emacs/lispref/symbols.texi

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999
@c   Free Software Foundation, Inc. 
@c See the file elisp.texi for copying conditions.
@setfilename ../info/symbols
@node Symbols, Evaluation, Hash Tables, Top
@chapter Symbols
@cindex symbol

  A @dfn{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 @ref{Variables}, and
@ref{Functions}.  For the precise read syntax for symbols, see
@ref{Symbol Type}.

  You can test whether an arbitrary Lisp object is a symbol
with @code{symbolp}:

@defun symbolp object
This function returns @code{t} if @var{object} is a symbol, @code{nil}
otherwise.
@end defun

@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.
@end menu

@node Symbol Components, Definitions, Symbols, Symbols
@section Symbol Components
@cindex symbol components

  Each symbol has four components (or ``cells''), each of which
references another object:

@table @asis
@item Print name
@cindex print name cell
The @dfn{print name cell} holds a string that names the symbol for
reading and printing.  See @code{symbol-name} in @ref{Creating Symbols}.

@item Value
@cindex value cell
The @dfn{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 @code{symbol-value} in
@ref{Accessing Variables}.

@item Function
@cindex function cell
The @dfn{function cell} holds the function definition of the symbol.
When a symbol is used as a function, its function definition is used in
its place.  This cell is also used to make a symbol stand for a keymap
or a keyboard macro, for editor command execution.  Because each symbol
has separate value and function cells, variables names and function names do
not conflict.  See @code{symbol-function} in @ref{Function Cells}.

@item Property list
@cindex property list cell
The @dfn{property list cell} holds the property list of the symbol.  See
@code{symbol-plist} in @ref{Property Lists}.
@end table

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

  The print name cell holds the string that is the name of the symbol.
Since symbols are represented textually by their names, it is important
not to have two symbols with the same name.  The Lisp reader ensures
this: every time it reads a symbol, it looks for an existing symbol with
the specified name before it creates a new one.  (In GNU Emacs Lisp,
this lookup uses a hashing algorithm and an obarray; see @ref{Creating
Symbols}.)

  The value cell holds the symbol's value as a variable
(@pxref{Variables}).  That is what you get if you evaluate the symbol as
a Lisp expression (@pxref{Evaluation}).  Any Lisp object is a legitimate
value.  Certain symbols have values that cannot be changed; these
include @code{nil} and @code{t}, and any symbol whose name starts with
@samp{:} (those are called @dfn{keywords}).  @xref{Constant Variables}.

  In normal usage, the function cell usually contains a function
(@pxref{Functions}) or a macro (@pxref{Macros}), as that is what the
Lisp interpreter expects to see there (@pxref{Evaluation}).  Keyboard
macros (@pxref{Keyboard Macros}), keymaps (@pxref{Keymaps}) and autoload
objects (@pxref{Autoloading}) are also sometimes stored in the function
cells of symbols.  We often refer to ``the function @code{foo}'' when we
really mean the function stored in the function cell of the symbol
@code{foo}.  We make the distinction only when necessary.

  The property list cell normally should hold a correctly formatted
property list (@pxref{Property Lists}), as a number of functions expect
to see a property list there.

  The function cell or the value cell may be @dfn{void}, which means
that the cell does not reference any object.  (This is not the same
thing as holding the symbol @code{void}, nor the same as holding the
symbol @code{nil}.)  Examining a function or value cell that is void
results in an error, such as @samp{Symbol's value as variable is void}.

  The four functions @code{symbol-name}, @code{symbol-value},
@code{symbol-plist}, and @code{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 @code{buffer-file-name}:

@example
(symbol-name 'buffer-file-name)
     @result{} "buffer-file-name"
(symbol-value 'buffer-file-name)
     @result{} "/gnu/elisp/symbols.texi"
(symbol-plist 'buffer-file-name)
     @result{} (variable-documentation 29529)
(symbol-function 'buffer-file-name)
     @result{} #<subr buffer-file-name>
@end example

@noindent
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 @code{(variable-documentation
29529)} which tells the documentation functions where to find the
documentation string for the variable @code{buffer-file-name} in the
@file{DOC-@var{version}} file.  (29529 is the offset from the beginning
of the @file{DOC-@var{version}} file to where that documentation string
begins---see @ref{Documentation Basics}.)  The function cell contains
the function for returning the name of the file.
@code{buffer-file-name} names a primitive function, which has no read
syntax and prints in hash notation (@pxref{Primitive Function Type}).  A
symbol naming a function written in Lisp would have a lambda expression
(or a byte-code object) in this cell.

