Do not design an element of font-lock-keywords
to match text which spans lines; this does not work reliably.
For details, see See Multiline Font Lock.
You can use case-fold in font-lock-defaults to specify
the value of font-lock-keywords-case-fold-search which says
whether search-based fontification should be case-insensitive.
Non-
nilmeans that regular expression matching for the sake offont-lock-keywordsshould be case-insensitive.
Next: Other Font Lock Variables, Previous: Search-based Fontification, Up: Font Lock Mode
23.6.3 Customizing Search-Based Fontification
You can use font-lock-add-keywords to add additional
search-based fontification rules to a major mode, and
font-lock-remove-keywords to remove rules.
This function adds highlighting keywords, for the current buffer or for major mode mode. The argument keywords should be a list with the same format as the variable
font-lock-keywords.If mode is a symbol which is a major mode command name, such as
c-mode, the effect is that enabling Font Lock mode in mode will add keywords tofont-lock-keywords. Calling with a non-nilvalue of mode is correct only in your ~/.emacs file.If mode is
nil, this function adds keywords tofont-lock-keywordsin the current buffer. This way of callingfont-lock-add-keywordsis usually used in mode hook functions.By default, keywords are added at the beginning of
font-lock-keywords. If the optional argument how isset, they are used to replace the value offont-lock-keywords. If how is any other non-nilvalue, they are added at the end offont-lock-keywords.Some modes provide specialized support you can use in additional highlighting patterns. See the variables
c-font-lock-extra-types,c++-font-lock-extra-types, andjava-font-lock-extra-types, for example.Warning: major mode functions must not call
font-lock-add-keywordsunder any circumstances, either directly or indirectly, except through their mode hooks. (Doing so would lead to incorrect behavior for some minor modes.) They should set up their rules for search-based fontification by settingfont-lock-keywords.
This function removes keywords from
font-lock-keywordsfor the current buffer or for major mode mode. As infont-lock-add-keywords, mode should be a major mode command name ornil. All the caveats and requirements forfont-lock-add-keywordsapply here too.
For example, this code
(font-lock-add-keywords 'c-mode
'(("\\<\\(FIXME\\):" 1 font-lock-warning-face prepend)
("\\<\\(and\\|or\\|not\\)\\>" . font-lock-keyword-face)))
adds two fontification patterns for C mode: one to fontify the word ‘FIXME’, even in comments, and another to fontify the words ‘and’, ‘or’ and ‘not’ as keywords.
That example affects only C mode proper. To add the same patterns to C mode and all modes derived from it, do this instead:
(add-hook 'c-mode-hook
(lambda ()
(font-lock-add-keywords nil
'(("\\<\\(FIXME\\):" 1 font-lock-warning-face prepend)
("\\<\\(and\\|or\\|not\\)\\>" .
font-lock-keyword-face)))))
Next: Levels of Font Lock, Previous: Customizing Keywords, Up: Font Lock Mode
23.6.4 Other Font Lock Variables
This section describes additional variables that a major mode can
set by means of other-vars in font-lock-defaults
(see Font Lock Basics).
If this variable is non-
nil, it should be a function that is called with no arguments, to choose an enclosing range of text for refontification for the command M-o M-o (font-lock-fontify-block).The function should report its choice by placing the region around it. A good choice is a range of text large enough to give proper results, but not too large so that refontification becomes slow. Typical values are
mark-defunfor programming modes ormark-paragraphfor textual modes.
This variable specifies additional properties (other than
font-lock-face) that are being managed by Font Lock mode. It is used byfont-lock-default-unfontify-region, which normally only manages thefont-lock-faceproperty. If you want Font Lock to manage other properties as well, you must specify them in a facespec infont-lock-keywordsas well as add them to this list. See Search-based Fontification.
Function to use for fontifying the buffer. The default value is
font-lock-default-fontify-buffer.
Function to use for unfontifying the buffer. This is used when turning off Font Lock mode. The default value is
font-lock-default-unfontify-buffer.
Function to use for fontifying a region. It should take two arguments, the beginning and end of the region, and an optional third argument verbose. If verbose is non-
nil, the function should print status messages. The default value isfont-lock-default-fontify-region.
Function to use for unfontifying a region. It should take two arguments, the beginning and end of the region. The default value is
font-lock-default-unfontify-region.
This function tells Font Lock mode to run the Lisp function function any time it has to fontify or refontify part of the current buffer. It calls function before calling the default fontification functions, and gives it two arguments, start and end, which specify the region to be fontified or refontified.
The optional argument contextual, if non-
nil, forces Font Lock mode to always refontify a syntactically relevant part of the buffer, and not just the modified lines. This argument can usually be omitted.
If function was previously registered as a fontification function using
jit-lock-register, this function unregisters it.
Next: Precalculated Fontification, Previous: Other Font Lock Variables, Up: Font Lock Mode
23.6.5 Levels of Font Lock
Many major modes offer three different levels of fontification. You
can define multiple levels by using a list of symbols for keywords
in font-lock-defaults. Each symbol specifies one level of
fontification; it is up to the user to choose one of these levels,
normally by setting font-lock-maximum-decoration (see Font Lock). The chosen level's symbol
value is used to initialize font-lock-keywords.
Here are the conventions for how to define the levels of fontification:
- Level 1: highlight function declarations, file directives (such as include or import directives), strings and comments. The idea is speed, so only the most important and top-level components are fontified.
- Level 2: in addition to level 1, highlight all language keywords, including type names that act like keywords, as well as named constant values. The idea is that all keywords (either syntactic or semantic) should be fontified appropriately.
- Level 3: in addition to level 2, highlight the symbols being defined in function and variable declarations, and all builtin function names, wherever they appear.
Next: Faces for Font Lock, Previous: Levels of Font Lock, Up: Font Lock Mode
23.6.6 Precalculated Fontification
Some major modes such as list-buffers and occur
construct the buffer text programmatically. The easiest way for them
to support Font Lock mode is to specify the faces of text when they
insert the text in the buffer.
The way to do this is to specify the faces in the text with the
special text property font-lock-face (see Special Properties). When Font Lock mode is enabled, this property controls
the display, just like the face property. When Font Lock mode
is disabled, font-lock-face has no effect on the display.
It is ok for a mode to use font-lock-face for some text and
also use the normal Font Lock machinery. But if the mode does not use
the normal Font Lock machinery, it should not set the variable
font-lock-defaults.
Next: Syntactic Font Lock, Previous: Precalculated Fontification, Up: Font Lock Mode
23.6.7 Faces for Font Lock
You can make Font Lock mode use any face, but several faces are
defined specifically for Font Lock mode. Each of these symbols is both
a face name, and a variable whose default value is the symbol itself.
Thus, the default value of font-lock-comment-face is
font-lock-comment-face. This means you can write
font-lock-comment-face in a context such as
font-lock-keywords where a face-name-valued expression is used.
font-lock-comment-face- Used (typically) for comments.
font-lock-comment-delimiter-face- Used (typically) for comments delimiters.
font-lock-doc-face- Used (typically) for documentation strings in the code.
font-lock-string-face- Used (typically) for string constants.
font-lock-keyword-face- Used (typically) for keywords—names that have special syntactic
significance, like
forandifin C. font-lock-builtin-face- Used (typically) for built-in function names.
font-lock-function-name-face- Used (typically) for the name of a function being defined or declared,
in a function definition or declaration.
font-lock-variable-name-face- Used (typically) for the name of a variable being defined or declared,
in a variable definition or declaration.
font-lock-type-face- Used (typically) for names of user-defined data types,
where they are defined and where they are used.
font-lock-constant-face- Used (typically) for constant names.
font-lock-preprocessor-face- Used (typically) for preprocessor commands.
font-lock-negation-char-face- Used (typically) for easily-overlooked negation characters.
font-lock-warning-face- Used (typically) for constructs that are peculiar, or that greatly
change the meaning of other text. For example, this is used for
‘;;;###autoload’ cookies in Emacs Lisp, and for
#errordirectives in C.
Next: Setting Syntax Properties, Previous: Faces for Font Lock, Up: Font Lock Mode
23.6.8 Syntactic Font Lock
Syntactic fontification uses the syntax table to find comments and
string constants (see Syntax Tables). It highlights them using
font-lock-comment-face and font-lock-string-face
(see Faces for Font Lock), or whatever
font-lock-syntactic-face-function chooses. There are several
variables that affect syntactic fontification; you should set them by
means of font-lock-defaults (see Font Lock Basics).
Non-
nilmeans Font Lock should not do syntactic fontification; it should only fontify based onfont-lock-keywords. The normal way for a mode to set this variable totis with keywords-only infont-lock-defaults.
This variable holds the syntax table to use for fontification of comments and strings. Specify it using syntax-alist in
font-lock-defaults. If this isnil, fontification uses the buffer's syntax table.
If this variable is non-
nil, it should be a function to move point back to a position that is syntactically at “top level” and outside of strings or comments. Font Lock uses this when necessary to get the right results for syntactic fontification.This function is called with no arguments. It should leave point at the beginning of any enclosing syntactic block. Typical values are
beginning-of-line(used when the start of the line is known to be outside a syntactic block), orbeginning-of-defunfor programming modes, orbackward-paragraphfor textual modes.If the value is
nil, Font Lock usessyntax-begin-functionto move back outside of any comment, string, or sexp. This variable is semi-obsolete; we recommend settingsyntax-begin-functioninstead.Specify this variable using syntax-begin in
font-lock-defaults.
A function to determine which face to use for a given syntactic element (a string or a comment). The function is called with one argument, the parse state at point returned by
parse-partial-sexp, and should return a face. The default value returnsfont-lock-comment-facefor comments andfont-lock-string-facefor strings.This can be used to highlighting different kinds of strings or comments differently. It is also sometimes abused together with
font-lock-syntactic-keywordsto highlight constructs that span multiple lines, but this is too esoteric to document here.Specify this variable using other-vars in
font-lock-defaults.
Next: Multiline Font Lock, Previous: Syntactic Font Lock, Up: Font Lock Mode
23.6.9 Setting Syntax Properties
Font Lock mode can be used to update syntax-table properties
automatically (see Syntax Properties). This is useful in
languages for which a single syntax table by itself is not sufficient.
This variable enables and controls updating
syntax-tableproperties by Font Lock. Its value should be a list of elements of this form:(matcher subexp syntax override laxmatch)The parts of this element have the same meanings as in the corresponding sort of element of
font-lock-keywords,(matcher subexp facespec override laxmatch)However, instead of specifying the value facespec to use for the
faceproperty, it specifies the value syntax to use for thesyntax-tableproperty. Here, syntax can be a string (as taken bymodify-syntax-entry), a syntax table, a cons cell (as returned bystring-to-syntax), or an expression whose value is one of those two types. override cannot beprependorappend.For example, an element of the form:
("\\$\\(#\\)" 1 ".")highlights syntactically a hash character when following a dollar character, with a SYNTAX of
"."(meaning punctuation syntax). Assuming that the buffer syntax table specifies hash characters to have comment start syntax, the element will only highlight hash characters that do not follow dollar characters as comments syntactically.An element of the form:
("\\('\\).\\('\\)" (1 "\"") (2 "\""))highlights syntactically both single quotes which surround a single character, with a SYNTAX of
"\""(meaning string quote syntax). Assuming that the buffer syntax table does not specify single quotes to have quote syntax, the element will only highlight single quotes of the form ‘'c'’ as strings syntactically. Other forms, such as ‘foo'bar’ or ‘'fubar'’, will not be highlighted as strings.Major modes normally set this variable with other-vars in
font-lock-defaults.
Previous: Setting Syntax Properties, Up: Font Lock Mode
23.6.10 Multiline Font Lock Constructs
Normally, elements of font-lock-keywords should not match
across multiple lines; that doesn't work reliably, because Font Lock
usually scans just part of the buffer, and it can miss a multi-line
construct that crosses the line boundary where the scan starts. (The
scan normally starts at the beginning of a line.)
Making elements that match multiline constructs work properly has two aspects: correct identification and correct rehighlighting. The first means that Font Lock finds all multiline constructs. The second means that Font Lock will correctly rehighlight all the relevant text when a multiline construct is changed—for example, if some of the text that was previously part of a multiline construct ceases to be part of it. The two aspects are closely related, and often getting one of them to work will appear to make the other also work. However, for reliable results you must attend explicitly to both aspects.
There are three ways to ensure correct identification of multiline constructs:
- Add a function to
font-lock-extend-region-functionsthat does the identification and extends the scan so that the scanned text never starts or ends in the middle of a multiline construct. - Use the
font-lock-fontify-region-functionhook similarly to extend the scan so that the scanned text never starts or ends in the middle of a multiline construct. - Somehow identify the multiline construct right when it gets inserted
into the buffer (or at any point after that but before font-lock
tries to highlight it), and mark it with a
font-lock-multilinewhich will instruct font-lock not to start or end the scan in the middle of the construct.
There are three ways to do rehighlighting of multiline constructs:
- Place a
font-lock-multilineproperty on the construct. This will rehighlight the whole construct if any part of it is changed. In some cases you can do this automatically by setting thefont-lock-multilinevariable, which see. - Make sure
jit-lock-contextuallyis set and rely on it doing its job. This will only rehighlight the part of the construct that follows the actual change, and will do it after a short delay. This only works if the highlighting of the various parts of your multiline construct never depends on text in subsequent lines. Sincejit-lock-contextuallyis activated by default, this can be an attractive solution. - Place a
jit-lock-defer-multilineproperty on the construct. This works only ifjit-lock-contextuallyis used, and with the same delay before rehighlighting, but likefont-lock-multiline, it also handles the case where highlighting depends on subsequent lines.
Next: Region to Fontify, Up: Multiline Font Lock
23.6.10.1 Font Lock Multiline
One way to ensure reliable rehighlighting of multiline Font Lock
constructs is to put on them the text property font-lock-multiline.
It should be present and non-nil for text that is part of a
multiline construct.
When Font Lock is about to highlight a range of text, it first
extends the boundaries of the range as necessary so that they do not
fall within text marked with the font-lock-multiline property.
Then it removes any font-lock-multiline properties from the
range, and highlights it. The highlighting specification (mostly
font-lock-keywords) must reinstall this property each time,
whenever it is appropriate.
Warning: don't use the font-lock-multiline property
on large ranges of text, because that will make rehighlighting slow.
If the
font-lock-multilinevariable is set tot, Font Lock will try to add thefont-lock-multilineproperty automatically on multiline constructs. This is not a universal solution, however, since it slows down Font Lock somewhat. It can miss some multiline constructs, or make the property larger or smaller than necessary.For elements whose matcher is a function, the function should ensure that submatch 0 covers the whole relevant multiline construct, even if only a small subpart will be highlighted. It is often just as easy to add the
font-lock-multilineproperty by hand.
The font-lock-multiline property is meant to ensure proper
refontification; it does not automatically identify new multiline
constructs. Identifying the requires that Font-Lock operate on large
enough chunks at a time. This will happen by accident on many cases,
which may give the impression that multiline constructs magically work.
If you set the font-lock-multiline variable non-nil,
this impression will be even stronger, since the highlighting of those
constructs which are found will be properly updated from then on.
But that does not work reliably.
To find multiline constructs reliably, you must either manually
place the font-lock-multiline property on the text before
Font-Lock looks at it, or use
font-lock-fontify-region-function.
Previous: Font Lock Multiline, Up: Multiline Font Lock
23.6.10.2 Region to Fontify after a Buffer Change
When a buffer is changed, the region that Font Lock refontifies is by default the smallest sequence of whole lines that spans the change. While this works well most of the time, sometimes it doesn't—for example, when a change alters the syntactic meaning of text on an earlier line.
You can enlarge (or even reduce) the region to fontify by setting one the following variables:
This buffer-local variable is either
nilor a function for Font-Lock to call to determine the region to scan and fontify.The function is given three parameters, the standard beg, end, and old-len from after-change-functions (see Change Hooks). It should return either a cons of the beginning and end buffer positions (in that order) of the region to fontify, or
nil(which means choose the region in the standard way). This function needs to preserve point, the match-data, and the current restriction. The region it returns may start or end in the middle of a line.Since this function is called after every buffer change, it should be reasonably fast.
23.7 Auto-indention of code
For programming languages, an important feature of a major mode is to
provide automatic indentation. This is controlled in Emacs by
indent-line-function (see Mode-Specific Indent).
Writing a good indentation function can be difficult and to a large
extent it is still a black art.
Many major mode authors will start by writing a simple indentation function that works for simple cases, for example by comparing with the indentation of the previous text line. For most programming languages that are not really line-based, this tends to scale very poorly: improving such a function to let it handle more diverse situations tends to become more and more difficult, resulting in the end with a large, complex, unmaintainable indentation function which nobody dares to touch.
A good indentation function will usually need to actually parse the text, according to the syntax of the language. Luckily, it is not necessary to parse the text in as much detail as would be needed for a compiler, but on the other hand, the parser embedded in the indentation code will want to be somewhat friendly to syntactically incorrect code.
Good maintainable indentation functions usually fall into 2 categories: either parsing forward from some “safe” starting point until the position of interest, or parsing backward from the position of interest. Neither of the two is a clearly better choice than the other: parsing backward is often more difficult than parsing forward because programming languages are designed to be parsed forward, but for the purpose of indentation it has the advantage of not needing to guess a “safe” starting point, and it generally enjoys the property that only a minimum of text will be analyzed to decide the indentation of a line, so indentation will tend to be unaffected by syntax errors in some earlier unrelated piece of code. Parsing forward on the other hand is usually easier and has the advantage of making it possible to reindent efficiently a whole region at a time, with a single parse.
Rather than write your own indentation function from scratch, it is often preferable to try and reuse some existing ones or to rely on a generic indentation engine. There are sadly few such engines. The CC-mode indentation code (used with C, C++, Java, Awk and a few other such modes) has been made more generic over the years, so if your language seems somewhat similar to one of those languages, you might try to use that engine. Another one is SMIE which takes an approach in the spirit of Lisp sexps and adapts it to non-Lisp languages.
Up: Auto-Indentation
23.7.1 Simple Minded Indentation Engine
SMIE is a package that provides a generic navigation and indentation engine. Based on a very simple parser using an “operator precedence grammar”, it lets major modes extend the sexp-based navigation of Lisp to non-Lisp languages as well as provide a simple to use but reliable auto-indentation.
Operator precedence grammar is a very primitive technology for parsing
compared to some of the more common techniques used in compilers.
It has the following characteristics: its parsing power is very limited,
and it is largely unable to detect syntax errors, but it has the
advantage of being algorithmically efficient and able to parse forward
just as well as backward. In practice that means that SMIE can use it
for indentation based on backward parsing, that it can provide both
forward-sexp and backward-sexp functionality, and that it
will naturally work on syntactically incorrect code without any extra
effort. The downside is that it also means that most programming
languages cannot be parsed correctly using SMIE, at least not without
resorting to some special tricks (see SMIE Tricks).
Next: Operator Precedence Grammars, Up: SMIE
23.7.1.1 SMIE Setup and Features
SMIE is meant to be a one-stop shop for structural navigation and
various other features which rely on the syntactic structure of code, in
particular automatic indentation. The main entry point is
smie-setup which is a function typically called while setting
up a major mode.
Setup SMIE navigation and indentation. grammar is a grammar table generated by
smie-prec2->grammar. rules-function is a set of indentation rules for use onsmie-rules-function. keywords are additional arguments, which can include the following keywords:
:forward-tokenfun: Specify the forward lexer to use.:backward-tokenfun: Specify the backward lexer to use.
Calling this function is sufficient to make commands such as
forward-sexp, backward-sexp, and transpose-sexps be
able to properly handle structural elements other than just the paired
parentheses already handled by syntax tables. For example, if the
provided grammar is precise enough, transpose-sexps can correctly
transpose the two arguments of a + operator, taking into account
the precedence rules of the language.
Calling `smie-setup' is also sufficient to make TAB indentation work in the expected way, and provides some commands that you can bind in the major mode keymap.
This command closes the most recently opened (and not yet closed) block.
This command is like
down-listbut it also pays attention to nesting of tokens other than parentheses, such asbegin...end.
Next: SMIE Grammar, Previous: SMIE setup, Up: SMIE
23.7.1.2 Operator Precedence Grammars
SMIE's precedence grammars simply give to each token a pair of
precedences: the left-precedence and the right-precedence. We say
T1 < T2 if the right-precedence of token T1 is less than
the left-precedence of token T2. A good way to read this
< is as a kind of parenthesis: if we find ... T1 something
T2 ... then that should be parsed as ... T1 (something T2 ...
rather than as ... T1 something) T2 .... The latter
interpretation would be the case if we had T1 > T2. If we have
T1 = T2, it means that token T2 follows token T1 in the same
syntactic construction, so typically we have "begin" = "end".
Such pairs of precedences are sufficient to express left-associativity
or right-associativity of infix operators, nesting of tokens like
parentheses and many other cases.
This function takes a prec2 grammar table and returns an alist suitable for use in
smie-setup. The prec2 table is itself meant to be built by one of the functions below.
This function takes several prec2 tables and merges them into a new prec2 table.
This function builds a prec2 table from a table of precedences precs. precs should be a list, sorted by precedence (for example
"+"will come before"*"), of elements of the form(assoc op...), where each op is a token that acts as an operator; assoc is their associativity, which can be eitherleft,right,assoc, ornonassoc. All operators in a given element share the same precedence level and associativity.
This function lets you specify the grammar using a BNF notation. It accepts a bnf description of the grammar along with a set of conflict resolution rules resolvers, and returns a prec2 table.
bnf is a list of nonterminal definitions of the form
(nonterm rhs1 rhs2...)where each rhs is a (non-empty) list of terminals (aka tokens) or non-terminals.Not all grammars are accepted:
- An rhs cannot be an empty list (an empty list is never needed, since SMIE allows all non-terminals to match the empty string anyway).
- An rhs cannot have 2 consecutive non-terminals: each pair of non-terminals needs to be separated by a terminal (aka token). This is a fundamental limitation of operator precedence grammars.
Additionally, conflicts can occur:
- The returned prec2 table holds constraints between pairs of tokens, and for any given pair only one constraint can be present: T1 < T2, T1 = T2, or T1 > T2.
- A token can be an
opener(something similar to an open-paren), acloser(like a close-paren), orneitherof the two (e.g. an infix operator, or an inner token like"else").Precedence conflicts can be resolved via resolvers, which is a list of precs tables (see
smie-precs->prec2): for each precedence conflict, if thoseprecstables specify a particular constraint, then the conflict is resolved by using this constraint instead, else a conflict is reported and one of the conflicting constraints is picked arbitrarily and the others are simply ignored.
Next: SMIE Lexer, Previous: Operator Precedence Grammars, Up: SMIE
23.7.1.3 Defining the Grammar of a Language
The usual way to define the SMIE grammar of a language is by defining a new global variable that holds the precedence table by giving a set of BNF rules. For example, the grammar definition for a small Pascal-like language could look like:
(require 'smie)
(defvar sample-smie-grammar
(smie-prec2->grammar
(smie-bnf->prec2
'((id)
(inst ("begin" insts "end")
("if" exp "then" inst "else" inst)
(id ":=" exp)
(exp))
(insts (insts ";" insts) (inst))
(exp (exp "+" exp)
(exp "*" exp)
("(" exps ")"))
(exps (exps "," exps) (exp)))
'((assoc ";"))
'((assoc ","))
'((assoc "+") (assoc "*")))))
A few things to note:
- The above grammar does not explicitly mention the syntax of function
calls: SMIE will automatically allow any sequence of sexps, such as
identifiers, balanced parentheses, or
begin ... endblocks to appear anywhere anyway. - The grammar category
idhas no right hand side: this does not mean that it can match only the empty string, since as mentioned any sequence of sexps can appear anywhere anyway. - Because non terminals cannot appear consecutively in the BNF grammar, it
is difficult to correctly handle tokens that act as terminators, so the
above grammar treats
";"as a statement separator instead, which SMIE can handle very well. - Separators used in sequences (such as
","and";"above) are best defined with BNF rules such as(foo (foo "separator" foo) ...)which generate precedence conflicts which are then resolved by giving them an explicit(assoc "separator"). - The
("(" exps ")")rule was not needed to pair up parens, since SMIE will pair up any characters that are marked as having paren syntax in the syntax table. What this rule does instead (together with the definition ofexps) is to make it clear that","should not appear outside of parentheses. - Rather than have a single precs table to resolve conflicts, it is preferable to have several tables, so as to let the BNF part of the grammar specify relative precedences where possible.
