@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
-@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999, 2001,
-@c 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Free Software Foundation, Inc.
+@c Copyright (C) 1990-1995, 1998-1999, 2001-2013 Free Software
+@c Foundation, Inc.
@c See the file elisp.texi for copying conditions.
-@setfilename ../../info/symbols
-@node Symbols, Evaluation, Hash Tables, Top
+@node Symbols
@chapter Symbols
@cindex symbol
@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}:
+ 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}
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
+* Symbol Properties:: Each symbol has a property list
for recording miscellaneous information.
@end menu
-@node Symbol Components, Definitions, Symbols, Symbols
+@node Symbol Components
@section Symbol Components
@cindex symbol components
@table @asis
@item Print name
@cindex print name cell
-The @dfn{print name cell} holds a string that names the symbol for
-reading and printing. See @code{symbol-name} in @ref{Creating Symbols}.
+The symbol's name.
@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}.
+The symbol's current value as a variable.
@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 names and function names do
-not conflict. See @code{symbol-function} in @ref{Function Cells}.
+The symbol's function definition. It can also hold a symbol, a
+keymap, or a keyboard macro.
@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}.
+The symbol's property list.
@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}.)
-
- The value cell holds the symbol's value as a variable
-(@pxref{Variables}). That is what you get if you evaluate the symbol as
-a Lisp expression (@pxref{Evaluation}). Any Lisp object is a legitimate
-value. Certain symbols have values that cannot be changed; these
-include @code{nil} and @code{t}, and any symbol whose name starts with
-@samp{:} (those are called @dfn{keywords}). @xref{Constant Variables}.
-
- 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 explicit only when necessary. In normal
-usage, the function cell usually contains a function
-(@pxref{Functions}) or a macro (@pxref{Macros}), 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 cells of symbols.
+@noindent
+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 @code{symbol-name} (@pxref{Creating
+Symbols}).
+
+ 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.
+@xref{Variables}, for details about how values are set and retrieved,
+including complications such as @dfn{local bindings} and @dfn{scoping
+rules}. Most symbols can have any Lisp object as a value, but certain
+special symbols have values that cannot be changed; these include
+@code{nil} and @code{t}, and any symbol whose name starts with
+@samp{:} (those are called @dfn{keywords}). @xref{Constant
+Variables}.
+
+ The function cell holds a symbol's function definition. Often, we
+refer to ``the function @code{foo}'' when we really mean the function
+stored in the function cell of @code{foo}; we make the distinction
+explicit only when necessary. Typically, the function cell is used to
+hold a function (@pxref{Functions}) or a macro (@pxref{Macros}).
+However, it can also be used to hold a symbol (@pxref{Function
+Indirection}), keyboard macro (@pxref{Keyboard Macros}), keymap
+(@pxref{Keymaps}), or autoload object (@pxref{Autoloading}). To get
+the contents of a symbol's function cell, use the function
+@code{symbol-function} (@pxref{Function Cells}).
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.
+property list. To get a symbol's property list, use the function
+@code{symbol-plist}. @xref{Symbol Properties}.
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
symbol @code{nil}.) Examining a function or value cell that 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}:
+ Because each symbol has separate value and function cells, variables
+names and function names do not conflict. For example, the symbol
+@code{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):
@example
-(symbol-name 'buffer-file-name)
- @result{} "buffer-file-name"
-(symbol-value 'buffer-file-name)
+buffer-file-name
@result{} "/gnu/elisp/symbols.texi"
(symbol-function 'buffer-file-name)
@result{} #<subr buffer-file-name>
-(symbol-plist 'buffer-file-name)
- @result{} (variable-documentation 29529)
@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-@var{version}} file. (29529 is the offset from the beginning
-of the @file{DOC-@var{version}} file to where that documentation string
-begins---see @ref{Documentation Basics}.) 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
+@node Definitions
@section Defining Symbols
@cindex definitions of symbols
- 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.
+ A @dfn{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.
@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}. For defining user option variables that can
-be customized, use @code{defcustom} (@pxref{Customization}).
+symbol as a @dfn{global variable}---a variable that can be accessed at
+any point in a Lisp program. @xref{Variables}, for details about
+variables. To define a customizable variable, use the
+@code{defcustom} macro, which also calls @code{defvar} as a subroutine
+(@pxref{Customization}).
+
+ In principle, you can assign a variable value to any symbol with
+@code{setq}, whether 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
+(@pxref{Variable Scoping}).
@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
+(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.) @code{defsubst} and @code{defalias} are two
other ways of defining a function. @xref{Functions}.
that cell can hold only one Lisp object at any given time.
@xref{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 @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{DOC-@var{version}} file. @xref{Accessing Documentation}.
-
-@node Creating Symbols, Property Lists, Definitions, Symbols
+ As previously noted, Emacs Lisp allows the same symbol to be defined
+both as a variable (e.g., with @code{defvar}) and as a function or
+macro (e.g., with @code{defun}). Such definitions do not conflict.
