Lecture 1: a partial introduction to Prolog

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1 Lecture 1: a partial introduction to Prolog First programs in Prolog including: working with objects to display information; change contents of an object; comparing and contrasting Prolog and Java; briefly introducing variables and assignment in Prolog.

2 Contact hours Lectures: Semester 1 Monday Thursday Labs/workshops: UG04 Semester 1 Monday Thursday Friday Catch up sessions: to be arranged on demand Introductory matters 1

3 Important Facts Assessment Examination: 80% - May/June 2013 Continuous assessment: Tutorial sheets - getting harder and harder. See Module Web page: ~pjh/modules/current/25433/25433.html Introductory matters 2

4 Teaching method: try it out How to study? Use the resources available: WWW Prolog notes textbooks tutorials demonstrators electronic communication Introductory matters 3

5 WWW teaching resource What the WWW material contains explanations of the essentials of Prolog self-test exercises complete set of solutions How to access it From the Module Web page: ~pjh/modules/current/25433/25433.html Introductory matters 4

6 Feedback Tutorial sheets designed to help you firm-up lecture material; extend your knowledge and experience of Prolog; to understand how Prolog differs from procedural languages; assess how well you are doing and take action to improve; identify topics you find difficult and ask for extra help. Tutorial sheets are handed back within two weeks. Introductory matters 5

7 Introducing Prolog - 1 Examples based on a bank account object. Specifically: displaying the balance of the bank account object changing the balance of the bank account object Based on the introductory example in: Core Java by Cay Horstmann. 1 - Introduction 6

8 The deeper purpose: Introducing Prolog - 2 to introduce the syntax of Prolog - how programs are typed on the screen; to introduce the semantics: declarative semantics: what Prolog programs mean; procedural semantics: how a Prolog program is executed. 1 - Introduction 7

9 The trivial pursuit Some differences between languages are trivial: Java:!= Prolog: =\= Some differences are a matter of style: Java: bankaccount Prolog: bank_account Some differences aren t trivial e.g. variables: Java: balance Prolog: Balance Some differences are at the heart of a language: Java: a = a + 5 Prolog: B is A Introduction 8

10 Step 1- output This example shows: most of the syntax of Prolog; Prolog passes objects in explicit parameters; how to output to the current stream in Prolog; everything is either true or it fails in Prolog. What does this rule do? displays a bank balance on the screen. 1 - Introduction 9

11 Your first Prolog rule display_balance(bank_account(no, Name, Balance)) :- write(name), write(' account no: '), write(no), nl, write('your balance is: '), write(balance), nl. 1 - Introduction 10

12 Your first Prolog rule display_balance(bank_account(no, Name, Balance)) :- write(name), write(' account no: '), functor the name of the rule write(no), nl, write('your balance is: '), write(balance), nl. 1 - Introduction 11

13 Your first Prolog rule display_balance(bank_account(no, Name, Balance)) :- write(name), write(' account no: '), write(no), nl, write('your balance parameter is: list '), write(balance), An object is passed nl. explicitly into this rule 1 - Introduction 12

14 Your first Prolog rule display_balance(bank_account(no, Name, Balance)) :- write(name), write(' account no: '), write(no), nl, write('your balance is: '), write(balance), nl. built-in predicates write/1 and nl/0 are part of Prolog 1 - Introduction 13

15 Your first Prolog rule display_balance(bank_account(no, Name, Balance)) :- write(name), write(' account no: '), write(no), nl, if write('your balance is: '), write(balance), nl. The head of the rule is true if the body of the rule can be proved true. 1 - Introduction 14

16 Your first Prolog rule display_balance(bank_account(no, Name, Balance)) :- write(name), write(' account no: '), write(no), nl, write('your balance is: '), write(balance), nl., usually means and in Prolog. Prolog is very fussy about full-stops. 1 - Introduction 15

17 A second rule write_message(message(text1, Text2)) :- write(text1), write(' : '), write(text2), nl. Yes! Prolog is an untyped language. This displays any message/2 object whatever type their parameters are. 1 - Introduction 16

18 Calling the second rule from the first display_balance(bank_account(no, Name, Balance)) :- write(name), write_message(message(' account no: ', No)), write_message(message( balance is: ', User-defined predicates are called just like Prolog s own predicates. Balance)). 1 - Introduction 17

19 Consolidation moment Prolog s built-in predicates and user-defined predicates look the same and behave the same. Every predicate in Prolog returns true (yes) or fail (no). All of you can now write a Prolog rule. 1 - Introduction 18

20 Consolidation moment Prolog s built-in predicates and user-defined predicates look the same and behave the same. Every predicate in Prolog returns true or fail. All of you can now write a Prolog rule. Hold onto this: you ll need it to understand the last example 1 - Introduction 19

21 Withdrawing money from an account request_withdrawal(bank_account) :- write('welcome to the bank'), nl, display_balance(bank_account), write('how much do you want? '), read(cash), withdraw_cash(bank_account, Cash), display_balance(bank_account). withdraw_cash(bank_account, Cash) :- write_message(message(bank_account, Cash)). 1 - Introduction 20

