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This is Info file ../../info/lispref.info, produced by Makeinfo version 1.68 from the input file lispref.texi. Edition History: GNU Emacs Lisp Reference Manual Second Edition (v2.01), May 1993 GNU Emacs Lisp Reference Manual Further Revised (v2.02), August 1993 Lucid Emacs Lisp Reference Manual (for 19.10) First Edition, March 1994 XEmacs Lisp Programmer's Manual (for 19.12) Second Edition, April 1995 GNU Emacs Lisp Reference Manual v2.4, June 1995 XEmacs Lisp Programmer's Manual (for 19.13) Third Edition, July 1995 XEmacs Lisp Reference Manual (for 19.14 and 20.0) v3.1, March 1996 XEmacs Lisp Reference Manual (for 19.15 and 20.1, 20.2) v3.2, April, May 1997 Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995 Free Software Foundation, Inc. Copyright (C) 1994, 1995 Sun Microsystems, Inc. Copyright (C) 1995, 1996 Ben Wing. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that this permission notice may be stated in a translation approved by the Foundation. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the section entitled "GNU General Public License" is included exactly as in the original, and provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that the section entitled "GNU General Public License" may be included in a translation approved by the Free Software Foundation instead of in the original English. File: lispref.info, Node: Sequence Functions, Next: Arrays, Up: Sequences Arrays Vectors Sequences ========= In XEmacs Lisp, a "sequence" is either a list, a vector, a bit vector, or a string. The common property that all sequences have is that each is an ordered collection of elements. This section describes functions that accept any kind of sequence. - Function: sequencep OBJECT Returns `t' if OBJECT is a list, vector, bit vector, or string, `nil' otherwise. - Function: copy-sequence SEQUENCE Returns a copy of SEQUENCE. The copy is the same type of object as the original sequence, and it has the same elements in the same order. Storing a new element into the copy does not affect the original SEQUENCE, and vice versa. However, the elements of the new sequence are not copies; they are identical (`eq') to the elements of the original. Therefore, changes made within these elements, as found via the copied sequence, are also visible in the original sequence. If the sequence is a string with extents or text properties, the extents and text properties in the copy are also copied, not shared with the original. (This means that modifying the extents or text properties of the original will not affect the copy.) However, the actual values of the properties are shared. *Note Extents::; *Note Text Properties::. See also `append' in *Note Building Lists::, `concat' in *Note Creating Strings::, `vconcat' in *Note Vectors::, and `bvconcat' in *Note Bit Vectors::, for other ways to copy sequences. (setq bar '(1 2)) => (1 2) (setq x (vector 'foo bar)) => [foo (1 2)] (setq y (copy-sequence x)) => [foo (1 2)] (eq x y) => nil (equal x y) => t (eq (elt x 1) (elt y 1)) => t ;; Replacing an element of one sequence. (aset x 0 'quux) x => [quux (1 2)] y => [foo (1 2)] ;; Modifying the inside of a shared element. (setcar (aref x 1) 69) x => [quux (69 2)] y => [foo (69 2)] ;; Creating a bit vector. (bit-vector 1 0 1 1 0 1 0 0) => #*10110100 - Function: length SEQUENCE Returns the number of elements in SEQUENCE. If SEQUENCE is a cons cell that is not a list (because the final CDR is not `nil'), a `wrong-type-argument' error is signaled. (length '(1 2 3)) => 3 (length ()) => 0 (length "foobar") => 6 (length [1 2 3]) => 3 (length #*01101) => 5 - Function: elt SEQUENCE INDEX This function returns the element of SEQUENCE indexed by INDEX. Legitimate values of INDEX are integers ranging from 0 up to one less than the length of SEQUENCE. If SEQUENCE is a list, then out-of-range values of INDEX return `nil'; otherwise, they trigger an `args-out-of-range' error. (elt [1 2 3 4] 2) => 3 (elt '(1 2 3 4) 2) => 3 (char-to-string (elt "1234" 2)) => "3" (elt #*00010000 3) => 1 (elt [1 2 3 4] 4) error-->Args out of range: [1 2 3 4], 4 (elt [1 2 3 4] -1) error-->Args out of range: [1 2 3 4], -1 This function generalizes `aref' (*note Array Functions::.) and `nth' (*note List Elements::.). File: lispref.info, Node: Arrays, Next: Array Functions, Prev: Sequence Functions, Up: Sequences Arrays Vectors Arrays ====== An "array" object has slots that hold a number of other Lisp objects, called the elements of the array. Any element of an array may be accessed in constant time. In contrast, an element of a list requires access time that is proportional to the position of the element in the list. When you create an array, you must specify how many elements it has. The amount of space allocated depends on the number of elements. Therefore, it is impossible to change the size of an array once it is created; you cannot add or remove elements. However, you can replace an element with a different value. XEmacs defines three types of array, all of which are one-dimensional: "strings", "vectors", and "bit vectors". A vector is a general array; its elements can be any Lisp objects. A string is a specialized array; its elements must be characters. A bit vector is another specialized array; its elements must be bits (an integer, either 0 or 1). Each type of array has its own read syntax. *Note String Type::, *Note Vector Type::, and *Note Bit Vector Type::. All kinds of array share these characteristics: * The first element of an array has index zero, the second element has index 1, and so on. This is called "zero-origin" indexing. For