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Scala - core features

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?ukasz W¨®jcik
?ukasz W¨®jcik

This document provides an introduction to Scala concepts and features compared to Java, including how to set up Scala, the Simple Build Tool (SBT), case classes, lazy definitions, imports, objects, pattern matching, collections, higher-order functions, partial functions, currying, implicit conversions, and implicit parameters. Useful Scala resources are also listed.

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Scala.compareTo(Java) to try or not to try? Based on http://github.com/jamie-allen/taxonomy-of-scala by Jamie Allen
Quick Start Download Scala from : - http://www.scala-lang.org/downloads Download Eclipse with Scala Worksheet : - http://typesafe.com/stack/scala_ide_download Download SBT (Simple Build Tool): - http://twitter.github.com/scala_school/sbt.html
SBT - Simple Build Tool SBT is a simple build tool, similar to maven. Helps you create, build, test, package your scala application. It includes dependency management which is defined in regular Scala file, allow to define simple build- plugins - everything simple and extensible and XML free.
Main concepts of Scala: - runs on JVM - statically typed - compiled into bytecode - Advanced type system with type inference and declaration-site variance - Function types (including anonymous) which support closures - Pattern-matching - Implicit parameters and conversions - Full interoperability with Java
Let's start! Create file hello.scala object HelloWorld extends App { println("Hello, World!") } Compile it : >scalac hello.scala Run it :) >scala HelloWorld "Hello, WOrld!"
- file can have whatever name - no semicolons - simplified syntax for many common cases More Java-like version of previous example (with main method): object HelloWorld2 { def main(args:Array[String])={ println("Hello 2") } } What is going on?
Case Classes case class Person(firstName: String = "Jamie", lastName: String = "Allen") val jamieDoe = Person(lastName = "Doe") res0: Person = Person(Jamie,Doe) ¡ñ Data Transfer Objects (DTOs) done right ¡ñ By default, class arguments are immutable & public ¡ñ Should never be extended ¡ñ Provide equals(), copy(), hashCode() and toString() implementations ¡ñ Don¡¯t have to use new keyword to create instances ¡ñ Named Parameters and Default arguments give us Builder pattern semantics
Lazy Definitions lazy val calculatedValue = piToOneMillionDecimalPoints() ¡ñ Excellent for deferring expensive operations until they are needed ¡ñ Reducing initial footprint ¡ñ Resolving ordering issues ¡ñ Implemented with a guard field and synchronization, ensuring it is created when necessary
Imports import scala.collection.immutable.Map class Person(val fName: String, val lName: String) { import scala.collection.mutable.{Map => MMap} val cars: MMap[String, String] = MMap() ... } ¡ñ Can be anywhere in a class ¡ñ Allow for selecting multiple classes from a package or using wildcards ¡ñ Aliasing ¡ñ Order matters!
Objects object Bootstrapper extends App { Person.createJamieAllen } object Person { def createJamieAllen = new Person("Jamie", "Allen") def createJamieDoe = new Person("Jamie", "Doe") val aConstantValue = "A constant value¡± } class Person(val firstName: String, val lastName: String) ¡ñ Singletons within a JVM process ¡ñ No private constructor histrionics ¡ñ Companion Objects, used for factories and constants
The apply() method Array(1, 2, 3) res0: Array[Int] = Array(1, 2, 3) res0(1) res1: Int = 2 ¡ñ In companion objects, it defines default behavior if no method is called on it ¡ñ In a class, it defines the same thing on an instance of the class
Tuples def firstPerson = (1, Person(firstName = ¡°Barbara¡±)) val (num: Int, person: Person) = firstPerson ¡ñ Binds you to an implementation ¡ñ Great way to group values without a DTO ¡ñ How to return multiple values, but wrapped in a single instance that you can bind to specific values
