Reading from a file

see('data.txt')

opens the file for reading. To switch back to the default input stream (i.e. the user, instead of a file), use the command seen.

display(Filename) :-
 see(Filename),
 read(Term),
 write(Term), nl,
 see(user),
 write('More terms: (y/n)'),
 read( Yes_No),
 perform( Yes_No, Filename).

perform( n,_).
perform( _,Filename) :- 
 display( Filename).

• The goal : see(Filename) succeeds only once, and cannot be re-satisfied on backtracking
• The first time it is satisfied for some file, that file - Filename is opened
• Reading of terms from file, in a top down manner, until seen is encountered
• If Filename is not instantiated or does not exist - then goal fails