R: Create a Generic Version of a Function

setGeneric {methods} | R Documentation

## Create a Generic Version of a Function

### Description

Create a generic version of the named function so that methods may be defined for it. A call to `setMethod` will call `setGeneric` automatically if applied to a non-generic function.

An explicit call to `setGeneric` is usually not required, but doesn't hurt and makes explicit that methods are being defined for a non-generic function.

Standard calls will be of the form:

`setGeneric(name)`

where `name` specifies an existing function, possibly in another package. An alternative when creating a new generic function in this package is:

`setGeneric(name, def)`

where the function definition `def` specifies the formal arguments and becomes the default method.

### Usage

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

### Arguments

`name` | The character string name of the generic function.
`def` | An optional function object, defining the non-generic version, to become the default method. This is equivalent in effect to assigning `def` as the function and then using the one-argument call to `setGeneric`. The following arguments are specialized, optionally used when creating a new generic function with non-standard features. They should not be used when the non-generic is in another package.
`group` | The name of the group generic function to which this function belongs. See Methods_Details for details of group generic functions in method selection and S4groupGeneric for existing groups.
`valueClass` | A character vector specifying one or more class names. The value returned by the generic function must have (or extend) this class, or one of the classes; otherwise, an error is generated.
`signature` | The vector of names from among the formal arguments to the function, that will be allowed in the signature of methods for this function, in calls to `setMethod`. By default and usually, this will be all formal arguments except `...`. A non-standard signature for the generic function may be used to exclude arguments that take advantage of lazy evaluation; in particular, if the argument may not be evaluated then it cannot be part of the signature. While `...` cannot be used as part of a general signature, it is possible to have this as the only element of the signature. Methods will then be selected if their signature matches all the `...` arguments. See the documentation for topic dotsMethods for details. It is not possible to mix `...` and other arguments in the signature. It's usually a mistake to omit arguments from the signature in the belief that this improves efficiency. For method selection, the arguments that are used in the signatures for the methods are what counts, and then only seriously on the first call to the function with that combination of classes.
`simpleInheritanceOnly` | Supply this argument as `TRUE` to require that methods selected be inherited through simple inheritance only; that is, from superclasses specified in the `contains=` argument to `setClass`, or by simple inheritance to a class union or other virtual class. Generic functions should require simple inheritance if they need to be assured that they get the complete original object, not one that has been transformed. Examples of functions requiring simple inheritance are `initialize`, because by definition it must return an object from the same class as its argument, and `show`, because it claims to give a full description of the object provided as its argument.
`useAsDefault` | Override the usual default method mechanism. Only relevant when defining a nonstandard generic function. See the section ‘Specialized Local Generics’. The remaining arguments are obsolete for normal applications.
`package` | The name of the package with which this function is associated. Should be determined automatically from the non-generic version.
`where` | Where to store the resulting objects as side effects. The default, to store in the package's namespace, is the only safe choice.
`genericFunction` | Obsolete.

### Value

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

### Basic Use

The `setGeneric` function is called to initialize a generic function as preparation for defining some methods for that function.

The simplest and most common situation is that `name` specifies an existing function, usually in another package. You now want to define methods for this function. In this case you should supply only `name`, for example:

`setGeneric("colSums")`

There must be an existing function of this name (in this case in package `"base"`). The non-generic function can be in the same package as the call, typically the case when you are creating a new function plus methods for it. When the function is in another package, it must be available by name, for example through an `importFrom()` directive in this package's `NAMESPACE` file. Not required for functions in `"base"`, which are implicitly imported.

A generic version of the function will be created in the current package. The existing function becomes the default method, and the package slot of the new generic function is set to the location of the original function (`"base"` in the example).

Two special types of non-generic should be noted. Functions that dispatch S3 methods by calling `UseMethod` are ordinary functions, not objects from the `"genericFunction"` class. They are made generic like any other function, but some special considerations apply to ensure that S4 and S3 method dispatch is consistent (see Methods_for_S3).

Primitive functions are handled in C code and don't exist as normal functions. A call to `setGeneric` is allowed in the simple form, but no actual generic function object is created. Method dispatch will take place in the C code. See the section on Primitive Functions for more details.

It's an important feature that the identical generic function definition is created in every package that uses the same `setGeneric()` call. When any of these packages is loaded into an R session, this function will be added to a table of generic functions, and will contain a methods table of all the available methods for the function.

