* Anonymous Functions:: Lambda expressions are functions with no names.
* Function Cells:: Accessing or setting the function definition
of a symbol.
+* Closures:: Functions that enclose a lexical environment.
* Obsolete Functions:: Declaring functions obsolete.
-* Inline Functions:: Defining functions that the compiler will open code.
+* Inline Functions:: Functions that the compiler will expand inline.
* Declaring Functions:: Telling the compiler that a function is defined.
* Function Safety:: Determining whether a function is safe to call.
* Related Topics:: Cross-references to specific Lisp primitives
@node What Is a Function
@section What Is a Function?
- In a general sense, a function is a rule for carrying on a computation
-given several values called @dfn{arguments}. The result of the
-computation is called the value of the function. The computation can
-also have side effects: lasting changes in the values of variables or
-the contents of data structures.
-
- Here are important terms for functions in Emacs Lisp and for other
-function-like objects.
+@cindex return value
+@cindex value of function
+@cindex argument
+ In a general sense, a function is a rule for carrying out a
+computation given input values called @dfn{arguments}. The result of
+the computation is called the @dfn{value} or @dfn{return value} of the
+function. The computation can also have side effects, such as lasting
+changes in the values of variables or the contents of data structures.
+
+ In most computer languages, every function has a name. But in Lisp,
+a function in the strictest sense has no name: it is an object which
+can @emph{optionally} be associated with a symbol (e.g.@: @code{car})
+that serves as the function name. @xref{Function Names}. When a
+function has been given a name, we usually also refer to that symbol
+as a ``function'' (e.g.@: we refer to ``the function @code{car}'').
+In this manual, the distinction between a function name and the
+function object itself is usually unimportant, but we will take note
+wherever it is relevant.
+
+ Certain function-like objects, called @dfn{special forms} and
+@dfn{macros}, also accept arguments to carry out computations.
+However, as explained below, these are not considered functions in
+Emacs Lisp.
+
+ Here are important terms for functions and function-like objects:
@table @dfn
-@item function
-@cindex function
-In Emacs Lisp, a @dfn{function} is anything that can be applied to
-arguments in a Lisp program. In some cases, we use it more
-specifically to mean a function written in Lisp. Special forms and
-macros are not functions.
+@item lambda expression
+A function (in the strict sense, i.e.@: a function object) which is
+written in Lisp. These are described in the following section.
+@ifnottex
+@xref{Lambda Expressions}.
+@end ifnottex
@item primitive
@cindex primitive
@cindex subr
@cindex built-in function
-A @dfn{primitive} is a function callable from Lisp that is written in C,
-such as @code{car} or @code{append}. These functions are also called
-@dfn{built-in functions}, or @dfn{subrs}. (Special forms are also
-considered primitives.)
-
-Usually the reason we implement a function as a primitive is either
-because it is fundamental, because it provides a low-level interface
-to operating system services, or because it needs to run fast.
-Primitives can be modified or added only by changing the C sources and
-recompiling the editor. See @ref{Writing Emacs Primitives}.
-
-@item lambda expression
-A @dfn{lambda expression} is a function written in Lisp.
-These are described in the following section.
-@ifnottex
-@xref{Lambda Expressions}.
-@end ifnottex
+A function which is callable from Lisp but is actually written in C.
+Primitives are also called @dfn{built-in functions}, or @dfn{subrs}.
+Examples include functions like @code{car} and @code{append}. In
+addition, all special forms (see below) are also considered
+primitives.
+
+Usually, a function is implemented as a primitive because it is a
+fundamental part of Lisp (e.g.@: @code{car}), or because it provides a
+low-level interface to operating system services, or because it needs
+to run fast. Unlike functions defined in Lisp, primitives can be
+modified or added only by changing the C sources and recompiling
+Emacs. See @ref{Writing Emacs Primitives}.
@item special form
-A @dfn{special form} is a primitive that is like a function but does not
-evaluate all of its arguments in the usual way. It may evaluate only
-some of the arguments, or may evaluate them in an unusual order, or
-several times. Many special forms are described in @ref{Control
-Structures}.
+A primitive that is like a function but does not evaluate all of its
+arguments in the usual way. It may evaluate only some of the
+arguments, or may evaluate them in an unusual order, or several times.
+Examples include @code{if}, @code{and}, and @code{while}.
+@xref{Special Forms}.
@item macro
@cindex macro
-A @dfn{macro} is a construct defined in Lisp by the programmer. It
-differs from a function in that it translates a Lisp expression that you
-write into an equivalent expression to be evaluated instead of the
-original expression. Macros enable Lisp programmers to do the sorts of
-things that special forms can do. @xref{Macros}, for how to define and
-use macros.
+A construct defined in Lisp, which differs from a function in that it
+translates a Lisp expression into another expression which is to be
+evaluated instead of the original expression. Macros enable Lisp
+programmers to do the sorts of things that special forms can do.
+@xref{Macros}.
@item command
@cindex command
-A @dfn{command} is an object that @code{command-execute} can invoke; it
-is a possible definition for a key sequence. Some functions are
-commands; a function written in Lisp is a command if it contains an
-interactive declaration (@pxref{Defining Commands}). Such a function
-can be called from Lisp expressions like other functions; in this case,
-the fact that the function is a command makes no difference.
+An object which can be invoked via the @code{command-execute}
+primitive, usually due to the user typing in a key sequence
+@dfn{bound} to that command. @xref{Interactive Call}. A command is
+usually a function; if the function is written in Lisp, it is made
+into a command by an @code{interactive} form in the function
+definition (@pxref{Defining Commands}). Commands that are functions
+can also be called from Lisp expressions, just like other functions.
