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Scala 2013 review

The document provides a comprehensive overview of JVM languages, focusing on Scala and its features such as static typing, functional programming, and language design trade-offs. It includes comparisons between Java and Scala, showcasing coding examples, meta-programming capabilities, and Scala's performance improvements for concurrent programming. Additionally, it discusses advanced language constructs and the use of macros for compile-time optimizations, demonstrating Scala's extensibility and efficiency in software development.

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2013  overview Sagie Davidovich singularityworld.com linkedin.com/in/sagied
2013  JVM  Languages  Landscape Static Object  Oriented Functional Dynamic
Who’s  using  scala
Performance  study  by   Google
Exponential  growth  in   demand
Thoughtworks   Technology  Radar  2012 Evaluate Consider Hold
Language  Design   Trade-­‐‑offs Expressiveness Performance Complexity
Positioning Object   Oriented JVM Statically   Typed Functional
Prof.  Martin  Odersky •  Designer of Java Generics •  Creator of JavaC •  Prof. at EPFL •  ACM Fellow •  Founder of
Class  Employee  -­‐‑  Java public class Employee implements Serializable { public boolean equals(Object o) { private final String firstName; if (this == o) { private final String lastName; return true;   } public Employee (String firstName, String lastName) { if (o == null || getClass() != o.getClass()) { this.firstName = firstName; return false; this.lastName = lastName; } } Employee employee = (Employee) o;   if (firstName != null ? !firstName.equals(employee.firstName) : public String getFirstName() { person.firstName != null) { return lastName; return false; } }   if (lastName != null ? !lastName.equals(employee.lastName) : employee.lastName != null) { public String getLastName() { return true; return firstName;   } } return true; Oops,  should  be  false   public Employee withFirstName(String firstName) {   } Anyone  noticed? return new Employee (firstName, lastName); } public int hashCode() { int result = firstName != null ? firstName.hashCode() : 0;   result = 31 * result + (lastName != null ? public Employee withLastName(String lastName) { lastName.hashCode() : 0); return new Employee (firstName, lastName); return result; } }     public String toString() { return "Employee(" + firstName + "," + lastName + ")"; } }
Class  Employee  -­‐‑  Scala case class Employee(firstName: String, lastName: String)! •  Constructor •  Copy  constructors •  Fields •  GeVers  /  (seVers) •  equals •  hashCode •  toString •  Recursive  decomposition •  Companion  object
Singletons Java public class OneAndOnly { private final static OneAndOnly INSTANCE = new OneAndOnly(); private OneAndOnly() {} public static OneAndOnly getInstance() { return OneAndOnly.INSTANCE; } } Scala object OneAndOnly
Everything  is  an  object 1.+(2) Same as 1+2
Every  block  returns  a  value   (no  statements,  only  expressions) def add (x:Int, y:Int) = x + {val tmp = y; tmp * 3}! ! if (x > 2) 3 else 4! ! val doubled = for(i <- (1 to 10)) yield i * 2!
Type  inference scala> val x = 3 x: Int = 3 scala> def x(y:Int) = y * 9 x: (y: Int)Int scala> def x(y:Int) = if (y == 1) List(y) else Set(y) x: (y: Int) Iterable[Int]
Higher  order  functions List(1, 2, 3).map((x: Int) => x + 1) = List(1, 2, 3).map(x => x + 1) // type is inferred = List(1, 2, 3).map(_ + 1) // placeholder notation
Universal  access  principle   import System._! ! class Clz {! def w = currentTimeMillis / 1000! val x = currentTimeMillis / 1000! var y = currentTimeMillis / 1000! lazy val z = currentTimeMillis / 1000! }!
Default  methods   apply  and  unapply object Square{ def apply(d: Double) = d * d def unapply(d: Double) = Some(math.sqrt(d)) } //apply val s = Square(3) // 9 // unaply 16 match { case Square(x) => x } // 4
Curried  functions •  Add new flow control and language constructs! def loop(n: Int)(body: => Any) {! (0 until n) foreach (n => body)! }! ! loop(2) {! println("IM IN YR LOOP!")! }! •  Multiple varargs lists are possible!! def crossProduct[L,R](left: L*)(right: R*) =
 for (l <- left; r <- right) yield (l,r)! crossProduct(1,2,3)(‘a’,’b’,’c’)! •  Partially applied functions! !
Multiple  inheritance  without   the  diamond  problem
Type  system
Type  system Strong Explicit  +  Inferred Traits Self  types Static  +  Dynamic Functional Erasure Nominal  +  Structural Type  aliases Nonnullable Monads Implicit Co/Contra-­‐‑variant Existential Value  +  Reference Case  classes Path  dependent Anonymous
Type  safety  (Java  example) // compiles fine in Java (arrays are co-variant!!) int[] ints = {3}; Object[] objects = ints; // ArrayStoreException in runtime objects[0] = new Object();
Structural  types object Closer { def using(closeable: { def close(): Unit }, f: => Unit) { try { f } finally { closeable.close } } }
Immutable  Collections
Mutable  Collections
Simpler  is  safer   Indexing  by  first  character Java List<String> keywords = Arrays.asList("Apple", "Banana", "Beer"); Map<Character, List<String>> result = new HashMap<Character, List<String>>(); for(String k : keywords) { char firstChar = k.charAt(1); Oops,  here’s  a  bug.   if(!result.containsKey(firstChar)) { Anyone  noticed? result.put(firstChar, new ArrayList<String>()); } result.get(firstChar).add(k); } for (List<String> list : result.values()) { Collections.sort(list); } Scala val keywords = List("Apple", "Banana", "Beer”) val result = keywords.sorted.groupBy(_.head)
Another  example ! Java! public List<String> empNames(ArrayList<Employee> employees) {! !ArrayList<String> result = new ArrayList<String>();! !for (Employee emp: employees) {! ! !result.add(emp.getName());! !}! !return result;! }! ! ! ! Scala! def empNames(employees: List[Employee]) = employees map getName!