+
+ These definition also act as guides for programming tools. For
+example, the @kbd{C-h f} and @kbd{C-h v} commands create help buffers
+containing links to the relevant variable, function, or macro
+definitions. @xref{Name Help,,, emacs, The GNU Emacs Manual}.
+
+@node Creating Symbols
@section Creating and Interning Symbols
@cindex reading symbols
@cindex CL note---symbol in obarrays
@quotation
-@b{Common Lisp note:} In Common Lisp, a single symbol may be interned in
-several obarrays.
+@b{Common Lisp note:} Unlike Common Lisp, Emacs Lisp does not provide
+for interning a single symbol in several obarrays.
@end quotation
Most of the functions below take a name and sometimes an obarray as
the function returns @var{name} if @var{name} is interned
in the specified obarray, and otherwise @code{nil}.
-@smallexample
+@example
(intern-soft "frazzle") ; @r{No such symbol exists.}
@result{} nil
(make-symbol "frazzle") ; @r{Create an uninterned one.}
(eq sym 'frazzle) ; @r{And it is the same one.}
@result{} t
@end group
-@end smallexample
+@end example
@end defun
@defvar obarray
omitted, it defaults to the value of @code{obarray}, the standard
obarray for ordinary symbols.
-@smallexample
+@example
(setq count 0)
@result{} 0
(defun count-syms (s)
@result{} nil
count
@result{} 1871
-@end smallexample
+@end example
See @code{documentation} in @ref{Accessing Documentation}, for another
example using @code{mapatoms}.
it returns @code{nil}.
@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. Each of the pairs associates a property name (usually a
-symbol) with a property or value.
-
- Every symbol has a cell that stores a property list (@pxref{Symbol
-Components}). This property list is used to record information about
-the symbol, such as its variable documentation and the name of the
-file where it was defined.
-
- Property lists can also be used in other contexts. For instance,
-you can assign property lists to character positions in a string or
-buffer. @xref{Text Properties}.
+@node Symbol Properties
+@section Symbol Properties
+@cindex symbol property
- 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 @code{eq}. Here is an example of a
-property list, found on the symbol @code{progn} when the compiler is
-loaded:
+ A symbol may possess any number of @dfn{symbol properties}, which
+can be used to record miscellaneous information about the symbol. For
+example, when a symbol has a @code{risky-local-variable} property with
+a non-@code{nil} value, that means the variable which the symbol names
+is a risky file-local variable (@pxref{File Local Variables}).
-@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.
+ Each symbol's properties and property values are stored in the
+symbol's property list cell (@pxref{Symbol Components}), in the form
+of a property list (@pxref{Property Lists}).
@menu
-* Plists and Alists:: Comparison of the advantages of property
- lists and association lists.
-* Symbol Plists:: Functions to access symbols' property lists.
-* Other Plists:: Accessing property lists stored elsewhere.
+* Symbol Plists:: Accessing symbol properties.
+* Standard Properties:: Standard meanings of symbol properties.
@end menu
-@node Plists and Alists
-@subsection Property Lists and Association Lists
-@cindex plist vs. alist
-@cindex alist vs. plist
-
-@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 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 @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 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.
-
@node Symbol Plists
-@subsection Property List Functions for Symbols
-
-@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. The return value is @var{plist}.
-
-@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
+@subsection Accessing Symbol Properties
-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
+ The following functions can be used to access symbol properties.
@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
+This function returns the value of the property named @var{property}
+in @var{symbol}'s property list. If there is no such property, it
+returns @code{nil}. 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
the property name @var{property}, replacing any previous property value.
The @code{put} function returns @var{value}.
-@smallexample
+@example
(put 'fly 'verb 'transitive)
@result{}'transitive
(put 'fly 'noun '(a buzzing little bug))
@result{} transitive
(symbol-plist 'fly)
@result{} (verb transitive noun (a buzzing little bug))
-@end smallexample
+@end example
@end defun
-@node Other Plists
-@subsection Property Lists Outside Symbols
-
- These functions are useful for manipulating property lists
-that are stored in places other than symbols:
-
-@defun plist-get plist property
-This returns the value of the @var{property} property stored in the
-property list @var{plist}. It accepts a malformed @var{plist}
-argument. If @var{property} is not found in the @var{plist}, it
-returns @code{nil}. For example,
-
-@example
-(plist-get '(foo 4) 'foo)
- @result{} 4
-(plist-get '(foo 4 bad) 'foo)
- @result{} 4
-(plist-get '(foo 4 bad) 'bad)
- @result{} @code{nil}
-(plist-get '(foo 4 bad) 'bar)
- @result{} nil
-@end example
+@defun symbol-plist symbol
+This function returns the property list of @var{symbol}.