Keyboard macros (strings and vectors) are commands also, even though
-they are not functions. A symbol is a command if its function
-definition is a command; such symbols can be invoked with @kbd{M-x}.
-The symbol is a function as well if the definition is a function.
-@xref{Interactive Call}.
-
-@item keystroke command
-@cindex keystroke command
-A @dfn{keystroke command} is a command that is bound to a key sequence
-(typically one to three keystrokes). The distinction is made here
-merely to avoid confusion with the meaning of ``command'' in non-Emacs
-editors; for Lisp programs, the distinction is normally unimportant.
+they are not functions. @xref{Keyboard Macros}. We say that a symbol
+is a command if its function cell contains a command (@pxref{Symbol
+Components}); such a @dfn{named command} can be invoked with
+@kbd{M-x}.
+
+@item closure
+A function object that is much like a lambda expression, except that
+it also encloses an ``environment'' of lexical variable bindings.
+@xref{Closures}.
@item byte-code function
-A @dfn{byte-code function} is a function that has been compiled by the
-byte compiler. @xref{Byte-Code Type}.
+A function that has been compiled by the byte compiler.
+@xref{Byte-Code Type}.
@item autoload object
@cindex autoload object
-An @dfn{autoload object} is a place-holder for a real function. If
-the autoload object is called, it will make Emacs load the file
-containing the definition of the real function, and then call the real
-function instead.
+A place-holder for a real function. If the autoload object is called,
+Emacs loads the file containing the definition of the real function,
+and then calls the real function. @xref{Autoload}.
@end table
+ You can use the function @code{functionp} to test if an object is a
+function:
+
@defun functionp object
This function returns @code{t} if @var{object} is any kind of
function, i.e.@: can be passed to @code{funcall}. Note that
-@code{functionp} returns @code{nil} for special forms (@pxref{Special
-Forms}).
+@code{functionp} returns @code{t} for symbols that are function names,
+and returns @code{nil} for special forms.
@end defun
-Unlike @code{functionp}, the next three functions do @emph{not}
-treat a symbol as its function definition.
+@noindent
+Unlike @code{functionp}, the next three functions do @emph{not} treat
+a symbol as its function definition.
@defun subrp object
This function returns @code{t} if @var{object} is a built-in function
@section Lambda Expressions
@cindex lambda expression
- A function written in Lisp is a list that looks like this:
+ A lambda expression is a function object written in Lisp. Here is
+an example:
@example
-(lambda (@var{arg-variables}@dots{})
- @r{[}@var{documentation-string}@r{]}
- @r{[}@var{interactive-declaration}@r{]}
- @var{body-forms}@dots{})
+(lambda (x)
+ "Return the hyperbolic cosine of X."
+ (* 0.5 (+ (exp x) (exp (- x)))))
@end example
@noindent
-Such a list is called a @dfn{lambda expression}. In Emacs Lisp, it
-actually is valid as an expression---it evaluates to itself. In some
-other Lisp dialects, a lambda expression is not a valid expression at
-all. In either case, its main use is not to be evaluated as an
-expression, but to be called as a function.
+In Emacs Lisp, such a list is valid as an expression---it evaluates to
+itself. But its main use is not to be evaluated as an expression, but
+to be called as a function.
+
+ A lambda expression, by itself, has no name; it is an @dfn{anonymous
+function}. Although lambda expressions can be used this way
+(@pxref{Anonymous Functions}), they are more commonly associated with
+symbols to make @dfn{named functions} (@pxref{Function Names}).
+Before going into these details, the following subsections describe
+the components of a lambda expression and what they do.
@menu
* Lambda Components:: The parts of a lambda expression.
@node Lambda Components
@subsection Components of a Lambda Expression
-@ifnottex
-
- A function written in Lisp (a ``lambda expression'') is a list that
-looks like this:
+ A lambda expression is a list that looks like this:
@example
(lambda (@var{arg-variables}@dots{})
[@var{interactive-declaration}]
@var{body-forms}@dots{})
@end example
-@end ifnottex
@cindex lambda list
The first element of a lambda expression is always the symbol
function is the value returned by the last element of the body.
@node Simple Lambda
-@subsection A Simple Lambda-Expression Example
+@subsection A Simple Lambda Expression Example
- Consider for example the following function:
+ Consider the following example:
@example
(lambda (a b c) (+ a b c))
4)} from left to right. Then it applies the lambda expression to the
argument values 1, 6 and 1 to produce the value 8.
- It is not often useful to write a lambda expression as the @sc{car} of
-a form in this way. You can get the same result, of making local
-variables and giving them values, using the special form @code{let}
-(@pxref{Local Variables}). And @code{let} is clearer and easier to use.
-In practice, lambda expressions are either stored as the function
-definitions of symbols, to produce named functions, or passed as
-arguments to other functions (@pxref{Anonymous Functions}).
-
- However, calls to explicit lambda expressions were very useful in the
-old days of Lisp, before the special form @code{let} was invented. At
-that time, they were the only way to bind and initialize local
-variables.
+ As these examples show, you can use a form with a lambda expression
+as its @sc{car} to make local variables and give them values. In the
+old days of Lisp, this technique was the only way to bind and
+initialize local variables. But nowadays, it is clearer to use the
+special form @code{let} for this purpose (@pxref{Local Variables}).
