Scala by Luc Duponcheel

Scala @ BeJUG
Luc Duponcheel
ImagineJ
September 2, 2009

Introduction

Chapter 0

Introduction

Types

Scala is a (taste)fully typed language

Types

Scala supports generic types

Expressions

Scala is an expression oriented language

Objects

Scala is an pure object oriented language

Functions

Scala is a functional language

Scalable language

Scala is a scalable language
Scala does not grow on a language basis
Scala grows on a library basis

Expression evaluation: Double

scala> 1.2.*(2.3)
res0: Double = 2.76

scala> 1.2 * 2.3
res1: Double = 2.76

Expression evaluation: String

scala> "Hello world".substring(2,9)
res2: java.lang.String = llo wor

scala> "Hello world" substring (2,9)
res3: java.lang.String = llo wor

Function result

def plus(x: Int) = (y: Int) => y + x

Function result

scala> :l plus.scala
Loading plus.scala...
plus: (x: Int)(Int) => Int

scala> plus(1)
res0: (Int) => Int = <function1>

Apply plus(1)

scala> plus(1).apply(1)
res1: Int = 2

scala> plus(1)(1)
res2: Int = 2

Function parameter

def applyToZero[X](f: Int => X) = f(0)

Function parameter

scala> :l applyToZero.scala
Loading applyToZero.scala...
applyToZero: [X]((Int) => X)X

scala> applyToZero (x => x + 1)
res0: Int = 1

Function parameter and result

def twice[X](f: X => X)
= (x: X) => f(f(x))


scala> :l twice.scala
Loading twice.scala...
twice: [X]((X) => X)(X) => X

scala> twice((x: Int) => x + 1)(0)
res0: Int = 2

Many parameter lists

def twice[X](f: X => X)(x: X) = f(f(x))


scala> :l twiceMany.scala
Loading twiceMany.scala...
twice: [X](f: (X) => X)(x: X)X

scala> twice((x: Int) => x + 1)(0)
res0: Int = 2

Placeholder syntax

scala> :l applyToZero.scala
Loading applyToZero.scala...
applyToZero: [X]((Int) => X)X

scala> applyToZero (_ + 1)
res0: Int = 1

Patterns

Chapter 2

Patterns

Compositional reuse

Compositional reuse is reuse of
code fragments
the building blocks, starting from atomic ones
code templates
the containers into which the building blocks
can be plugged

Natural numbers and digits

type Nat = Int
type Dig = Int

Structural recursion

def structRec[A]
(a: A, op: (Dig, A) => A) : Nat => A = {
(n: Nat) =>
if(n == 0) {
a
} else {
op(n % 10, structRec(a, op)(n / 10))
}
}

Using structural recursion

def sumOfDigits =
structRec[Int](0, _ + _)

def hasDigit(d: Dig) =
structRec[Boolean](false, _ == d || _)

def exists(p: Dig => Boolean) =
structRec[Boolean](false, p(_) || _)

Using structural recursion

scala> sumOfDigits(1234)
res0: Int = 10

scala> hasDigit(2)(1234)
res1: Boolean = true

scala> exists(_ == 2)(1234)

unfoldAndThenFold

6
---- | / |
| dig = 3 + 3 = rec |
-----------------------
| 2 + 1 |
| / |
| 1 + 0 |
| / |
| 0 0 |

List of digits

:
/
3 :
/
2 :
/
1 :
/
0

Sum of list of digits

+
/
3 +
/
2 +
/
1 +
/
0 0

Tail recursion

def tailRec[A]
(a: A, op: (Dig, A) => A) : Nat => A = {
(n: Nat) =>
if(n == 0) {
a
} else {
tailRec(op(n % 10, a), op)(n / 10)
}
}

Using tail recursion

def sumOfDigits =
tailRec[Int](0, _ + _)

def hasDigit(d: Dig) =
tailRec[Boolean](false, _ == d || _)

def exists(p: Dig => Boolean) =
tailRec[Boolean](false, p(_) || _)

