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 Variables, and Functions.  For the precise read syntax for symbols, see 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.

Symbol Components
=================

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

Print name
     The symbol's name.

Value
     The symbol's current value as a variable.

Function
     The symbol's function definition.  It can also hold a symbol, a keymap, or a keyboard macro.

Property list
     The symbol's property list.

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

The print name cell holds the string that is the name of a 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.  To get a symbol's name, use the function symbol-name (see Creating Symbols).  However, although each symbol has only one unique print name, it is nevertheless possible to refer to that same symbol via different alias names called ``shorthands'' (see Shorthands).

The value cell holds a symbol's value as a variable, which is what you get if the symbol itself is evaluated as a Lisp expression. See Variables, for details about how values are set and retrieved, including complications such as local bindings and scoping rules.  Most symbols can have any Lisp object as a value, but certain special symbols have values that cannot be changed; these include nil and t, and any symbol whose name starts with : (those are called keywords).  See Constant Variables.

The function cell holds a symbol's function definition.  Often, we refer to ``the function foo'' when we really mean the function stored in the function cell of foo; we make the distinction explicit only when necessary.  Typically, the function cell is used to hold a function (see Functions) or a macro (see Macros). However, it can also be used to hold a symbol (see Function Indirection), keyboard macro (see Keyboard Macros), keymap (see Keymaps), or autoload object (see Autoloading).  To get the contents of a symbol's function cell, use the function symbol-function (see Function Cells).

The property list cell normally should hold a correctly formatted property list.  To get a symbol's property list, use the function symbol-plist.  See Symbol Properties.

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.

Because each symbol has separate value and function cells, variables names and function names do not conflict.  For example, the symbol buffer-file-name has a value (the name of the file being visited in the current buffer) as well as a function definition (a primitive function that returns the name of the file):

     buffer-file-name
          => "/gnu/elisp/symbols.texi"
     (symbol-function 'buffer-file-name)
          => #<subr buffer-file-name>

Defining Symbols
================

A definition is a special kind of Lisp expression that announces your intention to use a symbol in a particular way.  It 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---a variable that can be accessed at any point in a Lisp program.  See Variables, for details about variables.  To define a customizable variable, use the defcustom macro, which also calls defvar as a subroutine (see Customization).

In principle, you can assign a variable value to any symbol with setq, whether or not it has first been defined as a variable. However, you ought to write a variable definition for each global variable that you want to use; otherwise, your Lisp program may not act correctly if it is evaluated with lexical scoping enabled (see Variable Scoping).

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 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 Macros.

As previously noted, Emacs Lisp allows the same symbol to be defined both as a variable (e.g., with defvar) and as a function or macro (e.g., with defun).  Such definitions do not conflict.

These definitions also act as guides for programming tools.  For example, the C-h f and C-h v commands create help buffers containing links to the relevant variable, function, or macro definitions.  See Name Help (The GNU Emacs Manual).

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 sequence of characters in the same context.  Failure to do so would cause complete confusion.

When the Lisp reader encounters a name that references a symbol in the source code, it reads all the characters of that name.  Then it looks up that name in a table called an obarray to find the symbol that the programmer meant.  The technique used in this lookup is called ``hashing'', an efficient method of looking something up by converting a sequence of characters to a number, known as a ``hash code''.  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 Hash Tables.)

When looking up names, the Lisp reader also considers ``shorthands''. If the programmer supplied them, this allows the reader to find a symbol even if its name isn't present in its full form in the source code.  Of course, the reader needs to be aware of some pre-established context about such shorthands, much as one needs context to be to able to refer uniquely to Jan Jones by just the name ``Jan'': it's probably fine when amongst the Joneses, or when Jan has been mentioned recently, but very ambiguous in any other situation. See Shorthands.

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 may want 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. As another example, a hypothetical telephone book program could intern the name of each looked up person's name as a symbol, even if the obarray did not contain it, so that it could attach information to that new symbol, such as the last time someone looked it up.

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.  Uninterned symbols are sometimes useful in generating Lisp code, see below.

