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1139 lines
38 KiB
Plaintext
@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 Free Software Foundation, Inc.
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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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* Inline Functions:: Defining functions that the compiler will open code.
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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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Usually the reason that a function is a primitives is because it is
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fundamental, because it provides a low-level interface to operating
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system services, or because it needs to run fast. Primitives can be
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modified or added only by changing the C sources and recompiling the
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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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@ifinfo
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@xref{Lambda Expressions}.
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@end ifinfo
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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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@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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@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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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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@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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@ifinfo
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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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@end ifinfo
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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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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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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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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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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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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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@subsection Advanced 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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accept an indefinite number of arguments, as the functions @code{list}
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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
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the @var{optional-vars}, and there cannot be any actual arguments beyond
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that unless the lambda list uses @code{&rest}. In that case, there may
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be any number of extra actual arguments.
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If actual arguments for the optional and rest variables are omitted,
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then they always default to @code{nil}. There is no way for the
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function to distinguish between an explicit argument of @code{nil} and
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an omitted argument. However, the body of the function is free to
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consider @code{nil} an abbreviation for some other meaningful value.
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This is what @code{substring} does; @code{nil} as the third argument to
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@code{substring} means to use the length of the string supplied.
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@cindex CL note---default optional arg
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@quotation
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@b{Common Lisp note:} Common Lisp allows the function to specify what
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default value to use when an optional argument is omitted; Emacs Lisp
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always uses @code{nil}.
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@end quotation
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For example, an argument list that looks like this:
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@example
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(a b &optional c d &rest e)
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@end example
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@noindent
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binds @code{a} and @code{b} to the first two actual arguments, which are
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required. If one or two more arguments are provided, @code{c} and
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@code{d} are bound to them respectively; any arguments after the first
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four are collected into a list and @code{e} is bound to that list. If
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there are only two arguments, @code{c} is @code{nil}; if two or three
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arguments, @code{d} is @code{nil}; if four arguments or fewer, @code{e}
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is @code{nil}.
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There is no way to have required arguments following optional
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ones---it would not make sense. To see why this must be so, suppose
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that @code{c} in the example were optional and @code{d} were required.
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Suppose three actual arguments are given; which variable would the third
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argument be for? Similarly, it makes no sense to have any more
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arguments (either required or optional) after a @code{&rest} argument.
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Here are some examples of argument lists and proper calls:
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@smallexample
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((lambda (n) (1+ n)) ; @r{One required:}
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1) ; @r{requires exactly one argument.}
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@result{} 2
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((lambda (n &optional n1) ; @r{One required and one optional:}
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(if n1 (+ n n1) (1+ n))) ; @r{1 or 2 arguments.}
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1 2)
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@result{} 3
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((lambda (n &rest ns) ; @r{One required and one rest:}
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(+ n (apply '+ ns))) ; @r{1 or more arguments.}
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1 2 3 4 5)
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@result{} 15
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@end smallexample
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@node Function Documentation
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@subsection Documentation Strings of Functions
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@cindex documentation of function
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A lambda expression may optionally have a @dfn{documentation string} just
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after the lambda list. This string does not affect execution of the
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function; it is a kind of comment, but a systematized comment which
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actually appears inside the Lisp world and can be used by the Emacs help
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facilities. @xref{Documentation}, for how the @var{documentation-string} is
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accessed.
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It is a good idea to provide documentation strings for all the
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functions in your program, even those that are only called from within
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your program. Documentation strings are like comments, except that they
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are easier to access.
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The first line of the documentation string should stand on its own,
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because @code{apropos} displays just this first line. It should consist
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of one or two complete sentences that summarize the function's purpose.
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The start of the documentation string is usually indented in the source file,
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but since these spaces come before the starting double-quote, they are not part of
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the string. Some people make a practice of indenting any additional
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lines of the string so that the text lines up in the program source.
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@emph{This is a mistake.} The indentation of the following lines is
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inside the string; what looks nice in the source code will look ugly
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when displayed by the help commands.
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You may wonder how the documentation string could be optional, since
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there are required components of the function that follow it (the body).
