| implicitGeneric {methods} | R Documentation |
The implicit generic mechanism stores generic versions of functions in a table in a package. The package does not want the current version of the function to be a generic, however, and retains the non-generic version.
When a call to setMethod or
setGeneric creates a generic version for one of these
functions, the object in the table is used.
This mechanism is only needed if special arguments were used to
create the generic; e.g., the signature or the valueClass
options.
Function implicitGeneric() returns the implicit
generic version, setGenericImplicit() turns a generic implicit,
prohibitGeneric() prevents your function from being made
generic, and registerImplicitGenerics() saves a set of implicit
generic definitions in the cached table of the current session.
implicitGeneric(name, where, generic) setGenericImplicit(name, where, restore = TRUE) prohibitGeneric(name, where) registerImplicitGenerics(what, where)
name |
Character string name of the function. |
where |
Package or environment in which to register the implicit generics. When using the functions from the top level of your own package source, this argument should be omitted. |
generic |
Obsolete, and likely to be deprecated. |
restore |
Should the non-generic version of the function be restored?. |
what |
Optional table of the implicit generics to register, but nearly always omitted, when it defaults to a standard metadata name. |
Multiple packages may define methods for the same function, to apply
to classes defined in that package. Arithmetic and other operators,
plot() and many other basic computations are typical
examples. It's essential that all such packages write methods for
the same definition of the generic function. So long as that
generic uses the default choice for signature and other parameters,
nothing needs to be done.
If the generic has special properties, these need to be ensured for
all packages creating methods for it. The simplest solution is just
to make the function generic in the package that originally owned
it. If for some reason the owner(s) of that package are unwilling
to do this, the alternative is to define the correct generic,
save it in a special table and restore the non-generic version by
calling setGenericImplicit.
Note that the package containing the function can define methods for the implicit generic as well; when the implicit generic is made a real generic, those methods will be included.
The usual reason for having a
non-default implicit generic is to provide a non-default signature,
and the usual reason for that is to allow lazy evaluation of
some arguments. All arguments in the signature of a
generic function must be evaluated at the time the function needs to
select a method.
In the base function with() in the example below, evaluation of the argument
expr must be delayed; therefore, it is excluded from the signature.
If you want to completely prohibit anyone from turning your function
into a generic, call prohibitGeneric().
Function implicitGeneric() returns the implicit generic
version of the named function. If there is no table of these or if
this function is not in the table, the result of a simple call
setGeneric(name) is returned.
Function implicitGeneric() returns the implicit generic
definition (and caches that definition the first time if it has to
construct it).
The other functions exist for their side effect and return nothing useful.
Implicit generic versions exist for some functions in the packages supplied in the distribution of R itself. These are stored in the ‘methods’ package itself and will always be available.
As emphasized repeatedly in the documentation,
setGeneric() calls for a function in another package
should never have non-default settings for arguments such as
signature.
The reasoning applies specially to functions in supplied packages,
since methods for these are likely to exist in multiple packages.
A call to implicitGeneric() will show the generic version.
### How we would make the function with() into a generic:
## Since the second argument, 'expr' is used literally, we want
## with() to only have "data" in the signature.
## Not run:
setGeneric("with", signature = "data")
## Now we could predefine methods for "with" if we wanted to.
## When ready, we store the generic as implicit, and restore the
original
setGenericImplicit("with")
## End(Not run)
implicitGeneric("with")
# (This implicit generic is stored in the 'methods' package.)