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Reputation: 23104

Make R invoke NextMethod when possible

I am trying to make R to invoke NextMethod when there is a next-class:

e = new.env()
class(e) = c(class(e), "c11", "c22")
print.c11 = function(e, ...) {
    print(e$x); 
    tryCatch({
        NextMethod()
    }, warning = function(w) {
    }, error = function(e) {
    }, finally = {
    })
}
print.c22 = function(e, ...) {
    print(e$y)
    tryCatch({
        NextMethod()
    }, warning = function(w) {
    }, error = function(e) {
    }, finally = {
    })
}
e$x = 111
e$y = 222
print(e)

This is a dirty hack which will cover all errors and warnings that the method might produce. How can this be done properly?

The above code kind of works without the tryCatch block, that's because, I think, the print.default function was invoked. Now take an example (modified) out of Hadley's advanced R book:

> baz <- function(x) UseMethod("baz", x)
> baz.c <- function(x) c("c", NextMethod())
> baz.d <- function(x) c("d", NextMethod())
> c <- structure(1, class = c("c", "d"))
> baz(c)
 Show Traceback

 Rerun with Debug
 Error in NextMethod() : no method to invoke

Ideally the NextMethod function should return NULL if there is no next class.

Upvotes: 1

Views: 436

Answers (1)

Martin Morgan
Martin Morgan

Reputation: 46856

Implement a default method, e.g., 

baz.default <- function(x) NULL

and

> c <- structure(1, class = c("c", "d"))
> baz(c)
[1] "c" "d"

Upvotes: 5

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