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A Pragmatic Introduction to Scala. Magnus Madsen. Presenting Scala:. An alternative to Java. Why I like Scala:. object-orientated and functional elegant and concise unrestrictive – gives freedom of choice Scala makes me a happier programmer!

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Presentation Transcript
presenting scala

An alternative

to Java

why i like scala
Why I like Scala:
    • object-orientated and functional
  • elegant and concise
  • unrestrictive – gives freedom of choice

Scala makes me a happier programmer!

Warning: Scala is the gateway drug to Haskell

a playground for fun stuff
A Playground for Fun Stuff:
  • Engineering Perspective:
    • Actor-based Programming
    • Embedded DSLs, and more ...
  • Research Perspective:
    • Higher-Kinded Types
    • Delimited Continuations
    • Abstract Types, and more ...
a used car analogy
A Used Car Analogy


classCar {

var frontRight: Wheel;

var frontLeft: Wheel;

var backRight: Wheel;

var backLeft: Wheel;


classWheel {




[…] I can honestly say if someone had shown me the Programming in Scala book back in 2003 I'd probably have never created Groovy.

James Strachan

(creator of Groovy)

case study minitajs
Case Study: MiniTAJS

An inter-procedural dataflow analysis

  • a scaled down version of TAJS
  • has lots of cool stuff:
    • abstract syntax trees, control flow graphs, etc.
    • lattices, transfer functions, etc.
  • about 4500 lines of code
    • of which 90-95% are in functional style
main scala

package dk.brics.minitajs

object Main {

def main(args: Array[String]) {

val options = Options.read(args.toList)




options scala

case class Options(inputFile: File,

context: Boolean,

recency: Boolean,

lazyprop: Boolean,


case classes the bread and butter
Case Classes: The bread and butter
  • A case class declaration:
    • case class Options(inputFile: File, ...)
  • Automatically gives us:
    • getters and setters
    • .equals() and .hashCode()
    • .toString()
    • .copy()
    • .apply()
    • .unapply()
options scala1

object Options {

def read(args: List[String]): Options = {

val context = args.exists(_ == "--context");

val lazyprop = args.exists(_ == "--lazy");


Options(new File(args.last), context, recency, ...);



bool scala

abstractsealedclass Bool {

defjoin(that: Bool): Bool = (this, that) match {

case(TrueBool, TrueBool) => TrueBool;

case (TrueBool, FalseBool) => AnyBool;




case object AnyBool extends Bool;

case object TrueBool extends Bool;

case object FalseBool extends Bool;

case objectNotBool extends Bool;

value scala

case class Value(..., bool: Bool, undef: Undef, ...) {

defisMaybeTrue: Boolean = (bool eq TrueBool)

|| (bool eq AnyBool);

defjoinUndef: Value

= copy(undef = undef.join(MaybeUndef));


latticeops scala

abstract classLatticeOps(...) extendsContextMixin


with PropagationMixin

trait PropagationMixin {

def propagate(state: BlockState, info: PropagateInfo,

lattice: Lattice): Lattice;


new LatticeOps(...) with CallSensitivity


with LazyPropagation


Problem: In functional programming argument lists grow and grow

Solution: Wrap arguments up inside a data type. In Scala this translates to a case class.


defpropagate(state: BlockState,

sourceContext: Context,

sourceBlock: BasicBlock,

targetContext: Context,

targetBlock: BasicBlock,

lattice: Lattice): Lattice

def propagate(state: BlockState,

info: PropagateInfo,

lattice: Lattice): Lattice



defpropagate(s: BlockState, i: PropagateInfo,

l: Lattice): Lattice = {

lattice.getState(i.targetContext, i.targetBlock) match{

caseReachable(targetState) => {

if (state != targetState) {

queue.enqueue(i.targetContext, i.targetBlock);


lattice.putState(i.targetContext, i.targetBlock,



caseUnreachable => {

queue.enqueue(i.targetContext, i.targetBlock);

lattice.putState(i.targetContext, i.targetBlock, s);




limited mutability
Limited Mutability

Problem: Whenever new flow enters a basicblock it must be added to the solver queue