@node Definitions, Creating Symbols, Symbol Components, Symbols
@section Defining Symbols
@cindex definition of a symbol

  A @dfn{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.

  @code{defvar} and @code{defconst} are special forms that define a
symbol as a global variable.  They are documented in detail in
@ref{Defining Variables}.  For defining user option variables that can
be customized, use @code{defcustom} (@pxref{Customization}).

  @code{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 @code{defun} gives the symbol its
definition as a function.)  @code{defsubst} and @code{defalias} are two
other ways of defining a function.  @xref{Functions}.

  @code{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.
@xref{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 @code{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
@emph{intended} to be used as variables, or as functions.  In addition,
utilities such as @file{etags} and @file{make-docfile} recognize
definitions, and add appropriate information to tag tables and the
@file{DOC-@var{version}} file.  @xref{Accessing Documentation}.

@node Creating Symbols, Property Lists, Definitions, Symbols
@section Creating and Interning Symbols
@cindex reading symbols

  To understand how symbols are created in GNU Emacs Lisp, you must know
how Lisp reads them.  Lisp must ensure that it finds the same symbol
every time it reads the same set of characters.  Failure to do so would
cause complete confusion.

@cindex symbol name hashing
@cindex hashing
@cindex obarray
@cindex bucket (in obarray)
  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 @dfn{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 @dfn{bucket} which holds all the symbols with a
given hash code; to look for a given name, it is sufficient to look
through all the symbols in the bucket for that name's hash code.  (The
same idea is used for general Emacs hash tables, but they are a
different data type; see @ref{Hash Tables}.)

@cindex interning
  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 @dfn{interning} it, and the
symbol is then called an @dfn{interned symbol}.

  Interning ensures that each obarray has just one symbol with any
particular name.  Other like-named symbols may exist, but not in the
same obarray.  Thus, the reader gets the same symbols for the same
names, as long as you keep reading with the same obarray.

  Interning usually happens automatically in the reader, but sometimes
other programs need to do it.  For example, after the @kbd{M-x} command
obtains the command name as a string using the minibuffer, it then
interns the string, to get the interned symbol with that name.

@cindex symbol equality
@cindex uninterned symbol
  No obarray contains all symbols; in fact, some symbols are not in any
obarray.  They are called @dfn{uninterned symbols}.  An uninterned
symbol has the same four cells as other symbols; however, the only way
to gain access to it is by finding it in some other object or as the
value of a variable.

  Creating an uninterned symbol is useful in generating Lisp code,
because an uninterned symbol used as a variable in the code you generate
cannot clash with any variables used in other Lisp programs.

  In Emacs Lisp, an obarray is actually a vector.  Each element of the
vector is a bucket; its value is either an interned symbol whose name
hashes to that bucket, or 0 if the bucket is empty.  Each interned
symbol has an internal link (invisible to the user) to the next symbol
in the bucket.  Because these links are invisible, there is no way to
find all the symbols in an obarray except using @code{mapatoms} (below).
The order of symbols in a bucket is not significant.

  In an empty obarray, every element is 0, so you can create an obarray
with @code{(make-vector @var{length} 0)}.  @strong{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.

  @strong{Do not try to put symbols in an obarray yourself.}  This does
not work---only @code{intern} can enter a symbol in an obarray properly.

@cindex CL note---symbol in obarrays
@quotation
@b{Common Lisp note:} In Common Lisp, a single symbol may be interned in
several obarrays.
@end quotation

  Most of the functions below take a name and sometimes an obarray as
arguments.  A @code{wrong-type-argument} error is signaled if the name
is not a string, or if the obarray is not a vector.

