rewritted the OOP intro, still needs some cleanup...

Signed-off-by: Alex A. Naanou <alex.nanou@gmail.com>

Slang/slang.js

@@ -128,7 +128,7 @@ var PRE_NAMESPACE = {
 			// XXX should we look ahead and count the explicitly closed
 			// 		via ']' and ']]' blocks???
 			// 		...at this point this seems a bit complex...
-			// 		...of there are more than one ']]' in a structure
+			// 		...if there are more than one ']]' in a structure
 			//		this might stop being deterministic...
 			code.splice(0, 0, cur)
 		}

js-oop.js

@@ -5,124 +5,405 @@
 *
 **********************************************************************/
 //
-// The value of 'this'
+// The basic prototype inheritance
+// -------------------------------
+//
+// First we'll create a  basic object a
+
+	var a = {
+		x: 1,
+		y: 2,
+	} 
+
+// Then we will create a new object using a as a "base" 
+
+	var b = Object.create(a)
+	b.z = 3
+
+// The object b now has both access to it's own attributes ('z') and 
+// attributes of a ('x' and 'y')
+
+	b.x					// -> 1
+	b.z					// -> 3
+
+// What we see is that if the attribute is not found in the current 
+// object it resolves to the object's "prototype" and so on, these 
+// chians can of any length.
+//
+// NOTE: there is also a second mechanism available but we'll discuss 
+// 		it a bit later in 
+//
+// A couple of easy ways to see these sets of attributes:
+	
+	Object.keys(b)		// -> z
+
+	for(var k in b){ console.log(k) }
+						// -> x, y, z
+
+// Another way to test if the attribute is "local" ("own"):
+
+	b.isOwnProperty('z')	// -> true
+	b.isOwnProperty('x')	// -> false
+
+
+// What happens under the hood is very simple:
+
+	b.__proto__ === a	// -> true
+
+
+// Thus, we could define our own create function like this:
+
+	function clone(from){
+		var o = {}
+		o.__proto__ = from
+		return o
+	}
+
+	var c = clone(b)
+
+// Out of curiosity let's see if .__proto__ is defined on a basic object
+
+	var x = {}
+
+	x.__proto__			// -> {}
+
+// Turns out it is, and it points to Object's prototype
+
+	x.__proto__ === Object.prototye
+						// -> true
+
+// We will discuss what this means and how we can use this in the next 
+// sections...
+//
+//
+//
+// The Constructor Mechanism
+// -------------------------
+//
+// JavaScript provides a second, complementary mechanism to inherit 
+// attributes, it resembles the class/object relationship in languages
+// like C++ but this resemblance is on the surface only as it still 
+// uses the same prototype mechanism as the above.  
+//
+// We will start by creating a constructor:
+	
+	function A(){
+		this.x = 1
+		this.y = 2
+	}
+
+// Technically a constructor is just a function, what makes it a 
+// "constructor" is how we use it...
+
+	var a = new A()
+
+
+// what 'new' does here is:
+// 	1) creates an empty object
+// 	2) sets a bunch of attributes on it
+// 	3) passes it to the constructor via 'this'
+// 	4) after the constructor returns, this object is returned
+//
+// We could write an equivalent (simplified) function:
+
+	function construct(func){
+		var obj = {}
+
+		// set some special attributes on obj...
+
+		return func.apply(obj)
+	}
+
+	var b = construct(A)
+
+// But what makes this interesting? At this point this all looks like 
+// all we did is moved attribute from a literal object notation to a 
+// constructor function, effectively adding complexity. What are we 
+// getting back from this?
+//
+// Let's look at a number of attributes:
+
+	a.__proto__		// -> {} 
+
+	a.constructor	// -> [Function A]
+
+
+// The answer lies in the attributes that are set on the object, lets 
+// write a more complete reference implementation:
+	
+	function construct(func, args){
+		var obj = {}
+
+		obj.constructor = func
+		obj.__proto__ = func.prototype
+
+		var res = func.apply(obj, args)
+		if(res instanceof Object){
+			return res
+		}
+
+		return obj
+	}
+
+	var b = construct(A)
+
+// Notice that we return the resulting object in a more complicated
+// way, this will come in handy later.
+// 
+// Also notice that 'prototype' from the end of the previous section.
+//
+// First let us cover the default. Each time a function is created in
+// JavaScript it will get a new empty object assigned to it's .prototype
+// attribute.
+// On the function level, in general, this is not used, but this is used
+// when we use the function as a constructor.
+//
+// As we can see from the code above, the resulting object's .__proto__
+// points to the constructor's .prototype, from the previous section 
+// this means that attributes accessed via that object are resolved to 
+// the prototype.
+// In the default case this is true.
+//
+// So if we add stuff to the constructor's .prototype they should get 
+// resolved from the object
+
