ML reference type and mutable data

These examples demonstrate mutable types in ML, i.e. types where data can be changed.

(* 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;;

(* using a local variable in a function *)
(* this should really have a loop*) 
let f x = 
  let r = ref 0 in
  if (x < 5) then (r := 6; !r)
  else (r := 8; !r);;

(* a random number generator*)
let current_rand = ref 0;;

let random () =
   current_rand := !current_rand * 25713 + 1345; 
   !current_rand;;

random();;

!current_rand;;

random();;

!current_rand;;

(* mutable types *)
(* I could make x mutable and y immutable *)
type mutable_pair = {mutable x: int; mutable y: int};;

let my_pair = {x = 5; y = 6};;

my_pair.x <- 99;;

my_pair.x;;