- Unless there is a very good reason to prefer
leftorright, it is usually preferable to mark operators as associative, usingassoc. For that reason"+"and"*"are defined above asassoc, although the language defines them formally as left associative.
Next: SMIE Tricks, Previous: SMIE Grammar, Up: SMIE
23.7.1.4 Defining Tokens
SMIE comes with a predefined lexical analyzer which uses syntax tables
in the following way: any sequence of characters that have word or
symbol syntax is considered a token, and so is any sequence of
characters that have punctuation syntax. This default lexer is
often a good starting point but is rarely actually correct for any given
language. For example, it will consider "2,+3" to be composed
of 3 tokens: "2", ",+", and "3".
To describe the lexing rules of your language to SMIE, you need 2 functions, one to fetch the next token, and another to fetch the previous token. Those functions will usually first skip whitespace and comments and then look at the next chunk of text to see if it is a special token. If so it should skip the token and return a description of this token. Usually this is simply the string extracted from the buffer, but it can be anything you want. For example:
(defvar sample-keywords-regexp
(regexp-opt '("+" "*" "," ";" ">" ">=" "<" "<=" ":=" "=")))
(defun sample-smie-forward-token ()
(forward-comment (point-max))
(cond
((looking-at sample-keywords-regexp)
(goto-char (match-end 0))
(match-string-no-properties 0))
(t (buffer-substring-no-properties
(point)
(progn (skip-syntax-forward "w_")
(point))))))
(defun sample-smie-backward-token ()
(forward-comment (- (point)))
(cond
((looking-back sample-keywords-regexp (- (point) 2) t)
(goto-char (match-beginning 0))
(match-string-no-properties 0))
(t (buffer-substring-no-properties
(point)
(progn (skip-syntax-backward "w_")
(point))))))
Notice how those lexers return the empty string when in front of parentheses. This is because SMIE automatically takes care of the parentheses defined in the syntax table. More specifically if the lexer returns nil or an empty string, SMIE tries to handle the corresponding text as a sexp according to syntax tables.
Next: SMIE Indentation, Previous: SMIE Lexer, Up: SMIE
23.7.1.5 Living With a Weak Parser
The parsing technique used by SMIE does not allow tokens to behave differently in different contexts. For most programming languages, this manifests itself by precedence conflicts when converting the BNF grammar.
Sometimes, those conflicts can be worked around by expressing the grammar slightly differently. For example, for Modula-2 it might seem natural to have a BNF grammar that looks like this:
...
(inst ("IF" exp "THEN" insts "ELSE" insts "END")
("CASE" exp "OF" cases "END")
...)
(cases (cases "|" cases) (caselabel ":" insts) ("ELSE" insts))
...
But this will create conflicts for "ELSE": on the one hand, the
IF rule implies (among many other things) that "ELSE" = "END";
but on the other hand, since "ELSE" appears within cases,
which appears left of "END", we also have "ELSE" > "END".
We can solve the conflict either by using:
...
(inst ("IF" exp "THEN" insts "ELSE" insts "END")
("CASE" exp "OF" cases "END")
("CASE" exp "OF" cases "ELSE" insts "END")
...)
(cases (cases "|" cases) (caselabel ":" insts))
...
or
...
(inst ("IF" exp "THEN" else "END")
("CASE" exp "OF" cases "END")
...)
(else (insts "ELSE" insts))
(cases (cases "|" cases) (caselabel ":" insts) (else))
...
Reworking the grammar to try and solve conflicts has its downsides, tho, because SMIE assumes that the grammar reflects the logical structure of the code, so it is preferable to keep the BNF closer to the intended abstract syntax tree.
Other times, after careful consideration you may conclude that those
conflicts are not serious and simply resolve them via the
resolvers argument of smie-bnf->prec2. Usually this is
because the grammar is simply ambiguous: the conflict does not affect
the set of programs described by the grammar, but only the way those
programs are parsed. This is typically the case for separators and
associative infix operators, where you want to add a resolver like
'((assoc "|")). Another case where this can happen is for the
classic dangling else problem, where you will use '((assoc
"else" "then")). It can also happen for cases where the conflict is
real and cannot really be resolved, but it is unlikely to pose a problem
in practice.
Finally, in many cases some conflicts will remain despite all efforts to
restructure the grammar. Do not despair: while the parser cannot be
made more clever, you can make the lexer as smart as you want. So, the
solution is then to look at the tokens involved in the conflict and to
split one of those tokens into 2 (or more) different tokens. E.g. if
the grammar needs to distinguish between two incompatible uses of the
token "begin", make the lexer return different tokens (say
"begin-fun" and "begin-plain") depending on which kind of
"begin" it finds. This pushes the work of distinguishing the
different cases to the lexer, which will thus have to look at the
surrounding text to find ad-hoc clues.
Next: SMIE Indentation Helpers, Previous: SMIE Tricks, Up: SMIE
23.7.1.6 Specifying Indentation Rules
Based on the provided grammar, SMIE will be able to provide automatic indentation without any extra effort. But in practice, this default indentation style will probably not be good enough. You will want to tweak it in many different cases.
SMIE indentation is based on the idea that indentation rules should be
as local as possible. To this end, it relies on the idea of
virtual indentation, which is the indentation that a particular
program point would have if it were at the beginning of a line.
Of course, if that program point is indeed at the beginning of a line,
its virtual indentation is its current indentation. But if not, then
SMIE uses the indentation algorithm to compute the virtual indentation
of that point. Now in practice, the virtual indentation of a program
point does not have to be identical to the indentation it would have if
we inserted a newline before it. To see how this works, the SMIE rule
for indentation after a { in C does not care whether the
{ is standing on a line of its own or is at the end of the
preceding line. Instead, these different cases are handled in the
indentation rule that decides how to indent before a {.
Another important concept is the notion of parent: The
parent of a token, is the head token of the nearest enclosing
syntactic construct. For example, the parent of an else is the
if to which it belongs, and the parent of an if, in turn,
is the lead token of the surrounding construct. The command
backward-sexp jumps from a token to its parent, but there are
some caveats: for openers (tokens which start a construct, like
if), you need to start with point before the token, while for
others you need to start with point after the token.
backward-sexp stops with point before the parent token if that is
the opener of the token of interest, and otherwise it stops with
point after the parent token.
SMIE indentation rules are specified using a function that takes two arguments method and arg where the meaning of arg and the expected return value depend on method.
method can be:
:after, in which case arg is a token and the function should return the offset to use for indentation after arg.:before, in which case arg is a token and the function should return the offset to use to indent arg itself.:elem, in which case the function should return either the offset to use to indent function arguments (if arg is the symbolarg) or the basic indentation step (if arg is the symbolbasic).:list-intro, in which case arg is a token and the function should return non-nilif the token is followed by a list of expressions (not separated by any token) rather than an expression.
When arg is a token, the function is called with point just before that token. A return value of nil always means to fallback on the default behavior, so the function should return nil for arguments it does not expect.
offset can be:
nil: use the default indentation rule.(column .column): indent to column column.- number: offset by number, relative to a base token which is
the current token for
:afterand its parent for:before.
Next: SMIE Indentation Example, Previous: SMIE Indentation, Up: SMIE
23.7.1.7 Helper Functions for Indentation Rules
SMIE provides various functions designed specifically for use in the
indentation rules function (several of those functions break if used in
another context). These functions all start with the prefix
smie-rule-.
Return non-
nilif the current token is hanging. A token is hanging if it is the last token on the line and if it is preceded by other tokens: a lone token on a line is not hanging.
Return non-
nilif the current token's parent is among parents.
Return non-nil if the current token's parent is actually a sibling. This is the case for example when the parent of a
","is just the previous",".
Return the proper offset to align the current token with the parent. If non-
nil, offset should be an integer giving an additional offset to apply.
Indent current token as a separator.
By separator, we mean here a token whose sole purpose is to separate various elements within some enclosing syntactic construct, and which does not have any semantic significance in itself (i.e. it would typically not exist as a node in an abstract syntax tree).
Such a token is expected to have an associative syntax and be closely tied to its syntactic parent. Typical examples are
","in lists of arguments (enclosed inside parentheses), or";"in sequences of instructions (enclosed in a{...}orbegin...endblock).method should be the method name that was passed to `smie-rules-function'.
Previous: SMIE Indentation Helpers, Up: SMIE
23.7.1.8 Sample Indentation Rules
Here is an example of an indentation function:
(eval-when-compile (require 'cl)) ;For the `case' macro.
(defun sample-smie-rules (kind token)
(case kind
(:elem (case token
(basic sample-indent-basic)))
(:after
(cond
((equal token ",") (smie-rule-separator kind))
((equal token ":=") sample-indent-basic)))
(:before
(cond
((equal token ",") (smie-rule-separator kind))
((member token '("begin" "(" "{"))
(if (smie-rule-hanging-p) (smie-rule-parent)))
((equal token "if")
(and (not (smie-rule-bolp)) (smie-rule-prev-p "else")
(smie-rule-parent)))))))
A few things to note:
- The first case indicates the basic indentation increment to use.
If
sample-indent-basicis nil, then SMIE uses the global settingsmie-indent-basic. The major mode could have setsmie-indent-basicbuffer-locally instead, but that is discouraged. - The two (identical) rules for the token
","make SMIE try to be more clever when the comma separator is placed at the beginning of lines. It tries to outdent the separator so as to align the code after the comma; for example:x = longfunctionname ( arg1 , arg2 ); - The rule for indentation after
":="exists because otherwise SMIE would treat":="as an infix operator and would align the right argument with the left one. - The rule for indentation before
"begin"is an example of the use of virtual indentation: This rule is used only when"begin"is hanging, which can happen only when"begin"is not at the beginning of a line. So this is not used when indenting"begin"itself but only when indenting something relative to this"begin". Concretely, this rule changes the indentation from:if x > 0 then begin dosomething(x); endto
if x > 0 then begin dosomething(x); end - The rule for indentation before
"if"is similar to the one for"begin", but where the purpose is to treat"else if"as a single unit, so as to align a sequence of tests rather than indent each test further to the right. This function does this only in the case where the"if"is not placed on a separate line, hence thesmie-rule-bolptest.If we know that the
"else"is always aligned with its"if"and is always at the beginning of a line, we can use a more efficient rule:((equal token "if") (and (not (smie-rule-bolp)) (smie-rule-prev-p "else") (save-excursion (sample-smie-backward-token) ;Jump before the "else". (cons 'column (current-column)))))The advantage of this formulation is that it reuses the indentation of the previous
"else", rather than going all the way back to the first"if"of the sequence.
Previous: Auto-Indentation, Up: Modes
23.8 Desktop Save Mode
Desktop Save Mode is a feature to save the state of Emacs from one session to another. The user-level commands for using Desktop Save Mode are described in the GNU Emacs Manual (see Saving Emacs Sessions). Modes whose buffers visit a file, don't have to do anything to use this feature.
For buffers not visiting a file to have their state saved, the major
mode must bind the buffer local variable desktop-save-buffer to
a non-nil value.
If this buffer-local variable is non-
nil, the buffer will have its state saved in the desktop file at desktop save. If the value is a function, it is called at desktop save with argument desktop-dirname, and its value is saved in the desktop file along with the state of the buffer for which it was called. When file names are returned as part of the auxiliary information, they should be formatted using the call(desktop-file-name file-name desktop-dirname)
For buffers not visiting a file to be restored, the major mode must
define a function to do the job, and that function must be listed in
the alist desktop-buffer-mode-handlers.
Alist with elements
(major-mode . restore-buffer-function)The function restore-buffer-function will be called with argument list
(buffer-file-name buffer-name desktop-buffer-misc)and it should return the restored buffer. Here desktop-buffer-misc is the value returned by the function optionally bound to
desktop-save-buffer.
24 Documentation
GNU Emacs Lisp has convenient on-line help facilities, most of which derive their information from the documentation strings associated with functions and variables. This chapter describes how to write good documentation strings for your Lisp programs, as well as how to write programs to access documentation.
Note that the documentation strings for Emacs are not the same thing as the Emacs manual. Manuals have their own source files, written in the Texinfo language; documentation strings are specified in the definitions of the functions and variables they apply to. A collection of documentation strings is not sufficient as a manual because a good manual is not organized in that fashion; it is organized in terms of topics of discussion.
For commands to display documentation strings, see Help. For the conventions for writing documentation strings, see Documentation Tips.
Next: Accessing Documentation, Up: Documentation
24.1 Documentation Basics
A documentation string is written using the Lisp syntax for strings, with double-quote characters surrounding the text of the string. This is because it really is a Lisp string object. The string serves as documentation when it is written in the proper place in the definition of a function or variable. In a function definition, the documentation string follows the argument list. In a variable definition, the documentation string follows the initial value of the variable.
When you write a documentation string, make the first line a
complete sentence (or two complete sentences) since some commands,
such as apropos, show only the first line of a multi-line
documentation string. Also, you should not indent the second line of
a documentation string, if it has one, because that looks odd when you
use C-h f (describe-function) or C-h v
(describe-variable) to view the documentation string. There
are many other conventions for doc strings; see Documentation Tips.
Documentation strings can contain several special substrings, which stand for key bindings to be looked up in the current keymaps when the documentation is displayed. This allows documentation strings to refer to the keys for related commands and be accurate even when a user rearranges the key bindings. (See Keys in Documentation.)
Emacs Lisp mode fills documentation strings to the width
specified by emacs-lisp-docstring-fill-column.
In Emacs Lisp, a documentation string is accessible through the function or variable that it describes:
- The documentation for a function is usually stored in the function
definition itself (see Lambda Expressions). The function
documentationknows how to extract it. You can also put function documentation in thefunction-documentationproperty of the function name. That is useful with definitions such as keyboard macros that can't hold a documentation string. - The documentation for a variable is stored in the variable's property
list under the property name
variable-documentation. The functiondocumentation-propertyknows how to retrieve it.
To save space, the documentation for preloaded functions and variables (including primitive functions and autoloaded functions) is stored in the file emacs/etc/DOC-version—not inside Emacs. The documentation strings for functions and variables loaded during the Emacs session from byte-compiled files are stored in those files (see Docs and Compilation).
The data structure inside Emacs has an integer offset into the file, or
a list containing a file name and an integer, in place of the
documentation string. The functions documentation and
documentation-property use that information to fetch the
documentation string from the appropriate file; this is transparent to
the user.
The emacs/lib-src directory contains two utilities that you can use to print nice-looking hardcopy for the file emacs/etc/DOC-version. These are sorted-doc and digest-doc.
Next: Keys in Documentation, Previous: Documentation Basics, Up: Documentation
24.2 Access to Documentation Strings
This function returns the documentation string that is recorded in symbol's property list under property property. It retrieves the text from a file if the value calls for that. If the property value isn't
nil, isn't a string, and doesn't refer to text in a file, then it is evaluated to obtain a string.The last thing this function does is pass the string through
substitute-command-keysto substitute actual key bindings, unless verbatim is non-nil.(documentation-property 'command-line-processed 'variable-documentation) ⇒ "Non-nil once command line has been processed" (symbol-plist 'command-line-processed) ⇒ (variable-documentation 188902) (documentation-property 'emacs 'group-documentation) ⇒ "Customization of the One True Editor."
This function returns the documentation string of function.
documentationhandles macros, named keyboard macros, and special forms, as well as ordinary functions.If function is a symbol, this function first looks for the
function-documentationproperty of that symbol; if that has a non-nilvalue, the documentation comes from that value (if the value is not a string, it is evaluated). If function is not a symbol, or if it has nofunction-documentationproperty, thendocumentationextracts the documentation string from the actual function definition, reading it from a file if called for.Finally, unless verbatim is non-
nil, it callssubstitute-command-keysso as to return a value containing the actual (current) key bindings.The function
documentationsignals avoid-functionerror if function has no function definition. However, it is OK if the function definition has no documentation string. In that case,documentationreturnsnil.
This function returns the documentation string of face as a face.
Here is an example of using the two functions, documentation and
documentation-property, to display the documentation strings for
several symbols in a ‘*Help*’ buffer.
(defun describe-symbols (pattern)
"Describe the Emacs Lisp symbols matching PATTERN.
All symbols that have PATTERN in their name are described
in the `*Help*' buffer."
(interactive "sDescribe symbols matching: ")
(let ((describe-func
(function
(lambda (s)
;; Print description of symbol.
(if (fboundp s) ; It is a function.
(princ
(format "%s\t%s\n%s\n\n" s
(if (commandp s)
(let ((keys (where-is-internal s)))
(if keys
(concat
"Keys: "
(mapconcat 'key-description
keys " "))
"Keys: none"))
"Function")
(or (documentation s)
"not documented"))))
(if (boundp s) ; It is a variable.
(princ
(format "%s\t%s\n%s\n\n" s
(if (user-variable-p s)
"Option " "Variable")
(or (documentation-property
s 'variable-documentation)
"not documented")))))))
sym-list)
;; Build a list of symbols that match pattern.
(mapatoms (function
(lambda (sym)
(if (string-match pattern (symbol-name sym))
(setq sym-list (cons sym sym-list))))))
;; Display the data.
(help-setup-xref (list 'describe-symbols pattern) (interactive-p))
(with-help-window (help-buffer)
(mapcar describe-func (sort sym-list 'string<)))))
The describe-symbols function works like apropos,
but provides more information.
(describe-symbols "goal")
---------- Buffer: *Help* ----------
goal-column Option
Semipermanent goal column for vertical motion, as set by ...
set-goal-column Keys: C-x C-n
Set the current horizontal position as a goal for C-n and C-p.
Those commands will move to this position in the line moved to
rather than trying to keep the same horizontal position.
With a non-nil argument, clears out the goal column
so that C-n and C-p resume vertical motion.
The goal column is stored in the variable `goal-column'.
temporary-goal-column Variable
Current goal column for vertical motion.
It is the column where point was
at the start of current run of vertical motion commands.
When the `track-eol' feature is doing its job, the value is 9999.
---------- Buffer: *Help* ----------
This function is used only during Emacs initialization, just before the runnable Emacs is dumped. It finds the file offsets of the documentation strings stored in the file filename, and records them in the in-core function definitions and variable property lists in place of the actual strings. See Building Emacs.
Emacs reads the file filename from the emacs/etc directory. When the dumped Emacs is later executed, the same file will be looked for in the directory
doc-directory. Usually filename is"DOC-version".
This variable holds the name of the directory which should contain the file
"DOC-version"that contains documentation strings for built-in and preloaded functions and variables.In most cases, this is the same as
data-directory. They may be different when you run Emacs from the directory where you built it, without actually installing it. See Definition of data-directory.
Next: Describing Characters, Previous: Accessing Documentation, Up: Documentation
24.3 Substituting Key Bindings in Documentation
When documentation strings refer to key sequences, they should use the
current, actual key bindings. They can do so using certain special text
sequences described below. Accessing documentation strings in the usual
way substitutes current key binding information for these special
sequences. This works by calling substitute-command-keys. You
can also call that function yourself.
Here is a list of the special sequences and what they mean:
\[command]- stands for a key sequence that will invoke command, or ‘M-x
command’ if command has no key bindings.
\{mapvar}- stands for a summary of the keymap which is the value of the variable
mapvar. The summary is made using
describe-bindings. \<mapvar>- stands for no text itself. It is used only for a side effect: it
specifies mapvar's value as the keymap for any following
‘\[command]’ sequences in this documentation string.
\=- quotes the following character and is discarded; thus, ‘\=\[’ puts ‘\[’ into the output, and ‘\=\=’ puts ‘\=’ into the output.
Please note: Each ‘\’ must be doubled when written in a string in Emacs Lisp.
This function scans string for the above special sequences and replaces them by what they stand for, returning the result as a string. This permits display of documentation that refers accurately to the user's own customized key bindings.
Here are examples of the special sequences:
(substitute-command-keys
"To abort recursive edit, type: \\[abort-recursive-edit]")
⇒ "To abort recursive edit, type: C-]"
(substitute-command-keys
"The keys that are defined for the minibuffer here are:
\\{minibuffer-local-must-match-map}")
⇒ "The keys that are defined for the minibuffer here are:
? minibuffer-completion-help
SPC minibuffer-complete-word
TAB minibuffer-complete
C-j minibuffer-complete-and-exit
RET minibuffer-complete-and-exit
C-g abort-recursive-edit
"
(substitute-command-keys
"To abort a recursive edit from the minibuffer, type\
\\<minibuffer-local-must-match-map>\\[abort-recursive-edit].")
⇒ "To abort a recursive edit from the minibuffer, type C-g."
There are other special conventions for the text in documentation strings—for instance, you can refer to functions, variables, and sections of this manual. See Documentation Tips, for details.
Next: Help Functions, Previous: Keys in Documentation, Up: Documentation
24.4 Describing Characters for Help Messages
These functions convert events, key sequences, or characters to textual descriptions. These descriptions are useful for including arbitrary text characters or key sequences in messages, because they convert non-printing and whitespace characters to sequences of printing characters. The description of a non-whitespace printing character is the character itself.
This function returns a string containing the Emacs standard notation for the input events in sequence. If prefix is non-
nil, it is a sequence of input events leading up to sequence and is included in the return value. Both arguments may be strings, vectors or lists. See Input Events, for more information about valid events.(key-description [?\M-3 delete]) ⇒ "M-3 <delete>" (key-description [delete] "\M-3") ⇒ "M-3 <delete>"See also the examples for
single-key-description, below.
This function returns a string describing event in the standard Emacs notation for keyboard input. A normal printing character appears as itself, but a control character turns into a string starting with ‘C-’, a meta character turns into a string starting with ‘M-’, and space, tab, etc. appear as ‘SPC’, ‘TAB’, etc. A function key symbol appears inside angle brackets ‘<...>’. An event that is a list appears as the name of the symbol in the car of the list, inside angle brackets.
If the optional argument no-angles is non-
nil, the angle brackets around function keys and event symbols are omitted; this is for compatibility with old versions of Emacs which didn't use the brackets.(single-key-description ?\C-x) ⇒ "C-x" (key-description "\C-x \M-y \n \t \r \f123") ⇒ "C-x SPC M-y SPC C-j SPC TAB SPC RET SPC C-l 1 2 3" (single-key-description 'delete) ⇒ "<delete>" (single-key-description 'C-mouse-1) ⇒ "<C-mouse-1>" (single-key-description 'C-mouse-1 t) ⇒ "C-mouse-1"
This function returns a string describing character in the standard Emacs notation for characters that appear in text—like
single-key-description, except that control characters are represented with a leading caret (which is how control characters in Emacs buffers are usually displayed). Another difference is thattext-char-descriptionrecognizes the 2**7 bit as the Meta character, whereassingle-key-descriptionuses the 2**27 bit for Meta.(text-char-description ?\C-c) ⇒ "^C" (text-char-description ?\M-m) ⇒ "\xed" (text-char-description ?\C-\M-m) ⇒ "\x8d" (text-char-description (+ 128 ?m)) ⇒ "M-m" (text-char-description (+ 128 ?\C-m)) ⇒ "M-^M"
This function is used mainly for operating on keyboard macros, but it can also be used as a rough inverse for
key-description. You call it with a string containing key descriptions, separated by spaces; it returns a string or vector containing the corresponding events. (This may or may not be a single valid key sequence, depending on what events you use; see Key Sequences.) If need-vector is non-nil, the return value is always a vector.
Previous: Describing Characters, Up: Documentation
24.5 Help Functions
Emacs provides a variety of on-line help functions, all accessible to the user as subcommands of the prefix C-h. For more information about them, see Help. Here we describe some program-level interfaces to the same information.
This function finds all “meaningful” symbols whose names contain a match for the apropos pattern pattern. An apropos pattern is either a word to match, a space-separated list of words of which at least two must match, or a regular expression (if any special regular expression characters occur). A symbol is “meaningful” if it has a definition as a function, variable, or face, or has properties.
The function returns a list of elements that look like this:
(symbol score fn-doc var-doc plist-doc widget-doc face-doc group-doc)Here, score is an integer measure of how important the symbol seems to be as a match, and the remaining elements are documentation strings for symbol's various roles (or
nil).It also displays the symbols in a buffer named ‘*Apropos*’, each with a one-line description taken from the beginning of its documentation string.
If do-all is non-
nil, or if the user optionapropos-do-allis non-nil, thenaproposalso shows key bindings for the functions that are found; it also shows all interned symbols, not just meaningful ones (and it lists them in the return value as well).
The value of this variable is a local keymap for characters following the Help key, C-h.