Map  combinator  paVern ! Java! public List<String> empNames(ArrayList<Employee> employees) {! !ArrayList<String> res = new ArrayList<String>();! !for (Employee emp: employees) {! ! !res.add(emp.getName());! !}! !return res;! }! ! ! ! Really  need  to  encapsulate?? Scala! def empNames(employees: List[Employee]) = employees map getName!
Another  example ! Java! public static int factorial(int n) {! int res = 1;! for (int i = 1; i <= n; i++) {! res *= i;! }! return res;! }! ! ! Scala! def factorial(n: Int) = (1 to n).reduce(_*_)!
Reduce  combinator  paVern ! Java! public static int factorial(int n) {! int res = 1;! for (int i = 1; i <= n; i++) {! res *= i;! }! return res;! }! ! ! Scala! def factorial(n: Int) = (1 to n).reduce(_ * _)!
Combinatorics o  (1 to 5) combinations 2 List(Vector(1, 2), Vector(1, 3), Vector(1, 4), Vector(1, 5), Vector(2, 3), Vector(2, 4), Vector(2, 5), Vector(3, 4), Vector(3, 5), Vector(4, 5)) o  (1 to 5).permutations "George W. Bush".split(" ").permutations List(Array(George, W., Bush), Array(George, Bush, W.), Array(W., George, Bush), Array(W., Bush, George), Array(Bush, George, W.), Array(Bush, W., George)) o  "George W. Bush".split(" ").toSet.subsets List(Set(), Set(George), Set(W.), Set(Bush), Set(George, W.), Set(George, Bush), Set(W., Bush), Set(George, W., Bush))
++ Collection  framework ++: +: /: /: ! :+ :: ::: : addString ! aggregate andThen apply applyOrElse asInstanceOf ! canEqual collect collectFirst combinations companion ! compose contains containsSlice copyToArray copyToBuffer ! corresponds count diff distinct drop ! dropRight dropWhile endsWith exists filter ! filterNot find flatMap flatten fold ! foldLeft foldRight forall foreach genericBuilder ! groupBy grouped hasDefiniteSize head headOption ! indexOf indexOfSlice indexWhere indices init ! inits intersect isDefinedAt isEmpty isInstanceOf ! isTraversableAgain iterator last lastIndexOf lastIndexOfSlice ! lastIndexWhere lastOption length lengthCompare lift ! map mapConserve max maxBy min ! minBy mkString nonEmpty orElse padTo ! par partition patch permutations prefixLength ! product productArity productElement productIterator productPrefix ! reduce reduceLeft reduceLeftOption reduceOption reduceRight ! reduceRightOption repr reverse reverseIterator reverseMap ! reverse_::: runWith sameElements scan scanLeft ! scanRight segmentLength seq size slice ! sliding sortBy sortWith sorted span ! splitAt startsWith stringPrefix sum tail ! tails take takeRight takeWhile to ! toArray toBuffer toIndexedSeq toIterable toIterator ! toList toMap toSeq toSet toStream ! toString toTraversable toVector transpose union ! unzip unzip3 updated view withFilter ! zip zipAll zipWithIndex !
Placeholder  syntax  for   anonymous  functions Placeholder Regular _ + 1 x => x + 1 _ * _ (x1, x2) => x1 * x2 (_: Int) * 2 (x: Int) => x * 2 if (_) x else y z => if (z) x else y _.map(f) x => x.map(f) _.map(_ + 1) x => x.map(y => y + 1)
Distributed  &  multicore   computing Core •  Parallel collections (myList.par.sum) •  Futures & promises •  Actors •  STM – makes Java heap ACID compatible 3rd  Party •  Hadoop •  Spark & Storm •  Plain old Java threads •  Scalding – Twitter’s Scala based MapReduce implementation
Calling  Hadoop  from   Scala Java public void map(LongWritable key, Text value, OutputCollector<Text, IntWritable> output, Reporter reporter) throws IOException { String line = value.toString(); StringTokenizer tokenizer = new StringTokenizer(line); while (tokenizer.hasMoreTokens()) { word.set(tokenizer.nextToken()); output.collect(word, one); } } Scala def map(key: LongWritable, value: Text, output: OutputCollector[Text, IntWritable], reporter: Reporter) = value split " " foreach (output collect (_, one))
Futures val from1 = future {conn1.fetch} val from2 = future {conn2.fetch}   from1 either from2 onSuccess println
Parallel  collections List(1, 2, 3, 4, 5).par filter (_ % 2 == 0) // ParVector(2, 4)
Spark val file = spark textFile "hdfs://...”
 ! file.flatMap(_ split " ")
 .map(word => (word, 1))
 .reduceByKey(_ + _)! !
