Assignments

1. Assignments L123Assg

2. l-value vs. r-value • Pascal/Ada: x := x + 1 • C/Java: x = x + 1 l-value = location, address, reference, … r-value = int, real, (sometimes even) address, … Environment binds an identifier to a location. env : ids -> locations Store binds a location to a value. store : locations -> values assign-op :location x value x store -> store L123Assg

3. Sharing • For functional subset, it is sufficient to model env as a function from ids to values. • For imperative programming that has both assignment and sharing, separating env and store is necessary. • E.g., sharing/aliasing Point p = new Point(); Point q = p; • E.g., call by reference void f(Point p){}; f(q); L123Assg

4. Side-effect causing Scheme primitives • Initialize variable: (define <var> <exp>) • Update variable: (set! <var> <exp>) • Other ops: display, write, set-car!, set-cdr!,... (set! x y) denotes location denotes value • Sequencing: (begin <exp1> <exp2> … <expn>) L123Assg

5. Extending object (not meta-) language and the interpreter to support variable assignment L123Assg

6. Modifying environment and the interpreter • An identifier denotes the address of a mutable data structure that holds a value (that is, it models a memory location). • This address is called a reference, and the contents of these references are modified by a variable assignment. • Variable reference (var-exp (id) (deref (apply-env-ref env id))) L123Assg

7. Introduction of variable assignment • Syntax <expression> ::= set <identifier> = <expression> • Abstract Syntax varassign-exp (id rhs-exp) • Semantics (varassign-exp (var exp) (set-ref!(apply-env-ref env id) (eval-expression rhs-exp env) ) ) l-value r-value L123Assg

8. Simulating letrec using let and assignment (letrec ((var1 exp1) (var2 exp2)) exp ) (* exp1 and exp2 are lambda-forms *) (let ((var1 ’*) (var2 ’*)) (set! var1 exp1) (set! var2 exp2) exp ) L123Assg

9. Simulating Objects and Classes L123Assg

10. Defining a stack object (define empty? '()) (define push! '()) (define pop! '()) (define top'()) (let ( (stk'()) ) (set! empty? (lambda() (null? stk))) (set! push! (lambda(x) (set! stk (cons x stk)))) (set! pop! (lambda() (set! stk (cdr stk)))) (set! top (lambda() (car stk))) ) L123Assg

11. > (empty?) #t > (push! 5) > (top) 5 > (pop!) > (empty?) #t Only one stack object. Scope of stk is limited (encapsulation), but its lifetime is not. representation pvt. methods public. Persistent state. Ops. share data and change state. Using the stack object L123Assg

12. stack object : Message passing style (define stack (let ( (stk '()) ) (lambda (msg) (case msg ((empty?) (lambda () (null? stk)) ) (( push!) (lambda (x) (set! stk (cons x stk))) ) (( pop! ) (lambda () (set! stk (cdr stk))) ) ((top) (lambda () (car stk)) ) (else 'error ) ) ))) L123Assg

13. > (stack 'empty?) > ((stack 'empty?)) #t > ((stack 'push!) 5) > ((stack 'top)) 5 > ((stack 'empty?)) () or #f > ((stack 'pop!)) > ((stack 'empty?)) #t (define s1 (make-stack)) (define s2 (make-stack)) ((s1 'push!) 1) ((s2 'push!) 2) ( (s1 'top) ) 1 ( (s1 'top) ) 1 ( (s2 'top) ) 2 Object vs. Class L123Assg

14. (define stack (let ((stk '()) ) (lambda (msg) (case msg ... ) )) ) (define make-stack (lambda() (let ((stk '()) ) (lambda (msg) (case msg ... ) )) )) Instance/Object vs. Class/Type L123Assg

15. stack class : Message passing style (define make-stack (lambda () (let ( (stk '()) ) (lambda (msg) (case msg ((empty?) (lambda () (null? stk)) ) (( push!) (lambda (x) (set! stk (cons x stk))) ) (( pop! ) (lambda () (set! stk (cdr stk))) ) ((top) (lambda () (car stk)) ) (else 'error ) ))) ) ) L123Assg

16. Template for class definition (define class-name (let ((class-var val)) (lambda () (let ((inst-var val)) (lambda (msg) (case msg ((m1) code) ((m2) code) ((c3) code) (else ’error) )) )) ) ) L123Assg

