OCaml reference types.

(* C:\Documents and Settings\Default\Desktop\4651\examples\references.ml *)

let x = ref 5;;

x := !x + 1;;

x;;

(* a record with two mutable fields *)
type changeme = {mutable n: int; mutable s: string};;

let y = {n = 3; s = "apple"};;

y.s <- "banana";;

y;;

let sum = ref 0;;

(* find_sum modifies a global mutable variable sum *)
(* () is the unit element, represents "do nothing" *) 

let rec find_sum = function 
[] -> ()         
| x :: xs -> sum := !sum + x; find_sum xs;;

let l = [2; 7; 9];;

find_sum l;;

sum;;

sum := 0;;

find_sum [];;

sum;;

sum := 0;;

find_sum [1; 2 ; 4; 8; -7];;

sum;;

(* references cells can reference polymorphic types *)
type poly = {mutable f : 'a. 'a -> 'a * int};;

let pair = {f = function x -> (x, 3)};;

pair.f 5;;

pair.f true;;

pair.f <- function x -> (x, 6);;

pair.f 5;;

pair.f true;;