signature STREAM = sig
  type fileId = int
  type fileOffset = int
  type pos = fileId * fileOffset
  type ppos (* pretty pos *)
  type pposCache

  type t
  type fileInfo = fileId * string * string

  val convert: t -> fileInfo

  val ppos2str: ppos -> string

  val getchar: t -> char option * t

  (* Will throw UngetcError, if applied at the beginning of the stream.
   * Can be always avoided, so is not provided in sig file *)
  val ungetc: t -> t

  val getPos: t -> pos
  val getPosAfterCharRead: t -> pos

  (* pos must come from t, see pos2pposWithFI *)
  val pos2ppos: pos -> t -> ppos
  (* #id pos must be equal to fileId of fileInfo,
   * otherwise InvalidFileInfo is thrown *)
  val pos2pposWithFI: pos -> fileInfo -> ppos

  (* Assumed to be called once for given pos, so will throw Unreachable on
   * second call *)
  val pposWithoutCol: ppos -> ppos

  val getSubstr: fileOffset -> fileOffset -> t -> string
  val isFirstOnLine: pos -> t -> bool

  (* throws IO.Io *)
  val create: string -> t

  val pposCacheInit: fileInfo -> pposCache
  val pposCacheAdvance: pos -> pposCache -> (int * int) * pposCache
  val pposCacheGetLine: pposCache -> int
  val pposCacheGetFname:  pposCache -> string
end