Linux Cubed Series 2: Applications/Linux Cubed Series 2 - Applications.iso/editors/emacs/xemacs/xemacs-1.006/xemacs-1/lib/xemacs-19.13/info/lispref.info-6

GNU Info File | 1995-09-01 | 47.4 KB | 1,196 lines

This is Info file ../../info/lispref.info, produced by Makeinfo-1.63
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

   Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995 Free Software
Foundation, Inc.  Copyright (C) 1994, 1995 Sun Microsystems, Inc.
Copyright (C) 1995 Amdahl Corporation.  Copyright (C) 1995 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: Association Lists,  Prev: Sets And Lists,  Up: Lists

Association Lists
=================

   An "association list", or "alist" for short, records a mapping from
keys to values.  It is a list of cons cells called "associations": the
CAR of each cell is the "key", and the CDR is the "associated value".(1)

   Here is an example of an alist.  The key `pine' is associated with
the value `cones'; the key `oak' is associated with `acorns'; and the
key `maple' is associated with `seeds'.

     '((pine . cones)
       (oak . acorns)
       (maple . seeds))

   The associated values in an alist may be any Lisp objects; so may the
keys.  For example, in the following alist, the symbol `a' is
associated with the number `1', and the string `"b"' is associated with
the *list* `(2 3)', which is the CDR of the alist element:

     ((a . 1) ("b" 2 3))

   Sometimes it is better to design an alist to store the associated
value in the CAR of the CDR of the element.  Here is an example:

     '((rose red) (lily white) (buttercup yellow))

Here we regard `red' as the value associated with `rose'.  One
advantage of this method is that you can store other related
information--even a list of other items--in the CDR of the CDR.  One
disadvantage is that you cannot use `rassq' (see below) to find the
element containing a given value.  When neither of these considerations
is important, the choice is a matter of taste, as long as you are
consistent about it for any given alist.

   Note that the same alist shown above could be regarded as having the
associated value in the CDR of the element; the value associated with
`rose' would be the list `(red)'.

   Association lists are often used to record information that you might
otherwise keep on a stack, since new associations may be added easily to
the front of the list.  When searching an association list for an
association with a given key, the first one found is returned, if there
is more than one.

   In Emacs Lisp, it is *not* an error if an element of an association
list is not a cons cell.  The alist search functions simply ignore such
elements.  Many other versions of Lisp signal errors in such cases.

   Note that property lists are similar to association lists in several
respects.  A property list behaves like an association list in which
each key can occur only once.  *Note Property Lists::, for a comparison
of property lists and association lists.

 - Function: assoc KEY ALIST
     This function returns the first association for KEY in ALIST.  It
     compares KEY against the alist elements using `equal' (*note
     Equality Predicates::.).  It returns `nil' if no association in
     ALIST has a CAR `equal' to KEY.  For example:

          (setq trees '((pine . cones) (oak . acorns) (maple . seeds)))
               => ((pine . cones) (oak . acorns) (maple . seeds))
          (assoc 'oak trees)
               => (oak . acorns)
          (cdr (assoc 'oak trees))
               => acorns
          (assoc 'birch trees)
               => nil

     Here is another example, in which the keys and values are not
     symbols:

          (setq needles-per-cluster
                '((2 "Austrian Pine" "Red Pine")
                  (3 "Pitch Pine")
                  (5 "White Pine")))

          (cdr (assoc 3 needles-per-cluster))
               => ("Pitch Pine")
          (cdr (assoc 2 needles-per-cluster))
               => ("Austrian Pine" "Red Pine")

 - Function: rassoc VALUE ALIST
     This function returns the first association with value VALUE in
     ALIST.  It returns `nil' if no association in ALIST has a CDR
     `equal' to VALUE.

     `rassoc' is like `assoc' except that it compares the CDR of each
     ALIST association instead of the CAR.  You can think of this as
     "reverse `assoc'", finding the key for a given value.

