START-INFO-DIR-ENTRY * Elisp: (elisp). The Emacs Lisp Reference Manual. END-INFO-DIR-ENTRY This Info file contains edition 2.8 of the GNU Emacs Lisp Reference Manual, corresponding to Emacs version 21.2. Published by the Free Software Foundation 59 Temple Place, Suite 330 Boston, MA 02111-1307 USA Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1998, 1999, 2000, 2001, 2002 Free Software Foundation, Inc. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with the Invariant Sections being "Copying", 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 freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development." This Info file contains edition 2.8 of the GNU Emacs Lisp Reference Manual, corresponding to GNU Emacs version 21.2. 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 2.8. 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. This includes most information about usage on VMS. 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 mail comments and corrections to bug-lisp-manual@gnu.org We let mail to this list accumulate unread until someone decides to apply the corrections. Months, and sometimes years, go by between updates. So please attach no significance to the lack of a reply--your mail _will_ be acted on in due time. If you want to contact the Emacs maintainers more quickly, send mail to `bug-gnu-emacs@gnu.org'. 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. *Note Common Lisp Extension: (cl)Top. 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. Conventions =========== This section explains the notational conventions that are used in this manual. You may want to skip this section and refer back to it later. Some Terms ---------- Throughout this manual, the phrases "the Lisp reader" and "the Lisp printer" refer to those routines in Lisp that convert textual representations of Lisp objects into actual Lisp objects, and vice versa. *Note Printed Representation::, for more details. You, the person reading this manual, are thought of as "the programmer" and are addressed as "you". "The user" is the person who uses Lisp programs, including those you write. Examples of Lisp code are formatted like this: `(list 1 2 3)'. Names that represent metasyntactic variables, or arguments to a function being described, are formatted like this: FIRST-NUMBER. `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 use `()' when we wish to emphasize that it means the empty list, and we use `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 (not 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. The same is true of any symbol whose name starts with a colon (`:'). *Note Constant Variables::. 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) 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. (progn (print 'foo) (print 'bar)) -| foo -| bar => bar 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 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 ---------- Format of Descriptions ---------------------- Functions, variables, macros, commands, user options, and special forms are described in this manual in a uniform format. The first line of a description contains the name of the item followed by its arguments, if any. The category--function, variable, or whatever--appears at the beginning of the line. The description follows on succeeding lines, sometimes with examples. A Sample Function Description ............................. 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 following argument name will have a value, as a variable, which is a list of all these remaining arguments. Do not write `&rest' when you call the function. Here is a description of an imaginary function `foo': - Function: foo integer1 &optional integer2 &rest integers The function `foo' subtracts INTEGER1 from INTEGER2, then adds all the rest of the arguments to the result. If INTEGER2 is not supplied, then the number 19 is used by default. (foo 1 5 3 9) => 16 (foo 5) => 14 More generally, (foo W X Y...) == (+ (- X W) Y...) 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. (*Note 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. *Note 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: - Special Form: count-loop (VAR [FROM TO [INC]]) BODY... This imaginary special form implements a loop that executes the BODY forms and then increments the variable VAR on each iteration. On the first iteration, the variable has the value FROM; on subsequent iterations, it is incremented by one (or by INC if that is given). The loop exits before executing BODY if VAR equals TO. Here is an example: (count-loop (i 0 10) (prin1 i) (princ " ") (prin1 (aref vector i)) (terpri)) If FROM and TO are omitted, VAR is bound to `nil' before the loop begins, and the loop exits if VAR is non-`nil' at the beginning of an iteration. Here is an example: (count-loop (done) (if (pending) (fixit) (setq done t))) In this special form, the arguments FROM and TO are optional, but must both be present or both absent. If they are present, INC may optionally be specified as well. These arguments are grouped with the argument VAR into a list, to distinguish them from BODY, which includes all remaining elements of the form. A Sample Variable Description ............................. A "variable" is a name that can hold a value. Although any variable can be set by the user, certain variables that exist specifically so that users can change them are called "user options". Ordinary variables and user options are described using a format like that for functions except that there are no arguments. Here is a description of the imaginary `electric-future-map' variable. - Variable: electric-future-map The value of this variable is a full keymap used by Electric Command Future mode. The functions in this map allow you to edit commands you have not yet thought about executing. User option descriptions have the same format, but `Variable' is replaced by `User Option'. Version Information =================== These facilities provide information about which version of Emacs is in use. - Command: emacs-version 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 20.3.5 (i486-pc-linux-gnulibc1, X toolkit) of Sat Feb 14 1998 on psilocin.gnu.org" Called interactively, the function prints the same information in the echo area. - Variable: emacs-build-time 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' (*note Time of Day::). emacs-build-time => (13623 62065 344633) - Variable: emacs-version The value of this variable is the version of Emacs being run. It is a string such as `"20.3.1"'. 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 `"20.3.9.1"', indicates an unreleased test version. The following two variables have existed since Emacs version 19.23: - Variable: emacs-major-version The major version number of Emacs, as an integer. For Emacs version 20.3, the value is 20. - Variable: emacs-minor-version The minor version number of Emacs, as an integer. For Emacs version 20.3, the value is 3. 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. Corrections were supplied by 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, 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, Skip Montanaro, John Gardiner Myers, Thomas A. Peterson, Francesco Potorti, Friedrich Pukelsheim, Arnold D. Robbins, Raul Rockwell, Per Starba"ck, Shinichirou Sugou, Kimmo Suominen, Edward Tharp, Bill Trost, Rickard Westman, Jean White, Matthew Wilding, Carl Witty, Dale Worley, Rusty Wright, and David D. Zuhn. Lisp Data Types *************** A Lisp "object" is a piece of data used and manipulated by Lisp programs. For our purposes, a "type" or "data type" is a set of possible objects. Every object belongs to at least one type. Objects of the same type have similar structures and may usually be used in the same contexts. Types can overlap, and objects can belong to two or more types. Consequently, we can ask whether an object belongs to a particular type, but not for "the" type of an object. A few fundamental object types are built into Emacs. These, from which all other types are constructed, are called "primitive types". Each object belongs to one and only one primitive type. These types include "integer", "float", "cons", "symbol", "string", "vector", "hash-table", "subr", and "byte-code function", plus several special types, such as "buffer", that are related to editing. (*Note Editing Types::.) Each primitive type has a corresponding Lisp function that checks whether an object is a member of that type. Note that Lisp is unlike many other languages in that Lisp objects are "self-typing": the primitive type of the 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. This chapter describes the purpose, printed representation, and read syntax of each of the standard types in GNU Emacs Lisp. Details on how to use these types can be found in later chapters. Printed Representation and Read Syntax ====================================== The "printed representation" of an object is the format of the output generated by the Lisp printer (the function `prin1') for that object. The "read syntax" of an object is the format of the input accepted by the Lisp reader (the function `read') for that object. *Note Read and Print::. Most objects have more than one possible read syntax. Some types of object have no read syntax, since it may not make sense to enter objects of these types directly in a Lisp program. Except for these cases, the printed representation of an object is also a read syntax for it. 