example, an array of four elements has indices 0, 1, 2, and 3. * The elements of an array may be referenced or changed with the functions `aref' and `aset', respectively (*note Array Functions::.). In principle, if you wish to have an array of text characters, you could use either a string or a vector. In practice, we always choose strings for such applications, for four reasons: * They usually occupy one-fourth the space of a vector of the same elements. (This is one-eighth the space for 64-bit machines such as the DEC Alpha, and may also be different when MULE support is compiled into XEmacs.) * Strings are printed in a way that shows the contents more clearly as characters. * Strings can hold extent and text properties. *Note Extents::; *Note Text Properties::. * Many of the specialized editing and I/O facilities of XEmacs accept only strings. For example, you cannot insert a vector of characters into a buffer the way you can insert a string. *Note Strings and Characters::. By contrast, for an array of keyboard input characters (such as a key sequence), a vector may be necessary, because many keyboard input characters are non-printable and are represented with symbols rather than with characters. *Note Key Sequence Input::. Similarly, when representing an array of bits, a bit vector has the following advantages over a regular vector: * They occupy 1/32nd the space of a vector of the same elements. (1/64th on 64-bit machines such as the DEC Alpha.) * Bit vectors are printed in a way that shows the contents more clearly as bits. File: lispref.info, Node: Array Functions, Next: Vectors, Prev: Arrays, Up: Sequences Arrays Vectors Functions that Operate on Arrays ================================ In this section, we describe the functions that accept strings, vectors, and bit vectors. - Function: arrayp OBJECT This function returns `t' if OBJECT is an array (i.e., a string, vector, or bit vector). (arrayp "asdf") => t (arrayp [a]) => t (arrayp #*101) => t - Function: aref ARRAY INDEX This function returns the INDEXth element of ARRAY. The first element is at index zero. (setq primes [2 3 5 7 11 13]) => [2 3 5 7 11 13] (aref primes 4) => 11 (elt primes 4) => 11 (aref "abcdefg" 1) => ?b (aref #*1101 2) => 0 See also the function `elt', in *Note Sequence Functions::. - Function: aset ARRAY INDEX OBJECT This function sets the INDEXth element of ARRAY to be OBJECT. It returns OBJECT. (setq w [foo bar baz]) => [foo bar baz] (aset w 0 'fu) => fu w => [fu bar baz] (setq x "asdfasfd") => "asdfasfd" (aset x 3 ?Z) => ?Z x => "asdZasfd" (setq bv #*1111) => #*1111 (aset bv 2 0) => 0 bv => #*1101 If ARRAY is a string and OBJECT is not a character, a `wrong-type-argument' error results. - Function: fillarray ARRAY OBJECT This function fills the array ARRAY with OBJECT, so that each element of ARRAY is OBJECT. It returns ARRAY. (setq a [a b c d e f g]) => [a b c d e f g] (fillarray a 0) => [0 0 0 0 0 0 0] a => [0 0 0 0 0 0 0] (setq s "When in the course") => "When in the course" (fillarray s ?-) => "------------------" (setq bv #*1101) => #*1101 (fillarray bv 0) => #*0000 If ARRAY is a string and OBJECT is not a character, a `wrong-type-argument' error results. The general sequence functions `copy-sequence' and `length' are often useful for objects known to be arrays. *Note Sequence Functions::. File: lispref.info, Node: Vectors, Next: Vector Functions, Prev: Array Functions, Up: Sequences Arrays Vectors Vectors ======= Arrays in Lisp, like arrays in most languages, are blocks of memory whose elements can be accessed in constant time. A "vector" is a general-purpose array; its elements can be any Lisp objects. (The other kind of array in XEmacs Lisp is the "string", whose elements must be characters.) Vectors in XEmacs serve as obarrays (vectors of symbols), although this is a shortcoming that should be fixed. They are also used internally as part of the representation of a byte-compiled function; if you print such a function, you will see a vector in it. In XEmacs Lisp, the indices of the elements of a vector start from zero and count up from there. Vectors are printed with square brackets surrounding the elements. Thus, a vector whose elements are the symbols `a', `b' and `a' is printed as `[a b a]'. You can write vectors in the same way in Lisp input. A vector, like a string or a number, is considered a constant for evaluation: the result of evaluating it is the same vector. This does not evaluate or even examine the elements of the vector. *Note Self-Evaluating Forms::. Here are examples of these principles: (setq avector [1 two '(three) "four" [five]]) => [1 two (quote (three)) "four" [five]] (eval avector) => [1 two (quote (three)) "four" [five]] (eq avector (eval avector)) => t File: lispref.info, Node: Vector Functions, Next: Bit Vectors, Prev: Vectors, Up: Sequences Arrays Vectors Functions That Operate on Vectors ================================= Here are some functions that relate to vectors: - Function: vectorp OBJECT This function returns `t' if OBJECT is a vector. (vectorp [a]) => t (vectorp "asdf") => nil - Function: vector &rest OBJECTS This function creates and returns a vector whose elements are the arguments, OBJECTS. (vector 'foo 23 [bar baz] "rats") => [foo 23 [bar baz] "rats"] (vector) => [] - Function: make-vector LENGTH OBJECT This function returns a new vector consisting of LENGTH elements, each initialized to OBJECT. (setq sleepy (make-vector 9 'Z)) => [Z Z Z Z Z Z Z Z Z] - Function: vconcat &rest SEQUENCES This function returns a new vector containing all the elements of the SEQUENCES. The arguments SEQUENCES may be lists, vectors, or strings. If no SEQUENCES are given, an empty vector is returned. The value is a newly constructed vector that is not `eq' to any existing vector. (setq a (vconcat '(A B C) '(D E F))) => [A B C D E F] (eq a (vconcat a)) => nil (vconcat) => [] (vconcat [A B C] "aa" '(foo (6 7))) => [A B C 97 97 foo (6 7)] The `vconcat' function also allows integers as arguments. It converts them to strings of digits, making up the decimal print representation of the integer, and then uses the strings instead of the original integers. *Don't use this feature; we plan to eliminate it. If you already use this feature, change your programs now!