Logic Programming

Lecture 1: Course orientation;

Getting started with Prolog

Assoc. Prof. Zeki Bayram

Based on slides by James Cheney

Logic Programming

What is Logic Programming?

• Logic Programming is a programming paradigm

• cf. object-oriented or functional programming

• Not a particular language (like Java or Haskell)

•Declarative philosophy

• specify problem, let computer search for answer

Logic Programming

The declarative dream

•Slogan: "Algorithm = Logic + Control"

•Would like to write a program that describes the solutions to the problem

•Computer then finds answers

•Programmer shouldn't have to think about how the system works

Logic Programming

The reality

•Purely declarative programming can only get you so far

•I/O, interaction with outside world, seem inherently "imperative"

•For efficiency/termination, sometimes need finer-grained control over search

Logic Programming

Prolog•Prolog is the best-known LP language

•Core based on first-order (predicate) logic

•Algorithmic realization via unification, search

•Many implementations that make it into a full-fledged programming language

• I/O, primitive ops, efficiency issues complicate declarative story

Logic Programming

Why learn LP?• LP often great for rapidly prototyping

algorithms/search strategies

• "Declarative" ideas arise in many areas of CS

• LP concepts very important in AI, databases, PL

• SAT solvers, model-checking, constraint programming

• Becoming important in program analysis, Semantic Web

• Learning a very different "way to think about problems" makes you a better programmer

Logic Programming

Course objectives• Theory

• first-order logic, semantics of LP

• unification, resolution proof search

• Programming

• basic LP techniques in Prolog

• how to use LP to solve interesting problems

• AI, parsing, search, symbolic processing

Logic Programming

Atoms•An atom is

• a sequence of alphanumeric characters

• usually starting with lower case letter

• or, a string enclosed in single quotes

•Examples:

•homer marge lisa bart

•‘Mr. Burns’ ’Principal Skinner’

Logic Programming

Variables

•A variable is a sequence of alphanumeric characters

•usually starting with an uppercase letter

•Examples:

•X Y Z Parent Child Foo

Logic Programming

Predicates•A predicate has the form

•p(t1,...,tn)

•where p is an atom and t1...tn are terms

•(For now a term is just an atom or variable)

•Examples:

•father(homer, bart)

•mother(marge, bart)

Logic Programming

Predicates (2)•A predicate has a name

• = atom p in p(t1,...,tn)

•and an arity

• = number of arguments (n)

•Predicates with same name but different arity are different

•We write foo/1, foo/2, ... to refer to these different predicates

Logic Programming

Facts

•A fact is an assertion that a predicate is true:

•father(homer, bart).

•mother(marge, bart).

•A collection of facts is sometimes called a knowledge base (or database).

Punctuation is Punctuation is important!important!

Logic Programming

Goals•A goal is a sequence of predicates

•p(t1,...,tn), ..., q(t1',...,tn').

•We interpret “,” as conjunction

•Logically, read as "p holds of t1...tn and ... and q holds of t1'...tm'"

•Predicates can be 0-ary

•Some built-ins: true, false, fail

Logic Programming

Answers•Given a goal, Prolog searches for

answers

• “yes” (possibly with answer substitution)

• “no”

•Substitutions are bindings of variables that make goal true

•Use “;” to see more answers

Logic Programming

Examples• ?- father(X,bart).

• X = homer ;

• no

• ?- father(X,Z), mother(Y,Z).

• X = homer, Y = marge, Z = bart ;

• X = homer, Y = marge, Z = lisa ;

• X = homer, Y = marge, Z = maggie ;

• no

Logic Programming

Rules• A rule is an assertion of the form

• p(ts) :- q(ts’), ..., r(ts’’).

• where ts, ts’, ts’’ are sequences of terms

• “p(ts) holds if q(ts’) holds and ... and r(ts’’) holds”

• Example:

• sibling(X,Y) :- parent(Z,X),

• parent(Z,Y).

Logic Programming

Miscellaneous

•Comments

•% single line comment

•/* multiple

• line

• comment */

Logic Programming

Consulting

•A Prolog program is a collection of facts and rules, or clauses

• stored in one or more files

•The predicate consult/1 loads the facts/rules in a file

•?- consult(‘simpsons.pro’).

Logic Programming

Consulting (2)

•If the file name ends with '.pl', can just write:

•?- consult(simpsons).

•Also, can just write

•?- [simpsons].

Logic Programming

A complete program

• /* hello.pl

• * James Cheney

• * Sept. 20, 2010

• */

• main :- write('hello world').

Logic Programming

Tracing

•trace/0 turns on tracing

•notrace/0 turns tracing off

•debugging/0 shows tracing status

•But: better to use Amzi Prolog’s built-in debugger

Logic Programming

Further reading•Quick Start Prolog notes (Dave Robertson)

•http://www.inf.ed.ac.uk/teaching/courses/lp/prolognotes.pdf

•Learn Prolog Now! (Blackburn, Bos, Striegnitz)

• online, free

•http://www.learnprolognow.org/

•Programming in Prolog (Clocksin & Mellish)

• a standard/classic text

Logic Programming

Exercises•Using simpsons.pro, write goal

bodies for:

• classmate(X,Y)

• employer(X)

• parent(X,Y)

• grandparent(X,Y)

Logic Programming

Next time

•Compound terms

•Equality and unification

•How Prolog searches for answers