When implementing a generic method for a new class:
scale.newclass <- function(x, center = TRUE, scale = TRUE) {
...
}
How to guarantee that the default values for additional arguments center and scale always stay consistent with their default values in scale.default?
In other words - if in the future scale.default changes it's default argument to scale = FALSE, how to make sure scale.newclass defaults are also adjusted to match?
It seems very unlikely that those values would ever change, but if you are really worried about, you could do something like
scale.newclass <- function(x, center = formals(scale.default)$center, scale = formals(scale.default)$scale) {
...
}
That will read the default values from the function signature.
The method scale.default inherits the default values of its arguments from the generic function scale:
identical(formals(scale), formals(scale.default))
## [1] TRUE
Those values will not change unless the generic function changes, and if the generic function changes, then you'll be warned about inconsistencies by R CMD check. So the example doesn't quite motivate the problem ...
Setting that aside, you can do this programmatically with formals<-:
formals(scale.a) <- formals(scale.default)
Well, that only works for generic functions not admitting a formal argument ...:
any(names(formals(scale)) == "...")
## [1] FALSE
In general, the formals of the two methods may not have the same length or the same names, and in that case you would want to be more careful:
f <- function(x, ...) UseMethod("f")
f.default <- function(x, a = 1, b = 2, c = a + b, ...) x
f.a <- function(x, a = 1, b = 1, c = a - b, ...) x
form.default <- formals(f.default)
form.a <- formals(f.a)
nms <- intersect(names(form.default), names(form.a))
formals(f.a)[nms] <- form.default[nms]
identical(formals(f.a), form.default)
## [1] TRUE
Still, my feeling is that these kinds of "precautions" can cause more problems than they solve in the long run ...
