\Header{substitute}{Substituting and Quoting Expressions}
\alias{quote}{substitute}
\keyword{programming}{substitute}
\keyword{data}{substitute}
\begin{Description}\relax
\code{substitute} returns the parse tree for the (unevaluated)
expression \code{expr}, substituting any variables bound in
\code{env}.

\code{quote} simply returns its argument. The argument is not evaluated
and can be any R expression.
\end{Description}
\begin{Usage}
\begin{verbatim}
substitute(expr, env)
quote(expr)
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{expr}] Any syntactically valid \R{} expression
\item[\code{env}] An environment or a list object.  Defaults to the
current evaluation environment.
\end{ldescription}
\end{Arguments}
\begin{Details}\relax
The typical use of \code{substitute} is to create informative labels
for data sets and plots.
The \code{myplot} example below shows a simple use of this facility.
It uses the functions \code{\Link{deparse}} and \code{substitute}
to create labels for a plot which are character string versions
of the actual arguments to the function \code{myplot}.

Substitution takes place by examining each component of the parse tree
as follows: If it is not a bound symbol in \code{env}, it is
unchanged. If it is a promise object, i.e., a formal argument to a
function or explicitly created using \code{\Link{delay}()}, the expression
slot of the promise replaces the symbol. If it is an ordinary
variable, its value is substituted, unless \code{env} is
\code{\Link{.GlobalEnv}} in which case the symbol is left unchanged.
\end{Details}
\begin{Value}
The \code{\Link{mode}} of the result is generally \code{"call"} but
may in principle be any type. In particular, single-variable
expressions have mode \code{"name"} and constants have the
appropriate base mode.
\end{Value}
\begin{Note}\relax
Substitute works on a purely lexical basis. There is no guarantee
that the resulting expression makes any sense.

Substituting and quoting often causes confusion when the argument is
\code{expression(...)}. The result is a call to the
\code{\Link{expression}} constructor function and needs to be evaluated
with \code{\Link{eval}} to give the actual expression object.
\end{Note}
\begin{References}\relax
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988)
\emph{The New S Language}.
Wadsworth \& Brooks/Cole.
\end{References}
\begin{SeeAlso}\relax
\code{\Link{missing}} for argument \dQuote{missingness},
\code{\Link{bquote}} for partial substitution,
\code{\Link{sQuote}} and \code{\Link{dQuote}} for adding quotation
marks to strings.
\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}
(s.e <- substitute(expression(a + b), list(a = 1)))  #> expression(1 + b)
(s.s <- substitute( a + b,            list(a = 1)))  #> 1 + b
c(mode(s.e), typeof(s.e)) #  "call", "language"
c(mode(s.s), typeof(s.s)) #   (the same)
# but:
(e.s.e <- eval(s.e))          #>  expression(1 + b)
c(mode(e.s.e), typeof(e.s.e)) #  "expression", "expression"

substitute(x <- x + 1, list(x=1)) # nonsense

myplot <- function(x, y)
    plot(x, y, xlab=deparse(substitute(x)),
         ylab=deparse(substitute(y)))

## Simple examples about lazy evaluation, etc:

f1 <- function(x, y = x)             { x <- x + 1; y }
s1 <- function(x, y = substitute(x)) { x <- x + 1; y }
s2 <- function(x, y) { if(missing(y)) y <- substitute(x); x <- x + 1; y }
a <- 10
f1(a)# 11
s1(a)# 11
s2(a)# a
typeof(s2(a))# "symbol"
\end{ExampleCode}
\end{Examples}