+Lambda expressions are mainly used as anonymous functions for passing
+as arguments to other functions (@pxref{Anonymous Functions}), or
+stored as symbol function definitions to produce named functions
+(@pxref{Function Names}).
@node Argument List
@subsection Other Features of Argument Lists
@subsection Documentation Strings of Functions
@cindex documentation of function
- A lambda expression may optionally have a @dfn{documentation string} just
-after the lambda list. This string does not affect execution of the
-function; it is a kind of comment, but a systematized comment which
-actually appears inside the Lisp world and can be used by the Emacs help
-facilities. @xref{Documentation}, for how the @var{documentation-string} is
-accessed.
+ A lambda expression may optionally have a @dfn{documentation string}
+just after the lambda list. This string does not affect execution of
+the function; it is a kind of comment, but a systematized comment
+which actually appears inside the Lisp world and can be used by the
+Emacs help facilities. @xref{Documentation}, for how the
+documentation string is accessed.
It is a good idea to provide documentation strings for all the
functions in your program, even those that are called only from within
@cindex named function
@cindex function name
- In most computer languages, every function has a name; the idea of a
-function without a name is nonsensical. In Lisp, a function in the
-strictest sense has no name. It is simply a list whose first element is
-@code{lambda}, a byte-code function object, or a primitive subr-object.
-
- However, a symbol can serve as the name of a function. This happens
-when you put the function in the symbol's @dfn{function cell}
-(@pxref{Symbol Components}). Then the symbol itself becomes a valid,
-callable function, equivalent to the list or subr-object that its
-function cell refers to. The contents of the function cell are also
-called the symbol's @dfn{function definition}. The procedure of using a
-symbol's function definition in place of the symbol is called
-@dfn{symbol function indirection}; see @ref{Function Indirection}.
-
- In practice, nearly all functions are given names in this way and
-referred to through their names. For example, the symbol @code{car} works
-as a function and does what it does because the primitive subr-object
-@code{#<subr car>} is stored in its function cell.
+ A symbol can serve as the name of a function. This happens when the
+symbol's @dfn{function cell} (@pxref{Symbol Components}) contains a
+function object (e.g.@: a lambda expression). Then the symbol itself
+becomes a valid, callable function, equivalent to the function object
+in its function cell.
+
+ The contents of the function cell are also called the symbol's
+@dfn{function definition}. The procedure of using a symbol's function
+definition in place of the symbol is called @dfn{symbol function
+indirection}; see @ref{Function Indirection}. If you have not given a
+symbol a function definition, its function cell is said to be
+@dfn{void}, and it cannot be used as a function.
+
+ In practice, nearly all functions have names, and are referred to by
+their names. You can create a named Lisp function by defining a
+lambda expression and putting it in a function cell (@pxref{Function
+Cells}). However, it is more common to use the @code{defun} special
+form, described in the next section.
+@ifnottex
+@xref{Defining Functions}.
+@end ifnottex
We give functions names because it is convenient to refer to them by
-their names in Lisp expressions. For primitive subr-objects such as
-@code{#<subr car>}, names are the only way you can refer to them: there
-is no read syntax for such objects. For functions written in Lisp, the
-name is more convenient to use in a call than an explicit lambda
-expression. Also, a function with a name can refer to itself---it can
-be recursive. Writing the function's name in its own definition is much
-more convenient than making the function definition point to itself
-(something that is not impossible but that has various disadvantages in
-practice).
-
- We often identify functions with the symbols used to name them. For
-example, we often speak of ``the function @code{car},'' not
-distinguishing between the symbol @code{car} and the primitive
-subr-object that is its function definition. For most purposes, the
-distinction is not important.
-
- Even so, keep in mind that a function need not have a unique name. While
-a given function object @emph{usually} appears in the function cell of only
-one symbol, this is just a matter of convenience. It is easy to store
-it in several symbols using @code{fset}; then each of the symbols is
-equally well a name for the same function.
-
- A symbol used as a function name may also be used as a variable; these
-two uses of a symbol are independent and do not conflict. (Some Lisp
-dialects, such as Scheme, do not distinguish between a symbol's value
-and its function definition; a symbol's value as a variable is also its
-function definition.) If you have not given a symbol a function
-definition, you cannot use it as a function; whether the symbol has a
-value as a variable makes no difference to this.
+their names in Lisp expressions. Also, a named Lisp function can
+easily refer to itself---it can be recursive. Furthermore, primitives
+can only be referred to textually by their names, since primitive
+function objects (@pxref{Primitive Function Type}) have no read
+syntax.
+
+ A function need not have a unique name. A given function object
+@emph{usually} appears in the function cell of only one symbol, but
+this is just a convention. It is easy to store it in several symbols
+using @code{fset}; then each of the symbols is a valid name for the
+same function.
+
+ Note that a symbol used as a function name may also be used as a
+variable; these two uses of a symbol are independent and do not
+conflict. (This is not the case in some dialects of Lisp, like
+Scheme.)
@node Defining Functions
@section Defining Functions
is called @dfn{defining a function}, and it is done with the
@code{defun} special form.
-@defspec defun name argument-list body-forms
+@defspec defun name argument-list body-forms...
@code{defun} is the usual way to define new Lisp functions. It
defines the symbol @var{name} as a function that looks like this:
value.
As described previously, @var{argument-list} is a list of argument
-names and may include the keywords @code{&optional} and @code{&rest}
-(@pxref{Lambda Expressions}). Also, the first two of the
-@var{body-forms} may be a documentation string and an interactive
-declaration.