+	A.prototype.x = 123
+	a.constructor.prototype.y = 321
+	a.__proto__.z = 333
+
+	// for illustration, some object own attributes
+	a.x = 'a!'
+	b.x = 'b!'
+
+	a.x				// -> 'a!'
+	a.y				// -> 321
+	a.z				// -> 333
+
+// These values are accessible from all objects constructed by A since
+// all of them point to A with both the .constructor and .__proto__ 
+// attributes
+
+	b.x				// -> 'b!'
+	b.y				// -> 321
+	b.z				// -> 333
+
+
+
+// Double inheritance:
 // -------------------
 //
+// NOTE: this might be implementation specific. Tested and works in IE11, V8
+//
+// There are actually three sources where JavaScript looks for attributes:
+// 	1) the actual object
+// 	2) .__proto__ 
+// 		as coverd in the first section
+// 	3) .constructor.prototype 
+// 		as explained in the previous section
+
+	var O = { 
+		o: 0
+	}
+
+	function A(){}
+	A.prototype.a = 1
+
+	var a = new A()
+	a.__proto__ = o
+
+// Now we can access both attributes inherited from 'O' and 'A'...
+
+	a.o				// -> 0
+	a.a				// -> 1
+
+
+// The check is done specifically in this order, thus attributes can 
+// "shadow" other attributes defined later in the chain.
+//
+// To show this let us define an attribute with the same name on both 
+// 'O' and 'A':
+
+	O.x = 'came from O'
+	A.prototype.x = 'came from A'
+
+	a.x				// -> 'came from O'
+
+
+// In both inheritance mechanisms, each step is checked via the same 
+// rules recursively, this enables inheritance chains.
+//
+// We will create a cahin:
+//
+// 		c -> b -> a
+//
+
+	var a = {x: 1}
+	var b = Object.create(a)
+	b.y = 2
+	var c = Object.create(b)
+	
+	c.x				// -> 1
+	c.y				// -> 2
+
+
+// Creating an inheritance chain via the constructor mechanism is a bit
+// more involved, and there are multiple ways to do this...
+//
+// Here we will create a similar chian:
+//
+// 		C -> B -> A
+//
+
+	function A(){}
+	A.prototype.x = 1
+
+	function B(){}
+	// NOTE: if this is done after an instance is created, that instances'
+	// 		.__proto__ will keep referencing the old prototype object.
+	// 		see the next constructor for a way around this...
+	B.prototype = Object.create(A.prototype)
+	B.prototype.y = 2
+
+	function C(){}
+	// NOTE: this is safer than Object.create as it does not overwrite
+	// 		the original object and thus will affect all existing 
+	// 		instances of C, if any were created before this point...
+	C.prototype.__proto__ = B.prototype
+
+	var c = new C()
+
+	c.x				// -> 1
+	c.y				// -> 2
+
+
+
+// Checking inheritance (instanceof)
+// ---------------------------------
+//
+// An object is considered an instance of its' constructor and all other 
+// constructors in the inheritance chain.
+
+	c instanceof C	// -> true
+	c instanceof B	// -> true
+	c instanceof A	// -> true
+	c instanceof Object
+					// -> true
+
+
+// This also works for manually created objects
+
+	var cc = construct(C)
+
+	cc instanceof C
+
+
+// But this will not work outside the constructor model, i.e. if the right 
+// parameter is not a function.
+
+	var x = {}
+	var y = Object.create(x)
+
+	try{
+		// this will fail as x is not a function...
+		y instanceof x
+	} catch(e){
+		console.log('error')
+	}
+
+
+// Again to make this simpler to understand we will implement our own
+// equivalent to instanceof:
+	
+	function isInstanceOf(obj, proto){
+		return proto instanceof Function 
+			&& (obj.__proto__ === proto.prototype ? true
+				// NOTE: the last in this chain is Object.prototype.__proto__ 
+				// 		and it is null
+				: obj.__proto__ == null ? false
+				// go down the chian...
+				: isInstanceOf(obj.__proto__, proto))
+	}
+
+	isInstanceOf(c, C)	// -> true
+	isInstanceOf(c, B)	// -> true
+	isInstanceOf(c, A)	// -> true
+	isInstanceOf(c, Object)
+						// -> true
+	isInstanceOf(c, function X(){})
+						// -> false
+
+
+// Checking type (typeof)
+// ----------------------
+//
+// What typeof returns in JavaScript is not too useful and sometimes 
+// even odd...
+
+	typeof c			// -> 'object'
+
+// This might differ from implementation to implementation but 
+// essentially the main thing typeof is useful for is distinguishing
+// between objects and non-objects (numbers, strings, ...etc.)
+
+	// non-objects
+	typeof 1			// -> 'number'
+	typeof Infinity		// -> 'number'
+	typeof 'a'			// -> 'string'
+	typeof undefined	// -> 'undefined'
+
+	// objects
+	typeof {}			// -> 'object'
+	typeof []			// -> 'object'
+
+	// the odd stuff...
+	typeof NaN			// -> 'number'
+	typeof null			// -> 'object'
+	typeof function(){}	// -> 'function'
+
+
+
+// Methods and the value of 'this'
+// -------------------------------
+//
+// A method is simply an attribute that references a function.
+
  	function f(){
  		console.log(this)
  		this.a = 1
  	}
+
 	var o = { f: f }
 
