Prolog

1
Prolog

• CSE 473 Autumn 2003

2
Today

1. First-order logic resolution proofs
2. Answer extraction
3. Resolution strategies
4. Prolog
5. The 5th Generation Project

3
FOL Resolution Proofs

• People who love happy people are happy.
• Rich people are happy.
• People love their spouse.
• Mothers love their children.
• Prove Bill Gates mother-in-law is happy.
• happy(x), rich(x), loves(x,y)spouse(Melinda,Bill
  )mother(MIL,Melinda)mother(Mary,Bill)

4
Answer Extraction

• Who is happy?
• Who loves Bill Gates?
• Who loves who?

5
Prolog Interpreter

• binding_list disprove(literal neglit)
• choose (clause c) such that (binding
  unify(head(c),neglit))
• if (no choice possible)
• backtrack to last choice
• for (each lit in body(c))
• binding binding U disprove(substitute(li
  t,binding))
• return binding

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Properties of Prolog Interpreter

• Tree-shaped proofs
• Efficient use of memory (just stack)
• Builds tree depth-first may enter infinite loop
  even on finite domains!
• Free implementation SWI Prolog
• http//www.swi-prolog.org/

7
happy.pl

• happy(X) - rich(X).
• happy(X) - loves(X,Y),happy(Y).
• loves(X,Y) - spouse(X,Y).
• loves(X,Y) - mother(X,Y).
• rich(bill).
• spouse(melinda,bill).
• mother(mil,melinda).
• mother(mary,bill).
• rich(paul).
• mother(barbara,henry).

8
toohappy.pl

• happy(X) - rich(X).
• happy(X) - loves(X,Y),happy(Y).
• loves(X,Y) - spouse(X,Y).
• loves(X,Y) - mother(X,Y).
• rich(bill).
• spouse(melinda,bill).
• mother(mil,melinda).
• mother(mary,bill).
• rich(paul).
• mother(barbara,henry).
• loves(bill,melinda).
• loves(henry,barbara).

9
hanoi.pl

• move(1,X,Y,_) -
• write('Move top disk from '),
• write(X),
• write(' to '),
• write(Y),
• nl.
• move(N,X,Y,Z) -
• Ngt1,
• M is N-1,
• move(M,X,Z,Y),
• move(1,X,Y,_),
• move(M,Z,Y,X).

10
nlp.pl

• Try s(a,man,loves,a,woman,).
• s(S0,S) - np(S0,S1), vp(S1,S).
• np(S0,S) - det(S0,S1), n(S1,S).
• vp(S0,S) - tv(S0,S1), np(S1,S).
• vp(S0,S) - v(S0,S).
• det(S0,S) - S0theS.
• det(S0,S) - S0aS.
• det(S0,S) - S0everyS.
• n(S0,S) - S0manS.
• n(S0,S) - S0womanS.
• n(S0,S) - S0parkS.
• tv(S0,S) - S0lovesS.
• tv(S0,S) - S0likesS.
• v(S0,S) - S0walksS.

11
Japanese 5th Generation Project

• 1982 1992 400M effort by Japanese
  corporate/government partnership to leapfrog the
  world in creating an epoch-making computer
• Approach hardware for parallel Prolog
• In the end, few results
• General purpose hardware evolved more rapidly
• Most significant software RD in Europe

12
Prolog Today

• Natural language processing
• Datalog database query and integrity
  constraints
• Function-free definite clauses bottom-up
  resolution strategy
• Expert systems diagnosis, classification,
• http//www.amzi.com/AdventureInProlog/