function filter(t, func)
  local ret = {}
  for i, v in ipairs(t) do
    ret[#ret+1] = func(v) and v or nil
  end
  return ret
end

function even(a) return a % 2 == 0 end

print(unpack(filter({1, 2, 3, 4 ,5, 6, 7, 8, 9, 10}, even)))