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Logic Programming

Logic Programming. Programming Lecture 1: Getting started with Prolog. Recap. Logic programming is a form of declarative programming "Algorithm = logic + control" Specify a problem, let computer find solution This does not always work out as well as we would wish

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Logic Programming

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  1. Logic Programming • Programming Lecture 1: • Getting started with Prolog

  2. Recap • Logic programming is a form of declarative programming • "Algorithm = logic + control" • Specify a problem, let computer find solution • This does not always work out as well as we would wish • Writing effective logic programs generally still requires pragmatic knowledge

  3. Why learn LP? • Learning a very different "way to think about problems" makes you a better programmer • LP works well for rapidly prototyping algorithms/search strategies, which can be transferred to mainstream language • "Declarative" ideas arise in many areas of CS and AI • LP concepts very important in AI, databases, PL • SAT solvers, model-checking, constraint programming • Becoming important in program analysis, Semantic Web • Learning how logic provides a foundation for computation can improve your understanding of both

  4. Getting started • There are many Prolog implementations & dialects • We’ll use SICStus Prolog • Free for UofE students • Can request through Windows support • Available on all DICE machines • You should become familiar with running on DICE • Tutorials, assignments, exams will be based on this version • Online documentation • http://sicstus.sics.se/documentation.html

  5. 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, many library copies

  6. Hello World • Prolog is an interactive language. • $ sicstus

  7. Hello World • Prolog is an interactive language. • $ sicstus • ?- Prompt

  8. Hello World • Prolog is an interactive language. • $ sicstus • ?- print(’hello world’). Goal

  9. Hello World • Prolog is an interactive language. • $ sicstus • ?- print(’hello world’). • hello world • yes Output response

  10. 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’

  11. Variables • A variable is a sequence of alphanumeric characters • usually starting with an uppercase letter • Examples: • X Y Z Parent Child Foo _Bar

  12. 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)

  13. 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

  14. Facts Punctuation is important! • 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).

  15. 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

  16. Answers • Given a goal, Prolog searches for answer(s) • “yes” (possibly with answer substitution) • “no” • Substitutions are bindings of variables that make goal true • Use “;” to see more answers

  17. 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

  18. 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).

  19. Miscellaneous • Comments • % single line comment • /* multiple • line • comment */ • To quit Sicstus, type • ?- halt. • (or just control-D)

  20. 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.pl’).

  21. Consulting (2) • If the file name ends with '.pl', can just write: • ?- consult(simpsons). • Also, can just write • ?- [simpsons].

  22. A complete program • /* hello.pl • * James Cheney • * Sept. 18, 2014 • */ • main :- print('hello world').

  23. Tracing • trace/0 turns on tracing • notrace/0 turns tracing off • debugging/0 shows tracing status

  24. More kinds of terms • Story so far... • Atoms: homer marge 'Mr. Burns' • Variables: X Y Z MR_BURNS • Also have... • Numbers: 1 2 3 42 -0.12345 • Lists[1,2,3] and other complex terms • Additional constants and infix operators

  25. Complex terms • A complex term is of the form • f(t1,...,tn) • where f is an atom and t1...tn are terms • Examples: • f(1,2) node(leaf,leaf) cons(42,nil) • household(homer, marge, bart, lisa, maggie)

  26. More about lists • Lists are built-in (and very useful) data structures • Syntax: • [1,2,3,4] • [a,[1,2,3],42,'forty-two'] • [a,b,c|Xs] • (Lots) More next week

  27. Exercises • Using simpsons.pl, write goal bodies for: • classmate(X,Y) • employer(X) • parent(X,Y) • grandparent(X,Y) • More in tutorial problems handout

  28. Next time • Equality and unification • How Prolog searches for answers • Reminder: Tutorials start in week 3

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