Tail  recursion  optimization Recursion in Java has an Achilles’ heel: the call stack def factorial(n: Int, res: Long = 1): Long = if(n == 0) res else factorial(n - 1, (res * n))
Safer  string  composition •  Standard string composition: “I’m “ + name + “, “ + age + “ years old” •  Formatted: “I’m %s, %d years old”.format(name, age) java.util.IllegalFormatConversionException •  Interpolated: s“I’m $name, $age years old”
Create  your  own   interpolations xml”””<body> <a href = “http://…”> $text</a> </body> ”””
Expression  simplification  –  Java public Expr simplify(Expr expr) { if (expr instanceof UnOp) { UnOp unOp = (UnOp) expr; if (unOp.getOperator().equals("-")) { if (unOp.getArg() instanceof UnOp) { UnOp arg = (UnOp) unOp.getArg(); if (arg.getOperator().equals("-")) return arg.getArg(); } } } if (expr instanceof BinOp) { BinOp binOp = (BinOp) expr; if (binOp.getRight() instanceof Number) { Number arg = (Number) binOp.getRight(); if (binOp.getOperator().equals("+") && arg.getNum() == 0) return binOp.getLeft(); } } return expr; }
PaVern  matching   Expression  simplification  -­‐‑  Scala def simplify(expr: Expr): Expr = expr match {! case UnOp("-", UnOp("-", e)) => simplify(e)! case BinOp("+", e, Number(0)) => simplify(e)! case _ => expr! }!
Value  classes Extensibility minus boxing overhead! ! class Meter(val v: Double) extends AnyVal {! def plus (that: Meter) = new Meter(v + that.v) ! }! ! Compiler generates! object Meter {! def plus(this: Meter, that: Meter) = new Meter(v + that.v)! }!
Compile-­‐‑time   Meta-­‐‑programming •  Language virtualization (overloading/overriding semantics of the original programming language to enable deep embedding of DSLs), •  Program reification (providing programs with means to inspect their own code), •  Self-optimization (self-application of domain-specific optimizations based on program reification), •  Algorithmic program construction (generation of code that is tedious to write with the abstractions supported by a programming language).
Meta  programming Language Operate  on Type   Expressive Runtime   safe ness Performance   overhead Textual   C,  Scala Text ✗ Low No preprocessors Template   C++ Text ✗ Low No systems Compile-­‐‑time   Haskell,   Abstract  (typed)   ✔ High No meta-­‐‑ Scala  2.10,   Syntax  trees programming   Nemerle Byte  code   Java,  Scala byte-­‐‑code ✗ Medium No manipulation “Dynamic”   Ruby,  Scala objects ✗ High Yes reflection Run-­‐‑time   Java,  Scala objects ✗ High Yes reflection
Scala  macros •  Full blown compile time meta- programming •  Access to the compiler API •  Written in Scala •  Hygienic •  Type-safe
Macros  –  safe  printf printf(format: String, params: Any*)! printf(“value is %d”, “boom”)! // runtime exception! java.util.IllegalFormatConversionException: d != java.lang.String! ! ! Macro implementation! def printf(format: String, params: Any*): Unit = macro printf_impl! def printf_impl(c: Context)(format: c.Expr[String], params: c.Expr[Any]*): c.Expr[Unit] = ...!
Efficient  Assert assert(2 + 2 == 4, "weird arithmetic") // 2 + 2 == 4 will be evaluated only if assertions were enabled
Macros  –  units  of  measure val gravityOnEarth = u(9.81, "m/s^2")! ! val heightOfMyOfficeWindow = u(3.5, "m")! ! val speedOfImpact =
 sqrt(2.0 * gravityOnEarth * heightOfMyOfficeWindow)! ! val monthsInAYear = 10b12! hVp://scalamacros.org/usecases/units-­‐‑of-­‐‑measure.html
Macros  –  type  providers type MySqlDb(connString: String) = macro ...! type MyDb = Base with MySqlDb("Server=127.0.0.1;Database=Foo;”)! val products = new MyDb().products! products.filter(p => p.name.startsWith("foo"))!  ! ! ! ! ! http://scalamacros.org/usecases/type-providers.html! Inspired by F# type providers!
Type  macros   Injecting  async  methods class D extends Lifter {! !def x = 2! !// def asyncX = future { 2 }! }! ! val d = new D! d.asyncX onComplete {! !case Success(x) => println(x)! !case Failure(_) => println("failed")! }!  hVp://scalamacros.org/talks/2012-­‐‑12-­‐‑18-­‐‑MacroParadise.pdf
More  uses  of  macros •  Ad-hoc code style and code-smell detector •  Advanced domain-specific languages •  Logging with minimal overhead •  Pre-compiled SQL queries •  GPU optimized code (see Scalaxy, ScalaCL) •  Macro annotations o  @Cached
Macro  annotations class atomic extends MacroAnnotation { def complete(defn: _) = macro(“backing field”) def typeCheck(defn: _) = macro("return defn itself”) } @atomic var fld: Int
GPU  compilation •  Enablers: Immutability, Macros •  ScalaCL Collections OpenCL-backed collections that look and behave like standard Scala collections (work in progress). •  ScalaCL Compiler Plugin optimizes Scala programs at compile-time, transforming regular Scala loops into faster code and transforming Scala functions given to ScalaCL Collections into OpenCL kernels.
