inheritance -- Lua style

gistfile1.lua

-- number
number = {}
function number:new(o)
    o = o or {}
    
    setmetatable(o, self)
    self.__index = self
    self.__add   = function(self, op)
        return self:new{value=(self:cast(self) + self:cast(op))}
    end
    
    return o
end

function number:cast(op)
    if (type(op) == 'table') then
        return op.value
    else
        return op
    end
end

n1 = number:new{value=10}
n2 = number:new{value=20}

print((n1 + n2).value)

-- money
money = number:new()
function money:cast(op)
    if (self.currency ~= op.currency) then
        error('unable to calculate with different currencies')
    end

    return number.cast(self, op)
end

m1 = money:new{value=10,currency='EUR'}
m2 = money:new{value=20,currency='EUR'}
m3 = money:new{value=30,currency='USD'}

sum = (m1 + m2)

print(sum.value)
print((sum + m3).value)