Playing with simple prototypal inheritance in Lua

gistfile1.lua

Object = {};

function Object:clone( o )
  return setmetatable( o, { __index = self } );
end

function Object:instanceOf( parent )
  local mt = getmetatable( self );

  if mt == nil or mt.__index == nil then
    return false;
  elseif mt.__index == parent then
    return true;
  else
    return mt.__index:instanceOf( parent );
  end
end

Person = Object:clone{ species = "human", hobbies = {} };
dom = Person:clone{};
alfie = dom:clone{};


-- working with primitives and assignment is simple
print( dom.name ); -- nil
print( alfie.name ); -- nil

dom.name = "Dom";
print( dom.name ); -- Dom
print( alfie.name ); -- Dom

alfie.name = "Alfie";
print( dom.name ); -- Dom
print( alfie.name ); -- Alfie


-- working with objects and mutation has some hazards
table.insert( dom.hobbies, "Geeking" );
print( unpack( dom.hobbies ) ); -- Geeking
print( unpack( alfie.hobbies ) ); -- Geeking

table.insert( alfie.hobbies, "Drooling" );
print( unpack( dom.hobbies ) ); -- Geeking Drooling
print( unpack( alfie.hobbies ) ); -- Geeking Drooling

Person.hobbies = {};
print( unpack( dom.hobbies ) ); -- <empty line>
print( unpack( alfie.hobbies ) ); -- <empty line>


-- instead, let's use a custom method to insert a hobby
function Person:addHobby( hobby )
  local hobbies = rawget( self, "hobbies" );

  -- incredibly naive shallow copy. good enough here
  if hobbies == nil then
    local hobbies = {};
    for k, v in pairs( self.hobbies ) do
      hobbies[ k ] = v;
    end
    self.hobbies = hobbies;
  end

  table.insert( self.hobbies, hobby );
end

dom:addHobby( "Geeking" );
print( unpack( dom.hobbies ) ); -- Geeking
print( unpack( alfie.hobbies ) ); -- Geeking

alfie:addHobby( "Drooling" );
print( unpack( dom.hobbies ) ); -- Geeking
print( unpack( alfie.hobbies ) ); -- Geeking Drooling

dom:addHobby( "Cycling" );
print( unpack( dom.hobbies ) ); -- Geeking Cycling
print( unpack( alfie.hobbies ) ); -- Geeking Drooling