From: Gavin Kistner <gavin@...>
Date: 2005-10-19T22:06:57+09:00
Subject: Re: Factory function like Array() for your own classes

On Oct 14, 2005, at 8:00 PM, Sean O'Halpin wrote:
> This makes it possible to define factory functions that work like the
> built-in Array() and String() functions, e.g. so that Thing(1,2,3) is
> the same as Thing.new(1,2,3).

A language-comparison-aside...

In Lua, every table (the core object unit; basically a Hash with some  
extra goodies) can be accessed directly OR called as a function, if  
the table's metatable has a __call property that points to a  
function. For example:

Person = {
   -- constructor
   new=function( self, inName, inAge )
     local theInstance = { name = inName or "John", age = inAge or 0 }
     -- make the instance inherit from the 'class'
     setmetatable( theInstance, self._instanceMetaTable )
     return theInstance
   end,

   -- methods and properties common to all instances
   prototype = {
     species = 'Human',
     greet = function( self )
       print( "Hello, my name is " .. self.name )
     end
   }
}

Person._instanceMetaTable = {
   __index = Person.prototype,
   __tostring = function( self )
     return string.format( '<Person name="%s" age="%d">', self.name,  
self.age )
   end
}

-- Here's the magic that makes the Person table callable as a function
setmetatable( Person, {
   __call = function( self, ... )
     return self:new( unpack( arg ) )
   end
} )

local gk = Person:new( 'Gavin', 32 )
local lk = Person( 'Lisa', 31 )

print( gk, lk )
--> <Person name="Gavin" age="32">  <Person name="Lisa" age="31">

Person.prototype.greet( gk )
--> Hello, my name is Gavin

gk:greet( )
--> Hello, my name is Gavin

lk:greet( )
--> Hello, my name is Lisa

-- Showing that Person isn't a function, just an object with properties
print( Person.prototype )
--> table: 0x303a40