Someone

function Parray(...)
  return setmetatable({...},
    {__index = function(self, k)
      if type(k) == "number" and  k < 0 then k = #self + 1 + k end
      return rawget(self, k) end 
  })
end 
> t = Parray(1,2,3,4,5,6,7,8)
> = t[7]
7
> = t[-2]
7