1994-03-21 22:27:20 +00:00
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@c -*-texinfo-*-
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@c This is part of the GNU Emacs Lisp Reference Manual.
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@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999, 2004
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2003-02-04 14:56:31 +00:00
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@c Free Software Foundation, Inc.
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1994-03-21 22:27:20 +00:00
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@c See the file elisp.texi for copying conditions.
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@setfilename ../info/functions
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@node Functions, Macros, Variables, Top
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@chapter Functions
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A Lisp program is composed mainly of Lisp functions. This chapter
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explains what functions are, how they accept arguments, and how to
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define them.
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@menu
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* What Is a Function:: Lisp functions vs. primitives; terminology.
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* Lambda Expressions:: How functions are expressed as Lisp objects.
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* Function Names:: A symbol can serve as the name of a function.
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* Defining Functions:: Lisp expressions for defining functions.
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* Calling Functions:: How to use an existing function.
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* Mapping Functions:: Applying a function to each element of a list, etc.
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* Anonymous Functions:: Lambda expressions are functions with no names.
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* Function Cells:: Accessing or setting the function definition
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of a symbol.
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2005-05-05 23:12:19 +00:00
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* Obsolete Functions:: Declaring functions obsolete.
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* Inline Functions:: Defining functions that the compiler will open code.
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2003-06-30 10:40:27 +00:00
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* Function Safety:: Determining whether a function is safe to call.
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1994-03-21 22:27:20 +00:00
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* Related Topics:: Cross-references to specific Lisp primitives
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that have a special bearing on how functions work.
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@end menu
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@node What Is a Function
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@section What Is a Function?
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In a general sense, a function is a rule for carrying on a computation
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given several values called @dfn{arguments}. The result of the
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computation is called the value of the function. The computation can
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also have side effects: lasting changes in the values of variables or
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the contents of data structures.
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Here are important terms for functions in Emacs Lisp and for other
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function-like objects.
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@table @dfn
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@item function
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@cindex function
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In Emacs Lisp, a @dfn{function} is anything that can be applied to
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arguments in a Lisp program. In some cases, we use it more
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specifically to mean a function written in Lisp. Special forms and
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macros are not functions.
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@item primitive
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@cindex primitive
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@cindex subr
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@cindex built-in function
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A @dfn{primitive} is a function callable from Lisp that is written in C,
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such as @code{car} or @code{append}. These functions are also called
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@dfn{built-in functions}, or @dfn{subrs}. (Special forms are also
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considered primitives.)
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1998-05-19 03:45:57 +00:00
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Usually the reason we implement a function as a primitive is either
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because it is fundamental, because it provides a low-level interface
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to operating system services, or because it needs to run fast.
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Primitives can be modified or added only by changing the C sources and
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recompiling the editor. See @ref{Writing Emacs Primitives}.
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@item lambda expression
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A @dfn{lambda expression} is a function written in Lisp.
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These are described in the following section.
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@ifnottex
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@xref{Lambda Expressions}.
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@end ifnottex
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1994-03-21 22:27:20 +00:00
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@item special form
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A @dfn{special form} is a primitive that is like a function but does not
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evaluate all of its arguments in the usual way. It may evaluate only
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some of the arguments, or may evaluate them in an unusual order, or
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several times. Many special forms are described in @ref{Control
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Structures}.
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@item macro
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@cindex macro
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A @dfn{macro} is a construct defined in Lisp by the programmer. It
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differs from a function in that it translates a Lisp expression that you
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write into an equivalent expression to be evaluated instead of the
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original expression. Macros enable Lisp programmers to do the sorts of
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things that special forms can do. @xref{Macros}, for how to define and
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use macros.
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@item command
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@cindex command
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A @dfn{command} is an object that @code{command-execute} can invoke; it
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is a possible definition for a key sequence. Some functions are
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commands; a function written in Lisp is a command if it contains an
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interactive declaration (@pxref{Defining Commands}). Such a function
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can be called from Lisp expressions like other functions; in this case,
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the fact that the function is a command makes no difference.
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Keyboard macros (strings and vectors) are commands also, even though
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they are not functions. A symbol is a command if its function
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definition is a command; such symbols can be invoked with @kbd{M-x}.
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The symbol is a function as well if the definition is a function.
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@xref{Command Overview}.
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@item keystroke command
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@cindex keystroke command
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A @dfn{keystroke command} is a command that is bound to a key sequence
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(typically one to three keystrokes). The distinction is made here
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merely to avoid confusion with the meaning of ``command'' in non-Emacs
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editors; for Lisp programs, the distinction is normally unimportant.
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@item byte-code function
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A @dfn{byte-code function} is a function that has been compiled by the
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byte compiler. @xref{Byte-Code Type}.
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@end table
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1998-02-28 01:53:53 +00:00
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@defun functionp object
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This function returns @code{t} if @var{object} is any kind of
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function, or a special form, or, recursively, a symbol whose function
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definition is a function or special form. (This does not include
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macros.)
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1998-02-28 01:53:53 +00:00
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@end defun
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2004-01-03 16:52:43 +00:00
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Unlike @code{functionp}, the next three functions do @emph{not}
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treat a symbol as its function definition.
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1994-03-21 22:27:20 +00:00
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@defun subrp object
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This function returns @code{t} if @var{object} is a built-in function
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(i.e., a Lisp primitive).
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@example
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@group
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(subrp 'message) ; @r{@code{message} is a symbol,}
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@result{} nil ; @r{not a subr object.}
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@end group
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@group
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(subrp (symbol-function 'message))
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@result{} t
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@end group
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@end example
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@end defun
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@defun byte-code-function-p object
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This function returns @code{t} if @var{object} is a byte-code
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function. For example:
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@example
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@group
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(byte-code-function-p (symbol-function 'next-line))
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@result{} t
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@end group
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@end example
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@end defun
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2000-11-29 14:55:23 +00:00
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@defun subr-arity subr
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@tindex subr-arity
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This function provides information about the argument list of a
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primitive, @var{subr}. The returned value is a pair
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@code{(@var{min} . @var{max})}. @var{min} is the minimum number of
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args. @var{max} is the maximum number or the symbol @code{many}, for a
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function with @code{&rest} arguments, or the symbol @code{unevalled} if
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@var{subr} is a special form.
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@end defun
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1994-03-21 22:27:20 +00:00
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@node Lambda Expressions
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@section Lambda Expressions
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@cindex lambda expression
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A function written in Lisp is a list that looks like this:
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@example
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(lambda (@var{arg-variables}@dots{})
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@r{[}@var{documentation-string}@r{]}
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@r{[}@var{interactive-declaration}@r{]}
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@var{body-forms}@dots{})
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@end example
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@noindent
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1995-06-06 19:21:15 +00:00
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Such a list is called a @dfn{lambda expression}. In Emacs Lisp, it
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actually is valid as an expression---it evaluates to itself. In some
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other Lisp dialects, a lambda expression is not a valid expression at
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all. In either case, its main use is not to be evaluated as an
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expression, but to be called as a function.
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1994-03-21 22:27:20 +00:00
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@menu
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* Lambda Components:: The parts of a lambda expression.
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* Simple Lambda:: A simple example.
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* Argument List:: Details and special features of argument lists.
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* Function Documentation:: How to put documentation in a function.
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@end menu
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@node Lambda Components
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@subsection Components of a Lambda Expression
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2000-01-05 15:18:01 +00:00
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@ifnottex
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1994-03-21 22:27:20 +00:00
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A function written in Lisp (a ``lambda expression'') is a list that
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looks like this:
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@example
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(lambda (@var{arg-variables}@dots{})
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[@var{documentation-string}]
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[@var{interactive-declaration}]
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@var{body-forms}@dots{})
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@end example
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2000-01-05 15:18:01 +00:00
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@end ifnottex
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@cindex lambda list
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The first element of a lambda expression is always the symbol
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@code{lambda}. This indicates that the list represents a function. The
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reason functions are defined to start with @code{lambda} is so that
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other lists, intended for other uses, will not accidentally be valid as
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functions.