* The proper way to convert an integer to a decimal number in this way is with `format' (*note Formatting Strings::.) or `number-to-string' (*note String Conversion::.). For other concatenation functions, see `mapconcat' in *Note Mapping Functions::, `concat' in *Note Creating Strings::, `append' in *Note Building Lists::, and `bvconcat' in *Note Bit Vector Functions::. The `append' function provides a way to convert a vector into a list with the same elements (*note Building Lists::.): (setq avector [1 two (quote (three)) "four" [five]]) => [1 two (quote (three)) "four" [five]] (append avector nil) => (1 two (quote (three)) "four" [five]) File: lispref.info, Node: Bit Vectors, Next: Bit Vector Functions, Prev: Vector Functions, Up: Sequences Arrays Vectors Bit Vectors =========== Bit vectors are specialized vectors that can only represent arrays of 1's and 0's. Bit vectors have a very efficient representation and are useful for representing sets of boolean (true or false) values. There is no limit on the size of a bit vector. You could, for example, create a bit vector with 100,000 elements if you really wanted to. Bit vectors have a special printed representation consisting of `#*' followed by the bits of the vector. For example, a bit vector whose elements are 0, 1, 1, 0, and 1, respectively, is printed as #*01101 Bit vectors are considered constants for evaluation, like vectors, strings, and numbers. *Note Self-Evaluating Forms::. File: lispref.info, Node: Bit Vector Functions, Prev: Bit Vectors, Up: Sequences Arrays Vectors Functions That Operate on Bit Vectors ===================================== Here are some functions that relate to bit vectors: - Function: bit-vector-p OBJECT This function returns `t' if OBJECT is a bit vector. (bit-vector-p #*01) => t (bit-vector-p [0 1]) => nil (bit-vector-p "01") => nil - Function: bitp OBJECT This function returns `t' if OBJECT is either 0 or 1. - Function: bit-vector &rest OBJECTS This function creates and returns a bit vector whose elements are the arguments OBJECTS. The elements must be either of the two integers 0 or 1. (bit-vector 0 0 0 1 0 0 0 0 1 0) => #*0001000010 (bit-vector) => #* - Function: make-bit-vector LENGTH OBJECT This function creates and returns a bit vector consisting of LENGTH elements, each initialized to OBJECT. (setq picket-fence (make-bit-vector 9 1)) => #*111111111 - Function: bvconcat &rest SEQUENCES This function returns a new bit vector containing all the elements of the SEQUENCES. The arguments SEQUENCES may be lists, vectors, or bit vectors, all of whose elements are the integers 0 or 1. If no SEQUENCES are given, an empty bit vector is returned. The value is a newly constructed bit vector that is not `eq' to any existing bit vector. (setq a (bvconcat '(1 1 0) '(0 0 1))) => #*110001 (eq a (bvconcat a)) => nil (bvconcat) => #* (bvconcat [1 0 0 0 0] #*111 '(0 0 0 0 1)) => #*1000011100001 For other concatenation functions, see `mapconcat' in *Note Mapping Functions::, `concat' in *Note Creating Strings::, `vconcat' in *Note Vector Functions::, and `append' in *Note Building Lists::. The `append' function provides a way to convert a bit vector into a list with the same elements (*note Building Lists::.): (setq bv #*00001110) => #*00001110 (append bv nil) => (0 0 0 0 1 1 1 0) File: lispref.info, Node: Symbols, Next: Evaluation, Prev: Sequences Arrays Vectors, Up: Top Symbols ******* A "symbol" is an object with a unique name. This chapter describes symbols, their components, their property lists, and how they are created and interned. Separate chapters describe the use of symbols as variables and as function names; see *Note Variables::, and *Note Functions::. For the precise read syntax for symbols, see *Note Symbol Type::. You can test whether an arbitrary Lisp object is a symbol with `symbolp': - Function: symbolp OBJECT This function returns `t' if OBJECT is a symbol, `nil' otherwise. * Menu: * Symbol Components:: Symbols have names, values, function definitions and property lists. * Definitions:: A definition says how a symbol will be used. * Creating Symbols:: How symbols are kept unique. * Symbol Properties:: Each symbol has a property list for recording miscellaneous information. File: lispref.info, Node: Symbol Components, Next: Definitions, Up: Symbols Symbol Components ================= Each symbol has four components (or "cells"), each of which references another object: Print name The "print name cell" holds a string that names the symbol for reading and printing. See `symbol-name' in *Note Creating Symbols::. Value The "value cell" holds the current value of the symbol as a variable. When a symbol is used as a form, the value of the form is the contents of the symbol's value cell. See `symbol-value' in *Note Accessing Variables::. Function The "function cell" holds the function definition of the symbol. When a symbol is used as a function, its function definition is used in its place. This cell is also used to make a symbol