Generic function called via second argument of foo(x, bar) in R

Question

I want to make a generic function predict() for 'foo' class in R so that it is invoked when second argument of predict() is of class 'foo':

class(y.foo) <- "foo"

predict(x, y.foo)

Is it possible? How to do it?

Answer 1

Try this. Here our foo method just outputs "foo" and in the real code it would be replaced with whatever foo method you have:

predict <- function(x, foo = NULL, ...) UseMethod("predict", foo)
predict.foo <- function(x, foo, ...) "foo" # replace with your foo method
predict.default <- function(x, foo = NULL, ...) if (is.null(foo)) 
         stats::predict(x, ...) else stats::predict(x, foo, ...)

Now test it out:

y.foo <- 1
class(y.foo) <- "foo"
predict(0, y.foo)
## [1] "foo"

fm <- lm(demand ~ Time, BOD)

predict(fm)
##        1        2        3        4        5        6 
## 10.24286 11.96429 13.68571 15.40714 17.12857 20.57143 

predict(fm, newdata = list(Time = 1:2))
##        1        2 
## 10.24286 11.96429 

predict(fm, list(Time = 1:2)) # same
##        1        2 
## 10.24286 11.96429