汎用のIO例外等を定義するストラクチャである．

signature IO =
sig
  exception BlockingNotSupported
  exception ClosedStream
  exception Io of {name : string, function : string, cause : exn}
  exception NonblockingNotSupported
  exception RandomAccessNotSupported
  datatype buffer_mode = NO_BUF | LINE_BUF | BLOCK_BUF
end