Assignments

1
Assignments
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 -gt locations
• Store binds a location to a value.
• store locations -gt values
• assign-op location x value x store -gt store

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)

4
Side-effect causing Scheme primitives

• Initialize variable (define ltvargt ltexpgt)
• Update variable (set! ltvargt ltexpgt)
• Other ops display, write,
• set-car!, set-cdr!,...
• (set! x y)
• denotes location denotes value
• Sequencing
• (begin ltexp1gt ltexp2gt ltexpngt)

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

7
Introduction of variable assignment

• Syntax
• ltexpressiongt
• set ltidentifiergt ltexpressiongt
• 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

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
• )

9
Simulating Objects and Classes
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)))
• )

11
Using the stack object

• gt (empty?)
• t
• gt (push! 5)
• gt (top)
• 5
• gt (pop!)
• gt (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.

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 ) )
• )))

13
Object vs. Class

• (define s1
• (make-stack))
• (define s2
• (make-stack))
• ((s1 'push!) 1)
• ((s2 'push!) 2)
• ( (s1 'top) )
• 1
• ( (s1 'top) )
• 1
• ( (s2 'top) )
• 2

• gt (stack 'empty?)
• gt ((stack 'empty?))
• t
• gt ((stack 'push!) 5)
• gt ((stack 'top))
• 5
• gt ((stack 'empty?))
• () or f
• gt ((stack 'pop!))
• gt ((stack 'empty?))
• t

14
Instance/Object vs. Class/Type

• (define stack
• (let ((stk '()) )
• (lambda (msg)
• (case msg
• ...
• )
• ))
• )

• (define make-stack
• (lambda()
• (let ((stk '()) )
• (lambda (msg)
• (case msg
• ...
• )
• ))
• ))

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 )
• )))
• )
• )

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) ))
• ) ) ) )

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))

18
make-stack
(make-stack) (make-stack)
19
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)
• ...

20

• ...
• 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

21
Approximate Environment
22
Rewrite in Java

• public 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()

23
Approximate Environment
24
Inheritance

• Share and reuse classes with modifications to
  fields and methods
• Improve programmer productivity and aid code
  evolution

25
Delegation

• (define make-bounded-stack
• (lambda (n)
• (let ((bound n) (stk (make-stack)))
• (lambda (message)
• (case message
• ((push!)
• (if (lt ((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))
• )
• ))))

26
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

27
Inheritance in Java

• class Boundedstackclass extends Stack
• private int bound
• Boundedstackclass()
• super()
• bound 10
• public void push(Object x)
• if (size() lt bound)
• super.push(x)
• else new Exception(Overflow)
• public void setbound(int x)
• bound x
• Specify only incremental changes.
• Access parents version of a method through super.

28
Composition and Delegation in Java

• class Boundedstackclass
• private int bound
• private Stack stk
• Boundedstackclass()
• stk new Stack()
• bound 10
• public void push(Object x)
• if (size() lt bound)
• stk.push(x)
• else new Exception(Overflow)
• public void setbound(int x)
• bound x
• public Object top()
• return stk.top()
• ...

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()

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()

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.