Prolog

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Prolog

• CS 171/271

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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

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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

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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

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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

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A simple Knowledge Base

• orbits(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).

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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).

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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).

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Another KB example

• parents(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).
  Whats 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).

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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!)

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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.)

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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)

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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)

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Recursion and Functions

• Make sure theyre not left-recursive
• ancestor(Person, Ancestor) - parent(Person,
  Ancestor).
• ancestor(Person, Ancestor) - parent(Person, P1),
  ancestor(P1, Ancestor).
• Functions are defined as relations
• fac(0,1).
• fac(N,F) - N gt 0, M is N - 1, fac(M,Fm), F
  is N Fm.

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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)

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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.

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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