This symbol is not a function; its function definition cell holds the keymap known as
help-map. It is defined in help.el as follows:(define-key global-map (string help-char) 'help-command) (fset 'help-command help-map)
The value of this variable is the help character—the character that Emacs recognizes as meaning Help. By default, its value is 8, which stands for C-h. When Emacs reads this character, if
help-formis a non-nilLisp expression, it evaluates that expression, and displays the result in a window if it is a string.Usually the value of
help-formisnil. Then the help character has no special meaning at the level of command input, and it becomes part of a key sequence in the normal way. The standard key binding of C-h is a prefix key for several general-purpose help features.The help character is special after prefix keys, too. If it has no binding as a subcommand of the prefix key, it runs
describe-prefix-bindings, which displays a list of all the subcommands of the prefix key.
The value of this variable is a list of event types that serve as alternative “help characters.” These events are handled just like the event specified by
help-char.
If this variable is non-
nil, its value is a form to evaluate whenever the characterhelp-charis read. If evaluating the form produces a string, that string is displayed.A command that calls
read-eventorread-charprobably should bindhelp-formto a non-nilexpression while it does input. (The time when you should not do this is when C-h has some other meaning.) Evaluating this expression should result in a string that explains what the input is for and how to enter it properly.Entry to the minibuffer binds this variable to the value of
minibuffer-help-form(see Definition of minibuffer-help-form).
This variable holds a function to print help for a prefix key. The function is called when the user types a prefix key followed by the help character, and the help character has no binding after that prefix. The variable's default value is
describe-prefix-bindings.
This function calls
describe-bindingsto display a list of all the subcommands of the prefix key of the most recent key sequence. The prefix described consists of all but the last event of that key sequence. (The last event is, presumably, the help character.)
The following two functions are meant for modes that want to provide help without relinquishing control, such as the “electric” modes. Their names begin with ‘Helper’ to distinguish them from the ordinary help functions.
This command pops up a window displaying a help buffer containing a listing of all of the key bindings from both the local and global keymaps. It works by calling
describe-bindings.
This command provides help for the current mode. It prompts the user in the minibuffer with the message ‘Help (Type ? for further options)’, and then provides assistance in finding out what the key bindings are, and what the mode is intended for. It returns
nil.This can be customized by changing the map
Helper-help-map.
This variable holds the name of the directory in which Emacs finds certain documentation and text files that come with Emacs.
This function returns the name of the help buffer, which is normally ‘*Help*’; if such a buffer does not exist, it is first created.
This macro evaluates the body forms, inserting any output they produce into a buffer named buffer-name like
with-output-to-temp-buffer(see Temporary Displays). (Usually, buffer-name should be the value returned by the functionhelp-buffer.) It also puts the specified buffer into Help mode and displays a message telling the user how to quit and scroll the help window.
This function updates the cross reference data in the ‘*Help*’ buffer, which is used to regenerate the help information when the user clicks on the ‘Back’ or ‘Forward’ buttons. Most commands that use the ‘*Help*’ buffer should invoke this function before clearing the buffer. The item argument should have the form
(funtion.args), where funtion is a function to call, with argument list args, to regenerate the help buffer. The interactive-p argument is non-nilif the calling command was invoked interactively; in that case, the stack of items for the ‘*Help*’ buffer's ‘Back’ buttons is cleared.
See describe-symbols example, for an example of using
help-buffer, with-help-window, and
help-setup-xref.
This macro defines a help command named fname that acts like a prefix key that shows a list of the subcommands it offers.
When invoked, fname displays help-text in a window, then reads and executes a key sequence according to help-map. The string help-text should describe the bindings available in help-map.
The command fname is defined to handle a few events itself, by scrolling the display of help-text. When fname reads one of those special events, it does the scrolling and then reads another event. When it reads an event that is not one of those few, and which has a binding in help-map, it executes that key's binding and then returns.
The argument help-line should be a single-line summary of the alternatives in help-map. In the current version of Emacs, this argument is used only if you set the option
three-step-helptot.This macro is used in the command
help-for-helpwhich is the binding of C-h C-h.
If this variable is non-
nil, commands defined withmake-help-screendisplay their help-line strings in the echo area at first, and display the longer help-text strings only if the user types the help character again.
Next: Backups and Auto-Saving, Previous: Documentation, Up: Top
25 Files
In Emacs, you can find, create, view, save, and otherwise work with files and file directories. This chapter describes most of the file-related functions of Emacs Lisp, but a few others are described in Buffers, and those related to backups and auto-saving are described in Backups and Auto-Saving.
Many of the file functions take one or more arguments that are file
names. A file name is actually a string. Most of these functions
expand file name arguments by calling expand-file-name, so that
~ is handled correctly, as are relative file names (including
‘../’). These functions don't recognize environment variable
substitutions such as ‘$HOME’. See File Name Expansion.
When file I/O functions signal Lisp errors, they usually use the
condition file-error (see Handling Errors). The error
message is in most cases obtained from the operating system, according
to locale system-message-locale, and decoded using coding system
locale-coding-system (see Locales).
Next: Saving Buffers, Up: Files
25.1 Visiting Files
Visiting a file means reading a file into a buffer. Once this is done, we say that the buffer is visiting that file, and call the file “the visited file” of the buffer.
A file and a buffer are two different things. A file is information recorded permanently in the computer (unless you delete it). A buffer, on the other hand, is information inside of Emacs that will vanish at the end of the editing session (or when you kill the buffer). Usually, a buffer contains information that you have copied from a file; then we say the buffer is visiting that file. The copy in the buffer is what you modify with editing commands. Such changes to the buffer do not change the file; therefore, to make the changes permanent, you must save the buffer, which means copying the altered buffer contents back into the file.
In spite of the distinction between files and buffers, people often refer to a file when they mean a buffer and vice-versa. Indeed, we say, “I am editing a file,” rather than, “I am editing a buffer that I will soon save as a file of the same name.” Humans do not usually need to make the distinction explicit. When dealing with a computer program, however, it is good to keep the distinction in mind.
Next: Subroutines of Visiting, Up: Visiting Files
25.1.1 Functions for Visiting Files
This section describes the functions normally used to visit files. For historical reasons, these functions have names starting with ‘find-’ rather than ‘visit-’. See Buffer File Name, for functions and variables that access the visited file name of a buffer or that find an existing buffer by its visited file name.
In a Lisp program, if you want to look at the contents of a file but
not alter it, the fastest way is to use insert-file-contents in a
temporary buffer. Visiting the file is not necessary and takes longer.
See Reading from Files.
This command selects a buffer visiting the file filename, using an existing buffer if there is one, and otherwise creating a new buffer and reading the file into it. It also returns that buffer.
Aside from some technical details, the body of the
find-filefunction is basically equivalent to:(switch-to-buffer (find-file-noselect filename nil nil wildcards))(See
switch-to-bufferin Displaying Buffers.)If wildcards is non-
nil, which is always true in an interactive call, thenfind-fileexpands wildcard characters in filename and visits all the matching files.When
find-fileis called interactively, it prompts for filename in the minibuffer.
This command visits filename, like
find-filedoes, but it does not perform any format conversions (see Format Conversion), character code conversions (see Coding Systems), or end-of-line conversions (see End of line conversion). The buffer visiting the file is made unibyte, and its major mode is Fundamental mode, regardless of the file name. File local variable specifications in the file (see File Local Variables) are ignored, and automatic decompression and adding a newline at the end of the file due torequire-final-newline(see require-final-newline) are also disabled.Note that if Emacs already has a buffer visiting the same file non-literally, it will not visit the same file literally, but instead just switch to the existing buffer. If you want to be sure of accessing a file's contents literally, you should create a temporary buffer and then read the file contents into it using
insert-file-contents-literally(see Reading from Files).
This function is the guts of all the file-visiting functions. It returns a buffer visiting the file filename. You may make the buffer current or display it in a window if you wish, but this function does not do so.
The function returns an existing buffer if there is one; otherwise it creates a new buffer and reads the file into it. When
find-file-noselectuses an existing buffer, it first verifies that the file has not changed since it was last visited or saved in that buffer. If the file has changed, this function asks the user whether to reread the changed file. If the user says ‘yes’, any edits previously made in the buffer are lost.Reading the file involves decoding the file's contents (see Coding Systems), including end-of-line conversion, and format conversion (see Format Conversion). If wildcards is non-
nil, thenfind-file-noselectexpands wildcard characters in filename and visits all the matching files.This function displays warning or advisory messages in various peculiar cases, unless the optional argument nowarn is non-
nil. For example, if it needs to create a buffer, and there is no file named filename, it displays the message ‘(New file)’ in the echo area, and leaves the buffer empty.The
find-file-noselectfunction normally callsafter-find-fileafter reading the file (see Subroutines of Visiting). That function sets the buffer major mode, parses local variables, warns the user if there exists an auto-save file more recent than the file just visited, and finishes by running the functions infind-file-hook.If the optional argument rawfile is non-
nil, thenafter-find-fileis not called, and thefind-file-not-found-functionsare not run in case of failure. What's more, a non-nilrawfile value suppresses coding system conversion and format conversion.The
find-file-noselectfunction usually returns the buffer that is visiting the file filename. But, if wildcards are actually used and expanded, it returns a list of buffers that are visiting the various files.(find-file-noselect "/etc/fstab") ⇒ #<buffer fstab>
This command selects a buffer visiting the file filename, but does so in a window other than the selected window. It may use another existing window or split a window; see Displaying Buffers.
When this command is called interactively, it prompts for filename.
This command selects a buffer visiting the file filename, like
find-file, but it marks the buffer as read-only. See Read Only Buffers, for related functions and variables.When this command is called interactively, it prompts for filename.
This command visits filename using View mode, returning to the previous buffer when you exit View mode. View mode is a minor mode that provides commands to skim rapidly through the file, but does not let you modify the text. Entering View mode runs the normal hook
view-mode-hook. See Hooks.When
view-fileis called interactively, it prompts for filename.
If this variable is non-
nil, then the variousfind-filecommands check for wildcard characters and visit all the files that match them (when invoked interactively or when their wildcards argument is non-nil). If this option isnil, then thefind-filecommands ignore their wildcards argument and never treat wildcard characters specially.
The value of this variable is a list of functions to be called after a file is visited. The file's local-variables specification (if any) will have been processed before the hooks are run. The buffer visiting the file is current when the hook functions are run.
This variable is a normal hook. See Hooks.
The value of this variable is a list of functions to be called when
find-fileorfind-file-noselectis passed a nonexistent file name.find-file-noselectcalls these functions as soon as it detects a nonexistent file. It calls them in the order of the list, until one of them returns non-nil.buffer-file-nameis already set up.This is not a normal hook because the values of the functions are used, and in many cases only some of the functions are called.
This buffer-local variable, if set to a non-
nilvalue, makessave-bufferbehave as if the buffer were visiting its file literally, i.e. without conversions of any kind. The commandfind-file-literallysets this variable's local value, but other equivalent functions and commands can do that as well, e.g. to avoid automatic addition of a newline at the end of the file. This variable us permanent local, so it is unaffected by changes of major modes.
Previous: Visiting Functions, Up: Visiting Files
25.1.2 Subroutines of Visiting
The find-file-noselect function uses two important subroutines
which are sometimes useful in user Lisp code: create-file-buffer
and after-find-file. This section explains how to use them.
This function creates a suitably named buffer for visiting filename, and returns it. It uses filename (sans directory) as the name if that name is free; otherwise, it appends a string such as ‘<2>’ to get an unused name. See also Creating Buffers.
Please note:
create-file-bufferdoes not associate the new buffer with a file and does not select the buffer. It also does not use the default major mode.(create-file-buffer "foo") ⇒ #<buffer foo> (create-file-buffer "foo") ⇒ #<buffer foo<2>> (create-file-buffer "foo") ⇒ #<buffer foo<3>>This function is used by
find-file-noselect. It usesgenerate-new-buffer(see Creating Buffers).
This function sets the buffer major mode, and parses local variables (see Auto Major Mode). It is called by
find-file-noselectand by the default revert function (see Reverting).If reading the file got an error because the file does not exist, but its directory does exist, the caller should pass a non-
nilvalue for error. In that case,after-find-fileissues a warning: ‘(New file)’. For more serious errors, the caller should usually not callafter-find-file.If warn is non-
nil, then this function issues a warning if an auto-save file exists and is more recent than the visited file.If noauto is non-
nil, that says not to enable or disable Auto-Save mode. The mode remains enabled if it was enabled before.If after-find-file-from-revert-buffer is non-
nil, that means this call was fromrevert-buffer. This has no direct effect, but some mode functions and hook functions check the value of this variable.If nomodes is non-
nil, that means don't alter the buffer's major mode, don't process local variables specifications in the file, and don't runfind-file-hook. This feature is used byrevert-bufferin some cases.The last thing
after-find-filedoes is call all the functions in the listfind-file-hook.
Next: Reading from Files, Previous: Visiting Files, Up: Files
25.2 Saving Buffers
When you edit a file in Emacs, you are actually working on a buffer that is visiting that file—that is, the contents of the file are copied into the buffer and the copy is what you edit. Changes to the buffer do not change the file until you save the buffer, which means copying the contents of the buffer into the file.
This function saves the contents of the current buffer in its visited file if the buffer has been modified since it was last visited or saved. Otherwise it does nothing.
save-bufferis responsible for making backup files. Normally, backup-option isnil, andsave-buffermakes a backup file only if this is the first save since visiting the file. Other values for backup-option request the making of backup files in other circumstances:
- With an argument of 4 or 64, reflecting 1 or 3 C-u's, the
save-bufferfunction marks this version of the file to be backed up when the buffer is next saved.- With an argument of 16 or 64, reflecting 2 or 3 C-u's, the
save-bufferfunction unconditionally backs up the previous version of the file before saving it.- With an argument of 0, unconditionally do not make any backup file.
This command saves some modified file-visiting buffers. Normally it asks the user about each buffer. But if save-silently-p is non-
nil, it saves all the file-visiting buffers without querying the user.The optional pred argument controls which buffers to ask about (or to save silently if save-silently-p is non-
nil). If it isnil, that means to ask only about file-visiting buffers. If it ist, that means also offer to save certain other non-file buffers—those that have a non-nilbuffer-local value ofbuffer-offer-save(see Killing Buffers). A user who says ‘yes’ to saving a non-file buffer is asked to specify the file name to use. Thesave-buffers-kill-emacsfunction passes the valuetfor pred.If pred is neither
tnornil, then it should be a function of no arguments. It will be called in each buffer to decide whether to offer to save that buffer. If it returns a non-nilvalue in a certain buffer, that means do offer to save that buffer.
This function writes the current buffer into file filename, makes the buffer visit that file, and marks it not modified. Then it renames the buffer based on filename, appending a string like ‘<2>’ if necessary to make a unique buffer name. It does most of this work by calling
set-visited-file-name(see Buffer File Name) andsave-buffer.If confirm is non-
nil, that means to ask for confirmation before overwriting an existing file. Interactively, confirmation is required, unless the user supplies a prefix argument.If filename is an existing directory, or a symbolic link to one,
write-fileuses the name of the visited file, in directory filename. If the buffer is not visiting a file, it uses the buffer name instead.
Saving a buffer runs several hooks. It also performs format conversion (see Format Conversion).
The value of this variable is a list of functions to be called before writing out a buffer to its visited file. If one of them returns non-
nil, the file is considered already written and the rest of the functions are not called, nor is the usual code for writing the file executed.If a function in
write-file-functionsreturns non-nil, it is responsible for making a backup file (if that is appropriate). To do so, execute the following code:(or buffer-backed-up (backup-buffer))You might wish to save the file modes value returned by
backup-bufferand use that (if non-nil) to set the mode bits of the file that you write. This is whatsave-buffernormally does. See Making Backup Files.The hook functions in
write-file-functionsare also responsible for encoding the data (if desired): they must choose a suitable coding system and end-of-line conversion (see Lisp and Coding Systems), perform the encoding (see Explicit Encoding), and setlast-coding-system-usedto the coding system that was used (see Encoding and I/O).If you set this hook locally in a buffer, it is assumed to be associated with the file or the way the contents of the buffer were obtained. Thus the variable is marked as a permanent local, so that changing the major mode does not alter a buffer-local value. On the other hand, calling
set-visited-file-namewill reset it. If this is not what you want, you might like to usewrite-contents-functionsinstead.Even though this is not a normal hook, you can use
add-hookandremove-hookto manipulate the list. See Hooks.
This works just like
write-file-functions, but it is intended for hooks that pertain to the buffer's contents, not to the particular visited file or its location. Such hooks are usually set up by major modes, as buffer-local bindings for this variable. This variable automatically becomes buffer-local whenever it is set; switching to a new major mode always resets this variable, but callingset-visited-file-namedoes not.If any of the functions in this hook returns non-
nil, the file is considered already written and the rest are not called and neither are the functions inwrite-file-functions.
This normal hook runs before a buffer is saved in its visited file, regardless of whether that is done normally or by one of the hooks described above. For instance, the copyright.el program uses this hook to make sure the file you are saving has the current year in its copyright notice.
This normal hook runs after a buffer has been saved in its visited file. One use of this hook is in Fast Lock mode; it uses this hook to save the highlighting information in a cache file.
If this variable is non-
nil, thensave-bufferprotects against I/O errors while saving by writing the new file to a temporary name instead of the name it is supposed to have, and then renaming it to the intended name after it is clear there are no errors. This procedure prevents problems such as a lack of disk space from resulting in an invalid file.As a side effect, backups are necessarily made by copying. See Rename or Copy. Yet, at the same time, saving a precious file always breaks all hard links between the file you save and other file names.
Some modes give this variable a non-
nilbuffer-local value in particular buffers.
This variable determines whether files may be written out that do not end with a newline. If the value of the variable is
t, thensave-buffersilently adds a newline at the end of the file whenever the buffer being saved does not already end in one. If the value of the variable is non-nil, but nott, thensave-bufferasks the user whether to add a newline each time the case arises.If the value of the variable is
nil, thensave-bufferdoesn't add newlines at all.nilis the default value, but a few major modes set it totin particular buffers.
See also the function set-visited-file-name (see Buffer File Name).
Next: Writing to Files, Previous: Saving Buffers, Up: Files
25.3 Reading from Files
You can copy a file from the disk and insert it into a buffer
using the insert-file-contents function. Don't use the user-level
command insert-file in a Lisp program, as that sets the mark.
This function inserts the contents of file filename into the current buffer after point. It returns a list of the absolute file name and the length of the data inserted. An error is signaled if filename is not the name of a file that can be read.
The function
insert-file-contentschecks the file contents against the defined file formats, and converts the file contents if appropriate and also calls the functions in the listafter-insert-file-functions. See Format Conversion. Normally, one of the functions in theafter-insert-file-functionslist determines the coding system (see Coding Systems) used for decoding the file's contents, including end-of-line conversion. However, if the file contains null bytes, it is by default visited without any code conversions; see inhibit-null-byte-detection, for how to control this behavior.If visit is non-
nil, this function additionally marks the buffer as unmodified and sets up various fields in the buffer so that it is visiting the file filename: these include the buffer's visited file name and its last save file modtime. This feature is used byfind-file-noselectand you probably should not use it yourself.If beg and end are non-
nil, they should be integers specifying the portion of the file to insert. In this case, visit must benil. For example,(insert-file-contents filename nil 0 500)inserts the first 500 characters of a file.
If the argument replace is non-
nil, it means to replace the contents of the buffer (actually, just the accessible portion) with the contents of the file. This is better than simply deleting the buffer contents and inserting the whole file, because (1) it preserves some marker positions and (2) it puts less data in the undo list.It is possible to read a special file (such as a FIFO or an I/O device) with
insert-file-contents, as long as replace and visit arenil.
This function works like
insert-file-contentsexcept that it does not do format decoding (see Format Conversion), does not do character code conversion (see Coding Systems), does not runfind-file-hook, does not perform automatic uncompression, and so on.
If you want to pass a file name to another process so that another
program can read the file, use the function file-local-copy; see
Magic File Names.
Next: File Locks, Previous: Reading from Files, Up: Files
25.4 Writing to Files
You can write the contents of a buffer, or part of a buffer, directly
to a file on disk using the append-to-file and
write-region functions. Don't use these functions to write to
files that are being visited; that could cause confusion in the
mechanisms for visiting.
This function appends the contents of the region delimited by start and end in the current buffer to the end of file filename. If that file does not exist, it is created. This function returns
nil.An error is signaled if filename specifies a nonwritable file, or a nonexistent file in a directory where files cannot be created.
When called from Lisp, this function is completely equivalent to:
(write-region start end filename t)
This function writes the region delimited by start and end in the current buffer into the file specified by filename.
If start is
nil, then the command writes the entire buffer contents (not just the accessible portion) to the file and ignores end.If start is a string, then
write-regionwrites or appends that string, rather than text from the buffer. end is ignored in this case.If append is non-
nil, then the specified text is appended to the existing file contents (if any). If append is an integer,write-regionseeks to that byte offset from the start of the file and writes the data from there.If mustbenew is non-
nil, thenwrite-regionasks for confirmation if filename names an existing file. If mustbenew is the symbolexcl, thenwrite-regiondoes not ask for confirmation, but instead it signals an errorfile-already-existsif the file already exists.The test for an existing file, when mustbenew is
excl, uses a special system feature. At least for files on a local disk, there is no chance that some other program could create a file of the same name before Emacs does, without Emacs's noticing.If visit is
t, then Emacs establishes an association between the buffer and the file: the buffer is then visiting that file. It also sets the last file modification time for the current buffer to filename's modtime, and marks the buffer as not modified. This feature is used bysave-buffer, but you probably should not use it yourself.If visit is a string, it specifies the file name to visit. This way, you can write the data to one file (filename) while recording the buffer as visiting another file (visit). The argument visit is used in the echo area message and also for file locking; visit is stored in
buffer-file-name. This feature is used to implementfile-precious-flag; don't use it yourself unless you really know what you're doing.The optional argument lockname, if non-
nil, specifies the file name to use for purposes of locking and unlocking, overriding filename and visit for that purpose.The function
write-regionconverts the data which it writes to the appropriate file formats specified bybuffer-file-formatand also calls the functions in the listwrite-region-annotate-functions. See Format Conversion.Normally,
write-regiondisplays the message ‘Wrote filename’ in the echo area. If visit is neithertnornilnor a string, then this message is inhibited. This feature is useful for programs that use files for internal purposes, files that the user does not need to know about.
The
with-temp-filemacro evaluates the body forms with a temporary buffer as the current buffer; then, at the end, it writes the buffer contents into file file. It kills the temporary buffer when finished, restoring the buffer that was current before thewith-temp-fileform. Then it returns the value of the last form in body.The current buffer is restored even in case of an abnormal exit via
throwor error (see Nonlocal Exits).See also
with-temp-bufferin The Current Buffer.
Next: Information about Files, Previous: Writing to Files, Up: Files
25.5 File Locks
When two users edit the same file at the same time, they are likely to interfere with each other. Emacs tries to prevent this situation from arising by recording a file lock when a file is being modified. (File locks are not implemented on Microsoft systems.) Emacs can then detect the first attempt to modify a buffer visiting a file that is locked by another Emacs job, and ask the user what to do. The file lock is really a file, a symbolic link with a special name, stored in the same directory as the file you are editing.
When you access files using NFS, there may be a small probability that you and another user will both lock the same file “simultaneously.” If this happens, it is possible for the two users to make changes simultaneously, but Emacs will still warn the user who saves second. Also, the detection of modification of a buffer visiting a file changed on disk catches some cases of simultaneous editing; see Modification Time.
This function returns
nilif the file filename is not locked. It returnstif it is locked by this Emacs process, and it returns the name of the user who has locked it if it is locked by some other job.(file-locked-p "foo") ⇒ nil
This function locks the file filename, if the current buffer is modified. The argument filename defaults to the current buffer's visited file. Nothing is done if the current buffer is not visiting a file, or is not modified, or if the system does not support locking.
This function unlocks the file being visited in the current buffer, if the buffer is modified. If the buffer is not modified, then the file should not be locked, so this function does nothing. It also does nothing if the current buffer is not visiting a file, or if the system does not support locking.
File locking is not supported on some systems. On systems that do not
support it, the functions lock-buffer, unlock-buffer and
file-locked-p do nothing and return nil.
This function is called when the user tries to modify file, but it is locked by another user named other-user. The default definition of this function asks the user to say what to do. The value this function returns determines what Emacs does next:
- A value of
tsays to grab the lock on the file. Then this user may edit the file and other-user loses the lock.- A value of
nilsays to ignore the lock and let this user edit the file anyway.- This function may instead signal a
file-lockederror, in which case the change that the user was about to make does not take place.The error message for this error looks like this:
error--> File is locked: file other-userwhere
fileis the name of the file and other-user is the name of the user who has locked the file.If you wish, you can replace the
ask-user-about-lockfunction with your own version that makes the decision in another way. The code for its usual definition is in userlock.el.