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1998-02-28 01:53:53 +00:00
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The second element is a list of symbols---the argument variable names.
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This is called the @dfn{lambda list}. When a Lisp function is called,
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the argument values are matched up against the variables in the lambda
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list, which are given local bindings with the values provided.
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@xref{Local Variables}.
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1994-04-29 10:07:31 +00:00
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The documentation string is a Lisp string object placed within the
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function definition to describe the function for the Emacs help
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facilities. @xref{Function Documentation}.
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1994-03-21 22:27:20 +00:00
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The interactive declaration is a list of the form @code{(interactive
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@var{code-string})}. This declares how to provide arguments if the
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function is used interactively. Functions with this declaration are called
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@dfn{commands}; they can be called using @kbd{M-x} or bound to a key.
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Functions not intended to be called in this way should not have interactive
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declarations. @xref{Defining Commands}, for how to write an interactive
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declaration.
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@cindex body of function
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The rest of the elements are the @dfn{body} of the function: the Lisp
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code to do the work of the function (or, as a Lisp programmer would say,
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``a list of Lisp forms to evaluate''). The value returned by the
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function is the value returned by the last element of the body.
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@node Simple Lambda
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@subsection A Simple Lambda-Expression Example
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Consider for example the following function:
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@example
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(lambda (a b c) (+ a b c))
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@end example
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@noindent
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We can call this function by writing it as the @sc{car} of an
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expression, like this:
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@example
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@group
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((lambda (a b c) (+ a b c))
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1 2 3)
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@end group
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@end example
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@noindent
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This call evaluates the body of the lambda expression with the variable
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@code{a} bound to 1, @code{b} bound to 2, and @code{c} bound to 3.
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Evaluation of the body adds these three numbers, producing the result 6;
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therefore, this call to the function returns the value 6.
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Note that the arguments can be the results of other function calls, as in
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this example:
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@example
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@group
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((lambda (a b c) (+ a b c))
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1 (* 2 3) (- 5 4))
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@end group
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@end example
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@noindent
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This evaluates the arguments @code{1}, @code{(* 2 3)}, and @code{(- 5
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1994-04-29 10:07:31 +00:00
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4)} from left to right. Then it applies the lambda expression to the
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argument values 1, 6 and 1 to produce the value 8.
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1994-03-21 22:27:20 +00:00
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It is not often useful to write a lambda expression as the @sc{car} of
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a form in this way. You can get the same result, of making local
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variables and giving them values, using the special form @code{let}
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(@pxref{Local Variables}). And @code{let} is clearer and easier to use.
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In practice, lambda expressions are either stored as the function
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definitions of symbols, to produce named functions, or passed as
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arguments to other functions (@pxref{Anonymous Functions}).
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However, calls to explicit lambda expressions were very useful in the
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old days of Lisp, before the special form @code{let} was invented. At
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that time, they were the only way to bind and initialize local
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variables.
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@node Argument List
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1998-02-28 01:53:53 +00:00
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@subsection Other Features of Argument Lists
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@kindex wrong-number-of-arguments
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@cindex argument binding
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@cindex binding arguments
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Our simple sample function, @code{(lambda (a b c) (+ a b c))},
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specifies three argument variables, so it must be called with three
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arguments: if you try to call it with only two arguments or four
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arguments, you get a @code{wrong-number-of-arguments} error.
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It is often convenient to write a function that allows certain
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arguments to be omitted. For example, the function @code{substring}
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accepts three arguments---a string, the start index and the end
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index---but the third argument defaults to the @var{length} of the
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string if you omit it. It is also convenient for certain functions to
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1994-04-29 10:07:31 +00:00
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accept an indefinite number of arguments, as the functions @code{list}
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1994-03-21 22:27:20 +00:00
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and @code{+} do.
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@cindex optional arguments
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@cindex rest arguments
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@kindex &optional
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@kindex &rest
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To specify optional arguments that may be omitted when a function
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is called, simply include the keyword @code{&optional} before the optional
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arguments. To specify a list of zero or more extra arguments, include the
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keyword @code{&rest} before one final argument.
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Thus, the complete syntax for an argument list is as follows:
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@example
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@group
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(@var{required-vars}@dots{}
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@r{[}&optional @var{optional-vars}@dots{}@r{]}
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@r{[}&rest @var{rest-var}@r{]})
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@end group
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@end example
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@noindent
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The square brackets indicate that the @code{&optional} and @code{&rest}
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clauses, and the variables that follow them, are optional.
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A call to the function requires one actual argument for each of the
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|
@var{required-vars}. There may be actual arguments for zero or more of
|
|
|
|
the @var{optional-vars}, and there cannot be any actual arguments beyond
|
|
|
|
that unless the lambda list uses @code{&rest}. In that case, there may
|
|
|
|
be any number of extra actual arguments.
|
|
|
|
|
|
|
|
If actual arguments for the optional and rest variables are omitted,
|
1994-04-29 10:07:31 +00:00
|
|
|
then they always default to @code{nil}. There is no way for the
|
1994-03-21 22:27:20 +00:00
|
|
|
function to distinguish between an explicit argument of @code{nil} and
|
1994-04-29 10:07:31 +00:00
|
|
|
an omitted argument. However, the body of the function is free to
|
|
|
|
consider @code{nil} an abbreviation for some other meaningful value.
|
|
|
|
This is what @code{substring} does; @code{nil} as the third argument to
|
|
|
|
@code{substring} means to use the length of the string supplied.
|
1994-03-21 22:27:20 +00:00
|
|
|
|
|
|
|
@cindex CL note---default optional arg
|
|
|
|
@quotation
|
|
|
|
@b{Common Lisp note:} Common Lisp allows the function to specify what
|
|
|
|
default value to use when an optional argument is omitted; Emacs Lisp
|
1998-02-28 01:53:53 +00:00
|
|
|
always uses @code{nil}. Emacs Lisp does not support ``supplied-p''
|
|
|
|
variables that tell you whether an argument was explicitly passed.
|
1994-03-21 22:27:20 +00:00
|
|
|
@end quotation
|
|
|
|
|
|
|
|
For example, an argument list that looks like this:
|
|
|
|
|
|
|
|
@example
|
|
|
|
(a b &optional c d &rest e)
|
|
|
|
@end example
|
|
|
|
|
|
|
|
@noindent
|
|
|
|
binds @code{a} and @code{b} to the first two actual arguments, which are
|
|
|
|
required. If one or two more arguments are provided, @code{c} and
|
|
|
|
@code{d} are bound to them respectively; any arguments after the first
|
|
|
|
four are collected into a list and @code{e} is bound to that list. If
|
|
|
|
there are only two arguments, @code{c} is @code{nil}; if two or three
|
|
|
|
arguments, @code{d} is @code{nil}; if four arguments or fewer, @code{e}
|
|
|
|
is @code{nil}.
|
|
|
|
|
|
|
|
There is no way to have required arguments following optional
|
|
|
|
ones---it would not make sense. To see why this must be so, suppose
|
|
|
|
that @code{c} in the example were optional and @code{d} were required.
|
2001-09-06 19:50:20 +00:00
|
|
|
Suppose three actual arguments are given; which variable would the
|
|
|
|
third argument be for? Would it be used for the @var{c}, or for
|
|
|
|
@var{d}? One can argue for both possibilities. Similarly, it makes
|
|
|
|
no sense to have any more arguments (either required or optional)
|
|
|
|
after a @code{&rest} argument.
