\Header{isSealedMethod}{Check for a Sealed Method or Class}
\alias{isSealedClass}{isSealedMethod}
\keyword{programming}{isSealedMethod}
\keyword{classes}{isSealedMethod}
\keyword{classes}{isSealedMethod}
\keyword{methods}{isSealedMethod}
\begin{Description}\relax
These functions check for either a method or a class that has been
\dQuote{sealed} when it was defined, and which therefore cannot be
re-defined.
\end{Description}
\begin{Usage}
\begin{verbatim}
isSealedMethod(f, signature, fdef, where)
isSealedClass(Class, where)
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{f}] The quoted name of the generic function. 
\item[\code{signature}] The class names in the method's signature, as
they would be supplied to \code{\Link{setMethod}}. 
\item[\code{fdef}] Optional, and usually omitted:  the generic function
definition for \code{f}. 
\item[\code{Class}] The quoted name of the class.
\item[\code{where}] where to search for the method or class definition.  By
default, searches from the top environment of the call to
\code{isSealedMethod} or \code{isSealedClass}, typically the
global environment or the namespace of a package containing a call
to one of the functions.
\end{ldescription}
\end{Arguments}
\begin{Details}\relax
In the R implementation of classes and methods, it is possible to
seal the definition of either a class or a method.  The basic
classes (numeric and other types of vectors, matrix and array data)
are sealed.  So also are the methods for the primitive functions on
those data types.  The effect is that programmers cannot re-define
the meaning of these basic data types and computations.  More
precisely, for primitive functions that depend on only one data
argument, methods cannot be specified for basic classes.  For
functions (such as the arithmetic operators) that depend on two
arguments, methods can be specified if \emph{one} of those arguments
is a basic class, but not if both are.

Programmers can seal other class and method definitions by using the
\code{sealed} argument to \code{\Link{setClass}} or \code{\Link{setMethod}}.
\end{Details}
\begin{Value}
The functions return \code{FALSE} if the method or class is not
sealed (including the case that it is not defined); \code{TRUE} if
it is.
\end{Value}
\begin{References}\relax
The R package \pkg{methods} implements, with a few exceptions, the
programming interface for classes and methods in the book
\emph{Programming with Data} (John M. Chambers, Springer, 1998), in
particular sections 1.6, 2.7, 2.8, and chapters 7 and 8.

While the programming interface for the \pkg{methods} package follows
the reference, the R software is an original implementation, so
details in the reference that reflect the S4 implementation may appear
differently in R.  Also, there are extensions to the programming
interface developed more recently than the reference.  For a
discussion of details and ongoing development, see the web page 
\url{http://developer.r-project.org/methodsPackage.html} and the
pointers from that page.
\end{References}
\begin{Examples}
\begin{ExampleCode}
## these are both TRUE
isSealedMethod("+", c("numeric", "character"))
isSealedClass("matrix")

setClass("track",
            representation(x="numeric", y="numeric"))
## but this is FALSE
isSealedClass("track")
## and so is this
isSealedClass("A Name for an undefined Class")
## and so are these, because only one of the two arguments is basic
isSealedMethod("+", c("track", "numeric"))
isSealedMethod("+", c("numeric", "track"))


\end{ExampleCode}
\end{Examples}