@defun symbol-name symbol
This function returns the string that is @var{symbol}'s name.  For example:

@example
@group
(symbol-name 'foo)
     @result{} "foo"
@end group
@end example

@strong{Warning:} Changing the string by substituting characters does
change the name of the symbol, but fails to update the obarray, so don't
do it!
@end defun

@defun make-symbol name
This function returns a newly-allocated, uninterned symbol whose name is
@var{name} (which must be a string).  Its value and function definition
are void, and its property list is @code{nil}.  In the example below,
the value of @code{sym} is not @code{eq} to @code{foo} because it is a
distinct uninterned symbol whose name is also @samp{foo}.

@example
(setq sym (make-symbol "foo"))
     @result{} foo
(eq sym 'foo)
     @result{} nil
@end example
@end defun

@defun intern name &optional obarray
This function returns the interned symbol whose name is @var{name}.  If
there is no such symbol in the obarray @var{obarray}, @code{intern}
creates a new one, adds it to the obarray, and returns it.  If
@var{obarray} is omitted, the value of the global variable
@code{obarray} is used.

@example
(setq sym (intern "foo"))
     @result{} foo
(eq sym 'foo)
     @result{} t

(setq sym1 (intern "foo" other-obarray))
     @result{} foo
(eq sym1 'foo)
     @result{} nil
@end example
@end defun

@cindex CL note---interning existing symbol
@quotation
@b{Common Lisp note:} In Common Lisp, you can intern an existing symbol
in an obarray.  In Emacs Lisp, you cannot do this, because the argument
to @code{intern} must be a string, not a symbol.
@end quotation

@defun intern-soft name &optional obarray
This function returns the symbol in @var{obarray} whose name is
@var{name}, or @code{nil} if @var{obarray} has no symbol with that name.
Therefore, you can use @code{intern-soft} to test whether a symbol with
a given name is already interned.  If @var{obarray} is omitted, the
value of the global variable @code{obarray} is used.

@smallexample
(intern-soft "frazzle")        ; @r{No such symbol exists.}
     @result{} nil
(make-symbol "frazzle")        ; @r{Create an uninterned one.}
     @result{} frazzle
@group
(intern-soft "frazzle")        ; @r{That one cannot be found.}
     @result{} nil
@end group
@group
(setq sym (intern "frazzle"))  ; @r{Create an interned one.}
     @result{} frazzle
@end group
@group
(intern-soft "frazzle")        ; @r{That one can be found!}
     @result{} frazzle
@end group
@group
(eq sym 'frazzle)              ; @r{And it is the same one.}
     @result{} t
@end group
@end smallexample
@end defun

@defvar obarray
This variable is the standard obarray for use by @code{intern} and
@code{read}.
@end defvar

@defun mapatoms function &optional obarray
This function calls @var{function} once with each symbol in the obarray
@var{obarray}.  Then it returns @code{nil}.  If @var{obarray} is
omitted, it defaults to the value of @code{obarray}, the standard
obarray for ordinary symbols.

@smallexample
(setq count 0)
     @result{} 0
(defun count-syms (s)
  (setq count (1+ count)))
     @result{} count-syms
(mapatoms 'count-syms)
     @result{} nil
count
     @result{} 1871
@end smallexample

See @code{documentation} in @ref{Accessing Documentation}, for another
example using @code{mapatoms}.
@end defun

@defun unintern symbol &optional obarray
This function deletes @var{symbol} from the obarray @var{obarray}.  If
@code{symbol} is not actually in the obarray, @code{unintern} does
nothing.  If @var{obarray} is @code{nil}, the current obarray is used.

If you provide a string instead of a symbol as @var{symbol}, it stands
for a symbol name.  Then @code{unintern} deletes the symbol (if any) in
the obarray which has that name.  If there is no such symbol,
@code{unintern} does nothing.

If @code{unintern} does delete a symbol, it returns @code{t}.  Otherwise
it returns @code{nil}.
@end defun

@node Property Lists,, Creating Symbols, Symbols
@section Property Lists
@cindex property list
@cindex plist

  A @dfn{property list} (@dfn{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.
@xref{Text Properties}.

