Many R-internal functions are generic and allow methods to be written for.
The following primitive and internal functions are generic,
i.e., you can write methods for them:
length,
length<-,
dimnames,
dimnames<-,
dim,
dim<-,
names,
names<-,
levels<-,
as.character,
as.complex,
as.double,
as.integer,
as.logical,
as.raw,
as.vector,
is.array,
is.matrix,
is.na,
is.nan,
is.numeric,
rep,
seq.int
(which dispatches methods for "seq")
and xtfrm
In addition, is.name is a synonym for is.symbol and
dispatches methods for the latter. Similarly, as.numeric
is a synonym for as.double and dispatches methods for the
latter, i.e., S3 methods are for as.double, whereas S4 methods
are to be written for as.numeric.
Note that all of the group generic functions are also internal/primitive and allow methods to be written for them.
.S3PrimitiveGenerics is a character vector listing the
primitives which are internal generic and not group generic.
Currently as.vector, cbind,
rbind and unlist are the internal
non-primitive functions which are internally generic.
For efficiency, internal dispatch only occurs on objects, that
is those for which is.object returns true.
methods for the methods which are available.