Calling `setGeneric()` is not strictly necessary before calling `setMethod()`. If the function specified in the call to `setMethod` is not generic, `setMethod` will execute the call to `setGeneric` itself. In the case that the non-generic is in another package, does not dispatch S3 methods and is not a primitive, a message is printed noting the creation of the generic function the first time `setMethod` is called.

The second common use of `setGeneric()` is to create a new generic function, unrelated to any existing function. See the `asRObject()` example below. This case can be handled just like the previous examples, with only the difference that the non-generic function exists in the current package. Again, the non-generic version becomes the default method. For clarity it's best for the assignment to immediately precede the call to `setGeneric()` in the source code.

Exactly the same result can be obtained by supplying the default as the `def` argument instead of assigning it. In some applications, there will be no completely general default method. While there is a special mechanism for this (see the ‘Specialized Local Generics’ section), the recommendation is to provide a default method that signals an error, but with a message that explains as clearly as you can why a non-default method is needed.

### Specialized Local Generics

The great majority of calls to `setGeneric()` should either have one argument to ensure that an existing function can have methods, or arguments `name` and `def` to create a new generic function and optionally a default method.

It is possible to create generic functions with nonstandard signatures, or functions that do additional computations besides method dispatch or that belong to a group of generic functions.

None of these mechanisms should be used with a non-generic function from a different package, because the result is to create a generic function that may not be consistent from one package to another. When any such options are used, the new generic function will be assigned with a package slot set to the current package, not the one in which the non-generic version of the function is found.

There is a mechanism to define a specialized generic version of a non-generic function, the `implicitGeneric` construction. This defines the generic version, but then reverts the function to it non-generic form, saving the implicit generic in a table to be activated when methods are defined. However, the mechanism can only legitimately be used either for a non-generic in the same package or by the `"methods"` package itself. And in the first case, there is no compelling reason not to simply make the function generic, with the non-generic as the default method. See `implicitGeneric` for details.

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. See the example `"authorNames"` below.

In this case, the `def` argument will define the nonstandard generic, not the default method. An existing non-generic of the same name and calling sequence should be pre-assigned. It will become the default method, as usual. (An alternative is the `useAsDefault` argument.)

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.

If the `def` argument calls `standardGeneric()` (with or without additional computations) and there is no existing non-generic version of the function, the generic is created without a default method. This is not usually a good idea: better to have a default method that signals an error with a message explaining why the default case is not defined.

A new generic function can be created belonging to an existing group by including the `group` argument. The argument list of the new generic must agree with that of the group. See `setGroupGeneric` for defining a new group generic. For the role of group generics in dispatching methods, see GroupGenericFunctions and section 10.5 of the second reference.

### 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 `implicitGeneric`), and become generic as soon as methods (including group methods) are defined on them. Others cannot be made generic.

Calling `setGeneric()` for the primitive functions in the base package differs in that it does not, in fact, generate an explicit generic function. Methods for primitives are selected and dispatched from the internal C code, to satisfy concerns for efficiency. The same is true for a few non-primitive functions that dispatch internally. These include `unlist` and `as.vector`.

Note, that the implementation restrict methods for primitive functions to signatures in which at least one of the classes in the signature is a formal S4 class. Otherwise the internal C code will not look for methods. This is a desirable restriction in principle, since optional packages should not be allowed to change the behavior of basic R computations on existing data types.

To see the generic version of a primitive function, use `getGeneric(name)`. The function `isGeneric` will tell you whether methods are defined for the function in the current session.

Note that S4 methods can only be set on those primitives which are ‘internal generic’, plus `%*%`.

### References

Chambers, John M. (2016) Extending R, Chapman & Hall. (Chapters 9 and 10.)

Chambers, John M. (2008) Software for Data Analysis: Programming with R Springer. (Section 10.5 for some details.)

### See Also

`Methods_Details` and the links there for a general discussion, `dotsMethods` for methods that dispatch on `...`, and `setMethod` for method definitions.

### Examples

    ## Specify that this package will define methods for plot()
    setGeneric("plot")

    ## create a new generic function, with a default method
    setGeneric("props", function(object) attributes(object))

    ###   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
          })

    ## the asRObject generic function, from package XR
    ## Its default method just returns object
    ## See the reference, Chapter 12 for methods

    setGeneric("asRObject", function(object, evaluator) {
            object
    })

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