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=NULL, valueClass=NULL, where=1, doAssign,
               myDispatch = FALSE, useAsDefault)

    setGroupGeneric(name, def, group=NULL, valueClass=NULL, knownMembers, where=1)

### 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`. The body will be set to the standard dispatch mechanism, unless you want to do something different (use argument `myDispatch` in this case. 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.
`where` | Where to store the resulting initial methods definition, and possibly the generic function; by default, stored into the global environment.
`doAssign` | You can supply `doAssign` as `TRUE` to force assigning a generic function even if one already exists. See the details section. The rule of not assigning generics for primitives cannot be overriden by setting `doAssign` to `TRUE`.
`myDispatch` | Normally, the body of a generic function does nothing except to dispatch methods (via a call to `standardGeneric`). The evaluation model, however, allows the generic to do any other computation it wants, it it calls `standardGeneric` at some point. Otherwise, `setGeneric` will ignore the body of the supplied generic unless `myDispatch` is set to `TRUE`. See the details section.
`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` does not supply one. Remember, though, you can also just assign the default you want (even one that generates an error) rather than relying on the prior situation.

Usually, calling `setGeneric` if there is already a generic function of this name has no effect. If you want to force a new definition, supply `doAssign = TRUE` (but it would be cleaner in most cases to remove the old generic before creating the new one; see `removeGeneric`). There is one absolute restriction: you cannot create an explicit generic for the primitive functions in the base library. These are dispatched from C code for efficiency and, also, are implicitly 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 function does not call `standardGeneric` you are in trouble, because none of the methods for the function will ever be dispatched. The computations in `setGeneric` look for such a call, and normally ignore the body of a `def` argument that has no such call. If you really need to supply such a definition (perhaps because you call something else that calls `standardGeneric`), you can force the use of `def` by supplying `myDispatch = TRUE`. By default, the generic function can return any object.

If `valueClass` is supplied, it can be either a vector of class names or an unevaluated expression. If an unevaluated expression is supplied (e.g., by using `quote`), the expression will be evaluated in the frame of the call to the generic function, after the method is dispatched. Therefore, the expression can involve any argument or locally defined object in that frame. The test is done in the form `is(object, class)`, so that a value from a class that extends a supplied value class will be accepted. An empty (i.e., zero length) vector of classes means anything is allowed (but it would be clearer programming to have an expression that evaluated to `"ANY"`).

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`.

### Author(s)

John Chambers

### References

The web page 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 Programming with Data (John M. Chambers, Springer, 1998). See this book for further details and examples.

### See Also

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

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