Ocaml's unusual object system in a nutshell

ocaml-object-system.ml

(* Source: http://pastebin.com/YsUs4Kfa *)

(* ocaml's object system is quite unique and rarely used, despite it actually being pretty awesome
 * I call it "statically typed duck typing" because that's what it is
 *)
 
(* let's define a """class"""
 * your basic example of a linked-list
 *)
let node (x, y) = object
	method hd = x
	method tl = y
	
	method nth n =
		if n = 0 then 
			x
		else 
			y#nth (n - 1)
	
	method length =
		1 + y#length
end

(* this isn't actually a class, this is a function called node that takes two arguments, and returns an object
 * objects can easily be created anonymously, in this case we simply create an anonynmous object inside the function body
 * this has methods and closes over the function arguments, this object is immutable
 * as such, there is really no "this/self" keyword used in this approach
 * this is your basic recursive implementation of a linked list's head, tail, indexing and length operations
 *)
 
(*now,  the type of this constructor is quite something and automatically inferred by the compiler and statically checked:
 *
 * node : 
 * 'a * (< length : int; nth : int -> 'a; .. > as 'b) -> 
 *  < hd : 'a; length : int; nth : int -> 'a; tl : 'b >
 * 
 * I've put it on two lines to make it clearer, the first is the input type. The second argument is the interesting one:
 * < length : int; nth : int -> 'a; .. > as 'b
 *
 * this reads an object with:
 * - a "length" method that returns an int
 * - a "nth" method that takes an int and returns something of the same type as the first argument to node.
 * - ANY FURTHER METHODS, this is what the dots indicate which is very important
 *
 * the quote before "a" means this is a type variable. 
 * This means that both instances of 'a can be any type, as long as they are the same one
 *
 * then the output type is our object. Which has all our methods,
 * the most important one is that its "tl" method returns 'b, which we gave as a shorhand to the type of our second argument.
 * obviously we return the tail we gave as second argument in tact.
 *)

(* we wil also make a nil object
 * this is not a function, OCaml just uses similar syntax to declare constants and functions
 * note that we give it hd and tl and nth methods, but raise exceptions on them. 
 * This is done on purpose to give it the appropriate types. 
 * After all. Our "node" constructor demanded that its second argument implement the "nth" and "length" methods.
 *)
exception Empty_list
let nil = object
	method hd = raise Empty_list 
	method tl = raise Empty_list
	
	method nth n = raise Empty_list
	
	method length = 0
end

(* so now we can make some linked lists
 * probably not the most convenient syntax but you get the point
 * this will all type check, all the numbers are the same type, which is what the implementation demanded
 *)
let int_list = node(0, node(1, node(2, node(3, nil))))

let () = Printf.printf "%d\n" int_list#length (* 3 *)

(* the constructors are polymorphic, 
 * the elements of the list  can be any type, as long as they are all the same type.
 *)
let string_list = node("zero", node("one", node("two", node("three", nil))))

let () = Printf.printf "%d\n" string_list#tl#length (* 2 *)

(* but this would be a static typing error, 
 *
 * let hetero_list = node(0, node("one", node(2.0, node(`Three, nil))))
 *)
 
(* now, here it gets interesting.
 * Remember, all our implementation of the linked list wanted was 
 * that the second argument implemented "nth" and "length" correctly.
 * so here's a fake parametic "structure" that is filled with factorials
 *)

let weird_object  = object
	val internal_array = [| 1,2,3,4 |]
	
	(* when we ask for the nth element, it in fact computes the factorial of said number *)
	method nth n = 
		let rec factorial n =
			if n = 0 then 
				1
			else 
				n * factorial (n - 1)
		in factorial n
		
	
	method length = 60
end

(* this array does not implement the hd and tl methods
 * but our linked list implementation didn't ask for that for its second elements
 * so, funnily enough, this works:
 *)

let wtf_structure = node(0, node(4, weird_object))

(* and so does this 
 *)
let () = Printf.printf "%d\n" wtf_structure#length   (* 62 *)
let () = Printf.printf "%d\n" (wtf_structure#nth 8)  (* 720 *)

(* but say we wanted to implement map on our custom list:
 *)
 
let rec map_obj f obj =
	if obj#length = 0 then
		obj
	else
		node (f obj#hd, map_obj f obj#tl)

(* so will this now work on our wtf_structure?
 * No! because the type of this function is inferred by the implementation that the second argument is:
 *
 * (< hd : 'a; length : int; nth : int -> 'a; tl : 'b > as 'b)
 * 
 * the "as 'b" part is important. 
 * It means that the "tl" method must in this case return an object which has all the methods listed there too.
 * and that's something our "weird object" doesn't provide.
 *)
 
(* conclusion
 * OCaml's object system allows for "statically typed duck typing",. 
 * meaning that the type of an object is just its methods and thier types
 * this system is surprisingly flexible. It statically checks if stuff quacks the right way.
 *
 * and yes, with explicit type annotations you can actually give a set of methods together a name
 * and there are even classes if you want them
 *)