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 Free Software Foundation, Inc. 
@c See the file elisp.texi for copying conditions.
@setfilename ../info/symbols
@node Symbols, Evaluation, Sequences Arrays Vectors, 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 which 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 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}.)

  In normal usage, the function cell usually contains a function or
macro, 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 cell 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 cell which 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} file.  (29529 is the offset from the beginning of the
@file{DOC} file to where that documentation string begins.)  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}.

  @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.)  @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 GNU 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{emacs/etc/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.

@cindex interning
  If a symbol with the desired name is found, then it is used.  If no
such symbol is found, then a new symbol is created and added to the
obarray bucket.  Adding a symbol to an obarray is called @dfn{interning}
it, and the symbol is then called an @dfn{interned symbol}.

@cindex symbol equality
@cindex uninterned symbol
  If a symbol is not in the obarray, then there is no way for Lisp to
find it when its name is read.  Such a symbol is called an
@dfn{uninterned symbol} relative to the obarray.  An uninterned symbol
has all the other characteristics of interned symbols; it has the same
four cells and they work in the usual way.

  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, and 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.
@strong{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 @code{eq} or @code{equal}.
However, this normally happens only as part of the abbrev mechanism
(@pxref{Abbrevs}).

@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

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!
@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 sym 'foo)
     @result{} nil
@end example
@end defun

@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
(intern-soft "frazzle")        ; @r{That one cannot be found.}
     @result{} nil
(setq sym (intern "frazzle"))  ; @r{Create an interned one.}
     @result{} frazzle
(intern-soft "frazzle")        ; @r{That one can be found!}
     @result{} frazzle
@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 call @var{function} for each symbol in the obarray
@var{obarray}.  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

@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 how to compile it, 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.  They are compared 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.

@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 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 @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 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.

@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