% File src/library/base/man/expression.Rd
% Part of the R package, https://www.R-project.org
% Copyright 1995-2021 R Core Team
% Distributed under GPL 2 or later

\name{expression}
\alias{expression}
\alias{is.expression}
\alias{as.expression}
\alias{as.expression.default}
\title{Unevaluated Expressions}
\description{
  Creates or tests for objects of mode and class \code{"expression"}.
}
\usage{
expression(\dots)

is.expression(x)
as.expression(x, \dots)
}
\arguments{
  \item{\dots}{\code{expression}: \R objects, typically calls, symbols
    or constants.\cr
    \code{as.expression}: arguments to be passed to methods.}
  \item{x}{an arbitrary \R object.}
}
\details{
  \sQuote{Expression} here is not being used in its colloquial sense,
  that of mathematical expressions.  Those are calls (see
  \code{\link{call}}) in \R, and an \R expression vector is a list of
  calls, symbols etc, for example as returned by \code{\link{parse}}.

  As an object of mode \code{"expression"} is a list, it can be
  subsetted by \code{[}, \code{[[} or \code{$}, the latter two extracting
  individual calls etc.  The replacement forms of these operators can be
  used to replace or delete elements.

  \code{expression} and \code{is.expression} are \link{primitive} functions.
  \code{expression} is \sQuote{special}: it does not evaluate its arguments.
}
\value{
  \code{expression} returns a vector of type \code{"expression"}
  containing its arguments (unevaluated).

  \code{is.expression} returns \code{TRUE} if \code{expr} is an
  expression object and \code{FALSE} otherwise.

  \code{as.expression} attempts to coerce its argument into an
  expression object.  It is generic, and only the default method is
  described here.  (The default method calls
  \code{as.vector(type = "expression")} and so may dispatch methods for
  \code{\link{as.vector}}.)  \code{NULL}, calls, symbols (see
  \code{\link{as.symbol}}) and pairlists are returned as the element of
  a length-one expression vector.  Atomic vectors are placed
  element-by-element into an expression vector (without using any
  names): \code{\link{list}}s have their type (\code{\link{typeof}})
  changed to an expression vector
  (keeping all attributes).
  Other types are not currently supported.
}
\references{
  Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988)
  \emph{The New S Language}.
  Wadsworth & Brooks/Cole.
}
\seealso{
  \code{\link{call}},
  \code{\link{eval}},
  \code{\link{function}}.
  Further,
  \code{\link{text}}, \code{\link{legend}}, and \code{\link{plotmath}}
  for plotting mathematical expressions.
}
\examples{
length(ex1 <- expression(1 + 0:9)) # 1
ex1
eval(ex1) # 1:10

length(ex3 <- expression(u, 2, u + 0:9)) # 3
mode(ex3 [3])   # expression
mode(ex3[[3]])  # call
## but not all components are 'call's :
sapply(ex3, mode  ) #  name  numeric  call
sapply(ex3, typeof) # symbol  double  language
rm(ex3)
}
\keyword{programming}
\keyword{dplot}