|
1994-03-21 22:27:20 +00:00
|
|
|
|
|
|
|
Here are some examples of argument lists and proper calls:
|
|
|
|
|
|
|
|
@smallexample
|
|
|
|
((lambda (n) (1+ n)) ; @r{One required:}
|
|
|
|
1) ; @r{requires exactly one argument.}
|
|
|
|
@result{} 2
|
|
|
|
((lambda (n &optional n1) ; @r{One required and one optional:}
|
|
|
|
(if n1 (+ n n1) (1+ n))) ; @r{1 or 2 arguments.}
|
|
|
|
1 2)
|
|
|
|
@result{} 3
|
|
|
|
((lambda (n &rest ns) ; @r{One required and one rest:}
|
|
|
|
(+ n (apply '+ ns))) ; @r{1 or more arguments.}
|
|
|
|
1 2 3 4 5)
|
|
|
|
@result{} 15
|
|
|
|
@end smallexample
|
|
|
|
|
|
|
|
@node Function Documentation
|
|
|
|
@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.
|
|
|
|
|
1995-06-06 19:21:15 +00:00
|
|
|
It is a good idea to provide documentation strings for all the
|
1998-04-20 17:43:57 +00:00
|
|
|
functions in your program, even those that are called only from within
|
1995-06-06 19:21:15 +00:00
|
|
|
your program. Documentation strings are like comments, except that they
|
|
|
|
are easier to access.
|
1994-03-21 22:27:20 +00:00
|
|
|
|
|
|
|
The first line of the documentation string should stand on its own,
|
|
|
|
because @code{apropos} displays just this first line. It should consist
|
|
|
|
of one or two complete sentences that summarize the function's purpose.
|
|
|
|
|
2005-02-14 10:11:48 +00:00
|
|
|
The start of the documentation string is usually indented in the
|
|
|
|
source file, but since these spaces come before the starting
|
|
|
|
double-quote, they are not part of the string. Some people make a
|
|
|
|
practice of indenting any additional lines of the string so that the
|
|
|
|
text lines up in the program source. @emph{That is a mistake.} The
|
|
|
|
indentation of the following lines is inside the string; what looks
|
|
|
|
nice in the source code will look ugly when displayed by the help
|
|
|
|
commands.
|
1994-03-21 22:27:20 +00:00
|
|
|
|
|
|
|
You may wonder how the documentation string could be optional, since
|
|
|
|
there are required components of the function that follow it (the body).
|
|
|
|
Since evaluation of a string returns that string, without any side effects,
|
|
|
|
it has no effect if it is not the last form in the body. Thus, in
|
|
|
|
practice, there is no confusion between the first form of the body and the
|
|
|
|
documentation string; if the only body form is a string then it serves both
|
|
|
|
as the return value and as the documentation.
|
|
|
|
|
2003-07-22 15:22:50 +00:00
|
|
|
The last line of the documentation string can specify calling
|
|
|
|
conventions different from the actual function arguments. Write
|
|
|
|
text like this:
|
|
|
|
|
|
|
|
@example
|
2004-01-03 16:52:43 +00:00
|
|
|
\(fn @var{arglist})
|
2003-07-22 15:22:50 +00:00
|
|
|
@end example
|
|
|
|
|
|
|
|
@noindent
|
2004-01-03 16:52:43 +00:00
|
|
|
following a blank line, at the beginning of the line, with no newline
|
2005-02-14 10:11:48 +00:00
|
|
|
following it inside the documentation string. (The @samp{\} is used
|
|
|
|
to avoid confusing the Emacs motion commands.) The calling convention
|
|
|
|
specified in this way appears in help messages in place of the one
|
|
|
|
derived from the actual arguments of the function.
|
|
|
|
|
|
|
|
This feature is particularly useful for macro definitions, since the
|
|
|
|
arguments written in a macro definition often do not correspond to the
|
|
|
|
way users think of the parts of the macro call.
|
2003-07-22 15:22:50 +00:00
|
|
|
|
1994-03-21 22:27:20 +00:00
|
|
|
@node Function Names
|
|
|
|
@section Naming a Function
|
|
|
|
@cindex function definition
|
|
|
|
@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
|
1998-04-20 17:43:57 +00:00
|
|
|
@code{lambda}, a byte-code function object, or a primitive subr-object.
|
1994-03-21 22:27:20 +00:00
|
|
|
|
|
|
|
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.
|
|
|
|
|
|
|
|
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
|
2005-02-14 10:11:48 +00:00
|
|
|
subr-object that is its function definition. For most purposes, the
|
|
|
|
distinction is not important.
|
1994-03-21 22:27:20 +00:00
|
|
|
|
|
|
|
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.
|
|
|
|
|
1998-05-19 03:45:57 +00:00
|
|
|
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.
|
1994-03-21 22:27:20 +00:00
|
|
|
|
|
|
|
@node Defining Functions
|
1994-04-26 20:24:51 +00:00
|
|
|
@section Defining Functions
|
1994-03-21 22:27:20 +00:00
|
|
|
@cindex defining a function
|
|
|
|
|
|
|
|
We usually give a name to a function when it is first created. This
|
|
|
|
is called @dfn{defining a function}, and it is done with the
|
|
|
|
@code{defun} special form.
|
|
|
|
|
|
|
|
@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:
|
|
|
|
|
|
|
|
@example
|
|
|
|
(lambda @var{argument-list} . @var{body-forms})
|
|
|
|
@end example
|
|
|
|
|
|
|
|
@code{defun} stores this lambda expression in the function cell of
|
|
|
|
@var{name}. It returns the value @var{name}, but usually we ignore this
|
|
|
|
value.
|
|
|
|
|
2005-06-18 13:57:17 +00:00
|
|
|
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
|
1998-02-28 01:53:53 +00:00
|
|
|
@var{body-forms} may be a documentation string and an interactive
|
1994-03-21 22:27:20 +00:00
|
|
|
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}.
|
|
|
|
|
|
|
|
Here are some examples:
|
|
|
|
|
|
|
|
@example
|
|
|
|
@group
|
|
|
|
(defun foo () 5)
|
|
|
|
@result{} foo
|
|
|
|
@end group
|
|
|
|
@group
|
|
|
|
(foo)
|
|
|
|
@result{} 5
|
|
|
|
@end group
|
|
|
|
|
|
|
|
@group
|
|
|
|
(defun bar (a &optional b &rest c)
|
|
|
|
(list a b c))
|
|
|
|
@result{} bar
|
|
|
|
@end group
|
|
|
|
@group
|
|
|
|
(bar 1 2 3 4 5)
|
|
|
|
@result{} (1 2 (3 4 5))
|
|
|
|
@end group
|
|
|
|
@group
|
|
|
|
(bar 1)
|
|
|
|
@result{} (1 nil nil)
|
|
|
|
@end group
|
|
|
|
@group
|
|
|
|
(bar)
|
|
|
|
@error{} Wrong number of arguments.
|
|
|
|
@end group
|
|
|
|
|
|
|
|
@group
|
|
|
|
(defun capitalize-backwards ()
|
|
|
|
"Upcase the last letter of a word."
|
|
|
|
(interactive)
|
|
|
|
(backward-word 1)
|
|
|
|
(forward-word 1)
|
|
|
|
(backward-char 1)
|
|
|
|
(capitalize-word 1))
|
|
|
|
@result{} capitalize-backwards
|
|
|
|
@end group
|
|
|
|
@end example
|
|
|
|
|
|
|
|
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.
|
|
|
|
@end defspec
|
|
|
|
|
2004-01-03 16:52:43 +00:00
|
|
|
@defun defalias name definition &optional docstring
|
2004-06-23 16:40:04 +00:00
|
|
|
@anchor{Definition of defalias}
|
1994-03-21 22:27:20 +00:00
|
|
|
This special form defines the symbol @var{name} as a function, with
|
1994-04-29 10:07:31 +00:00
|
|
|
definition @var{definition} (which can be any valid Lisp function).
|
2004-01-03 16:52:43 +00:00
|
|
|
It returns @var{definition}.
|
|
|
|
|
|
|
|
If @var{docstring} is non-@code{nil}, it becomes the function
|
|
|
|
documentation of @var{name}. Otherwise, any documentation provided by
|
|
|
|
@var{definition} is used.
|
1995-06-06 19:21:15 +00:00
|
|
|
|
|
|
|
The proper place to use @code{defalias} is where a specific function
|
|
|
|
name is being defined---especially where that name appears explicitly in
|
|
|
|
the source file being loaded. This is because @code{defalias} records
|
|
|
|
which file defined the function, just like @code{defun}
|
|
|
|
(@pxref{Unloading}).
|
|
|
|
|
|
|
|
By contrast, in programs that manipulate function definitions for other
|
|
|
|
purposes, it is better to use @code{fset}, which does not keep such
|
2005-05-05 23:12:19 +00:00
|
|
|
records. @xref{Function Cells}.
|
1994-03-21 22:27:20 +00:00
|
|
|
@end defun
|
|
|
|
|
2003-06-30 10:40:27 +00:00
|
|
|
You cannot create a new primitive function with @code{defun} or
|
2004-01-03 16:52:43 +00:00
|
|
|
@code{defalias}, but you can use them to change the function definition of
|
2003-06-30 10:40:27 +00:00
|
|
|
any symbol, even one such as @code{car} or @code{x-popup-menu} whose
|
|
|
|
normal definition is a primitive. However, this is risky: for
|
|
|
|
instance, it is next to impossible to redefine @code{car} without
|
|
|
|
breaking Lisp completely. Redefining an obscure function such as
|
|
|
|
@code{x-popup-menu} is less dangerous, but it still may not work as
|
|
|
|
you expect. If there are calls to the primitive from C code, they
|
|
|
|
call the primitive's C definition directly, so changing the symbol's
|
|
|
|
definition will have no effect on them.
|
|
|
|
|
1995-06-06 19:21:15 +00:00
|
|
|
See also @code{defsubst}, which defines a function like @code{defun}
|
|
|
|
and tells the Lisp compiler to open-code it. @xref{Inline Functions}.
|
|
|
|
|
1994-03-21 22:27:20 +00:00
|
|
|
@node Calling Functions
|
|
|
|
@section Calling Functions
|
|
|
|
@cindex function invocation
|
|
|
|
@cindex calling a function
|
|
|
|
|
|
|
|
Defining functions is only half the battle. Functions don't do
|
|
|
|
anything until you @dfn{call} them, i.e., tell them to run. Calling a
|
|
|
|
function is also known as @dfn{invocation}.
|
|
|
|
|
1994-04-29 10:07:31 +00:00
|
|
|
The most common way of invoking a function is by evaluating a list.
|
|
|
|
For example, evaluating the list @code{(concat "a" "b")} calls the
|
|
|
|
function @code{concat} with arguments @code{"a"} and @code{"b"}.
|
|
|
|
@xref{Evaluation}, for a description of evaluation.
|
1994-03-21 22:27:20 +00:00
|
|
|
|
2005-02-14 10:11:48 +00:00
|
|
|
When you write a list as an expression in your program, you specify
|
|
|
|
which function to call, and how many arguments to give it, in the text
|
|
|
|
of the program. Usually that's just what you want. Occasionally you
|
|
|
|
need to compute at run time which function to call. To do that, use
|
|
|
|
the function @code{funcall}. When you also need to determine at run
|
|
|
|
time how many arguments to pass, use @code{apply}.
|
1994-03-21 22:27:20 +00:00
|
|
|
|
|
|
|
@defun funcall function &rest arguments
|
|
|
|
@code{funcall} calls @var{function} with @var{arguments}, and returns
|
|
|
|
whatever @var{function} returns.
|
|
|
|
|
|
|
|
Since @code{funcall} is a function, all of its arguments, including
|
|
|
|
@var{function}, are evaluated before @code{funcall} is called. This
|
|
|
|
means that you can use any expression to obtain the function to be
|
2005-02-14 10:11:48 +00:00
|
|
|
called. It also means that @code{funcall} does not see the
|
|
|
|
expressions you write for the @var{arguments}, only their values.
|
|
|
|
These values are @emph{not} evaluated a second time in the act of
|
|
|
|
calling @var{function}; the operation of @code{funcall} is like the
|
|
|
|
normal procedure for calling a function, once its arguments have
|
|
|
|
already been evaluated.
|
1994-03-21 22:27:20 +00:00
|
|
|
|
|
|
|
The argument @var{function} must be either a Lisp function or a
|
|
|
|
primitive function. Special forms and macros are not allowed, because
|
|
|
|
they make sense only when given the ``unevaluated'' argument
|
|
|
|
expressions. @code{funcall} cannot provide these because, as we saw
|
|
|
|
above, it never knows them in the first place.
|
|
|
|
|
|
|
|
@example
|
|
|
|
@group
|
|
|
|
(setq f 'list)
|
|
|
|
@result{} list
|
|
|
|
@end group
|
|
|
|
@group
|
|
|
|
(funcall f 'x 'y 'z)
|
|
|
|
@result{} (x y z)
|
|
|
|
@end group
|
|
|
|
@group
|
|
|
|
(funcall f 'x 'y '(z))
|
|
|
|
@result{} (x y (z))
|
|
|
|
@end group
|
|
|
|
@group
|
|
|
|
(funcall 'and t nil)
|
|
|
|
@error{} Invalid function: #<subr and>
|
|
|
|
@end group
|
|
|
|
@end example
|
|
|
|
|
1999-10-26 13:08:02 +00:00
|
|
|
Compare these examples with the examples of @code{apply}.
|
1994-03-21 22:27:20 +00:00
|
|
|
@end defun
|
|
|
|
|
|
|
|
@defun apply function &rest arguments
|
|
|
|
@code{apply} calls @var{function} with @var{arguments}, just like
|
|
|
|
@code{funcall} but with one difference: the last of @var{arguments} is a
|
1998-02-28 01:53:53 +00:00
|
|
|
list of objects, which are passed to @var{function} as separate
|
|
|
|
arguments, rather than a single list. We say that @code{apply}
|
|
|
|
@dfn{spreads} this list so that each individual element becomes an
|
|
|
|
argument.
|
1994-03-21 22:27:20 +00:00
|
|
|
|
|
|
|
@code{apply} returns the result of calling @var{function}. As with
|
|
|
|
@code{funcall}, @var{function} must either be a Lisp function or a
|
|
|
|
primitive function; special forms and macros do not make sense in
|
|
|
|
@code{apply}.
|
|
|
|
|
|
|
|
@example
|
|
|
|
@group
|
|
|
|
(setq f 'list)
|
|
|
|
@result{} list
|
|
|
|
@end group
|
|
|
|
@group
|
|
|
|
(apply f 'x 'y 'z)
|
|
|
|
@error{} Wrong type argument: listp, z
|
|
|
|
@end group
|
|
|
|
@group
|
|
|
|
(apply '+ 1 2 '(3 4))
|
|
|
|
@result{} 10
|
|
|
|
@end group
|
|
|
|
@group
|
|
|
|
(apply '+ '(1 2 3 4))
|
|
|
|
@result{} 10
|
|
|
|
@end group
|
|
|
|
|
|
|
|
@group
|
|
|
|
(apply 'append '((a b c) nil (x y z) nil))
|
|
|
|
@result{} (a b c x y z)
|
|
|
|
@end group
|
|
|
|
@end example
|
|
|
|
|
2004-01-03 16:52:43 +00:00
|
|
|
For an interesting example of using @code{apply}, see @ref{Definition
|
|
|
|
of mapcar}.
|
1994-03-21 22:27:20 +00:00
|
|
|
@end defun
|
|
|
|
|
|
|
|
@cindex functionals
|
|
|
|
It is common for Lisp functions to accept functions as arguments or
|
|
|
|
find them in data structures (especially in hook variables and property
|
|
|
|
lists) and call them using @code{funcall} or @code{apply}. Functions
|
|
|
|
that accept function arguments are often called @dfn{functionals}.
