R in a Nutshell, 2nd Edition

Everything in R Is an Object

In the last few sections, most examples of objects were objects that stored data: vectors, lists, and other data structures. However, everything in R is an object: functions, symbols, and even R expressions.

For example, function names in R are really symbol objects that point to function objects. (That relationship is, in turn, stored in an environment object.) You can assign a symbol to refer to a numeric object and then change the symbol to refer to a function:

> x <- 1
> x
[1] 1
> x(2)
Error: could not find function "x"
> x <- function(i) i^2
> x
function(i) i^2
> x(2)
[1] 4

You can even use R code to construct new functions. If you really wanted to, you could write a function that modifies its own definition.