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Scala: functional programming for the imperative mind

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Scala: functional programming for the imperative mind

  1. 1. Scala Functional programming for the imperative mind
  2. 2. Scala Functional programming for the imperative mind ‘of the nature of or expressing a command; commanding.’ - imperative.
(n.d.).
 Dictionary.com
  3. 3. Scala ‘imperative Functional programmingisfor the imperative mind programming a programming paradigm that describes computation in terms of statements that change a program state.’ -
Imperative
programming Wikipedia.com
  4. 4. Outline • Introduction • Functional programming • Scala features • Java to Scala in three steps • Scala community • Wrap up
  5. 5. Roots • Created by Martin Odersky • Supported by EPFL Switzerland
  6. 6. What is Scala? A programming language ... • Runs on JVM (and .Net) • Statically typed • Object Oriented • Functional • From scripting to enterprise apps
  7. 7. Scalable language • Scalable language constructs: • Composition ‘in the small’ • Composition ‘in the large’ • Java interoperability • Performance on par with Java
  8. 8. Scalable language • Scalable language constructs: • Composition ‘in the small’ • Composition ‘in the large’ • Java interoperability • Performance on par with Java Fuses functional and object oriented paradigms
  9. 9. What is Functional Programming
  10. 10. Functional programming • Focus on functions, not state fx=x+1 • Functions are values vs. • Recursion, not loops x=x+1 • Immutability (‘referential transparency’) • Schools of thought: pure and impure (‘pragmatic’)
  11. 11. Functional programming • Focus on functions, not state fx=x+1 • Functions are values vs. • Recursion, not loops x=x+1 • Immutability (‘referential transparency’) • Schools of thought: pure and impure (‘pragmatic’)
  12. 12. Functional programming • Focus on functions, not state • Functions are values f g x = g(x) • Recursion, not loops • Immutability (‘referential transparency’) • Schools of thought: pure and impure (‘pragmatic’)
  13. 13. Functional programming • Focus on functions, not state f0=0 fx= • Functions are values f (x - 1) vs. • Recursion, not loops for(i=x; i>0 ; i--) { • Immutability (‘referential transparency’)x--; } • Schools of thought: pure and impure (‘pragmatic’)
  14. 14. Functional programming • Focus on functions, not state • Functions are values • Recursion, not loops • Immutability (‘referential transparency’) • Schools of thought: pure and impure (‘pragmatic’)
  15. 15. Functional programming Why should I care about FP? • Concurrency: FP ‘killer app’ • Higher order functions: expressiveness boost • Type-system: when present, often superior • It is coming to a language near you (C#, Java 8?)
  16. 16. FP in Scala • First-class functions: functions are objects with pleasing syntax • Immutability • Algebraic data-types and pattern matching • Parametric polymorphism (e.g. generics)
  17. 17. What are Scala’s Features
  18. 18. Scala is like Java... (Generic) classes: class Foo[T], or: class Foo[+T] public class Foo<T> Methods: def m(s : String) : Unit = .. public void m(String s) {..} Bean properties: private Foo foo; Real properties: getFoo() {..}, setFoo(..) {..} var foo: Foo Fully interoperable with existing Java Mature, large amount of proven libraries code Class/Interface distinction, single Abstract classes, traits (restricted inheritance. multiple inheritance)
  19. 19. Pure object orientation No primitive types:
  20. 20. Pure object orientation No primitive types:
  21. 21. Pure object orientation Every operation is a method call: 1+3 1.+(3)
  22. 22. Pure object orientation Every operation is a method call: 1+3 1.+(3) Console.println(“hi”) Console println “hi”
  23. 23. Pure object orientation Every operation is a method call: 1+3 1.+(3) Console.println(“hi”) Console println “hi” Since operators are methods, operator overloading is trivial.
  24. 24. Pure object orientation Every operation is a method call: 1+3 1.+(3) Console.println(“hi”) Console println “hi”
  25. 25. Pure object orientation Every operation is a method call: 1+3 1.+(3) Console.println(“hi”) Console println “hi”
  26. 26. Pure object orientation No static members, but singleton objects:
  27. 27. Type inference • Types may be omitted in declarations • Does not mean there is no type! • Inference is local only • Var/val: mutable vs. immutable
  28. 28. Type inference
  29. 29. Everything is an expression
  30. 30. Everything is an expression
  31. 31. Everything is an expression
  32. 32. Everything is an expression
  33. 33. Functional objects • Functions are first-class values • Function literals:
  34. 34. Functional objects • Functions are first-class values • Function literals: (x: Int) => x * 2
  35. 35. Functional objects • Functions are first-class values • Function literals: (x: Int) => x * 2 val double = (x: Int) => x * 2
  36. 36. Functional objects • Functions are first-class values • Function literals: (x: Int) => x * 2 val double = (x: Int) => x * 2 double(2) == 4 What is the type of double?
  37. 37. Functional types val double = (x: Int) => x * 2 has type (Int) => Int
  38. 38. Functional types val double = (x: Int) => x * 2 has type (Int) => Int Function1[Int,Int]
  39. 39. Passing functions Since functions are values, we can pass them around:
  40. 40. Passing functions But we can do this with anonymous classes...
  41. 41. Passing functions But we can do this with anonymous classes...
  42. 42. Passing functions But we can do this with anonymous classes...