An  extremely  hack-­‐‑able  compiler > scala -Xshow-phases! phase name id description! ---------- -- -----------! parser 1 parse source into ASTs, perform simple desugaring! namer 2 resolve names, attach symbols to named trees! packageobjects 3 load package objects! typer 4 the meat and potatoes: type the trees! patmat 5 translate match expressions! superaccessors 6 add super accessors in traits and nested classes! extmethods 7 add extension methods for inline classes! pickler 8 serialize symbol tables! refchecks 9 reference/override checking, translate nested objects! selectiveanf 10 ANF pre-transform for @cps! selectivecps 11 @cps-driven transform of selectiveanf assignments! uncurry 12 uncurry, translate function values to anonymous classes! tailcalls 13 replace tail calls by jumps! specialize 14 @specialized-driven class and method specialization! explicitouter 15 this refs to outer pointers, translate patterns! erasure 16 erase types, add interfaces for traits! posterasure 17 clean up erased inline classes! lazyvals 18 allocate bitmaps, translate lazy vals into lazified defs! lambdalift 19 move nested functions to top level! constructors 20 move field definitions into constructors! flatten 21 eliminate inner classes! mixin 22 mixin composition! cleanup 23 platform-specific cleanups, generate reflective calls! icode 24 generate portable intermediate code! inliner 25 optimization: do inlining! inlinehandlers 26 optimization: inline exception handlers! closelim 27 optimization: eliminate uncalled closures! dce 28 optimization: eliminate dead code! jvm 29 generate JVM bytecode! terminal 30 The last phase in the compiler chain!
Type-­‐‑safe  failures try { 2 / x } catch { case e:Throwable => 0} in 2.10: Try (2 / 0) // returns Failure Try (2 / 1) // returns Success Try (2 / x) getOrElse (0) List(0,1,0,2).map (x=> Try(2 / x)) filter(_.isSuccess) // List(Success(2), Success(1))
Implicit  parameters implicit val db = getDB! def store(employee: Employee)(implicit db: DB)! ! store(emp1oyee1) //db is passed implicitly!
Implicit  methods implicit def toEuro(x: Currency): Euro = …! ! def transfer(amount: Euro) {! println(”transferred” + amount)! }! ! // dollar is automatically converter to euro! transfer(Dollar(3.0))! transferred 2.25 Euros!
Implicit  classes ! implicit class StringExtensions(s: String) {! def words = s split " "! }! ! ! “hello world”.words // Array(hello, world)!
Environments -­‐‑  Currently  the  most  mature -­‐‑  Formally  supported  by  TypeSafe ublime   -­‐‑  Through  Ensime Scala  REPL
DSLs  (accounting) Rules to calculate an employee's paycheck:! employee's gross salary for 2 weeks! minus deductions for! federalIncomeTax, which is 25% of gross! stateIncomeTax, which is 5% of gross! insurancePremiums, which are 500 in gross’s currency! retirementFundContributions are 10% of gross! ! hVp://ofps.oreilly.com/titles/9780596155957/DomainSpecificLanguages.html
DSLs  (JSON) ("person" ->! ("name" -> "Joe") ~! ("age" -> 35) ~! ("spouse" ->! ("person" ->! ("name" -> "Marilyn") ~! ("age" -> 33)! )! )! )!
DSL  (XPath  /  json) val titles = xml "channel" "item" "title“ val titles2 = json "feed" "entry" "title" @ "$t" github.com/razie/snakked
DSLs   testing  frameworks val emptyStack = new Stack[String] evaluating { emptyStack.pop() } should produce [NoSuchElementException]
Full  access  to  the  compiler import tools.reflect.ToolBox! import reflect.runtime.{currentMirror => cm}! ! val tb = cm.mkToolBox()! ! scala> val tree = tb parse "(5 + 9) * 3"! tree: tb.u.Tree = 5.$plus(9).$times(3)! ! scala> tb eval tree! res1: Any = 42!
Reflection val take = typeOf[List[_]].member(TermName("take")).asMethod! ! reflect(List(1,3,2,4)).reflectMethod(take)(2) // List(1,2)!
Testing •  ScalaCheck o  Auto-generation of inputs and mocks. o  Composable check units o  Inspired by Haskell QuickCheck •  ScalaTest o  TDD, BDD, FeatureSpec o  Selenium DSL •  Specs2 o  TDD, BDD, Datatables support o  Integration with Mockito, EasyMock and JMock o  "hello world" must be matching("h.* w.*") •  JUnit
ScalaCheck   (inspired  by  Haskell  QuickCheck) scala> val sqrt = forAll {(n: Int) => math.sqrt(n*n) == n } scala> propSqrt.check ! Falsified after 1 passed tests: > -1
BDD  with  ScalaTest describe("A Stack") { it("should throw NoSuchElementException if an empty stack is popped") in { val emptyStack = new Stack[String] evaluating { emptyStack.pop() } should produce [NoSuchElementException] } it("should pop values in last-in-first-out order") { … } }
How  to  learn •  twitter.github.com/scala_school •  Effective Scala •  Coursera Scala course (50K students last year) •  Simply scala (interactive learning) •  Programming in scala book •  Stackoverflow
Contributing   docs.scala-­‐‑lang.org/sips
Contributing   github.com/scala
Things  to  be  aware  of ✗  Scala Compiler has to do much more => slower ✗  Tooling is not perfect (debugging, synthetic frames, macros support) Getting better every day. ✗  Language is rapidly evolving. Deprecations should be expected.
Scala 2013 review

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Scala 2013 review

  • 1.
    2013  overview Sagie Davidovich singularityworld.com linkedin.com/in/sagied
  • 2.
    2013  JVM  Languages Landscape Static Object  Oriented Functional Dynamic
  • 3.
  • 4.
  • 5.
  • 6.
    Thoughtworks   Technology  Radar  2012 Evaluate Consider Hold
  • 7.
