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\name{setGeneric}
\alias{setGeneric}
\title{Define a New Generic Function}
\description{
 Create a new generic function of the given name, for which formal methods can
 then be defined.
}
\usage{
setGeneric(name, def, group=NULL, valueClass=NULL, where=1)
}
\arguments{
  \item{name}{ The character string name of the generic function.  In
    the simplest and most common case, a function of this name is
    already defined.  The existing function may be non-generic or
    already a generic (see the details).
  }
  \item{def}{ An optional function object, defining the generic.  This
    argument is usually only needed (and is then required) if there is
    no current function of this name. In that case, the formal arguments
    and default values for the generic are taken from \code{def}.  See
    also argument \code{useAsDefault}.
  }
  \item{group}{ Optionally, a character string giving the group of
    generic functions to which this function belongs.  Methods can be
    defined for the corresponding group generic, and these will then
    define methods for this specific generic function, if no method has
    been explicitly defined for the corresponding signature.  See the
    references for more discussion.
  }
  \item{valueClass}{ An optional character string defining the class to
    asserted to be given to the value returned by this generic
    function. By default, the generic function can return any object.
    (At the moment, the assertion supplied in this argument is not
    enforced or checked.)
   }
  \item{where}{ Where to store the resulting generic function; by
    default, stored into the global environment.
  }
  \item{useAsDefault}{ Optional instructions as to the default method.
    Useful values include: \code{TRUE}, forcing argument \code{def} to be
    used as the default method; \code{NULL}, forcing the new generic to
    have no default method (calls that don't match a defined method
    generate an error in this case); or a function object, which then
    becomes the default method.

    See the Details section for more information on how generic
    functions are set up, and on the rules for picking the default
    method.
  }
}
\details{
  The \code{setGeneric} function is called to initialize a generic
  function in an environment (usually the global environment), as
  prepartion for defining some methods for that function.

  The simplest and most common situation is that \code{name} is already
  a function, either an ordinary non-generic function or else a generic
  function in another environment.  In the second case, essentially all
  you will be doing is to make a copy of the existing generic, without
  its methods, in the current environment.  \emph{Only} the \code{name}
  argument will be used.  The new generic \emph{must} agree with the
  current one in essentially everything except for the methods defined.

  The other simple and common situation is that you already have a
  non-generic function (probably in the same place you plan to save the
  generic), and now want to turn this function into a generic.  In this
  case, too, you will most often supply only \code{name}.  The existing
  function becomes the default method, and the special \code{group} and
  \code{valueClass} properties remain unspecified.
}
\value{
  The \code{setGeneric} function exists for its side effect:  saving the
  generic function to allow methods to be specified later.  It returns \code{name}.
}
\references{
The web page \url{http://www.omegahat.org/RSMethods/index.html} is the primary documentation.

The functions in this package emulate the facility for classes and methods described in
\emph{Programming with Data}, (John M. Chambers, Springer, 1998).  See this book
for further details and examples.
}

\author{
  John Chambers
}

\seealso{
  \code{\link{Methods}} for a discussion of other functions to
  specify and manipulate the methods of generic functions.
}

\keyword{ programming }
\keyword{ methods }