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% File src/library/base/man/function.Rd
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% Part of the R package, https://www.R-project.org
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% Copyright 1995-2023 R Core Team
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% Distributed under GPL 2 or later
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\name{function}
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\alias{function}
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\alias{return}
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\alias{closure}
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\title{Function Definition}
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\usage{
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\special{function( arglist ) expr}
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\special{\\( arglist ) expr}
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\special{return(value)}
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}
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\description{
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  These functions provide the base mechanisms for defining
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  new functions in the \R language.
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}
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\arguments{
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  \item{arglist}{empty or one or more (comma-separated) \samp{name} or
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    \samp{name = expression} terms
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    and/or the special token \code{\link{...}}.}
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  \item{expr}{an expression.}
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  \item{value}{an expression.}
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}
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\details{
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  The names in an argument list can be back-quoted non-standard names
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  (see \sQuote{\link{backquote}}).
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  If \code{value} is missing, \code{NULL} is returned.  If it is a
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  single expression, the value of the evaluated expression is returned.
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  (The expression is evaluated as soon as \code{return} is called, in
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  the evaluation frame of the function and before any
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  \code{\link{on.exit}} expression is evaluated.)
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  If the end of a function is reached without calling \code{return}, the
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  value of the last evaluated expression is returned.
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  The shorthand form \code{\\(x) x + 1} is parsed as \code{function(x) x
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    + 1}. It may be helpful in making code containing simple function
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  expressions more readable.
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}
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\section{Technical details}{
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  This type of function is not the only type in \R: they are called
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  \emph{closures} (a name with origins in LISP) to distinguish them from
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  \link{primitive} functions.
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  A closure has three components, its \code{\link{formals}} (its argument
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  list), its \code{\link{body}} (\code{expr} in the \sQuote{Usage}
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  section) and its \code{\link{environment}} which provides the
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  enclosure of the evaluation frame when the closure is used.
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  There is an optional further component if the closure has been
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  byte-compiled.  This is not normally user-visible, but is indicated
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  when functions are printed.
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}
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\seealso{
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  \code{\link{args}}.
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  \code{\link{formals}}, \code{\link{body}} and
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  \code{\link{environment}} for accessing the component parts of a
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  function.
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  \code{\link{debug}} for debugging; using \code{\link{invisible}} inside
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  \code{return(.)} for returning \emph{invisibly}.
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}
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\references{
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  \bibshow{R:Becker+Chambers+Wilks:1988}
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}
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\examples{
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norm <- function(x) sqrt(x\%*\%x)
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norm(1:4)
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## An anonymous function:
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(function(x, y){ z <- x^2 + y^2; x+y+z })(0:7, 1)
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}
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\keyword{programming}