  43. 43. Passing functions But we can do this with anonymous classes... Well, sort of... but: • You need explicit interfaces (no function types) • Verbose • Doesn’t scale (syntactically and semantically) • No true closures:
  44. 44. Passing functions But we can do this with anonymous classes... Well, sort of... but: • You need explicit interfaces (no function types) • Verbose • Doesn’t scale (syntactically and semantically) • No true closures:
  45. 45. Traits • Compare trait with abstract class • No interfaces, but: completely abstract traits • Can mixin multiple traits, statically and dynamically
  46. 46. Traits as rich interfaces Java interfaces have two consumers with conflicting interests: 1) Implementors 2) Users
  47. 47. Traits as rich interfaces Java interfaces have two consumers with conflicting interests: 1) Implementors 2) Users
  48. 48. Traits as stackable modifications • Situation: IntQueue interface (abstract trait), IntQueueImpl implementation • We want to add logging and filtering to any IntQueue implementation
  49. 49. Traits as stackable modifications • Situation: IntQueue interface (abstract trait), IntQueueImpl implementation • We want to add logging and filtering to any IntQueue implementation
  50. 50. Traits as stackable modifications • Situation: IntQueue interface (abstract trait), IntQueueImpl implementation • We want to add logging and filtering to any IntQueue implementation
  51. 51. Pattern matching
  52. 52. Pattern matching
  53. 53. Pattern matching Yes, it prints 9
  54. 54. Pattern matching • No more instanceof/typecasts • No more visitor pattern
  55. 55. Pattern matching • No more instanceof/typecasts • No more visitor pattern No more NullPointerException:
  56. 56. Pattern matching & XML • Scala has XML literals, can be matched • Other literals can be matched as well
  57. 57. Language feature or Library Support
  58. 58. Actors • Message-based concurrency • Actors exchange immutable messages • Extract them by pattern matching
  59. 59. Actors • Message-based concurrency • Actors exchange immutable messages • Extract them by pattern matching Looks like language feature, but is a library
  60. 60. Other library features • Enums • Map ‘syntax’ • Events • Using ‘keyword’ (e.g. Java 7 ‘automatic resource management.’) • Virtually all other Project Coin proposals
  61. 61. Other library features • Enums • Map ‘syntax’ • Events • Using ‘keyword’ (e.g. Java 7 ‘automatic resource management.’) • Virtually all other Project Coin proposals
  62. 62. Other library features • Enums • Map ‘syntax’ • Events • Using ‘keyword’ (e.g. Java 7 ‘automatic resource management.’) • Virtually all other Project Coin proposals
  63. 63. Other library features • Enums • Map ‘syntax’ • Events • Using ‘keyword’ (e.g. Java 7 ‘automatic resource management.’) • Virtually all other Project Coin proposals Lesson: choose language core wisely, all else will follow...
  64. 64. Lift webframework In own words: ✓Seaside's highly granular sessions and security ✓Rails fast flash-to-bang ✓Django's quot;more than just CRUD is includedquot; ✓Wicket's designer-friendly templating style • Heavy use of actors for async features • Integrated O/R mapping (surprisingly little boilerplate code)
  65. 65. From Java to Scala In Three steps
  66. 66. Requirements • Person entity with age property • Method to separate minors and adults • Input: List[Person] • Output: list of minors, list of adults • One pass over input
  67. 67. Requirements • Person entity with age property • Method to separate minors and adults • Input: List[Person] • Output: list of minors, list of adults • One pass over input
  68. 68. What happens in the Scala Community
  69. 69. Scala progression • Current version: 2.7.4 • Version 2.8 beta coming up: • Package objects • Named and default parameters • Many library improvements
  70. 70. Tool support • Major IDEs (Eclipse, IntelliJ, NetBeans) supported • Maven support • Scaladoc • SBaz package manager
  71. 71. Wrapping up with Concluding Remarks
  72. 72. Scala hitting mainstream? Reports of first switchers Twitter, SAP, LinkedIn, Sony Pictures
  73. 73. Scala hitting mainstream? Reports of first switchers Twitter, SAP, LinkedIn, Sony Pictures April 2009: top-30 of TIOBE index
  74. 74. Scala hitting mainstream? Reports of first switchers Twitter, SAP, LinkedIn, Sony Pictures April 2009: top-30 of TIOBE index Lots of books appearing
  75. 75. Scala hitting mainstream? “If I were to pick a language today other than Java, it would be Scala” James Gosling, ‘Father of Java’
  76. 76. Scala hitting mainstream? “If Java programmers want to use features that aren't present in the language, I think they're probably best off using another language that targets the JVM, such a Scala and Groovy.” Joshua Bloch Author of ‘Effective Java’
  77. 77. Pro’s and cons • Complexity • Java interoperability • Java -> Scala • Hides accidental complexity harder than Scala -> Java • Expressiveness • Type-system may • Uniform, extensible be intimidating language
  78. 78. Conclusion • Scala feels like ‘cleaned up Java on stereoids’ • Small core (takes some time to see it as such) provides broad options • Type inference brings ‘dynamic language’ feel • Adoptation growing because of: • Java interoperability • Growing discontent with Java
  79. 79. Conclusion • Scala feels like ‘cleaned up Java on stereoids’ • Small core (takes some time to see it as such) provides broad options • Type inference brings ‘dynamic language’ feel • Adoptation growing because of: • Java interoperability • Growing discontent with Java Scala provides deep features, but at the same time helps getting things done.
  80. 80. More information http://www.scala-lang.org http://liftweb.net Article Java Magazine 1/2009
  81. 81. Questions?

