Scope of methods invoked by UseMethod

Question

Contrast the following two code snippets:

1)

> y <- 1
> g <- function(x) { 
+   y <- 2
+   UseMethod("g")
+ }
> g.numeric <- function(x) y
> g(10)
[1] 2

2)

> x <- 1
> g <- function(x) {
+   x <- 2
+   UseMethod("g")
+ }
> g.numeric <- function(y) x
> g(10)
[1] 1

In the first snippet, g.numeric's free variable (namely, "y") is evaluated in g's local environment, whereas in the second snippet, g.numeric's free variable (namely "x") is evaluated in the global environment. How so?

Answer 1

As it says in Writing R Extensions:

A method must have all the arguments of the generic, including … if the generic does.

Your second example does not (g(x) vs g.numeric(y)). If you redefine g <- function(y), everything works the same as your first example.

> x <- 1
> g <- function(y) {
+   x <- 2
+   UseMethod("g")
+ }
> g.numeric <- function(y) x
> g(10)
[1] 2