4.6.7.6 Finding the Current Input Stream

• current_input(Stream) unifies Stream with the current input stream.
• If the current input stream is user_input, then seeing(S) unifies S with user. Otherwise, if the current input stream was opened by see(F), then seeing(S) unifies S with F. Otherwise, if the current input stream was opened by open/[3,4], then seeing(S) unifies S with the corresponding stream object.

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

  /* nonvar(FileNameOrStream), */
  see(FileNameOrStream),
  …
  seeing(FileNameOrStream)
  

  WARNING: The sequence

  seeing(File),
  …
  set_input(File),
  

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

  seeing(FileOrStream),
  …
  see(FileOrStream)
  

  and

  current_input(Stream),
  …
  set_input(Stream)