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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 Variables, and Functions and Commands. For the precise read syntax for symbols, see Symbol Type.
You can test whether an arbitrary Lisp object is a symbol
with symbolp
:
This function returns t
if object is a symbol, nil
otherwise.
13.1 Symbol Components | Symbols have names, values, function definitions and property lists. | |
13.2 Defining Symbols | A definition says how a symbol will be used. | |
13.3 Creating and Interning Symbols | How symbols are kept unique. | |
13.4 Symbol Properties | Each symbol has a property list for recording miscellaneous information. |
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Each symbol has four components (or “cells”), each of which references another object:
The print name cell holds a string that names the symbol for
reading and printing. See symbol-name
in Creating and Interning Symbols.
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
Accessing Variable Values.
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 Accessing Function Cell Contents.
The property list cell holds the property list of the symbol. See
symbol-plist
in Symbol Properties.
The print name cell always holds a string, and cannot be changed. The other three cells can be set individually to any specified Lisp object.
The print name cell holds the string that is the name of the symbol. Since symbols are represented textually by their names, it is important not to have two symbols with the same name. The Lisp reader ensures this: every time it reads a symbol, it looks for an existing symbol with the specified name before it creates a new one. (In XEmacs Lisp, this lookup uses a hashing algorithm and an obarray; see Creating and Interning Symbols.)
In normal usage, the function cell usually contains a function or
macro, as that is what the Lisp interpreter expects to see there
(see section Evaluation). Keyboard macros (see section Keyboard Macros),
keymaps (see section Keymaps) and autoload objects (see section 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 (see section Property Lists), as a number of functions expect to see a property list there.
The function cell or the value cell may be void, which means
that the cell does not reference any object. (This is not the same
thing as holding the symbol void
, nor the same as holding the
symbol nil
.) Examining a cell that is void results in an error,
such as ‘Symbol's value as variable is void’.
The four functions symbol-name
, symbol-value
,
symbol-plist
, and symbol-function
return the contents of
the four cells of a symbol. Here as an example we show the contents of
the four cells of the symbol buffer-file-name
:
(symbol-name 'buffer-file-name) ⇒ "buffer-file-name" (symbol-value 'buffer-file-name) ⇒ "/gnu/elisp/symbols.texi" (symbol-plist 'buffer-file-name) ⇒ (variable-documentation 29529) (symbol-function 'buffer-file-name) ⇒ #<subr buffer-file-name> |
Because this symbol is the variable which holds the name of the file
being visited in the current buffer, the value cell contents we see are
the name of the source file of this chapter of the XEmacs Lisp Reference
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 (see section Primitive Function Type). A
symbol naming a function written in Lisp would have a lambda expression
(or a byte-code object) in this cell.
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A definition in Lisp is a special operator that announces your intention to use a certain symbol in a particular way. In XEmacs Lisp, you can define a symbol as a variable, or define it as a function (or macro), or both independently.
A definition construct typically specifies a value or meaning for the symbol for one kind of use, plus documentation for its meaning when used in this way. Thus, when you define a symbol as a variable, you can supply an initial value for the variable, plus documentation for the variable.
defvar
and defconst
are special operators that define a
symbol as a global variable. They are documented in detail in
Defining Global Variables.
defun
defines a symbol as a function, creating a lambda
expression and storing it in the function cell of the symbol. This
lambda expression thus becomes the function definition of the symbol.
(The term “function definition”, meaning the contents of the function
cell, is derived from the idea that defun
gives the symbol its
definition as a function.) defsubst
, define-function
and
defalias
are other ways of defining a function.
See section Functions and Commands.
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 Macros.
In XEmacs Lisp, a definition is not required in order to use a symbol
as a variable or function. Thus, you can make a symbol a global
variable with setq
, whether you define it first or not. The real
purpose of definitions is to guide programmers and programming tools.
They inform programmers who read the code that certain symbols are
intended to be used as variables, or as functions. In addition,
utilities such as ‘etags’ and ‘make-docfile’ recognize
definitions, and add appropriate information to tag tables and the
‘DOC’ file. See section Access to Documentation Strings.
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To understand how symbols are created in XEmacs Lisp, you must know how Lisp reads them. Lisp must ensure that it finds the same symbol every time it reads the same set of characters. Failure to do so would cause complete confusion.