|
|
|
|
|
1995-06-06 19:21:15 +00:00
|
|
|
Sometimes, when you call a functional, it is useful to supply a no-op
|
|
|
|
function as the argument. Here are two different kinds of no-op
|
1994-03-21 22:27:20 +00:00
|
|
|
function:
|
|
|
|
|
|
|
|
@defun identity arg
|
|
|
|
This function returns @var{arg} and has no side effects.
|
|
|
|
@end defun
|
|
|
|
|
|
|
|
@defun ignore &rest args
|
|
|
|
This function ignores any arguments and returns @code{nil}.
|
|
|
|
@end defun
|
|
|
|
|
|
|
|
@node Mapping Functions
|
|
|
|
@section Mapping Functions
|
|
|
|
@cindex mapping functions
|
|
|
|
|
2004-01-03 16:52:43 +00:00
|
|
|
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.
|
1998-04-20 17:43:57 +00:00
|
|
|
|
|
|
|
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
|
|
|
|
over a char-table in a way that deals properly with its sparse nature,
|
|
|
|
use the function @code{map-char-table} (@pxref{Char-Tables}).
|
1994-03-21 22:27:20 +00:00
|
|
|
|
|
|
|
@defun mapcar function sequence
|
2004-06-23 16:40:04 +00:00
|
|
|
@anchor{Definition of mapcar}
|
1994-04-29 10:07:31 +00:00
|
|
|
@code{mapcar} applies @var{function} to each element of @var{sequence}
|
|
|
|
in turn, and returns a list of the results.
|
1994-03-21 22:27:20 +00:00
|
|
|
|
1998-04-20 17:43:57 +00:00
|
|
|
The argument @var{sequence} can be any kind of sequence except a
|
|
|
|
char-table; that is, a list, a vector, a bool-vector, or a string. The
|
1994-03-21 22:27:20 +00:00
|
|
|
result is always a list. The length of the result is the same as the
|
|
|
|
length of @var{sequence}.
|
|
|
|
|
|
|
|
@smallexample
|
|
|
|
@group
|
|
|
|
@exdent @r{For example:}
|
|
|
|
|
|
|
|
(mapcar 'car '((a b) (c d) (e f)))
|
|
|
|
@result{} (a c e)
|
|
|
|
(mapcar '1+ [1 2 3])
|
|
|
|
@result{} (2 3 4)
|
|
|
|
(mapcar 'char-to-string "abc")
|
|
|
|
@result{} ("a" "b" "c")
|
|
|
|
@end group
|
|
|
|
|
|
|
|
@group
|
|
|
|
;; @r{Call each function in @code{my-hooks}.}
|
|
|
|
(mapcar 'funcall my-hooks)
|
|
|
|
@end group
|
|
|
|
|
|
|
|
@group
|
1998-04-20 17:43:57 +00:00
|
|
|
(defun mapcar* (function &rest args)
|
1994-03-21 22:27:20 +00:00
|
|
|
"Apply FUNCTION to successive cars of all ARGS.
|
|
|
|
Return the list of results."
|
|
|
|
;; @r{If no list is exhausted,}
|
2004-01-03 16:52:43 +00:00
|
|
|
(if (not (memq nil args))
|
1998-04-20 17:43:57 +00:00
|
|
|
;; @r{apply function to @sc{car}s.}
|
2003-02-04 14:56:31 +00:00
|
|
|
(cons (apply function (mapcar 'car args))
|
|
|
|
(apply 'mapcar* function
|
1994-03-21 22:27:20 +00:00
|
|
|
;; @r{Recurse for rest of elements.}
|
|
|
|
(mapcar 'cdr args)))))
|
|
|
|
@end group
|
|
|
|
|
|
|
|
@group
|
|
|
|
(mapcar* 'cons '(a b c) '(1 2 3 4))
|
|
|
|
@result{} ((a . 1) (b . 2) (c . 3))
|
|
|
|
@end group
|
|
|
|
@end smallexample
|
|
|
|
@end defun
|
|
|
|
|
2000-04-12 17:20:46 +00:00
|
|
|
@defun mapc function sequence
|
|
|
|
@tindex mapc
|
|
|
|
@code{mapc} is like @code{mapcar} except that @var{function} is used for
|
|
|
|
side-effects only---the values it returns are ignored, not collected
|
|
|
|
into a list. @code{mapc} always returns @var{sequence}.
|
|
|
|
@end defun
|
|
|
|
|
1994-03-21 22:27:20 +00:00
|
|
|
@defun mapconcat function sequence separator
|
|
|
|
@code{mapconcat} applies @var{function} to each element of
|
|
|
|
@var{sequence}: the results, which must be strings, are concatenated.
|
|
|
|
Between each pair of result strings, @code{mapconcat} inserts the string
|
|
|
|
@var{separator}. Usually @var{separator} contains a space or comma or
|
|
|
|
other suitable punctuation.
|
|
|
|
|
|
|
|
The argument @var{function} must be a function that can take one
|
1998-04-20 17:43:57 +00:00
|
|
|
argument and return a string. The argument @var{sequence} can be any
|
|
|
|
kind of sequence except a char-table; that is, a list, a vector, a
|
|
|
|
bool-vector, or a string.
|
2003-02-04 14:56:31 +00:00
|
|
|
|
1994-03-21 22:27:20 +00:00
|
|
|
@smallexample
|
|
|
|
@group
|
|
|
|
(mapconcat 'symbol-name
|
|
|
|
'(The cat in the hat)
|
|
|
|
" ")
|
|
|
|
@result{} "The cat in the hat"
|
|
|
|
@end group
|
|
|
|
|
|
|
|
@group
|
|
|
|
(mapconcat (function (lambda (x) (format "%c" (1+ x))))
|
|
|
|
"HAL-8000"
|
|
|
|
"")
|
|
|
|
@result{} "IBM.9111"
|
|
|
|
@end group
|
|
|
|
@end smallexample
|
|
|
|
@end defun
|
|
|
|
|
|
|
|
@node Anonymous Functions
|
|
|
|
@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 usually functions
|
|
|
|
are 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.
|
|
|
|
|
|
|
|
Any method of creating such a list makes a valid function. Even this:
|
|
|
|
|
|
|
|
@smallexample
|
|
|
|
@group
|
2002-01-12 22:40:36 +00:00
|
|
|
(setq silly (append '(lambda (x)) (list (list '+ (* 3 4) 'x))))
|
1994-03-21 22:27:20 +00:00
|
|
|
@result{} (lambda (x) (+ 12 x))
|
|
|
|
@end group
|
|
|
|
@end smallexample
|
|
|
|
|
|
|
|
@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}.
|
|
|
|
|
|
|
|
Here is how we might call this function:
|
|
|
|
|
|
|
|
@example
|
|
|
|
@group
|
|
|
|
(funcall silly 1)
|
|
|
|
@result{} 13
|
|
|
|
@end group
|
|
|
|
@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
|
1998-02-28 01:53:53 +00:00
|
|
|
your program. For example, 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.