Using tail recursion

scala> sumOfDigits(1234)
res0: Int = 10

scala> hasDigit(2)(1234)

scala> exists(_ == 2)(1234)

iterateAndAccumulate

6 = rec
| / |
| 0 6 |
| / |
| 1 + 5 |
| ------------
| 2| + 3 = acc |
--------| / |
| dig = 3 + 0 |

Control

Chapter 3

Control

Conditional construct

def _if
(cond: Boolean)
(block: () => Unit)
= cond match {
case true => block()
case false => ()
}

Conditional expression

scala> :l if.scala
Loading if.scala...
_if: (Boolean)(() => Unit)Unit

scala> _if(Math.random<0.5)(() =>
| println("ok")
| )
ok

Loop construct

def _while
(cond: () => Boolean)
(block: () => Unit): Unit
= if(cond()) {
block()
_while(cond)(block)
}

Loop expression

scala> :l while.scala
Loading while.scala...
_while: (() => Boolean)(() => Unit)Unit

scala> _while(() => Math.random<0.5)(() =>
| println ("ok")
| )
ok

Conditional construct

def _if
(cond: Boolean)
(block: => Unit)
= cond match {
case true => block
case false => ()
}

Loop construct

def _while
(cond: => Boolean)
(block: => Unit): Unit
= if(cond) {
block
_while(cond)(block)
}

Conditional expression

scala> :l if_cbn.scala
Loading if_cbn.scala...
_if: (Boolean)(=> Unit)Unit

scala> _if(Math.random<0.5) {
| println ("ok")
| }
ok

Loop expression

scala> :l while_cbn.scala
Loading while_cbn.scala...
_while: (=> Boolean)(=> Unit)Unit

scala> _while(Math.random<0.5) {
| println ("ok")
| }
ok

Instance constants

class C {
val re = 0.0
val im = 0.0
}

Instance constants: usage

scala> :l complex01.scala
Loading complex01.scala...
defined class C

scala> new C
res0: C = C@151fe8a

toString

class C {
// ...
override def toString
= re + " + " + im + "*" + "i"
}

toString: usage

defined class C

scala> new C
res0: C = 0.0 + 0.0*i

Class parameters

class C(r: Double, i: Double) {
val re = r
val im = i
// ...
}

Class parameters: usage

defined class C

scala> new C(-1, -1)
res0: C = -1.0 + -1.0*i

toString again

override def toString = {
val sgnim = if(im<0.0){" - "}else{" + "}
val absim = if(im<0.0){ -im }else{ im }

if(im == 0.0) { re + "" } else {
if(re == 0.0) { im + "*i" } else {
re + sgnim + absim + "*i" } }
}

toString again: usage

defined class C

scala> new C(-1, -1)
res0: C = -1.0 - 1.0*i

scala> new C(-1, +1)
res1: C = -1.0 + 1.0*i

Additive operators

// ...
def +(c: C) =
new C(re + c.re, im + c.im)
def -(c: C) =
new C(re - c.re, im - c.im)
// ...
}

Additive operators: usage

defined class C

scala> new C(+1,+1) + new C(-3,+2)
res1: C = -2.0 + 3.0*i

scala> new C(+1,+1) - new C(-3,+2)
res2: C = 4.0 - 1.0*i

Multiplicative operators

// ...
def *(c: C) = new C(re*c.re - im*c.im,
im*c.re + re*c.im)
def /(c: C) = {
val d = c.re*c.re + c.im*c.im
new C((re*c.re + im*c.im) / d,
(im*c.re - re*c.im) / d) }
// ...

Multiplicative operators: usage

defined class C

scala> new C(+1,+1) * new C(-3,+2)
res0: C = -5.0 - 1.0*i

scala> new C(-5,-1) / new C(-3,+2)
res1: C = 1.0 + 1.0*i

Negation operator

// ...
def unary_- = new C(-re, -im)
// ...
}

Negation operator: usage

defined class C

scala> - new C(-5,-1)
res0: C = 5.0 + 1.0*i

scala> - new C(5,1)
res1: C = -5.0 - 1.0*i

The complex number i

// ...
}

object C {
val i = new C(0.0, 1.0)
}

The complex number i: usage

...