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Since evaluation of a string returns that string, without any side effects,
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it has no effect if it is not the last form in the body. Thus, in
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practice, there is no confusion between the first form of the body and the
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documentation string; if the only body form is a string then it serves both
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as the return value and as the documentation.
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@node Function Names
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@section Naming a Function
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@cindex function definition
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@cindex named function
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@cindex function name
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In most computer languages, every function has a name; the idea of a
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function without a name is nonsensical. In Lisp, a function in the
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strictest sense has no name. It is simply a list whose first element is
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@code{lambda}, or a primitive subr-object.
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However, a symbol can serve as the name of a function. This happens
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when you put the function in the symbol's @dfn{function cell}
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(@pxref{Symbol Components}). Then the symbol itself becomes a valid,
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callable function, equivalent to the list or subr-object that its
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function cell refers to. The contents of the function cell are also
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called the symbol's @dfn{function definition}. The procedure of using a
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symbol's function definition in place of the symbol is called
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@dfn{symbol function indirection}; see @ref{Function Indirection}.
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In practice, nearly all functions are given names in this way and
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referred to through their names. For example, the symbol @code{car} works
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as a function and does what it does because the primitive subr-object
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@code{#<subr car>} is stored in its function cell.
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We give functions names because it is convenient to refer to them by
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their names in Lisp expressions. For primitive subr-objects such as
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@code{#<subr car>}, names are the only way you can refer to them: there
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is no read syntax for such objects. For functions written in Lisp, the
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name is more convenient to use in a call than an explicit lambda
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expression. Also, a function with a name can refer to itself---it can
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be recursive. Writing the function's name in its own definition is much
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more convenient than making the function definition point to itself
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(something that is not impossible but that has various disadvantages in
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practice).
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We often identify functions with the symbols used to name them. For
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example, we often speak of ``the function @code{car}'', not
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distinguishing between the symbol @code{car} and the primitive
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subr-object that is its function definition. For most purposes, there
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is no need to distinguish.
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Even so, keep in mind that a function need not have a unique name. While
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a given function object @emph{usually} appears in the function cell of only
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one symbol, this is just a matter of convenience. It is easy to store
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it in several symbols using @code{fset}; then each of the symbols is
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equally well a name for the same function.
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A symbol used as a function name may also be used as a variable;
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these two uses of a symbol are independent and do not conflict.
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@node Defining Functions
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@section Defining Functions
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@cindex defining a function
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We usually give a name to a function when it is first created. This
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is called @dfn{defining a function}, and it is done with the
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@code{defun} special form.
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@defspec defun name argument-list body-forms
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@code{defun} is the usual way to define new Lisp functions. It
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defines the symbol @var{name} as a function that looks like this:
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@example
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(lambda @var{argument-list} . @var{body-forms})
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@end example
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@code{defun} stores this lambda expression in the function cell of
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@var{name}. It returns the value @var{name}, but usually we ignore this
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value.
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As described previously (@pxref{Lambda Expressions}),
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@var{argument-list} is a list of argument names and may include the
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keywords @code{&optional} and @code{&rest}. Also, the first two forms
|
|
in @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}.
|
|
|
|
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
|
|
|
|
@defun defalias name definition
|
|
This special form defines the symbol @var{name} as a function, with
|
|
definition @var{definition} (which can be any valid Lisp function).
|
|
|
|
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
|
|
records.
|
|
@end defun
|
|
|
|
See also @code{defsubst}, which defines a function like @code{defun}
|
|
and tells the Lisp compiler to open-code it. @xref{Inline Functions}.
|
|
|
|
@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}.
|
|
|
|
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.
|
|
|
|
When you write a list as an expression in your program, the function
|
|
name is part of the program. This means that you choose which function
|
|
to call, and how many arguments to give it, when you write the program.
|
|
Usually that's just what you want. Occasionally you need to decide at
|
|
run time which function to call. To do that, use the functions
|
|
@code{funcall} and @code{apply}.
|
|
|
|
@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
|
|
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};
|
|
@code{funcall} enters the normal procedure for calling a function at the
|
|
place where the arguments have already been evaluated.
|
|
|
|
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
|
|
|
|
Compare these example with the examples of @code{apply}.