|
|
|
|
|
2003-02-04 14:56:31 +00:00
|
|
|
Here we define a function @code{change-property} which
|
1998-02-28 01:53:53 +00:00
|
|
|
uses a function as its third argument:
|
1994-03-21 22:27:20 +00:00
|
|
|
|
|
|
|
@example
|
|
|
|
@group
|
1998-02-28 01:53:53 +00:00
|
|
|
(defun change-property (symbol prop function)
|
|
|
|
(let ((value (get symbol prop)))
|
|
|
|
(put symbol prop (funcall function value))))
|
1994-03-21 22:27:20 +00:00
|
|
|
@end group
|
1998-02-28 01:53:53 +00:00
|
|
|
@end example
|
|
|
|
|
|
|
|
@noindent
|
|
|
|
Here we define a function that uses @code{change-property},
|
1998-04-20 17:43:57 +00:00
|
|
|
passing it a function to double a number:
|
1998-02-28 01:53:53 +00:00
|
|
|
|
|
|
|
@example
|
1994-03-21 22:27:20 +00:00
|
|
|
@group
|
1998-02-28 01:53:53 +00:00
|
|
|
(defun double-property (symbol prop)
|
2002-01-11 01:05:49 +00:00
|
|
|
(change-property symbol prop '(lambda (x) (* 2 x))))
|
1994-03-21 22:27:20 +00:00
|
|
|
@end group
|
|
|
|
@end example
|
|
|
|
|
|
|
|
@noindent
|
|
|
|
In such cases, we usually use the special form @code{function} instead
|
1998-02-28 01:53:53 +00:00
|
|
|
of simple quotation to quote the anonymous function, like this:
|
1994-03-21 22:27:20 +00:00
|
|
|
|
|
|
|
@example
|
|
|
|
@group
|
1998-02-28 01:53:53 +00:00
|
|
|
(defun double-property (symbol prop)
|
1998-05-19 03:45:57 +00:00
|
|
|
(change-property symbol prop
|
|
|
|
(function (lambda (x) (* 2 x)))))
|
1994-03-21 22:27:20 +00:00
|
|
|
@end group
|
|
|
|
@end example
|
|
|
|
|
1998-02-28 01:53:53 +00:00
|
|
|
Using @code{function} instead of @code{quote} makes a difference if you
|
|
|
|
compile the function @code{double-property}. For example, if you
|
|
|
|
compile the second definition of @code{double-property}, the anonymous
|
|
|
|
function is compiled as well. By contrast, if you compile the first
|
|
|
|
definition which uses ordinary @code{quote}, the argument passed to
|
|
|
|
@code{change-property} is the precise list shown:
|
1994-03-21 22:27:20 +00:00
|
|
|
|
|
|
|
@example
|
|
|
|
(lambda (x) (* x 2))
|
|
|
|
@end example
|
|
|
|
|
|
|
|
@noindent
|
|
|
|
The Lisp compiler cannot assume this list is a function, even though it
|
1998-02-28 01:53:53 +00:00
|
|
|
looks like one, since it does not know what @code{change-property} will
|
1998-05-19 03:45:57 +00:00
|
|
|
do with the list. Perhaps it will check whether the @sc{car} of the third
|
1998-02-28 01:53:53 +00:00
|
|
|
element is the symbol @code{*}! Using @code{function} tells the
|
|
|
|
compiler it is safe to go ahead and compile the constant function.
|
1994-03-21 22:27:20 +00:00
|
|
|
|
2002-01-11 01:05:49 +00:00
|
|
|
Nowadays it is possible to omit @code{function} entirely, like this:
|
|
|
|
|
|
|
|
@example
|
|
|
|
@group
|
|
|
|
(defun double-property (symbol prop)
|
|
|
|
(change-property symbol prop (lambda (x) (* 2 x))))
|
|
|
|
@end group
|
|
|
|
@end example
|
|
|
|
|
|
|
|
@noindent
|
|
|
|
This is because @code{lambda} itself implies @code{function}.
|
|
|
|
|
1994-03-21 22:27:20 +00:00
|
|
|
We sometimes write @code{function} instead of @code{quote} when
|
|
|
|
quoting the name of a function, but this usage is just a sort of
|
1998-02-28 01:53:53 +00:00
|
|
|
comment:
|
1994-03-21 22:27:20 +00:00
|
|
|
|
|
|
|
@example
|
|
|
|
(function @var{symbol}) @equiv{} (quote @var{symbol}) @equiv{} '@var{symbol}
|
1998-05-19 03:45:57 +00:00
|
|
|
@end example
|
|
|
|
|
1999-09-17 06:59:04 +00:00
|
|
|
@cindex @samp{#'} syntax
|
1998-05-19 03:45:57 +00:00
|
|
|
The read syntax @code{#'} is a short-hand for using @code{function}.
|
2003-02-04 14:56:31 +00:00
|
|
|
For example,
|
1998-05-19 03:45:57 +00:00
|
|
|
|
|
|
|
@example
|
|
|
|
#'(lambda (x) (* x x))
|
|
|
|
@end example
|
|
|
|
|
|
|
|
@noindent
|
|
|
|
is equivalent to
|
|
|
|
|
|
|
|
@example
|
|
|
|
(function (lambda (x) (* x x)))
|
1994-03-21 22:27:20 +00:00
|
|
|
@end example
|
|
|
|
|
1998-02-28 01:53:53 +00:00
|
|
|
@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
|
|
|
|
|
2004-01-03 16:52:43 +00:00
|
|
|
@xref{describe-symbols example}, for a realistic example using
|
|
|
|
@code{function} and an anonymous function.
|
1994-03-21 22:27:20 +00:00
|
|
|
|
|
|
|
@node Function Cells
|
|
|
|
@section Accessing Function Cell Contents
|
|
|
|
|
|
|
|
The @dfn{function definition} of a symbol is the object stored in the
|
|
|
|
function cell of the symbol. The functions described here access, test,
|
|
|
|
and set the function cell of symbols.
|
|
|
|
|
2004-01-03 16:52:43 +00:00
|
|
|
See also the function @code{indirect-function}. @xref{Definition of
|
|
|
|
indirect-function}.
|
1994-04-29 10:07:31 +00:00
|
|
|
|
1994-03-21 22:27:20 +00:00
|
|
|
@defun symbol-function symbol
|
|
|
|
@kindex void-function
|
|
|
|
This returns the object in the function cell of @var{symbol}. If the
|
|
|
|
symbol's function cell is void, a @code{void-function} error is
|
|
|
|
signaled.
|
|
|
|
|
|
|
|
This function does not check that the returned object is a legitimate
|
|
|
|
function.
|
|
|
|
|
|
|
|
@example
|
|
|
|
@group
|
|
|
|
(defun bar (n) (+ n 2))
|
|
|
|
@result{} bar
|
|
|
|
@end group
|
|
|
|
@group
|
|
|
|
(symbol-function 'bar)
|
|
|
|
@result{} (lambda (n) (+ n 2))
|
|
|
|
@end group
|
|
|
|
@group
|
|
|
|
(fset 'baz 'bar)
|
|
|
|
@result{} bar
|
|
|
|
@end group
|
|
|
|
@group
|
|
|
|
(symbol-function 'baz)
|
|
|
|
@result{} bar
|
|
|
|
@end group
|
|
|
|
@end example
|
|
|
|
@end defun
|
|
|
|
|
|
|
|
@cindex void function cell
|
|
|
|
If you have never given a symbol any function definition, we say that
|
|
|
|
that symbol's function cell is @dfn{void}. In other words, the function
|
|
|
|
cell does not have any Lisp object in it. If you try to call such a symbol
|
|
|
|
as a function, it signals a @code{void-function} error.
|
|
|
|
|
|
|
|
Note that void is not the same as @code{nil} or the symbol
|
|
|
|
@code{void}. The symbols @code{nil} and @code{void} are Lisp objects,
|
|
|
|
and can be stored into a function cell just as any other object can be
|
|
|
|
(and they can be valid functions if you define them in turn with
|
1994-04-29 10:07:31 +00:00
|
|
|
@code{defun}). A void function cell contains no object whatsoever.
|
1994-03-21 22:27:20 +00:00
|
|
|
|
|
|
|
You can test the voidness of a symbol's function definition with
|
|
|
|
@code{fboundp}. After you have given a symbol a function definition, you
|
|
|
|
can make it void once more using @code{fmakunbound}.
|
|
|
|
|
|
|
|
@defun fboundp symbol
|
|
|
|
This function returns @code{t} if the symbol has an object in its
|
|
|
|
function cell, @code{nil} otherwise. It does not check that the object
|
|
|
|
is a legitimate function.
