Some S3 brainteasers

s3.md

• UseMethod() has a special way of calling methods which has some weird consequences. Predict what the following code will return, then run it and read the help for UseMethod() to figure out what's going on. Write down the rules in the simplest form possible.

  y <- 1
  g <- function(x) { 
    y <- 2
    UseMethod("g")
  }
  g.numeric <- function(x) y
  g(10)
  
  h <- function(x) {
    x <- 10
    UseMethod("h")
  }
  h.character <- function(x) paste("char", x)
  h.numeric <- function(x) paste("num", x)
  
  h("a")

• Internal generics don't dispatch on the implicit class of base types. Carefully read ?"internal generic" to determine why the length of f and g is different in the example below. What function helps distinguish between the behaviour of f and g?

  f <- function() 1
  g <- function() 2
  class(g) <- "function"
  class(f)
  class(g)
  
  length.function <- function(x) "function"
  
  length(f)
  length(g)

For extra fun, some internal generics define their dispatch rules quite arbitrarily. cbind() and row() for example, are unique in supporting multiple dispatch, while (nearly) all other methods dispatch on only the first argument:

x <- structure(1:10, class="foo")
cbind.foo <- function(...) "FOO!"
cbind(1:10, x) 
cbind(data.frame(a=1:10), x)
cbind(matrix(1:10, nrow=10), x)