From: Gavin Kistner <gavin@...>
Date: 2005-08-17T21:57:10+09:00
Subject: Re: Prototype-based / Ruby question

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On Aug 17, 2005, at 2:21 AM, zimba.tm@gmail.com wrote:
> As I understand the definition, prototype based languages don't
> instanciate objects from a class, but rather clone an existing model
> object. Ruby's classes are objects.

 From research on the web, it appears that the above definition is  
the commonly-held one. I had always thought that 'prototype-based'  
referred to the object-chaining available in languages like  
Javascript (where the chain uses the word 'prototype') or Lua (the  
__index metatable property). Is there another term to cover this sort  
of 'inheritance'? Or is this an important aspect of prototype-based  
languages for representing inheritance?

[Non-Ruby Code Samples Follow]
For those who don't know what I'm talking about, the following  
JavaScript and Lua examples show how you can make simple 'instance'  
objects have access to a common 'class' object, including changing  
the methods and properties on that class after instantiation.


JavaScript:
Mammal = function( ){
   this.isAlive = true;
}
Mammal.prototype.hasHair = true;
Mammal.prototype.die = function( ){
   this.isAlive = false;
}

Person = function( inName ){
   this.name = inName;
}
Person.prototype = new Mammal;
//other 'best practice' inheritance details omitted for clarity
Person.prototype.toString = function(){
   return "[Person '"+this.name+"']";
}

var gk = new Person( "Gavin Kistner" );
gk.nickname = "Phrogz"; // instance property
alert( gk ); // [Person 'Gavin Kistner']
alert( gk.isAlive ); // true
alert( gk.hasHair ); // true
Person.prototype.eat = function(){
   if ( !this.weight ) this.weight = 170;
   this.weight += 1;
}
gk.eat( );
alert( gk.weight ); // 171
gk.die( );
alert( gk.isAlive ); // false

In the above, when gk.hasHair is referenced, JavaScript looks:
   * In the gk object for a hasHair property
   * In the constructor's (Person) prototype object for a hasHair  
Property
   * In the constructor's prototype's prototype object for a hasHair  
Property (where it is found)

When the die() method is invoked on gk, it is found on the Mammal  
prototype, but 'this' is still the gk object. While reading the value  
of 'isAlive' on gk searches up the prototype chain for that property,  
setting the value creates a new property on the gk object itself.


Lua:
Mammal = {
   isAlive = true, --by default, overridden per instance if needed
   hasHair = true,
   die = function( self )
     self.isAlive = false
   end
}

Person = {
   new = function( inClass, inName )
     local instance = { name = inName }
     setmetatable( instance, inClass.inheritTable )
     return instance
   end,
   proto = {
     -- empty to begin with
   },
   inheritTable = {
     __tostring = function(self) return "[Person '"..self.name.."']" end
   }
}
Person.inheritTable.__index = Person.proto
--inherit properties from Mammal
setmetatable( Person.proto, { __index = Mammal } )

local gk = Person:new( "Gavin Kistner" )
gk.nickname = "Phrogz"

print( gk )         --> [Person 'Gavin Kistner']
print( gk.isAlive ) --> true
print( gk.hasHair ) --> true
Person.proto.eat = function( self )
   if not self.weight then self.weight = 170 end
   self.weight = self.weight + 1
end

gk:eat( )
print( gk.weight )  --> 171
gk:die( )
print( gk.isAlive ) --> false

In Lua, every 'table' (hash object) can have another 'metatable'  
associated with it. While the table holds information about the  
object itself, the metatable holds information about that object's  
inheritance chain, string conversion, operator overloading, and more.  
If Lua cannot find a property for an object in the object's table, it  
sees if the object has a metatable with an __index property pointing  
to another table; if so, it asks that table for the value.

Like the JS example, the Lua above has three main objects - the gk  
table, the Person.proto table, and the Mammal table. (The Person  
table itself is a nice namespace wrapper for the proto, new, and  
inheritTable properties.) Like JS, any number of Person instances can  
be created, and they will all share common 'eat' and 'die' methods  
from their ancestors.


In both of the above cases, a full-fledged instance with all the  
methods is not copied upon creation, but instead a lightweight  
instance is created with pointers that the language can use to find  
'inherited' methods or properties.

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