|
|
@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
|
|
list of arguments to give to @var{function}, rather than a single
|
|
argument. We also say that @code{apply} @dfn{spreads} this list so that
|
|
each individual element becomes an argument.
|
|
|
|
@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
|
|
|
|
For an interesting example of using @code{apply}, see the description of
|
|
@code{mapcar}, in @ref{Mapping Functions}.
|
|
@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}.
|
|
|
|
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
|
|
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
|
|
|
|
A @dfn{mapping function} applies a given function to each element of a
|
|
list or other collection. Emacs Lisp has three such functions;
|
|
@code{mapcar} and @code{mapconcat}, which scan a list, are described
|
|
here. For the third mapping function, @code{mapatoms}, see
|
|
@ref{Creating Symbols}.
|
|
|
|
@defun mapcar function sequence
|
|
@code{mapcar} applies @var{function} to each element of @var{sequence}
|
|
in turn, and returns a list of the results.
|
|
|
|
The argument @var{sequence} may be a list, a vector, or a string. The
|
|
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
|
|
(defun mapcar* (f &rest args)
|
|
"Apply FUNCTION to successive cars of all ARGS.
|
|
Return the list of results."
|
|
;; @r{If no list is exhausted,}
|
|
(if (not (memq 'nil args))
|
|
;; @r{apply function to @sc{CAR}s.}
|
|
(cons (apply f (mapcar 'car args))
|
|
(apply 'mapcar* f
|
|
;; @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
|
|
|
|
@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
|
|
argument and return a string.
|
|
|
|
@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
|
|
(setq silly (append '(lambda (x)) (list (list '+ (* 3 4) 'x))))
|
|
@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
|
|
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. Here we pass an anonymous function that multiplies
|
|
a number by two:
|
|
|
|
@example
|
|
@group
|
|
(defun double-each (list)
|
|
(mapcar '(lambda (x) (* 2 x)) list))
|
|
@result{} double-each
|
|
@end group
|
|
@group
|
|
(double-each '(2 11))
|
|
@result{} (4 22)
|
|
@end group
|
|
@end example
|
|
|
|
@noindent
|
|
In such cases, we usually use the special form @code{function} instead
|
|
of simple quotation to quote the anonymous 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
|
|
|
|
Using @code{function} instead of @code{quote} makes a difference
|
|
inside a function or macro that you are going to compile. For example:
|
|
|
|
@example
|
|
@group
|
|
(defun double-each (list)
|
|
(mapcar (function (lambda (x) (* 2 x))) list))
|
|
@result{} double-each
|
|
@end group
|
|
@group
|
|
(double-each '(2 11))
|
|
@result{} (4 22)
|
|
@end group
|
|
@end example
|
|
|
|
@noindent
|
|
If this definition of @code{double-each} is compiled, the anonymous
|
|
function is compiled as well. By contrast, in the previous definition
|
|
where ordinary @code{quote} is used, the argument passed to
|
|
@code{mapcar} 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{mapcar} does with the
|
|
list. Perhaps @code{mapcar} will check that the @sc{car} of the third
|
|
element is the symbol @code{*}! The advantage of @code{function} is
|
|
that it tells the compiler to go ahead and compile the constant
|
|
function.
|
|
|
|
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
|
|
|
|
See @code{documentation} in @ref{Accessing Documentation}, for a
|
|
realistic example using @code{function} and an anonymous function.
|
|
|
|
@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.
|
|
|
|
See also the function @code{indirect-function} in @ref{Function
|
|
Indirection}.
|
|
|
|
@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
|
|
@code{defun}). A void function cell contains no object whatsoever.
|
|
|
|
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
|
|
subsequent attempt to access this cell will cause a @code{void-function}
|
|
error. (See also @code{makunbound}, in @ref{Local Variables}.)
|
|
|
|
@example
|
|
@group
|
|
(defun foo (x) x)
|
|
@result{} x
|
|
@end group
|
|
@group
|
|
(foo 1)
|
|
@result{}1
|
|
@end group
|
|
@group
|
|
(fmakunbound 'foo)
|
|
@result{} x
|
|
@end group
|
|
@group
|
|
(foo 1)
|
|
@error{} Symbol's function definition is void: foo
|
|
@end group
|
|
@end example
|
|
@end defun
|
|
|
|
@defun fset symbol object
|
|
This function stores @var{object} in the function cell of @var{symbol}.