17. (define make-stack (let ((pushed 0)) (lambda () (let ((stk '()) (local-pushed 0) ) (lambda (message) (case message ((empty?) (lambda () (null? stk))) ((push!) (lambda (x) (set! pushed (+ pushed 1)) (set! local-pushed (+ local-pushed 1)) (set! stk (cons x stk)))) ((pop!) (lambda () (if (null? stk) (error "Stack: Underflow") (begin (set! pushed (- pushed 1)) (set! local-pushed (- local-pushed 1)) (set! stk (cdr stk)))))) ((top) (lambda () (if (null? stk) (error "Stack: Underflow") (car stk)))) ((local-pushed) (lambda () local-pushed)) ((pushed) (lambda () pushed)) (else (error "Stack: Invalid message" message)))))))) L123Assg

18. (lambda () (let (…) … ) ) pushed = 0 (lambda (msg) … ) (lambda (msg) … ) stk = () local-pushed = 0 stk = () local-pushed = 0 make-stack (make-stack)(make-stack) L123Assg

19. Rewrite in Java publicclass Stackclass { static int pushed; private Vector stk; private int localpushed; static { pushed := 0 } public Stackclass(){ localpushed = 0; stk = new Vector(); } public boolean empty() { return stk.isEmpty() }; publicvoid push(Object x){ pushed++; localpushed++; stk.addElement(x); } ... L123Assg

20. ... publicvoid pop(){ if (stk.isEmpty()) throw new Exception(“Stack Empty”); else { --pushed; --localpushed; stk.removeElementAt(stk.size()-1); } } public Object top(){ if (empty()) thrownew Exception(“Stack Empty”);elsereturn stk.lastElement(); } public int pushed() { return pushed; } public int localpushed(){ return localpushed; } } L123Assg

21. Approximate Environment L123Assg

22. Rewrite in Java public class Test { public static void main(String [] args) { Stackclassstack1 = new Stackclass(); Stackclassstack1 = new Stackclass(); stack1.push(new Integer(7)); stack1.top(); stack2.push(new Integer(6)); stack2.push(new Integer(8)); stack2.top(); stack1.localpushed(); stack2.localpushed(); stack1.pushed(); stack2.pushed(); } } L123Assg

23. Approximate Environment L123Assg

24. Inheritance Share and reuse classes with modifications to fields and methods Improve programmer productivity and aid code evolution L123Assg

25. Delegation (define make-bounded-stack (lambda (n) (let ((bound n) (stk (make-stack))) (lambda (message) (case message ((push!) (if (< ((stk 'local-pushed)) bound) (stk 'push!) (error ”Overflow”))) ((set-bound!) (lambda (x) (set! bound x))) ((set-stack!) (lambda (s) (set! stk s))) (else(stk message)) ) )))) L123Assg

26. Code sharing by organization of objects. Delegate message handling. Dynamic and flexible. When and how to delegate can depend on system state. E.g., Java 1.1 Event Model Code sharing by organization of classes. Single vs multiple inheritance. Static but efficient. Type determines message interpretation. E.g., Java 1.0 Event Model Delegation vs Inheritance L123Assg

27. Inheritance in Java class Boundedstackclass extends Stack { private int bound; Boundedstackclass() { super(); bound = 10; } publicvoid push(Object x) { if (size() < bound) super.push(x); elsenew Exception(”Overflow”); } publicvoid setbound(int x){ bound = x; } } • Specify only incremental changes. • Access parent’s version of a method through super. L123Assg

28. Composition and Delegation in Java class Boundedstackclass { private int bound; privateStack stk; Boundedstackclass() { stk = new Stack(); bound = 10; } publicvoid push(Object x) { if (size() < bound) stk.push(x); elsenew Exception(”Overflow”); } publicvoid setbound(int x){ bound = x; } public Object top() { returnstk.top(); } ... Explicit list of other delegated stack methods } L123Assg

29. Scoping: Lexical vs Static class A { static int cv; int iv; } class Test { public static void main(String [] args){ int iv, cv; class B extends A { void print() { System.out.print( “”+ cv + iv ); /*error*/ System.out.println( “”+this.cv +this.iv ); } } new B(). print(); } } L123Assg

30. Binding: Dynamic vs Static class A { static int cv = 10; int iv = 70; int f() {return 40;} void print() { System.out.println( “”+ cv + iv + f()); }} class B extends A { static int cv = 33 ; int iv = 88; int f() {return 55;} } class Test2 { public static void main(String [] args){ new A(). print(); new B(). print(); }} L123Assg

31. Advanced Topics • Multiple Inheritance • E.g., C++, Eiffel. • Meta-classes • E.g., Smalltalk. • Inner/Nested classes • E.g., Java. • Polymorphic Static Typing • E.g., ML, Haskell. L123Assg