setGeneric {methods}R Documentation

Define a New Generic Function

Description

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.

Usage

setGeneric(name, def= , group=list(), valueClass=character(), where= ,
           package= , signature= , useAsDefault= , genericFunction= )

setGroupGeneric(name, def= , group=list(), valueClass=character(),
                knownMembers=list(), package= , where= )

Arguments

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).
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 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 def is not the default method; use argument useAsDefault if you want to specify the default separately.
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.
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.
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).
where Where to store the resulting initial methods definition, and possibly the generic function; by default, stored into the top-level environment.
signature Optionally, the signature of arguments in the function that can be used in methods for this generic. By default, all arguments other than ... can be used. The signature argument can prohibit methods from using some arguments. The argument, if provided, is a vector of formal argument names.
genericFunction The object to be used as a (nonstandard) generic function definition. Supply this explicitly only if you know what you are doing!
useAsDefault Override the usual choice of default argument (an existing non-generic function or no default if there is no such function). Argument useAsDefault can be supplied, either as a function to use for the default, or as a logical value. 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. TRUE says to use the existing function as default, unconditionally (hardly ever needed as an explicit argument). See the section on details.
knownMembers (For 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.

Details

The 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 name is already an ordinary non-generic function, and you now want to turn this function into a generic. In this case you will most often supply only name. The existing function becomes the default method, and the special group and 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, standardGeneric(name) where name is the quoted name of the generic function.

When calling setGeneric in this form, you would normally supply the def argument as a function of this form. If not told otherwise, 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 useAsDefault. That argument can be the function you want to be the default method. You can supply the argument as 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 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 library. These are dispatched from C code for efficiency and are not to be redefined in any case.

As mentioned, the body of a generic function usually does nothing except for dispatching methods by a call to 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 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 not call 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 valueClass is supplied, it should be a vector of class names; the value returned by a method is then required to satisfy 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 setGroupGeneric function behaves like 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.

Value

The setGeneric function exists for its side effect: saving the generic function to allow methods to be specified later. It returns name.

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 S language definition. Primitive functions are eligible to have methods, but are handled differently by setGeneric and setGroupGeneric. A call to setGeneric for a primitive function does not create a new definition of the function, and the call is allowed only to “turn on” methods for that function.

A call to setGeneric for a primitive causes the evaluator to look for methods for that generic; a call to setGroupGeneric for any of the groups that include primitives ("Arith", "Logic", "Compare", "Ops", "Math", "Math2", "Summary", and "Complex") does the same for each of the functions in that group.

You usually only need to use either function if the methods are being defined only for the group generic. Defining a method for a primitive function, say "+", by a call to setMethod turns on method dispatch for that function. But in R defining a method for the corresponding group generic, "Arith", does not currently turn on method dispatch (for efficiency reasons). If there are no non-group methods for the functions, you have two choices.

You can turn on method dispatch for all the functions in the group by calling setGroupGeneric("Arith"), or you can turn on method dispatch for only some of the functions by calling setGeneric("+"), etc. Note that in either case you should give the name of the generic function as the only argument.

References

The R package methods implements, with a few exceptions, the programming interface for classes and methods in the book 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 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 http://developer.r-project.org/methodsPackage.html and the pointers from that page.

See Also

Methods for a discussion of other functions to specify and manipulate the methods of generic functions.

Examples



###   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(rnorm(10)))

t1 - 100

1/t1



[Package methods version 2.1.0 Index]