|
|
The result is @var{object}. Normally @var{object} should be a function
|
|
or the name of a function, but this is not checked.
|
|
|
|
There are three normal uses of this function:
|
|
|
|
@itemize @bullet
|
|
@item
|
|
Copying one symbol's function definition to another. (In other words,
|
|
making an alternate name for a function.)
|
|
|
|
@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.
|
|
|
|
@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
|
|
|
|
Here are examples of the first two uses:
|
|
|
|
@example
|
|
@group
|
|
;; @r{Give @code{first} the same definition @code{car} has.}
|
|
(fset 'first (symbol-function 'car))
|
|
@result{} #<subr car>
|
|
@end group
|
|
@group
|
|
(first '(1 2 3))
|
|
@result{} 1
|
|
@end group
|
|
|
|
@group
|
|
;; @r{Make the symbol @code{car} the function definition of @code{xfirst}.}
|
|
(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
|
|
@end example
|
|
|
|
See also the related function @code{defalias}, in @ref{Defining
|
|
Functions}.
|
|
@end defun
|
|
|
|
When writing a function that extends a previously defined function,
|
|
the following idiom is sometimes used:
|
|
|
|
@example
|
|
(fset 'old-foo (symbol-function 'foo))
|
|
(defun foo ()
|
|
"Just like old-foo, except more so."
|
|
@group
|
|
(old-foo)
|
|
(more-so))
|
|
@end group
|
|
@end example
|
|
|
|
@noindent
|
|
This does not work properly if @code{foo} has been defined to autoload.
|
|
In such a case, when @code{foo} calls @code{old-foo}, Lisp attempts
|
|
to define @code{old-foo} by loading a file. Since this presumably
|
|
defines @code{foo} rather than @code{old-foo}, it does not produce the
|
|
proper results. The only way to avoid this problem is to make sure the
|
|
file is loaded before moving aside the old definition of @code{foo}.
|
|
|
|
But it is unmodular and unclean, in any case, for a Lisp file to
|
|
redefine a function defined elsewhere.
|
|
|
|
@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
|
|
inline function would execute. But the macro would have a limitation:
|
|
you can use it only explicitly---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. (@xref{Macros}.) 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}.)
|
|
|
|
Inline functions can be used and open-coded later on in the same file,
|
|
following the definition, just like macros.
|
|
|
|
@c Emacs versions prior to 19 did not have inline functions.
|
|
|
|
@node Related Topics
|
|
@section Other Topics Related to Functions
|
|
|
|
Here is a table of several functions that do things related to
|
|
function calling and function definitions. They are documented
|
|
elsewhere, but we provide cross references here.
|
|
|
|
@table @code
|
|
@item apply
|
|
See @ref{Calling Functions}.
|
|
|
|
@item autoload
|
|
See @ref{Autoload}.
|
|
|
|
@item call-interactively
|
|
See @ref{Interactive Call}.
|
|
|
|
@item commandp
|
|
See @ref{Interactive Call}.
|
|
|
|
@item documentation
|
|
See @ref{Accessing Documentation}.
|
|
|
|
@item eval
|
|
See @ref{Eval}.
|
|
|
|
@item funcall
|
|
See @ref{Calling Functions}.
|
|
|
|
@item ignore
|
|
See @ref{Calling Functions}.
|
|
|
|
@item indirect-function
|
|
See @ref{Function Indirection}.
|
|
|
|
@item interactive
|
|
See @ref{Using Interactive}.
|
|
|
|
@item interactive-p
|
|
See @ref{Interactive Call}.
|
|
|
|
@item mapatoms
|
|
See @ref{Creating Symbols}.
|
|
|
|
@item mapcar
|
|
See @ref{Mapping Functions}.
|
|
|
|
@item mapconcat
|
|
See @ref{Mapping Functions}.
|
|
|
|
@item undefined
|
|
See @ref{Key Lookup}.
|
|
@end table
|
|
|