Scope in generic functions R

Question

If you define a variable inside a generic function, it is available to the method. For example:

g <- function(x) {
  y <- 2
  UseMethod("g")
}
g.default <- function() y
g()
[1] 2

But if the variable you define has the same name as the function parameter, this does not happen. It seems that R deletes that variable before calling the method:

g <- function(x) {
  x <- 2
  UseMethod("g")
}
g.default <- function() x
g()
Error in g.default() : object 'x' not found

Could someone explain exactly what is going on here?

Answer 1

The following comments from the C source file that defines do_usemethod at least hint at what's going on. See especially the second sentence of the second enumerated item.

Basically, it looks like (due to dumb application of rule in that second point), the value of x does not get copied over because the C code checks to see if it's among the formals, sees that it is, and so excludes if from the list of variables inserted into the method's evaluation environment.

/* usemethod - calling functions need to evaluate the object
* (== 2nd argument). They also need to ensure that the
* argument list is set up in the correct manner.
*
* 1. find the context for the calling function (i.e. the generic)
* this gives us the unevaluated arguments for the original call
*
* 2. create an environment for evaluating the method and insert
* a handful of variables (.Generic, .Class and .Method) into
* that environment. Also copy any variables in the env of the
* generic that are not formal (or actual) arguments.
*
* 3. fix up the argument list; it should be the arguments to the
* generic matched to the formals of the method to be invoked */