-
-There is no conflict if the same symbol @var{name} is also used as a
-variable, since the symbol's value cell is independent of the function
-cell. @xref{Symbol Components}.
+names and may include the keywords @code{&optional} and @code{&rest}.
+Also, the first two of the @var{body-forms} may be a documentation
+string and an interactive declaration. @xref{Lambda Components}.
Here are some examples:
@group
(defun capitalize-backwards ()
- "Upcase the last letter of a word."
+ "Upcase the last letter of the word at point."
(interactive)
(backward-word 1)
(forward-word 1)
Be careful not to redefine existing functions unintentionally.
@code{defun} redefines even primitive functions such as @code{car}
-without any hesitation or notification. Redefining a function already
-defined is often done deliberately, and there is no way to distinguish
-deliberate redefinition from unintentional redefinition.
+without any hesitation or notification. Emacs does not prevent you
+from doing this, because redefining a function is sometimes done
+deliberately, and there is no way to distinguish deliberate
+redefinition from unintentional redefinition.
@end defspec
@cindex function aliases
definition will have no effect on them.
See also @code{defsubst}, which defines a function like @code{defun}
-and tells the Lisp compiler to open-code it. @xref{Inline Functions}.
+and tells the Lisp compiler to perform inline expansion on it.
+@xref{Inline Functions}.
@node Calling Functions
@section Calling Functions
This function ignores any arguments and returns @code{nil}.
@end defun
- Emacs Lisp functions can also be user-visible @dfn{commands}. A
-command is a function that has an @dfn{interactive} specification.
-You may want to call these functions as if they were called
-interactively. See @ref{Interactive Call} for details on how to do
-that.
+ Some functions are user-visible @dfn{commands}, which can be called
+interactively (usually by a key sequence). It is possible to invoke
+such a command exactly as though it was called interactively, by using
+the @code{call-interactively} function. @xref{Interactive Call}.
@node Mapping Functions
@section Mapping Functions
A @dfn{mapping function} applies a given function (@emph{not} a
special form or macro) to each element of a list or other collection.
-Emacs Lisp has several such functions; @code{mapcar} and
-@code{mapconcat}, which scan a list, are described here.
-@xref{Definition of mapatoms}, for the function @code{mapatoms} which
-maps over the symbols in an obarray. @xref{Definition of maphash},
-for the function @code{maphash} which maps over key/value associations
-in a hash table.
+Emacs Lisp has several such functions; this section describes
+@code{mapcar}, @code{mapc}, and @code{mapconcat}, which map over a
+list. @xref{Definition of mapatoms}, for the function @code{mapatoms}
+which maps over the symbols in an obarray. @xref{Definition of
+maphash}, for the function @code{maphash} which maps over key/value
+associations in a hash table.
These mapping functions do not allow char-tables because a char-table
is a sparse array whose nominal range of indices is very large. To map
@section Anonymous Functions
@cindex anonymous function
- In Lisp, a function is a list that starts with @code{lambda}, a
-byte-code function compiled from such a list, or alternatively a
-primitive subr-object; names are ``extra.'' Although functions are
-usually defined with @code{defun} and given names at the same time, it
-is occasionally more concise to use an explicit lambda expression---an
-anonymous function. Such a list is valid wherever a function name is.
+ Although functions are usually defined with @code{defun} and given
+names at the same time, it is sometimes convenient to use an explicit
+lambda expression---an @dfn{anonymous function}. Anonymous functions
+are valid wherever function names are. They are often assigned as
+variable values, or as arguments to functions; for instance, you might
+pass one as the @var{function} argument to @code{mapcar}, which
+applies that function to each element of a list (@pxref{Mapping
+Functions}). @xref{describe-symbols example}, for a realistic example
+of this.
+
+ When defining a lambda expression that is to be used as an anonymous
+function, you can in principle use any method to construct the list.
+But typically you should use the @code{lambda} macro, or the
+@code{function} special form, or the @code{#'} read syntax:
+
+@defmac lambda args body...
+This macro returns an anonymous function with argument list @var{args}
+and body forms given by @var{body}. In effect, this macro makes
+@code{lambda} forms ``self-quoting'': evaluating a form whose @sc{car}
+is @code{lambda} yields the form itself:
+
+@example
+(lambda (x) (* x x))
+ @result{} (lambda (x) (* x x))
+@end example
+
+The @code{lambda} form has one other effect: it tells the Emacs
+evaluator and byte-compiler that its argument is a function, by using
+@code{function} as a subroutine (see below).
+@end defmac
- Any method of creating such a list makes a valid function. Even this:
+@defspec function function-object
+@cindex function quoting
+This special form returns @var{function-object} without evaluating it.
+In this, it is similar to @code{quote} (@pxref{Quoting}). But unlike
+@code{quote}, it also serves as a note to the Emacs evaluator and
+byte-compiler that @var{function-object} is intended to be used as a
+function. Assuming @var{function-object} is a valid lambda
+expression, this has two effects:
-@smallexample
-@group
-(setq silly (append '(lambda (x)) (list (list '+ (* 3 4) 'x))))
-@result{} (lambda (x) (+ 12 x))
-@end group
-@end smallexample
+@itemize
+@item
+When the code is byte-compiled, @var{function-object} is compiled into
+a byte-code function object (@pxref{Byte Compilation}).