+// Thus we call the attribute .f of object o a method.
+//
+//
+// 'this' is a reserved word and is available in the context of a function
+// execution, not just in methods, but what value it references depends
+// on how that function is called...
 // 
-// 'this' is always the "context" of the function call, in JS a context
+// a simple way to think about is that 'this' always points to the 
-// can be:
+// "context" of the function call.
+//
+// This context can be:
 // 	- implicit
 // 		- root context
- 			f()			// 'window' or 'module' is implied, this is 
+ 			f()
- 						//	equivalent to: window.f()
+						// 'window', 'global' or 'module' is implied, 
+						// in strict mode this is null.
+						// the same as:
+						// 		window.f()
 // 		- 'new' context
  			new f()		// here a context will be created and passed to
- 						//	'f's 'this', for more details on what 'new'
+ 						// 'f's 'this', for more details on what 'new'
- 						//	does see the next section.
+ 						// does see: "The Constructor Mechanism" section.
 // 	- explicit:
-// 		- the object on the left side of "." or the [ ] attribute access:
+// 		- the object on the left side of "." or the [ ] operators:
  			o.f()		// o is the context
  			o['f']()	// the same as the above
 // 		- the object explicitly passed to .call(..) or .apply(..) methods
 // 		  as first argument:
  		  	f.call(o)	// o is the context
-//
+ 		  	f.apply(o)	// o is the context
-//
-//
-// What's 'new'?
-// -------------
-//
- 	function O(){
- 		this.attr = 123
- 	}
- 	var o = new O()
-
-//
-// 'new' creates a new context object and passes it to the function 
-// being called (the constructor), this new object has several special
-// attributes set:
-// 	o.constructor	-- references the object (constructor) used to 
-// 						construct the object o (instance)
-// 	o.__proto__		-- references the constructor prototype, same as:
- 							o.constructor.prototype === o.__proto__
-// 						this is used to identify the object via:
- 							o instanceof O
-//
-// The 'new' expression returns the context object after it has been
-// populated by the constructor function.
-//
-// NOTE: when using 'new', the function/constructor return value is 
-// 		ignored.
-//
-//
-// The values set on 'this' by the constructor are instance attributes,
-// i.e. they are stored in the specific instance being created.
-//
-// NOTE: calling the constructor is not required, but not calling it
-// 		will not run the initialization code that would otherwise 
-// 		populate the object. i.e. no instance values will be created.
-// 
-//
-// The instance has another type of attribute accessible through it, an 
-// attribute that's not stored in the object, but rather in it's 
-// prototype (o.__proto__), or rather the constructor's prototype 
-// (o.constructor.prototype). For more details see the next section.
-//
-//
-//
-// The object's 'prototype'
-// ------------------------
-//
- 	function f(){}
- 	console.log(f.prototype)			// f {}
-
-//
-// The unique prototype object is created on every function definition.
-// The prototype is instance-like of the function to which it is assigned.
-//
-// NOTE: since the prototype has .__proto__ set to Object, technically
-// 		it's not a strict instance of the type it defines, so:
- 			f.prototype instanceof f
-// 		will return false.
-//
-// Each unresolved attribute access on the "instance" will resolve to its
-// prototype (o.__proto__ or o.constructor.prototype). 
-//
- 	O.prototype.x = 321
- 	console.log(o.x)					// 321
-
-//
-// Since the prototype is a JS object that adheres to the same rules as 
-// any other object, if the attr is not resolved in it directly, it will 
-// be searched in its prototype, and so on.
-// This principle enables us to implement inheritance.
-//
-	var OO = function(){
-		// call the base constructor...
-		O.call(this)
-	}
-	// chain the two prototypes...
-	OO.prototype = new O()
-	// this is to make things coherent for oo below...
-	OO.prototype.constructor = OO
-
-	var oo = new OO()
-
-	console.log(oo.x)					// 321
-	console.log(oo.constructor.name)	// 'OO'
-
-//
-// So in the example above local attributes (in 'oo') will be searched 
-// first, then oo.__proto__ (instance of O), then in O.prototype, and 
-// then in Object.prototype.
-//
-// 	oo -> oo.__proto__ -> (oo.__proto__).__proto__ -> ...
 //
 //
 //