    Language  Design   Trade-­‐‑offs Expressiveness Performance Complexity
  • 8.
    Positioning Object   Oriented JVM Statically   Typed Functional
  • 9.
    Prof.  Martin  Odersky •  Designer of Java Generics •  Creator of JavaC •  Prof. at EPFL •  ACM Fellow •  Founder of
  • 10.
    Class  Employee  -­‐‑ Java public class Employee implements Serializable { public boolean equals(Object o) { private final String firstName; if (this == o) { private final String lastName; return true;   } public Employee (String firstName, String lastName) { if (o == null || getClass() != o.getClass()) { this.firstName = firstName; return false; this.lastName = lastName; } } Employee employee = (Employee) o;   if (firstName != null ? !firstName.equals(employee.firstName) : public String getFirstName() { person.firstName != null) { return lastName; return false; } }   if (lastName != null ? !lastName.equals(employee.lastName) : employee.lastName != null) { public String getLastName() { return true; return firstName;   } } return true; Oops,  should  be  false   public Employee withFirstName(String firstName) {   } Anyone  noticed? return new Employee (firstName, lastName); } public int hashCode() { int result = firstName != null ? firstName.hashCode() : 0;   result = 31 * result + (lastName != null ? public Employee withLastName(String lastName) { lastName.hashCode() : 0); return new Employee (firstName, lastName); return result; } }     public String toString() { return "Employee(" + firstName + "," + lastName + ")"; } }
  • 11.
    Class  Employee  -­‐‑ Scala case class Employee(firstName: String, lastName: String)! •  Constructor •  Copy  constructors •  Fields •  GeVers  /  (seVers) •  equals •  hashCode •  toString •  Recursive  decomposition •  Companion  object
  • 12.
    Singletons Java public class OneAndOnly{ private final static OneAndOnly INSTANCE = new OneAndOnly(); private OneAndOnly() {} public static OneAndOnly getInstance() { return OneAndOnly.INSTANCE; } } Scala object OneAndOnly
  • 13.
    Everything  is  an object 1.+(2) Same as 1+2
  • 14.
    Every  block  returns a  value   (no  statements,  only  expressions) def add (x:Int, y:Int) = x + {val tmp = y; tmp * 3}! ! if (x > 2) 3 else 4! ! val doubled = for(i <- (1 to 10)) yield i * 2!
  • 15.
    Type  inference scala> valx = 3 x: Int = 3 scala> def x(y:Int) = y * 9 x: (y: Int)Int scala> def x(y:Int) = if (y == 1) List(y) else Set(y) x: (y: Int) Iterable[Int]
  • 16.
    Higher  order  functions List(1,2, 3).map((x: Int) => x + 1) = List(1, 2, 3).map(x => x + 1) // type is inferred = List(1, 2, 3).map(_ + 1) // placeholder notation
  • 17.
    Universal  access  principle  import System._! ! class Clz {! def w = currentTimeMillis / 1000! val x = currentTimeMillis / 1000! var y = currentTimeMillis / 1000! lazy val z = currentTimeMillis / 1000! }!
  • 18.
    Default  methods   apply  and  unapply object Square{ def apply(d: Double) = d * d def unapply(d: Double) = Some(math.sqrt(d)) } //apply val s = Square(3) // 9 // unaply 16 match { case Square(x) => x } // 4
  • 19.
    Curried  functions •  Addnew flow control and language constructs! def loop(n: Int)(body: => Any) {! (0 until n) foreach (n => body)! }! ! loop(2) {! println("IM IN YR LOOP!")! }! •  Multiple varargs lists are possible!! def crossProduct[L,R](left: L*)(right: R*) =
 for (l <- left; r <- right) yield (l,r)! crossProduct(1,2,3)(‘a’,’b’,’c’)! •  Partially applied functions! !
  • 20.
    Multiple  inheritance  without  the  diamond  problem
  • 21.
  • 22.
    Type  system Strong Explicit  +  Inferred Traits Self  types Static  +  Dynamic Functional Erasure Nominal  +  Structural Type  aliases Nonnullable Monads Implicit Co/Contra-­‐‑variant Existential Value  +  Reference Case  classes Path  dependent Anonymous
  • 23.
    Type  safety  (Java example) // compiles fine in Java (arrays are co-variant!!) int[] ints = {3}; Object[] objects = ints; // ArrayStoreException in runtime objects[0] = new Object();
  • 24.
    Structural  types object Closer{ def using(closeable: { def close(): Unit }, f: => Unit) { try { f } finally { closeable.close } } }
  • 25.
  • 26.
  • 27.
    Simpler  is  safer  Indexing  by  first  character Java List<String> keywords = Arrays.asList("Apple", "Banana", "Beer"); Map<Character, List<String>> result = new HashMap<Character, List<String>>(); for(String k : keywords) { char firstChar = k.charAt(1); Oops,  here’s  a  bug.   if(!result.containsKey(firstChar)) { Anyone  noticed? result.put(firstChar, new ArrayList<String>()); } result.get(firstChar).add(k); } for (List<String> list : result.values()) { Collections.sort(list); } Scala val keywords = List("Apple", "Banana", "Beer”) val result = keywords.sorted.groupBy(_.head)
  • 28.
    Another  example ! Java! public List<String>empNames(ArrayList<Employee> employees) {! !ArrayList<String> result = new ArrayList<String>();! !for (Employee emp: employees) {! ! !result.add(emp.getName());! !}! !return result;! }! ! ! ! Scala! def empNames(employees: List[Employee]) = employees map getName!