-@noindent
-This computes a list that looks like @code{(lambda (x) (+ 12 x))} and
-makes it the value (@emph{not} the function definition!) of
-@code{silly}.
+@item
+When lexical binding is enabled, @var{function-object} is converted
+into a closure. @xref{Closures}.
+@end itemize
+@end defspec
- Here is how we might call this function:
+@cindex @samp{#'} syntax
+The read syntax @code{#'} is a short-hand for using @code{function}.
+The following forms are all equivalent:
@example
-@group
-(funcall silly 1)
-@result{} 13
-@end group
+(lambda (x) (* x x))
+(function (lambda (x) (* x x)))
+#'(lambda (x) (* x x))
@end example
-@noindent
-It does @emph{not} work to write @code{(silly 1)}, because this
-function is not the @emph{function definition} of @code{silly}. We
-have not given @code{silly} any function definition, just a value as a
-variable.
-
- Most of the time, anonymous functions are constants that appear in
-your program. For instance, you might want to pass one as an argument
-to the function @code{mapcar}, which applies any given function to
-each element of a list (@pxref{Mapping Functions}).
-@xref{describe-symbols example}, for a realistic example of this.
-
In the following example, we define a @code{change-property}
function that takes a function as its third argument, followed by a
@code{double-property} function that makes use of
@end example
@noindent
-In the @code{double-property} function, we did not quote the
-@code{lambda} form. This is permissible, because a @code{lambda} form
-is @dfn{self-quoting}: evaluating the form yields the form itself.
+Note that we do not quote the @code{lambda} form.
-Whether or not you quote a @code{lambda} form makes a difference if
-you compile the code (@pxref{Byte Compilation}). If the @code{lambda}
-form is unquoted, as in the above example, the anonymous function is
-also compiled. Suppose, however, that we quoted the @code{lambda}
-form:
+ If you compile the above code, the anonymous function is also
+compiled. This would not happen if, say, you had constructed the
+anonymous function by quoting it as a list:
@example
@group
@end example
@noindent
-If you compile this, the argument passed to @code{change-property} is
-the precise list shown:
-
-@example
-(lambda (x) (* x 2))
-@end example
-
-@noindent
-The Lisp compiler cannot assume this list is a function, even though
-it looks like one, since it does not know what @code{change-property}
-will do with the list. Perhaps it will check whether the @sc{car} of
-the third element is the symbol @code{*}!
-
-@findex function
-The @code{function} special form explicitly tells the byte-compiler
-that its argument is a function:
-
-@defspec function function-object
-@cindex function quoting
-This special form returns @var{function-object} without evaluating it.
-In this, it is equivalent to @code{quote}. However, it serves as a
-note to the Emacs Lisp compiler that @var{function-object} is intended
-to be used only as a function, and therefore can safely be compiled.
-Contrast this with @code{quote}, in @ref{Quoting}.
-@end defspec
-
-@cindex @samp{#'} syntax
-The read syntax @code{#'} is a short-hand for using @code{function}.
-Generally, it is not necessary to use either @code{#'} or
-@code{function}; just use an unquoted @code{lambda} form instead.
-(Actually, @code{lambda} is a macro defined using @code{function}.)
-The following forms are all equivalent:
-
-@example
-#'(lambda (x) (* x x))
-(function (lambda (x) (* x x)))
-(lambda (x) (* x x))
-@end example
-
- We sometimes write @code{function} instead of @code{quote} when
-quoting the name of a function, but this usage is just a sort of
-comment:
-
-@example
-(function @var{symbol}) @equiv{} (quote @var{symbol}) @equiv{} '@var{symbol}
-@end example
+In that case, the anonymous function is kept as a lambda expression in
+the compiled code. The byte-compiler cannot assume this list is a
+function, even though it looks like one, since it does not know that
+@code{change-property} intends to use it as a function.
@node Function Cells
@section Accessing Function Cell Contents
this is not checked. The argument @var{symbol} is an ordinary evaluated
argument.
-There are three normal uses of this function:
+The primary use of this function is as a subroutine by constructs that
+define or alter functions, like @code{defadvice} (@pxref{Advising
+Functions}). (If @code{defun} were not a primitive, it could be
+written as a Lisp macro using @code{fset}.) You can also use it to
+give a symbol a function definition that is not a list, e.g.@: a
+keyboard macro (@pxref{Keyboard Macros}):
-@itemize @bullet
-@item
-Copying one symbol's function definition to another---in other words,
-making an alternate name for a function. (If you think of this as the
-definition of the new name, you should use @code{defalias} instead of
-@code{fset}; see @ref{Definition of defalias}.)
+@example
+;; @r{Define a named keyboard macro.}
+(fset 'kill-two-lines "\^u2\^k")
+ @result{} "\^u2\^k"
+@end example
-@item
-Giving a symbol a function definition that is not a list and therefore
-cannot be made with @code{defun}. For example, you can use @code{fset}
-to give a symbol @code{s1} a function definition which is another symbol
-@code{s2}; then @code{s1} serves as an alias for whatever definition
-@code{s2} presently has. (Once again use @code{defalias} instead of
-@code{fset} if you think of this as the definition of @code{s1}.)
+It you wish to use @code{fset} to make an alternate name for a
+function, consider using @code{defalias} instead. @xref{Definition of
+defalias}.
+@end defun
-@item
-In constructs for defining or altering functions. If @code{defun}
-were not a primitive, it could be written in Lisp (as a macro) using
-@code{fset}.