22 Withdrawing money from an account request_withdrawal(bank_account) :- write('welcome to the bank'), nl, display_balance(bank_account), write('how much do you want? '), read(cash), withdraw_cash(bank_account, Cash), display_balance(bank_account). read/1 gets input from the current stream - i.e. the keyboard withdraw_cash(bank_account, Cash) :- write_message(message(bank_account, Cash)). 1 - Introduction 21

23 Withdrawing money from an account request_withdrawal(bank_account) :- write('welcome to the bank'), nl, display_balance(bank_account), write('how much do you want? '), read(cash), This is a stub - until we withdraw_cash(bank_account, Cash), display_balance(bank_account). write the proper code. withdraw_cash(bank_account, Cash) :- write_message(message(bank_account, Cash)). 1 - Introduction 22

24 Withdrawing money from an account withdraw_cash(bank_account(_no, _Name, Balance), Cash) :- Balance is Balance - Cash. This won t work because it is illogical for a variable to have two values simultaneously. 1 - Introduction 23

25 Withdrawing money from an account withdraw_cash(bank_account(_no, _Name, Balance), Cash) :- New_Balance is Balance - Cash. This won t work because there is no way of sending this variable back to the calling rule. 1 - Introduction 24

26 Withdrawing money from an account withdraw_cash(bank_account(no, Name, Old_Balance), Cash, bank_account(no, Name, New_Balance)) :- New_Balance is Old_Balance - Cash. 1 - Introduction 25

27 Withdrawing money from an account We can only communicate with the withdraw_cash(bank_account(no, calling Name, rule through a Old_Balance), parameter. Cash, bank_account(no, Name, New_Balance)) :- New_Balance is Old_Balance - Cash. 1 - Introduction 26

28 Withdrawing money from an account withdraw_cash(bank_account(no, Name, Old_Balance), in the parameters Cash, bank_account(no, Name, New_Balance)) :- New_Balance is Old_Balance - Cash. The variable has to be 1 - Introduction 27

29 We have to change the calling rule request_withdrawal(old_bank_account) :- write('welcome to the bank'), nl, display_balance(old_bank_account), write('how much do you want? '), read(cash), withdraw_cash(old_bank_account, Cash, New_Bank_Account), display_balance(new_bank_account). 1 - Introduction 28

30 We have to change the calling rule request_withdrawal(old_bank_account) :- write('welcome to the bank'), nl, arguments display_balance(old_bank_account), write('how much do you want? '), read(cash), withdraw_cash(old_bank_account, Cash, New_Bank_Account), display_balance(new_bank_account). This call now has three 1 - Introduction 29

31 Consolidation moment Variables can t be reassigned - it s not logical. Var is Var +1 is as logical as saying: 1 is For your rules to communicate with other rules, you have to: send information in through the parameter list; send information back out through the parameter list. 1 - Introduction 30

32 But banks aren t usually generous These days, banks don t seem to like people going overdrawn. We can change the code to stop customers going overdrawn - but it involves a massive step in your knowledge of Prolog: You are about to learn how to write IF - THEN - ELSE in Prolog 1 - Introduction 31

33 Declaring what we know about being in the black All we need to do is to add a test that the account isn t overdrawn: withdraw_cash(bank_account(no, Name, Old_Balance), Cash, bank_account(no, Name, New_Balance)) :- New_Balance is Old_Balance - Cash, New_Balance >= Introduction 32

34 But if the customer takes too much the program will fail Yes - but sometimes we want failure usually we want local failure - but that s for another day. Here, we want the customer to get a helpful message and then to continue the banking system. So we add an alternative rule: 1 - Introduction 33

35 Declaring what we know about trying to be in the red Note: this rule has the same name and parameters as the previous rule. withdraw_cash(bank_account(no, Name, Old_Balance), Cash, bank_account(no, Name, Old_Balance)) :- New_Balance is Old_Balance - Cash, New_Balance < 0.0, write_message(message( Account, in the red')). 1 - Introduction 34

36 If at first you don t succeed, try, try again When there is a choice of rules, Prolog tries the first and, if it works, returns true and carries on If the rule fails, it tries the next, and the next and the next, etc, until: either it finds a true rule or it fails (catastrophically) 1 - Introduction 35

37 A question Why can t Prolog have the ++ operator? For instance: A_Variable Introduction 36

38 What s ahead? Your lab session in: at 4 pm today. Collecting a tutorial sheet which will introduce you to the basics of Prolog. Sadly, the tutorial sheet is not available on the WWW. 1 - Introduction 37

39 Finally: a message from our sponsor Of course, that is irony. Irony is saying one thing and meaning the opposite! 1 - Introduction 38

CSC209. Software Tools and Systems Programming. https://mcs.utm.utoronto.ca/~209

CSC209. Software Tools and Systems Programming. https://mcs.utm.utoronto.ca/~209 CSC209 Software Tools and Systems Programming https://mcs.utm.utoronto.ca/~209 What is this Course About? Software Tools Using them Building them Systems Programming Quirks of C The file system System