Pattern Matching Examples name match { case "Lisa" => println("Found Lisa¡±) case Person("Bob") => println("Found Bob¡±) case "Karen" | "Michelle" => println("Found Karen or Michelle¡±) case Seq("Dave", "John") => println("Got Dave before John¡±) case Seq("Dave", "John", _*) => println("Got Dave before John¡±) case ("Susan", "Steve") => println("Got Susan and Steve¡±) case x: Int if x > 5 => println("got value greater than 5: " + x) case x => println("Got something that wasn't an Int: " + x) case _ => println("Not found¡±) } ¡ñ A gateway drug for Scala ¡ñ Extremely powerful and readable ¡ñ Not compiled down to lookup/table switch unless you use the @switch annotation,
Scala Collections val myMap = Map(1 -> "one", 2 -> "two", 3 -> "three") val mySet = Set(1, 4, 2, 8) val myList = List(1, 2, 8, 3, 3, 4) val myVector = Vector(1, 2, 3...) ¡ñ You have the choice of mutable or immutable collection instances, immutable by default ¡ñ Rich implementations, extremely flexible
Rich Collection Functionality val numbers = 1 to 20 // Range(1, 2, 3, ... 20) numbers.head // Int = 1 numbers.tail // Range(2, 3, 4, ... 20) numbers.take(5) // Range(1, 2, 3, 4, 5) numbers.drop(5) // Range(6, 7, 8, ... 20) ¡ñ There are many methods available to you in the Scala collections library ¡ñ Spend 5 minutes every day going over the ScalaDoc for one collection class
Higher Order Functions val names = List("Barb", "May", "Jon") names map(_.toUpperCase) res0: List[java.lang.String] = List(BARB, MAY, JON) ¡ñ Really methods in Scala ¡ñ Applying closures to collections
Higher Order Functions val names = List("Barb", "May", "Jon") names map(_.toUpperCase) res0: List[java.lang.String] = List(BARB, MAY, JON) names flatMap(_.toUpperCase) res1: List[Char] = List(B, A, R, B, M, A, Y, J, O, N) names filter (_.contains("a")) res2: List[java.lang.String] = List(Barb, May) val numbers = 1 to 20 // Range(1, 2, 3, ... 20) numbers.groupBy(_ % 3) res3: Map[Int, IndexedSeq[Int]] = Map(1 -> Vector(1, 4, 7, 10, 13, 16, 19), 2 -> Vector(2, 5, 8, 11, 14, 17, 20), 0 - > Vector(3, 6, 9, 12, 15, 18))
Partial Functions class MyActor extends Actor { def receive = { case s: String => println("Got a String: " + s) case i: Int => println("Got an Int: " + i) case x => println("Got something else: " + x) } } ¡ñ A simple match without the match keyword ¡ñ The receive block in Akka actors is an excellent example ¡ñ Is characterized by what "isDefinedAt" in the case statements
Currying def product(i: Int)(j: Int) = i * j val doubler = product(2)_ doubler(3) // Int = 6 doubler(4) // Int = 8 val tripler = product(3)_ tripler(4) // Int = 12 tripler(5) // Int = 15 ¡ñ Take a function that takes n parameters as separate argument lists ¡ñ ¡°Curry¡± it to create a new function that only takes one parameter ¡ñ Fix on a value and use it to apply a specific implementation of a product with semantic value ¡ñ Have to be defined explicitly as such in Scala ¡ñ The _ is what explicitly marks this as curried
Implicit Conversions case class Person(firstName: String, lastName: String) implicit def PersonToInt(p: Person) = p.toString.head. toInt val me = Person("Jamie", "Allen") val weird = 1 + me res0: Int = 81 ¡ñ Looks for definitions at compile time that will satisfy type incompatibilities ¡ñ Modern IDEs will warn you with an underline when they are in use ¡ñ Limit scope as much as possible (see Josh Suereth's NE Scala 2011)
Implicit Parameters def executeFutureWithTimeout(f: Future)(implicit t: Timeout) implicit val t: Timeout = Timeout(20, TimeUnit. MILLISECONDS) executeFutureWithTimeout(Future {proxy.getCustomer(id)}) ¡ñ Allow you to define default parameter values that are only overridden if you do so explicitly ¡ñ Handy to avoid code duplication
Usefull resources: https://www.coursera.org/course/progfun - Lessons and Examples on Coursera - good staring point!!! http://twitter.github.com/scala_school/ http://docs.scala-lang.org/tutorials/tour/tour-of-scala.html http://stackoverflow.com/tags/scala/info http://docs.scala-lang.org/cheatsheets/ :):) http://github.com/jamie-allen/taxonomy-of-scala (oryginal content of pres.) http://www.playframework.com/ - Play Framework!!!