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. Every type has a printed representation. Some types have no read syntax--for example, the buffer type has none. Objects of these types are printed in "hash notation": the characters `#<' followed by a descriptive string (typically the type name followed by the name of the object), and closed with a matching `>'. Hash notation cannot be read at all, so the Lisp reader signals the error `invalid-read-syntax' whenever it encounters `#<'. (current-buffer) => # When you evaluate an expression interactively, the Lisp interpreter first reads the textual representation of it, producing a Lisp object, and then evaluates that object (*note Evaluation::). However, evaluation and reading are separate activities. Reading returns the Lisp object represented by the text that is read; the object may or may not be evaluated later. *Note Input Functions::, for a description of `read', the basic function for reading objects. Comments ======== A "comment" is text that is written in a program only for the sake of humans that read the program, and that has no effect on the meaning of the program. In Lisp, a semicolon (`;') starts a comment if it is not within a string or character constant. The comment continues to the end of line. The Lisp reader discards comments; they do not become part of the Lisp objects which represent the program within the Lisp system. The `#@COUNT' construct, which skips the next COUNT characters, is useful for program-generated comments containing binary data. The Emacs Lisp byte compiler uses this in its output files (*note Byte Compilation::). It isn't meant for source files, however. *Note Comment Tips::, for conventions for formatting comments. Programming Types ================= There are two general categories of types in Emacs Lisp: those having to do with Lisp programming, and those having to do with editing. The former exist in many Lisp implementations, in one form or another. The latter are unique to Emacs Lisp. Integer Type ------------ The range of values for integers in Emacs Lisp is -134217728 to 134217727 (28 bits; i.e., -2**27 to 2**27 - 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+ 134217727)' is -134217728 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. 268435457 ; Also the integer 1 on a 28-bit implementation. *Note Numbers::, for more information. Floating Point Type ------------------- Floating point numbers are the computer equivalent of scientific notation. The precise number of significant figures and the range of possible exponents is machine-specific; Emacs always uses the C data type `double' to store the value. The printed representation for floating point numbers requires either a decimal point (with at least one digit following), an exponent, or both. For example, `1500.0', `15e2', `15.0e2', `1.5e3', and `.15e4' are five ways of writing a floating point number whose value is 1500. They are all equivalent. *Note Numbers::, for more information. 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 not often used in programs. It is far more common to work with _strings_, which are sequences composed of characters. *Note String Type::. Characters in strings, buffers, and files are currently limited to the range of 0 to 524287--nineteen bits. But not all values in that range are valid character codes. Codes 0 through 127 are ASCII codes; the rest are non-ASCII (*note Non-ASCII Characters::). Characters that represent keyboard input have a much wider range, to encode modifier keys such as Control, Meta and Shift. 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 a very bad idea. 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 space 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, return, del, and escape as `?\a', `?\b', `?\t', `?\n', `?\v', `?\f', `?\r', `?\d', and `?\e', respectively. Thus, ?\a => 7 ; `C-g' ?\b => 8 ; backspace, , `C-h' ?\t => 9 ; tab, , `C-i' ?\n => 10 ; newline, `C-j' ?\v => 11 ; vertical tab, `C-k' ?\f => 12 ; formfeed character, `C-l' ?\r => 13 ; carriage return, , `C-m' ?\e => 27 ; escape character, , `C-[' ?\\ => 92 ; backslash character, `\' ?\d => 127 ; delete character, These sequences which start with backslash are also known as "escape sequences", because backslash plays the role of an escape character; this usage has nothing to do with the character . Control characters may 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 character as the control equivalent of `?': ?\^? => 127 ?\C-? => 127 As a result, it is currently not possible to represent the character `Control-?', which is a meaningful input character under X, using `\C-'. It is not easy to change this, as various Lisp files refer to in this way. For representing control characters to be found in files or strings, we recommend the `^' syntax; for control characters in keyboard input, we prefer the `C-' syntax. Which one you use does not affect the meaning of the program, but may guide the understanding of people who read it. A "meta character" is a character typed with the modifier key. The integer that represents such a character has the 2**27 bit set (which on most machines makes it a negative number). 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. (In Emacs versions 18 and older, this convention was used for characters outside of strings as well.) 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'. 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'. Numerically, the bit values are 2**22 for alt, 2**23 for super and 2**24 for hyper. Finally, 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. 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. 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', instead of an actual whitespace character such as a tab. 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 use, the 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 sixth example because the rest of the name makes it invalid as a number. foo ; A symbol named `foo'. FOO ; A symbol named `FOO', different from `foo'. char-to-string ; A symbol named `char-to-string'. 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 (*note Creating Symbols::). To prevent interning, you can write `#:' before the name of the symbol. 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 (*note Text Properties::), but vectors do not support text properties, even when their elements happen to be characters. Lists, strings and the other array types 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. *Note Sequences Arrays Vectors::. It is generally impossible to read the same sequence twice, since sequences are always created anew upon reading. If you read the read syntax for a sequence twice, you get two sequences with equal contents. There is one exception: the empty list `()' always stands for the same object, `nil'. 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. *Note 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. 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. 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. 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'. 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 are examples of lists written in Lisp syntax: (A 2 "A") ; A list of three elements. () ; A list of no elements (the empty list). nil ; A list of no elements (the empty list). ("A ()") ; A list of one element: the string `"A ()"'. (A ()) ; A list of two elements: `A' and the empty list. (A nil) ; Equivalent to the previous. ((A B C)) ; A list of one element ; (which is a list of three elements). Here is the list `(A ())', or equivalently `(A nil)', depicted with boxes and arrows: --- --- --- --- | | |--> | | |--> nil --- --- --- --- | | | | --> A --> nil Dotted Pair Notation .................... "Dotted pair notation" is an alternative 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 therefore more general than list syntax. In the 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 Association List Type ..................... An "association list" or "alist" is a specially-constructed list whose elements are cons cells. In each element, the CAR is considered a "key", and the CDR is considered an "associated value". (In some cases, the associated value is stored in the CAR of the CDR.) Association lists are often used as stacks, since it is easy to add or remove associations at the front of the list. For example, (setq alist-of-colors '((rose . red) (lily . white) (buttercup . yellow))) sets the variable `alist-of-colors' to an alist of three elements. In the first element, `rose' is the key and `red' is the value. *Note Association Lists::, for a further explanation of alists and for functions that work on alists. *Note Hash Tables::, for another kind of lookup table, which is much faster for handling a large number of keys. 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 an array of arrays.) Each type of array has its own read syntax; see the following sections for details. The array type is contained in the sequence type and contains the string type, the vector type, the bool-vector type, and the char-table type. String Type ----------- A "string" is an array of characters. Strings are used for many purposes in Emacs, as can be expected in a text editor; for example, as the names of Lisp symbols, as messages for the user, and to represent text extracted from buffers. Strings in Lisp are constants: evaluation of a string returns the same string. *Note Strings and Characters::, for functions that operate on strings. Syntax for Strings .................. The read syntax for strings 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." 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. Using a multibyte hex escape forces the string to multibyte. 