 - Function: assq KEY ALIST
     This function is like `assoc' in that it returns the first
     association for KEY in ALIST, but it makes the comparison using
     `eq' instead of `equal'.  `assq' returns `nil' if no association
     in ALIST has a CAR `eq' to KEY.  This function is used more often
     than `assoc', since `eq' is faster than `equal' and most alists
     use symbols as keys.  *Note Equality Predicates::.

          (setq trees '((pine . cones) (oak . acorns) (maple . seeds)))
               => ((pine . cones) (oak . acorns) (maple . seeds))
          (assq 'pine trees)
               => (pine . cones)

     On the other hand, `assq' is not usually useful in alists where the
     keys may not be symbols:

          (setq leaves
                '(("simple leaves" . oak)
                  ("compound leaves" . horsechestnut)))

          (assq "simple leaves" leaves)
               => nil
          (assoc "simple leaves" leaves)
               => ("simple leaves" . oak)

 - Function: rassq VALUE ALIST
     This function returns the first association with value VALUE in
     ALIST.  It returns `nil' if no association in ALIST has a CDR `eq'
     to VALUE.

     `rassq' is like `assq' except that it compares the CDR of each
     ALIST association instead of the CAR.  You can think of this as
     "reverse `assq'", finding the key for a given value.

     For example:

          (setq trees '((pine . cones) (oak . acorns) (maple . seeds)))

          (rassq 'acorns trees)
               => (oak . acorns)
          (rassq 'spores trees)
               => nil

     Note that `rassq' cannot search for a value stored in the CAR of
     the CDR of an element:

          (setq colors '((rose red) (lily white) (buttercup yellow)))

          (rassq 'white colors)
               => nil

     In this case, the CDR of the association `(lily white)' is not the
     symbol `white', but rather the list `(white)'.  This becomes
     clearer if the association is written in dotted pair notation:

          (lily white) == (lily . (white))

 - Function: copy-alist ALIST
     This function returns a two-level deep copy of ALIST: it creates a
     new copy of each association, so that you can alter the
     associations of the new alist without changing the old one.

          (setq needles-per-cluster
                '((2 . ("Austrian Pine" "Red Pine"))
                  (3 . ("Pitch Pine"))
                  (5 . ("White Pine"))))
          =>
          ((2 "Austrian Pine" "Red Pine")
           (3 "Pitch Pine")
           (5 "White Pine"))

          (setq copy (copy-alist needles-per-cluster))
          =>
          ((2 "Austrian Pine" "Red Pine")
           (3 "Pitch Pine")
           (5 "White Pine"))

          (eq needles-per-cluster copy)
               => nil
          (equal needles-per-cluster copy)
               => t
          (eq (car needles-per-cluster) (car copy))
               => nil
          (cdr (car (cdr needles-per-cluster)))
               => ("Pitch Pine")
          (eq (cdr (car (cdr needles-per-cluster)))
              (cdr (car (cdr copy))))
               => t

     This example shows how `copy-alist' makes it possible to change
     the associations of one copy without affecting the other:

          (setcdr (assq 3 copy) '("Martian Vacuum Pine"))
          (cdr (assq 3 needles-per-cluster))
               => ("Pitch Pine")

   ---------- Footnotes ----------

   (1)  This usage of "key" is not related to the term "key sequence";
it means a value used to look up an item in a table.  In this case, the
table is the alist, and the alist associations are the items.


File: lispref.info,  Node: Sequences Arrays Vectors,  Next: Symbols,  Prev: Lists,  Up: Top

Sequences, Arrays, and Vectors
******************************

   Recall that the "sequence" type is the union of three other Lisp
types: lists, vectors, and strings.  In other words, any list is a
sequence, any vector is a sequence, and any string is a sequence.  The
common property that all sequences have is that each is an ordered
collection of elements.

   An "array" is a single primitive object that has a slot for each
elements.  All the elements are accessible in constant time, but the
length of an existing array cannot be changed.  Strings and vectors are
the two types of arrays.

   A list is a sequence of elements, but it is not a single primitive
object; it is made of cons cells, one cell per element.  Finding the
Nth element requires looking through N cons cells, so elements farther
from the beginning of the list take longer to access.  But it is
possible to add elements to the list, or remove elements.