Next: Changing Files, Previous: File Locks, Up: Files
25.6 Information about Files
The functions described in this section all operate on strings that designate file names. With a few exceptions, all the functions have names that begin with the word ‘file’. These functions all return information about actual files or directories, so their arguments must all exist as actual files or directories unless otherwise noted.
Next: Kinds of Files, Up: Information about Files
25.6.1 Testing Accessibility
These functions test for permission to access a file in specific ways. Unless explicitly stated otherwise, they recursively follow symbolic links for their file name arguments, at all levels (at the level of the file itself and at all levels of parent directories).
This function returns
tif a file named filename appears to exist. This does not mean you can necessarily read the file, only that you can find out its attributes. (On Unix and GNU/Linux, this is true if the file exists and you have execute permission on the containing directories, regardless of the protection of the file itself.)If the file does not exist, or if fascist access control policies prevent you from finding the attributes of the file, this function returns
nil.Directories are files, so
file-exists-preturnstwhen given a directory name. However, symbolic links are treated specially;file-exists-preturnstfor a symbolic link name only if the target file exists.
This function returns
tif a file named filename exists and you can read it. It returnsnilotherwise.(file-readable-p "files.texi") ⇒ t (file-exists-p "/usr/spool/mqueue") ⇒ t (file-readable-p "/usr/spool/mqueue") ⇒ nil
This function returns
tif a file named filename exists and you can execute it. It returnsnilotherwise. On Unix and GNU/Linux, if the file is a directory, execute permission means you can check the existence and attributes of files inside the directory, and open those files if their modes permit.
This function returns
tif the file filename can be written or created by you, andnilotherwise. A file is writable if the file exists and you can write it. It is creatable if it does not exist, but the specified directory does exist and you can write in that directory.In the third example below, foo is not writable because the parent directory does not exist, even though the user could create such a directory.
(file-writable-p "~/foo") ⇒ t (file-writable-p "/foo") ⇒ nil (file-writable-p "~/no-such-dir/foo") ⇒ nil
This function returns
tif you have permission to open existing files in the directory whose name as a file is dirname; otherwise (or if there is no such directory), it returnsnil. The value of dirname may be either a directory name (such as /foo/) or the file name of a file which is a directory (such as /foo, without the final slash).Example: after the following,
(file-accessible-directory-p "/foo") ⇒ nilwe can deduce that any attempt to read a file in /foo/ will give an error.
This function opens file filename for reading, then closes it and returns
nil. However, if the open fails, it signals an error using string as the error message text.
This function returns
tif deleting the file filename and then creating it anew would keep the file's owner unchanged. It also returnstfor nonexistent files.If filename is a symbolic link, then, unlike the other functions discussed here,
file-ownership-preserved-pdoes not replace filename with its target. However, it does recursively follow symbolic links at all levels of parent directories.
This function returns
tif the file filename1 is newer than file filename2. If filename1 does not exist, it returnsnil. If filename1 does exist, but filename2 does not, it returnst.In the following example, assume that the file aug-19 was written on the 19th, aug-20 was written on the 20th, and the file no-file doesn't exist at all.
(file-newer-than-file-p "aug-19" "aug-20") ⇒ nil (file-newer-than-file-p "aug-20" "aug-19") ⇒ t (file-newer-than-file-p "aug-19" "no-file") ⇒ t (file-newer-than-file-p "no-file" "aug-19") ⇒ nilYou can use
file-attributesto get a file's last modification time as a list of two numbers. See File Attributes.
Next: Truenames, Previous: Testing Accessibility, Up: Information about Files
25.6.2 Distinguishing Kinds of Files
This section describes how to distinguish various kinds of files, such as directories, symbolic links, and ordinary files.
If the file filename is a symbolic link, the
file-symlink-pfunction returns the (non-recursive) link target as a string. (Determining the file name that the link points to from the target is nontrivial.) First, this function recursively follows symbolic links at all levels of parent directories.If the file filename is not a symbolic link (or there is no such file),
file-symlink-preturnsnil.(file-symlink-p "foo") ⇒ nil (file-symlink-p "sym-link") ⇒ "foo" (file-symlink-p "sym-link2") ⇒ "sym-link" (file-symlink-p "/bin") ⇒ "/pub/bin"
The next two functions recursively follow symbolic links at all levels for filename.
This function returns
tif filename is the name of an existing directory,nilotherwise.(file-directory-p "~rms") ⇒ t (file-directory-p "~rms/lewis/files.texi") ⇒ nil (file-directory-p "~rms/lewis/no-such-file") ⇒ nil (file-directory-p "$HOME") ⇒ nil (file-directory-p (substitute-in-file-name "$HOME")) ⇒ t
This function returns
tif the file filename exists and is a regular file (not a directory, named pipe, terminal, or other I/O device).
Next: File Attributes, Previous: Kinds of Files, Up: Information about Files
25.6.3 Truenames
The truename of a file is the name that you get by following symbolic links at all levels until none remain, then simplifying away ‘.’ and ‘..’ appearing as name components. This results in a sort of canonical name for the file. A file does not always have a unique truename; the number of distinct truenames a file has is equal to the number of hard links to the file. However, truenames are useful because they eliminate symbolic links as a cause of name variation.
The function
file-truenamereturns the truename of the file filename. The argument must be an absolute file name.This function does not expand environment variables. Only
substitute-in-file-namedoes that. See Definition of substitute-in-file-name.If you may need to follow symbolic links preceding ‘..’ appearing as a name component, you should make sure to call
file-truenamewithout prior direct or indirect calls toexpand-file-name, as otherwise the file name component immediately preceding ‘..’ will be “simplified away” beforefile-truenameis called. To eliminate the need for a call toexpand-file-name,file-truenamehandles ‘~’ in the same way thatexpand-file-namedoes. See Functions that Expand Filenames.
This function follows symbolic links, starting with filename, until it finds a file name which is not the name of a symbolic link. Then it returns that file name. This function does not follow symbolic links at the level of parent directories.
If you specify a number for limit, then after chasing through that many links, the function just returns what it has even if that is still a symbolic link.
To illustrate the difference between file-chase-links and
file-truename, suppose that /usr/foo is a symbolic link to
the directory /home/foo, and /home/foo/hello is an
ordinary file (or at least, not a symbolic link) or nonexistent. Then
we would have:
(file-chase-links "/usr/foo/hello")
;; This does not follow the links in the parent directories.
⇒ "/usr/foo/hello"
(file-truename "/usr/foo/hello")
;; Assuming that /home is not a symbolic link.
⇒ "/home/foo/hello"
See Buffer File Name, for related information.
Next: Locating Files, Previous: Truenames, Up: Information about Files
25.6.4 Other Information about Files
This section describes the functions for getting detailed information about a file, other than its contents. This information includes the mode bits that control access permission, the owner and group numbers, the number of names, the inode number, the size, and the times of access and modification.
This function returns the mode bits of filename, as an integer. The mode bits are also called the file permissions, and they specify access control in the usual Unix fashion. If the low-order bit is 1, then the file is executable by all users, if the second-lowest-order bit is 1, then the file is writable by all users, etc.
The highest value returnable is 4095 (7777 octal), meaning that everyone has read, write, and execute permission, that the SUID bit is set for both others and group, and that the sticky bit is set.
If filename does not exist,
file-modesreturnsnil.This function recursively follows symbolic links at all levels.
(file-modes "~/junk/diffs") ⇒ 492 ; Decimal integer. (format "%o" 492) ⇒ "754" ; Convert to octal. (set-file-modes "~/junk/diffs" 438) ⇒ nil (format "%o" 438) ⇒ "666" ; Convert to octal. % ls -l diffs -rw-rw-rw- 1 lewis 0 3063 Oct 30 16:00 diffs
If the filename argument to the next two functions is a symbolic link, then these function do not replace it with its target. However, they both recursively follow symbolic links at all levels of parent directories.
This functions returns the number of names (i.e., hard links) that file filename has. If the file does not exist, then this function returns
nil. Note that symbolic links have no effect on this function, because they are not considered to be names of the files they link to.% ls -l foo* -rw-rw-rw- 2 rms 4 Aug 19 01:27 foo -rw-rw-rw- 2 rms 4 Aug 19 01:27 foo1 (file-nlinks "foo") ⇒ 2 (file-nlinks "doesnt-exist") ⇒ nil
This function returns a list of attributes of file filename. If the specified file cannot be opened, it returns
nil. The optional parameter id-format specifies the preferred format of attributes UID and GID (see below)—the valid values are'stringand'integer. The latter is the default, but we plan to change that, so you should specify a non-nilvalue for id-format if you use the returned UID or GID.The elements of the list, in order, are:
tfor a directory, a string for a symbolic link (the name linked to), ornilfor a text file.- The number of names the file has. Alternate names, also known as hard links, can be created by using the
add-name-to-filefunction (see Changing Files).- The file's UID, normally as a string. However, if it does not correspond to a named user, the value is an integer or a floating point number.
- The file's GID, likewise.
- The time of last access, as a list of two integers. The first integer has the high-order 16 bits of time, the second has the low 16 bits. (This is similar to the value of
current-time; see Time of Day.) Note that on some FAT-based filesystems, only the date of last access is recorded, so this time will always hold the midnight of the day of last access.- The time of last modification as a list of two integers (as above). This is the last time when the file's contents were modified.
- The time of last status change as a list of two integers (as above). This is the time of the last change to the file's access mode bits, its owner and group, and other information recorded in the filesystem for the file, beyond the file's contents.
- The size of the file in bytes. If the size is too large to fit in a Lisp integer, this is a floating point number.
- The file's modes, as a string of ten letters or dashes, as in ‘ls -l’.
tif the file's GID would change if file were deleted and recreated;nilotherwise.- The file's inode number. If possible, this is an integer. If the inode number is too large to be represented as an integer in Emacs Lisp, but still fits into a 32-bit integer, then the value has the form
(high.low), where low holds the low 16 bits. If the inode is wider than 32 bits, the value is of the form(high middle.low), wherehighholds the high 24 bits, middle the next 24 bits, and low the low 16 bits.- The filesystem number of the device that the file is on. Depending on the magnitude of the value, this can be either an integer or a cons cell, in the same manner as the inode number. This element and the file's inode number together give enough information to distinguish any two files on the system—no two files can have the same values for both of these numbers.
For example, here are the file attributes for files.texi:
(file-attributes "files.texi" 'string) ⇒ (nil 1 "lh" "users" (19145 42977) (19141 59576) (18340 17300) 122295 "-rw-rw-rw-" nil (5888 2 . 43978) (15479 . 46724))and here is how the result is interpreted:
nil- is neither a directory nor a symbolic link.
1- has only one name (the name files.texi in the current default directory).
"lh"- is owned by the user with name "lh".
"users"- is in the group with name "users".
(19145 42977)- was last accessed on Oct 5 2009, at 10:01:37.
(19141 59576)- last had its contents modified on Oct 2 2009, at 13:49:12.
(18340 17300)- last had its status changed on Feb 2 2008, at 12:19:00.
122295- is 122295 bytes long. (It may not contain 122295 characters, though, if some of the bytes belong to multibyte sequences, and also if the end-of-line format is CR-LF.)
"-rw-rw-rw-"- has a mode of read and write access for the owner, group, and world.
nil- would retain the same GID if it were recreated.
(5888 2 . 43978)- has an inode number of 6473924464520138.
(15479 . 46724)- is on the file-system device whose number is 1014478468.
On MS-DOS, there is no such thing as an “executable” file mode bit.
So Emacs considers a file executable if its name ends in one of the
standard executable extensions, such as .com, .bat,
.exe, and some others. Files that begin with the Unix-standard
‘#!’ signature, such as shell and Perl scripts, are also considered
as executable files. This is reflected in the values returned by
file-modes and file-attributes. Directories are also
reported with executable bit set, for compatibility with Unix.
Previous: File Attributes, Up: Information about Files
25.6.5 How to Locate Files in Standard Places
This section explains how to search for a file in a list of directories (a path). One example is when you need to look for a program's executable file, e.g., to find out whether a given program is installed on the user's system. Another example is the search for Lisp libraries (see Library Search). Such searches generally need to try various possible file name extensions, in addition to various possible directories. Emacs provides a function for such a generalized search for a file.
This function searches for a file whose name is filename in a list of directories given by path, trying the suffixes in suffixes. If it finds such a file, it returns the full absolute file name of the file (see Relative File Names); otherwise it returns
nil.The optional argument suffixes gives the list of file-name suffixes to append to filename when searching.
locate-filetries each possible directory with each of these suffixes. If suffixes isnil, or(""), then there are no suffixes, and filename is used only as-is. Typical values of suffixes areexec-suffixes(see exec-suffixes),load-suffixes,load-file-rep-suffixesand the return value of the functionget-load-suffixes(see Load Suffixes).Typical values for path are
exec-path(see exec-path) when looking for executable programs orload-path(see load-path) when looking for Lisp files. If filename is absolute, path has no effect, but the suffixes in suffixes are still tried.The optional argument predicate, if non-
nil, specifies the predicate function to use for testing whether a candidate file is suitable. The predicate function is passed the candidate file name as its single argument. If predicate isnilor unspecified,locate-fileusesfile-readable-pas the default predicate. Useful non-default predicates includefile-executable-p,file-directory-p, and other predicates described in Kinds of Files.For compatibility, predicate can also be one of the symbols
executable,readable,writable,exists, or a list of one or more of these symbols.
This function searches for the executable file of the named program and returns the full absolute name of the executable, including its file-name extensions, if any. It returns
nilif the file is not found. The functions searches in all the directories inexec-pathand tries all the file-name extensions inexec-suffixes.
Next: File Names, Previous: Information about Files, Up: Files
25.7 Changing File Names and Attributes
The functions in this section rename, copy, delete, link, and set the modes of files.
In the functions that have an argument newname, if a file by the name of newname already exists, the actions taken depend on the value of the argument ok-if-already-exists:
- Signal a
file-already-existserror if ok-if-already-exists isnil. - Request confirmation if ok-if-already-exists is a number.
- Replace the old file without confirmation if ok-if-already-exists is any other value.
The next four commands all recursively follow symbolic links at all
levels of parent directories for their first argument, but, if that
argument is itself a symbolic link, then only copy-file
replaces it with its (recursive) target.
This function gives the file named oldname the additional name newname. This means that newname becomes a new “hard link” to oldname.
In the first part of the following example, we list two files, foo and foo3.
% ls -li fo* 81908 -rw-rw-rw- 1 rms 29 Aug 18 20:32 foo 84302 -rw-rw-rw- 1 rms 24 Aug 18 20:31 foo3Now we create a hard link, by calling
add-name-to-file, then list the files again. This shows two names for one file, foo and foo2.(add-name-to-file "foo" "foo2") ⇒ nil % ls -li fo* 81908 -rw-rw-rw- 2 rms 29 Aug 18 20:32 foo 81908 -rw-rw-rw- 2 rms 29 Aug 18 20:32 foo2 84302 -rw-rw-rw- 1 rms 24 Aug 18 20:31 foo3Finally, we evaluate the following:
(add-name-to-file "foo" "foo3" t)and list the files again. Now there are three names for one file: foo, foo2, and foo3. The old contents of foo3 are lost.
(add-name-to-file "foo1" "foo3") ⇒ nil % ls -li fo* 81908 -rw-rw-rw- 3 rms 29 Aug 18 20:32 foo 81908 -rw-rw-rw- 3 rms 29 Aug 18 20:32 foo2 81908 -rw-rw-rw- 3 rms 29 Aug 18 20:32 foo3This function is meaningless on operating systems where multiple names for one file are not allowed. Some systems implement multiple names by copying the file instead.
See also
file-nlinksin File Attributes.
This command renames the file filename as newname.
If filename has additional names aside from filename, it continues to have those names. In fact, adding the name newname with
add-name-to-fileand then deleting filename has the same effect as renaming, aside from momentary intermediate states.
This command copies the file oldname to newname. An error is signaled if oldname does not exist. If newname names a directory, it copies oldname into that directory, preserving its final name component.
If time is non-
nil, then this function gives the new file the same last-modified time that the old one has. (This works on only some operating systems.) If setting the time gets an error,copy-filesignals afile-date-errorerror. In an interactive call, a prefix argument specifies a non-nilvalue for time.This function copies the file modes, too.
If argument preserve-uid-gid is
nil, we let the operating system decide the user and group ownership of the new file (this is usually set to the user running Emacs). If preserve-uid-gid is non-nil, we attempt to copy the user and group ownership of the file. This works only on some operating systems, and only if you have the correct permissions to do so.
This command makes a symbolic link to filename, named newname. This is like the shell command ‘ln -s filename newname’.
This function is not available on systems that don't support symbolic links.
This command deletes the file filename, like the shell command ‘rm filename’. If the file has multiple names, it continues to exist under the other names.
A suitable kind of
file-errorerror is signaled if the file does not exist, or is not deletable. (On Unix and GNU/Linux, a file is deletable if its directory is writable.)If filename is a symbolic link,
delete-filedoes not replace it with its target, but it does follow symbolic links at all levels of parent directories.See also
delete-directoryin Create/Delete Dirs.
This function sets mode bits of filename to mode (which must be an integer when the function is called non-interactively). Only the low 12 bits of mode are used.
Interactively, mode is read from the minibuffer using
read-file-modes, which accepts mode bits either as a number or as a character string representing the mode bits symbolically. See the description ofread-file-modesbelow for the supported forms of symbolic notation for mode bits.This function recursively follows symbolic links at all levels for filename.
This function sets the default file protection for new files created by Emacs and its subprocesses. Every file created with Emacs initially has this protection, or a subset of it (
write-regionwill not give a file execute permission even if the default file protection allows execute permission). On Unix and GNU/Linux, the default protection is the bitwise complement of the “umask” value.The argument mode must be an integer. On most systems, only the low 9 bits of mode are meaningful. You can use the Lisp construct for octal character codes to enter mode; for example,
(set-default-file-modes ?\644)Saving a modified version of an existing file does not count as creating the file; it preserves the existing file's mode, whatever that is. So the default file protection has no effect.
This function reads file mode bits from the minibuffer. The optional argument prompt specifies a non-default prompt. Second optional argument base-file is the name of a file on whose permissions to base the mode bits that this function returns, if what the user types specifies mode bits relative to permissions of an existing file.
If user input represents an octal number, this function returns that number. If it is a complete symbolic specification of mode bits, as in
"u=rwx", the function converts it to the equivalent numeric value usingfile-modes-symbolic-to-numberand returns the result. If the specification is relative, as in"o+g", then the permissions on which the specification is based are taken from the mode bits of base-file. If base-file is omitted ornil, the function uses0as the base mode bits. The complete and relative specifications can be combined, as in"u+r,g+rx,o+r,g-w". See File Permissions, for detailed description of symbolic mode bits specifications.
This subroutine converts a symbolic specification of file mode bits in modes into the equivalent numeric value. If the symbolic specification is based on an existing file, that file's mode bits are taken from the optional argument base-modes; if that argument is omitted or
nil, it defaults to zero, i.e. no access rights at all.
This function sets the access and modification times of filename to time. The return value is
tif the times are successfully set, otherwise it isnil. time defaults to the current time and must be in the format returned bycurrent-time(see Time of Day).
Next: Contents of Directories, Previous: Changing Files, Up: Files
25.8 File Names
Files are generally referred to by their names, in Emacs as elsewhere. File names in Emacs are represented as strings. The functions that operate on a file all expect a file name argument.
In addition to operating on files themselves, Emacs Lisp programs often need to operate on file names; i.e., to take them apart and to use part of a name to construct related file names. This section describes how to manipulate file names.
The functions in this section do not actually access files, so they can operate on file names that do not refer to an existing file or directory.
On MS-DOS and MS-Windows, these functions (like the function that actually operate on files) accept MS-DOS or MS-Windows file-name syntax, where backslashes separate the components, as well as Unix syntax; but they always return Unix syntax. This enables Lisp programs to specify file names in Unix syntax and work properly on all systems without change.
Next: Relative File Names, Up: File Names
25.8.1 File Name Components
The operating system groups files into directories. To specify a file, you must specify the directory and the file's name within that directory. Therefore, Emacs considers a file name as having two main parts: the directory name part, and the nondirectory part (or file name within the directory). Either part may be empty. Concatenating these two parts reproduces the original file name.
On most systems, the directory part is everything up to and including the last slash (backslash is also allowed in input on MS-DOS or MS-Windows); the nondirectory part is the rest.
For some purposes, the nondirectory part is further subdivided into the name proper and the version number. On most systems, only backup files have version numbers in their names.
This function returns the directory part of filename, as a directory name (see Directory Names), or
nilif filename does not include a directory part.On GNU and Unix systems, a string returned by this function always ends in a slash. On MS-DOS it can also end in a colon.
(file-name-directory "lewis/foo") ; Unix example ⇒ "lewis/" (file-name-directory "foo") ; Unix example ⇒ nil
This function returns the nondirectory part of filename.
(file-name-nondirectory "lewis/foo") ⇒ "foo" (file-name-nondirectory "foo") ⇒ "foo" (file-name-nondirectory "lewis/") ⇒ ""
This function returns filename with any file version numbers, backup version numbers, or trailing tildes discarded.
If keep-backup-version is non-
nil, then true file version numbers understood as such by the file system are discarded from the return value, but backup version numbers are kept.(file-name-sans-versions "~rms/foo.~1~") ⇒ "~rms/foo" (file-name-sans-versions "~rms/foo~") ⇒ "~rms/foo" (file-name-sans-versions "~rms/foo") ⇒ "~rms/foo"
This function returns filename's final “extension,” if any, after applying
file-name-sans-versionsto remove any version/backup part. The extension, in a file name, is the part that follows the last ‘.’ in the last name component (minus any version/backup part).This function returns
nilfor extensionless file names such as foo. It returns""for null extensions, as in foo.. If the last component of a file name begins with a ‘.’, that ‘.’ doesn't count as the beginning of an extension. Thus, .emacs's “extension” isnil, not ‘.emacs’.If period is non-
nil, then the returned value includes the period that delimits the extension, and if filename has no extension, the value is"".
This function returns filename minus its extension, if any. The version/backup part, if present, is only removed if the file has an extension. For example,
(file-name-sans-extension "foo.lose.c") ⇒ "foo.lose" (file-name-sans-extension "big.hack/foo") ⇒ "big.hack/foo" (file-name-sans-extension "/my/home/.emacs") ⇒ "/my/home/.emacs" (file-name-sans-extension "/my/home/.emacs.el") ⇒ "/my/home/.emacs" (file-name-sans-extension "~/foo.el.~3~") ⇒ "~/foo" (file-name-sans-extension "~/foo.~3~") ⇒ "~/foo.~3~"Note that the ‘.~3~’ in the two last examples is the backup part, not an extension.
Next: Directory Names, Previous: File Name Components, Up: File Names
25.8.2 Absolute and Relative File Names
All the directories in the file system form a tree starting at the root directory. A file name can specify all the directory names starting from the root of the tree; then it is called an absolute file name. Or it can specify the position of the file in the tree relative to a default directory; then it is called a relative file name. On Unix and GNU/Linux, an absolute file name starts with a slash or a tilde (‘~’), and a relative one does not. On MS-DOS and MS-Windows, an absolute file name starts with a slash or a backslash, or with a drive specification ‘x:/’, where x is the drive letter.
This function returns
tif file filename is an absolute file name,nilotherwise.(file-name-absolute-p "~rms/foo") ⇒ t (file-name-absolute-p "rms/foo") ⇒ nil (file-name-absolute-p "/user/rms/foo") ⇒ t
Given a possibly relative file name, you can convert it to an
absolute name using expand-file-name (see File Name Expansion). This function converts absolute file names to relative
names:
This function tries to return a relative name that is equivalent to filename, assuming the result will be interpreted relative to directory (an absolute directory name or directory file name). If directory is omitted or
nil, it defaults to the current buffer's default directory.On some operating systems, an absolute file name begins with a device name. On such systems, filename has no relative equivalent based on directory if they start with two different device names. In this case,
file-relative-namereturns filename in absolute form.(file-relative-name "/foo/bar" "/foo/") ⇒ "bar" (file-relative-name "/foo/bar" "/hack/") ⇒ "../foo/bar"
Next: File Name Expansion, Previous: Relative File Names, Up: File Names
25.8.3 Directory Names
A directory name is the name of a directory. A directory is actually a kind of file, so it has a file name, which is related to the directory name but not identical to it. (This is not quite the same as the usual Unix terminology.) These two different names for the same entity are related by a syntactic transformation. On GNU and Unix systems, this is simple: a directory name ends in a slash, whereas the directory's name as a file lacks that slash. On MS-DOS the relationship is more complicated.
