Prolog

CS 171/271

Before we begin…

Download and install SWI-Prolog, version 5.6.4: http://gollem.science.uva.nl/cgi-bin/nph-download/SWI-Prolog/w32pl564.exe

What is Prolog? A logic programming language

“Prolog” stands for “Programming in Logic” (or “Programmation en Logique” in French)

Designed in 1972 by Alain Colmerauer and Robert Kowalski (much later than Lisp)

Used in AI programming, expert systems, and computational linguistics Functional programming (ala Lisp) is possible,

but very cumbersome

Logic Programming Creates logical models that describe

the world in which a problem exists Uses first-order logic (but can also be

used with propositional logic) A subset of declarative programming

Creates a set of conditions that describe a solution space

Two phases: declaration and interpretation

Step 1: Declaration Creating a knowledge base (KB) of

sentences describing the world Declaring facts

fun(ai). (AI is fun!) likes(mark,bacon). (Mark likes bacon)

Defining rules heartbroken(X) :- loves(X,Y), not(loves(Y,X)). (X is heartbroken if X loves Y and Y does not love X… *sniff*)

Sentences end with a period

A simple Knowledge Baseorbits(mercury, sun).orbits(venus, sun).orbits(earth, sun).orbits(mars, sun).

orbits(moon, earth).

orbits(phobos, mars).orbits(deimos, mars).

planet(P) :- orbits(P,sun).satellite(S) :- orbits(S,P), planet(P).

Step 2: Interpretation

Deriving new sentences from the sentences in the KB by making queries

Uses a Prolog interpreter SWI-Prolog, GNU Prolog, etc.

Knowledge bases must first be loaded/consulted using consult(‘filename.pl’).

Some simple queries?- consult(‘solar.pl’).% Is the moon a satellite??- satellite(moon).% Is the sun a planet??- planet(sun).% Is Uranus a planet??- planet(uranus).% What are the planets??- planet(Planet).% What objects orbit Mars??- orbits(X, mars).% Is the moon made of cheese??- made_of_cheese(moon).

Another KB exampleparents(william, diana, charles).parents(henry, diana, charles).parents(charles, elizabeth, philip).parents(diana, frances, edward).parents(anne, elizabeth, philip).parents(andrew, elizabeth, philip).parents(edwardW, elizabeth, philip).married(diana, charles).married(elizabeth, philip).married(frances, edward).married(anne, mark).

parent(C,M) :- parents(C,M,D).parent(C,D) :- parents(C,M,D).sibling(X,Y) :- parents(X,M,D), parents(Y,M,D). % What’s wrong with this?

aORuDirect(C, A) :- parent(C,P), sibling(P,A).aORuMarr(C, A) :- aORuDirect(C,X), married(X,A).aORuMarr(C, A) :- aORuDirect(C,X), married(A,X).aORu(C,A) :- aORuDirect(C,A).aORu(C,A) :- aORuMarr(C,A).

Predicates Used to express facts or statements

about objects and their relationships Examples: likes heartbroken orbits Have arguments

teaches(jpv,cs171). (2 arguments) Names/atoms are alphanumeric, can

contain underscores, begin with a lowercase letter

Meanings of predicates, arguments, and order of arguments are arbitrary (but should be descriptive and consistent!)

Variables Can be used to stand for any object Are instantiated during matching Examples: Planet X Scope is statement-wide Usually used in rules, but can also be used in facts

goes_to_heaven(Dog) :- dog(Dog). (All dogs go to heaven.)

must_perish(Thing). (All things must perish.) Variable names are alphanumeric, can contain

underscores, must begin with an uppercase letter Use the underscore to indicate an anonymous variable

female(X) :- mother(X,_). (All mothers are female, regardless of who the child is.)

Queries Describe problems to be solved Consist of goals to be satisfied

Example: father(X) :- male(X), parent(X,_). Goals are male(X) and parent (X,_).

Goals are checked against facts in the KB Rules are satisfied if all the goals in the “if” part (in

the body, separated by commas) are satisfied Variables can take on any value

Press semi-colon to look for further matches Uses backtracking to satisfy goals in all

possible ways (“brute-forcing” it)

Prolog and FOL Rules are usually in the form of Horn

clauses :- is reversed

Backtracking in Prolog is known in FOL terms as backward chaining Begins with a goal (the query) Recursively builds a set of substitutions that

satisfy the goals necessary to conclude the goal All variables in Prolog are assumed to be

universally quantified (though skolemization can still be done)

Recursion and Functions% Make sure they’re not left-recursiveancestor(Person, Ancestor) :- parent(Person, Ancestor).

ancestor(Person, Ancestor) :- parent(Person, P1), ancestor(P1, Ancestor).

% Functions are defined as relationsfac(0,1).fac(N,F) :- N > 0, M is N - 1, fac(M,Fm), F is N * Fm.

Other Prolog features

Support for tree structures and lists (similar to Haskell)

Many built-in predicates (arithmetic, logic, I/O, etc.) and operators

Can parse context-free grammars through definite clause grammars (DCGs)

Towers of Hanoi in Prolog

hanoi(N) :- move(N,a,b,c). move(0,_,_,_).move(N,From,To,Using) :- M is N-1, move(M,From,Using,To), print_move(From,To), move(M,Using,To,From).

print_move(From,To) :- write([move,disk,from,From,to,To]), nl.

Resources SWI-Prolog

http://www.swi-prolog.org/ Visual Prolog

http://www.visual-prolog.com/ GNU Prolog

http://gnu-prolog.inria.fr/ InterProlog (Java)

http://www.declarativa.com/interprolog/ P# (.NET)

http://www.dcs.ed.ac.uk/home/jjc/psharp/psharp-1.1.4/dlpsharp.html