Finding the current output stream

• current_output(Stream) unifies Stream with the current output

  stream.

• telling(S) unifies S with the current output stream. This is exactly the same as current_output(S), except that S will be unified with a filename if the current output stream was opened by tell/1.

  A common usage of telling/1 is

            tell('Some File Name')
            ...
            telling('Some File Name')
            

  telling/1 should succeed if the current input stream was not changed (or if changed, restored). It succeeds for any filename (including user) and any stream object opened by open/3 (see ref-iou-sfh-opn), but fails for user_output and any stream object opened by tell/1 (see ref-iou-sfh-opn). Passing file names to tell/1 is the only DEC-10 Prolog usage of telling/1, so Quintus Prolog is compatible with this usage.

  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)