   The following diagram shows the relationship between these types:

               ___________________________________
              |                                   |
              |          Sequence                 |
              |  ______   ______________________  |
              | |      | |                      | |
              | | List | |         Array        | |
              | |      | |  ________   _______  | |
              | |______| | |        | |       | | |
              |          | | Vector | | String| | |
              |          | |________| |_______| | |
              |          |______________________| |
              |___________________________________|

   The elements of vectors and lists may be any Lisp objects.  The
elements of strings are all characters.

* Menu:

* Sequence Functions::    Functions that accept any kind of sequence.
* Arrays::                Characteristics of arrays in Emacs Lisp.
* Array Functions::       Functions specifically for arrays.
* Vectors::               Special characteristics of Emacs Lisp vectors.
* Vector Functions::      Functions specifically for vectors.


File: lispref.info,  Node: Sequence Functions,  Next: Arrays,  Up: Sequences Arrays Vectors

Sequences
=========

   In Emacs Lisp, a "sequence" is either a list, a 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, 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 text properties, the property
     list in the copy is itself a copy, not shared with the original's
     property list.  However, the actual values of the properties are
     shared.  *Note Text Properties::.

     See also `append' in *Note Building Lists::, `concat' in *Note
     Creating Strings::, and `vconcat' in *Note Vectors::, for others
     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)]

 - 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

 - 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 [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 two types of array, both of which are one-dimensional:
"strings" and "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 (i.e., integers between 0 and 255).  Each type of
array has its own read syntax.  *Note String Type::, and *Note Vector
Type::.

   Both 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 occupy one-fourth the space of a vector of the same elements.

   * Strings are printed in a way that shows the contents more clearly
     as characters.

   * Strings can hold text properties.  *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 outside the range that will fit in a string.  *Note Key
Sequence Input::.


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 both strings
and vectors.

 - Function: arrayp OBJECT
     This function returns `t' if OBJECT is an array (i.e., either a
     vector or a string).

          (arrayp [a])
          => t
          (arrayp "asdf")
          => 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)
               => 98           ; `b' is ASCII code 98.

     See also the function `elt', in *Note Sequence Functions::.

 - Function: aset ARRAY INDEX OBJECT
     This function sets the INDEXth element of ARRAY to be OBJECT.  It
     returns OBJECT.

          (setq w [foo bar baz])
               => [foo bar baz]
          (aset w 0 'fu)
               => fu
          w
               => [fu bar baz]

          (setq x "asdfasfd")
               => "asdfasfd"
          (aset x 3 ?Z)
               => 90
          x
               => "asdZasfd"

     If ARRAY is a string and OBJECT is not a character, a
     `wrong-type-argument' error results.

 - Function: fillarray ARRAY OBJECT
     This function fills the array ARRAY with OBJECT, so that each
     element of ARRAY is OBJECT.  It returns ARRAY.

          (setq a [a b c d e f g])
               => [a b c d e f g]
          (fillarray a 0)
               => [0 0 0 0 0 0 0]
          a
               => [0 0 0 0 0 0 0]
          (setq s "When in the course")
               => "When in the course"
          (fillarray s ?-)
               => "------------------"

     If ARRAY is a string and OBJECT is not a character, a
     `wrong-type-argument' error results.

   The general sequence functions `copy-sequence' and `length' are
often useful for objects known to be arrays.  *Note Sequence
Functions::.


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 Emacs Lisp is the "string", whose elements must be
characters.)  Vectors in Emacs serve as syntax tables (vectors of
integers), as obarrays (vectors of symbols), and in keymaps (vectors of
commands).  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 Emacs Lisp, the indices of the elements of a vector start from
zero and count up from there.

   Vectors are printed with square brackets surrounding the elements.
Thus, a vector whose elements are the symbols `a', `b' and `a' is
printed as `[a b a]'.  You can write vectors in the same way in Lisp
input.

   A vector, like a string or a number, is considered a constant for
evaluation: the result of evaluating it is the same vector.  This does
not evaluate or even examine the elements of the vector.  *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,  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::, and
     `append' in *Note Building Lists::.

   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: 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.
* Property Lists::           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 Property Lists::.

