Forward Chaining in Prolog

1
Forward Chainingin Prolog
2
FC Core

• add(P) adds P to DB and triggers forward
  chaining rules.
• add(P) - clause(P,true), !.
• add(P) -
• dbug("Adding p.n",P),
• assert(P),
• foreach(ifAdded(P,Actions), call(Actions)).
• remove(P) removes P from DB, triggering
  ifRemoved rules.
• remove(P) -
• dbug("Removing p.n",P),
• retract(P),
• foreach(ifRemoved(P,Actions), call(Actions)).

3
Mapping rules into triggers

• AgtB adds forward chaining rule to execute B
  whenever the facts
• in A have all been added to the DB.
• LHS is a conjunction
• ((P1,P2)gtQ) - !, (P1gt(P2gtQ)).
• LHS is a disjunction
• ((P1P2)gtQ) - !, (P1gtQ), (P2gtQ).
• LHS is atomic. trigger already in DB?
• (PgtQ) - ifAdded(P,Q), !.
• LHS is atomic. Add trigger to DB
• (PgtQ) -
• assert(ifAdded(P,Q)),
• foreach(clause(P,true),fcDo(Q)).

4
If-removed rules

• A/gtB adds ifRemoved rule to execute B whenever
  the
• assertions in A have all been removed from the
  database.
• ((P1,P2)/gtQ) - !, (P1/gt(P2/gtQ)).
• ((P1P2)/gtQ) - !, (P1/gtQ), (P2/gtQ).
• (P/gtQ) - assert(ifRemoved(P,Q)).
• (gtX) - add(X).

5
A test file

• FCTEST
• - spouse(P1,P2) gt add(spouse(P2,P1)).
• - spouse(P1,P2) /gt remove(spouse(P2,P1)).
• - gt spouse(adam,eve)).
• - spouse(X,Y), female(X) gt add(wife(X,Y)).
• - gt female(eve).
• - female(X), male(X) gt add(contradiction(female(
  X), male(X))).
• - contradiction(P,Q) gt format(CONTRADICTION
  w and w.\n, P,Q).

6
Fc in action

• ?- fctest.
• Adding spouse(adam,eve).
• Adding spouse(eve,adam).
• yes
• ?- listing(ifAdded).
• ifAdded(spouse(A,B), add(spouse(B,A))).
• ifAdded(a(A), (b(A),c(A)gtadd(d(A)))).
• yes
• ?- add(b(1)).
• Adding b(1).
• yes
• ?- add(a(1)).
• Adding a(1).
• yes
• ?- listing(ifAdded).
• ifAdded(spouse(A,B), add(spouse(B,A))).
• ifAdded(a(A), (b(A),c(A)gtadd(d(A)))).
• ifAdded(b(1), (c(1)gtadd(d(1)))).
• ifAdded(c(1), add(d(1))).

• ?- add(c(1)).
• Adding c(1).
• Adding d(1).
• Yes
• ?- b(X) gt add(foo(X)), add(bar(X)).
• Adding foo(1).
• Adding bar(1).
• yes
• ?- listing(ifAdded).
• ifAdded(spouse(A,B), add(spouse(B,A))).
• ifAdded(a(A), (b(A),c(A)gtadd(d(A)))).
• ifAdded(b(1), (c(1)gtadd(d(1)))).
• ifAdded(c(1), add(d(1))).
• ifAdded(b(A), (add(foo(A)),add(bar(A)))).
• yes