  • 29.
    Map  combinator  paVern ! Java! publicList<String> empNames(ArrayList<Employee> employees) {! !ArrayList<String> res = new ArrayList<String>();! !for (Employee emp: employees) {! ! !res.add(emp.getName());! !}! !return res;! }! ! ! ! Really  need  to  encapsulate?? Scala! def empNames(employees: List[Employee]) = employees map getName!
  • 30.
    Another  example ! Java! public staticint factorial(int n) {! int res = 1;! for (int i = 1; i <= n; i++) {! res *= i;! }! return res;! }! ! ! Scala! def factorial(n: Int) = (1 to n).reduce(_*_)!
  • 31.
    Reduce  combinator  paVern ! Java! publicstatic int factorial(int n) {! int res = 1;! for (int i = 1; i <= n; i++) {! res *= i;! }! return res;! }! ! ! Scala! def factorial(n: Int) = (1 to n).reduce(_ * _)!
  • 32.
    Combinatorics o  (1 to5) combinations 2 List(Vector(1, 2), Vector(1, 3), Vector(1, 4), Vector(1, 5), Vector(2, 3), Vector(2, 4), Vector(2, 5), Vector(3, 4), Vector(3, 5), Vector(4, 5)) o  (1 to 5).permutations "George W. Bush".split(" ").permutations List(Array(George, W., Bush), Array(George, Bush, W.), Array(W., George, Bush), Array(W., Bush, George), Array(Bush, George, W.), Array(Bush, W., George)) o  "George W. Bush".split(" ").toSet.subsets List(Set(), Set(George), Set(W.), Set(Bush), Set(George, W.), Set(George, Bush), Set(W., Bush), Set(George, W., Bush))
  • 33.
    ++ Collection  framework ++: +: /: /: ! :+ :: ::: : addString ! aggregate andThen apply applyOrElse asInstanceOf ! canEqual collect collectFirst combinations companion ! compose contains containsSlice copyToArray copyToBuffer ! corresponds count diff distinct drop ! dropRight dropWhile endsWith exists filter ! filterNot find flatMap flatten fold ! foldLeft foldRight forall foreach genericBuilder ! groupBy grouped hasDefiniteSize head headOption ! indexOf indexOfSlice indexWhere indices init ! inits intersect isDefinedAt isEmpty isInstanceOf ! isTraversableAgain iterator last lastIndexOf lastIndexOfSlice ! lastIndexWhere lastOption length lengthCompare lift ! map mapConserve max maxBy min ! minBy mkString nonEmpty orElse padTo ! par partition patch permutations prefixLength ! product productArity productElement productIterator productPrefix ! reduce reduceLeft reduceLeftOption reduceOption reduceRight ! reduceRightOption repr reverse reverseIterator reverseMap ! reverse_::: runWith sameElements scan scanLeft ! scanRight segmentLength seq size slice ! sliding sortBy sortWith sorted span ! splitAt startsWith stringPrefix sum tail ! tails take takeRight takeWhile to ! toArray toBuffer toIndexedSeq toIterable toIterator ! toList toMap toSeq toSet toStream ! toString toTraversable toVector transpose union ! unzip unzip3 updated view withFilter ! zip zipAll zipWithIndex !
  • 34.
    Placeholder  syntax  for  anonymous  functions Placeholder Regular _ + 1 x => x + 1 _ * _ (x1, x2) => x1 * x2 (_: Int) * 2 (x: Int) => x * 2 if (_) x else y z => if (z) x else y _.map(f) x => x.map(f) _.map(_ + 1) x => x.map(y => y + 1)
  • 35.
    Distributed  &  multicore  computing Core •  Parallel collections (myList.par.sum) •  Futures & promises •  Actors •  STM – makes Java heap ACID compatible 3rd  Party •  Hadoop •  Spark & Storm •  Plain old Java threads •  Scalding – Twitter’s Scala based MapReduce implementation
  • 36.
    Calling  Hadoop  from  Scala Java public void map(LongWritable key, Text value, OutputCollector<Text, IntWritable> output, Reporter reporter) throws IOException { String line = value.toString(); StringTokenizer tokenizer = new StringTokenizer(line); while (tokenizer.hasMoreTokens()) { word.set(tokenizer.nextToken()); output.collect(word, one); } } Scala def map(key: LongWritable, value: Text, output: OutputCollector[Text, IntWritable], reporter: Reporter) = value split " " foreach (output collect (_, one))
  • 37.
    Futures val from1 =future {conn1.fetch} val from2 = future {conn2.fetch}   from1 either from2 onSuccess println
  • 38.
    Parallel  collections List(1, 2,3, 4, 5).par filter (_ % 2 == 0) // ParVector(2, 4)
  • 39.
    Spark val file =spark textFile "hdfs://...”
 ! file.flatMap(_ split " ")
 .map(word => (word, 1))
 .reduceByKey(_ + _)! !
  • 40.
    Tail  recursion  optimization Recursionin Java has an Achilles’ heel: the call stack def factorial(n: Int, res: Long = 1): Long = if(n == 0) res else factorial(n - 1, (res * n))
  • 41.
    Safer  string  composition • Standard string composition: “I’m “ + name + “, “ + age + “ years old” •  Formatted: “I’m %s, %d years old”.format(name, age) java.util.IllegalFormatConversionException •  Interpolated: s“I’m $name, $age years old”
  • 42.
