CS 152: Programming Language Paradigms

Prolog: Resolution, Unification, & Cuts

Prof. Tom Austin San José State University References for Prolog

• "Learn Prolog Now", http://www.learnprolognow.org • SWI-Prolog website (contains manual and tutorials), http://www.swi-prolog.org • "NLP with Prolog in the IBM Watson System", http://www.cs.nmsu.edu/ALP/2011/03/ natural-language-processing-with-prolog-in- the-ibm-watson-system/ Review: Facts likes(batman, gotham). likes(batman, justice). likes(ras_al_ghul, justice). likes(ras_al_ghul, revenge). Review: Queries & Variables

What do Batman and Ra's al Ghul both like? X is a comma variable is "and" ?- likes(batman,X), likes(ras_al_ghul,X).

How does Prolog resolve queries?

Through 2 processes: • Resolution • Unification Resolution & Unification

• Resolution: The process of matching facts & rules to perform inferencing – infer: derive logical conclusions from the rules. – If a subgoal matches the head of another rule, we can replace it with the body of the matching rule. • Unification: Instantiation of variables via pattern matching

Query: likes(batman, X), likes(ras_al_ghul, X).

Knowledge Base: likes(batman, gotham). likes(batman, justice). likes(ras_al_ghul, justice). likes(ras_al_ghul, revenge). Query: likes(batman, X), likes(ras_al_ghul, X). Finds match for first sub-query; Knowledge Base: sets a marker likes(batman, gotham). likes(batman, justice). likes(ras_al_ghul, justice). likes(ras_al_ghul, revenge). Query: likes(batman, gotham), likes(ras_al_ghul, gotham). Knowledge Base: likes(batman, gotham). X is bound likes(batman, justice). to gotham likes(ras_al_ghul, justice). likes(ras_al_ghul, revenge). Query: likes(batman, gotham), likes(ras_al_ghul, No match found: gotham). fails and backtracks Knowledgeto marker Base: likes(batman, gotham). likes(batman, justice). likes(ras_al_ghul, justice). likes(ras_al_ghul, revenge). Query: likes(batman, X), likes(ras_al_ghul, X).

Knowledge Base: Finds another match for first likes(batman, gotham). sub-query likes(batman, justice). likes(ras_al_ghul, justice). likes(ras_al_ghul, revenge). Query: likes(batman, justice), likes(ras_al_ghul, justice). Knowledge Base: X is bound to likes(batman, gotham). justice likes(batman, justice). likes(ras_al_ghul, justice). likes(ras_al_ghul, revenge). Query: likes(batman, justice), likes(ras_al_ghul, justice). Knowledge Base: likes(batman, gotham). Match found, and the result likes(batman, justice). is returned likes(ras_al_ghul, justice). likes(ras_al_ghul, revenge). villain(joker). More villain(penguin). facts: villain(catwoman). villain(scarecrow).

kills_people(joker). kills_people(penguin). power(scarecrow, fear).

romantic_interest(catwoman). romantic_interest(talia). Rules Queries scary(V) :- villain(V),

"Head" of kills_people(V). the rule scary(V) :- villain(V), power(V,_). Who is scary? (in-class) Math in Prolog Arithmetic in Prolog heists(joker, 97). heists(penguin, 18). heists(catwoman, 31). heists(scarecrow, 42). combined_heists(X, Y, Total) :- heists(X,XN), heists(Y,YN), Total = XN + YN. ?- combined_heists(catwoman, scarecrow, T). T = 31+42. Using "is" operator combined_heists(X, Y, Total) :- heists(X,XN), heists(Y,YN), Total is XN + YN. ... ?- combined_heists(catwoman, scarecrow, T). T = 73.

The Cut Operator

"Learn Prolog Now" section 10.2 The Cut Operator

Motivation: • Prolog may needlessly backtrack • We wish to stop the backtracking to optimize our code. max example (no cuts) max(X,Y,Y):- X =< Y. max(X,Y,X):- X > Y. Using max ?- max(2,3,M). M = 3 ; Why continue false. the search?

?- max(2,1,M). M = 2 ; false. Two types of cuts (!)

• A green cut – improves performance or memory usage – Does not alter results • A red cut – controls resolution to prevent future matches – changes the results – is considered "bad form" max example (no cuts) max(X,Y,Y):- X =< Y.

max(X,Y,X):- X > Y. If true, no need to keep searching max example, with green cut max(X,Y,Y):- X =< Y, !. max(X,Y,X):- X > Y. Red Cut Example

Batman is enemies with all villains, unless the villain is also a romantic interest. Red Cut Example enemy(batman, X) :- romantic_interest(X), !, No backtracking once fail. we make it here. enemy(batman, X) :- villain(X). Red Cut Example bad_breakup(batman, talia). bad_breakup(batman, poison_ivy). enemy(batman, X) :- romantic_interest(X), !, bad_breakup(batman, X). enemy(batman, X) :- villain(X). Avoiding red cut bad_breakup(batman, talia). bad_breakup(batman, poison_ivy). enemy(batman, X) :- villain(X), \+ romantic_interest(X). enemy(batman, X) :- villain(X), bad_breakup(batman,X). Lab

For the rest of the class, continue with the lab from last class.