   The print name cell always holds a string, and cannot be changed.
The other three cells can be set individually to any specified Lisp
object.

   The print name cell holds the string that is the name of the symbol.
Since symbols are represented textually by their names, it is important
not to have two symbols with the same name.  The Lisp reader ensures
this: every time it reads a symbol, it looks for an existing symbol with
the specified name before it creates a new one.  (In Emacs 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 Emacs Lisp Manual.
The property list cell contains the list `(variable-documentation
29529)' which tells the documentation functions where to find the
documentation string for the variable `buffer-file-name' in the `DOC'
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 Emacs Lisp,
you can define a symbol as a variable, or define it as a function (or
macro), or both independently.

   A definition construct typically specifies a value or meaning for the
symbol for one kind of use, plus documentation for its meaning when used
in this way.  Thus, when you define a symbol as a variable, you can
supply an initial value for the variable, plus documentation for the
variable.

   `defvar' and `defconst' are special forms that define a symbol as a
global variable.  They are documented in detail in *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' and `defalias' are two other ways of
defining a function.  *Note Functions::.

   `defmacro' defines a symbol as a macro.  It creates a macro object
and stores it in the function cell of the symbol.  Note that a given
symbol can be a macro or a function, but not both at once, because both
macro and function definitions are kept in the function cell, and that
cell can hold only one Lisp object at any given time.  *Note Macros::.

   In Emacs Lisp, a definition is not required in order to use a symbol
as a variable or function.  Thus, you can make a symbol a global
variable with `setq', whether you define it first or not.  The real
purpose of definitions is to guide programmers and programming tools.
They inform programmers who read the code that certain symbols are
*intended* to be used as variables, or as functions.  In addition,
utilities such as `etags' and `make-docfile' recognize definitions, and
add appropriate information to tag tables and the
`emacs/etc/DOC-VERSION' file. *Note Accessing Documentation::.


File: lispref.info,  Node: Creating Symbols,  Next: Property Lists,  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 Emacs Lisp, an obarray is actually a vector.  Each element of the
vector is a bucket; its value is either an interned symbol whose name
hashes to that bucket, or 0 if the bucket is empty.  Each interned
symbol has an internal link (invisible to the user) to the next symbol
in the bucket.  Because these links are invisible, there is no way to
find all the symbols in an obarray except using `mapatoms' (below).
The order of symbols in a bucket is not significant.

   In an empty obarray, every element is 0, 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: Property Lists,  Prev: Creating Symbols,  Up: Symbols

Property Lists
==============

   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.

   Character positions in a string or buffer can also have property
lists.  *Note Text Properties::.

   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: Property Lists

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: Property Lists

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: Property Lists

Property Lists Outside Symbols
------------------------------

   These two functions are useful for manipulating property lists that
are stored in places other than symbols:

 - Function: plist-get PLIST PROPERTY
     This returns the value of the PROPERTY property stored in the
     property list PLIST.  For example,

          (plist-get '(foo 4) 'foo)
               => 4

 - Function: plist-put PLIST PROPERTY VALUE
     This stores VALUE as the value of the PROPERTY property in the
     property list PLIST.  It may modify PLIST destructively, or it may
     construct a new list structure without altering the old.  The
     function returns the modified property list, so you can store that
     back in the place where you got PLIST.  For example,

          (setq my-plist '(bar t foo 4))
               => (bar t foo 4)
          (setq my-plist (plist-put my-plist 'foo 69))
               => (bar t foo 69)
          (setq my-plist (plist-put my-plist 'quux '(a)))
               => (quux (a) bar t foo 5)


File: lispref.info,  Node: Evaluation,  Next: Control Structures,  Prev: Symbols,  Up: Top

Evaluation
**********

   The "evaluation" of expressions in Emacs Lisp is performed by the
"Lisp interpreter"--a program that receives a Lisp object as input and
computes its "value as an expression".  How it does this depends on the
data type of the object, according to rules described in this chapter.
The interpreter runs automatically to evaluate portions of your
program, but can also be called explicitly via the Lisp primitive
function `eval'.

* 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.