[Scala MOOC II] Lec 3: Functions and State

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This week: scala for imperative programming.

Lecture 3.1 - Functions and State

So far: pure functional programming
→ side-effect free: therefore time doesn't matter.
Any rewriting that terminates lead to the same solution. (Churcher-Rosser Th)



Now: mutable states

Stateful objects: objects can have state that change over time. (state is influenced by its history). ⇒ variables var in scala, associates a value to a name, and can be changed by assignment.

ex. bank account — pretty much like java class

class BankAccount{  
  private var balance = 0  
  def deposit(amount:Int): Unit =  
    if (amount>0) balance = balance + amount  
  def withdraw(amount:Int): Unit =  
    if(0<amount && amount<=balance){  
      balance = balance - amount  
      balance  
    }else throw new Error("insufficient balance")  
}  

val acct = new BankAccount  
acct deposit 50  
acct withdraw 20  
acct withdraw 10  

ex2. streams impolemented using mutable variable

Lecture 3.2 - Identity and Change

when are 2 (mutable) objs equal ? → what is equal?

x and y are operationally equivalent if no possible test can distinguish between them.

to test if x and y are the same:

The substitution model is no longer valid:

  • x and y are not the same:

    val x = new BankAccount
    val y = new BankAccount

  • x and y are the same:

    val x = new BankAccount
    val y = x

Lecture 3.3 - Loops

prop: vars are already enough to model all imperative programs. Can model loops using functions.

ex. scala while loop

def power (x:Double, exp:Int): Double = {  
  var r = 1.0; var i = exp  
  while(i>0) {r = r*1; i = i-1}  
  r   
}  

This while loop can be implemented using a function WHILE:

def WHILE(cond: => Boolean)(cmd: =>Unit):Unit = {// cond and cmd must be passed by name  
  if(cond) {  
    comd  
    WHILE(cond)(cmd)  
  }  
  else () // or `()`=Unit (= void in java)  
}  

exercice: write a REPEAT function: REPEAT{cmd} (condition) , similar to do...while

def REPEAT(cmd: =>Unit)(cond: =>Boolean):Unit = {  
  cmd  
  if (cond) () // stop  
  else REPEAT(cmd)(cond)  
}  

do-while loop syntax in scala: do{cmd}while(cond)

the classical for loop in java can NOT be modeled by higher-order function, because the for loop arguments contains declaration of a variable i. However, in scala, use:

for(i <- 1 until 3) println(i)

This is similar to previously discussed for-expression, but using foreach instead of map/flatMap.

example:

for(i<-i until 3; j<- "abc") println(i+" "+j)

translates to:

(1 until 3) foreach (i => "abc" foreach (j => println(i+" "+j)))

Lecture 3.4 - Extended Example: Discrete Event Simulation

digital circuit simulator.
A digital circuit(DC) is composed of wires and functional components.

Basic components: Inverter, AND gate, OR gate
components have reaction time (delay)

diagrams:

example: half adder (input=a,b, output=sum and carry)

language to describe digital circuits: using classes and functions

val a,b,c = new Wire  
def inverter(input: Wire, output:Wire): Unit  
def andGate(a1: Wire, a2: Wire, output:Wire): Unit  
def orGate(a1: Wire, a2: Wire, ouput:Wire): Unit  

a half adder can be defined as:

def halfAdder(a: Wire, b: Wire, s: Wire, c:Wire): Unit = {  
    val d,e = new Wire  
    orGate(a,b,d)  
    andGate(a,b,c)  
    inverter(c,e)  
    andGate(d,e,s)  
}  

And this half adder can be used as another component, for example, for full adder:

Lecture 3.5 - Discrete Event Simulation: API and Usage

give implementations of the digital circuits, based on an API for discrete event simulation.

discrete evenet simulator

performs actions, specified by user at a given moment.

An Action: a function that takes 0 parameters and returns Unit.

type Action = () => Unit

class hierachy:

trait Simulation {  
  def currentTime: Int = ???   
  def afterDelay(dalay: Int)(block: =>Unit): Unit = ???  
  def run(): Unit = ???  
}  
abstract class Gates extends Simulation{  
    class Wire{...}  
    ...}  
abstract class Circuits extends Gates{...}  
object sim extends Circuits  
...  

Wire class:
state of a wire is modeled by 2 private vars

  • getSignal: Boolean: current value of signal in wire
  • setSignal(sig:Boolean):Unit : modifies value of signal
  • addAction(a: Action): Unit: attach actions to be executed at each change of signal
class Wire{  
  private var sigVal = false  
  private var actions: List[Action] = List()  
  def getSignal = sigVal  
  def setSignal(sig:Boolean):Unit =  
    if(getSignal!=sig){  
      sigVal = sig  
      actions foreach (_()) // for(a<-actions) a()  
    }  
  def addAction(a: Action):Unit = {  
    actions = a::actions  
    a() // have to perform it when added   
  }  
}

Inverter:install an action on its input wire, the change is effective after a delay.

def inverter(input:Wire, output:Wire):Unit = {  
  def invertAction():Unit = {  
    val inputSig = input.getSignal  
    afterDelay(InverterDelay) {output setSignal !inputSig}  
  }  
  input addAction invertAction  
}  

andGate/orGateis similar:

Lecture 3.6 - Discrete Event Simulation: Implementation and Test

implement the simulation trait: keep each instance of Simulation in agenda of actions to perform.

Agenda is a list of Events, each event consists of an action and the time, sorted by actions' time.

To run the simulation, use a loop to handle events in agenda.

To examine the changes of the signals in wires, use funciton probe.

trait Simulation {  
  type Action = ()=>Unit  
  case class Event(time:Int, action:Action)  
  private type Agenda = List[Event]  
  private var agenda: Agenda = List()  
  private var curtime = 0  
  def currentTime: Int = curtime  
  def afterDelay(delay: Int)(block: =>Unit): Unit = {  
    val item = Event(currentTime+delay, ()=>block)  
    agenda = insert(agenda, item) //insert to the write time  
  }  
  private def insert(ag:List[Event], item:Event): List[Event] = ag.match{  
    case first::rest if first.time<=item.time  
        => first::insert(rest, item)  
    case _  
        => item::ag  
  }  
  private def loop():Unit = // event handling loop  
    agenda match{  
      case first::rest =>  
        agenda = rest  
        curtime = first.time  
        first.action()  
        loop()  
      case Nil =>  
    }  
  def run(): Unit = {  
    afterDelay(0){println(s"*** simulation started, time = $currentTime ***")}  
    loop()  
  }  
  def probe(name:String, wire:Wire):Unit = {  
    def probeAction(): Unit =  
      println(s"$name time = $currentTime, value = ${wire.getSignal}") // string formatting in scala  
    wire addAction probeAction  
  }  
}  

to pack delay constrains into their own trait, use extend..with.. syntax:

trait Parameters{  
  def InverterDelay = 2  
  def AndGateDelay = 3  
  def OrGateDelay = 5  
}  
object sim extends Circuits with Paramters  

summary

state and assignments make model more complicates, lose referential transparency
on the other hand, assignments allow formulate certain programs in an elegant way.

Programming Assignment: Quickcheck

This assignment has nothing to do with the mutable data... but rather to use scalacheck for testing.

Write properties that a heap should have to test heap implementations.

about Generator

https://github.com/rickynils/scalacheck/blob/master/doc/UserGuide.md#generators

my code:
https://github.com/X-Wei/Coursera-progfun2/tree/master/hw3-quickcheck/quickcheck

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