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). This forces a unibyte string. *Note Text Representations::, for more information about the two text representations. Nonprinting Characters in Strings ................................. You can use the same backslash escape-sequences in a string constant as in character literals (but do not use the question mark that begins a character constant). For example, you can write a string containing the nonprinting characters tab and `C-a', with commas and spaces between them, like this: `"\t, \C-a"'. *Note Character Type::, for a description of the read syntax for characters. However, not all of the characters you can write with backslash escape-sequences are valid in strings. The only control characters that a string can hold are the ASCII control characters. Strings do not distinguish case in ASCII control characters. Properly speaking, strings cannot hold meta characters; but when a string is to be used as a key sequence, there is a special convention that provides a way to represent meta versions of ASCII characters in a string. If you use the `\M-' syntax to indicate a meta character in a string constant, this sets the 2**7 bit of the character in the string. If the string is used in `define-key' or `lookup-key', this numeric code is translated into the equivalent meta character. *Note Character Type::. Strings cannot hold characters that have the hyper, super, or alt modifiers. Text Properties in Strings .......................... A string can hold properties for the characters it contains, in addition to the characters themselves. This enables programs that copy text between strings and buffers to copy the text's properties with no special effort. *Note Text Properties::, for an explanation of what text properties mean. Strings with text properties use a special read and print syntax: #("CHARACTERS" PROPERTY-DATA...) where PROPERTY-DATA consists of zero or more elements, in groups of three as follows: BEG END PLIST The elements BEG and END are integers, and together specify a range of indices in the string; PLIST is the property list for that range. For example, #("foo bar" 0 3 (face bold) 3 4 nil 4 7 (face italic)) represents a string whose textual contents are `foo bar', in which the first three characters have a `face' property with value `bold', and the last three have a `face' property with value `italic'. (The fourth character has no text properties, so its property list is `nil'. It is not actually necessary to mention ranges with `nil' as the property list, since any characters not mentioned in any range will default to having no properties.) Vector Type ----------- A "vector" is a one-dimensional array of elements of any type. It takes a constant amount of time to access any element of a vector. (In a list, the access time of an element is proportional to the distance of the element from the beginning of the list.) The printed representation of a vector consists of a left square bracket, the elements, and a right square bracket. This is also the read syntax. Like numbers and strings, vectors are considered constants for evaluation. [1 "two" (three)] ; A vector of three elements. => [1 "two" (three)] *Note Vectors::, for functions that work with vectors. 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. *Note Char-Tables::, for special functions to operate on char-tables. Uses of char-tables include: * Case tables (*note Case Tables::). * Character category tables (*note Categories::). * Display tables (*note Display Tables::). * Syntax tables (*note Syntax Tables::). Bool-Vector Type ---------------- A "bool-vector" is a one-dimensional array of elements that 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. If the length is not a multiple of 8, the printed representation shows extra elements, but these extras really make no difference. (make-bool-vector 3 t) => #&3"\007" (make-bool-vector 3 nil) => #&3"\0" ;; These are equal since only the first 3 bits are used. (equal #&3"\377" #&3"\007") => t 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. Hash tables are a new feature in Emacs 21; they have no read syntax, and print using hash notation. *Note Hash Tables::. (make-hash-table) => # Function Type ------------- Just as functions in other programming languages are executable, "Lisp function" objects are pieces of executable code. However, functions in Lisp are primarily Lisp objects, and only secondarily the text which represents them. These Lisp objects are lambda expressions: lists whose first element is the symbol `lambda' (*note Lambda Expressions::). In most programming languages, it is impossible to have a function without a name. In Lisp, a function has no intrinsic name. A lambda expression is also called an "anonymous function" (*note Anonymous Functions::). A named function in Lisp is actually a symbol with a valid function in its function cell (*note Defining Functions::). Most of the time, functions are called when their names are written in Lisp expressions in Lisp programs. However, you can construct or obtain a function object at run time and then call it with the primitive functions `funcall' and `apply'. *Note Calling Functions::. Macro Type ---------- A "Lisp macro" is a user-defined construct that extends the Lisp language. It is represented as an object much like a function, but with different argument-passing semantics. A Lisp macro has the form of a list whose first element is the symbol `macro' and whose CDR is a Lisp function object, including the `lambda' symbol. Lisp macro objects are usually defined with the built-in `defmacro' function, but any list that begins with `macro' is a macro as far as Emacs is concerned. *Note Macros::, for an explanation of how to write a macro. *Warning*: Lisp macros and keyboard macros (*note Keyboard Macros::) are entirely different things. When we use the word "macro" without qualification, we mean a Lisp macro, not a keyboard macro. Primitive Function Type ----------------------- A "primitive function" is a function callable from Lisp but written in the C programming language. Primitive functions are also called "subrs" or "built-in functions". (The word "subr" is derived from "subroutine".) Most primitive functions evaluate all their arguments when they are called. A primitive function that does not evaluate all its arguments is called a "special form" (*note Special Forms::). It does not matter to the caller of a function whether the function is primitive. However, this does matter if you try to redefine a primitive with a function written in Lisp. The reason is that the primitive function may be called directly from C code. Calls to the redefined function from Lisp will use the new definition, but calls from C code may still use the built-in definition. Therefore, *we discourage redefinition of primitive functions*. The term "function" refers to all Emacs functions, whether written in Lisp or C. *Note Function Type::, for information about the functions written in Lisp. Primitive functions have no read syntax and print in hash notation with the name of the subroutine. (symbol-function 'car) ; Access the function cell ; of the symbol. => # (subrp (symbol-function 'car)) ; Is this a primitive function? => t ; Yes. 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. *Note 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 `['. Autoload Type ------------- An "autoload object" is a list whose first element is the symbol `autoload'. It is stored as the function definition of a symbol, where it serves as a placeholder for the real definition. The autoload object says that the real definition is found in a file of Lisp code that should be loaded when necessary. It contains the name of the file, plus some other information about the real definition. After the file has been loaded, the symbol should have a new function definition that is not an autoload object. The new definition is then called as if it had been there to begin with. From the user's point of view, the function call works as expected, using the function definition in the loaded file. An autoload object is usually created with the function `autoload', which stores the object in the function cell of a symbol. *Note Autoload::, for more details. Editing Types ============= The types in the previous section are used for general programming purposes, and most of them are common to most Lisp dialects. Emacs Lisp provides several additional data types for purposes connected with editing. Buffer Type ----------- A "buffer" is an object that holds text that can be edited (*note Buffers::). Most buffers hold the contents of a disk file (*note Files::) so they can be edited, but some are used for other purposes. Most buffers are also meant to be seen by the user, and therefore displayed, at some time, in a window (*note Windows::). But a buffer need not be displayed in any window. 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. Each buffer has a designated position called "point" (*note Positions::). At any time, one buffer is the "current buffer". Most editing commands act on the contents of the current buffer in the neighborhood of point. Many of the standard Emacs functions manipulate or test the characters in the current buffer; a whole chapter in this manual is devoted to describing these functions (*note Text::). Several other data structures are associated with each buffer: * a local syntax table (*note Syntax Tables::); * a local keymap (*note Keymaps::); and, * a list of buffer-local variable bindings (*note Buffer-Local Variables::). * overlays (*note Overlays::). * text properties for the text in the buffer (*note Text Properties::). The local keymap and variable list contain entries that individually override global bindings or values. These are used to customize the behavior of programs in different buffers, without actually changing the programs. A buffer may be "indirect", which means it shares the text of another buffer, but presents it differently. *Note Indirect Buffers::. Buffers have no read syntax. They print in hash notation, showing the buffer name. (current-buffer) => # Marker Type ----------- A "marker" denotes a position in a specific buffer. Markers therefore have two components: one for the buffer, and one for the position. Changes in the buffer's text automatically relocate the position value as necessary to ensure that the marker always points between the same two characters in the buffer. Markers have no read syntax. They print in hash notation, giving the current character position and the name of the buffer. (point-marker) => # *Note Markers::, for information on how to test, create, copy, and move markers. Window Type ----------- A "window" describes the portion of the terminal screen that Emacs uses to display a buffer. Every window has one associated buffer, whose contents appear in the window. By contrast, a given buffer may appear in one window, no window, or several windows. Though many windows may exist simultaneously, at any time one window is designated the "selected window". This is the window where the cursor is (usually) displayed when Emacs