    Create  your  own  interpolations xml”””<body> <a href = “http://…”> $text</a> </body> ”””
  • 43.
    Expression  simplification  – Java public Expr simplify(Expr expr) { if (expr instanceof UnOp) { UnOp unOp = (UnOp) expr; if (unOp.getOperator().equals("-")) { if (unOp.getArg() instanceof UnOp) { UnOp arg = (UnOp) unOp.getArg(); if (arg.getOperator().equals("-")) return arg.getArg(); } } } if (expr instanceof BinOp) { BinOp binOp = (BinOp) expr; if (binOp.getRight() instanceof Number) { Number arg = (Number) binOp.getRight(); if (binOp.getOperator().equals("+") && arg.getNum() == 0) return binOp.getLeft(); } } return expr; }
  • 44.
    PaVern  matching   Expression  simplification  -­‐‑  Scala def simplify(expr: Expr): Expr = expr match {! case UnOp("-", UnOp("-", e)) => simplify(e)! case BinOp("+", e, Number(0)) => simplify(e)! case _ => expr! }!
  • 45.
    Value  classes Extensibility minus boxing overhead! ! class Meter(val v: Double) extends AnyVal {! def plus (that: Meter) = new Meter(v + that.v) ! }! ! Compiler generates! object Meter {! def plus(this: Meter, that: Meter) = new Meter(v + that.v)! }!
  • 46.
    Compile-­‐‑time   Meta-­‐‑programming •  Language virtualization (overloading/overriding semantics of the original programming language to enable deep embedding of DSLs), •  Program reification (providing programs with means to inspect their own code), •  Self-optimization (self-application of domain-specific optimizations based on program reification), •  Algorithmic program construction (generation of code that is tedious to write with the abstractions supported by a programming language).
  • 47.
    Meta  programming Language Operate  on Type   Expressive Runtime   safe ness Performance   overhead Textual   C,  Scala Text ✗ Low No preprocessors Template   C++ Text ✗ Low No systems Compile-­‐‑time   Haskell,   Abstract  (typed)   ✔ High No meta-­‐‑ Scala  2.10,   Syntax  trees programming   Nemerle Byte  code   Java,  Scala byte-­‐‑code ✗ Medium No manipulation “Dynamic”   Ruby,  Scala objects ✗ High Yes reflection Run-­‐‑time   Java,  Scala objects ✗ High Yes reflection
  • 48.
    Scala  macros •  Fullblown compile time meta- programming •  Access to the compiler API •  Written in Scala •  Hygienic •  Type-safe
  • 49.
    Macros  –  safe printf printf(format: String, params: Any*)! printf(“value is %d”, “boom”)! // runtime exception! java.util.IllegalFormatConversionException: d != java.lang.String! ! ! Macro implementation! def printf(format: String, params: Any*): Unit = macro printf_impl! def printf_impl(c: Context)(format: c.Expr[String], params: c.Expr[Any]*): c.Expr[Unit] = ...!
  • 50.
    Efficient  Assert assert(2 +2 == 4, "weird arithmetic") // 2 + 2 == 4 will be evaluated only if assertions were enabled
  • 51.
    Macros  –  units of  measure val gravityOnEarth = u(9.81, "m/s^2")! ! val heightOfMyOfficeWindow = u(3.5, "m")! ! val speedOfImpact =
 sqrt(2.0 * gravityOnEarth * heightOfMyOfficeWindow)! ! val monthsInAYear = 10b12! hVp://scalamacros.org/usecases/units-­‐‑of-­‐‑measure.html
  • 52.
    Macros  –  type providers type MySqlDb(connString: String) = macro ...! type MyDb = Base with MySqlDb("Server=127.0.0.1;Database=Foo;”)! val products = new MyDb().products! products.filter(p => p.name.startsWith("foo"))!  ! ! ! ! ! http://scalamacros.org/usecases/type-providers.html! Inspired by F# type providers!
  • 53.
    Type  macros   Injecting  async  methods class D extends Lifter {! !def x = 2! !// def asyncX = future { 2 }! }! ! val d = new D! d.asyncX onComplete {! !case Success(x) => println(x)! !case Failure(_) => println("failed")! }!  hVp://scalamacros.org/talks/2012-­‐‑12-­‐‑18-­‐‑MacroParadise.pdf
  • 54.
    More  uses  of macros •  Ad-hoc code style and code-smell detector •  Advanced domain-specific languages •  Logging with minimal overhead •  Pre-compiled SQL queries •  GPU optimized code (see Scalaxy, ScalaCL) •  Macro annotations o  @Cached
  • 55.
    Macro  annotations class atomicextends MacroAnnotation { def complete(defn: _) = macro(“backing field”) def typeCheck(defn: _) = macro("return defn itself”) } @atomic var fld: Int
  • 56.
    GPU  compilation •  Enablers:Immutability, Macros •  ScalaCL Collections OpenCL-backed collections that look and behave like standard Scala collections (work in progress). •  ScalaCL Compiler Plugin optimizes Scala programs at compile-time, transforming regular Scala loops into faster code and transforming Scala functions given to ScalaCL Collections into OpenCL kernels.
  • 57.