Editor's Notes

  • Intro, gaan het hebben over Scala
    Wellicht al eens gehoord over FP, lijkt groeiende interesse
    wat is imperatief? Ook wel procedureel, met state, het is wat we in Java eigenlijk doen.
  • Intro, gaan het hebben over Scala
    Wellicht al eens gehoord over FP, lijkt groeiende interesse
    wat is imperatief? Ook wel procedureel, met state, het is wat we in Java eigenlijk doen.
  • Intro, gaan het hebben over Scala
    Wellicht al eens gehoord over FP, lijkt groeiende interesse
    wat is imperatief? Ook wel procedureel, met state, het is wat we in Java eigenlijk doen.

  • Odersky: co-designer generics, original author javac.

    Wellicht had hier oracle logo moeten staan :)
  • *Voordelen JVM (enorm veel tijd in optimalisatie, platform agnostisch etc.), ook wat nadelen (niet echt ingericht op FP constructies).
    * OO, maar dan ook puur, en met extra functionaliteit
    * Scripting: REPL shell
  • Scala redelijk uniek in samenvoegen OO+FP
    OCaml en F# zijn toch meer FP met een OO systeem er aan geplakt.

  • Recursie niet alleen in functies, ook in data (bomen, lijsten)
  • Recursie niet alleen in functies, ook in data (bomen, lijsten)
  • Recursie niet alleen in functies, ook in data (bomen, lijsten)
  • Recursie niet alleen in functies, ook in data (bomen, lijsten)
  • Recursie niet alleen in functies, ook in data (bomen, lijsten)
  • Recursie niet alleen in functies, ook in data (bomen, lijsten)
  • Recursie niet alleen in functies, ook in data (bomen, lijsten)
  • Recursie niet alleen in functies, ook in data (bomen, lijsten)
  • Recursie niet alleen in functies, ook in data (bomen, lijsten)
  • Recursie niet alleen in functies, ook in data (bomen, lijsten)
  • Recursie niet alleen in functies, ook in data (bomen, lijsten)
  • Nieuwe paradigma&#x2019;s hebben killer-app nodig
    OO had GUIs, bij FP concurrency?

