8. 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 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:

Function: symbolp object

This function returns t if object is a symbol, nil otherwise.

8.1 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 8.3 Creating and Interning 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 11.7 Accessing Variable Values.

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 names and function names do not conflict. See symbol-function in 12.8 Accessing Function Cell Contents.

Property list

The property list cell holds the property list of the symbol. See 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.

8.2 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 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.

8.3 Creating and Interning 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.

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.

Function: symbol-name symbol

This function returns the string that is symbol's name. For example:

(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!

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 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.

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.

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

Variable: obarray

This variable is the standard obarray for use by intern and read.

Function: mapatoms function &optional obarray

This function calls function once with each symbol in the obarray obarray. Then 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 24.2 Access to Documentation Strings, 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.

8.4 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. 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

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.

8.4.2 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 (see section 36. Abbrevs and Abbrev Expansion).

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))

8.4.3 Property Lists Outside Symbols

These 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))) => (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)))

Function: plist-member plist property

This returns non-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.