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: Unlike Common Lisp, Emacs Lisp does not provide for interning the same name in several different ``packages'', thus creating multiple symbols with the same name but different packages. Emacs Lisp provides a different namespacing system called ``shorthands'' (see Shorthands).

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: Never alter the string returned by that function. Doing that might make Emacs dysfunctional, and might even crash Emacs.

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.

 -- 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: gensym &optional prefix
     This function returns a symbol using make-symbol, whose name is made by appending gensym-counter to PREFIX and incrementing that counter, guaranteeing that no two calls to this function will generate a symbol with the same name.  The prefix defaults to "g".

To avoid problems when accidentally interning printed representation of generated code (see Printed Representation), it is recommended to use gensym instead of make-symbol.

 -- 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 Accessing Documentation, for another example using mapatoms.

 -- Function: unintern symbol 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.

Symbol Properties
=================

A symbol may possess any number of symbol properties, which can be used to record miscellaneous information about the symbol.  For example, when a symbol has a risky-local-variable property with a non-nil value, that means the variable which the symbol names is a risky file-local variable (see File Local Variables).

Each symbol's properties and property values are stored in the symbol's property list cell (see Symbol Components), in the form of a property list (see Property Lists).

Accessing Symbol Properties
---------------------------

The following functions can be used to access symbol properties.

 -- Function: get symbol property
     This function returns the value of the property named PROPERTY in SYMBOL's property list.  If there is no such property, it returns nil.  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))

 -- 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.  The return value is PLIST.

          (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 Abbrevs).

     You could define put in terms of setplist and plist-put, as follows:

          (defun put (symbol prop value)
            (setplist symbol
                      (plist-put (symbol-plist symbol) prop value)))

 -- Function: function-get symbol property &optional autoload
     This function is identical to get, except that if SYMBOL is the name of a function alias, it looks in the property list of the symbol naming the actual function.  See Defining Functions.  If the optional argument AUTOLOAD is non-nil, and SYMBOL is auto-loaded, this function will try to autoload it, since autoloading might set PROPERTY of SYMBOL.  If AUTOLOAD is the symbol macro, only try autoloading if SYMBOL is an auto-loaded macro.

 -- Function: function-put function property value
     This function sets PROPERTY of FUNCTION to VALUE. FUNCTION should be a symbol.  This function is preferred to calling put for setting properties of a function, because it will allow us some day to implement remapping of old properties to new ones.

Standard Symbol Properties
--------------------------

Here, we list the symbol properties which are used for special purposes in Emacs.  In the following table, whenever we say ``the named function'', that means the function whose name is the relevant symbol; similarly for ``the named variable'' etc.

:advertised-binding
     This property value specifies the preferred key binding, when showing documentation, for the named function.  See Keys in Documentation.

char-table-extra-slots
     The value, if non-nil, specifies the number of extra slots in the named char-table type.  See Char-Tables.

customized-face
face-defface-spec
saved-face
theme-face
     These properties are used to record a face's standard, saved, customized, and themed face specs.  Do not set them directly; they are managed by defface and related functions.  See Defining Faces.

customized-value
saved-value
standard-value
theme-value
     These properties are used to record a customizable variable's standard value, saved value, customized-but-unsaved value, and themed values. Do not set them directly; they are managed by defcustom and related functions.  See Variable Definitions.

definition-name
     This property is used to find the definition of a symbol in the source code, when it might be hard to find the definition by textual search of the source file.  For example, a define-derived-mode (see Derived Modes) might define a mode-specific function or a variable implicitly; or your Lisp program might generate a run-time call to defun to define a function (see Defining Functions).  In these and similar cases, the definition-name property of the symbol should be another symbol whose definition can be found by textual search and whose code defines the original symbol. In the example with define-derived-mode, the value of this property of the functions and variables it defines should be the mode symbol.  The Emacs Help commands such as C-h f (see Help (The GNU Emacs Manual)) use this property to show the definition of a symbol via a button in the *Help* buffer where the symbol's documentation is shown.

disabled
     If the value is non-nil, the named function is disabled as a command.  See Disabling Commands.