  The property names and values in a property list can be any Lisp
objects, but the names are usually symbols.  Property list functions
compare the property names using @code{eq}.  Here is an example of a
property list, found on the symbol @code{progn} when the compiler is
loaded:

@example
(lisp-indent-function 0 byte-compile byte-compile-progn)
@end example

@noindent
Here @code{lisp-indent-function} and @code{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.
@end menu

@node Plists and Alists
@subsection Property Lists and Association Lists

@cindex property lists vs association lists
  Association lists (@pxref{Association Lists}) are very similar to
property lists.  In contrast to association lists, the order of the
pairs in the property list is not significant since the property names
must be distinct.

  Property lists are better than association lists for attaching
information to various Lisp function names or variables.  If your
program keeps all of its associations in one association list, it will
typically need to search that entire list each time it checks for an
association.  This could be slow.  By contrast, if you keep the same
information in the property lists of the function names or variables
themselves, each search will scan only the length of one property list,
which is usually short.  This is why the documentation for a variable is
recorded in a property named @code{variable-documentation}.  The byte
compiler likewise uses properties to record those functions needing
special treatment.

  However, association lists have their own advantages.  Depending on
your application, it may be faster to add an association to the front of
an association list than to update a property.  All properties for a
symbol are stored in the same property list, so there is a possibility
of a conflict between different uses of a property name.  (For this
reason, it is a good idea to choose property names that are probably
unique, such as by beginning the property name with the program's usual
name-prefix for variables and functions.)  An association list may be
used like a stack where associations are pushed on the front of the list
and later discarded; this is not possible with a property list.

@node Symbol Plists
@subsection Property List Functions for Symbols

@defun symbol-plist symbol
This function returns the property list of @var{symbol}.
@end defun

@defun setplist symbol plist
This function sets @var{symbol}'s property list to @var{plist}.
Normally, @var{plist} should be a well-formed property list, but this is
not enforced.

@smallexample
(setplist 'foo '(a 1 b (2 3) c nil))
     @result{} (a 1 b (2 3) c nil)
(symbol-plist 'foo)
     @result{} (a 1 b (2 3) c nil)
@end smallexample

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
(@pxref{Abbrevs}).
@end defun

@defun get symbol property
This function finds the value of the property named @var{property} in
@var{symbol}'s property list.  If there is no such property, @code{nil}
is returned.  Thus, there is no distinction between a value of
@code{nil} and the absence of the property.

The name @var{property} is compared with the existing property names
using @code{eq}, so any object is a legitimate property.

See @code{put} for an example.
@end defun

@defun put symbol property value
This function puts @var{value} onto @var{symbol}'s property list under
the property name @var{property}, replacing any previous property value.
The @code{put} function returns @var{value}.

@smallexample
(put 'fly 'verb 'transitive)
     @result{}'transitive
(put 'fly 'noun '(a buzzing little bug))
     @result{} (a buzzing little bug)
(get 'fly 'verb)
     @result{} transitive
(symbol-plist 'fly)
     @result{} (verb transitive noun (a buzzing little bug))
@end smallexample
@end defun

@node Other Plists
@subsection Property Lists Outside Symbols

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

@defun plist-get plist property
This returns the value of the @var{property} property
stored in the property list @var{plist}.  For example,

@example
(plist-get '(foo 4) 'foo)
     @result{} 4
@end example
@end defun

@defun plist-put plist property value
This stores @var{value} as the value of the @var{property} property in
the property list @var{plist}.  It may modify @var{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 @var{plist}.  For example,

@example
(setq my-plist '(bar t foo 4))
     @result{} (bar t foo 4)
(setq my-plist (plist-put my-plist 'foo 69))
     @result{} (bar t foo 69)
(setq my-plist (plist-put my-plist 'quux '(a)))
     @result{} (bar t foo 69 quux (a))
@end example
@end defun

  You could define @code{put} in terms of @code{plist-put} as follows:

@example
(defun put (symbol prop value)
  (setplist symbol
            (plist-put (symbol-plist symbol) prop value)))
@end example