Topic A: Introduction to Prolog

Transcription

1 Topic A: Introduction to Prolog Recommended Exercises and Readings From Programming in Prolog (5 th Ed.) Exercises: 1.2, 1.3, 1.4, Readings: Chapters 1 and Prolog Prolog: Programming in Logic A logic programming language Expresses the logic of the computation without describing its control flow Prolog Creating a Prolog program Write facts Write rules involving the facts Running a Prolog program Write a query (or goal) Let Prolog s engine search for an answer Search processes similar to those used by Prolog are an important part of AI 3 4

2 Your First Prolog Knowledge Base Grapes, plums, apples and pears are fruit Alice has grapes Bob has apples and plums Charles has apples and pears Diane has plums, celery and carrots Create a Prolog knowledge base that includes these facts Your First Prolog Knowledge Base Load the knowledge base into Prolog Choose Consult from the File menu Query the facts Are apples fruit? Does Bob have plums? Does Bob have celery? Are bananas fruit? Is celery a vegetable? 5 6 Rules Rules allow the knowledge base to contain conditional knowledge Something is true if something else is true Add rules that state that a food is healthy if it is a fruit Add additional rules that state that a food is healthy if it is a vegetable Variables A variable is a placeholder for information that will be determined in the future Any name that starts with an uppercase letter is a variable Write Prolog goals that answer the following questions: What items does Alice have? Who has pears? Which items are vegetables? Which items are healthy? 7 8

3 Conjunction and Disjunction Conjunction And Both items must be true for the result to be true Denoted by a, in Prolog Disjunction Or At least one item must be true for the result to be true Denoted by a ; in Prolog, or by having two separate rules Variables A variable can appear more than once in a Prolog goal Create a query that determines who has both apples and plums Variables can appear in Prolog rules Rewrite our rule for healthy so that it doesn t have to list off every healthy item All fruits are healthy and all vegetables are healthy 9 10 Modus Ponens A rule of logical inference Given A rule that says a is true if b is true Knowledge that b is true Then Conclude that a is true Prolog uses this rule (repeatedly) to answer queries Fruits and Vegetables Botanically speaking A fruit is a seed bearing structure that develops from the ovary of a flowering plant A vegetable is some other part of a plant typically a leaf, a stem or a root But from a culinary point of view Some items that are botanically fruits are used as vegetables Tomato, bell peppers, egg plants, And they are shelved with the vegetables in the store 11 12

4 Fruits and Vegetables Add rules to the knowledge base that make tomatoes, bell peppers and egg plants both fruits and vegetables A Final Thought on this Example Knowledge is knowing that a tomato is a fruit. Add a rule that says that an item causes confusion if it is both a fruit and a vegetable Wisdom is knowing not to put it in a fruit salad Terms Prolog knowledge bases are constructed from three types of terms Constants Must be a number or an atom Atoms begin with a lowercase letter, look like an operator (a collection of symbols), or are quoted inside single quotes Examples: 1, 31, lisa, james, +, :, 'Apple', Variables Must begin with an uppercase letter or an underscore Examples: X, _123, ListOfValues, Terms Prolog knowledge bases are constructed from three types of terms Compound term Consists of multiple terms f(t 1, t 2,, t n ) The atom f is referred to as a functor The terms t 1 t n are referred to as components The number of terms inside the parentheses is the arity of the term f 15 16

5 Matching How does Prolog determine whether or not a goal is true? How does it identify the values for variables that satisfy a goal? Matching is performed between the goal and the knowledge base Matching Two terms, t 1 and t 2, match if: t 1 and t 2 are both the same constant term t 1 is an uninstantiated variable and t 2 is any other term t 1 is instantiated to t 2 t 2 is an uninstantiated variable and t 1 is any other term t 2 is instantiated to t 1 t 1 and t 2 both uninstantiated variables t 1 and t 2 now co refer t 1 and t 2 are both compound terms with the same functor name and arity And all corresponding terms inside parentheses match And all variable instantiations within the parentheses are compatible No other terms match Example: Family Tree Create a knowledge base that describes a family Facts Identify each person as male or female Identify each parent / child relationship Rules Describe more complex relationships like sibling, brother, sister, aunt, uncle, grandfather, grandmother, cousin, Goals / Queries Test if two people have a certain relationship Identify everyone who is related to a certain person in a given way Summary A Prolog knowledgebase consists of Facts Rules Goals / Queries can answer questions based on the information in the knowledgebase Is a statement true? What values can be used to make a statement true? The programmer does not specify the control flow for the program 19 21

6 A good example for a cut: An item is okforschool if it is not junk and it s not a nut 22