scala> import C.i
import C.i

scala> i * i
res0: C = -1.0

Converting Double

// ...
}

object C {
//...
implicit def toC(d: Double)
= new C(d, 0.0)
}

Converting Double: usage

...

scala> (1.0 + 1.0*i) / i
res0: C = 1.0 - 1.0*i

scala> 1.0 + 1.0*i / i
res1: C = 2.0

Space

class Space[X] extends Actor {
// ...
}

Worker

abstract class Worker[X](space: Space[X])
extends Actor {
// ...
}

Work

object Types {
type Work[X] = PartialFunction[X,Unit]
}

Put and Reg

case class Put[X](x: X)

case class Reg[X](
work: Work[X]
worker: Worker[X])

App

case class App[X](work: Work[X], x: X)

Space information

private var ps: List[Put[X]] = Nil
private var rs: List[Reg[X]] = Nil

Space functionality: part one

A worker can put objects x into the space
using space ! Put(x)
A worker can register work with the space
using space ! Reg(work, worker)

Space functionality: part two

A space can notify a worker when an object
x, to which the work he registered with the
space can be applied, has been put into the
space using worker ! App(work, x)

reacting to a Put: part one

case p@Put(x: X) => {
val frs =
rs filter(r => r.work.isDefinedAt(x)
if(frs == Nil) {
ps ::= p.asInstanceOf[Put[X]]
}

reacting to a Put: part two

else {
val fr = frs.last
rs = rs filter (r => !(r eq fr))
fr.worker ! App(fr.work, x)
}

reacting to a Reg: part one

case r@Reg(work, worker) => {
val fps =
ps filter (p => work.isDefinedAt(p.x))
if(fps == Nil) {
rs ::= r.asInstanceOf[Reg[X]]
}

reacting to a Reg: part two

else {
val fp = fps.last
ps = ps filter (p => !(p eq fp))
worker ! App(work, fp.x)
}

Worker: part one

abstract class Worker[X](space: Space[X])
extends Actor {
protected var isAcceptingMoreWork = true
protected def registerForWork()
def put(x: X) {
space ! Put(x) }
def reg(work: Work[X]) {
space ! Reg(work, this) }

Worker: part two

private def applyZeroOrMoreTimes() {
loop { react {
case App(work, x) => {
work.apply(x)
if(isAcceptingMoreWork) {
registerForWork()
} } } }
}

Worker: part three

private def applyOneOrMoreTimes() {
registerForWork()
applyZeroOrMoreTimes()
}

Worker: part four

def act() {
applyOneOrMoreTimes()
}

PingPong

sealed abstract class PingPong
case object Ping extends PingPong
case object Pong extends PingPong
case object Over extends PingPong
case object Done extends PingPong

Player

abstract class Player(
name: String
table: Space[PingPong]
)
extends Worker[PingPong](table) {
override def toString = name
}

Pinger: part one

protected def registerForWork() =
reg {
case Pong => {
if (Math.random < 0.95) {
println(this + " ping")
put(Ping)
} else {
put(Over)
}

Pinger: part two

case Done => {
println(this + " stop")
isAcceptingMoreWork = false
}
}
}
}

Ponger: part one

reg {
case Ping => {
if (Math.random < 0.95) {
println(this + " pong")
put(Pong)
} else {
put(Over)
}

Ponger: part two

case Done => {
}
}
}
}

Umpire: part one

class Umpire(
name: String,
players: List[Player],
table: Space[PingPong])
extends Worker[PingPong](table) {
override def toString = name

Umpire: part two

reg {
case Over => {
println(this + " done")
for { _ <- players } put(Done)
}
}

theTable

class Table extends Space[PingPong]

val theTable = new Table

thePlayers

val thePlayers =
Pinger("pinger_1", true) ::
Pinger("pinger_2", false) ::
Ponger("ponger_1") ::
Ponger("ponger_2") ::
Nil

theUmpire

val theUmpire =
new Umpire
("_umpire_", thePlayers, theTable)

main

theTable.start
thePlayers.foreach(_.start)
theUmpire.start
val startPinger = thePlayers.head
startPinger.put(Ping)

Scala by Luc Duponcheel

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