The difference between a directory name and its name as a file is
subtle but crucial. When an Emacs variable or function argument is
described as being a directory name, a file name of a directory is not
acceptable. When file-name-directory returns a string, that is
always a directory name.
The following two functions convert between directory names and file names. They do nothing special with environment variable substitutions such as ‘$HOME’, and the constructs ‘~’, ‘.’ and ‘..’.
This function returns a string representing filename in a form that the operating system will interpret as the name of a directory. On most systems, this means appending a slash to the string (if it does not already end in one).
(file-name-as-directory "~rms/lewis") ⇒ "~rms/lewis/"
This function returns a string representing dirname in a form that the operating system will interpret as the name of a file. On most systems, this means removing the final slash (or backslash) from the string.
(directory-file-name "~lewis/") ⇒ "~lewis"
Given a directory name, you can combine it with a relative file name
using concat:
(concat dirname relfile)
Be sure to verify that the file name is relative before doing that. If you use an absolute file name, the results could be syntactically invalid or refer to the wrong file.
If you want to use a directory file name in making such a
combination, you must first convert it to a directory name using
file-name-as-directory:
(concat (file-name-as-directory dirfile) relfile)
Don't try concatenating a slash by hand, as in
;;; Wrong!
(concat dirfile "/" relfile)
because this is not portable. Always use
file-name-as-directory.
To convert a directory name to its abbreviation, use this function:
This function returns an abbreviated form of filename. It applies the abbreviations specified in
directory-abbrev-alist(see File Aliases), then substitutes ‘~’ for the user's home directory if the argument names a file in the home directory or one of its subdirectories. If the home directory is a root directory, it is not replaced with ‘~’, because this does not make the result shorter on many systems.You can use this function for directory names and for file names, because it recognizes abbreviations even as part of the name.
Next: Unique File Names, Previous: Directory Names, Up: File Names
25.8.4 Functions that Expand Filenames
Expansion of a file name means converting a relative file name to an absolute one. Since this is done relative to a default directory, you must specify the default directory name as well as the file name to be expanded. Expansion also simplifies file names by eliminating redundancies such as ./ and name/../.
This function converts filename to an absolute file name. If directory is supplied, it is the default directory to start with if filename is relative. (The value of directory should itself be an absolute directory name or directory file name; it may start with ‘~’.) Otherwise, the current buffer's value of
default-directoryis used. For example:(expand-file-name "foo") ⇒ "/xcssun/users/rms/lewis/foo" (expand-file-name "../foo") ⇒ "/xcssun/users/rms/foo" (expand-file-name "foo" "/usr/spool/") ⇒ "/usr/spool/foo" (expand-file-name "$HOME/foo") ⇒ "/xcssun/users/rms/lewis/$HOME/foo"If the part of the combined file name before the first slash is ‘~’, it expands to the value of the HOME environment variable (usually your home directory). If the part before the first slash is ‘~user’ and if user is a valid login name, it expands to user's home directory.
Filenames containing ‘.’ or ‘..’ are simplified to their canonical form:
(expand-file-name "bar/../foo") ⇒ "/xcssun/users/rms/lewis/foo"In some cases, a leading ‘..’ component can remain in the output:
(expand-file-name "../home" "/") ⇒ "/../home"This is for the sake of filesystems that have the concept of a “superroot” above the root directory /. On other filesystems, /../ is interpreted exactly the same as /.
Note that
expand-file-namedoes not expand environment variables; onlysubstitute-in-file-namedoes that.Note also that
expand-file-namedoes not follow symbolic links at any level. This results in a difference between the wayfile-truenameandexpand-file-nametreat ‘..’. Assuming that ‘/tmp/bar’ is a symbolic link to the directory ‘/tmp/foo/bar’ we get:(file-truename "/tmp/bar/../myfile") ⇒ "/tmp/foo/myfile" (expand-file-name "/tmp/bar/../myfile") ⇒ "/tmp/myfile"If you may need to follow symbolic links preceding ‘..’, you should make sure to call
file-truenamewithout prior direct or indirect calls toexpand-file-name. See Truenames.
The value of this buffer-local variable is the default directory for the current buffer. It should be an absolute directory name; it may start with ‘~’. This variable is buffer-local in every buffer.
expand-file-nameuses the default directory when its second argument isnil.The value is always a string ending with a slash.
default-directory ⇒ "/user/lewis/manual/"
This function replaces environment variable references in filename with the environment variable values. Following standard Unix shell syntax, ‘$’ is the prefix to substitute an environment variable value. If the input contains ‘$$’, that is converted to ‘$’; this gives the user a way to “quote” a ‘$’.
The environment variable name is the series of alphanumeric characters (including underscores) that follow the ‘$’. If the character following the ‘$’ is a ‘{’, then the variable name is everything up to the matching ‘}’.
Calling
substitute-in-file-nameon output produced bysubstitute-in-file-nametends to give incorrect results. For instance, use of ‘$$’ to quote a single ‘$’ won't work properly, and ‘$’ in an environment variable's value could lead to repeated substitution. Therefore, programs that call this function and put the output where it will be passed to this function need to double all ‘$’ characters to prevent subsequent incorrect results.Here we assume that the environment variable
HOME, which holds the user's home directory name, has value ‘/xcssun/users/rms’.(substitute-in-file-name "$HOME/foo") ⇒ "/xcssun/users/rms/foo"After substitution, if a ‘~’ or a ‘/’ appears immediately after another ‘/’, the function discards everything before it (up through the immediately preceding ‘/’).
(substitute-in-file-name "bar/~/foo") ⇒ "~/foo" (substitute-in-file-name "/usr/local/$HOME/foo") ⇒ "/xcssun/users/rms/foo" ;; /usr/local/ has been discarded.
Next: File Name Completion, Previous: File Name Expansion, Up: File Names
25.8.5 Generating Unique File Names
Some programs need to write temporary files. Here is the usual way to construct a name for such a file:
(make-temp-file name-of-application)
The job of make-temp-file is to prevent two different users or
two different jobs from trying to use the exact same file name.
This function creates a temporary file and returns its name. Emacs creates the temporary file's name by adding to prefix some random characters that are different in each Emacs job. The result is guaranteed to be a newly created empty file. On MS-DOS, this function can truncate the string prefix to fit into the 8+3 file-name limits. If prefix is a relative file name, it is expanded against
temporary-file-directory.(make-temp-file "foo") ⇒ "/tmp/foo232J6v"When
make-temp-filereturns, the file has been created and is empty. At that point, you should write the intended contents into the file.If dir-flag is non-
nil,make-temp-filecreates an empty directory instead of an empty file. It returns the file name, not the directory name, of that directory. See Directory Names.If suffix is non-
nil,make-temp-fileadds it at the end of the file name.To prevent conflicts among different libraries running in the same Emacs, each Lisp program that uses
make-temp-fileshould have its own prefix. The number added to the end of prefix distinguishes between the same application running in different Emacs jobs. Additional added characters permit a large number of distinct names even in one Emacs job.
The default directory for temporary files is controlled by the
variable temporary-file-directory. This variable gives the user
a uniform way to specify the directory for all temporary files. Some
programs use small-temporary-file-directory instead, if that is
non-nil. To use it, you should expand the prefix against
the proper directory before calling make-temp-file.
In older Emacs versions where make-temp-file does not exist,
you should use make-temp-name instead:
(make-temp-name
(expand-file-name name-of-application
temporary-file-directory))
This function generates a string that can be used as a unique file name. The name starts with string, and has several random characters appended to it, which are different in each Emacs job. It is like
make-temp-fileexcept that it just constructs a name, and does not create a file. Another difference is that string should be an absolute file name. On MS-DOS, this function can truncate the string prefix to fit into the 8+3 file-name limits.
This variable specifies the directory name for creating temporary files. Its value should be a directory name (see Directory Names), but it is good for Lisp programs to cope if the value is a directory's file name instead. Using the value as the second argument to
expand-file-nameis a good way to achieve that.The default value is determined in a reasonable way for your operating system; it is based on the
TMPDIR,TMPandTEMPenvironment variables, with a fall-back to a system-dependent name if none of these variables is defined.Even if you do not use
make-temp-fileto create the temporary file, you should still use this variable to decide which directory to put the file in. However, if you expect the file to be small, you should usesmall-temporary-file-directoryfirst if that is non-nil.
This variable specifies the directory name for creating certain temporary files, which are likely to be small.
If you want to write a temporary file which is likely to be small, you should compute the directory like this:
(make-temp-file (expand-file-name prefix (or small-temporary-file-directory temporary-file-directory)))
Next: Standard File Names, Previous: Unique File Names, Up: File Names
25.8.6 File Name Completion
This section describes low-level subroutines for completing a file name. For higher level functions, see Reading File Names.
This function returns a list of all possible completions for a file whose name starts with partial-filename in directory directory. The order of the completions is the order of the files in the directory, which is unpredictable and conveys no useful information.
The argument partial-filename must be a file name containing no directory part and no slash (or backslash on some systems). The current buffer's default directory is prepended to directory, if directory is not absolute.
In the following example, suppose that ~rms/lewis is the current default directory, and has five files whose names begin with ‘f’: foo, file~, file.c, file.c.~1~, and file.c.~2~.
(file-name-all-completions "f" "") ⇒ ("foo" "file~" "file.c.~2~" "file.c.~1~" "file.c") (file-name-all-completions "fo" "") ⇒ ("foo")
This function completes the file name filename in directory directory. It returns the longest prefix common to all file names in directory directory that start with filename. If predicate is non-
nilthen it ignores possible completions that don't satisfy predicate, after calling that function with one argument, the expanded absolute file name.If only one match exists and filename matches it exactly, the function returns
t. The function returnsnilif directory directory contains no name starting with filename.In the following example, suppose that the current default directory has five files whose names begin with ‘f’: foo, file~, file.c, file.c.~1~, and file.c.~2~.
(file-name-completion "fi" "") ⇒ "file" (file-name-completion "file.c.~1" "") ⇒ "file.c.~1~" (file-name-completion "file.c.~1~" "") ⇒ t (file-name-completion "file.c.~3" "") ⇒ nil
file-name-completionusually ignores file names that end in any string in this list. It does not ignore them when all the possible completions end in one of these suffixes. This variable has no effect onfile-name-all-completions.A typical value might look like this:
completion-ignored-extensions ⇒ (".o" ".elc" "~" ".dvi")If an element of
completion-ignored-extensionsends in a slash ‘/’, it signals a directory. The elements which do not end in a slash will never match a directory; thus, the above value will not filter out a directory named foo.elc.
Previous: File Name Completion, Up: File Names
25.8.7 Standard File Names
Most of the file names used in Lisp programs are entered by the user.
But occasionally a Lisp program needs to specify a standard file name
for a particular use—typically, to hold customization information
about each user. For example, abbrev definitions are stored (by
default) in the file ~/.abbrev_defs; the completion
package stores completions in the file ~/.completions. These are
two of the many standard file names used by parts of Emacs for certain
purposes.
Various operating systems have their own conventions for valid file
names and for which file names to use for user profile data. A Lisp
program which reads a file using a standard file name ought to use, on
each type of system, a file name suitable for that system. The function
convert-standard-filename makes this easy to do.
This function alters the file name filename to fit the conventions of the operating system in use, and returns the result as a new string.
The recommended way to specify a standard file name in a Lisp program
is to choose a name which fits the conventions of GNU and Unix systems,
usually with a nondirectory part that starts with a period, and pass it
to convert-standard-filename instead of using it directly. Here
is an example from the completion package:
(defvar save-completions-file-name
(convert-standard-filename "~/.completions")
"*The file name to save completions to.")
On GNU and Unix systems, and on some other systems as well,
convert-standard-filename returns its argument unchanged. On
some other systems, it alters the name to fit the system's conventions.
For example, on MS-DOS the alterations made by this function include converting a leading ‘.’ to ‘_’, converting a ‘_’ in the middle of the name to ‘.’ if there is no other ‘.’, inserting a ‘.’ after eight characters if there is none, and truncating to three characters after the ‘.’. (It makes other changes as well.) Thus, .abbrev_defs becomes _abbrev.def, and .completions becomes _complet.ion.
Next: Create/Delete Dirs, Previous: File Names, Up: Files
25.9 Contents of Directories
A directory is a kind of file that contains other files entered under various names. Directories are a feature of the file system.
Emacs can list the names of the files in a directory as a Lisp list,
or display the names in a buffer using the ls shell command. In
the latter case, it can optionally display information about each file,
depending on the options passed to the ls command.
This function returns a list of the names of the files in the directory directory. By default, the list is in alphabetical order.
If full-name is non-
nil, the function returns the files' absolute file names. Otherwise, it returns the names relative to the specified directory.If match-regexp is non-
nil, this function returns only those file names that contain a match for that regular expression—the other file names are excluded from the list. On case-insensitive filesystems, the regular expression matching is case-insensitive.If nosort is non-
nil,directory-filesdoes not sort the list, so you get the file names in no particular order. Use this if you want the utmost possible speed and don't care what order the files are processed in. If the order of processing is visible to the user, then the user will probably be happier if you do sort the names.(directory-files "~lewis") ⇒ ("#foo#" "#foo.el#" "." ".." "dired-mods.el" "files.texi" "files.texi.~1~")An error is signaled if directory is not the name of a directory that can be read.
This is similar to
directory-filesin deciding which files to report on and how to report their names. However, instead of returning a list of file names, it returns for each file a list(filename.attributes), where attributes is whatfile-attributeswould return for that file. The optional argument id-format has the same meaning as the corresponding argument tofile-attributes(see Definition of file-attributes).
This function expands the wildcard pattern pattern, returning a list of file names that match it.
If pattern is written as an absolute file name, the values are absolute also.
If pattern is written as a relative file name, it is interpreted relative to the current default directory. The file names returned are normally also relative to the current default directory. However, if full is non-
nil, they are absolute.
This function inserts (in the current buffer) a directory listing for directory file, formatted with
lsaccording to switches. It leaves point after the inserted text. switches may be a string of options, or a list of strings representing individual options.The argument file may be either a directory name or a file specification including wildcard characters. If wildcard is non-
nil, that means treat file as a file specification with wildcards.If full-directory-p is non-
nil, that means the directory listing is expected to show the full contents of a directory. You should specifytwhen file is a directory and switches do not contain ‘-d’. (The ‘-d’ option tolssays to describe a directory itself as a file, rather than showing its contents.)On most systems, this function works by running a directory listing program whose name is in the variable
insert-directory-program. If wildcard is non-nil, it also runs the shell specified byshell-file-name, to expand the wildcards.MS-DOS and MS-Windows systems usually lack the standard Unix program
ls, so this function emulates the standard Unix programlswith Lisp code.As a technical detail, when switches contains the long ‘--dired’ option,
insert-directorytreats it specially, for the sake of dired. However, the normally equivalent short ‘-D’ option is just passed on toinsert-directory-program, as any other option.
This variable's value is the program to run to generate a directory listing for the function
insert-directory. It is ignored on systems which generate the listing with Lisp code.
25.10 Creating, Copying and Deleting Directories
Most Emacs Lisp file-manipulation functions get errors when used on
files that are directories. For example, you cannot delete a directory
with delete-file. These special functions exist to create and
delete directories.
This command creates a directory named dirname. If parents is non-
nil, as is always the case in an interactive call, that means to create the parent directories first, if they don't already exist.
mkdiris an alias for this.
This command copies the directory named dirname to newname. If newname names an existing directory, dirname will be copied to a subdirectory there.
It always sets the file modes of the copied files to match the corresponding original file.
The third arg keep-time non-
nilmeans to preserve the modification time of the copied files. A prefix arg makes keep-time non-nil.Noninteractively, the last argument parents says whether to create parent directories if they don't exist. Interactively, this happens by default.
This command deletes the directory named dirname. The function
delete-filedoes not work for files that are directories; you must usedelete-directoryfor them. If recursive isnil, and the directory contains any files,delete-directorysignals an error.
delete-directoryonly follows symbolic links at the level of parent directories.
Next: Format Conversion, Previous: Create/Delete Dirs, Up: Files
25.11 Making Certain File Names “Magic”
You can implement special handling for certain file names. This is called making those names magic. The principal use for this feature is in implementing remote file names (see Remote Files).
To define a kind of magic file name, you must supply a regular expression to define the class of names (all those that match the regular expression), plus a handler that implements all the primitive Emacs file operations for file names that do match.
The variable file-name-handler-alist holds a list of handlers,
together with regular expressions that determine when to apply each
handler. Each element has this form:
(regexp . handler)
All the Emacs primitives for file access and file name transformation
check the given file name against file-name-handler-alist. If
the file name matches regexp, the primitives handle that file by
calling handler.
The first argument given to handler is the name of the primitive, as a symbol; the remaining arguments are the arguments that were passed to that primitive. (The first of these arguments is most often the file name itself.) For example, if you do this:
(file-exists-p filename)
and filename has handler handler, then handler is called like this:
(funcall handler 'file-exists-p filename)
When a function takes two or more arguments that must be file names, it checks each of those names for a handler. For example, if you do this:
(expand-file-name filename dirname)
then it checks for a handler for filename and then for a handler for dirname. In either case, the handler is called like this:
(funcall handler 'expand-file-name filename dirname)
The handler then needs to figure out whether to handle filename or dirname.
If the specified file name matches more than one handler, the one whose match starts last in the file name gets precedence. This rule is chosen so that handlers for jobs such as uncompression are handled first, before handlers for jobs such as remote file access.
Here are the operations that a magic file name handler gets to handle:
access-file, add-name-to-file,
byte-compiler-base-file-name,
copy-directory, copy-file,
delete-directory, delete-file,
diff-latest-backup-file,
directory-file-name,
directory-files,
directory-files-and-attributes,
dired-compress-file, dired-uncache,
expand-file-name,
file-accessible-directory-p,
file-attributes,
file-directory-p,
file-executable-p, file-exists-p,
file-local-copy, file-remote-p,
file-modes, file-name-all-completions,
file-name-as-directory,
file-name-completion,
file-name-directory,
file-name-nondirectory,
file-name-sans-versions, file-newer-than-file-p,
file-ownership-preserved-p,
file-readable-p, file-regular-p, file-symlink-p,
file-truename, file-writable-p,
find-backup-file-name,
get-file-buffer,
insert-directory,
insert-file-contents,
load,
make-auto-save-file-name,
make-directory,
make-directory-internal,
make-symbolic-link,
process-file,
rename-file, set-file-modes, set-file-times,
set-visited-file-modtime, shell-command,
start-file-process,
substitute-in-file-name,
unhandled-file-name-directory,
vc-registered,
verify-visited-file-modtime,
write-region.
Handlers for insert-file-contents typically need to clear the
buffer's modified flag, with (set-buffer-modified-p nil), if the
visit argument is non-nil. This also has the effect of
unlocking the buffer if it is locked.
The handler function must handle all of the above operations, and possibly others to be added in the future. It need not implement all these operations itself—when it has nothing special to do for a certain operation, it can reinvoke the primitive, to handle the operation “in the usual way.” It should always reinvoke the primitive for an operation it does not recognize. Here's one way to do this:
(defun my-file-handler (operation &rest args)
;; First check for the specific operations
;; that we have special handling for.
(cond ((eq operation 'insert-file-contents) ...)
((eq operation 'write-region) ...)
...
;; Handle any operation we don't know about.
(t (let ((inhibit-file-name-handlers
(cons 'my-file-handler
(and (eq inhibit-file-name-operation operation)
inhibit-file-name-handlers)))
(inhibit-file-name-operation operation))
(apply operation args)))))
When a handler function decides to call the ordinary Emacs primitive for
the operation at hand, it needs to prevent the primitive from calling
the same handler once again, thus leading to an infinite recursion. The
example above shows how to do this, with the variables
inhibit-file-name-handlers and
inhibit-file-name-operation. Be careful to use them exactly as
shown above; the details are crucial for proper behavior in the case of
multiple handlers, and for operations that have two file names that may
each have handlers.
Handlers that don't really do anything special for actual access to the
file—such as the ones that implement completion of host names for
remote file names—should have a non-nil safe-magic
property. For instance, Emacs normally “protects” directory names
it finds in PATH from becoming magic, if they look like magic
file names, by prefixing them with ‘/:’. But if the handler that
would be used for them has a non-nil safe-magic
property, the ‘/:’ is not added.
A file name handler can have an operations property to
declare which operations it handles in a nontrivial way. If this
property has a non-nil value, it should be a list of
operations; then only those operations will call the handler. This
avoids inefficiency, but its main purpose is for autoloaded handler
functions, so that they won't be loaded except when they have real
work to do.
Simply deferring all operations to the usual primitives does not
work. For instance, if the file name handler applies to
file-exists-p, then it must handle load itself, because
the usual load code won't work properly in that case. However,
if the handler uses the operations property to say it doesn't
handle file-exists-p, then it need not handle load
nontrivially.
This variable holds a list of handlers whose use is presently inhibited for a certain operation.
The operation for which certain handlers are presently inhibited.
This function returns the handler function for file name file, or
nilif there is none. The argument operation should be the operation to be performed on the file—the value you will pass to the handler as its first argument when you call it. If operation equalsinhibit-file-name-operation, or if it is not found in theoperationsproperty of the handler, this function returnsnil.
This function copies file filename to an ordinary non-magic file on the local machine, if it isn't on the local machine already. Magic file names should handle the
file-local-copyoperation if they refer to files on other machines. A magic file name that is used for other purposes than remote file access should not handlefile-local-copy; then this function will treat the file as local.If filename is local, whether magic or not, this function does nothing and returns
nil. Otherwise it returns the file name of the local copy file.
This function tests whether filename is a remote file. If filename is local (not remote), the return value is
nil. If filename is indeed remote, the return value is a string that identifies the remote system.This identifier string can include a host name and a user name, as well as characters designating the method used to access the remote system. For example, the remote identifier string for the filename
/sudo::/some/fileis/sudo:root@localhost:.If
file-remote-preturns the same identifier for two different filenames, that means they are stored on the same file system and can be accessed locally with respect to each other. This means, for example, that it is possible to start a remote process accessing both files at the same time. Implementors of file handlers need to ensure this principle is valid.identification specifies which part of the identifier shall be returned as string. identification can be the symbol
method,userorhost; any other value is handled likeniland means to return the complete identifier string. In the example above, the remoteuseridentifier string would beroot.If connected is non-
nil, this function returnsnileven if filename is remote, if Emacs has no network connection to its host. This is useful when you want to avoid the delay of making connections when they don't exist.
This function returns the name of a directory that is not magic. It uses the directory part of filename if that is not magic. For a magic file name, it invokes the file name handler, which therefore decides what value to return. If filename is not accessible from a local process, then the file name handler should indicate it by returning
nil.This is useful for running a subprocess; every subprocess must have a non-magic directory to serve as its current directory, and this function is a good way to come up with one.
Previous: Magic File Names, Up: Files
25.12 File Format Conversion
Emacs performs several steps to convert the data in a buffer (text,
text properties, and possibly other information) to and from a
representation suitable for storing into a file. This section describes
the fundamental functions that perform this format conversion,
namely insert-file-contents for reading a file into a buffer,
and write-region for writing a buffer into a file.
Next: Format Conversion Round-Trip, Up: Format Conversion
25.12.1 Overview
The function insert-file-contents:
- initially, inserts bytes from the file into the buffer;
- decodes bytes to characters as appropriate;
- processes formats as defined by entries in
format-alist; and - calls functions in
after-insert-file-functions.
The function write-region:
- initially, calls functions in
write-region-annotate-functions; - processes formats as defined by entries in
format-alist; - encodes characters to bytes as appropriate; and
- modifies the file with the bytes.
This shows the symmetry of the lowest-level operations; reading and writing handle things in opposite order. The rest of this section describes the two facilities surrounding the three variables named above, as well as some related functions. Coding Systems, for details on character encoding and decoding.
25.12.2 Round-Trip Specification
The most general of the two facilities is controlled by the variable
format-alist, a list of file format specifications, which
describe textual representations used in files for the data in an Emacs
buffer. The descriptions for reading and writing are paired, which is
why we call this “round-trip” specification
(see Format Conversion Piecemeal, for non-paired specification).