face-documentation
     The value stores the documentation string of the named face.  This is set automatically by defface.  See Defining Faces.

history-length
     The value, if non-nil, specifies the maximum minibuffer history length for the named history list variable.  See Minibuffer History.

interactive-form
     The value is an interactive form for the named function.  Normally, you should not set this directly; use the interactive special form instead.  See Interactive Call.

menu-enable
     The value is an expression for determining whether the named menu item should be enabled in menus.  See Simple Menu Items.

mode-class
     If the value is special, the named major mode is special. See Major Mode Conventions.

permanent-local
     If the value is non-nil, the named variable is a buffer-local variable whose value should not be reset when changing major modes. See Creating Buffer-Local.

permanent-local-hook
     If the value is non-nil, the named function should not be deleted from the local value of a hook variable when changing major modes.  See Setting Hooks.

pure
     If the value is non-nil, the named function is considered to be pure (see What Is a Function).  Calls with constant arguments can be evaluated at compile time.  This may shift run time errors to compile time.  Not to be confused with pure storage (see Pure Storage).

risky-local-variable
     If the value is non-nil, the named variable is considered risky as a file-local variable.  See File Local Variables.

safe-function
     If the value is non-nil, the named function is considered generally safe for evaluation.  See Function Safety.

safe-local-eval-function
     If the value is non-nil, the named function is safe to call in file-local evaluation forms.  See File Local Variables.

safe-local-variable
     The value specifies a function for determining safe file-local values for the named variable.  See File Local Variables.  Since this value is consulted when loading files, the function should be efficient and should ideally not lead to loading any libraries to determine the safeness (e.g., it should not be an autoloaded function).

side-effect-free
     A non-nil value indicates that the named function is free of side effects (see What Is a Function), so the byte compiler may ignore a call whose value is unused.  If the property's value is error-free, the byte compiler may even delete such unused calls.  In addition to byte compiler optimizations, this property is also used for determining function safety (see Function Safety).

undo-inhibit-region
     If non-nil, the named function prevents the undo operation from being restricted to the active region, if undo is invoked immediately after the function.  See Undo.

variable-documentation
     If non-nil, this specifies the named variable's documentation string.  This is set automatically by defvar and related functions.  See Defining Faces.

Shorthands
==========

The symbol shorthands, sometimes known as ``renamed symbols'', are symbolic forms found in Lisp source.  They're just like regular symbolic forms, except that when the Lisp reader encounters them, it produces symbols which have a different and usually longer print name (see Symbol Components).

It is useful to think of shorthands as abbreviating the full names of intended symbols.  Despite this, do not confuse shorthands with the Abbrev system (see Abbrevs).

Shorthands make Emacs Lisp's namespacing etiquette easier to work with.  Since all symbols are stored in a single obarray (see Creating Symbols), programmers commonly prefix each symbol name with the name of the library where it originates.  For example, the functions text-property-search-forward and text-property-search-backward both belong to the text-property-search.el library (see Loading).  By properly prefixing symbol names, one effectively prevents clashes between similarly named symbols which belong to different libraries and thus do different things.  However, this practice commonly originates very long symbols names, which are inconvenient to type and read after a while.  Shorthands solve these issues in a clean way.

 -- Variable: read-symbol-shorthands
     This variable's value is an alist whose elements have the form (SHORTHAND-PREFIX . LONGHAND-PREFIX).  Each element instructs the Lisp reader to read every symbol form which starts with SHORTHAND-PREFIX as if it started with LONGHAND-PREFIX instead.

     This variable may only be set in file-local variables (see Local Variables in Files (The GNU Emacs Manual)).

Here's an example of shorthands usage in a hypothetical string manipulating library some-nice-string-utils.el.

     (defun some-nice-string-utils-split (separator s &optional omit-nulls)
       "A match-data saving variant of `split-string'."
       (save-match-data (split-string s separator omit-nulls)))

     (defun some-nice-string-utils-lines (s)
       "Split string S at newline characters into a list of strings."
       (some-nice-string-utils-split "\\(\r\n\\|[\n\r]\\)" s))

As can be seen, it's quite tedious to read or develop this code since the symbol names to type are so long.  We can use shorthands to alleviate that.

