Defining binary indexing operators

Assume we have a function like:

foo <- function(x, y)

how is it possible to define a binary indexing operator, denoted by $, 

so

that

x$y

functions the same as

foo(x, y)

  Here is an example. Note that $ does not evaluate y so you have
to do it yourself:

x <- structure(3, class = "myclass")
y <- 5
foo <- function(x,y) x+y
"$.myclass" <- function(x, i) { i <- eval.parent(parse(text=i)); foo(x, i) 
}
x$y # structure(8, class = "myclass")