This list contains one format definition for each defined file format. Each format definition is a list of this form:
(name doc-string regexp from-fn to-fn modify mode-fn preserve)
Here is what the elements in a format definition mean:
- name
- The name of this format.
- doc-string
- A documentation string for the format.
- regexp
- A regular expression which is used to recognize files represented in
this format. If
nil, the format is never applied automatically. - from-fn
- A shell command or function to decode data in this format (to convert
file data into the usual Emacs data representation).
A shell command is represented as a string; Emacs runs the command as a filter to perform the conversion.
If from-fn is a function, it is called with two arguments, begin and end, which specify the part of the buffer it should convert. It should convert the text by editing it in place. Since this can change the length of the text, from-fn should return the modified end position.
One responsibility of from-fn is to make sure that the beginning of the file no longer matches regexp. Otherwise it is likely to get called again.
- to-fn
- A shell command or function to encode data in this format—that is, to
convert the usual Emacs data representation into this format.
If to-fn is a string, it is a shell command; Emacs runs the command as a filter to perform the conversion.
If to-fn is a function, it is called with three arguments: begin and end, which specify the part of the buffer it should convert, and buffer, which specifies which buffer. There are two ways it can do the conversion:
- By editing the buffer in place. In this case, to-fn should return the end-position of the range of text, as modified.
- By returning a list of annotations. This is a list of elements of the
form
(position.string), where position is an integer specifying the relative position in the text to be written, and string is the annotation to add there. The list must be sorted in order of position when to-fn returns it.When
write-regionactually writes the text from the buffer to the file, it intermixes the specified annotations at the corresponding positions. All this takes place without modifying the buffer.
- modify
- A flag,
tif the encoding function modifies the buffer, andnilif it works by returning a list of annotations. - mode-fn
- A minor-mode function to call after visiting a file converted from this
format. The function is called with one argument, the integer 1;
that tells a minor-mode function to enable the mode.
- preserve
- A flag,
tifformat-write-fileshould not remove this format frombuffer-file-format.
The function insert-file-contents automatically recognizes file
formats when it reads the specified file. It checks the text of the
beginning of the file against the regular expressions of the format
definitions, and if it finds a match, it calls the decoding function for
that format. Then it checks all the known formats over again.
It keeps checking them until none of them is applicable.
Visiting a file, with find-file-noselect or the commands that use
it, performs conversion likewise (because it calls
insert-file-contents); it also calls the mode function for each
format that it decodes. It stores a list of the format names in the
buffer-local variable buffer-file-format.
This variable states the format of the visited file. More precisely, this is a list of the file format names that were decoded in the course of visiting the current buffer's file. It is always buffer-local in all buffers.
When write-region writes data into a file, it first calls the
encoding functions for the formats listed in buffer-file-format,
in the order of appearance in the list.
This command writes the current buffer contents into the file file in a format based on format, which is a list of format names. It constructs the actual format starting from format, then appending any elements from the value of
buffer-file-formatwith a non-nil preserve flag (see above), if they are not already present in format. It then updatesbuffer-file-formatwith this format, making it the default for future saves. Except for the format argument, this command is similar towrite-file. In particular, confirm has the same meaning and interactive treatment as the corresponding argument towrite-file. See Definition of write-file.
This command finds the file file, converting it according to format format. It also makes format the default if the buffer is saved later.
The argument format is a list of format names. If format is
nil, no conversion takes place. Interactively, typing just <RET> for format specifiesnil.
This command inserts the contents of file file, converting it according to format format. If beg and end are non-
nil, they specify which part of the file to read, as ininsert-file-contents(see Reading from Files).The return value is like what
insert-file-contentsreturns: a list of the absolute file name and the length of the data inserted (after conversion).The argument format is a list of format names. If format is
nil, no conversion takes place. Interactively, typing just <RET> for format specifiesnil.
This variable specifies the format to use for auto-saving. Its value is a list of format names, just like the value of
buffer-file-format; however, it is used instead ofbuffer-file-formatfor writing auto-save files. If the value ist, the default, auto-saving uses the same format as a regular save in the same buffer. This variable is always buffer-local in all buffers.
Previous: Format Conversion Round-Trip, Up: Format Conversion
25.12.3 Piecemeal Specification
In contrast to the round-trip specification described in the previous
subsection (see Format Conversion Round-Trip), you can use the variables
after-insert-file-functions and write-region-annotate-functions
to separately control the respective reading and writing conversions.
Conversion starts with one representation and produces another representation. When there is only one conversion to do, there is no conflict about what to start with. However, when there are multiple conversions involved, conflict may arise when two conversions need to start with the same data.
This situation is best understood in the context of converting text
properties during write-region. For example, the character at
position 42 in a buffer is ‘X’ with a text property foo. If
the conversion for foo is done by inserting into the buffer, say,
‘FOO:’, then that changes the character at position 42 from
‘X’ to ‘F’. The next conversion will start with the wrong
data straight away.
To avoid conflict, cooperative conversions do not modify the buffer,
but instead specify annotations, a list of elements of the form
(position . string), sorted in order of increasing
position.
If there is more than one conversion, write-region merges their
annotations destructively into one sorted list. Later, when the text
from the buffer is actually written to the file, it intermixes the
specified annotations at the corresponding positions. All this takes
place without modifying the buffer.
In contrast, when reading, the annotations intermixed with the text
are handled immediately. insert-file-contents sets point to
the beginning of some text to be converted, then calls the conversion
functions with the length of that text. These functions should always
return with point at the beginning of the inserted text. This
approach makes sense for reading because annotations removed by the
first converter can't be mistakenly processed by a later converter.
Each conversion function should scan for the annotations it
recognizes, remove the annotation, modify the buffer text (to set a
text property, for example), and return the updated length of the
text, as it stands after those changes. The value returned by one
function becomes the argument to the next function.
A list of functions for
write-regionto call. Each function in the list is called with two arguments: the start and end of the region to be written. These functions should not alter the contents of the buffer. Instead, they should return annotations.As a special case, a function may return with a different buffer current. Emacs takes this to mean that the current buffer contains altered text to be output. It therefore changes the start and end arguments of the
write-regioncall, giving them the values ofpoint-minandpoint-maxin the new buffer, respectively. It also discards all previous annotations, because they should have been dealt with by this function.
The value of this variable, if non-
nil, should be a function. This function is called, with no arguments, afterwrite-regionhas completed.If any function in
write-region-annotate-functionsreturns with a different buffer current, Emacs callswrite-region-post-annotation-functionmore than once. Emacs calls it with the last buffer that was current, and again with the buffer before that, and so on back to the original buffer.Thus, a function in
write-region-annotate-functionscan create a buffer, give this variable the local value ofkill-bufferin that buffer, set up the buffer with altered text, and make the buffer current. The buffer will be killed afterwrite-regionis done.
Each function in this list is called by
insert-file-contentswith one argument, the number of characters inserted, and with point at the beginning of the inserted text. Each function should leave point unchanged, and return the new character count describing the inserted text as modified by the function.
We invite users to write Lisp programs to store and retrieve text properties in files, using these hooks, and thus to experiment with various data formats and find good ones. Eventually we hope users will produce good, general extensions we can install in Emacs.
We suggest not trying to handle arbitrary Lisp objects as text property names or values—because a program that general is probably difficult to write, and slow. Instead, choose a set of possible data types that are reasonably flexible, and not too hard to encode.
26 Backups and Auto-Saving
Backup files and auto-save files are two methods by which Emacs tries to protect the user from the consequences of crashes or of the user's own errors. Auto-saving preserves the text from earlier in the current editing session; backup files preserve file contents prior to the current session.
Next: Auto-Saving, Up: Backups and Auto-Saving
26.1 Backup Files
A backup file is a copy of the old contents of a file you are editing. Emacs makes a backup file the first time you save a buffer into its visited file. Thus, normally, the backup file contains the contents of the file as it was before the current editing session. The contents of the backup file normally remain unchanged once it exists.
Backups are usually made by renaming the visited file to a new name. Optionally, you can specify that backup files should be made by copying the visited file. This choice makes a difference for files with multiple names; it also can affect whether the edited file remains owned by the original owner or becomes owned by the user editing it.
By default, Emacs makes a single backup file for each file edited. You can alternatively request numbered backups; then each new backup file gets a new name. You can delete old numbered backups when you don't want them any more, or Emacs can delete them automatically.
Next: Rename or Copy, Up: Backup Files
26.1.1 Making Backup Files
This function makes a backup of the file visited by the current buffer, if appropriate. It is called by
save-bufferbefore saving the buffer the first time.If a backup was made by renaming, the return value is a cons cell of the form (modes . backupname), where modes are the mode bits of the original file, as returned by
file-modes(see Other Information about Files), and backupname is the name of the backup. In all other cases, that is, if a backup was made by copying or if no backup was made, this function returnsnil.
This buffer-local variable says whether this buffer's file has been backed up on account of this buffer. If it is non-
nil, the backup file has been written. Otherwise, the file should be backed up when it is next saved (if backups are enabled). This is a permanent local;kill-all-local-variablesdoes not alter it.
This variable determines whether or not to make backup files. If it is non-
nil, then Emacs creates a backup of each file when it is saved for the first time—provided thatbackup-inhibitedisnil(see below).The following example shows how to change the
make-backup-filesvariable only in the Rmail buffers and not elsewhere. Setting itnilstops Emacs from making backups of these files, which may save disk space. (You would put this code in your init file.)(add-hook 'rmail-mode-hook (lambda () (set (make-local-variable 'make-backup-files) nil)))
This variable's value is a function to be called on certain occasions to decide whether a file should have backup files. The function receives one argument, an absolute file name to consider. If the function returns
nil, backups are disabled for that file. Otherwise, the other variables in this section say whether and how to make backups.The default value is
normal-backup-enable-predicate, which checks for files intemporary-file-directoryandsmall-temporary-file-directory.
If this variable is non-
nil, backups are inhibited. It records the result of testingbackup-enable-predicateon the visited file name. It can also coherently be used by other mechanisms that inhibit backups based on which file is visited. For example, VC sets this variable non-nilto prevent making backups for files managed with a version control system.This is a permanent local, so that changing the major mode does not lose its value. Major modes should not set this variable—they should set
make-backup-filesinstead.
This variable's value is an alist of filename patterns and backup directory names. Each element looks like
(regexp . directory)Backups of files with names matching regexp will be made in directory. directory may be relative or absolute. If it is absolute, so that all matching files are backed up into the same directory, the file names in this directory will be the full name of the file backed up with all directory separators changed to ‘!’ to prevent clashes. This will not work correctly if your filesystem truncates the resulting name.
For the common case of all backups going into one directory, the alist should contain a single element pairing ‘"."’ with the appropriate directory name.
If this variable is
nil, or it fails to match a filename, the backup is made in the original file's directory.On MS-DOS filesystems without long names this variable is always ignored.
This variable's value is a function to use for making backups instead of the default
make-backup-file-name. A value ofnilgives the defaultmake-backup-file-namebehavior. See Naming Backup Files.This could be buffer-local to do something special for specific files. If you define it, you may need to change
backup-file-name-pandfile-name-sans-versionstoo.
Next: Numbered Backups, Previous: Making Backups, Up: Backup Files
26.1.2 Backup by Renaming or by Copying?
There are two ways that Emacs can make a backup file:
- Emacs can rename the original file so that it becomes a backup file, and then write the buffer being saved into a new file. After this procedure, any other names (i.e., hard links) of the original file now refer to the backup file. The new file is owned by the user doing the editing, and its group is the default for new files written by the user in that directory.
- Emacs can copy the original file into a backup file, and then overwrite the original file with new contents. After this procedure, any other names (i.e., hard links) of the original file continue to refer to the current (updated) version of the file. The file's owner and group will be unchanged.
The first method, renaming, is the default.
The variable backup-by-copying, if non-nil, says to use
the second method, which is to copy the original file and overwrite it
with the new buffer contents. The variable file-precious-flag,
if non-nil, also has this effect (as a sideline of its main
significance). See Saving Buffers.
If this variable is non-
nil, Emacs always makes backup files by copying.
The following three variables, when non-nil, cause the second
method to be used in certain special cases. They have no effect on the
treatment of files that don't fall into the special cases.
If this variable is non-
nil, Emacs makes backups by copying for files with multiple names (hard links).This variable is significant only if
backup-by-copyingisnil, since copying is always used when that variable is non-nil.
If this variable is non-
nil, Emacs makes backups by copying in cases where renaming would change either the owner or the group of the file.The value has no effect when renaming would not alter the owner or group of the file; that is, for files which are owned by the user and whose group matches the default for a new file created there by the user.
This variable is significant only if
backup-by-copyingisnil, since copying is always used when that variable is non-nil.
This variable, if non-
nil, specifies the same behavior asbackup-by-copying-when-mismatch, but only for certain user-id values: namely, those less than or equal to a certain number. You set this variable to that number.Thus, if you set
backup-by-copying-when-privileged-mismatchto 0, backup by copying is done for the superuser only, when necessary to prevent a change in the owner of the file.The default is 200.
Next: Backup Names, Previous: Rename or Copy, Up: Backup Files
26.1.3 Making and Deleting Numbered Backup Files
If a file's name is foo, the names of its numbered backup versions are foo.~v~, for various integers v, like this: foo.~1~, foo.~2~, foo.~3~, ..., foo.~259~, and so on.
This variable controls whether to make a single non-numbered backup file or multiple numbered backups.
nil- Make numbered backups if the visited file already has numbered backups; otherwise, do not. This is the default.
never- Do not make numbered backups.
- anything else
- Make numbered backups.
The use of numbered backups ultimately leads to a large number of backup versions, which must then be deleted. Emacs can do this automatically or it can ask the user whether to delete them.
The value of this variable is the number of newest versions to keep when a new numbered backup is made. The newly made backup is included in the count. The default value is 2.
The value of this variable is the number of oldest versions to keep when a new numbered backup is made. The default value is 2.
If there are backups numbered 1, 2, 3, 5, and 7, and both of these
variables have the value 2, then the backups numbered 1 and 2 are kept
as old versions and those numbered 5 and 7 are kept as new versions;
backup version 3 is excess. The function find-backup-file-name
(see Backup Names) is responsible for determining which backup
versions to delete, but does not delete them itself.
If this variable is
t, then saving a file deletes excess backup versions silently. If it isnil, that means to ask for confirmation before deleting excess backups. Otherwise, they are not deleted at all.
This variable specifies how many of the newest backup versions to keep in the Dired command . (
dired-clean-directory). That's the same thingkept-new-versionsspecifies when you make a new backup file. The default is 2.
Previous: Numbered Backups, Up: Backup Files
26.1.4 Naming Backup Files
The functions in this section are documented mainly because you can customize the naming conventions for backup files by redefining them. If you change one, you probably need to change the rest.
This function returns a non-
nilvalue if filename is a possible name for a backup file. It just checks the name, not whether a file with the name filename exists.(backup-file-name-p "foo") ⇒ nil (backup-file-name-p "foo~") ⇒ 3The standard definition of this function is as follows:
(defun backup-file-name-p (file) "Return non-nil if FILE is a backup file \ name (numeric or not)..." (string-match "~\\'" file))Thus, the function returns a non-
nilvalue if the file name ends with a ‘~’. (We use a backslash to split the documentation string's first line into two lines in the text, but produce just one line in the string itself.)This simple expression is placed in a separate function to make it easy to redefine for customization.
This function returns a string that is the name to use for a non-numbered backup file for file filename. On Unix, this is just filename with a tilde appended.
The standard definition of this function, on most operating systems, is as follows:
(defun make-backup-file-name (file) "Create the non-numeric backup file name for FILE..." (concat file "~"))You can change the backup-file naming convention by redefining this function. The following example redefines
make-backup-file-nameto prepend a ‘.’ in addition to appending a tilde:(defun make-backup-file-name (filename) (expand-file-name (concat "." (file-name-nondirectory filename) "~") (file-name-directory filename))) (make-backup-file-name "backups.texi") ⇒ ".backups.texi~"Some parts of Emacs, including some Dired commands, assume that backup file names end with ‘~’. If you do not follow that convention, it will not cause serious problems, but these commands may give less-than-desirable results.
This function computes the file name for a new backup file for filename. It may also propose certain existing backup files for deletion.
find-backup-file-namereturns a list whose car is the name for the new backup file and whose cdr is a list of backup files whose deletion is proposed. The value can also benil, which means not to make a backup.Two variables,
kept-old-versionsandkept-new-versions, determine which backup versions should be kept. This function keeps those versions by excluding them from the cdr of the value. See Numbered Backups.In this example, the value says that ~rms/foo.~5~ is the name to use for the new backup file, and ~rms/foo.~3~ is an “excess” version that the caller should consider deleting now.
(find-backup-file-name "~rms/foo") ⇒ ("~rms/foo.~5~" "~rms/foo.~3~")
This function returns the name of the most recent backup file for filename, or
nilif that file has no backup files.Some file comparison commands use this function so that they can automatically compare a file with its most recent backup.
26.2 Auto-Saving
Emacs periodically saves all files that you are visiting; this is called auto-saving. Auto-saving prevents you from losing more than a limited amount of work if the system crashes. By default, auto-saves happen every 300 keystrokes, or after around 30 seconds of idle time. See Auto Save, for information on auto-save for users. Here we describe the functions used to implement auto-saving and the variables that control them.
This buffer-local variable is the name of the file used for auto-saving the current buffer. It is
nilif the buffer should not be auto-saved.buffer-auto-save-file-name ⇒ "/xcssun/users/rms/lewis/#backups.texi#"
When used interactively without an argument, this command is a toggle switch: it turns on auto-saving of the current buffer if it is off, and vice versa. With an argument arg, the command turns auto-saving on if the value of arg is
t, a nonempty list, or a positive integer. Otherwise, it turns auto-saving off.
This function returns a non-
nilvalue if filename is a string that could be the name of an auto-save file. It assumes the usual naming convention for auto-save files: a name that begins and ends with hash marks (‘#’) is a possible auto-save file name. The argument filename should not contain a directory part.(make-auto-save-file-name) ⇒ "/xcssun/users/rms/lewis/#backups.texi#" (auto-save-file-name-p "#backups.texi#") ⇒ 0 (auto-save-file-name-p "backups.texi") ⇒ nilThe standard definition of this function is as follows:
(defun auto-save-file-name-p (filename) "Return non-nil if FILENAME can be yielded by..." (string-match "^#.*#$" filename))This function exists so that you can customize it if you wish to change the naming convention for auto-save files. If you redefine it, be sure to redefine the function
make-auto-save-file-namecorrespondingly.
This function returns the file name to use for auto-saving the current buffer. This is just the file name with hash marks (‘#’) prepended and appended to it. This function does not look at the variable
auto-save-visited-file-name(described below); callers of this function should check that variable first.(make-auto-save-file-name) ⇒ "/xcssun/users/rms/lewis/#backups.texi#"Here is a simplified version of the standard definition of this function:
(defun make-auto-save-file-name () "Return file name to use for auto-saves \ of current buffer.." (if buffer-file-name (concat (file-name-directory buffer-file-name) "#" (file-name-nondirectory buffer-file-name) "#") (expand-file-name (concat "#%" (buffer-name) "#"))))This exists as a separate function so that you can redefine it to customize the naming convention for auto-save files. Be sure to change
auto-save-file-name-pin a corresponding way.
If this variable is non-
nil, Emacs auto-saves buffers in the files they are visiting. That is, the auto-save is done in the same file that you are editing. Normally, this variable isnil, so auto-save files have distinct names that are created bymake-auto-save-file-name.When you change the value of this variable, the new value does not take effect in an existing buffer until the next time auto-save mode is reenabled in it. If auto-save mode is already enabled, auto-saves continue to go in the same file name until
auto-save-modeis called again.
This function returns
tif the current buffer has been auto-saved since the last time it was read in or saved.
This function marks the current buffer as auto-saved. The buffer will not be auto-saved again until the buffer text is changed again. The function returns
nil.
The value of this variable specifies how often to do auto-saving, in terms of number of input events. Each time this many additional input events are read, Emacs does auto-saving for all buffers in which that is enabled. Setting this to zero disables autosaving based on the number of characters typed.
The value of this variable is the number of seconds of idle time that should cause auto-saving. Each time the user pauses for this long, Emacs does auto-saving for all buffers in which that is enabled. (If the current buffer is large, the specified timeout is multiplied by a factor that increases as the size increases; for a million-byte buffer, the factor is almost 4.)
If the value is zero or
nil, then auto-saving is not done as a result of idleness, only after a certain number of input events as specified byauto-save-interval.
If this variable is non-
nil, buffers that are visiting files have auto-saving enabled by default. Otherwise, they do not.
This function auto-saves all buffers that need to be auto-saved. It saves all buffers for which auto-saving is enabled and that have been changed since the previous auto-save.
If any buffers are auto-saved,
do-auto-savenormally displays a message saying ‘Auto-saving...’ in the echo area while auto-saving is going on. However, if no-message is non-nil, the message is inhibited.If current-only is non-
nil, only the current buffer is auto-saved.
This function deletes the current buffer's auto-save file if
delete-auto-save-filesis non-nil. It is called every time a buffer is saved.Unless force is non-
nil, this function only deletes the file if it was written by the current Emacs session since the last true save.
This variable is used by the function
delete-auto-save-file-if-necessary. If it is non-nil, Emacs deletes auto-save files when a true save is done (in the visited file). This saves disk space and unclutters your directory.
This function adjusts the current buffer's auto-save file name if the visited file name has changed. It also renames an existing auto-save file, if it was made in the current Emacs session. If the visited file name has not changed, this function does nothing.
The value of this buffer-local variable is the length of the current buffer, when it was last read in, saved, or auto-saved. This is used to detect a substantial decrease in size, and turn off auto-saving in response.
If it is −1, that means auto-saving is temporarily shut off in this buffer due to a substantial decrease in size. Explicitly saving the buffer stores a positive value in this variable, thus reenabling auto-saving. Turning auto-save mode off or on also updates this variable, so that the substantial decrease in size is forgotten.
If it is −2, that means this buffer should disregard changes in buffer size; in particular, it should not shut off auto-saving temporarily due to changes in buffer size.
This variable (if non-
nil) specifies a file for recording the names of all the auto-save files. Each time Emacs does auto-saving, it writes two lines into this file for each buffer that has auto-saving enabled. The first line gives the name of the visited file (it's empty if the buffer has none), and the second gives the name of the auto-save file.When Emacs exits normally, it deletes this file; if Emacs crashes, you can look in the file to find all the auto-save files that might contain work that was otherwise lost. The
recover-sessioncommand uses this file to find them.The default name for this file specifies your home directory and starts with ‘.saves-’. It also contains the Emacs process ID and the host name.
After Emacs reads your init file, it initializes
auto-save-list-file-name(if you have not already set it non-nil) based on this prefix, adding the host name and process ID. If you set this tonilin your init file, then Emacs does not initializeauto-save-list-file-name.
Previous: Auto-Saving, Up: Backups and Auto-Saving
26.3 Reverting
If you have made extensive changes to a file and then change your mind
about them, you can get rid of them by reading in the previous version
of the file with the revert-buffer command. See Reverting a Buffer.
This command replaces the buffer text with the text of the visited file on disk. This action undoes all changes since the file was visited or saved.
By default, if the latest auto-save file is more recent than the visited file, and the argument ignore-auto is
nil,revert-bufferasks the user whether to use that auto-save instead. When you invoke this command interactively, ignore-auto istif there is no numeric prefix argument; thus, the interactive default is not to check the auto-save file.Normally,
revert-bufferasks for confirmation before it changes the buffer; but if the argument noconfirm is non-nil,revert-bufferdoes not ask for confirmation.Normally, this command reinitializes the buffer's major and minor modes using
normal-mode. But if preserve-modes is non-nil, the modes remain unchanged.Reverting tries to preserve marker positions in the buffer by using the replacement feature of
insert-file-contents. If the buffer contents and the file contents are identical before the revert operation, reverting preserves all the markers. If they are not identical, reverting does change the buffer; in that case, it preserves the markers in the unchanged text (if any) at the beginning and end of the buffer. Preserving any additional markers would be problematical.
You can customize how revert-buffer does its work by setting
the variables described in the rest of this section.
This variable holds a list of files that should be reverted without query. The value is a list of regular expressions. If the visited file name matches one of these regular expressions, and the file has changed on disk but the buffer is not modified, then
revert-bufferreverts the file without asking the user for confirmation.
