[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
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 11. Variables, and 12. Functions. For the precise read syntax for symbols, see 2.3.4 Symbol Type.
You can test whether an arbitrary Lisp object is a symbol
with symbolp
:
t
if object is a symbol, nil
otherwise.
8.1 Symbol Components Symbols have names, values, function definitions and property lists. 8.2 Defining Symbols A definition says how a symbol will be used. 8.3 Creating and Interning Symbols How symbols are kept unique. 8.4 Property Lists Each symbol has a property list for recording miscellaneous information.
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
Each symbol has four components (or "cells"), each of which references another object:
symbol-name
in 8.3 Creating and Interning Symbols.
symbol-value
in
11.7 Accessing Variable Values.
symbol-function
in 12.8 Accessing Function Cell Contents.
symbol-plist
in 8.4 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 GNU Emacs Lisp, this lookup uses a hashing algorithm and an obarray; see 8.3 Creating and Interning Symbols.)
The value cell holds the symbol's value as a variable
(see section 11. Variables). That is what you get if you evaluate the symbol as
a Lisp expression (see section 9. Evaluation). Any Lisp object is a legitimate
value. Certain symbols have values that cannot be changed; these
include nil
and t
, and any symbol whose name starts with
`:' (those are called keywords). See section 11.2 Variables that Never Change.
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 explicit only when necessary. In normal
usage, the function cell usually contains a function
(see section 12. Functions) or a macro (see section 13. Macros), as that is what the
Lisp interpreter expects to see there (see section 9. Evaluation). Keyboard
macros (see section 21.15 Keyboard Macros), keymaps (see section 22. Keymaps) and
autoload objects (see section 9.2.8 Autoloading) are also sometimes stored in
the function cells of symbols.
The property list cell normally should hold a correctly formatted property list (see section 8.4 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 function or value 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-version' file. (29529 is the offset from the beginning
of the `DOC-version' file to where that documentation string
begins--see 24.1 Documentation Basics.) 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 (see section 2.3.15 Primitive Function Type). A
symbol naming a function written in Lisp would have a lambda expression
(or a byte-code object) in this cell.
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
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
11.5 Defining Global Variables. For defining user option variables that can
be customized, use defcustom
(see section 14. Writing Customization Definitions).
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. See section 12. 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.
See section 13. 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
`DOC-version' file. See section 24.2 Access to Documentation Strings.
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
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.
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. (The same idea is used for general Emacs hash tables, but they are a different data type; see 7. Hash Tables.)
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.
Interning usually happens automatically in the reader, but sometimes other programs need to do it. For example, after the 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.
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.
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 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 (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.
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.
(symbol-name 'foo) => "foo" |
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!
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 |
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 sym1 'foo) => nil |
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 intern
must be a string, not a symbol.
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.
The argument name may also be a symbol; in that case,
the function returns name if name is interned
in the specified obarray, and otherwise nil
.
(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 |
intern
and
read
.
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 24.2 Access to Documentation Strings, for another
example using mapatoms
.
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
.
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
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. See section 32.19 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 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.
8.4.1 Property Lists and Association Lists Comparison of the advantages of property lists and association lists. 8.4.2 Property List Functions for Symbols Functions to access symbols' property lists. 8.4.3 Property Lists Outside Symbols Accessing property lists stored elsewhere.
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
Association lists (see section 5.8 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 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.
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
(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 (see section 36. Abbrevs and Abbrev Expansion).
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.
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)) |
[ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
These functions are useful for manipulating property lists that are stored in places other than symbols:
(plist-get '(foo 4) 'foo) => 4 |
(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))) => (bar t foo 69 quux (a)) |
You could define put
in terms of plist-put
as follows:
(defun put (symbol prop value) (setplist symbol (plist-put (symbol-plist symbol) prop value))) |
nil
if plist contains the given
property. Unlike plist-get
, this allows you to distinguish
between a missing property and a property with the value nil
.
The value is actually the tail of plist whose car
is
property.
[ << ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |