4.6.7.7 Finding the Current Output Stream

• current_output(Stream) unifies Stream with the current output

  stream.

• If the current output stream is user_output, then telling(S) unifies S with user. Otherwise, if the current output stream was opened by tell(F), then telling(S) unifies S with F. Otherwise, if the current output stream was opened by open/[3,4], then telling(S) unifies S with the corresponding stream object.

  telling/1 can be used to verify that a section of code leaves the current output stream unchanged as follows:

  /* nonvar(FileNameOrStream), */
  tell(FileNameOrStream),
  …
  telling(FileNameOrStream)
  

  WARNING: The sequence

  telling(File),
  …
  set_output(File),
  

  will signal an error if the current output stream was opened by tell/1. The only sequences that are guaranteed to succeed are

  telling(FileOrStream),
  …
  tell(FileOrStream)
  

  and

  current_output(Stream),
  …
  set_output(Stream)