17 LogicProg

# 17 LogicProg - 1 Logic Programming Scott Chapter 11 2 Logic...

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Unformatted text preview: 1 Logic Programming Scott, Chapter 11 2 Logic Programming • Recall – functional programming • computing with functions • foundation: lambda calculus – imperative programming • computing with side effects • Turing machine 3 • Logic programming – computing with relations – foundation: predicate calculus – while functional and imperative languages implement the full power of the lambda calculus or Turing machine, logic programming only implements a restriction of the predicate calculus – logic programming is still surprisingly powerful 4 • The most widely used logic programming language is Prolog (1972) – Roussel and Colmerauer at Aix-Marseille – Kowalski at Edinburgh • We will focus on Prolog 5 • Computing in Prolog: – The programmer provides database of facts and rules of inference – Queries formulated as assertions involving facts in database – Program execution = attempting to prove an assertion. 6 Synax <term> ::= <constant> | <variable> | <structure> <constant> ::= <number> | <atom> <structure> ::= <functor> ( <term>(,<term>)* ) <functor> ::= <atom> <atom> ::= <lowercase>(<letter>| some other things )* <variable> ::= <uppercase>(<letter>| some other things )* Example: near (house, X, 22, distance(Y)) constants: house, 22 variables: X,Y functors: distance, near structures: distance(Y), near (house, X, 22, distance(Y)) 7 • A database is constructed out of facts and rules – constants representing people: tom,dick,harry, jane, judy, mary – predicates: fatherOf, motherOf, parentOf, grandparentOf, siblingOf Facts: fatherOf(tom,dick). %read “the father of tom is dick” fatherOf(dick,harry). fatherOf(jane,harry). motherOf(tom,judy). motherOf(dick,mary). motherOf(jane,mary). %period terminates facts and rules, %which may use more than one line 8 • While we choose the facts to have some interpretation, to Prolog, they are just structural building blocks with no meaning except the explicitly stated relationships • We can think of fatherOf(tom,dick) as a predicate asserting that tom is the father of dick, or a data structure (tree) where the functor fatherOf is the root, and tom and dick are children. 9 query: request to prove an assertion built of predicates ?- fatherOf(dick,harry). Yes ?- fatherOf(harry,tom). No Database fatherOf(tom,dick). fatherOf(dick,harry). fatherOf(jane,harry). motherOf(tom,judy). motherOf(dick,mary). motherOf(jane,mary). 10 • Queries may contain variables. – Prolog tries to find an assignment of variables that make the predicate true. – One solution is given. The user may type ; to get another. ?- fatherOf(X,harry) X = dick ; X = jane ; No ?- motherOf(X,Y) X = tom, Y = judy ; X = dick, Y = mary ; X = jane, Y = mary No Database fatherOf(tom,dick)....
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17 LogicProg - 1 Logic Programming Scott Chapter 11 2 Logic...

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