stand for a keymap or a keyboard macro, for editor command execution. Because each symbol has separate value and function cells, variables and function names do not conflict. See `symbol-function' in *Note Function Cells::. Property list The "property list cell" holds the property list of the symbol. See `symbol-plist' in *Note Symbol Properties::. The print name cell always holds a string, and cannot be changed. The other three cells can be set individually to any specified Lisp object. The print name cell holds the string that is the name of the symbol. Since symbols are represented textually by their names, it is important not to have two symbols with the same name. The Lisp reader ensures this: every time it reads a symbol, it looks for an existing symbol with the specified name before it creates a new one. (In XEmacs Lisp, this lookup uses a hashing algorithm and an obarray; see *Note Creating Symbols::.) In normal usage, the function cell usually contains a function or macro, as that is what the Lisp interpreter expects to see there (*note Evaluation::.). Keyboard macros (*note Keyboard Macros::.), keymaps (*note Keymaps::.) and autoload objects (*note Autoloading::.) are also sometimes stored in the function cell of symbols. We often refer to "the function `foo'" when we really mean the function stored in the function cell of the symbol `foo'. We make the distinction only when necessary. The property list cell normally should hold a correctly formatted property list (*note Property Lists::.), as a number of functions expect to see a property list there. The function cell or the value cell may be "void", which means that the cell does not reference any object. (This is not the same thing as holding the symbol `void', nor the same as holding the symbol `nil'.) Examining a cell that is void results in an error, such as `Symbol's value as variable is void'. The four functions `symbol-name', `symbol-value', `symbol-plist', and `symbol-function' return the contents of the four cells of a symbol. Here as an example we show the contents of the four cells of the symbol `buffer-file-name': (symbol-name 'buffer-file-name) => "buffer-file-name" (symbol-value 'buffer-file-name) => "/gnu/elisp/symbols.texi" (symbol-plist 'buffer-file-name) => (variable-documentation 29529) (symbol-function 'buffer-file-name) => #<subr buffer-file-name> Because this symbol is the variable which holds the name of the file being visited in the current buffer, the value cell contents we see are the name of the source file of this chapter of the XEmacs Lisp Manual. The property list cell contains the list `(variable-documentation 29529)' which tells the documentation functions where to find the documentation string for the variable `buffer-file-name' in the `DOC' file. (29529 is the offset from the beginning of the `DOC' file to where that documentation string begins.) The function cell contains the function for returning the name of the file. `buffer-file-name' names a primitive function, which has no read syntax and prints in hash notation (*note Primitive Function Type::.). A symbol naming a function written in Lisp would have a lambda expression (or a byte-code object) in this cell. File: lispref.info, Node: Definitions, Next: Creating Symbols, Prev: Symbol Components, Up: Symbols Defining Symbols ================ A "definition" in Lisp is a special form that announces your intention to use a certain symbol in a particular way. In XEmacs Lisp, you can define a symbol as a variable, or define it as a function (or macro), or both independently. A definition construct typically specifies a value or meaning for the symbol for one kind of use, plus documentation for its meaning when used in this way. Thus, when you define a symbol as a variable, you can supply an initial value for the variable, plus documentation for the variable. `defvar' and `defconst' are special forms that define a symbol as a global variable. They are documented in detail in *Note Defining Variables::. `defun' defines a symbol as a function, creating a lambda expression and storing it in the function cell of the symbol. This lambda expression thus becomes the function definition of the symbol. (The term "function definition", meaning the contents of the function cell, is derived from the idea that `defun' gives the symbol its definition as a function.) `defsubst', `define-function' and `defalias' are other ways of defining a function. *Note Functions::. `defmacro' defines a symbol as a macro. It creates a macro object and stores it in the function cell of the symbol. Note that a given symbol can be a macro or a function, but not both at once, because both macro and function definitions are kept in the function cell, and that cell can hold only one Lisp object at any given time. *Note Macros::. In XEmacs Lisp, a definition is not required in order to use a symbol as a variable or function. Thus, you can make a symbol a global variable with `setq', whether you define it first or not. The real purpose of definitions is to guide programmers and programming tools. They inform programmers who read the code that certain symbols are *intended* to be used as variables, or as functions. In addition, utilities such as `etags' and `make-docfile' recognize definitions, and add appropriate information to tag tables and the `DOC' file. *Note Accessing Documentation::. File: lispref.info, Node: Creating Symbols, Next: Symbol Properties, Prev: Definitions, Up: Symbols Creating and Interning Symbols ============================== To understand how symbols are created in XEmacs Lisp, you must know how Lisp reads them. Lisp