is ready for a command. The selected window usually displays the current buffer, but this is not necessarily the case. Windows are grouped on the screen into frames; each window belongs to one and only one frame. *Note Frame Type::. Windows have no read syntax. They print in hash notation, giving the window number and the name of the buffer being displayed. The window numbers exist to identify windows uniquely, since the buffer displayed in any given window can change frequently. (selected-window) => # *Note Windows::, for a description of the functions that work on windows. Frame Type ---------- A "frame" is a rectangle on the screen that contains one or more Emacs windows. A frame initially contains a single main window (plus perhaps a minibuffer window) which you can subdivide vertically or horizontally into smaller windows. Frames have no read syntax. They print in hash notation, giving the frame's title, plus its address in core (useful to identify the frame uniquely). (selected-frame) => # *Note Frames::, for a description of the functions that work on frames. Window Configuration Type ------------------------- A "window configuration" stores information about the positions, sizes, and contents of the windows in a frame, so you can recreate the same arrangement of windows later. Window configurations do not have a read syntax; their print syntax looks like `#'. *Note Window Configurations::, for a description of several functions related to window configurations. Frame Configuration Type ------------------------ A "frame configuration" stores information about the positions, sizes, and contents of the windows in all frames. It is actually a list whose CAR is `frame-configuration' and whose CDR is an alist. Each alist element describes one frame, which appears as the CAR of that element. *Note Frame Configurations::, for a description of several functions related to frame configurations. Process Type ------------ The word "process" usually means a running program. Emacs itself runs in a process of this sort. However, in Emacs Lisp, a process is a Lisp object that designates a subprocess created by the Emacs process. Programs such as shells, GDB, ftp, and compilers, running in subprocesses of Emacs, extend the capabilities of Emacs. An Emacs subprocess takes textual input from Emacs and returns textual output to Emacs for further manipulation. Emacs can also send signals to the subprocess. Process objects have no read syntax. They print in hash notation, giving the name of the process: (process-list) => (#) *Note Processes::, for information about functions that create, delete, return information about, send input or signals to, and receive output from processes. Stream Type ----------- A "stream" is an object that can be used as a source or sink for characters--either to supply characters for input or to accept them as output. Many different types can be used this way: markers, buffers, strings, and functions. Most often, input streams (character sources) obtain characters from the keyboard, a buffer, or a file, and output streams (character sinks) send characters to a buffer, such as a `*Help*' buffer, or to the echo area. The object `nil', in addition to its other meanings, may be used as a stream. It stands for the value of the variable `standard-input' or `standard-output'. Also, the object `t' as a stream specifies input using the minibuffer (*note Minibuffers::) or output in the echo area (*note The Echo Area::). Streams have no special printed representation or read syntax, and print as whatever primitive type they are. *Note Read and Print::, for a description of functions related to streams, including parsing and printing functions. Keymap Type ----------- A "keymap" maps keys typed by the user to commands. This mapping controls how the user's command input is executed. A keymap is actually a list whose CAR is the symbol `keymap'. *Note Keymaps::, for information about creating keymaps, handling prefix keys, local as well as global keymaps, and changing key bindings. Overlay Type ------------ An "overlay" specifies properties that apply to a part of a buffer. Each overlay applies to a specified range of the buffer, and contains a property list (a list whose elements are alternating property names and values). Overlay properties are used to present parts of the buffer temporarily in a different display style. Overlays have no read syntax, and print in hash notation, giving the buffer name and range of positions. *Note Overlays::, for how to create and use overlays. Read Syntax for Circular Objects ================================ In Emacs 21, to represent shared or circular structure within a complex of Lisp objects, you can use the reader constructs `#N=' and `#N#'. Use `#N=' before an object to label it for later reference; subsequently, you can use `#N#' to refer the same object in another place. Here, N is some integer. For example, here is how to make a list in which the first element recurs as the third element: (#1=(a) b #1#) This differs from ordinary syntax such as this ((a) b (a)) which would result in a list whose first and third elements look alike but are not the same Lisp object. This shows the difference: (prog1 nil (setq x '(#1=(a) b #1#))) (eq (nth 0 x) (nth 2 x)) => t (setq x '((a) b (a))) (eq (nth 0 x) (nth 2 x)) => nil You can also use the same syntax to make a circular structure, which appears as an "element" within itself. Here is an example: #1=(a #1#) This makes a list whose second element is the list itself. Here's how you can see that it really works: (prog1 nil (setq x '#1=(a #1#))) (eq x (cadr x)) => t The Lisp printer can produce this syntax to record circular and shared structure in a Lisp object, if you bind the variable `print-circle' to a non-`nil' value. *Note Output Variables::. Type Predicates =============== The Emacs Lisp interpreter itself does not perform type checking on the actual arguments passed to functions when they are called. It could not do so, since function arguments in Lisp do not have declared data types, as they do in other programming languages. It is therefore up to the individual function to test whether each actual argument belongs to a type that the function can use. All built-in functions do check the types of their actual arguments when appropriate, and signal a `wrong-type-argument' error if an argument is of the wrong type. For example, here is what happens if you pass an argument to `+' that it cannot handle: (+ 2 'a) error--> Wrong type argument: number-or-marker-p, a If you want your program to handle different types differently, you must do explicit type checking. The most common way to check the type of an object is to call a "type predicate" function. Emacs has a type predicate for each type, as well as some predicates for combinations of types. A type predicate function takes one argument; it returns `t' if the argument belongs to the appropriate type, and `nil' otherwise. Following a general Lisp convention for predicate functions, most type predicates' names end with `p'. Here is an example which uses the predicates `listp' to check for a list and `symbolp' to check for a symbol. (defun add-on (x) (cond ((symbolp x) ;; If X is a symbol, put it on LIST. (setq list (cons x list))) ((listp x) ;; If X is a list, add its elements to LIST. (setq list (append x list))) (t ;; We handle only symbols and lists. (error "Invalid argument %s in add-on" x)))) Here is a table of predefined type predicates, in alphabetical order, with references to further information. `atom' *Note atom: List-related Predicates. `arrayp' *Note arrayp: Array Functions. `bool-vector-p' *Note bool-vector-p: Bool-Vectors. `bufferp' *Note bufferp: Buffer Basics. `byte-code-function-p' *Note byte-code-function-p: Byte-Code Type. `case-table-p' *Note case-table-p: Case Tables. `char-or-string-p' *Note char-or-string-p: Predicates for Strings. `char-table-p' *Note char-table-p: Char-Tables. `commandp' *Note commandp: Interactive Call. `consp' *Note consp: List-related Predicates. `display-table-p' *Note display-table-p: Display Tables. `floatp' *Note floatp: Predicates on Numbers. `frame-configuration-p' *Note frame-configuration-p: Frame Configurations. `frame-live-p' *Note frame-live-p: Deleting Frames. `framep' *Note framep: Frames. `functionp' *Note functionp: Functions. `integer-or-marker-p' *Note integer-or-marker-p: Predicates on Markers. `integerp' *Note integerp: Predicates on Numbers. `keymapp' *Note keymapp: Creating Keymaps. `keywordp' *Note Constant Variables::. `listp' *Note listp: List-related Predicates. `markerp' *Note markerp: Predicates on Markers. `wholenump' *Note wholenump: Predicates on Numbers. `nlistp' *Note nlistp: List-related Predicates. `numberp' *Note numberp: Predicates on Numbers. `number-or-marker-p' *Note number-or-marker-p: Predicates on Markers. `overlayp' *Note overlayp: Overlays. `processp' *Note processp: Processes. `sequencep' *Note sequencep: Sequence Functions. `stringp' *Note stringp: Predicates for Strings. `subrp' *Note subrp: Function Cells. `symbolp' *Note symbolp: Symbols. `syntax-table-p' *Note syntax-table-p: Syntax Tables. `user-variable-p' *Note user-variable-p: Defining Variables. `vectorp' *Note vectorp: Vectors. `window-configuration-p' *Note window-configuration-p: Window Configurations. `window-live-p' *Note window-live-p: Deleting Windows. `windowp' *Note windowp: Basic Windows. The most general way to check the type of an object is to call the function `type-of'. Recall that each object belongs to one and only one primitive type; `type-of' tells you which one (*note Lisp Data Types::). But `type-of' knows nothing about non-primitive types. In most cases, it is more convenient to use type predicates than `type-of'. - Function: type-of object This function returns a symbol naming the primitive type of OBJECT. The value is one of the symbols `symbol', `integer', `float', `string', `cons', `vector', `char-table', `bool-vector', `hash-table', `subr', `compiled-function', `marker', `overlay', `window', `buffer', `frame', `process', or `window-configuration'. (type-of 1) => integer (type-of 