|
|
|
|
@end defun
|
|
|
|
|
|
|
|
@defun fmakunbound symbol
|
|
|
|
This function makes @var{symbol}'s function cell void, so that a
|
2004-01-03 16:52:43 +00:00
|
|
|
subsequent attempt to access this cell will cause a
|
|
|
|
@code{void-function} error. It returns @var{symbol}. (See also
|
|
|
|
@code{makunbound}, in @ref{Void Variables}.)
|
1994-03-21 22:27:20 +00:00
|
|
|
|
|
|
|
@example
|
|
|
|
@group
|
|
|
|
(defun foo (x) x)
|
1998-02-28 01:53:53 +00:00
|
|
|
@result{} foo
|
1994-03-21 22:27:20 +00:00
|
|
|
@end group
|
|
|
|
@group
|
1994-04-29 10:07:31 +00:00
|
|
|
(foo 1)
|
|
|
|
@result{}1
|
|
|
|
@end group
|
|
|
|
@group
|
1994-03-21 22:27:20 +00:00
|
|
|
(fmakunbound 'foo)
|
1998-02-28 01:53:53 +00:00
|
|
|
@result{} foo
|
1994-03-21 22:27:20 +00:00
|
|
|
@end group
|
|
|
|
@group
|
|
|
|
(foo 1)
|
|
|
|
@error{} Symbol's function definition is void: foo
|
|
|
|
@end group
|
|
|
|
@end example
|
|
|
|
@end defun
|
|
|
|
|
1996-12-26 22:40:52 +00:00
|
|
|
@defun fset symbol definition
|
1998-02-28 01:53:53 +00:00
|
|
|
This function stores @var{definition} in the function cell of
|
|
|
|
@var{symbol}. The result is @var{definition}. Normally
|
|
|
|
@var{definition} should be a function or the name of a function, but
|
|
|
|
this is not checked. The argument @var{symbol} is an ordinary evaluated
|
|
|
|
argument.
|
1994-03-21 22:27:20 +00:00
|
|
|
|
|
|
|
There are three normal uses of this function:
|
|
|
|
|
|
|
|
@itemize @bullet
|
|
|
|
@item
|
1998-04-20 17:43:57 +00:00
|
|
|
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
|
2004-01-03 16:52:43 +00:00
|
|
|
@code{fset}; see @ref{Definition of defalias}.)
|
1994-03-21 22:27:20 +00:00
|
|
|
|
|
|
|
@item
|
|
|
|
Giving a symbol a function definition that is not a list and therefore
|
1994-04-29 10:07:31 +00:00
|
|
|
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
|
1998-04-20 17:43:57 +00:00
|
|
|
@code{s2} presently has. (Once again use @code{defalias} instead of
|
|
|
|
@code{fset} if you think of this as the definition of @code{s1}.)
|
1994-03-21 22:27:20 +00:00
|
|
|
|
|
|
|
@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
|
|
|
|
|
1998-04-20 17:43:57 +00:00
|
|
|
Here are examples of these uses:
|
1994-03-21 22:27:20 +00:00
|
|
|
|
|
|
|
@example
|
|
|
|
@group
|
1998-04-20 17:43:57 +00:00
|
|
|
;; @r{Save @code{foo}'s definition in @code{old-foo}.}
|
|
|
|
(fset 'old-foo (symbol-function 'foo))
|
1994-03-21 22:27:20 +00:00
|
|
|
@end group
|
|
|
|
|
|
|
|
@group
|
|
|
|
;; @r{Make the symbol @code{car} the function definition of @code{xfirst}.}
|
1998-04-20 17:43:57 +00:00
|
|
|
;; @r{(Most likely, @code{defalias} would be better than @code{fset} here.)}
|
1994-03-21 22:27:20 +00:00
|
|
|
(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
|
|
|
|
|
|
|
|
@group
|
|
|
|
;; @r{Define a named keyboard macro.}
|
|
|
|
(fset 'kill-two-lines "\^u2\^k")
|
|
|
|
@result{} "\^u2\^k"
|
|
|
|
@end group
|
1994-04-29 10:07:31 +00:00
|
|
|
|
1998-04-20 17:43:57 +00:00
|
|
|
@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
|
|
|
|
@end example
|
1994-03-21 22:27:20 +00:00
|
|
|
@end defun
|
|
|
|
|
2005-02-14 10:11:48 +00:00
|
|
|
@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}).
|
1995-06-06 19:21:15 +00:00
|
|
|
|
2005-05-05 23:12:19 +00:00
|
|
|
@node Obsolete Functions
|
|
|
|
@section Declaring Functions Obsolete
|
|
|
|
|
|
|
|
You can use @code{make-obsolete} to declare a function obsolete. This
|
|
|
|
indicates that the function may be removed at some stage in the future.
|
|
|
|
|
2005-06-15 23:09:44 +00:00
|
|
|
@defun make-obsolete obsolete-name current-name &optional when
|
2005-05-05 23:12:19 +00:00
|
|
|
This function makes the byte compiler warn that the function
|
2005-06-15 23:09:44 +00:00
|
|
|
@var{obsolete-name} is obsolete. If @var{current-name} is a symbol, the
|
|
|
|
warning message says to use @var{current-name} instead of
|
|
|
|
@var{obsolete-name}. @var{current-name} does not need to be an alias for
|
|
|
|
@var{obsolete-name}; it can be a different function with similar
|
|
|
|
functionality. If @var{current-name} is a string, it is the warning
|
|
|
|
message.
|
2005-05-05 23:12:19 +00:00
|
|
|
|
|
|
|
If provided, @var{when} should be a string indicating when the function
|
|
|
|
was first made obsolete---for example, a date or a release number.
|
|
|
|
@end defun
|
|
|
|
|
|
|
|
You can define a function as an alias and declare it obsolete at the
|
|
|
|
same time using the macro @code{define-obsolete-function-alias}.
|
|
|
|
|
2005-06-15 23:09:44 +00:00
|
|
|
@defmac define-obsolete-function-alias obsolete-name current-name &optional when docstring
|
2005-06-18 13:57:17 +00:00
|
|
|
This macro marks the function @var{obsolete-name} obsolete and also
|
|
|
|
defines it as an alias for the function @var{current-name}. It is
|
|
|
|
equivalent to the following:
|
2005-05-05 23:12:19 +00:00
|
|
|
|
|
|
|
@example
|
2005-06-18 13:57:17 +00:00
|
|
|
(defalias @var{obsolete-name} @var{current-name} @var{docstring})
|
|
|
|
(make-obsolete @var{obsolete-name} @var{current-name} @var{when})
|
2005-05-05 23:12:19 +00:00
|
|
|
@end example
|
|
|
|
@end defmac
|
|
|
|
|
1994-03-21 22:27:20 +00:00
|
|
|
@node Inline Functions
|
|
|
|
@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.
|
|
|
|
|
|
|
|
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. Since the flexibility of
|
|
|
|
redefining functions is an important feature of Emacs, you should not
|
|
|
|
make a function inline unless its speed is really crucial.
|
|
|
|
|
|
|
|
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.
|
|
|
|
|
|
|
|
It's possible to define a macro to expand into the same code that an
|
1998-04-20 17:43:57 +00:00
|
|
|
inline function would execute. (@xref{Macros}.) But the macro would be
|
|
|
|
limited to direct use in expressions---a macro cannot be called with
|
1994-03-21 22:27:20 +00:00
|
|
|
@code{apply}, @code{mapcar} and so on. Also, it takes some work to
|
1998-04-20 17:43:57 +00:00
|
|
|
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}.)
|
1994-03-21 22:27:20 +00:00
|
|
|
|
1994-04-29 10:07:31 +00:00
|
|
|
Inline functions can be used and open-coded later on in the same file,
|
1994-03-21 22:27:20 +00:00
|
|
|
following the definition, just like macros.
