References in ML:

  -- An ML reference is a value which is the address of a location 
     in the memory. 
  -- The value in the location is the content. 
  -- The content can be read through the dereferenceing operation and
     changed through an assignment operation.

[1] Creating a reference 

  Using the constructor: ref
  
    -- Ref is an ML function.
    -- It expects an argument, the initial content of the reference.
    -- It returns a new address every time it is called.  

- val x = ref 2;
val x = ref 2 : int ref

- val y = ref 5;
val y = ref 5 : int ref


[2] Dereferencing

  Using the function: !

    -- When ! is applied to a reference, it returns its contents.

- !x;
val it = 2 : int

- !y;
val it = 5 : int


[3] Updating

  Using the assignment operator: := 
  
    -- := operation updates the value of a reference.
    -- The type of := is unit. 

- x := !x + 2;
val it = () : unit
- x;
val it = ref 4 : int ref
- !x;
val it = 4 : int

- y := !x + 5;
val it = () : unit
- y;
val it = ref 9 : int ref
- !y;
val it = 9 : int


[4] Sequencing

  Syntax: (E1; E2; ... ; En)

    -- The expressions are evaluated from left to right.
    -- The result is the value of En. 
    -- The value of E1 to En-1 will be discarded. They are used
       for side effects. 

- (x := !x + 3; y := !x + 4);
val it = () : unit
- x;
val it = ref 7 : int ref
- y;
val it = ref 11 : int ref


[5] More examples

  Tuples:

- val a = ref 2;
val a = ref 2 : int ref
- val b= ref 3;  
val b = ref 3 : int ref
- (a, b);
val it = (ref 2,ref 3) : int ref * int ref

  Lists:

- val c = ref 4;
val c = ref 4 : int ref
- val s = [a, b, c];
val s = [ref 2,ref 3,ref 4] : int ref list
- hd s;
val it = ref 2 : int ref
- tl s;
val it = [ref 3,ref 4] : int ref list