'nil) => symbol (type-of '()) ; `()' is `nil'. => symbol (type-of '(x)) => cons Equality Predicates =================== Here we describe two functions that test for equality between any two objects. Other functions test equality between objects of specific types, e.g., strings. For these predicates, see the appropriate chapter describing the data type. - Function: eq object1 object2 This function returns `t' if OBJECT1 and OBJECT2 are the same object, `nil' otherwise. The "same object" means that a change in one will be reflected by the same change in the other. `eq' returns `t' if 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 are `eq'. For other types (e.g., lists, vectors, strings), two arguments with the same contents or elements are not necessarily `eq' to each other: they are `eq' only if they are the same object. (eq 'foo 'foo) => t (eq 456 456) => t (eq "asdf" "asdf") => nil (eq '(1 (2 (3))) '(1 (2 (3)))) => nil (setq foo '(1 (2 (3)))) => (1 (2 (3))) (eq foo foo) => t (eq foo '(1 (2 (3)))) => nil (eq [(1 2) 3] [(1 2) 3]) => nil (eq (point-marker) (point-marker)) => nil The `make-symbol' function returns an uninterned symbol, distinct from the symbol that is used if you write the name in a Lisp expression. Distinct symbols with the same name are not `eq'. *Note Creating Symbols::. (eq (make-symbol "foo") 'foo) => nil - Function: equal object1 object2 This function returns `t' if OBJECT1 and OBJECT2 have equal components, `nil' otherwise. Whereas `eq' tests if its arguments are the same object, `equal' looks inside nonidentical arguments to see if their elements or contents are the same. So, if two objects are `eq', they are `equal', but the converse is not always true. (equal 'foo 'foo) => t (equal 456 456) => t (equal "asdf" "asdf") => t (eq "asdf" "asdf") => nil (equal '(1 (2 (3))) '(1 (2 (3)))) => t (eq '(1 (2 (3))) '(1 (2 (3)))) => nil (equal [(1 2) 3] [(1 2) 3]) => t (eq [(1 2) 3] [(1 2) 3]) => nil (equal (point-marker) (point-marker)) => t (eq (point-marker) (point-marker)) => nil Comparison of strings is case-sensitive, but does not take account of text properties--it compares only the characters in the strings. A unibyte string never equals a multibyte string unless the contents are entirely ASCII (*note Text Representations::). (equal "asdf" "ASDF") => nil However, two distinct buffers are never considered `equal', even if their textual contents are the same. The test for equality is implemented recursively; for example, given two cons cells X and Y, `(equal X Y)' returns `t' if and only if both the expressions below return `t': (equal (car X) (car Y)) (equal (cdr X) (cdr Y)) Because of this recursive method, circular lists may therefore cause infinite recursion (leading to an error). Numbers ******* GNU Emacs supports two numeric data types: "integers" and "floating point numbers". Integers are whole numbers such as -3, 0, 7, 13, and 511. Their values are exact. Floating point numbers are numbers with fractional parts, such as -4.5, 0.0, or 2.71828. They can also be expressed in exponential notation: 1.5e2 equals 150; in this example, `e2' stands for ten to the second power, and that is multiplied by 1.5. Floating point values are not exact; they have a fixed, limited amount of precision. Integer Basics ============== The range of values for an integer depends on the machine. The minimum range is -134217728 to 134217727 (28 bits; i.e., -2**27 to 2**27 - 1), but some machines may provide a wider range. Many examples in this chapter assume an integer has 28 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. 268435457 ; Also the integer 1, due to overflow. 0 ; The integer 0. -0 ; The integer 0. In addition, the Lisp reader recognizes a syntax for integers in bases other than 10: `#BINTEGER' reads INTEGER in binary (radix 2), `#OINTEGER' reads INTEGER in octal (radix 8), `#XINTEGER' reads INTEGER in hexadecimal (radix 16), and `#RADIXrINTEGER' reads INTEGER in radix RADIX (where RADIX is between 2 and 36, inclusivley). Case is not significant for the letter after `#' (`B', `O', etc.) that denotes the radix. To understand how various functions work on integers, especially the bitwise operators (*note Bitwise Operations::), it is often helpful to view the numbers in their binary form. In 28-bit binary, the decimal integer 5 looks like this: 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: 1111 1111 1111 1111 1111 1111 1111 -1 is represented as 28 ones. (This is called "two's complement" notation.) The negative integer, -5, is creating by subtracting 4 from -1. In binary, the decimal integer 4 is 100. Consequently, -5 looks like this: 1111 1111 1111 1111 1111 1111 1011 In this implementation, the largest 28-bit binary integer value is 134,217,727 in decimal. In binary, it looks like this: 0111 1111 1111 1111 1111 1111 1111 Since the arithmetic functions do not check whether integers go outside their range, when you add 1 to 134,217,727, the value is the negative integer -134,217,728: (+ 1 134217727) => -134217728 => 1000 0000 0000 0000 0000 0000 0000 Many of the functions described in this chapter accept markers for arguments in place of numbers. (*Note Markers::.) Since the actual arguments to such functions may be either numbers or markers, we often give these arguments the name NUMBER-OR-MARKER. When the argument value is a marker, its position value is used and its buffer is ignored. 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, `(sqrt -1.0)' 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. 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'. In addition, the value `-0.0' is distinguishable from ordinary zero in IEEE floating point (although `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): - Function: logb number This function returns the binary exponent of NUMBER. More precisely, the value is the logarithm of NUMBER base 2, rounded down to an integer. (logb 10) => 3 (logb 10.0e20) => 69 Type Predicates for Numbers =========================== The functions in this section test whether the argument is a number or whether it is a certain sort of number. The functions `integerp' and `floatp' can take any type of Lisp object as argument (the predicates would not be of much use otherwise); but the `zerop' predicate requires a number as its argument. See also `integer-or-marker-p' and `number-or-marker-p', in *Note Predicates on Markers::. - Function: floatp object This predicate tests whether its argument is a floating point number and returns `t' if so, `nil' otherwise. `floatp' does not exist in Emacs versions 18 and earlier. - Function: integerp object This predicate tests whether its argument is an integer, and returns `t' if so, `nil' otherwise. - Function: numberp object This predicate tests whether its argument is a number (either integer or floating point), and returns `t' if so, `nil' otherwise. - Function: wholenump object The `wholenump' predicate (whose name comes from the phrase "whole-number-p") tests to see whether its argument is a nonnegative integer, and returns `t' if so, `nil' otherwise. 0 is considered non-negative. `natnump' is an obsolete synonym for `wholenump'. - Function: zerop number This predicate tests whether its argument is zero, and returns `t' if so, `nil' otherwise. The argument must be a number. These two forms are equivalent: `(zerop x)' == `(= x 0)'. 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. There is another wrinkle: because floating point arithmetic is not exact, it is often a bad idea to check for equality of two floating point values. Usually it is better to test for approximate equality. Here's a function to do this: (defvar fuzz-factor 1.0e-6) (defun approx-equal (x y) (or (and (= x 0) (= y 0)) (< (/ (abs (- x y)) (max (abs x) (abs y))) fuzz-factor))) Common Lisp note: Comparing numbers in Common Lisp always requires `=' because Common Lisp implements multi-word integers, and two distinct integer objects can have the same numeric value. Emacs Lisp can have just one integer object for any given value because it has a limited range of integer values. - Function: = number-or-marker1 number-or-marker2 This function tests whether its arguments are numerically equal, and returns `t' if so, `nil' otherwise. - Function: /= number-or-marker1 number-or-marker2 This function tests whether its arguments are numerically equal, and returns `t' if they are not, and `nil' if they are. - Function: < number-or-marker1 number-or-marker2 This function tests whether its first argument is strictly less than its second argument. It returns `t' if so, `nil' otherwise. - Function: <= number-or-marker1 number-or-marker2 This function tests whether its first argument is less than or equal to its second argument. It returns `t' if so, `nil' otherwise. - Function: > number-or-marker1 number-or-marker2 This function tests whether its first argument is strictly greater than its second argument. It returns `t' if so, `nil' otherwise. - Function: >= number-or-marker1 number-or-marker2 This function tests whether its first argument is greater than or equal to its second argument. It returns `t' if so, `nil' otherwise. - Function: max number-or-marker &rest numbers-or-markers This function returns the largest of its arguments. If any of the argument is floating-point, the value is returned as floating point, even if it was given as an integer. (max 20) => 20 (max 1 2.5) => 2.5 (max 1 3 2.5) => 3.0 - Function: min number-or-marker &rest numbers-or-markers This function returns the smallest of its arguments. If any of the argument is floating-point, the value is returned as floating point, even if it was given as an integer. (min -4 1) => -4 - Function: abs number This function returns the absolute value of NUMBER. Numeric Conversions =================== To convert an integer to floating point, use the function `float'. - Function: float number This returns NUMBER converted to floating point. If NUMBER is already a floating point number, `float' returns it unchanged. There are four functions to convert floating point numbers to integers; they differ in how they round. These functions accept integer arguments