- (e1); (e2): Evaluate e1 first, evaluate e2 next. Then return teh value of e2.

- Type of doubly linked lists:
  type dll = { value : int; mutable succ : dll option; mutable pred : dll option };;

- Question: How do we make a doubly linked list with the elements [3; 4]?
  How to do this with rec?

  let dll3 = { value = 3; pred=None; succ=None };;
  let dll4 = { value = 4; pred=dll3; succ=None };;
  dll3.pred <- dll4;;

  let dll3 = { value = 3; pred=None; succ=dll4 } and
      dll4 = { value = 4; pred=dll3; succ=None };;

  let rec even x = even (x - 2) under some situations

  let rec even x = !odd (x - 1) under some situations and <== The and keyword allows us to define mutually recursive functions
      rec odd x = !even (x - 1) under some conditions.

====================

let rec even x = if (x == 0) then true else if (x == 1) then false else not (even (x - 1));;
let rec odd x = if (x == 0) then false else if (x == 1) then true else not (odd (x - 1));;

let rec even x = if (x == 0) then true else odd (x - 1) and
        odd x = if (x == 0) then false else even (x - 1);;

===================

"Tying the Knot"

let rec even x = if (x = 0) then true else if (x = 1) then false else not (even (x - 1));;

let recursive_placeholder = ref fun x -> false
let even x = if (x = 0) then true else if (x = 1) then false else not ((!recursive_placeholder) (x - 1));;
recursive_placeholder := fun x -> even x
recursive_placeholder := even

========================================================================================================================

Module

- Header files, imports, exports
- Packages, interfaces, classes, inheritance
- Compilation units

- Namespaces: Prevent name collisions
  C++ namespaces, Java packages
- Abstraction: C++ / Java / Python class
  Code reuse: "Draw method for a button is the same as the draw method for a scroll bar = Draw method for widgets"
    Inheritance
  Syntactic organization into files (Header files, #use in the toplevel, input / include in Latex)

- Module: "Functor"