Writing to file

• Assign current output stream to file name
• Done via the tell(Filename) command
• If ANY file already opened for writing - tell will write to that file
• told closes the file and switches output stream back to default value - the user
• In MacProlog also have seeing/0 - which returns the current input stream - (file currently opened)

copy(Filename) :-
 tell(user),
 write( 'Term: '),
 read(Term),
 tell(Filename),
 perform(Term, Filename).

perform( stop,_) :- told.
perform( Term, Filename) :-
 write(Term), nl,
 copy(Filename).

• If arguments of tell not instantiated, tell(Filename) fails
• If argument instantiated to a filename that does not exist, a new file of that name is created and output stream switched
• Over writes an existing file