-@end itemize
+@node Closures
+@section Closures
-Here are examples of these uses:
+ As explained in @ref{Variable Scoping}, Emacs can optionally enable
+lexical binding of variables. When lexical binding is enabled, any
+named function that you create (e.g.@: with @code{defun}), as well as
+any anonymous function that you create using the @code{lambda} macro
+or the @code{function} special form or the @code{#'} syntax
+(@pxref{Anonymous Functions}), is automatically converted into a
+closure.
-@example
-@group
-;; @r{Save @code{foo}'s definition in @code{old-foo}.}
-(fset 'old-foo (symbol-function 'foo))
-@end group
+ A closure is a function that also carries a record of the lexical
+environment that existed when the function was defined. When it is
+invoked, any lexical variable references within its definition use the
+retained lexical environment. In all other respects, closures behave
+much like ordinary functions; in particular, they can be called in the
+same way as ordinary functions.
-@group
-;; @r{Make the symbol @code{car} the function definition of @code{xfirst}.}
-;; @r{(Most likely, @code{defalias} would be better than @code{fset} here.)}
-(fset 'xfirst 'car)
- @result{} car
-@end group
-@group
-(xfirst '(1 2 3))
- @result{} 1
-@end group
-@group
-(symbol-function 'xfirst)
- @result{} car
-@end group
-@group
-(symbol-function (symbol-function 'xfirst))
- @result{} #<subr car>
-@end group
+ @xref{Lexical Binding}, for an example of using a closure.
-@group
-;; @r{Define a named keyboard macro.}
-(fset 'kill-two-lines "\^u2\^k")
- @result{} "\^u2\^k"
-@end group
+ Currently, an Emacs Lisp closure object is represented by a list
+with the symbol @code{closure} as the first element, a list
+representing the lexical environment as the second element, and the
+argument list and body forms as the remaining elements:
-@group
-;; @r{Here is a function that alters other functions.}
-(defun copy-function-definition (new old)
- "Define NEW with the same function definition as OLD."
- (fset new (symbol-function old)))
-@end group
+@example
+;; @r{lexical binding is enabled.}
+(lambda (x) (* x x))
+ @result{} (closure (t) (x) (* x x))
@end example
-@end defun
- @code{fset} is sometimes used to save the old definition of a
-function before redefining it. That permits the new definition to
-invoke the old definition. But it is unmodular and unclean for a Lisp
-file to redefine a function defined elsewhere. If you want to modify
-a function defined by another package, it is cleaner to use
-@code{defadvice} (@pxref{Advising Functions}).
+@noindent
+However, the fact that the internal structure of a closure is
+``exposed'' to the rest of the Lisp world is considered an internal
+implementation detail. For this reason, we recommend against directly
+examining or altering the structure of closure objects.
@node Obsolete Functions
@section Declaring Functions Obsolete
@section Inline Functions
@cindex inline functions
-@findex defsubst
-You can define an @dfn{inline function} by using @code{defsubst} instead
-of @code{defun}. An inline function works just like an ordinary
-function except for one thing: when you compile a call to the function,
-the function's definition is open-coded into the caller.
+@defmac defsubst name argument-list body-forms...
+Define an inline function. The syntax is exactly the same as
+@code{defun} (@pxref{Defining Functions}).
+@end defmac
+
+ You can define an @dfn{inline function} by using @code{defsubst}
+instead of @code{defun}. An inline function works just like an
+ordinary function except for one thing: when you byte-compile a call
+to the function (@pxref{Byte Compilation}), the function's definition
+is expanded into the caller.
-Making a function inline makes explicit calls run faster. But it also
-has disadvantages. For one thing, it reduces flexibility; if you
-change the definition of the function, calls already inlined still use
-the old definition until you recompile them.
+ Making a function inline often makes its function calls run faster.
+But it also has disadvantages. For one thing, it reduces flexibility;
+if you change the definition of the function, calls already inlined
+still use the old definition until you recompile them.
-Another disadvantage is that making a large function inline can increase
-the size of compiled code both in files and in memory. Since the speed
-advantage of inline functions is greatest for small functions, you
-generally should not make large functions inline.
+ Another disadvantage is that making a large function inline can
+increase the size of compiled code both in files and in memory. Since
+the speed advantage of inline functions is greatest for small
+functions, you generally should not make large functions inline.
-Also, inline functions do not behave well with respect to debugging,
+ Also, inline functions do not behave well with respect to debugging,
tracing, and advising (@pxref{Advising Functions}). Since ease of
debugging and the flexibility of redefining functions are important
features of Emacs, you should not make a function inline, even if it's
small, unless its speed is really crucial, and you've timed the code
to verify that using @code{defun} actually has performance problems.
-It's possible to define a macro to expand into the same code that an
-inline function would execute. (@xref{Macros}.) But the macro would be
-limited to direct use in expressions---a macro cannot be called with
-@code{apply}, @code{mapcar} and so on. Also, it takes some work to
-convert an ordinary function into a macro. To convert it into an inline
-function is very easy; simply replace @code{defun} with @code{defsubst}.
-Since each argument of an inline function is evaluated exactly once, you
-needn't worry about how many times the body uses the arguments, as you
-do for macros. (@xref{Argument Evaluation}.)
+ It's possible to define a macro to expand into the same code that an
+inline function would execute (@pxref{Macros}). But the macro would
+be limited to direct use in expressions---a macro cannot be called
+with @code{apply}, @code{mapcar} and so on. Also, it takes some work
+to convert an ordinary function into a macro. To convert it into an
+inline function is easy; just replace @code{defun} with
+@code{defsubst}. Since each argument of an inline function is
+evaluated exactly once, you needn't worry about how many times the
+body uses the arguments, as you do for macros.
-Inline functions can be used and open-coded later on in the same file,
-following the definition, just like macros.
+ After an inline function is defined, its inline expansion can be
+performed later on in the same file, just like macros.
@node Declaring Functions
@section Telling the Compiler that a Function is Defined
expand the definition file name relative to the directory of the file
that contains the @code{declare-function} call.
- You can also say that a function is defined by C code by specifying a
-file name ending in @samp{.c} or @samp{.m}. @code{check-declare-file}
-looks for these files in the C source code directory. This is useful
-only when you call a function that is defined only on certain systems.
-Most of the primitive functions of Emacs are always defined so they will
-never give you a warning.
+ You can also say that a function is a primitive by specifying a file
+name ending in @samp{.c} or @samp{.m}. This is useful only when you
+call a primitive that is defined only on certain systems. Most
+primitives are always defined, so they will never give you a warning.
Sometimes a file will optionally use functions from an external package.
If you prefix the filename in the @code{declare-function} statement with
A @dfn{variable} is a name used in a program to stand for a value.
In Lisp, each variable is represented by a Lisp symbol
-(@pxref{Symbols}). The symbol's name serves as the variable name, and
-the symbol's value cell holds the variable's value@footnote{Strictly
-speaking, the symbol's value cell always holds the variable's current
-value under the default @dfn{dynamic binding} rules. Under
-@dfn{lexical binding} rules, the value cell holds the variable's
-@dfn{global value}. @xref{Variable Scoping}, for details.}.
-@xref{Symbol Components}.
-
- In Emacs Lisp, the use of a symbol as a variable is independent of
-its use as a function name.
+(@pxref{Symbols}). The variable name is simply the symbol's name, and
+the variable's value is stored in the symbol's value cell@footnote{To
+be precise, under the default @dfn{dynamic binding} rules the value
+cell always holds the variable's current value, but this is not the
+case under @dfn{lexical binding} rules. @xref{Variable Scoping}, for
+details.}. @xref{Symbol Components}. In Emacs Lisp, the use of a
+symbol as a variable is independent of its use as a function name.
As previously noted in this manual, a Lisp program is represented
primarily by Lisp objects, and only secondarily as text. The textual
with new values. Sometimes it is useful to give a variable a
@dfn{local value}---a value that takes effect only within a certain
part of a Lisp program. When a variable has a local value, we say
-that it has a @dfn{local binding}, or that it is a @dfn{local
-variable}.
+that it is @dfn{locally bound} to that value, and that it is a
+@dfn{local variable}.
For example, when a function is called, its argument variables
receive local values, which are the actual arguments supplied to the
rule} determines where in a program a local binding takes effect. The
default scoping rule in Emacs Lisp is called @dfn{dynamic scoping},
which simply states that the current binding at any given point in the
-execution of a program is the most recently-created local binding for
-that variable that still exists. For details about dynamic scoping,
-and an alternative scoping rule called @dfn{lexical scoping},
-@xref{Variable Scoping}.
+execution of a program is the most recently-created binding for that
+variable that still exists. For details about dynamic scoping, and an
+alternative scoping rule called @dfn{lexical scoping}, @xref{Variable
+Scoping}.
The special forms @code{let} and @code{let*} exist to create local
bindings:
@cindex @code{void-variable} error
@cindex void variable
- If you have never given a symbol any value as a global variable, we
-say that that symbol's global value is @dfn{void}. Note that this
-does @emph{not} mean the value is @code{nil}. The symbol @code{nil}
-is a Lisp object and can be the value of a variable, just as any other
-object can be; but it is still a value.
-
- More precisely, a variable is void if its symbol has an unassigned
-value cell (@pxref{Symbol Components}). Under Emacs Lisp's default
-dynamic binding rules, the value cell stores the variable's current
-(local or global) value; if a variable is void, trying to evaluate the
-variable signals a @code{void-variable} error rather than a value.
-But when a variable is lexically bound, it can have a local value
-which is determined by the lexical environment, even if the value cell
-is empty and the variable is technically void. @xref{Variable
-Scoping}.
+ We say that a variable is void if its symbol has an unassigned value
+cell (@pxref{Symbol Components}). Under Emacs Lisp's default dynamic
+binding rules (@pxref{Variable Scoping}), the value cell stores the
+variable's current (local or global) value. Note that an unassigned
+value cell is @emph{not} the same as having @code{nil} in the value
+cell. The symbol @code{nil} is a Lisp object and can be the value of
+a variable, just as any other object can be; but it is still a value.
+If a variable is void, trying to evaluate the variable signals a
+@code{void-variable} error rather than a value.
+
+ Under lexical binding rules, the value cell only holds the
+variable's global value, i.e.@: the value outside of any lexical
+binding contruct. When a variable is lexically bound, the local value
+is determined by the lexical environment; the variable may have a
+local value if its symbol's value cell is unassigned.
@defun makunbound symbol
This function empties out the value cell of @var{symbol}, making the
variable void. It returns @var{symbol}.
-If @var{symbol} has a (dynamic) local binding, @code{makunbound} voids
+If @var{symbol} has a dynamic local binding, @code{makunbound} voids
the current binding, and this voidness lasts only as long as the local
binding is in effect. Afterwards, the previously shadowed local or
global binding is reexposed; then the variable will no longer be void,
function to extract the value.
@defun symbol-value symbol
-This function returns the value of @var{symbol}. This is the value in
-the symbol's value cell, which is where the variable's current
-(dynamic) value is stored. If the variable has no local binding, this
-is simply its global value.
+This function returns the value stored in @var{symbol}'s value cell.
+This is where the variable's current (dynamic) value is stored. If
+the variable has no local binding, this is simply its global value.
+If the variable is void, a @code{void-variable} error is signaled.
If the variable is lexically bound, the value reported by
-@code{symbol-value} is the dynamic value, and not the local lexical
-value (which is determined by the lexical environment rather than the
-symbol's value cell). @xref{Variable Scoping}.
+@code{symbol-value} is not necessarily the same as the variable's
+lexical value, which is determined by the lexical environment rather
+than the symbol's value cell. @xref{Variable Scoping}.
@example
@group
@result{} 5
@end group
@end example
-
-A @code{void-variable} error is signaled if @var{symbol} is void as a
-variable.
@end defun
@node Setting Variables
@example
@group
-(defun getx ()
- x) ; @r{@code{x} is used ``free'' in this function.}
-
(let ((x 1)) ; @r{@code{x} is lexically bound.}
(+ x 3))
@result{} 4
+(defun getx ()
+ x) ; @r{@code{x} is used ``free'' in this function.}
+
(let ((x 1)) ; @r{@code{x} is lexically bound.}
(getx))
@error{} Symbol's value as variable is void: x
within the construct and their local values. When the Lisp evaluator
wants the current value of a variable, it looks first in the lexical
environment; if the variable is not specified in there, it looks in
-the symbol's value cell, where the dynamical value is stored.
+the symbol's value cell, where the dynamic value is stored.
@cindex closures
Lexical bindings have indefinite extent. Even after a binding
construct has finished executing, its lexical environment can be
``kept around'' in Lisp objects called @dfn{closures}. A closure is
-created whenever you evaluate a lambda expression (@pxref{Lambda
-Expressions}) with lexical binding enabled. It is represented by a
-list whose @sc{car} is the symbol @code{closure}. It is a function,
-in the sense that it can be passed as an argument to @code{funcall};
-when called as a function, any lexical variable references within its
-definition will use the retained lexical environment.
+created when you create a named or anonymous function with lexical
+binding enabled. @xref{Closures}, for details.
- Here is an example which illustrates the use of a closure:
+ When a closure is called as a function, any lexical variable
+references within its definition use the retained lexical environment.
+Here is an example:
@example
(defvar my-ticker nil) ; @r{We will use this dynamically bound}
@end group
@end example
- Note that references to @code{foo} in @var{body} access the
+@noindent
+Note that references to @code{foo} in @var{body} access the
buffer-local binding of buffer @samp{b}.
When a file specifies local variable values, these become buffer-local
property value. Lambda expressions should be quoted so that
@code{describe-variable} can display the predicate.
+ When defining a user option using @code{defcustom}, you can set its
+@code{safe-local-variable} property by adding the arguments
+@code{:safe @var{function}} to @code{defcustom} (@pxref{Variable
+Definitions}).
+
@defopt safe-local-variable-values
This variable provides another way to mark some variable values as
safe. It is a list of cons cells @code{(@var{var} . @var{val})},
@end defun
@c @cindex risky local variable Duplicates risky-local-variable
- Some variables are considered @dfn{risky}. A variable whose name
-ends in any of @samp{-command}, @samp{-frame-alist}, @samp{-function},
+ Some variables are considered @dfn{risky}. If a variable is risky,
+it is never entered automatically into
+@code{safe-local-variable-values}; Emacs always queries before setting
+a risky variable, unless the user explicitly allows a value by
+customizing @code{safe-local-variable-values} directly.
+
+ Any variable whose name has a non-@code{nil}
+@code{risky-local-variable} property is considered risky. When you
+define a user option using @code{defcustom}, you can set its
+@code{risky-local-variable} property by adding the arguments
+@code{:risky @var{value}} to @code{defcustom} (@pxref{Variable
+Definitions}). In addition, any variable whose name ends in any of
+@samp{-command}, @samp{-frame-alist}, @samp{-function},
@samp{-functions}, @samp{-hook}, @samp{-hooks}, @samp{-form},
@samp{-forms}, @samp{-map}, @samp{-map-alist}, @samp{-mode-alist},
-@samp{-program}, or @samp{-predicate} is considered risky. The
-variables @samp{font-lock-keywords}, @samp{font-lock-keywords}
-followed by a digit, and @samp{font-lock-syntactic-keywords} are also
-considered risky. Finally, any variable whose name has a
-non-@code{nil} @code{risky-local-variable} property is considered
-risky.
+@samp{-program}, or @samp{-predicate} is automatically considered
+risky. The variables @samp{font-lock-keywords},
+@samp{font-lock-keywords} followed by a digit, and
+@samp{font-lock-syntactic-keywords} are also considered risky.
@defun risky-local-variable-p sym
This function returns non-@code{nil} if @var{sym} is a risky variable,
based on the above criteria.
@end defun
- If a variable is risky, it will not be entered automatically into
-@code{safe-local-variable-values} as described above. Therefore,
-Emacs will always query before setting a risky variable, unless the
-user explicitly allows the setting by customizing
-@code{safe-local-variable-values} directly.
-
@defvar ignored-local-variables
This variable holds a list of variables that should not be given local
values by files. Any value specified for one of these variables is
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