     (defun snu-split (separator s &optional omit-nulls)
       "A match-data saving variation on `split-string'."
       (save-match-data (split-string s separator omit-nulls)))

     (defun snu-lines (s)
       "Split string S into a list of strings on newline characters."
       (snu-split "\\(\r\n\\|[\n\r]\\)" s))

     ;; Local Variables:
     ;; read-symbol-shorthands: (("snu-" . "some-nice-string-utils-"))
     ;; End:

Even though the two excerpts look different, they are quite identical after the Lisp reader processes them.  Both will lead to the very same symbols being interned (see Creating Symbols).  Thus loading or byte-compiling any of the two files has equivalent results.  The shorthands snu-split and snu-lines used in the second version are not interned in the obarray.  This is easily seen by moving point to the location where the shorthands are used and waiting for ElDoc (see Local Variables in Files (The GNU Emacs Manual)) to hint at the true full name of the symbol under point in the echo area.

Since read-symbol-shorthands is a file-local variable, it is possible that multiple libraries depending on some-nice-string-utils-lines.el refer to the same symbols under different shorthands, or not using shorthands at all.  In the next example, the my-tricks.el library refers to the symbol some-nice-string-utils-lines using the sns- prefix instead of snu-.

     (defun t-reverse-lines (s) (string-join (reverse (sns-lines s)) "\n")

     ;; Local Variables:
     ;; read-symbol-shorthands: (("t-" . "my-tricks-")
     ;;                          ("sns-" . "some-nice-string-utils-"))
     ;; End:

Exceptions
----------

There are two exceptions to rules governing Shorthand transformations:

  * Symbol forms comprised entirely of characters in the Emacs Lisp symbol constituent class (see Syntax Class Table) are not transformed. For example, it's possible to use - or /= as shorthand prefixes, but that won't shadow the arithmetic functions of those names.

  * Symbol forms whose names start with #_ are not transformed.

Symbols with Position
=====================

A symbol with position is a symbol, the bare symbol, together with an unsigned integer called the position.  These objects are intended for use by the byte compiler, which records in them the position of each symbol occurrence and uses those positions in warning and error messages.

The printed representation of a symbol with position uses the hash notation outlined in Printed Representation.  It looks like #<symbol foo at 12345>.  It has no read syntax.  You can cause just the bare symbol to be printed by binding the variable print-symbols-bare to non-nil around the print operation.  The byte compiler does this before writing its output to the compiled Lisp file.

For most purposes, when the flag variable symbols-with-pos-enabled is non-nil, symbols with positions behave just as bare symbols do.  For example, (eq #<symbol foo at 12345> foo) has a value t when that variable is set (but nil when it isn't set).  Most of the time in Emacs this variable is nil, but the byte compiler binds it to t when it runs.

Typically, symbols with position are created by the byte compiler calling the reader function read-positioning-symbols (see Input Functions).  One can also be created with the function position-symbol.

 -- Variable: symbols-with-pos-enabled
     When this variable is non-nil, symbols with position behave like the contained bare symbol.  Emacs runs a little more slowly in this case.

 -- Variable: print-symbols-bare
     When bound to non-nil, the Lisp printer prints only the bare symbol of a symbol with position, ignoring the position.

 -- Function: symbol-with-pos-p symbol.
     This function returns t if SYMBOL is a symbol with position, nil otherwise.

 -- Function: bare-symbol symbol
     This function returns the bare symbol contained in SYMBOL, or SYMBOL itself if it is already a bare symbol.  For any other type of object, it signals an error.

 -- Function: symbol-with-pos-pos symbol
     This function returns the position, a number, from a symbol with position.  For any other type of object, it signals an error.

 -- Function: position-symbol sym pos
     Make a new symbol with position.  SYM is either a bare symbol or a symbol with position, and supplies the symbol part of the new object.  POS is either an integer which becomes the number part of the new object, or a symbol with position whose position is used. Emacs signals an error if either argument is invalid.