cs784(prasad)l123assg1 assignments. cs784(prasad)l123assg2 l-value vs. r-value pascal/ada: x := x +...

cs784(Prasad) L123Assg 1

Assignments

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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

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SharingSharing

• 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);

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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>)

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Extending object (not meta-) language and the interpreter to support variable

assignment

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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)))

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Introduction of variable assignment• Syntax <expression> ::= set <identifier> = <expression>

• Abstract Syntaxvarassign-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

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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)

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Simulating Objects and Classes

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Defining a stack object

(define empty? '())

(define push! '())

(define pop! '())

(define top '())

(let ( (stkstk '()) )

(set! empty? (lambda() (null? stkstk)))

(set! push! (lambda(x)

(set! stkstk (cons x stkstk))))

(set! pop! (lambda()

(set! stkstk (cdr stkstk))))

(set! top (lambda() (car stkstk)))

)

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Using the stack object

> (empty?)

#t

> (push! 5)

> (top)

5

> (pop!)

> (empty?)

#t

• Only oneone stack object.• ScopeScope of stkstk is

limited (encapsulation), but its lifetimelifetime is not.– representation pvtpvt.– methods publicpublic.

• PersistentPersistent state.• Ops. share datashare data and

change statechange state.

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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 ) ) )))

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Object vs. Class

> (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

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Instance/Object vs. Class/Type

(define stack

(let ((stk '()) )

(lambda (msg)

(case msg

...

)

))

)

(define make-make-stack

(lambda()(lambda()

(let ((stk '()) )

(lambda (msg)

(case msg

...

)

))

)))

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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 ) )))

))

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Template for classclass 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) ))) ) ) )

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(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))))) )))

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make-stack

(lambda () (let (…) … ))

pushed = 0

(make-stack) (make-stack)

(lambda (msg) … )

stk = ()local-pushed = 0

(lambda (msg) … )

stk = ()local-pushed = 0

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Rewrite in Java public class 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() }; public void push(Object x){ pushed++; localpushed++; stk.addElement(x);

} ...

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... public void pop(){

if (stk.isEmpty()) throw new Exception(“Stack Empty”); else { --pushed; --localpushed;

stk.removeElementAt(stk.size()-1); }

} public Object top(){ if (empty())

throw new Exception(“Stack Empty”); else return stk.lastElement();

} public int pushed() { return pushed; } public int localpushed(){ return localpushed; }}

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Approximate Environment

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Rewrite in Javapublic class Test { public static void main(String [] args) { Stackclass stack1 = new Stackclass(); Stackclass stack1 = 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(); }}

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Approximate Environment

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Inheritance

ShareShare and reusereuse classes with modifications to fields and methods

Improve programmer productivityproductivity and aid code evolutionevolution

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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)) )

))))

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Delegation vs Inheritance

• 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

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Inheritance in Java class Boundedstackclass extends Stack { private int bound; Boundedstackclass() { super(); bound = 10; } public void push(Object x) { if (size() < bound) super.push(x); else new Exception(”Overflow”); } public void setbound(int x){ bound = x; } }

• Specify only incremental changes.• Access parent’s version of a method through super.

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Composition and Delegation in Javaclass Boundedstackclass { private int bound; private Stack stk; Boundedstackclass() { stk = new Stack(); bound = 10; } public void push(Object x) { if (size() < bound) stk.push(x); else new Exception(”Overflow”); } public void setbound(int x){ bound = x; } public Object top() { return stk.top(); } ... Explicit list of other delegated stack methods }

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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(); } }

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Binding: Dynamic vs Staticclass 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();}}

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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.