C++-style functors in R.

functors_in_R.R

## A simple example of a functor in R.

## A simple function of one argument. Since y is neither defined inside the function body or
## given as an argument, R will search the enclosing environment for y. Since R is lazy, it 
## won't do this when the function is defined, but only when the function is called (and y
## is needed.)
f = function(x) x^2 + y^2

## This changes the enclosing environment of f to a new environment. It was the global
## environment before, and we don't want that. 
environment(f) = new.env()

## Adding a class attribute to f is needed to make the generics below work.
class(f) = "f"

## The magic is here. The generic system R allows you to define class-specific methods for most
## common functions, even $ and []. 

`$.f` = function(f, y) environment(f)[[y]]

`$<-.f` = function(f, y, value) {
  environment(f)[[y]] = value
  invisible(f)
}

## Finally we can assign a value to y. 
f$y = 5
f(3)

## Note that the approach of using attr(f, "y") = 5 would not work,  since the function
## would not be able to access the attribute directly. Furthermore, the call by reference
## semantics can have a large effect on performance.

## This idea could probably be mixed with R6 in order to make callable objects in that
## framework as well.