    An  extremely  hack-­‐‑able compiler > scala -Xshow-phases! phase name id description! ---------- -- -----------! parser 1 parse source into ASTs, perform simple desugaring! namer 2 resolve names, attach symbols to named trees! packageobjects 3 load package objects! typer 4 the meat and potatoes: type the trees! patmat 5 translate match expressions! superaccessors 6 add super accessors in traits and nested classes! extmethods 7 add extension methods for inline classes! pickler 8 serialize symbol tables! refchecks 9 reference/override checking, translate nested objects! selectiveanf 10 ANF pre-transform for @cps! selectivecps 11 @cps-driven transform of selectiveanf assignments! uncurry 12 uncurry, translate function values to anonymous classes! tailcalls 13 replace tail calls by jumps! specialize 14 @specialized-driven class and method specialization! explicitouter 15 this refs to outer pointers, translate patterns! erasure 16 erase types, add interfaces for traits! posterasure 17 clean up erased inline classes! lazyvals 18 allocate bitmaps, translate lazy vals into lazified defs! lambdalift 19 move nested functions to top level! constructors 20 move field definitions into constructors! flatten 21 eliminate inner classes! mixin 22 mixin composition! cleanup 23 platform-specific cleanups, generate reflective calls! icode 24 generate portable intermediate code! inliner 25 optimization: do inlining! inlinehandlers 26 optimization: inline exception handlers! closelim 27 optimization: eliminate uncalled closures! dce 28 optimization: eliminate dead code! jvm 29 generate JVM bytecode! terminal 30 The last phase in the compiler chain!
  • 58.
    Type-­‐‑safe  failures try {2 / x } catch { case e:Throwable => 0} in 2.10: Try (2 / 0) // returns Failure Try (2 / 1) // returns Success Try (2 / x) getOrElse (0) List(0,1,0,2).map (x=> Try(2 / x)) filter(_.isSuccess) // List(Success(2), Success(1))
  • 59.
    Implicit  parameters implicit valdb = getDB! def store(employee: Employee)(implicit db: DB)! ! store(emp1oyee1) //db is passed implicitly!
  • 60.
    Implicit  methods implicit deftoEuro(x: Currency): Euro = …! ! def transfer(amount: Euro) {! println(”transferred” + amount)! }! ! // dollar is automatically converter to euro! transfer(Dollar(3.0))! transferred 2.25 Euros!
  • 61.
    Implicit  classes ! implicit classStringExtensions(s: String) {! def words = s split " "! }! ! ! “hello world”.words // Array(hello, world)!
  • 62.
    Environments -­‐‑  Currently  the  most  mature -­‐‑  Formally  supported  by  TypeSafe ublime   -­‐‑  Through  Ensime Scala  REPL
  • 63.
    DSLs  (accounting) Rules tocalculate an employee's paycheck:! employee's gross salary for 2 weeks! minus deductions for! federalIncomeTax, which is 25% of gross! stateIncomeTax, which is 5% of gross! insurancePremiums, which are 500 in gross’s currency! retirementFundContributions are 10% of gross! ! hVp://ofps.oreilly.com/titles/9780596155957/DomainSpecificLanguages.html
  • 64.
    DSLs  (JSON) ("person" ->! ("name" -> "Joe") ~! ("age" -> 35) ~! ("spouse" ->! ("person" ->! ("name" -> "Marilyn") ~! ("age" -> 33)! )! )! )!
  • 65.
    DSL  (XPath  / json) val titles = xml "channel" "item" "title“ val titles2 = json "feed" "entry" "title" @ "$t" github.com/razie/snakked
  • 66.
    DSLs   testing  frameworks val emptyStack = new Stack[String] evaluating { emptyStack.pop() } should produce [NoSuchElementException]
  • 67.
    Full  access  to the  compiler import tools.reflect.ToolBox! import reflect.runtime.{currentMirror => cm}! ! val tb = cm.mkToolBox()! ! scala> val tree = tb parse "(5 + 9) * 3"! tree: tb.u.Tree = 5.$plus(9).$times(3)! ! scala> tb eval tree! res1: Any = 42!
  • 68.
    Reflection val take =typeOf[List[_]].member(TermName("take")).asMethod! ! reflect(List(1,3,2,4)).reflectMethod(take)(2) // List(1,2)!
  • 69.
    Testing •  ScalaCheck o  Auto-generation of inputs and mocks. o  Composable check units o  Inspired by Haskell QuickCheck •  ScalaTest o  TDD, BDD, FeatureSpec o  Selenium DSL •  Specs2 o  TDD, BDD, Datatables support o  Integration with Mockito, EasyMock and JMock o  "hello world" must be matching("h.* w.*") •  JUnit
  • 70.
    ScalaCheck   (inspired  by  Haskell  QuickCheck) scala> val sqrt = forAll {(n: Int) => math.sqrt(n*n) == n } scala> propSqrt.check ! Falsified after 1 passed tests: > -1
  • 71.
    BDD  with  ScalaTest describe("AStack") { it("should throw NoSuchElementException if an empty stack is popped") in { val emptyStack = new Stack[String] evaluating { emptyStack.pop() } should produce [NoSuchElementException] } it("should pop values in last-in-first-out order") { … } }
  • 72.
    How  to  learn •  twitter.github.com/scala_school •  Effective Scala •  Coursera Scala course (50K students last year) •  Simply scala (interactive learning) •  Programming in scala book •  Stackoverflow
  • 73.
    Contributing   docs.scala-­‐‑lang.org/sips
  • 74.
  • 75.
    Things  to  be aware  of ✗  Scala Compiler has to do much more => slower ✗  Tooling is not perfect (debugging, synthetic frames, macros support) Getting better every day. ✗  Language is rapidly evolving. Deprecations should be expected.