The function selectMethod() returns the method that
would be selected for a call to function f if the arguments had
classes as specified by signature. Failing to find a method
is an error, unless argument optional = TRUE, in which case
NULL is returned.
The function findMethod() returns a list of
environments that contain a method for the specified function and signature; by
default, these are a subset of the packages in the current search
list. See section “Using findMethod()” for details.
The function getMethod() returns the method corresponding to the
function and signature supplied similarly to selectMethod, but
without using inheritance or group generics.
The functions hasMethod() and
existsMethod() test whether selectMethod() or
getMethod(), respectively, finds a matching method.
selectMethod(f, signature, optional = FALSE, useInherited =,
mlist = , fdef = , verbose = , doCache = ) findMethod(f, signature, where)
getMethod(f, signature = character(), where, optional = FALSE,
mlist, fdef)
existsMethod(f, signature = character(), where)
hasMethod(f, signature = character(), where)
a generic function or the character-string name of one.
the signature of classes to match to the arguments
of f. See the details below.
the environment in which to look for the
method(s). By default, if the call comes from the command line, the table of methods defined in the generic
function itself is used, except for findMethod (see the
section below).
if the selection in selectMethod does not find
a valid method an error is generated, unless optional is
TRUE, in which case the value returned is NULL.
optional arguments
to getMethod and selectMethod for internal use. Avoid
these: some will work as expected and others will not, and none of
them is required for normal use of the functions. But see the
section “Methods for as()” for nonstandard inheritance.
The call to selectMethod or getMethod returns the selected method, if
one is found.
(This class extends function, so you can use the result
directly as a function if that is what you want.)
Otherwise an error is thrown if optional is FALSE and NULL is returned if
optional is TRUE.
The returned method object is a
'>MethodDefinition object, except that the default method for a primitive function is required to be the primitive itself.
Note therefore that the only reliable test that the search failed is
is.null().
The returned value of findMethod is a list of
environments in which a corresponding method was found; that is, a
table of methods including the one specified.
As its name suggests, this function is intended to behave like
find, which produces a list of the packages on the
current search list which have, and have exported, the object named.
That's what findMethod does also, by default. The
“exported” part in this case means that the package's namespace
has an exportMethods directive for this generic function.
An important distinction is that the absence of such a directive does not prevent methods from the package from being called once the package is loaded. Otherwise, the code in the package could not use un-exported methods.
So, if your question is whether loading package thisPkg will define a
method for this function and signature, you need to ask that question
about the namespace of the package:
findMethod(f, signature, where = asNamespace("thisPkg"))
If the package did not export the method, attaching it and calling
findMethod with no where argument will not find the
method.
Notice also that the length of the signature must be what the
corresponding package used. If thisPkg had only methods for
one argument, only length-1 signatures will match (no trailing
"ANY"), even if another currently loaded package had signatures
with more arguments.
The function setAs allows packages to define methods for
coercing one class of objects to another class. This works internally
by defining methods for the generic function coerce(from,
to),
which can not be called directly.
The R evaluator selects
methods for this purpose using a different form of inheritance. While
methods can be inherited for the object being coerced, they cannot
inherit for the target class, since the result would not be a valid
object from that class.
If you want to
examine the selection procedure, you must supply the optional argument
useInherited = c(TRUE, FALSE) to selectMethod.
The signature argument specifies classes, corresponding to
formal arguments of the generic function; to be precise, to the
signature slot of the generic function object. The argument
may be a vector of strings identifying classes, and may be named or
not. Names, if supplied, match the names of those formal arguments
included in the signature of the generic. That signature is normally
all the arguments except …. However, generic functions can be
specified with only a subset of the arguments permitted, or with the
signature taking the arguments in a different order.
It's a good idea to name the arguments in the signature to avoid
confusion, if you're dealing with a generic that does something
special with its signature. In any case, the elements of the
signature are matched to the formal signature by the same rules used
in matching arguments in function calls (see
match.call).
The strings in the signature may be class names, "missing" or
"ANY". See Methods_Details for the meaning of these in method
selection. Arguments not supplied in the signature implicitly
correspond to class "ANY"; in particular, giving an empty
signature means to look for the default method.
A call to getMethod returns the method for a particular
function and signature. The search for the method makes no use of
inheritance.
The function selectMethod also looks for a method given the
function and signature, but makes full use of the method dispatch
mechanism; i.e., inherited methods and group generics are taken into
account just as they would be in dispatching a method for the
corresponding signature, with the one exception that conditional
inheritance is not used. Like getMethod, selectMethod
returns NULL or generates an error if
the method is not found, depending on the argument optional.
Both selectMethod and getMethod will normally use the
current version of the generic function in the R session, which has a
table of the methods obtained from all the packages loaded in the
session. Optional arguments can cause a search for the generic function from a
specified environment, but this is rarely a useful idea. In contrast,
findMethod has a different default and the optional
where= argument may be needed. See the section “Using
findMethod()”.
The functions existsMethod and hasMethod return
TRUE or FALSE according to whether a method is found,
the first corresponding to getMethod (no inheritance) and the
second to selectMethod.
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 10.6 for some details of method selection.)
Methods_Details for the details of method
selection; GenericFunctions for other functions
manipulating methods and generic function objects;
'>MethodDefinition for the class that represents
method definitions.
# NOT RUN {
testFun <- function(x)x
setGeneric("testFun")
setMethod("testFun", "numeric", function(x)x+1)
hasMethod("testFun", "numeric") # TRUE
hasMethod("testFun", "integer") #TRUE, inherited
existsMethod("testFun", "integer") #FALSE
hasMethod("testFun") # TRUE, default method
hasMethod("testFun", "ANY")
# }
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