Potential Solution: We could modify all functions to return a pair where the last component is the set of basicblocks that must be enqueued

but the stack is deep
But the stack is deep

Solver.Solve ->

BlockTransfer.transfer ->

BlockTransfer.transferCallBlock ->

LatticeOps.functionEntry ->

LazyPropagation.functionEntry ->


  • Not feasible to modify all return types
  • Instead we use a mutable queue!
but the stack is still deep
But the stack is still deep!

How do we get a reference to the queue???

  • We could use a global reference

Or we could use Scala's implicits:

class BlockTransfer(...)(implicit q: Queue)

class LatticeOps(...)(implicitq: Queue)

some of the bad stuff
Some of the Bad Stuff
  • Death Traps
  • Debugging
  • Compiler Warnings
  • Compilation Times
death trap
Death Trap

case class BasicBlock(var successors: Set[BasicBlock]);

val a = BasicBlock(Set.empty);

a.succesors = Set(a);

a == a;

compilation is slow
Compilation is Slow
  • Compiling miniTAJS takes 35 seconds
    • 4500 lines of code
    • 113 classes + 40 objects = 580 .class files
  • Why?
    • Scalac is written in Scala - i.e. it runs on the JVM
    • Scalac must type-check both Java and Scala
    • Scalac must do local type inference
functions objects does it work
Functions + ObjectsDoes it work?

No, not really


functions objects
Functions + Objects
  • Fundamental problem:
    • Functional Programming = Immutability
    • Object-orientated Programming = Mutability
  • Immutable objects are not really objects
  • Mutating functions are not really functions
functions objects1
Functions + Objects

A proposed solution:

  • Split the program into FP and OO parts
  • Decide whether some data should be immutable or mutable (i.e. targed for FP or OO programming)
  • Prefer immutable data, otherwise use mutable data

Not a silver bullet

recent history
Recent History
  • Scala 2.10 Milestone 1 (Januar 2012)
  • New Eclipse Plugin (January 2012)
  • New IntelliJ IDEA Plugin (December 2011)
  • Scala 2.9 (May 2011)
    • parallel collections
  • Scala 2.8 (July 2010)
    • new collections framework
critical mass
Critical Mass?
  • Introduction to the Art of Programming Using Scala (October 2012)
  • Scala for the Impatient (March 2012)
  • Scala in Action (April 2012)
  • Scala in Depth (April 2012)
  • Actors in Scala (Januar 2012)
  • Pro Scala: Monadic Design Patterns for the Web (August 2011)
  • Programming in Scala 2nd (Januar 2011)
recommended websites
Recommended Websites
  • Official Scala website
    • http://scala-lang.org/
  • Daily Scala – small code sniplets
    • http://daily-scala.blogspot.com/
  • CodeCommit – "Scala for Java Refugees"
    • http://www.codecommit.com/blog/
  • StackOverflow
    • http://stackoverflow.com/

is viable alternative to Java

  • object-orientated and functional
  • has useful features not found in Java
  • runs on the JVM and interacts with Java
  • is fun!

Thank You!

(now go download Scala)


Me>I need [what turns out to be virtual types]

Erik> You could use an extra-linguistic solution

Me> What do you mean "extra-linguistic"?

Erik> A perl script...

embedded dsls
Embedded DSLs

Scala has syntactic flexibility:

object Button {

def onClick(f: => Unit) { ... }


Button.onClick(() => println("Hello"!));

But you can also write:

Button.onClick {



historical anecdote
Historical Anecdote

BETA was supposed to be called Scala:

For many years the name SCALA was a candidate for a new name – SCALA could mean SCAndinavian LAnguage, and in Latin it means ladder and could be interpreted as meaning something ‘going up’.

The When, Why and Why Not

of the BETA Programming Language