| setGeneric {methods} | R Documentation |
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.
setGeneric(name, def, group=NULL, valueClass=NULL, where=1, doAssign,
myDispatch = FALSE, useAsDefault)
setGroupGeneric(name, def, group=NULL, valueClass=NULL, knownMembers, where=1)
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 Note that |
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 |
myDispatch |
Normally, the body of a generic function does nothing except to
dispatch methods (via a call to |
useAsDefault |
Override the usual choice of default argument (an existing
non-generic function or no default if there is no such function).
Argument |
knownMembers |
(For |
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.
The setGeneric function exists for its side effect: saving the
generic function to allow methods to be specified later. It returns
name.
John Chambers
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.
Methods for a discussion of other functions to specify
and manipulate the methods of generic functions.