\HeaderA{setGeneric}{Define a New Generic Function}{setGeneric}
\aliasA{setGroupGeneric}{setGeneric}{setGroupGeneric}
\keyword{programming}{setGeneric}
\keyword{methods}{setGeneric}
\begin{Description}\relax
Create a new generic function of the given name, for which formal
methods can then be defined.  Typically, an existing non-generic
function becomes the default method, but there is much optional
control.  See the details section.
\end{Description}
\begin{Usage}
\begin{verbatim}
setGeneric(name, def= , group=list(), valueClass=character(),
           where= , package= , signature= , useAsDefault= ,
           genericFunction= )

setGroupGeneric(name, def= , group=list(), valueClass=character(),
                knownMembers=list(), package= , where= )
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{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[\code{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}.  You
can also supply this argument if you want the generic function to
do something other than just dispatch methods (an advanced topic
best left alone unless you are sure you want it).

Note that \code{def} is \emph{not} the default method; use argument
\code{useAsDefault} if you want to specify the default separately.

\item[\code{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[\code{valueClass}] An optional character vector or unevaluated
expression.  The value returned by the generic function must
have (or extend) this class, or one of the classes; otherwise,
an error is generated.  See the
details section for supplying an expression.

\item[\code{package}] The name of the package with which this function is
associated.  Usually determined automatically (as the package
containing the non-generic version if there is one, or else the
package where this generic is to be saved).

\item[\code{where}] Where to store the resulting initial methods definition,
and possibly the generic function; by
default, stored into the top-level environment.

\item[\code{signature}] Optionally, the signature of arguments in the function that can
be used in methods for this generic.  By default, all arguments
other than \code{...} can be used.  The signature argument can
prohibit methods from using some arguments.  The argument, if
provided, is a vector of formal argument names.

\item[\code{genericFunction}] The object to be used as a (nonstandard)
generic function definition.  Supply this explicitly \emph{only} if
you know what you are doing!

\item[\code{useAsDefault}] Override the usual choice of default argument (an existing
non-generic function or no default if there is no such function).
Argument \code{useAsDefault} can be supplied,
either as a function to use for the default, or as a logical value.
\code{FALSE} says not to have a default method at all, so that an
error occurs if there is not an explicit or inherited method for a
call.
\code{TRUE} says to use the existing function as default,
unconditionally (hardly ever needed as an explicit argument).
See the section on details.

\item[\code{knownMembers}] (For \code{setGroupGeneric} only.)  The names of functions that are
known to be members of this group.  This information is used to
reset cached definitions of the member generics when information
about the group generic is changed.

\end{ldescription}
\end{Arguments}
\begin{Details}\relax
The \code{setGeneric} function is called to initialize a generic
function in an environment (usually the global environment), as
preparation for defining some methods for that function.

The simplest and most common situation is that \code{name} is already
an ordinary non-generic non-primitive function, and you now want to turn this
function into a generic.  In this case 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.

A second situation is that you want to create a new, generic
function, unrelated to any existing function.  In this case, you
need to supply a skeleton of the function definition, to define the
arguments for the function.  The body of a generic function is
usually a standard form, \code{standardGeneric(name)} where
\code{name} is the quoted name of the generic function.

When calling \code{setGeneric} in this form, you would normally
supply the \code{def} argument as a function of this form.  If not
told otherwise, \code{setGeneric} will try to find a non-generic
version of the function to use as a default.  If you don't want this
to happen, supply the argument \code{useAsDefault}.  That argument
can be the function you want to be the default method.  You can supply
the argument as \code{FALSE} to force no default (i.e., to cause an error if
there is not direct or inherited method on call to the function).

The same no-default situation occurs if there is no non-generic form
of the function, and \code{useAsDefault=FALSE}.  Remember, though, you
can also just assign the default you want (even one that generates an
error) rather than relying on the prior situation.

You cannot (and never need to) create an explicit generic for the
primitive functions in the base package.  Those which are generic are
dispatched from C code for efficiency and the others cannot be made
generic.  If you want to define a generic with the same name as a
primitive but unrelated to it, you must specify both \code{def} and
\code{useAsDefault} (as a function or \code{FALSE}).

As mentioned, the body of a generic function usually does nothing
except for dispatching methods by a call to \code{standardGeneric}.
Under some circumstances you might just want to do some additional
computation in the generic function itself.  As long as your
function eventually calls \code{standardGeneric} that is permissible
(though perhaps not a good idea, in that it makes the behavior of
your function different from the usual S model).  If your explicit
definition of the generic function
does \emph{not} call \code{standardGeneric} you are in trouble,
because none of the methods for the function will ever be
dispatched.

By default, the generic function can return any object.  If
\code{valueClass} is supplied, it should be a vector of class names;
the value returned by a method is then required to satisfy
\code{is(object, Class)} for one of the specified classes.  An empty
(i.e., zero length) vector of classes means anything is allowed.  Note
that more complicated requirements on the result can be specified
explicitly, by defining a non-standard generic function.

The \code{setGroupGeneric} function behaves like \code{setGeneric}
except that it constructs a group generic function, differing in two
ways from an ordinary generic function.  First, this function cannot
be called directly, and the body of the function created will contain
a stop call with this information.  Second, the group generic
function contains information about the known members of the group,
used to keep the members up to date when the group definition
changes, through changes in the search list or direct specification
of methods, etc.
\end{Details}
\begin{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}.
\end{Value}
\begin{Section}{Generic Functions and Primitive Functions}
A number of the basic \R{} functions are specially implemented as
primitive functions, to be evaluated directly in the underlying C
code rather than by evaluating an \R{} language definition.  Most have
implicit generics (see \code{\LinkA{implicitGeneric}{implicitGeneric}}), and become
generic as soon as methods (including group methods) are defined on
them.  Others cannot be made generic.

Currently those with implicit generics are those which are
\LinkA{internal generic}{internal generic} (which includes all members of the group
generics) and \code{\LinkA{\%*\%}{.Rpcent.*.Rpcent.}}.
\end{Section}
\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 see \code{?\LinkA{Methods}{Methods}}
and the links from that documentation.
\end{References}
\begin{SeeAlso}\relax
\code{\LinkA{Methods}{Methods}} for a discussion of other functions to specify
and manipulate the methods of generic functions.
\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}


###   A non-standard generic function.  It insists that the methods
###   return a non-empty character vector (a stronger requirement than
###    valueClass = "character" in the call to setGeneric)

setGeneric("authorNames",
    function(text) {
      value <- standardGeneric("authorNames")
      if(!(is(value, "character") && any(nchar(value)>0)))
        stop("authorNames methods must return non-empty strings")
      value
      })



## An example of group generic methods, using the class
## "track"; see the documentation of setClass for its definition

## define a method for the Arith group

setMethod("Arith", c("track", "numeric"),
 function(e1, e2) {
  e1@y <- callGeneric(e1@y , e2)
  e1
})

setMethod("Arith", c("numeric", "track"),
 function(e1, e2) {
  e2@y <- callGeneric(e1, e2@y)
  e2
})

## now arithmetic operators  will dispatch methods:

t1 <- new("track", x=1:10, y=sort(stats::rnorm(10)))

t1 - 100
1/t1


\end{ExampleCode}
\end{Examples}