|
|
|
|
|
2003-06-30 10:40:27 +00:00
|
|
|
@node Function Safety
|
2005-05-05 23:12:19 +00:00
|
|
|
@section Determining whether a Function is Safe to Call
|
2002-09-28 18:45:56 +00:00
|
|
|
@cindex function safety
|
|
|
|
@cindex safety of functions
|
2003-06-30 10:40:27 +00:00
|
|
|
|
2003-10-23 20:47:07 +00:00
|
|
|
Some major modes such as SES call functions that are stored in user
|
|
|
|
files. (@inforef{Top, ,ses}, for more information on SES.) User
|
|
|
|
files sometimes have poor pedigrees---you can get a spreadsheet from
|
|
|
|
someone you've just met, or you can get one through email from someone
|
|
|
|
you've never met. So it is risky to call a function whose source code
|
|
|
|
is stored in a user file until you have determined that it is safe.
|
2002-09-28 18:45:56 +00:00
|
|
|
|
|
|
|
@defun unsafep form &optional unsafep-vars
|
2003-10-05 13:55:00 +00:00
|
|
|
Returns @code{nil} if @var{form} is a @dfn{safe} Lisp expression, or
|
2003-06-30 10:40:27 +00:00
|
|
|
returns a list that describes why it might be unsafe. The argument
|
2002-09-28 18:45:56 +00:00
|
|
|
@var{unsafep-vars} is a list of symbols known to have temporary
|
|
|
|
bindings at this point; it is mainly used for internal recursive
|
|
|
|
calls. The current buffer is an implicit argument, which provides a
|
|
|
|
list of buffer-local bindings.
|
|
|
|
@end defun
|
|
|
|
|
|
|
|
Being quick and simple, @code{unsafep} does a very light analysis and
|
|
|
|
rejects many Lisp expressions that are actually safe. There are no
|
2003-06-30 10:40:27 +00:00
|
|
|
known cases where @code{unsafep} returns @code{nil} for an unsafe
|
|
|
|
expression. However, a ``safe'' Lisp expression can return a string
|
|
|
|
with a @code{display} property, containing an associated Lisp
|
|
|
|
expression to be executed after the string is inserted into a buffer.
|
|
|
|
This associated expression can be a virus. In order to be safe, you
|
|
|
|
must delete properties from all strings calculated by user code before
|
2002-09-28 18:45:56 +00:00
|
|
|
inserting them into buffers.
|
|
|
|
|
2003-06-30 10:40:27 +00:00
|
|
|
@ignore
|
2002-09-28 18:45:56 +00:00
|
|
|
What is a safe Lisp expression? Basically, it's an expression that
|
|
|
|
calls only built-in functions with no side effects (or only innocuous
|
|
|
|
ones). Innocuous side effects include displaying messages and
|
|
|
|
altering non-risky buffer-local variables (but not global variables).
|
|
|
|
|
|
|
|
@table @dfn
|
|
|
|
@item Safe expression
|
|
|
|
@itemize
|
|
|
|
@item
|
|
|
|
An atom or quoted thing.
|
|
|
|
@item
|
|
|
|
A call to a safe function (see below), if all its arguments are
|
|
|
|
safe expressions.
|
|
|
|
@item
|
2003-06-30 10:40:27 +00:00
|
|
|
One of the special forms @code{and}, @code{catch}, @code{cond},
|
|
|
|
@code{if}, @code{or}, @code{prog1}, @code{prog2}, @code{progn},
|
|
|
|
@code{while}, and @code{unwind-protect}], if all its arguments are
|
|
|
|
safe.
|
2002-09-28 18:45:56 +00:00
|
|
|
@item
|
2003-06-30 10:40:27 +00:00
|
|
|
A form that creates temporary bindings (@code{condition-case},
|
|
|
|
@code{dolist}, @code{dotimes}, @code{lambda}, @code{let}, or
|
|
|
|
@code{let*}), if all args are safe and the symbols to be bound are not
|
|
|
|
explicitly risky (see @pxref{File Local Variables}).
|
2002-09-28 18:45:56 +00:00
|
|
|
@item
|
2003-06-30 10:40:27 +00:00
|
|
|
An assignment using @code{add-to-list}, @code{setq}, @code{push}, or
|
|
|
|
@code{pop}, if all args are safe and the symbols to be assigned are
|
|
|
|
not explicitly risky and they already have temporary or buffer-local
|
|
|
|
bindings.
|
2002-09-28 18:45:56 +00:00
|
|
|
@item
|
|
|
|
One of [apply, mapc, mapcar, mapconcat] if the first argument is a
|
|
|
|
safe explicit lambda and the other args are safe expressions.
|
|
|
|
@end itemize
|
|
|
|
|
|
|
|
@item Safe function
|
|
|
|
@itemize
|
|
|
|
@item
|
|
|
|
A lambda containing safe expressions.
|
|
|
|
@item
|
|
|
|
A symbol on the list @code{safe-functions}, so the user says it's safe.
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@item
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2003-06-30 10:40:27 +00:00
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A symbol with a non-@code{nil} @code{side-effect-free} property.
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2002-09-28 18:45:56 +00:00
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@item
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2003-06-30 10:40:27 +00:00
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A symbol with a non-@code{nil} @code{safe-function} property. Value t
|
2002-09-28 18:45:56 +00:00
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indicates a function that is safe but has innocuous side effects.
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Other values will someday indicate functions with classes of side
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effects that are not always safe.
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@end itemize
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The @code{side-effect-free} and @code{safe-function} properties are
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provided for built-in functions and for low-level functions and macros
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defined in @file{subr.el}. You can assign these properties for the
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functions you write.
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@end table
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2003-06-30 10:40:27 +00:00
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@end ignore
|
1994-03-21 22:27:20 +00:00
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@node Related Topics
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@section Other Topics Related to Functions
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Here is a table of several functions that do things related to
|
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|
function calling and function definitions. They are documented
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elsewhere, but we provide cross references here.
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@table @code
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@item apply
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See @ref{Calling Functions}.
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@item autoload
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|
See @ref{Autoload}.
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@item call-interactively
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|
|
See @ref{Interactive Call}.
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@item commandp
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|
See @ref{Interactive Call}.
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@item documentation
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|
See @ref{Accessing Documentation}.
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@item eval
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|
See @ref{Eval}.
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@item funcall
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|
|
See @ref{Calling Functions}.
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|
|
1998-04-20 17:43:57 +00:00
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@item function
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|
|
See @ref{Anonymous Functions}.
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|
|
1994-03-21 22:27:20 +00:00
|
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|
@item ignore
|
|
|
|
See @ref{Calling Functions}.
|
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|
|
@item indirect-function
|
2004-02-18 04:40:53 +00:00
|
|
|
See @ref{Function Indirection}.
|
1994-03-21 22:27:20 +00:00
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@item interactive
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|
|
See @ref{Using Interactive}.
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@item interactive-p
|
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|
|
See @ref{Interactive Call}.
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|
@item mapatoms
|
|
|
|
See @ref{Creating Symbols}.
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|
|
@item mapcar
|
|
|
|
See @ref{Mapping Functions}.
|
|
|
|
|
1998-04-20 17:43:57 +00:00
|
|
|
@item map-char-table
|
|
|
|
See @ref{Char-Tables}.
|
|
|
|
|
1994-03-21 22:27:20 +00:00
|
|
|
@item mapconcat
|
|
|
|
See @ref{Mapping Functions}.
|
|
|
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|
|
@item undefined
|
2004-02-18 04:40:53 +00:00
|
|
|
See @ref{Functions for Key Lookup}.
|
1994-03-21 22:27:20 +00:00
|
|
|
@end table
|
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|
2003-09-01 15:45:59 +00:00
|
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|
@ignore
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arch-tag: 39100cdf-8a55-4898-acba-595db619e8e2
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@end ignore
|