More Related Content

Scala - core features

  • 1. Scala.compareTo(Java) to try or not to try? Based on http://github.com/jamie-allen/taxonomy-of-scala by Jamie Allen
  • 2. Quick Start Download Scala from : - http://www.scala-lang.org/downloads Download Eclipse with Scala Worksheet : - http://typesafe.com/stack/scala_ide_download Download SBT (Simple Build Tool): - http://twitter.github.com/scala_school/sbt.html
  • 3. SBT - Simple Build Tool SBT is a simple build tool, similar to maven. Helps you create, build, test, package your scala application. It includes dependency management which is defined in regular Scala file, allow to define simple build- plugins - everything simple and extensible and XML free.
  • 4. Main concepts of Scala: - runs on JVM - statically typed - compiled into bytecode - Advanced type system with type inference and declaration-site variance - Function types (including anonymous) which support closures - Pattern-matching - Implicit parameters and conversions - Full interoperability with Java
  • 5. Let's start! Create file hello.scala object HelloWorld extends App { println("Hello, World!") } Compile it : >scalac hello.scala Run it :) >scala HelloWorld "Hello, WOrld!"
  • 6. - file can have whatever name - no semicolons - simplified syntax for many common cases More Java-like version of previous example (with main method): object HelloWorld2 { def main(args:Array[String])={ println("Hello 2") } } What is going on?
  • 7. Case Classes case class Person(firstName: String = "Jamie", lastName: String = "Allen") val jamieDoe = Person(lastName = "Doe") res0: Person = Person(Jamie,Doe) ¡ñ Data Transfer Objects (DTOs) done right ¡ñ By default, class arguments are immutable & public ¡ñ Should never be extended ¡ñ Provide equals(), copy(), hashCode() and toString() implementations ¡ñ Don¡¯t have to use new keyword to create instances ¡ñ Named Parameters and Default arguments give us Builder pattern semantics
  • 8. Lazy Definitions lazy val calculatedValue = piToOneMillionDecimalPoints() ¡ñ Excellent for deferring expensive operations until they are needed ¡ñ Reducing initial footprint ¡ñ Resolving ordering issues ¡ñ Implemented with a guard field and synchronization, ensuring it is created when necessary
  • 9. Imports import scala.collection.immutable.Map class Person(val fName: String, val lName: String) { import scala.collection.mutable.{Map => MMap} val cars: MMap[String, String] = MMap() ... } ¡ñ Can be anywhere in a class ¡ñ Allow for selecting multiple classes from a package or using wildcards ¡ñ Aliasing ¡ñ Order matters!
  • 10. Objects object Bootstrapper extends App { Person.createJamieAllen } object Person { def createJamieAllen = new Person("Jamie", "Allen") def createJamieDoe = new Person("Jamie", "Doe") val aConstantValue = "A constant value¡± } class Person(val firstName: String, val lastName: String) ¡ñ Singletons within a JVM process ¡ñ No private constructor histrionics ¡ñ Companion Objects, used for factories and constants
  • 11. The apply() method Array(1, 2, 3) res0: Array[Int] = Array(1, 2, 3) res0(1) res1: Int = 2 ¡ñ In companion objects, it defines default behavior if no method is called on it ¡ñ In a class, it defines the same thing on an instance of the class
  • 12. Tuples def firstPerson = (1, Person(firstName = ¡°Barbara¡±)) val (num: Int, person: Person) = firstPerson ¡ñ Binds you to an implementation ¡ñ Great way to group values without a DTO ¡ñ How to return multiple values, but wrapped in a single instance that you can bind to specific values
