03-60-440 Principles of Programming Languages (2011F)

1. 03-60-440Principles of Programming Languages (2011F) Jianguo Lu School of Computer Science University of Windsor Jlu at uwindsor

2. It is about programming languages… • This course involves several programming languages: • Scheme (Functional) • Prolog (Logic) • AspectJ • XSLT • This course is more than excursions to various languages …

3. It is about the fundamental question • What is a programming language?

4. What is a program • It is a sequence of statements! • Imperative paradigm • It is a group of interacting objects! • OO paradigm • It is a function! • Functional paradigm • It is a set of rules! • Logic paradigm • …

5. It is about classification, history, styles (paradigms), concepts common in many languages • Programming language classification and history • Syntax and semantics of programming languages • Syntax is covered in 60-214. Won’t repeat it. • Axiomatic semantics • Paradigms of programming • Imperative and declarative programming • OOP: Abstract data type, inheritance, polymorphism, AOP (Aspect Oriented Programming) • Functional programming, lambda calculus, Scheme, MapReduce • XML Programming: XSLT • Logic programming • Concepts in programming languages • Side-effect, type checking, strong typing, overload, override, coercion, call-by-value, call-by-reference, polymorphism, multiple inheritance, generics, dynamic binding, imperative and declarative programming, …

6. Language description: Axiomatic Semantics • Language description: Syntax (covered in 03-60-214), semantics. • Formal methods to describe semantics: operational, denotational, axiomatic … • Axiomatic semantics is also used to prove the correctness of programs • Java has Assertion to describe the meaning of program • Axiomatic semantics uses a formal system (axiomatic system) to describe the meaning of programs • For example, {x=0} x= x+1; y= x; { x=1AND y=1} Axiom Inference rule

7. Imperative programming • Describes computation in terms of a program state and statements that change the program state. • Imperative programs are a sequence of commands for the computer to perform. • The hardware implementation of almost all computers is imperative. • In contrast to declarative programming Insertion sort in Java void (int[ ] A) { int j; for (int i = 1; i < A.length; i++) { int a = A[i]; for (j = i -1; j >=0 && A[j] > a; j- -) A[j + 1] = A[j]; A[j + 1] = a; } } insertionSort

8. Functional Programming • Functional programming treats computation as the evaluation of mathematical functions. • It is more heavily used in academia than in industry. • The strength of a functional paradigm is the removal of side-effects during computation. This has uses in • program verification, for checking the correctness of programs, • program optimisation. One particular use in program optimisation is to transform programs for parallel programming. • The idea is used by google etc. z = f(sqrt(2), sqrt(2)); we can factor out sqrt(2) and write s = sqrt(2); z = f(s, s);

9. Functional Programming vs. Imperative Programming • In strict functional programming, there is no explicit memory allocation and no explicit variable assignment, so side effects of function evaluation are eliminated. • Looping is accomplished through the more general functional construct of recursion. Sort in ML: funinsertsort [] = [] | insertsort (x::xs) = let fun insert (x:real, []) = [x] | insert (x:real, y::ys) = if x<=y then x::y::ys else y::insert(x, ys) in insert(x, insertsort xs) end;

10. sort function defined in Scheme language (define (insert x l) ( if (null? l) (list x) (if (<= x (car l)) (cons x l) (cons (car l) (insert x (cdr l)))))) (define (isort l) (if (null? l) () (insert (car l) (isort (cdr l)))))

11. Logic Programming • Program is a set of facts and rules. • Based on first-order predicate logic • Original motivation: study of mechanical theorem proving • Used in Artificial Intelligence, databases, expert systems. • Insertion sort in prolog isort([ ],[ ]). isort([X|UnSorted],AllSorted) :- isort(UnSorted,Sorted), insert(X,Sorted,AllSorted). insert(X, [ ], [X]). insert(X, [Y|L], [X, Y|L]) :- X =< Y. insert(X, [Y|L], [Y|IL]) :- X > Y, insert(X, L, IL).

12. OOP • Abstract Data Type • Multiple inheritance in Java • Polymorphism • Overloading • Generics • Dynamic binding

13. Distributed Object and Object Persistency from www.agiledata.org

14. AOP—Aspect Oriented Programming • A new programming paradigm • Example void transfer(Account fromAccount, Account toAccount, int amount) { if (fromAccount.getBalance() < amount) { throw new InsufficientFundsException(); } fromAccount.withdraw(amount); toAccount.deposit(amount); } • However, in a real-world banking application, this transfer method is not adequate. We need to: • Include security checks to verify that the current user has the authorization to perform this operation. • Enclose the operation in a database transaction in order to prevent accidental data loss. • Log the operation to the system log. And so on.

15. void transfer(Account fromAccount, Account toAccount, int amount) { if (!getCurrentUser().canPerform(OP_TRANSFER)) { throw new SecurityException(); } if (fromAccount.getBalance() < amount) { throw new InsufficientFundsException(); } Transaction tx = database.newTransaction(); try { fromAccount.withdraw(amount); toAcount.deposit(amount); tx.commit(); systemLog.logOperation(OP_TRANSFER, fromAccount, toAccount, amount); } catch(Exception e) { tx.rollback(); } } The code has lost its elegance and simplicity various new concerns tangled with the basic functionality (business logic concern). The transactions, security, logging, etc. all exemplify cross-cutting concerns. Implementation of crosscutting concerns are scattered across numerous methods. Change of the implementation would require a major effort. Solution: Separate different concerns. Aspect Oriented Programming

16. XSLT XSLT processor XML XSLT • It is an XML-based language • Used to transform XML; • The language itself is in XML. • It is a declarative language • Does not list an imperative sequence of actions to perform in a stateful environment, • Consists of a template rules, specifies what to add to the result.

17. Robert Sabastia, Von Roy &Haridi, John Mitchell, Benjemin Pierce, Glynn Winskel, Kenneth Louden, Seyed Roosta, Pratt & Zelkowitz

18. Times and places • Course web site: http://cs.uwindsor.ca/~jlu/440 • Office: 5111 Lambton Tower • Email: jlu at uwindsor • Office hours: Tuesday & Thursday 2:30-3:30 • Midterms: October 20, in class time • Final exam:  December 15, 15:30 • Examinations are closed books and closed notes. • The only valid excuse for missing an exam is a documented medical emergency. A missed exam without medical documentation will result in a mark of zero.

19. Grade

20. Three assignments (15 %): • Aspect oriented programming; • Functional and Logic programming; • XSLT programming. • TA will help you with your assignments • There is no text book. • Class attendance is important ! • Exams will cover whatever is taught in class; • Exams will cover the assignments.