Some major modes customize revert-buffer by making
buffer-local bindings for these variables:
The value of this variable is the function to use to revert this buffer. If non-
nil, it should be a function with two optional arguments to do the work of reverting. The two optional arguments, ignore-auto and noconfirm, are the arguments thatrevert-bufferreceived. If the value isnil, reverting works the usual way.Modes such as Dired mode, in which the text being edited does not consist of a file's contents but can be regenerated in some other fashion, can give this variable a buffer-local value that is a function to regenerate the contents.
The value of this variable, if non-
nil, specifies the function to use to insert the updated contents when reverting this buffer. The function receives two arguments: first the file name to use; second,tif the user has asked to read the auto-save file.The reason for a mode to set this variable instead of
revert-buffer-functionis to avoid duplicating or replacing the rest of whatrevert-bufferdoes: asking for confirmation, clearing the undo list, deciding the proper major mode, and running the hooks listed below.
This normal hook is run by
revert-bufferbefore inserting the modified contents—but only ifrevert-buffer-functionisnil.
This normal hook is run by
revert-bufferafter inserting the modified contents—but only ifrevert-buffer-functionisnil.
Next: Windows, Previous: Backups and Auto-Saving, Up: Top
27 Buffers
A buffer is a Lisp object containing text to be edited. Buffers are used to hold the contents of files that are being visited; there may also be buffers that are not visiting files. While several buffers may exist at one time, only one buffer is designated the current buffer at any time. Most editing commands act on the contents of the current buffer. Each buffer, including the current buffer, may or may not be displayed in any windows.
Next: Current Buffer, Up: Buffers
27.1 Buffer Basics
A buffer is a Lisp object containing text to be edited. Buffers are used to hold the contents of files that are being visited; there may also be buffers that are not visiting files. Although several buffers normally exist, only one buffer is designated the current buffer at any time. Most editing commands act on the contents of the current buffer. Each buffer, including the current buffer, may or may not be displayed in any windows.
Buffers in Emacs editing are objects that have distinct names and hold text that can be edited. Buffers appear to Lisp programs as a special data type. You can think of the contents of a buffer as a string that you can extend; insertions and deletions may occur in any part of the buffer. See Text.
A Lisp buffer object contains numerous pieces of information. Some of this information is directly accessible to the programmer through variables, while other information is accessible only through special-purpose functions. For example, the visited file name is directly accessible through a variable, while the value of point is accessible only through a primitive function.
Buffer-specific information that is directly accessible is stored in
buffer-local variable bindings, which are variable values that are
effective only in a particular buffer. This feature allows each buffer
to override the values of certain variables. Most major modes override
variables such as fill-column or comment-column in this
way. For more information about buffer-local variables and functions
related to them, see Buffer-Local Variables.
For functions and variables related to visiting files in buffers, see Visiting Files and Saving Buffers. For functions and variables related to the display of buffers in windows, see Buffers and Windows.
Next: Buffer Names, Previous: Buffer Basics, Up: Buffers
27.2 The Current Buffer
There are, in general, many buffers in an Emacs session. At any time, one of them is designated as the current buffer. This is the buffer in which most editing takes place, because most of the primitives for examining or changing text in a buffer operate implicitly on the current buffer (see Text). Normally the buffer that is displayed on the screen in the selected window is the current buffer, but this is not always so: a Lisp program can temporarily designate any buffer as current in order to operate on its contents, without changing what is displayed on the screen.
The way to designate a current buffer in a Lisp program is by calling
set-buffer. The specified buffer remains current until a new one
is designated.
When an editing command returns to the editor command loop, the
command loop designates the buffer displayed in the selected window as
current, to prevent confusion: the buffer that the cursor is in when
Emacs reads a command is the buffer that the command will apply to.
(See Command Loop.) Therefore, set-buffer is not the way to
switch visibly to a different buffer so that the user can edit it. For
that, you must use the functions described in Displaying Buffers.
Warning: Lisp functions that change to a different current buffer
should not depend on the command loop to set it back afterwards.
Editing commands written in Emacs Lisp can be called from other programs
as well as from the command loop; it is convenient for the caller if
the subroutine does not change which buffer is current (unless, of
course, that is the subroutine's purpose). Therefore, you should
normally use set-buffer within a save-current-buffer or
save-excursion (see Excursions) form that will restore the
current buffer when your function is done. Here, as an example, is a
simplified version of the command append-to-buffer:
(defun append-to-buffer (buffer start end)
"Append to specified buffer the text of the region."
(interactive "BAppend to buffer: \nr")
(let ((oldbuf (current-buffer)))
(save-current-buffer
(set-buffer (get-buffer-create buffer))
(insert-buffer-substring oldbuf start end))))
This function binds a local variable to record the current buffer, and
then save-current-buffer arranges to make it current again.
Next, set-buffer makes the specified buffer current. Finally,
insert-buffer-substring copies the string from the original
current buffer to the specified (and now current) buffer.
If the buffer appended to happens to be displayed in some window, the next redisplay will show how its text has changed. Otherwise, you will not see the change immediately on the screen. The buffer becomes current temporarily during the execution of the command, but this does not cause it to be displayed.
If you make local bindings (with let or function arguments) for
a variable that may also have buffer-local bindings, make sure that the
same buffer is current at the beginning and at the end of the local
binding's scope. Otherwise you might bind it in one buffer and unbind
it in another! There are two ways to do this. In simple cases, you may
see that nothing ever changes the current buffer within the scope of the
binding. Otherwise, use save-current-buffer or
save-excursion to make sure that the buffer current at the
beginning is current again whenever the variable is unbound.
Do not rely on using set-buffer to change the current buffer
back, because that won't do the job if a quit happens while the wrong
buffer is current. For instance, in the previous example, it would
have been wrong to do this:
(let ((oldbuf (current-buffer)))
(set-buffer (get-buffer-create buffer))
(insert-buffer-substring oldbuf start end)
(set-buffer oldbuf))
Using save-current-buffer, as we did, handles quitting, errors,
and throw, as well as ordinary evaluation.
This function returns the current buffer.
(current-buffer) ⇒ #<buffer buffers.texi>
This function makes buffer-or-name the current buffer. buffer-or-name must be an existing buffer or the name of an existing buffer. The return value is the buffer made current.
This function does not display the buffer in any window, so the user cannot necessarily see the buffer. But Lisp programs will now operate on it.
The
save-current-bufferspecial form saves the identity of the current buffer, evaluates the body forms, and finally restores that buffer as current. The return value is the value of the last form in body. The current buffer is restored even in case of an abnormal exit viathrowor error (see Nonlocal Exits).If the buffer that used to be current has been killed by the time of exit from
save-current-buffer, then it is not made current again, of course. Instead, whichever buffer was current just before exit remains current.
The
with-current-buffermacro saves the identity of the current buffer, makes buffer-or-name current, evaluates the body forms, and finally restores the current buffer. buffer-or-name must specify an existing buffer or the name of an existing buffer.The return value is the value of the last form in body. The current buffer is restored even in case of an abnormal exit via
throwor error (see Nonlocal Exits).
The
with-temp-buffermacro evaluates the body forms with a temporary buffer as the current buffer. It saves the identity of the current buffer, creates a temporary buffer and makes it current, evaluates the body forms, and finally restores the previous current buffer while killing the temporary buffer. By default, undo information (see Undo) is not recorded in the buffer created by this macro (but body can enable that, if needed).The return value is the value of the last form in body. You can return the contents of the temporary buffer by using
(buffer-string)as the last form.The current buffer is restored even in case of an abnormal exit via
throwor error (see Nonlocal Exits).See also
with-temp-filein Writing to Files.
Next: Buffer File Name, Previous: Current Buffer, Up: Buffers
27.3 Buffer Names
Each buffer has a unique name, which is a string. Many of the functions that work on buffers accept either a buffer or a buffer name as an argument. Any argument called buffer-or-name is of this sort, and an error is signaled if it is neither a string nor a buffer. Any argument called buffer must be an actual buffer object, not a name.
Buffers that are ephemeral and generally uninteresting to the user
have names starting with a space, so that the list-buffers and
buffer-menu commands don't mention them (but if such a buffer
visits a file, it is mentioned). A name starting with
space also initially disables recording undo information; see
Undo.
This function returns the name of buffer as a string. buffer defaults to the current buffer.
If
buffer-namereturnsnil, it means that buffer has been killed. See Killing Buffers.(buffer-name) ⇒ "buffers.texi" (setq foo (get-buffer "temp")) ⇒ #<buffer temp> (kill-buffer foo) ⇒ nil (buffer-name foo) ⇒ nil foo ⇒ #<killed buffer>
This function renames the current buffer to newname. An error is signaled if newname is not a string.
Ordinarily,
rename-buffersignals an error if newname is already in use. However, if unique is non-nil, it modifies newname to make a name that is not in use. Interactively, you can make unique non-nilwith a numeric prefix argument. (This is how the commandrename-uniquelyis implemented.)This function returns the name actually given to the buffer.
This function returns the buffer specified by buffer-or-name. If buffer-or-name is a string and there is no buffer with that name, the value is
nil. If buffer-or-name is a buffer, it is returned as given; that is not very useful, so the argument is usually a name. For example:(setq b (get-buffer "lewis")) ⇒ #<buffer lewis> (get-buffer b) ⇒ #<buffer lewis> (get-buffer "Frazzle-nots") ⇒ nilSee also the function
get-buffer-createin Creating Buffers.
This function returns a name that would be unique for a new buffer—but does not create the buffer. It starts with starting-name, and produces a name not currently in use for any buffer by appending a number inside of ‘<...>’. It starts at 2 and keeps incrementing the number until it is not the name of an existing buffer.
If the optional second argument ignore is non-
nil, it should be a string, a potential buffer name. It means to consider that potential buffer acceptable, if it is tried, even it is the name of an existing buffer (which would normally be rejected). Thus, if buffers named ‘foo’, ‘foo<2>’, ‘foo<3>’ and ‘foo<4>’ exist,(generate-new-buffer-name "foo") ⇒ "foo<5>" (generate-new-buffer-name "foo" "foo<3>") ⇒ "foo<3>" (generate-new-buffer-name "foo" "foo<6>") ⇒ "foo<5>"See the related function
generate-new-bufferin Creating Buffers.
Next: Buffer Modification, Previous: Buffer Names, Up: Buffers
27.4 Buffer File Name
The buffer file name is the name of the file that is visited in
that buffer. When a buffer is not visiting a file, its buffer file name
is nil. Most of the time, the buffer name is the same as the
nondirectory part of the buffer file name, but the buffer file name and
the buffer name are distinct and can be set independently.
See Visiting Files.
This function returns the absolute file name of the file that buffer is visiting. If buffer is not visiting any file,
buffer-file-namereturnsnil. If buffer is not supplied, it defaults to the current buffer.(buffer-file-name (other-buffer)) ⇒ "/usr/user/lewis/manual/files.texi"
This buffer-local variable contains the name of the file being visited in the current buffer, or
nilif it is not visiting a file. It is a permanent local variable, unaffected bykill-all-local-variables.buffer-file-name ⇒ "/usr/user/lewis/manual/buffers.texi"It is risky to change this variable's value without doing various other things. Normally it is better to use
set-visited-file-name(see below); some of the things done there, such as changing the buffer name, are not strictly necessary, but others are essential to avoid confusing Emacs.
This buffer-local variable holds the abbreviated truename of the file visited in the current buffer, or
nilif no file is visited. It is a permanent local, unaffected bykill-all-local-variables. See Truenames, and Definition of abbreviate-file-name.
This buffer-local variable holds the file number and directory device number of the file visited in the current buffer, or
nilif no file or a nonexistent file is visited. It is a permanent local, unaffected bykill-all-local-variables.The value is normally a list of the form
(filenum devnum). This pair of numbers uniquely identifies the file among all files accessible on the system. See the functionfile-attributes, in File Attributes, for more information about them.If
buffer-file-nameis the name of a symbolic link, then both numbers refer to the recursive target.
This function returns the buffer visiting file filename. If there is no such buffer, it returns
nil. The argument filename, which must be a string, is expanded (see File Name Expansion), then compared against the visited file names of all live buffers. Note that the buffer'sbuffer-file-namemust match the expansion of filename exactly. This function will not recognize other names for the same file.(get-file-buffer "buffers.texi") ⇒ #<buffer buffers.texi>In unusual circumstances, there can be more than one buffer visiting the same file name. In such cases, this function returns the first such buffer in the buffer list.
This is like
get-file-buffer, except that it can return any buffer visiting the file possibly under a different name. That is, the buffer'sbuffer-file-namedoes not need to match the expansion of filename exactly, it only needs to refer to the same file. If predicate is non-nil, it should be a function of one argument, a buffer visiting filename. The buffer is only considered a suitable return value if predicate returns non-nil. If it can not find a suitable buffer to return,find-buffer-visitingreturnsnil.
If filename is a non-empty string, this function changes the name of the file visited in the current buffer to filename. (If the buffer had no visited file, this gives it one.) The next time the buffer is saved it will go in the newly-specified file.
This command marks the buffer as modified, since it does not (as far as Emacs knows) match the contents of filename, even if it matched the former visited file. It also renames the buffer to correspond to the new file name, unless the new name is already in use.
If filename is
nilor the empty string, that stands for “no visited file.” In this case,set-visited-file-namemarks the buffer as having no visited file, without changing the buffer's modified flag.Normally, this function asks the user for confirmation if there already is a buffer visiting filename. If no-query is non-
nil, that prevents asking this question. If there already is a buffer visiting filename, and the user confirms or query is non-nil, this function makes the new buffer name unique by appending a number inside of ‘<...>’ to filename.If along-with-file is non-
nil, that means to assume that the former visited file has been renamed to filename. In this case, the command does not change the buffer's modified flag, nor the buffer's recorded last file modification time as reported byvisited-file-modtime(see Modification Time). If along-with-file isnil, this function clears the recorded last file modification time, after whichvisited-file-modtimereturns zero.When the function
set-visited-file-nameis called interactively, it prompts for filename in the minibuffer.
This buffer-local variable specifies a string to display in a buffer listing where the visited file name would go, for buffers that don't have a visited file name. Dired buffers use this variable.
Next: Modification Time, Previous: Buffer File Name, Up: Buffers
27.5 Buffer Modification
Emacs keeps a flag called the modified flag for each buffer, to
record whether you have changed the text of the buffer. This flag is
set to t whenever you alter the contents of the buffer, and
cleared to nil when you save it. Thus, the flag shows whether
there are unsaved changes. The flag value is normally shown in the mode
line (see Mode Line Variables), and controls saving (see Saving Buffers) and auto-saving (see Auto-Saving).
Some Lisp programs set the flag explicitly. For example, the function
set-visited-file-name sets the flag to t, because the text
does not match the newly-visited file, even if it is unchanged from the
file formerly visited.
The functions that modify the contents of buffers are described in Text.
This function returns
tif the buffer buffer has been modified since it was last read in from a file or saved, ornilotherwise. If buffer is not supplied, the current buffer is tested.
This function marks the current buffer as modified if flag is non-
nil, or as unmodified if the flag isnil.Another effect of calling this function is to cause unconditional redisplay of the mode line for the current buffer. In fact, the function
force-mode-line-updateworks by doing this:(set-buffer-modified-p (buffer-modified-p))
Like
set-buffer-modified-p, but does not force redisplay of mode lines.
This command marks the current buffer as unmodified, and not needing to be saved. If arg is non-
nil, it marks the buffer as modified, so that it will be saved at the next suitable occasion. Interactively, arg is the prefix argument.Don't use this function in programs, since it prints a message in the echo area; use
set-buffer-modified-p(above) instead.
This function returns buffer's modification-count. This is a counter that increments every time the buffer is modified. If buffer is
nil(or omitted), the current buffer is used. The counter can wrap around occasionally.
This function returns buffer's character-change modification-count. Changes to text properties leave this counter unchanged; however, each time text is inserted or removed from the buffer, the counter is reset to the value that would be returned by
buffer-modified-tick. By comparing the values returned by twobuffer-chars-modified-tickcalls, you can tell whether a character change occurred in that buffer in between the calls. If buffer isnil(or omitted), the current buffer is used.
Next: Read Only Buffers, Previous: Buffer Modification, Up: Buffers
27.6 Buffer Modification Time
Suppose that you visit a file and make changes in its buffer, and meanwhile the file itself is changed on disk. At this point, saving the buffer would overwrite the changes in the file. Occasionally this may be what you want, but usually it would lose valuable information. Emacs therefore checks the file's modification time using the functions described below before saving the file. (See File Attributes, for how to examine a file's modification time.)
This function compares what buffer has recorded for the modification time of its visited file against the actual modification time of the file as recorded by the operating system. The two should be the same unless some other process has written the file since Emacs visited or saved it.
The function returns
tif the last actual modification time and Emacs's recorded modification time are the same,nilotherwise. It also returnstif the buffer has no recorded last modification time, that is ifvisited-file-modtimewould return zero.It always returns
tfor buffers that are not visiting a file, even ifvisited-file-modtimereturns a non-zero value. For instance, it always returnstfor dired buffers. It returnstfor buffers that are visiting a file that does not exist and never existed, butnilfor file-visiting buffers whose file has been deleted.
This function clears out the record of the last modification time of the file being visited by the current buffer. As a result, the next attempt to save this buffer will not complain of a discrepancy in file modification times.
This function is called in
set-visited-file-nameand other exceptional places where the usual test to avoid overwriting a changed file should not be done.
This function returns the current buffer's recorded last file modification time, as a list of the form
(high low). (This is the same format thatfile-attributesuses to return time values; see File Attributes.)If the buffer has no recorded last modification time, this function returns zero. This case occurs, for instance, if the buffer is not visiting a file or if the time has been explicitly cleared by
clear-visited-file-modtime. Note, however, thatvisited-file-modtimereturns a list for some non-file buffers too. For instance, in a Dired buffer listing a directory, it returns the last modification time of that directory, as recorded by Dired.For a new buffer visiting a not yet existing file, high is −1 and low is 65535, that is, 2**16 - 1.
This function updates the buffer's record of the last modification time of the visited file, to the value specified by time if time is not
nil, and otherwise to the last modification time of the visited file.If time is neither
nilnor zero, it should have the form(high.low)or(high low), in either case containing two integers, each of which holds 16 bits of the time.This function is useful if the buffer was not read from the file normally, or if the file itself has been changed for some known benign reason.
This function is used to ask a user how to proceed after an attempt to modify an buffer visiting file filename when the file is newer than the buffer text. Emacs detects this because the modification time of the file on disk is newer than the last save-time of the buffer. This means some other program has probably altered the file.
Depending on the user's answer, the function may return normally, in which case the modification of the buffer proceeds, or it may signal a
file-supersessionerror with data(filename), in which case the proposed buffer modification is not allowed.This function is called automatically by Emacs on the proper occasions. It exists so you can customize Emacs by redefining it. See the file userlock.el for the standard definition.
See also the file locking mechanism in File Locks.
Next: The Buffer List, Previous: Modification Time, Up: Buffers
27.7 Read-Only Buffers
If a buffer is read-only, then you cannot change its contents, although you may change your view of the contents by scrolling and narrowing.
Read-only buffers are used in two kinds of situations:
- A buffer visiting a write-protected file is normally read-only.
Here, the purpose is to inform the user that editing the buffer with the aim of saving it in the file may be futile or undesirable. The user who wants to change the buffer text despite this can do so after clearing the read-only flag with C-x C-q.
- Modes such as Dired and Rmail make buffers read-only when altering the
contents with the usual editing commands would probably be a mistake.
The special commands of these modes bind
buffer-read-onlytonil(withlet) or bindinhibit-read-onlytotaround the places where they themselves change the text.
This buffer-local variable specifies whether the buffer is read-only. The buffer is read-only if this variable is non-
nil.
If this variable is non-
nil, then read-only buffers and, depending on the actual value, some or all read-only characters may be modified. Read-only characters in a buffer are those that have non-nilread-onlyproperties (either text properties or overlay properties). See Special Properties, for more information about text properties. See Overlays, for more information about overlays and their properties.If
inhibit-read-onlyist, allread-onlycharacter properties have no effect. Ifinhibit-read-onlyis a list, thenread-onlycharacter properties have no effect if they are members of the list (comparison is done witheq).
This command toggles whether the current buffer is read-only. It is intended for interactive use; do not use it in programs. At any given point in a program, you should know whether you want the read-only flag on or off; so you can set
buffer-read-onlyexplicitly to the proper value,tornil.If arg is non-
nil, it should be a raw prefix argument.toggle-read-onlysetsbuffer-read-onlytotif the numeric value of that prefix argument is positive and tonilotherwise. See Prefix Command Arguments.
This function signals a
buffer-read-onlyerror if the current buffer is read-only. See Using Interactive, for another way to signal an error if the current buffer is read-only.
Next: Creating Buffers, Previous: Read Only Buffers, Up: Buffers
27.8 The Buffer List
The buffer list is a list of all live buffers. The order of the
buffers in this list is based primarily on how recently each buffer has
been displayed in a window. Several functions, notably
other-buffer, use this ordering. A buffer list displayed for the
user also follows this order.
Creating a buffer adds it to the end of the buffer list, and killing a
buffer removes it from that list. A buffer moves to the front of this
list whenever it is chosen for display in a window (see Displaying Buffers) or a window displaying it is selected (see Selecting Windows). A buffer moves to the end of the list when it is buried (see
bury-buffer, below). There are no functions available to the
Lisp programmer which directly manipulate the buffer list.
In addition to the fundamental buffer list just described, Emacs
maintains a local buffer list for each frame, in which the buffers that
have been displayed (or had their windows selected) in that frame come
first. (This order is recorded in the frame's buffer-list frame
parameter; see Buffer Parameters.) Buffers never displayed in
that frame come afterward, ordered according to the fundamental buffer
list.
This function returns the buffer list, including all buffers, even those whose names begin with a space. The elements are actual buffers, not their names.
If frame is a frame, this returns frame's local buffer list. If frame is
nilor omitted, the fundamental buffer list is used: the buffers appear in order of most recent display or selection, regardless of which frames they were displayed on.(buffer-list) ⇒ (#<buffer buffers.texi> #<buffer *Minibuf-1*> #<buffer buffer.c> #<buffer *Help*> #<buffer TAGS>) ;; Note that the name of the minibuffer ;; begins with a space! (mapcar (function buffer-name) (buffer-list)) ⇒ ("buffers.texi" " *Minibuf-1*" "buffer.c" "*Help*" "TAGS")
The list returned by buffer-list is constructed specifically;
it is not an internal Emacs data structure, and modifying it has no
effect on the order of buffers. If you want to change the order of
buffers in the fundamental buffer list, here is an easy way:
(defun reorder-buffer-list (new-list)
(while new-list
(bury-buffer (car new-list))
(setq new-list (cdr new-list))))
With this method, you can specify any order for the list, but there is no danger of losing a buffer or adding something that is not a valid live buffer.
To change the order or value of a specific frame's buffer list, set
that frame's buffer-list parameter with
modify-frame-parameters (see Parameter Access).
This function returns the first buffer in the buffer list other than buffer. Usually, this is the buffer appearing in the most recently selected window (in frame frame or else the selected frame, see Input Focus), aside from buffer. Buffers whose names start with a space are not considered at all.
If buffer is not supplied (or if it is not a live buffer), then
other-bufferreturns the first buffer in the selected frame's local buffer list. (If frame is non-nil, it returns the first buffer in frame's local buffer list instead.)If frame has a non-
nilbuffer-predicateparameter, thenother-bufferuses that predicate to decide which buffers to consider. It calls the predicate once for each buffer, and if the value isnil, that buffer is ignored. See Buffer Parameters.If visible-ok is
nil,other-bufferavoids returning a buffer visible in any window on any visible frame, except as a last resort. If visible-ok is non-nil, then it does not matter whether a buffer is displayed somewhere or not.If no suitable buffer exists, the buffer ‘*scratch*’ is returned (and created, if necessary).
This function returns the last buffer in frame's buffer list other than BUFFER. If frame is omitted or
nil, it uses the selected frame's buffer list.The argument visible-ok is handled as with
other-buffer, see above. If no suitable buffer can be found, the buffer ‘*scratch*’ is returned.