  • 13. Pattern Matching Examples name match { case "Lisa" => println("Found Lisa¡±) case Person("Bob") => println("Found Bob¡±) case "Karen" | "Michelle" => println("Found Karen or Michelle¡±) case Seq("Dave", "John") => println("Got Dave before John¡±) case Seq("Dave", "John", _*) => println("Got Dave before John¡±) case ("Susan", "Steve") => println("Got Susan and Steve¡±) case x: Int if x > 5 => println("got value greater than 5: " + x) case x => println("Got something that wasn't an Int: " + x) case _ => println("Not found¡±) } ¡ñ A gateway drug for Scala ¡ñ Extremely powerful and readable ¡ñ Not compiled down to lookup/table switch unless you use the @switch annotation,
  • 14. Scala Collections val myMap = Map(1 -> "one", 2 -> "two", 3 -> "three") val mySet = Set(1, 4, 2, 8) val myList = List(1, 2, 8, 3, 3, 4) val myVector = Vector(1, 2, 3...) ¡ñ You have the choice of mutable or immutable collection instances, immutable by default ¡ñ Rich implementations, extremely flexible
  • 15. Rich Collection Functionality val numbers = 1 to 20 // Range(1, 2, 3, ... 20) numbers.head // Int = 1 numbers.tail // Range(2, 3, 4, ... 20) numbers.take(5) // Range(1, 2, 3, 4, 5) numbers.drop(5) // Range(6, 7, 8, ... 20) ¡ñ There are many methods available to you in the Scala collections library ¡ñ Spend 5 minutes every day going over the ScalaDoc for one collection class
  • 16. Higher Order Functions val names = List("Barb", "May", "Jon") names map(_.toUpperCase) res0: List[java.lang.String] = List(BARB, MAY, JON) ¡ñ Really methods in Scala ¡ñ Applying closures to collections
  • 17. Higher Order Functions val names = List("Barb", "May", "Jon") names map(_.toUpperCase) res0: List[java.lang.String] = List(BARB, MAY, JON) names flatMap(_.toUpperCase) res1: List[Char] = List(B, A, R, B, M, A, Y, J, O, N) names filter (_.contains("a")) res2: List[java.lang.String] = List(Barb, May) val numbers = 1 to 20 // Range(1, 2, 3, ... 20) numbers.groupBy(_ % 3) res3: Map[Int, IndexedSeq[Int]] = Map(1 -> Vector(1, 4, 7, 10, 13, 16, 19), 2 -> Vector(2, 5, 8, 11, 14, 17, 20), 0 - > Vector(3, 6, 9, 12, 15, 18))
  • 18. Partial Functions class MyActor extends Actor { def receive = { case s: String => println("Got a String: " + s) case i: Int => println("Got an Int: " + i) case x => println("Got something else: " + x) } } ¡ñ A simple match without the match keyword ¡ñ The receive block in Akka actors is an excellent example ¡ñ Is characterized by what "isDefinedAt" in the case statements
  • 19. Currying def product(i: Int)(j: Int) = i * j val doubler = product(2)_ doubler(3) // Int = 6 doubler(4) // Int = 8 val tripler = product(3)_ tripler(4) // Int = 12 tripler(5) // Int = 15 ¡ñ Take a function that takes n parameters as separate argument lists ¡ñ ¡°Curry¡± it to create a new function that only takes one parameter ¡ñ Fix on a value and use it to apply a specific implementation of a product with semantic value ¡ñ Have to be defined explicitly as such in Scala ¡ñ The _ is what explicitly marks this as curried
  • 20. Implicit Conversions case class Person(firstName: String, lastName: String) implicit def PersonToInt(p: Person) = p.toString.head. toInt val me = Person("Jamie", "Allen") val weird = 1 + me res0: Int = 81 ¡ñ Looks for definitions at compile time that will satisfy type incompatibilities ¡ñ Modern IDEs will warn you with an underline when they are in use ¡ñ Limit scope as much as possible (see Josh Suereth's NE Scala 2011)
  • 21. Implicit Parameters def executeFutureWithTimeout(f: Future)(implicit t: Timeout) implicit val t: Timeout = Timeout(20, TimeUnit. MILLISECONDS) executeFutureWithTimeout(Future {proxy.getCustomer(id)}) ¡ñ Allow you to define default parameter values that are only overridden if you do so explicitly ¡ñ Handy to avoid code duplication
  • 22. Usefull resources: https://www.coursera.org/course/progfun - Lessons and Examples on Coursera - good staring point!!! http://twitter.github.com/scala_school/ http://docs.scala-lang.org/tutorials/tour/tour-of-scala.html http://stackoverflow.com/tags/scala/info http://docs.scala-lang.org/cheatsheets/ :):) http://github.com/jamie-allen/taxonomy-of-scala (oryginal content of pres.) http://www.playframework.com/ - Play Framework!!!