also, and return such arguments unchanged. - Function: truncate number This returns NUMBER, converted to an integer by rounding towards zero. (truncate 1.2) => 1 (truncate 1.7) => 1 (truncate -1.2) => -1 (truncate -1.7) => -1 - Function: floor number &optional divisor This returns NUMBER, converted to an integer by rounding downward (towards negative infinity). If DIVISOR is specified, `floor' divides NUMBER by DIVISOR and then converts to an integer; this uses the kind of division operation that corresponds to `mod', rounding downward. An `arith-error' results if DIVISOR is 0. (floor 1.2) => 1 (floor 1.7) => 1 (floor -1.2) => -2 (floor -1.7) => -2 (floor 5.99 3) => 1 - Function: ceiling number This returns NUMBER, converted to an integer by rounding upward (towards positive infinity). (ceiling 1.2) => 2 (ceiling 1.7) => 2 (ceiling -1.2) => -1 (ceiling -1.7) => -1 - Function: round number 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 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+ 134217727)' may evaluate to -134217728, depending on your hardware. - Function: 1+ number-or-marker This function returns NUMBER-OR-MARKER plus 1. For example, (setq foo 4) => 4 (1+ foo) => 5 This function is not analogous to the C operator `++'--it does not increment a variable. It just computes a sum. Thus, if we continue, foo => 4 If you want to increment the variable, you must use `setq', like this: (setq foo (1+ foo)) => 5 - Function: 1- number-or-marker This function returns NUMBER-OR-MARKER minus 1. - Function: + &rest numbers-or-markers This function adds its arguments together. When given no arguments, `+' returns 0. (+) => 0 (+ 1) => 1 (+ 1 2 3 4) => 10 - Function: - &optional number-or-marker &rest more-numbers-or-markers The `-' function serves two purposes: negation and subtraction. When `-' has a single argument, the value is the negative of the argument. When there are multiple arguments, `-' subtracts each of the MORE-NUMBERS-OR-MARKERS from NUMBER-OR-MARKER, cumulatively. If there are no arguments, the result is 0. (- 10 1 2 3 4) => 0 (- 10) => -10 (-) => 0 - Function: * &rest numbers-or-markers This function multiplies its arguments together, and returns the product. When given no arguments, `*' returns 1. (*) => 1 (* 1) => 1 (* 1 2 3 4) => 24 - Function: / dividend divisor &rest divisors This function divides DIVIDEND by DIVISOR and returns the quotient. If there are additional arguments DIVISORS, then it divides DIVIDEND by each divisor in turn. Each argument may be a number or a marker. If all the arguments are integers, 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-error' error is signaled. (*Note Errors::.) Floating point division by zero returns either infinity or a NaN if your machine supports IEEE floating point; otherwise, it signals an `arith-error' error. (/ 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 The result of `(/ -17 6)' could in principle be -3 on some machines. - Function: % dividend divisor 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-error' results if DIVISOR is 0. (% 9 4) => 1 (% -9 4) => -1 (% 9 -4) => 1 (% -9 -4) => -1 For any two integers DIVIDEND and DIVISOR, (+ (% DIVIDEND DIVISOR) (* (/ DIVIDEND DIVISOR) DIVISOR)) always equals DIVIDEND. - Function: mod dividend divisor This function returns the value of DIVIDEND modulo DIVISOR; in other words, the remainder after division of DIVIDEND by DIVISOR, but with the same sign as DIVISOR. The arguments must be numbers or markers. Unlike `%', `mod' returns 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-error' results 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) => .5 For any two numbers DIVIDEND and DIVISOR, (+ (mod DIVIDEND DIVISOR) (* (floor DIVIDEND DIVISOR) DIVISOR)) always equals DIVIDEND, subject to rounding error if either argument is floating point. For `floor', see *Note Numeric Conversions::. Rounding Operations =================== The functions `ffloor', `fceiling', `fround', and `ftruncate' take a floating point argument and return a floating point result whose value is a nearby integer. `ffloor' returns the nearest integer below; `fceiling', the nearest integer above; `ftruncate', the nearest integer in the direction towards zero; `fround', the nearest integer. - Function: ffloor float This function rounds FLOAT to the next lower integral value, and returns that value as a floating point number. - Function: fceiling float This function rounds FLOAT to the next higher integral value, and returns that value as a floating point number. - Function: ftruncate float This function rounds FLOAT towards zero to an integral value, and returns that value as a floating point number. - Function: fround float This function rounds FLOAT to the nearest integral value, and returns that value as a floating point number. Bitwise Operations on Integers ============================== In a computer, an integer is represented as a binary number, a sequence of "bits" (digits which are either zero or one). A bitwise operation acts on the individual bits of such a sequence. For example, "shifting" moves the whole sequence left or right one or more places, reproducing the same pattern "moved over". The bitwise operations in Emacs Lisp apply only to integers. - Function: lsh integer1 count `lsh', which is an abbreviation for "logical shift", shifts the bits in INTEGER1 to the left COUNT places, or to the right if COUNT is negative, bringing zeros into the vacated bits. If COUNT is negative, `lsh' shifts zeros into the leftmost (most-significant) bit, producing a positive result even if INTEGER1 is negative. Contrast this with `ash', below. Here are two examples of `lsh', shifting a pattern of bits one place to the left. We show only the low-order eight bits of the binary pattern; the rest are all zero. (lsh 5 1) => 10 ;; Decimal 5 becomes decimal 10. 00000101 => 00001010 (lsh 7 1) => 14 ;; Decimal 7 becomes decimal 14. 00000111 => 00001110 As the examples illustrate, shifting the pattern of bits one place to the left produces a number that is twice the value of the previous number. Shifting a pattern of bits two places to the left produces results like this (with 8-bit binary numbers): (lsh 3 2) => 12 ;; Decimal 3 becomes decimal 12. 00000011 => 00001100 On the other hand, shifting one place to the right looks like this: (lsh 6 -1) => 3 ;; Decimal 6 becomes decimal 3. 00000110 => 00000011 (lsh 5 -1) => 2 ;; Decimal 5 becomes decimal 2. 00000101 => 00000010 As the example illustrates, shifting one place to the right divides the value of a positive integer by two, rounding downward. The function `lsh', like all Emacs Lisp arithmetic functions, does not check for overflow, so shifting left can discard significant bits and change the sign of the number. For example, left shifting 134,217,727 produces -2 on a 28-bit machine: (lsh 134217727 1) ; left shift => -2 In binary, in the 28-bit implementation, the argument looks like this: ;; Decimal 134,217,727 0111 1111 1111 1111 1111 1111 1111 which becomes the following when left shifted: ;; Decimal -2 1111 1111 1111 1111 1111 1111 1110 - Function: ash integer1 count `ash' ("arithmetic shift") shifts the bits in INTEGER1 to the left COUNT places, or to the right if COUNT is negative. `ash' gives the same results as `lsh' except when INTEGER1 and COUNT are both negative. In that case, `ash' puts ones in the empty bit positions on the left, while `lsh' puts zeros in those bit positions. Thus, with `ash', shifting the pattern of bits one place to the right looks like this: (ash -6 -1) => -3 ;; Decimal -6 becomes decimal -3. 1111 1111 1111 1111 1111 1111 1010 => 1111 1111 1111 1111 1111 1111 1101 In contrast, shifting the pattern of bits one place to the right with `lsh' looks like this: (lsh -6 -1) => 134217725 ;; Decimal -6 becomes decimal 134,217,725. 1111 1111 1111 1111 1111 1111 1010 => 0111 1111 1111 1111 1111 1111 1101 Here are other examples: ; 28-bit binary values (lsh 5 2) ; 5 = 0000 0000 0000 0000 0000 0000 0101 => 20 ; = 0000 0000 0000 0000 0000 0001 0100 (ash 5 2) => 20 (lsh -5 2) ; -5 = 1111 1111 1111 1111 1111 1111 1011 => -20 ; = 1111 1111 1111 1111 1111 1110 1100 (ash -5 2) => -20 (lsh 5 -2) ; 5 = 0000 0000 0000 0000 0000 0000 0101 => 1 ; = 0000 0000 0000 0000 0000 0000 0001 (ash 5 -2) => 1 (lsh -5 -2) ; -5 = 1111 1111 1111 1111 1111 1111 1011 => 4194302 ; = 0011 1111 1111 1111 1111 1111 1110 (ash -5 -2) ; -5 = 1111 1111 1111 1111 1111 1111 1011 => -2 ; = 1111 1111 1111 1111 1111 1111 1110 - Function: logand &rest ints-or-markers This function returns the "logical and" of the arguments: the Nth bit is set in the result if, and only if, the Nth bit is set in all the arguments. ("Set" means that the value of the bit is 1 rather than 0.) For example, using 4-bit binary numbers, the "logical and" of 13 and 12 is 12: 1101 combined with 1100 produces 1100. In both the binary numbers, the leftmost two bits are set (i.e., they are 1's), so the leftmost two bits of the returned value are set. However, for the rightmost two bits, each is zero in at least one of the arguments, so the rightmost two bits of the returned value are 0's. Therefore, (logand 13 12) => 12 If `logand' is not passed any argument, it returns a value of -1. This number is an identity element for `logand' because its binary representation consists entirely of ones. If `logand' is passed just one argument, it returns that argument. ; 28-bit binary values (logand 14 13) ; 14 = 0000 0000 0000 0000 0000 0000 1110 ; 13 = 0000 0000 0000 0000 0000 0000 1101 => 12 ; 12 = 0000 0000 0000 0000 0000 0000 1100 (logand 14 13 4) ; 14 = 0000 0000 0000 0000 0000 0000 1110 ; 13 = 0000 0000 0000 0000 0000 0000 1101 ; 4 = 0000 0000 0000 0000 0000 0000 0100 => 4 ; 4 = 0000 0000 0000 0000 0000 0000 0100 (logand) => -1 ; -1 = 1111 1111 1111 1111 1111 1111 1111 - Function: logior &rest ints-or-markers This function returns the "inclusive or" of its arguments: the Nth bit is set in the result if, and only if, the Nth bit is set in at least one of the arguments. If there are no arguments, the result is zero, which is an identity element for this operation. If `logior' is passed just one argument, it