@end defun
-@defun plist-put plist property value
-This stores @var{value} as the value of the @var{property} property in
-the property list @var{plist}. It may modify @var{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 @var{plist}. For example,
+@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. The return value is @var{plist}.
@example
-(setq my-plist '(bar t foo 4))
- @result{} (bar t foo 4)
-(setq my-plist (plist-put my-plist 'foo 69))
- @result{} (bar t foo 69)
-(setq my-plist (plist-put my-plist 'quux '(a)))
- @result{} (bar t foo 69 quux (a))
+(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 example
-@end defun
- You could define @code{put} in terms of @code{plist-put} as follows:
+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}).
+
+You could define @code{put} in terms of @code{setplist} and
+@code{plist-put}, as follows:
@example
(defun put (symbol prop value)
(setplist symbol
(plist-put (symbol-plist symbol) prop value)))
@end example
-
-@defun lax-plist-get plist property
-Like @code{plist-get} except that it compares properties
-using @code{equal} instead of @code{eq}.
-@end defun
-
-@defun lax-plist-put plist property value
-Like @code{plist-put} except that it compares properties
-using @code{equal} instead of @code{eq}.
@end defun
-@defun plist-member plist property
-This returns non-@code{nil} if @var{plist} contains the given
-@var{property}. Unlike @code{plist-get}, this allows you to distinguish
-between a missing property and a property with the value @code{nil}.
-The value is actually the tail of @var{plist} whose @code{car} is
-@var{property}.
+@defun function-get symbol property
+This function is identical to @code{get}, except that if @var{symbol}
+is the name of a function alias, it looks in the property list of the
+symbol naming the actual function. @xref{Defining Functions}.
@end defun
-@ignore
- arch-tag: 8750b7d2-de4c-4923-809a-d35fc39fd8ce
-@end ignore
+@node Standard Properties
+@subsection 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.
+
+@table @code
+@item :advertised-binding
+This property value specifies the preferred key binding, when showing
+documentation, for the named function. @xref{Keys in Documentation}.
+
+@item char-table-extra-slots
+The value, if non-@code{nil}, specifies the number of extra slots in
+the named char-table type. @xref{Char-Tables}.
+
+@item customized-face
+@itemx face-defface-spec
+@itemx saved-face
+@itemx 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 @code{defface} and related functions. @xref{Defining
+Faces}.
+
+@item customized-value
+@itemx saved-value
+@itemx standard-value
+@itemx 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 @code{defcustom} and
+related functions. @xref{Variable Definitions}.
+
+@item disabled
+If the value is non-@code{nil}, the named function is disabled as a
+command. @xref{Disabling Commands}.
+
+@item face-documentation
+The value stores the documentation string of the named face. This is
+set automatically by @code{defface}. @xref{Defining Faces}.
+
+@item history-length
+The value, if non-@code{nil}, specifies the maximum minibuffer history
+length for the named history list variable. @xref{Minibuffer
+History}.
+
+@item interactive-form
+The value is an interactive form for the named function. Normally,
+you should not set this directly; use the @code{interactive} special
+form instead. @xref{Interactive Call}.
+
+@item menu-enable
+The value is an expression for determining whether the named menu item
+should be enabled in menus. @xref{Simple Menu Items}.
+
+@item mode-class
+If the value is @code{special}, the named major mode is ``special''.
+@xref{Major Mode Conventions}.
+
+@item permanent-local
+If the value is non-@code{nil}, the named variable is a buffer-local
+variable whose value should not be reset when changing major modes.
+@xref{Creating Buffer-Local}.
+
+@item permanent-local-hook
+If the value is non-@code{nil}, the named function should not be
+deleted from the local value of a hook variable when changing major
+modes. @xref{Setting Hooks}.
+
+@item pure
+This property is used internally to mark certain named functions for
+byte compiler optimization. Do not set it.
+
+@item risky-local-variable
+If the value is non-@code{nil}, the named variable is considered risky
+as a file-local variable. @xref{File Local Variables}.
+
+@item safe-function
+If the value is non-@code{nil}, the named function is considered
+generally safe for evaluation. @xref{Function Safety}.
+
+@item safe-local-eval-function
+If the value is non-@code{nil}, the named function is safe to call in
+file-local evaluation forms. @xref{File Local Variables}.
+
+@item safe-local-variable
+The value specifies a function for determining safe file-local values
+for the named variable. @xref{File Local Variables}.
+
+@item side-effect-free
+A non-@code{nil} value indicates that the named function is free of
+side-effects, for determining function safety (@pxref{Function
+Safety}) as well as for byte compiler optimizations. Do not set it.
+
+@item variable-documentation
+If non-@code{nil}, this specifies the named vaariable's documentation
+string. This is set automatically by @code{defvar} and related
+functions. @xref{Defining Faces}.
+@end table
HCoop / bpt / emacs.git / blobdiff