Consulting Files

Contains three clauses consult, reconsult and listing

• Possible to place all predicates with the same name in a separate file (module)
• This module is then loaded into a complete program - done via consult(Filename)
• Possible to consult a list of file
• reconsult(Filename) - same as consult, except that all previous predicate definitions (of the same name) are deleted from the program and Filename
• listing(P) displays predicates with name P - regardless of the arity
• listing - without argument, shows the complete program

plays(X,Y) :-
 plays(X, Y, _).

plays(X, piano) :-
 plays(X, keyboard).

plays(X, keyboard) :-
 plays(X, guitar).

also, consider a file named 'beatles', which contains the following :

plays(john,guitar,rhythm).
plays(george, guitar, solo).
plays(paul, guitar, bass).
plays(ringo, drums).

and consider the dialog (queries) with Prolog

Q: listing
A: yes

Q: consult(beatles)
A: yes

Q: listing
A: plays(john,guitar,rhythm).
plays(george, guitar, solo).
plays(paul, guitar, bass).
plays(ringo, drums).
yes

Q: reconsult(plays)
A: yes

Q: listing
A: plays(john,guitar,rhythm).
plays(george, guitar, solo).
plays(paul, guitar, bass).

plays(X,Y) :-
 plays(X, Y, _).
plays(X, piano) :-
 plays(X, keyboard).
plays(X, keyboard) :-
 plays(X, guitar).
yes

• Note that reconsult(plays) resulted in the removal of plays(ringo,drums)
• Also, only one occurrence of the required play clause was removed - hence, the reconsult clause only needs to be satisfied once (i.e. no backtracking)