This command puts buffer-or-name at the end of the buffer list, without changing the order of any of the other buffers on the list. This buffer therefore becomes the least desirable candidate for
other-bufferto return. The argument can be either a buffer itself or the name of one.
bury-bufferoperates on each frame'sbuffer-listparameter as well as the fundamental buffer list; therefore, the buffer that you bury will come last in the value of(buffer-listframe)and in the value of(buffer-list).If buffer-or-name is
nilor omitted, this means to bury the current buffer. In addition, if the buffer is displayed in the selected window, this switches to some other buffer (obtained usingother-buffer) in the selected window. See Displaying Buffers. But if the selected window is dedicated to its buffer, it deletes that window if there are other windows left on its frame. Otherwise, if the selected window is the only window on its frame, it iconifies that frame. If buffer-or-name is displayed in some other window, it remains displayed there.To replace a buffer in all the windows that display it, use
replace-buffer-in-windows. See Buffers and Windows.
This command switches to the last buffer in the local buffer list of the selected frame. More precisely, it calls the function
switch-to-buffer(see Displaying Buffers), to display the buffer returned bylast-buffer, see above, in the selected window.
Next: Killing Buffers, Previous: The Buffer List, Up: Buffers
27.9 Creating Buffers
This section describes the two primitives for creating buffers.
get-buffer-create creates a buffer if it finds no existing buffer
with the specified name; generate-new-buffer always creates a new
buffer and gives it a unique name.
Other functions you can use to create buffers include
with-output-to-temp-buffer (see Temporary Displays) and
create-file-buffer (see Visiting Files). Starting a
subprocess can also create a buffer (see Processes).
This function returns a buffer named buffer-or-name. The buffer returned does not become the current buffer—this function does not change which buffer is current.
buffer-or-name must be either a string or an existing buffer. If it is a string and a live buffer with that name already exists,
get-buffer-createreturns that buffer. If no such buffer exists, it creates a new buffer. If buffer-or-name is a buffer instead of a string, it is returned as given, even if it is dead.(get-buffer-create "foo") ⇒ #<buffer foo>The major mode for a newly created buffer is set to Fundamental mode. (The default value of the variable
major-modeis handled at a higher level; see Auto Major Mode.) If the name begins with a space, the buffer initially disables undo information recording (see Undo).
This function returns a newly created, empty buffer, but does not make it current. If there is no buffer named name, then that is the name of the new buffer. If that name is in use, this function adds suffixes of the form ‘<n>’ to name, where n is an integer. It tries successive integers starting with 2 until it finds an available name.
An error is signaled if name is not a string.
(generate-new-buffer "bar") ⇒ #<buffer bar> (generate-new-buffer "bar") ⇒ #<buffer bar<2>> (generate-new-buffer "bar") ⇒ #<buffer bar<3>>The major mode for the new buffer is set to Fundamental mode. The default value of the variable
major-modeis handled at a higher level. See Auto Major Mode.See the related function
generate-new-buffer-namein Buffer Names.
Next: Indirect Buffers, Previous: Creating Buffers, Up: Buffers
27.10 Killing Buffers
Killing a buffer makes its name unknown to Emacs and makes the memory space it occupied available for other use.
The buffer object for the buffer that has been killed remains in
existence as long as anything refers to it, but it is specially marked
so that you cannot make it current or display it. Killed buffers retain
their identity, however; if you kill two distinct buffers, they remain
distinct according to eq although both are dead.
If you kill a buffer that is current or displayed in a window, Emacs automatically selects or displays some other buffer instead. This means that killing a buffer can in general change the current buffer. Therefore, when you kill a buffer, you should also take the precautions associated with changing the current buffer (unless you happen to know that the buffer being killed isn't current). See Current Buffer.
If you kill a buffer that is the base buffer of one or more indirect buffers, the indirect buffers are automatically killed as well.
The buffer-name of a killed buffer is nil. You can use
this feature to test whether a buffer has been killed:
(defun buffer-killed-p (buffer)
"Return t if BUFFER is killed."
(not (buffer-name buffer)))
This function kills the buffer buffer-or-name, freeing all its memory for other uses or to be returned to the operating system. If buffer-or-name is
nilor omitted, it kills the current buffer.Any processes that have this buffer as the
process-bufferare sent theSIGHUPsignal, which normally causes them to terminate. (The basic meaning ofSIGHUPis that a dialup line has been disconnected.) See Signals to Processes.If the buffer is visiting a file and contains unsaved changes,
kill-bufferasks the user to confirm before the buffer is killed. It does this even if not called interactively. To prevent the request for confirmation, clear the modified flag before callingkill-buffer. See Buffer Modification.This function calls
replace-buffer-in-windowsfor cleaning up all windows currently displaying the buffer to be killed.Killing a buffer that is already dead has no effect.
This function returns
tif it actually killed the buffer. It returnsnilif the user refuses to confirm or if buffer-or-name was already dead.(kill-buffer "foo.unchanged") ⇒ t (kill-buffer "foo.changed") ---------- Buffer: Minibuffer ---------- Buffer foo.changed modified; kill anyway? (yes or no) yes ---------- Buffer: Minibuffer ---------- ⇒ t
After confirming unsaved changes,
kill-buffercalls the functions in the listkill-buffer-query-functions, in order of appearance, with no arguments. The buffer being killed is the current buffer when they are called. The idea of this feature is that these functions will ask for confirmation from the user. If any of them returnsnil,kill-bufferspares the buffer's life.
This is a normal hook run by
kill-bufferafter asking all the questions it is going to ask, just before actually killing the buffer. The buffer to be killed is current when the hook functions run. See Hooks. This variable is a permanent local, so its local binding is not cleared by changing major modes.
This variable, if non-
nilin a particular buffer, tellssave-buffers-kill-emacsandsave-some-buffers(if the second optional argument to that function ist) to offer to save that buffer, just as they offer to save file-visiting buffers. See Definition of save-some-buffers. The variablebuffer-offer-saveautomatically becomes buffer-local when set for any reason. See Buffer-Local Variables.
This variable, if non-
nilin a particular buffer, tellssave-buffers-kill-emacsandsave-some-buffersto save this buffer (if it's modified) without asking the user. The variable automatically becomes buffer-local when set for any reason.
This function returns
tif object is a buffer which has not been killed,nilotherwise.
Next: Swapping Text, Previous: Killing Buffers, Up: Buffers
27.11 Indirect Buffers
An indirect buffer shares the text of some other buffer, which is called the base buffer of the indirect buffer. In some ways it is the analogue, for buffers, of a symbolic link among files. The base buffer may not itself be an indirect buffer.
The text of the indirect buffer is always identical to the text of its base buffer; changes made by editing either one are visible immediately in the other. This includes the text properties as well as the characters themselves.
In all other respects, the indirect buffer and its base buffer are completely separate. They have different names, independent values of point, independent narrowing, independent markers and overlays (though inserting or deleting text in either buffer relocates the markers and overlays for both), independent major modes, and independent buffer-local variable bindings.
An indirect buffer cannot visit a file, but its base buffer can. If you try to save the indirect buffer, that actually saves the base buffer.
Killing an indirect buffer has no effect on its base buffer. Killing the base buffer effectively kills the indirect buffer in that it cannot ever again be the current buffer.
This creates and returns an indirect buffer named name whose base buffer is base-buffer. The argument base-buffer may be a live buffer or the name (a string) of an existing buffer. If name is the name of an existing buffer, an error is signaled.
If clone is non-
nil, then the indirect buffer originally shares the “state” of base-buffer such as major mode, minor modes, buffer local variables and so on. If clone is omitted ornilthe indirect buffer's state is set to the default state for new buffers.If base-buffer is an indirect buffer, its base buffer is used as the base for the new buffer. If, in addition, clone is non-
nil, the initial state is copied from the actual base buffer, not from base-buffer.
This function creates and returns a new indirect buffer that shares the current buffer's base buffer and copies the rest of the current buffer's attributes. (If the current buffer is not indirect, it is used as the base buffer.)
If display-flag is non-
nil, that means to display the new buffer by callingpop-to-buffer. If norecord is non-nil, that means not to put the new buffer to the front of the buffer list.
This function returns the base buffer of buffer, which defaults to the current buffer. If buffer is not indirect, the value is
nil. Otherwise, the value is another buffer, which is never an indirect buffer.
Next: Buffer Gap, Previous: Indirect Buffers, Up: Buffers
27.12 Swapping Text Between Two Buffers
Specialized modes sometimes need to let the user access from the same buffer several vastly different types of text. For example, you may need to display a summary of the buffer text, in addition to letting the user access the text itself.
This could be implemented with multiple buffers (kept in sync when the user edits the text), or with narrowing (see Narrowing). But these alternatives might sometimes become tedious or prohibitively expensive, especially if each type of text requires expensive buffer-global operations in order to provide correct display and editing commands.
Emacs provides another facility for such modes: you can quickly swap
buffer text between two buffers with buffer-swap-text. This
function is very fast because it doesn't move any text, it only
changes the internal data structures of the buffer object to point to
a different chunk of text. Using it, you can pretend that a group of
two or more buffers are actually a single virtual buffer that holds
the contents of all the individual buffers together.
This function swaps the text of the current buffer and that of its argument buffer. It signals an error if one of the two buffers is an indirect buffer (see Indirect Buffers) or is a base buffer of an indirect buffer.
All the buffer properties that are related to the buffer text are swapped as well: the positions of point and mark, all the markers, the overlays, the text properties, the undo list, the value of the
enable-multibyte-charactersflag (see enable-multibyte-characters), etc.
If you use buffer-swap-text on a file-visiting buffer, you
should set up a hook to save the buffer's original text rather than
what it was swapped with. write-region-annotate-functions
works for this purpose. You should probably set
buffer-saved-size to −2 in the buffer, so that changes
in the text it is swapped with will not interfere with auto-saving.
Previous: Swapping Text, Up: Buffers
27.13 The Buffer Gap
Emacs buffers are implemented using an invisible gap to make insertion and deletion faster. Insertion works by filling in part of the gap, and deletion adds to the gap. Of course, this means that the gap must first be moved to the locus of the insertion or deletion. Emacs moves the gap only when you try to insert or delete. This is why your first editing command in one part of a large buffer, after previously editing in another far-away part, sometimes involves a noticeable delay.
This mechanism works invisibly, and Lisp code should never be affected by the gap's current location, but these functions are available for getting information about the gap status.
28 Windows
This chapter describes most of the functions and variables related to Emacs windows. See Frames and Windows, for how windows relate to frames. See Display, for information on how text is displayed in windows.
Next: Splitting Windows, Up: Windows
28.1 Basic Concepts of Emacs Windows
A window in Emacs is the physical area of the screen in which a buffer is displayed. The term is also used to refer to a Lisp object that represents that screen area in Emacs Lisp. It should be clear from the context which is meant.
Emacs groups windows into frames; see Frames. A frame represents an area of screen available for Emacs to use. Each frame always contains at least one window, but you can subdivide it vertically or horizontally into multiple, nonoverlapping Emacs windows.
In each frame, at any time, one and only one window is designated as
selected within the frame. The frame's cursor appears in that
window, but the other windows have “non-selected” cursors, normally
less visible. (See Cursor Parameters, for customizing this.) At
any time, one frame is the selected frame; and the window selected
within that frame is the selected window. The selected window's
buffer is usually the current buffer (except when set-buffer has
been used); see Current Buffer.
For practical purposes, a window exists only while it is displayed in a frame. Once removed from the frame, the window is effectively deleted and should not be used, even though there may still be references to it from other Lisp objects; see Deleting Windows. Restoring a saved window configuration is the only way for a window no longer on the screen to come back to life; see Window Configurations.
Users create multiple windows so they can look at several buffers at once. Lisp libraries use multiple windows for a variety of reasons, but most often to display related information. In Rmail, for example, you can move through a summary buffer in one window while the other window shows messages one at a time as they are reached.
The meaning of “window” in Emacs is similar to what it means in the context of general-purpose window systems such as X, but not identical. The X Window System places X windows on the screen; Emacs uses one or more X windows as frames, and subdivides them into Emacs windows. When you use Emacs on a character-only terminal, Emacs treats the whole terminal screen as one frame.
Most window systems support arbitrarily located overlapping windows. In contrast, Emacs windows are tiled; they never overlap, and together they fill the whole screen or frame. Because of the way in which Emacs creates new windows (see Splitting Windows) and resizes them (see Resizing Windows), not all conceivable tilings of windows on an Emacs frame are actually possible.
Next: Deleting Windows, Previous: Basic Windows, Up: Windows
28.2 Splitting Windows
The functions described below are the primitives used to split a window into two windows. They do not accept a buffer as an argument. Rather, the two “halves” of the split window initially display the same buffer previously visible in the window that was split.
This function splits a new window out of window's screen area. It returns the new window. The default for window is the selected window. When you split the selected window, it remains selected.
If horizontal is non-
nil, then window splits into two side by side windows. The original window keeps the leftmost size columns, and gives the rest of the columns to the new window. Otherwise, window splits into windows one above the other, the original window keeps the upper size lines and gives the rest of the lines to the new window. The original window window is therefore the left-hand or upper of the two, and the new window is the right-hand or lower.If size is omitted or
nil, then window is divided evenly into two parts. (If there is an odd line, it is allocated to the new window.) Whensplit-windowis called interactively, all its arguments arenil.If splitting would result in making a window that is smaller than
window-min-heightorwindow-min-width(see Resizing Windows),split-windowsignals an error and does not split the window at all.The following example starts with one window on a screen that is 50 lines high by 80 columns wide; then it splits the window.
(setq w (selected-window)) ⇒ #<window 8 on windows.texi> (window-edges) ; Edges in order: ⇒ (0 0 80 50) ; left--top--right--bottom ;; Returns window created (setq w2 (split-window w 15)) ⇒ #<window 28 on windows.texi> (window-edges w2) ⇒ (0 15 80 50) ; Bottom window; ; top is line 15 (window-edges w) ⇒ (0 0 80 15) ; Top windowThe screen looks like this:
__________ | | line 0 | w | |__________| | | line 15 | w2 | |__________| line 50 column 0 column 80Next, split the top window horizontally:
(setq w3 (split-window w 35 t)) ⇒ #<window 32 on windows.texi> (window-edges w3) ⇒ (35 0 80 15) ; Left edge at column 35 (window-edges w) ⇒ (0 0 35 15) ; Right edge at column 35 (window-edges w2) ⇒ (0 15 80 50) ; Bottom window unchangedNow the screen looks like this:
column 35 __________ | | | line 0 | w | w3 | |___|______| | | line 15 | w2 | |__________| line 50 column 0 column 80Normally, Emacs indicates the border between two side-by-side windows with a scroll bar (see Scroll Bars), or with ‘|’ characters. The display table can specify alternative border characters; see Display Tables.
This function splits the selected window into two windows, one above the other, leaving the upper of the two windows selected, with size lines. (If size is negative, then the lower of the two windows gets −size lines and the upper window gets the rest, but the upper window is still the one selected.) However, if
split-window-keep-point(see below) isnil, then either window can be selected.In other respects, this function is similar to
split-window. In particular, the upper window is the original one and the return value is the new, lower window.
If this variable is non-
nil(the default), thensplit-window-verticallybehaves as described above.If it is
nil, thensplit-window-verticallyadjusts point in each of the two windows to avoid scrolling. (This is useful on slow terminals.) It selects whichever window contains the screen line that point was previously on.This variable affects the behavior of
split-window-verticallyonly. It has no effect on the other functions described here.
This function splits the selected window into two windows side-by-side, leaving the selected window on the left with size columns. If size is negative, the rightmost window gets −size columns, but the leftmost window still remains selected.
This function is basically an interface to
split-window. You could define a simplified version of the function like this:(defun split-window-horizontally (&optional arg) "Split selected window into two windows, side by side..." (interactive "P") (let ((size (and arg (prefix-numeric-value arg)))) (and size (< size 0) (setq size (+ (window-width) size))) (split-window nil size t)))
This function returns non-
nilif there is only one window. The argument no-mini, if non-nil, means don't count the minibuffer even if it is active; otherwise, the minibuffer window is counted when it is active.The argument all-frames specifies which frames to consider. Here are the possible values and their meanings:
nil- Count the windows in the selected frame, plus the minibuffer used by that frame even if it lies in some other frame.
t- Count all windows in all existing frames.
visible- Count all windows in all visible frames.
- 0
- Count all windows in all visible or iconified frames.
- anything else
- Count precisely the windows in the selected frame, and no others.
Next: Selecting Windows, Previous: Splitting Windows, Up: Windows
28.3 Deleting Windows
A window remains visible on its frame unless you delete it by calling certain functions that delete windows. A deleted window cannot appear on the screen, but continues to exist as a Lisp object until there are no references to it. There is no way to cancel the deletion of a window aside from restoring a saved window configuration (see Window Configurations). Restoring a window configuration also deletes any windows that aren't part of that configuration.
When you delete a window, the space it took up is given to one of its sibling windows adjacent to it.
This function returns
nilif window is deleted, andtotherwise.Warning: Erroneous information or fatal errors may result from using a deleted window as if it were live.
This function removes window from display and returns
nil. The default for window is the selected window. An error is signaled if window is the only window on its frame.
This function makes window the only window on its frame, by deleting the other windows in that frame. The default for window is the selected window. The return value is
nil.
This function deletes all windows showing buffer-or-name. If there are no windows showing buffer-or-name, it does nothing. The optional argument buffer-or-name may be a buffer or the name of an existing buffer and defaults to the current buffer.
delete-windows-onoperates frame by frame. If a frame has several windows showing different buffers, then those showing buffer-or-name are removed, and the others expand to fill the space. If all windows in some frame are showing buffer-or-name (including the case where there is only one window), then the frame winds up with a single window showing another buffer chosen withother-buffer(see The Buffer List). If, however, the window showing buffer-or-name is dedicated to its buffer (see Dedicated Windows), and there are other frames left, that window's frame is deleted.The optional argument frame specifies which frames to operate on. This function does not use it in quite the same way as the other functions which scan all windows; specifically, the values
tandnilhave the opposite of their meanings in other functions. Here are the full details:
- If it is
nil, operate on all frames.- If it is
t, operate on the selected frame.- If it is
visible, operate on all visible frames.- If it is 0, operate on all visible or iconified frames.
- If it is a frame, operate on that frame.
This function always returns
nil.
Next: Cyclic Window Ordering, Previous: Deleting Windows, Up: Windows
28.4 Selecting Windows
When a window is selected, the buffer in the window becomes the current buffer, and the cursor will appear in it.
This function returns the selected window. This is the window in which the cursor appears and to which many commands apply.
This function makes window the selected window. The cursor then appears in window (after redisplay). Unless window was already selected,
select-windowmakes window's buffer the current buffer. The return value is window.Normally, window's selected buffer is moved to the front of the buffer list (see The Buffer List) and window becomes the most recently selected window. But if norecord is non-
nil, the buffer list remains unchanged and window does not become the most recently selected one.(setq w (next-window)) (select-window w) ⇒ #<window 65 on windows.texi>
This macro records the selected frame, as well as the selected window of each frame, executes forms in sequence, then restores the earlier selected frame and windows. It also saves and restores the current buffer. It returns the value of the last form in forms.
This macro does not save or restore anything about the sizes, arrangement or contents of windows; therefore, if forms change them, the change persists. If the previously selected window of some frame is no longer live at the time of exit from forms, that frame's selected window is left alone. If the previously selected window is no longer live, then whatever window is selected at the end of forms remains selected. The current buffer is restored if and only if it is still live when exiting forms.
This macro changes neither the ordering of recently selected windows nor the buffer list.
This macro selects window, executes forms in sequence, then restores the previously selected window and current buffer. The ordering of recently selected windows and the buffer list remain unchanged unless you deliberately change them within forms, for example, by calling
select-windowwith argument norecordnil.
The following functions choose one of the windows on the screen, offering various criteria for the choice.
This function returns the window least recently “used” (that is, selected) among a set of candidate windows. If any full-width windows are present, it only considers these.
The selected window is returned if it is the only candidate. A minibuffer window is never a candidate. A dedicated window (see Dedicated Windows) is never a candidate unless the optional argument dedicated is non-
nil.The optional argument frame specifies which windows are considered.
- If it is
nil, consider windows on the selected frame.- If it is
t, consider windows on all frames.- If it is
visible, consider windows on all visible frames.- If it is 0, consider windows on all visible or iconified frames.
- If it is a frame, consider windows on that frame.
This function returns the window with the largest area (height times width). If there are no side-by-side windows, then this is the window with the most lines. A minibuffer window is never a candidate. A dedicated window (see Dedicated Windows) is never a candidate unless the optional argument dedicated is non-
nil.If there are two candidate windows of the same size, this function prefers the one that comes first in the cyclic ordering of windows, starting from the selected window (see Cyclic Window Ordering).
The optional argument frame specifies which set of windows to consider, see
get-lru-windowabove.
This function returns a window satisfying predicate. It cycles through all visible windows using
walk-windows(see Cyclic Window Ordering), calling predicate on each one of them with that window as its argument. The function returns the first window for which predicate returns a non-nilvalue; if that never happens, it returns default (which defaults tonil).The optional arguments minibuf and all-frames specify the set of windows to include in the scan. See the description of
next-windowin Cyclic Window Ordering, for details.
Next: Buffers and Windows, Previous: Selecting Windows, Up: Windows
28.5 Cyclic Ordering of Windows
When you use the command C-x o (other-window) to select
some other window, it moves through the windows on the screen in a
specific order. For any given configuration of windows, this order
never varies. It is called the cyclic ordering of windows.
For a particular frame, this ordering generally goes from top to bottom, and from left to right. But it may go down first or go right first, depending on the order in which windows were split.
If the first split was vertical (into windows one above each other), and then the subwindows were split horizontally, then the ordering is left to right in the top of the frame, and then left to right in the next lower part of the frame, and so on. If the first split was horizontal, the ordering is top to bottom in the left part, and so on. In general, within each set of siblings at any level in the window tree (see Window Tree), the order is left to right, or top to bottom.
This function returns the window following window in the cyclic ordering of windows. This is the window C-x o selects if typed when window is selected. The default for window is the selected window.
The value of the optional argument minibuf specifies whether the minibuffer is included in the window order. Normally, when minibuf is
nil, the minibuffer is included only if it is currently “active”; this matches the behavior of C-x o. (The minibuffer window is active while the minibuffer is in use; see Minibuffers.)If minibuf is
t, the cyclic ordering includes the minibuffer window even if it is not active. If minibuf is neithertnornil, the minibuffer window is not included even if it is active.The optional argument all-frames specifies which frames to consider. Here are the possible values and their meanings:
nil- Consider all the windows in window's frame, plus the minibuffer used by that frame even if it lies in some other frame. If the minibuffer counts (as determined by minibuf), then all windows on all frames that share that minibuffer count too.
t- Consider all windows in all existing frames.
visible- Consider all windows in all visible frames. (To get useful results, you must ensure window is in a visible frame.)
- 0
- Consider all windows in all visible or iconified frames.
- a frame
- Consider all windows on that frame.
- anything else
- Consider precisely the windows in window's frame, and no others.
This example assumes there are two windows, both displaying the buffer ‘windows.texi’:
(selected-window) ⇒ #<window 56 on windows.texi> (next-window (selected-window)) ⇒ #<window 52 on windows.texi> (next-window (next-window (selected-window))) ⇒ #<window 56 on windows.texi>
This function returns the window preceding window in the cyclic ordering of windows. The other arguments specify which windows to include in the cycle, as in
next-window.
This function selects another window in the cyclic ordering of windows. count specifies the number of windows to skip in the ordering, starting with the selected window, before making the selection. If count is a positive number, it skips count windows forwards. count negative means skip −count windows backwards. If count is zero, it does not skip any window, thus re-selecting the selected window. In an interactive call, count is the numeric prefix argument.
The optional argument all-frames has the same meaning as in
next-window, but the minibuf argument ofnext-windowis always effectivelynil. This function returnsnil.
This function cycles through all windows. It calls the function
proconce for each window, with the window as its sole argument.The optional arguments minibuf and all-frames specify the set of windows to include in the walk. See
next-window, above, for details.
This function returns a list of all windows on frame, starting with window. The default for frame is the selected frame; the default for window is the selected window.
The value of minibuf specifies if the minibuffer window shall be included in the result list. If minibuf is
t, the result always includes the minibuffer window. If minibuf isnilor omitted, that includes the minibuffer window if it is active. If minibuf is neithernilnort, the result never includes the minibuffer window.
Next: Displaying Buffers, Previous: Cyclic Window Ordering, Up: Windows
28.6 Buffers and Windows
This section describes low-level functions to examine windows or to display buffers in windows in a precisely controlled fashion. See Displaying Buffers, for related functions that find a window to use and specify a buffer for it. The functions described there are easier to use, but they employ heuristics in choosing or creating a window; use the functions described here when you need complete control.