# Logic Programming and Prolog Danielle and Joseph Bennett 24 April 2007.

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Logic Programming and Prolog Danielle and Joseph Bennett 24 April 2007

What is Logic Programming?

“The use of mathematical logic for computer programming.” (Wikipedia) “A declarative, relational style of programming based on first-order logic.” (Dictionary.com)

History of Logic Programming Came about in 1960s and 1970s due to debates about using declarative or procedural representations in AI Stanford and Edinburgh – declarative MIT - procedural Developed Planner in 1969 (first language in the proceduralistic paradigm).

Systems for Logic Programming ALF CLP ECLiPSe Elf Fish Flang Gödel KLIC LIFE MONA Oz System RELFUN SAMPLE XSB Just to name a few….

Systems for Logic Programming, cont. Prolog is the most common system Prolog has many variations: &-Prolog, And-parallel Prolog. ACE, And-Or-parallel Prolog. Actor Prolog Andorra-I, an Or- and (deterministic) and-parallel Prolog. Aurora, Or-parallel Prolog. cu-Prolog, a constraint logic programming language lambda Prolog LeanTaP, a small theorem prover written in SICStus Prolog. Logtalk, an extension for Object-Oriented Programming in Prolog. Mixtus, an automatic partial evaluator for full Prolog. Muse, Or-parallel Prolog.

Developed in 1972 by Alain Colmerauer and Philippe Roussel Name comes from “PROgramming in LOGic” A solution to the debate about which kinds of logic to use

Prolog Structure Prolog facts – a database of predicates and associations. Prolog rules – define new predicates by using Prolog facts. Note: Prolog considers capital letters to denote variables, not predicates.

Prolog Structure – Queries A query searches the database for the first fact that satisfies its goal. If a fact is found it either unifies the variable with a constant or returns yes. If a fact is not found that meets that condition it returns no.

Prolog Structure – Queries, cont. Use a semi-colon to request subsequent answers. In other words, a semi-colon signifies disjunction. A comma signifies conjunction.

Prolog Structure - Unification A query resolves by unifying all of its elements. A constant unifies with itself and any variable. Scope is limited to the rule in which a variable occurs. When a variable is unified with a constant in a rule, all instances of that variable in that rule are unified to that constant.

Prolog Structure - Backtracking In more complex examples, Prolog uses backtracking to find possible solutions. Prolog will attempt to resolve the first fact of its rule, unifying any variables with the first constant that satisfies that fact It then attempts to resolve the rest of that rules facts. If it is unable to do so under those conditions it “backs up” and tries again with the next available unifying constant.

More Than Just Information Prolog rules can also be used write programs that do more than find the answers to simple database queries. append([], L, L). append([H|T], L, [H|L1]):-append(T, L, L1). This will append a list to another list recursively. A binary tree can be defined as follows tree(nil). tree(node(_, Left, Right):-tree(left), tree(right).

How Prolog Works Example from pages 45-46 of our text: instructor (bebis, cs365) instructor (looney, cs311) instructor (yuksel, cs446) enrolled (joseph, cs311) enrolled (joseph, cs365) enrolled (joseph, cs446) enrolled (danielle, cs365) enrolled (danielle, cs446) This is the database of Prolog facts.

How Prolog Works, cont. Prolog rules: teaches (P,S) :- instructor (P,C), enrolled (S,C) This is to say that an instructor only teaches if he teaches a class and students are enrolled in that class.

How Prolog Works, cont. Prolog answers queries based off of the database that has been given. ?enrolled (joseph, cs365) yes ?enrolled (X, cs365) joseph danielle ?teaches (X, joseph) bebis looney yuksel

How Prolog Works, cont. Imagine what happens if we expand the database: instructor (bebis, cs365) instructor (looney, cs311) instructor (yuksel, cs446) instructor (helfand, cs493) instructor (quint, math486) enrolled (ben, cs365) enrolled (bill, cs365) enrolled (bill, cs446) enrolled (brian, cs311) enrolled (brian, cs365) enrolled (brittney, cs311) enrolled (brittney, cs365) enrolled (brittney, cs446) enrolled (cody, cs311) enrolled (cody, cs365) enrolled (danielle, cs365) enrolled (danielle, cs446) enrolled (danielle, cs493) enrolled (david, cs365) enrolled (javier, cs365) enrolled (jeffrey, cs365) enrolled (jessica, cs311) enrolled (jessica, cs446) enrolled (jessica, math486) enrolled (joel, cs365) enrolled (joseph, cs311) enrolled (joseph, cs365) enrolled (joseph, cs446) enrolled (joseph, cs493) enrolled (joseph, math486) enrolled (kellen, cs365) enrolled (matts, cs311) enrolled (matts, cs365) enrolled (mattw, cs311) enrolled (mattw, cs365) enrolled (mattw, cs446) enrolled (miran, cs365) enrolled (ryan, cs365) enrolled (samuel, cs365) enrolled (shane, cs311) enrolled (shane, cs365) enrolled (shane, cs446) enrolled (tiffany, cs311) enrolled (tiffany, cs365) enrolled (tiffany, cs446)

How Prolog Works, cont. ?enrolled (X, cs365) ben bill brian brittney cody danielle david javier jeffrey joel joseph kellen matts mattw miran ryan samuel shane tiffany This list now gives us the entire roster of students in CS 365.

How Prolog Works, cont. Queries can be more complicated to compare more data: classmates (S1, S2) :- enrolled (S1, C), enrolled (S2, C) ?classmates (joseph, danielle) yes ?classmates (joseph, jessica) yes ?classmates (jessica, danielle) no

How Prolog Works, cont. classmates (S1, S2, C) :- enrolled (S1, C), enrolled (S2, C) ?classmates (joseph, danielle, C) cs365 cs446 cs493 no ?classmates (joseph, jessica, C) math ?classmates (jessica, danielle, C) no

Free Prolog Access SWI-Prolog http://www.swi-prolog.org/ YAProlog http://www.ncc.up.pt/~vsc/Yap/ Strawberry Prolog http://www.dobrev.com/

Sources http://www.afm.sbu.ac.uk/logic-prog/ http://en.wikipedia.org/wiki/Logic_programming http://dictionary.reference.com/browse/logic%20program ming http://dictionary.reference.com/browse/logic%20program ming Pages 45-46 of our textbook CS 326 lectures on Prolog (written by Dr. Mircea Nicolescu)

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