must ensure that it finds the same symbol every time it reads the same set of characters. Failure to do so would cause complete confusion. When the Lisp reader encounters a symbol, it reads all the characters of the name. Then it "hashes" those characters to find an index in a table called an "obarray". Hashing is an efficient method of looking something up. For example, instead of searching a telephone book cover to cover when looking up Jan Jones, you start with the J's and go from there. That is a simple version of hashing. Each element of the obarray is a "bucket" which holds all the symbols with a given hash code; to look for a given name, it is sufficient to look through all the symbols in the bucket for that name's hash code. If a symbol with the desired name is found, the reader uses that symbol. If the obarray does not contain a symbol with that name, the reader makes a new symbol and adds it to the obarray. Finding or adding a symbol with a certain name is called "interning" it, and the symbol is then called an "interned symbol". Interning ensures that each obarray has just one symbol with any particular name. Other like-named symbols may exist, but not in the same obarray. Thus, the reader gets the same symbols for the same names, as long as you keep reading with the same obarray. No obarray contains all symbols; in fact, some symbols are not in any obarray. They are called "uninterned symbols". An uninterned symbol has the same four cells as other symbols; however, the only way to gain access to it is by finding it in some other object or as the value of a variable. In XEmacs Lisp, an obarray is actually a vector. Each element of the vector is a bucket; its value is either an interned symbol whose name hashes to that bucket, or 0 if the bucket is empty. Each interned symbol has an internal link (invisible to the user) to the next symbol in the bucket. Because these links are invisible, there is no way to find all the symbols in an obarray except using `mapatoms' (below). The order of symbols in a bucket is not significant. In an empty obarray, every element is 0, and you can create an obarray with `(make-vector LENGTH 0)'. *This is the only valid way to create an obarray.* Prime numbers as lengths tend to result in good hashing; lengths one less than a power of two are also good. *Do not try to put symbols in an obarray yourself.* This does not work--only `intern' can enter a symbol in an obarray properly. *Do not try to intern one symbol in two obarrays.* This would garble both obarrays, because a symbol has just one slot to hold the following symbol in the obarray bucket. The results would be unpredictable. It is possible for two different symbols to have the same name in different obarrays; these symbols are not `eq' or `equal'. However, this normally happens only as part of the abbrev mechanism (*note Abbrevs::.). Common Lisp note: In Common Lisp, a single symbol may be interned in several obarrays. Most of the functions below take a name and sometimes an obarray as arguments. A `wrong-type-argument' error is signaled if the name is not a string, or if the obarray is not a vector. - Function: symbol-name SYMBOL This function returns the string that is SYMBOL's name. For example: (symbol-name 'foo) => "foo" Changing the string by substituting characters, etc, does change the name of the symbol, but fails to update the obarray, so don't do it! - Function: make-symbol NAME This function returns a newly-allocated, uninterned symbol whose name is NAME (which must be a string). Its value and function definition are void, and its property list is `nil'. In the example below, the value of `sym' is not `eq' to `foo' because it is a distinct uninterned symbol whose name is also `foo'. (setq sym (make-symbol "foo")) => foo (eq sym 'foo) => nil - Function: intern NAME &optional OBARRAY This function returns the interned symbol whose name is NAME. If there is no such symbol in the obarray OBARRAY, `intern' creates a new one, adds it to the obarray, and returns it. If OBARRAY is omitted, the value of the global variable `obarray' is used. (setq sym (intern "foo")) => foo (eq sym 'foo) => t (setq sym1 (intern "foo" other-obarray)) => foo (eq sym 'foo) => nil - Function: intern-soft NAME &optional OBARRAY This function returns the symbol in OBARRAY whose name is NAME, or `nil' if OBARRAY has no symbol with that name. Therefore, you can use `intern-soft' to test whether a symbol with a given name is already interned. If OBARRAY is omitted, the value of the global variable `obarray' is used. (intern-soft "frazzle") ; No such symbol exists. => nil (make-symbol "frazzle") ; Create an uninterned one. => frazzle (intern-soft "frazzle") ; That one cannot be found. => nil (setq sym (intern "frazzle")) ; Create an interned one. => frazzle (intern-soft "frazzle") ; That one can be found! => frazzle (eq sym 'frazzle) ; And it is the same one. => t - Variable: obarray This variable is the standard obarray for use by `intern' and `read'. - Function: mapatoms FUNCTION &optional OBARRAY This function calls FUNCTION for each symbol in the obarray OBARRAY. It returns `nil'. If OBARRAY is omitted, it defaults to the value of `obarray', the standard obarray for ordinary symbols. (setq count 0) => 0 (defun count-syms (s) (setq count (1+ count))) => count-syms (mapatoms 'count-syms) => nil count => 1871 See `documentation' in *Note Accessing Documentation::, for another example using `mapatoms'. - Function: unintern SYMBOL &optional OBARRAY This function deletes SYMBOL from the obarray OBARRAY. If `symbol' is not actually in the obarray, `unintern' does