Lua streaming line reader for text files

local function each_line(path)
  local file, err = io.open(path, 'r')
  if not file then error(('cannot open %s: %s'):format(path, err), 0) end
  local n = 0
  return function()
    if not file then return nil end
    local line = file:read('l')
    if line then
      n = n + 1
      return line, n
    end
    file:close()
    file = nil
  end
end

return each_line