When the Lisp reader encounters a symbol, it reads all the characters of the name. Then it “hashes” those characters to find an index in a table called an obarray. Hashing is an efficient method of looking something up. For example, instead of searching a telephone book cover to cover when looking up Jan Jones, you start with the J’s and go from there. That is a simple version of hashing. Each element of the obarray is a bucket which holds all the symbols with a given hash code; to look for a given name, it is sufficient to look through all the symbols in the bucket for that name’s hash code.
If a symbol with the desired name is found, the reader uses that symbol. If the obarray does not contain a symbol with that name, the reader makes a new symbol and adds it to the obarray. Finding or adding a symbol with a certain name is called interning it, and the symbol is then called an interned symbol.
Interning ensures that each obarray has just one symbol with any particular name. Other like-named symbols may exist, but not in the same obarray. Thus, the reader gets the same symbols for the same names, as long as you keep reading with the same obarray.
No obarray contains all symbols; in fact, some symbols are not in any obarray. They are called uninterned symbols. An uninterned symbol has the same four cells as other symbols; however, the only way to gain access to it is by finding it in some other object or as the value of a variable.
In XEmacs Lisp, an obarray is actually a vector. Each element of the
vector is a bucket; its value is either an interned symbol whose name
hashes to that bucket, or 0 if the bucket is empty. Each interned
symbol has an internal link (invisible to the user) to the next symbol
in the bucket. Because these links are invisible, there is no way to
find all the symbols in an obarray except using mapatoms
(below).
The order of symbols in a bucket is not significant.
In an empty obarray, every element is 0, and you can create an obarray
with (make-vector length 0)
. This is the only
valid way to create an obarray. Prime numbers as lengths tend
to result in good hashing; lengths one less than a power of two are also
good.
Do not try to put symbols in an obarray yourself. This does
not work—only intern
can enter a symbol in an obarray properly.
Do not try to intern one symbol in two obarrays. This would
garble both obarrays, because a symbol has just one slot to hold the
following symbol in the obarray bucket. The results would be
unpredictable.
It is possible for two different symbols to have the same name in
different obarrays; these symbols are not eq
or equal
.
However, this normally happens only as part of the abbrev mechanism
(see section Abbrevs And Abbrev Expansion).
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.
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!
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 |
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 |
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
|
This variable is the standard obarray for use by intern
and
read
.
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 Access to Documentation Strings, for another
example using mapatoms
.
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
.
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A property list (plist for short) is a list of paired elements, often 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.
Some objects which are not symbols also have property lists associated with them, and XEmacs provides a full complement of functions for working with property lists. See section Property Lists.
The property names and values in a property list can be any Lisp
objects, but the names are usually symbols. They are compared using
eq
. Here is an example of a property list, found on the symbol
progn
when the compiler is loaded:
(lisp-indent-function 0 byte-compile byte-compile-progn) |
Here lisp-indent-function
and byte-compile
are property
names, and the other two elements are the corresponding values.
13.4.1 Property Lists and Association Lists | Comparison of the advantages of property lists and association lists. | |
13.4.2 Property List Functions for Objects | Functions to access objects’ property lists. | |
13.4.3 Property Lists Not Associated with Objects | Accessing property lists stored elsewhere. |
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Association lists (see section 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.
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Once upon a time, only symbols had property lists. Now, several other object types, including strings, extents, faces and glyphs also have property lists.
This function returns the property list of symbol.
This function returns the property list of object. If
object is a symbol, this is identical to 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
(see section Abbrevs And Abbrev Expansion). But generally, its use is discouraged. Use
put
instead. setplist
can only be used with symbols, not
other object types.
This function finds the value of the property named property in
object’s property list. If there is no such property,
default
(which itself defaults to nil
) is returned.
property is compared with the existing properties using eq
,
so any object is a legitimate property.
See put
for an example.
This function puts value onto object’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 (object-plist 'fly) ⇒ (verb transitive noun (a buzzing little bug)) |
This function removes the entry for property from the property
list of object. It returns t
if the property was
indeed found and removed, or nil
if there was no such property.
(This function was probably omitted from Emacs originally because,
since get
did not allow a default, it was very difficult
to distinguish between a missing property and a property whose value
was nil
; thus, setting a property to nil
was close
enough to remprop
for most purposes.)
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