nothing. If OBARRAY is `nil', the current obarray is used. If you provide a string instead of a symbol as SYMBOL, it stands for a symbol name. Then `unintern' deletes the symbol (if any) in the obarray which has that name. If there is no such symbol, `unintern' does nothing. If `unintern' does delete a symbol, it returns `t'. Otherwise it returns `nil'. File: lispref.info, Node: Symbol Properties, Prev: Creating Symbols, Up: Symbols Symbol Properties ================= A "property list" ("plist" for short) is a list of paired elements stored in the property list cell of a symbol. Each of the pairs associates a property name (usually a symbol) with a property or value. Property lists are generally used to record information about a symbol, such as its documentation as a variable, the name of the file where it was defined, or perhaps even the grammatical class of the symbol (representing a word) in a language-understanding system. Many objects other than symbols can have property lists associated with them, and XEmacs provides a full complement of functions for working with property lists. *Note Property Lists::. The property names and values in a property list can be any Lisp objects, but the names are usually symbols. They are compared using `eq'. Here is an example of a property list, found on the symbol `progn' when the compiler is loaded: (lisp-indent-function 0 byte-compile byte-compile-progn) Here `lisp-indent-function' and `byte-compile' are property names, and the other two elements are the corresponding values. * Menu: * Plists and Alists:: Comparison of the advantages of property lists and association lists. * Symbol Plists:: Functions to access symbols' property lists. * Other Plists:: Accessing property lists stored elsewhere. File: lispref.info, Node: Plists and Alists, Next: Symbol Plists, Up: Symbol Properties Property Lists and Association Lists ------------------------------------ Association lists (*note Association Lists::.) are very similar to property lists. In contrast to association lists, the order of the pairs in the property list is not significant since the property names must be distinct. Property lists are better than association lists for attaching information to various Lisp function names or variables. If all the associations are recorded in one association list, the program will need to search that entire list each time a function or variable is to be operated on. By contrast, if the information is recorded in the property lists of the function names or variables themselves, each search will scan only the length of one property list, which is usually short. This is why the documentation for a variable is recorded in a property named `variable-documentation'. The byte compiler likewise uses properties to record those functions needing special treatment. However, association lists have their own advantages. Depending on your application, it may be faster to add an association to the front of an association list than to update a property. All properties for a symbol are stored in the same property list, so there is a possibility of a conflict between different uses of a property name. (For this reason, it is a good idea to choose property names that are probably unique, such as by including the name of the library in the property name.) An association list may be used like a stack where associations are pushed on the front of the list and later discarded; this is not possible with a property list. File: lispref.info, Node: Symbol Plists, Next: Other Plists, Prev: Plists and Alists, Up: Symbol Properties Property List Functions for Symbols ----------------------------------- - Function: symbol-plist SYMBOL This function returns the property list of SYMBOL. - Function: setplist SYMBOL PLIST This function sets SYMBOL's property list to PLIST. Normally, PLIST should be a well-formed property list, but this is not enforced. (setplist 'foo '(a 1 b (2 3) c nil)) => (a 1 b (2 3) c nil) (symbol-plist 'foo) => (a 1 b (2 3) c nil) For symbols in special obarrays, which are not used for ordinary purposes, it may make sense to use the property list cell in a nonstandard fashion; in fact, the abbrev mechanism does so (*note Abbrevs::.). - Function: get SYMBOL PROPERTY This function finds the value of the property named PROPERTY in SYMBOL's property list. If there is no such property, `nil' is returned. Thus, there is no distinction between a value of `nil' and the absence of the property. The name PROPERTY is compared with the existing property names using `eq', so any object is a legitimate property. See `put' for an example. - Function: put SYMBOL PROPERTY VALUE This function puts VALUE onto SYMBOL's property list under the property name PROPERTY, replacing any previous property value. The `put' function returns VALUE. (put 'fly 'verb 'transitive) =>'transitive (put 'fly 'noun '(a buzzing little bug)) => (a buzzing little bug) (get 'fly 'verb) => transitive (symbol-plist 'fly) => (verb transitive noun (a buzzing little bug)) File: lispref.info, Node: Other Plists, Prev: Symbol Plists, Up: Symbol Properties Property Lists Outside Symbols ------------------------------ These functions are useful for manipulating property lists that are stored in places other than symbols: - Function: getf PLIST PROPERTY &optional DEFAULT This returns the value of the PROPERTY property stored in the property list PLIST. For example, (getf '(foo 4) 'foo) => 4 - Function: putf PLIST PROPERTY VALUE This stores VALUE as the value of the PROPERTY property in the property list PLIST. It may modify PLIST destructively, or it may construct a new list structure without altering the old. The function returns the modified property list, so you can store that back in the place where you got PLIST. For example, (setq my-plist '(bar t foo 4)) => (bar t foo 4) (setq my-plist (putf my-plist 'foo 69)) => (bar t foo 69) (setq my-plist (putf my-plist 'quux '(a))) => (quux (a) bar t foo 5) - Function: plists-eq A B This function returns non-`nil' if property lists A and B are `eq'. This means that the property lists have the same values for all the same properties, where comparison between values is done using `eq'. - Function: plists-equal A B This function returns non-`nil' if property lists A and B are `equal'. Both of the above functions do order-insensitive comparisons. (plists-eq '(a 1 b 2 c nil) '(b 2 a 1)) => t (plists-eq '(foo "hello" bar "goodbye") '(bar "goodbye" foo "hello")) => nil (plists-equal '(foo "hello" bar "goodbye") '(bar "goodbye" foo "hello")) => t File: lispref.info, Node: Evaluation, Next: Control Structures, Prev: Symbols, Up: Top Evaluation ********** The "evaluation" of expressions in XEmacs Lisp is performed by the "Lisp interpreter"--a program that receives a Lisp object as input and computes its "value as an expression". How it does this depends on the data type of the object, according to rules described in this chapter. The interpreter runs automatically to evaluate portions of your program, but can also be called explicitly via the Lisp primitive function `eval'. * Menu: * Intro Eval:: Evaluation in the scheme of things. * Eval:: How to invoke the Lisp interpreter explicitly. * Forms:: How various sorts of objects are evaluated. * Quoting:: Avoiding evaluation (to put constants in the program). File: lispref.info, Node: Intro Eval, Next: Eval, Up: Evaluation Introduction to Evaluation ========================== The Lisp interpreter, or evaluator, is the program that computes the value of an expression that is given to it. When a function written in Lisp is called, the evaluator computes the value of the function by evaluating the expressions in the function body. Thus, running any Lisp program really means running the Lisp interpreter. How the evaluator handles an object depends primarily on the data type of the object. A Lisp object that is intended for evaluation is called an "expression" or a "form". The fact that expressions are data objects and not merely text is one of the fundamental differences between Lisp-like languages and typical programming languages. Any object can be evaluated, but in practice only numbers, symbols, lists and strings are evaluated very often. It is very common to read a Lisp expression and then evaluate the expression, but reading and evaluation are separate activities, and either can be performed alone. Reading per se does not evaluate anything; it converts the printed representation of a Lisp object to the object itself. It is up to the caller of `read' whether this object is a form to be evaluated, or serves some entirely different purpose. *Note Input Functions::. Do not confuse evaluation with command key interpretation. The editor command loop translates keyboard input into a command (an interactively callable function) using the active keymaps, and then uses `call-interactively' to invoke the command. The execution of the command itself involves evaluation if the command is written in Lisp, but that is not a part of command key interpretation itself. *Note Command Loop::. Evaluation is a recursive process. That is, evaluation of a form may call `eval' to evaluate parts of the form. For example, evaluation of a function call first evaluates each argument of the function call, and then evaluates each form in the function body. Consider evaluation of the form `(car x)': the subform `x' must first be evaluated recursively, so that its value can be passed as an argument to the function `car'. Evaluation of a function call ultimately calls the function specified in it. *Note Functions::. The execution of the function may itself work by evaluating the function definition; or the function may be a Lisp primitive implemented in C, or it may be a byte-compiled function (*note Byte Compilation::.). The evaluation of forms takes place in a context called the "environment", which consists of the current values and bindings of all Lisp variables.(1) Whenever the form refers to a variable without creating a new binding for it, the value of the binding in the current environment is used. *Note Variables::. Evaluation of a form may create new environments for recursive evaluation by binding variables (*note Local Variables::.). These environments are temporary and vanish by the time evaluation of the form is complete. The form may also make changes that persist; these changes are called "side effects". An example of a form that produces side effects is `(setq foo 1)'. The details of what evaluation means for each kind of form are described below (*note Forms::.). ---------- Footnotes ---------- (1) This definition of "environment" is specifically not intended to include all the data that can affect the result of a program. File: lispref.info, Node: Eval, Next: Forms, Prev: Intro Eval, Up: Evaluation Eval ==== Most often, forms are evaluated automatically, by virtue of their occurrence in a program being run. On rare occasions, you may need to write code that evaluates a form that is computed at run time, such as after reading a form from text being edited or getting one from a property list. On these occasions, use the `eval' function. *Note:* it is generally cleaner and more flexible to call functions that are stored in data structures, rather