returns that argument. ; 28-bit binary values (logior 12 5) ; 12 = 0000 0000 0000 0000 0000 0000 1100 ; 5 = 0000 0000 0000 0000 0000 0000 0101 => 13 ; 13 = 0000 0000 0000 0000 0000 0000 1101 (logior 12 5 7) ; 12 = 0000 0000 0000 0000 0000 0000 1100 ; 5 = 0000 0000 0000 0000 0000 0000 0101 ; 7 = 0000 0000 0000 0000 0000 0000 0111 => 15 ; 15 = 0000 0000 0000 0000 0000 0000 1111 - Function: logxor &rest ints-or-markers This function returns the "exclusive or" of its arguments: the Nth bit is set in the result if, and only if, the Nth bit is set in an odd number of the arguments. If there are no arguments, the result is 0, which is an identity element for this operation. If `logxor' is passed just one argument, it returns that argument. ; 28-bit binary values (logxor 12 5) ; 12 = 0000 0000 0000 0000 0000 0000 1100 ; 5 = 0000 0000 0000 0000 0000 0000 0101 => 9 ; 9 = 0000 0000 0000 0000 0000 0000 1001 (logxor 12 5 7) ; 12 = 0000 0000 0000 0000 0000 0000 1100 ; 5 = 0000 0000 0000 0000 0000 0000 0101 ; 7 = 0000 0000 0000 0000 0000 0000 0111 => 14 ; 14 = 0000 0000 0000 0000 0000 0000 1110 - Function: lognot integer This function returns the logical complement of its argument: the Nth bit is one in the result if, and only if, the Nth bit is zero in INTEGER, and vice-versa. (lognot 5) => -6 ;; 5 = 0000 0000 0000 0000 0000 0000 0101 ;; becomes ;; -6 = 1111 1111 1111 1111 1111 1111 1010 Standard Mathematical Functions =============================== These mathematical functions allow integers as well as floating point numbers as arguments. - Function: sin arg - Function: cos arg - Function: tan arg These are the ordinary trigonometric functions, with argument measured in radians. - Function: asin arg The value of `(asin ARG)' is a number between -pi/2 and pi/2 (inclusive) whose sine is ARG; if, however, ARG is out of range (outside [-1, 1]), then the result is a NaN. - Function: acos arg The value of `(acos ARG)' is a number between 0 and pi (inclusive) whose cosine is ARG; if, however, ARG is out of range (outside [-1, 1]), then the result is a NaN. - Function: atan arg The value of `(atan ARG)' is a number between -pi/2 and pi/2 (exclusive) whose tangent is ARG. - Function: exp arg 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. - Function: log arg &optional base 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, the result is a NaN. - Function: log10 arg This function returns the logarithm of ARG, with base 10. If ARG is negative, the result is a NaN. `(log10 X)' == `(log X 10)', at least approximately. - Function: expt x y This function returns X raised to power Y. If both arguments are integers and Y is positive, the result is an integer; in this case, it is truncated to fit the range of possible integer values. - Function: sqrt arg This returns the square root of ARG. If ARG is negative, the value is a NaN. 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. - Function: random &optional limit This function returns a pseudo-random integer. Repeated calls return a series of pseudo-random integers. If LIMIT is a positive integer, the value is chosen to be nonnegative and less than LIMIT. 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. Strings and Characters ********************** A string in Emacs Lisp is an array that contains an ordered sequence of characters. Strings are used as names of symbols, buffers, and files; to send messages to users; to hold text being copied between buffers; and for many other purposes. Because strings are so important, Emacs Lisp has many functions expressly for manipulating them. Emacs Lisp programs use strings more often than individual characters. *Note Strings of Events::, for special considerations for strings of keyboard character events. 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. 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. (*Note Sequences Arrays Vectors::.) For example, you can access or change individual characters in a string using the functions `aref' and `aset' (*note Array Functions::). There are two text representations for non-ASCII characters in Emacs strings (and in buffers): unibyte and multibyte (*note Text Representations::). An ASCII character always occupies one byte in a string; in fact, when a string is all ASCII, there is no real difference between the unibyte and multibyte representations. For most Lisp programming, you don't need to be concerned with these two representations. Sometimes key sequences are represented as strings. When a string is a key sequence, string elements in the range 128 to 255 represent meta characters (which are large integers) rather than character codes in the range 128 to 255. Strings cannot hold characters that have the hyper, super or alt modifiers; they can hold ASCII control characters, but no other control characters. They do not distinguish case in ASCII control characters. If you want to store such characters in a sequence, such as a key sequence, you must use a vector instead of a string. *Note Character Type::, for more information about the representation of meta and other modifiers for keyboard input characters. Strings are useful for holding regular expressions. You can also match regular expressions against strings (*note Regexp Search::). The functions `match-string' (*note Simple Match Data::) and `replace-match' (*note Replacing Match::) are useful for decomposing and modifying strings based on regular expression matching. Like a buffer, a string can contain text properties for the characters in it, as well as the characters themselves. *Note Text Properties::. All the Lisp primitives that copy text from strings to buffers or other strings also copy the properties of the characters being copied. *Note Text::, for information about functions that display strings or copy them into buffers. *Note Character Type::, and *Note String Type::, for information about the syntax of characters and strings. *Note Non-ASCII Characters::, for functions to convert between text representations and to encode and decode character codes. The Predicates for Strings ========================== For more information about general sequence and array predicates, see *Note Sequences Arrays Vectors::, and *Note Arrays::. - Function: stringp object This function returns `t' if OBJECT is a string, `nil' otherwise. - Function: char-or-string-p object This function returns `t' if OBJECT is a string or a character (i.e., an integer), `nil' otherwise. Creating Strings ================ The following functions create strings, either from scratch, or by putting strings together, or by taking them apart. - Function: make-string count character This function returns a string made up of COUNT repetitions of CHARACTER. If COUNT is negative, an error is signaled. (make-string 5 ?x) => "xxxxx" (make-string 0 ?x) => "" Other functions to compare with this one include `char-to-string' (*note String Conversion::), `make-vector' (*note Vectors::), and `make-list' (*note Building Lists::). - Function: string &rest characters This returns a string containing the characters CHARACTERS. (string ?a ?b ?c) => "abc" - Function: substring string start &optional end This function returns a new string which consists of those characters from STRING in the range from (and including) the character at the index START up to (but excluding) the character at the index END. The first character is at index zero. (substring "abcdefg" 0 3) => "abc" Here the index for `a' is 0, the index for `b' is 1, and the index for `c' is 2. Thus, three letters, `abc', are copied from the string `"abcdefg"'. The index 3 marks the character position up to which the substring is copied. The character whose index is 3 is actually the fourth character in the string. A negative number counts from the end of the string, so that -1 signifies the index of the last character of the string. For example: (substring "abcdefg" -3 -1) => "ef" In this example, the index for `e' is -3, the index for `f' is -2, and the index for `g' is -1. Therefore, `e' and `f' are included, and `g' is excluded. When `nil' is used as an index, it stands for the length of the string. Thus, (substring "abcdefg" -3 nil) => "efg" Omitting the argument END is equivalent to specifying `nil'. It follows that `(substring STRING 0)' returns a copy of all of STRING. (substring "abcdefg" 0) => "abcdefg" But we recommend `copy-sequence' for this purpose (*note Sequence Functions::). If the characters copied from STRING have text properties, the properties are copied into the new string also. *Note Text Properties::. `substring' also accepts a vector for the first argument. For example: (substring [a b (c) "d"] 1 3) => [b (c)] A `wrong-type-argument' error is signaled if either START or END is not an integer or `nil'. An `args-out-of-range' error is signaled if START indicates a character following END, or if either integer is out of range for STRING. Contrast this function with `buffer-substring' (*note Buffer Contents::), which returns a string containing a portion of the text in the current buffer. The beginning of a string is at index 0, but the beginning of a buffer is at index 1. - Function: concat &rest sequences This function returns a new string consisting of the characters in the arguments passed to it (along with their text properties, if any). The arguments may be strings, lists of numbers, or vectors of numbers; they are not themselves changed. If `concat' receives no arguments, it returns an empty string. (concat "abc" "-def") => "abc-def" (concat "abc" (list 120 121) [122]) => "abcxyz" ;; `nil' is an empty sequence. (concat "abc" nil "-def") => "abc-def" (concat "The " "quick brown " "fox.") => "The quick brown fox." (concat) => "" The `concat' function always constructs a new string that is not `eq' to any existing string. In Emacs versions before 21, when an argument was an integer (not a sequence of integers), it was converted to a string of digits making up the decimal printed representation of the integer. This obsolete usage no longer works. The proper way to convert an integer to its decimal printed form is with `format' (*note Formatting Strings::) or `number-to-string' (*note String Conversion::). For information about other concatenation functions, see the description of `mapconcat' in *Note Mapping Functions::, `vconcat' in *Note Vectors::, and `append' in *Note Building Lists::. - Function: split-string string separators This function splits STRING into substrings at matches for the regular expression SEPARATORS. Each match for SEPARATORS defines a splitting point; the substrings between the splitting points are made into a list, which is the value returned by `split-string'. If SEPARATORS is `nil' (or omitted), the default is `"[ \f\t\n\r\v]+"'. For example, (split-string "Soup is good food" "o") => ("S" "up is g" "" "d f" "" "d") (split-string "Soup is good food" "o+") => ("S" "up is g" "d f" "d") When there is a match adjacent to the beginning or end of the string, this does not cause a null string to appear at the beginning or end of the list: (split-string "out to moo" "o+") => ("ut t" " m") Empty matches do count, when not adjacent to another match: (split-string "Soup is good food" "o*") =>("S" "u" "p" " " "i" "s" " " "g" "d" " " "f" "d") (split-string "Nice doggy!" "") =>("N" "i" "c" "e" " " "d" "o" "g" "g" "y" "!") Modifying Strings ================= The most basic way to alter the contents of an existing string is with `aset' (*note Array Functions::). `(aset STRING IDX CHAR)' stores CHAR into STRING at index IDX. Each character occupies one or more bytes, and if CHAR needs a different number of bytes from the character already present at that index, `aset' signals an error. A more powerful function is `store-substring': - Function: store-substring string idx obj This function alters part of the contents of the string STRING, by storing OBJ starting at index IDX. The argument OBJ may be either a character or a (smaller) string. Since it is impossible to change the length of an existing string, it is an error if OBJ doesn't fit within STRING's actual length, or if any new character requires a different number of bytes from the character currently present at that point in STRING. Comparison of Characters and Strings ==================================== - Function: char-equal character1 character2 This function returns `t' if the arguments represent the same character, `nil' otherwise. This function ignores differences in case if `case-fold-search' is non-`nil'. (char-equal ?x ?x) => t (let ((case-fold-search nil)) (char-equal ?x ?X)) => nil - Function: string= string1 string2 This function returns `t' if the characters of the two strings match exactly. Case is always significant, regardless of `case-fold-search'. (string= "abc" "abc") => t (string= "abc" "ABC") => nil (string= "ab" "ABC") => nil The function `string=' ignores the text properties of the two strings. When `equal' (*note Equality Predicates::) compares two strings, it uses `string='. If the strings contain non-ASCII characters, and one is unibyte while the other is multibyte, then they cannot be equal. *Note Text Representations::. - Function: string-equal string1 string2 `string-equal' is another name for `string='. - Function: string< string1 string2 This function compares two strings a character at a time. It scans both the strings at the same time to find the first pair of corresponding characters that do not match. If the lesser character of these two is the character from STRING1, then STRING1 is less, and this function returns `t'. If the lesser character is the one from STRING2, then STRING1 is greater, and this function returns `nil'. If the two strings match entirely, the value is `nil'. Pairs of characters are compared according to their character codes. Keep in mind that lower case letters have higher numeric values in the ASCII character set than their upper case counterparts; digits and many punctuation characters have a lower numeric value than upper case letters. An ASCII character is less than any non-ASCII character; a unibyte non-ASCII character is always less than any multibyte non-ASCII character (*note Text Representations::). (string< "abc" "abd") => t (string< "abd" "abc") => nil (string< "123" "abc") => t When the strings have different lengths, and they match up to the length of STRING1, then the result is `t'. If they match up to the length of STRING2, the result is `nil'. A string of no characters is less than any other string. (string< "" "abc") => t (string< "ab" "abc") => t (string< "abc" "") => nil (string< "abc" "ab") => nil (string< "" "") => nil - Function: string-lessp string1 string2 `string-lessp' is another name for `string<'. - Function: compare-strings string1 start1 end1 string2 start2 end2 &optional ignore-case 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 (`nil' means the end of the string). The specified part of STRING2 runs from index START2 up to index END2 (`nil' means the end of the string). The strings are both converted to multibyte for the comparison (*note Text Representations::) so that a unibyte string can be equal to a multibyte string. 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. - Function: assoc-ignore-case key alist This function works like `assoc', except that KEY must be a string, and comparison is done using `compare-strings', ignoring case differences. *Note Association Lists::. - Function: assoc-ignore-representation key alist This function works like `assoc', except that KEY must be a string, and comparison is done using `compare-strings'. Case differences are significant. See also `compare-buffer-substrings' in *Note Comparing Text::, for a way to compare text in buffers. The function `string-match', which matches a regular expression against a string, can be used for a kind of string comparison; see *Note Regexp Search::. Conversion of Characters and Strings ==================================== This section describes functions for conversions between characters, strings and integers. `format' and `prin1-to-string' (*note Output Functions::) can also convert Lisp objects into strings. `read-from-string' (*note Input Functions::) can "convert" a string representation of a Lisp object into an object. The functions `string-make-multibyte' and `string-make-unibyte' convert the text representation of a string (*note Converting Representations::). *Note Documentation::, for functions that produce textual descriptions of text characters and general input events (`single-key-description' and `text-char-description'). These functions are used primarily for making help messages. - Function: char-to-string character This function returns a new string containing one character, CHARACTER. This function is semi-obsolete because the function `string' is more general. *Note Creating Strings::. - Function: string-to-char string This function returns the first character in STRING. If the string is empty, the function returns 0. The value is also 0 when the first character of STRING is the null character, ASCII code 0. (string-to-char "ABC") => 65 (string-to-char "xyz") => 120 (string-to-char "") => 0 (string-to-char "\000") => 0 This function may be eliminated in the future if it does not seem useful enough to retain. - Function: number-to-string number This function returns a string consisting of the printed base-ten representation of NUMBER, which may be an integer or a floating point number. The returned value starts with a minus sign if the argument is negative. (number-to-string 256) => "256" (number-to-string -23) => "-23" (number-to-string -23.5) => "-23.5" `int-to-string' is a semi-obsolete alias for this function. See also the function `format' in *Note Formatting Strings::. - Function: string-to-number string &optional base This function returns the numeric value of the characters in STRING. If BASE is non-`nil', integers are converted in that base. If BASE is `nil', then base ten is used. Floating point conversion always uses base ten; we have not implemented other radices for floating point numbers, because that would be much more work and does not seem useful. If STRING looks like an integer but its value is too large to fit into a Lisp integer, `string-to-number' returns a floating point result. The parsing skips spaces and tabs at the beginning of STRING, then reads as much of STRING as it can interpret as a number. (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, nor a plus or minus sign, nor the leading dot of a floating point number, this function returns 0. (string-to-number "256") => 256 (string-to-number "25 is a perfect square.") => 25 (string-to-number "X256") => 0 (string-to-number "-4.5") => -4.5 (string-to-number "1e5") => 100000.0 `string-to-int' is an obsolete alias for this function. Here are some other functions that can convert to or from a string: `concat' `concat' can convert a vector or a list into a string. *Note Creating Strings::. `vconcat' `vconcat' can convert a string into a vector. *Note Vector Functions::. `append' `append' can convert a string into a list. *Note Building Lists::. Formatting Strings ================== "Formatting" means constructing a string by substitution of computed values at various places in a constant string. This constant string controls how the other values are printed, as well as where they appear; it is called a "format string". Formatting is often useful for computing messages to be displayed. In fact, the functions `message' and `error' provide the same formatting feature described here; they differ from `format' only in how they use the result of formatting. - Function: format string &rest objects This function returns a new string that is made by copying STRING and then replacing any format specification in the copy with encodings of the corresponding OBJECTS. The arguments OBJECTS are the computed values to be formatted. The characters in STRING, other than the format specifications, are copied directly into the output; starting in Emacs 21, if they have text properties, these are copied into the output also. 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." 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 unpredictable behavior. 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 specificatio