The essense of functional programming in R.

functional.r

# Value of x depends on order foo, bar are called. They have the
# side effect of altering global state so they aren't pure
# functions

x = 0

foo <- function() {
    x <<- (x+1)
}

bar <- function() {
    x <<- (x+2)
}

# Calling fetch has the side-effect of going out to the web and
# reading a file. Network could be down, the url could fail...
# either way it is not guaranteed to produce the same output every
# time so its not a pure function.

fetch <- function() {
    read.csv(url('http://www.cyclismo.org/tutorial/R/simple.csv'))
}

# Higher order functions, functions which return functions.

add <- function(x) {
    addf <- function(y) {
        return (x+y)
    }
}

addOne <- add(1)
print(addOne(5))

### Closures

fizz <- function(x) {
    x <- x+1
    bang <- function(y) {
        y <- x + y
    }
}

first  = fizz(1)
second = first(2)
third  = first(3)
fourth = fizz(1)(1)

print(first)
print(second)
print(third)
print(fourth)

# "map" function, every functional programmers favorite example .
# You can do this natively in R but lets recreate it using
# recursion and higher order functions, because why not. :) 

map <- function(f, arr) {
    map_ <- function(ar) {
        if (length(ar) > 1) {
            append( f(ar[1]), map_(ar[(2):length(ar)]) );
        } else {
            return( f(ar[1]) )
        }
    }
    map_(arr)
}

f = function(x){x %% 3}
print(map(f, c(1:25)))