than to evaluate expressions stored in data structures. Using functions provides the ability to pass information to them as arguments. The functions and variables described in this section evaluate forms, specify limits to the evaluation process, or record recently returned values. Loading a file also does evaluation (*note Loading::.). - Function: eval FORM This is the basic function for performing evaluation. It evaluates FORM in the current environment and returns the result. How the evaluation proceeds depends on the type of the object (*note Forms::.). Since `eval' is a function, the argument expression that appears in a call to `eval' is evaluated twice: once as preparation before `eval' is called, and again by the `eval' function itself. Here is an example: (setq foo 'bar) => bar (setq bar 'baz) => baz ;; `eval' receives argument `bar', which is the value of `foo' (eval foo) => baz (eval 'foo) => bar The number of currently active calls to `eval' is limited to `max-lisp-eval-depth' (see below). - Command: eval-region START END &optional STREAM This function evaluates the forms in the current buffer in the region defined by the positions START and END. It reads forms from the region and calls `eval' on them until the end of the region is reached, or until an error is signaled and not handled. If STREAM is supplied, `standard-output' is bound to it during the evaluation. You can use the variable `load-read-function' to specify a function for `eval-region' to use instead of `read' for reading expressions. *Note How Programs Do Loading::. `eval-region' always returns `nil'. - Command: eval-buffer BUFFER &optional STREAM This is like `eval-region' except that it operates on the whole contents of BUFFER. - Variable: max-lisp-eval-depth This variable defines the maximum depth allowed in calls to `eval', `apply', and `funcall' before an error is signaled (with error message `"Lisp nesting exceeds max-lisp-eval-depth"'). This counts internal uses of those functions, such as for calling the functions mentioned in Lisp expressions, and recursive evaluation of function call arguments and function body forms. This limit, with the associated error when it is exceeded, is one way that Lisp avoids infinite recursion on an ill-defined function. The default value of this variable is 200. If you set it to a value less than 100, Lisp will reset it to 100 if the given value is reached. `max-specpdl-size' provides another limit on nesting. *Note Local Variables::. - Variable: values The value of this variable is a list of the values returned by all the expressions that were read from buffers (including the minibuffer), evaluated, and printed. The elements are ordered most recent first. (setq x 1) => 1 (list 'A (1+ 2) auto-save-default) => (A 3 t) values => ((A 3 t) 1 ...) This variable is useful for referring back to values of forms recently evaluated. It is generally a bad idea to print the value of `values' itself, since this may be very long. Instead, examine particular elements, like this: ;; Refer to the most recent evaluation result. (nth 0 values) => (A 3 t) ;; That put a new element on, ;; so all elements move back one. (nth 1 values) => (A 3 t) ;; This gets the element that was next-to-most-recent ;; before this example. (nth 3 values) => 1 File: lispref.info, Node: Forms, Next: Quoting, Prev: Eval, Up: Evaluation Kinds of Forms ============== A Lisp object that is intended to be evaluated is called a "form". How XEmacs evaluates a form depends on its data type. XEmacs has three different kinds of form that are evaluated differently: symbols, lists, and "all other types". This section describes all three kinds, starting with "all other types" which are self-evaluating forms. * Menu: * Self-Evaluating Forms:: Forms that evaluate to themselves. * Symbol Forms:: Symbols evaluate as variables. * Classifying Lists:: How to distinguish various sorts of list forms. * Function Indirection:: When a symbol appears as the car of a list, we find the real function via the symbol. * Function Forms:: Forms that call functions. * Macro Forms:: Forms that call macros. * Special Forms:: "Special forms" are idiosyncratic primitives, most of them extremely important. * Autoloading:: Functions set up to load files containing their real definitions. File: lispref.info, Node: Self-Evaluating Forms, Next: Symbol Forms, Up: Forms Self-Evaluating Forms --------------------- A "self-evaluating form" is any form that is not a list or symbol. Self-evaluating forms evaluate to themselves: the result of evaluation is the same object that was evaluated. Thus, the number 25 evaluates to 25, and the string `"foo"' evaluates to the string `"foo"'. Likewise, evaluation of a vector does not cause evaluation of the elements of the vector--it returns the same vector with its contents unchanged. '123 ; An object, shown without evaluation. => 123 123 ; Evaluated as usual--result is the same. => 123 (eval '123) ; Evaluated "by hand"--result is the same. => 123 (eval (eval '123)) ; Evaluating twice changes nothing. => 123 It is common to write numbers, characters, strings, and even vectors in Lisp code, taking advantage of the fact that they self-evaluate. However, it is quite unusual to do this for types that lack a read syntax, because there's no way to write them textually. It is possible to construct Lisp expressions containing these types by means of a Lisp program. Here is an example: ;; Build an expression containing a buffer object. (setq buffer (list 'print (current-buffer))) => (print #<buffer eval.texi>) ;; Evaluate it. (eval buffer) -| #<buffer eval.texi> => #<buffer eval.texi>