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!

Warning: Scala is the gateway drug to Haskell

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 Analogyclass Car { var frontRight: Wheel; var frontLeft: Wheel; var backRight: Wheel; var backLeft: Wheel;}

class Wheel { ...}

Real

Quote

[…] 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

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.scalapackage dk.brics.minitajs

object Main { def main(args: Array[String]) { val options = Options.read(args.toList) Analysis.run(options) }}

Options.scalacase class Options(inputFile: File, context: Boolean, recency: Boolean, lazyprop: Boolean, ...);

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.scalaobject 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.scalaabstract sealed class Bool { def join(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 object NotBool extends Bool;

Value.scalacase class Value(..., bool: Bool, undef: Undef, ...) { def isMaybeTrue: Boolean = (bool eq TrueBool) || (bool eq AnyBool); def joinUndef: Value = copy(undef = undef.join(MaybeUndef));}

LatticeOps.scalaabstract class LatticeOps(...) extends ContextMixin with RecencyMixin with PropagationMixintrait PropagationMixin { def propagate(state: BlockState, info: PropagateInfo, lattice: Lattice): Lattice;}

new LatticeOps(...) with CallSensitivity with RecencyAbstraction with LazyPropagation

Arguments

Arguments

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.

Exampledef propagate(state: BlockState, sourceContext: Context, sourceBlock: BasicBlock, targetContext: Context, targetBlock: BasicBlock, lattice: Lattice): Lattice

def propagate(state: BlockState, info: PropagateInfo, lattice: Lattice): Lattice

def propagate(s: BlockState, i: PropagateInfo, l: Lattice): Lattice = { lattice.getState(i.targetContext, i.targetBlock) match { case Reachable(targetState) => { if (state != targetState) { queue.enqueue(i.targetContext, i.targetBlock); } lattice.putState(i.targetContext, i.targetBlock, targetState.join(s)); } case Unreachable => { queue.enqueue(i.targetContext, i.targetBlock); lattice.putState(i.targetContext, i.targetBlock, s); } }}

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

Solver.Solve -> BlockTransfer.transfer -> BlockTransfer.transferCallBlock -> LatticeOps.functionEntry -> LazyPropagation.functionEntry -> LazyPropagation.propagate

• Not feasible to modify all return types• Instead we use a mutable queue!

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(...)(implicit q: Queue)

Scaladoc

Some of the Bad Stuff

• Death Traps• Debugging• Compiler Warnings• Compilation Times

Death Trapcase class BasicBlock(var successors:

Set[BasicBlock]);

val a = BasicBlock(Set.empty);a.succesors = Set(a);

a == a;

Difficult to Debug

Cryptic Compiler Warnings

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 + ObjectsDoes it work?

No, not really(but...)

Functions + Objects

• Fundamental problem:– Functional Programming = Immutability– Object-orientated Programming = Mutability

• Immutable objects are not really objects• Mutating functions are not really functions

Functions + Objects

A proposed solution:1. Split the program into FP and OO parts2. Decide whether some data should be

immutable or mutable (i.e. targed for FP or OO programming)

3. Prefer immutable data, otherwise use mutable data

Not a silver bullet

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?

• 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 Books

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/

Summary

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)

AddendumMe> I need [what turns out to be virtual types]Erik> You could use an extra-linguistic solutionMe> What do you mean "extra-linguistic"?

Erik> A perl script...

Last Code Slide: Real Code

Embedded DSLs

Scala has syntactic flexibility: object Button { def onClick(f: => Unit) { ... } } Button.onClick(() => println("Hello"!));

But you can also write: Button.onClick { println("Hello"); }

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