    Typesysteem: niet noodzakelijk voor FP, traditioneel wel focus
  • Zonder diep in te gaan, voor we features gaan bekijken, dit is hoe Scala FP bevat.

  • *default modifier is public
    *Type after identifier, type Unit == void
    *zowel Scala->Java als Java->Scala interop. Dus, elkaars classes instantieren overerven etc.
    *Abstract class ipv interfaces
  • Extenden van Java class (extends ook voor interfaces)
    Type Unit -> void in Java
    hashCode -> hashCode() , haakjes weglaten.
    Soort van autoboxing, maar beter
  • * + is methode naam, kan je zelf op een class implementeren
    * Leestekens legale identifiers in Scala
    * Operator notatie ook te gebruiken met &#x2018;normale&#x2019; methode namen
    * In Java discussie: operator overloadig BigInts (en wat voor matrix etc.)? Scala: just do it.
  • * + is methode naam, kan je zelf op een class implementeren
    * Leestekens legale identifiers in Scala
    * Operator notatie ook te gebruiken met &#x2018;normale&#x2019; methode namen
    * In Java discussie: operator overloadig BigInts (en wat voor matrix etc.)? Scala: just do it.
  • * + is methode naam, kan je zelf op een class implementeren
    * Leestekens legale identifiers in Scala
    * Operator notatie ook te gebruiken met &#x2018;normale&#x2019; methode namen
    * In Java discussie: operator overloadig BigInts (en wat voor matrix etc.)? Scala: just do it.
  • * + is methode naam, kan je zelf op een class implementeren
    * Leestekens legale identifiers in Scala
    * Operator notatie ook te gebruiken met &#x2018;normale&#x2019; methode namen
    * In Java discussie: operator overloadig BigInts (en wat voor matrix etc.)? Scala: just do it.
  • * + is methode naam, kan je zelf op een class implementeren
    * Leestekens legale identifiers in Scala
    * Operator notatie ook te gebruiken met &#x2018;normale&#x2019; methode namen
    * In Java discussie: operator overloadig BigInts (en wat voor matrix etc.)? Scala: just do it.
  • *Java statics niet echt OO. Scala: Classes->te instantieren, singleton objects->&#xE9;&#xE9;n instantie
    *Zelfde naam: companions -> toegang tot private members
    *haakjes mogen weg bij methode aanroep zonder params; puntkomma&#x2019;s ook optioneel!
    *main methode op object Main, entrypoint applicatie
  • * Vorige slide goed opgelet: geen return type!







  • * Zijn natuurlijk allemaal op ISKA over closures geweest, maar toch een opfrisser
    * Java heeft Anon. classes -> beperkingen, verbose (interface nodig), daarom weinig gebruikt
  • * Zijn natuurlijk allemaal op ISKA over closures geweest, maar toch een opfrisser
    * Java heeft Anon. classes -> beperkingen, verbose (interface nodig), daarom weinig gebruikt
  • * Zijn natuurlijk allemaal op ISKA over closures geweest, maar toch een opfrisser
    * Java heeft Anon. classes -> beperkingen, verbose (interface nodig), daarom weinig gebruikt
  • * Zijn natuurlijk allemaal op ISKA over closures geweest, maar toch een opfrisser
    * Java heeft Anon. classes -> beperkingen, verbose (interface nodig), daarom weinig gebruikt
  • * Function0 tot Function22 op deze manier beschikbaar

  • * Function0 tot Function22 op deze manier beschikbaar

  • * Function0 tot Function22 op deze manier beschikbaar

  • * Function0 tot Function22 op deze manier beschikbaar

  • * Function0 tot Function22 op deze manier beschikbaar

  • * Function0 tot Function22 op deze manier beschikbaar

  • * Function0 tot Function22 op deze manier beschikbaar


  • Scala gaat nog verder, zelf control structures maken (by-name params)
  • * First trait/class with extends, then 0 or more times with
    * Traits can have any member: defs, abstract defs, fields traits can extend from each other
  • * Traits are used in this fashion a lot for the Scala collection libs
  • * abstract override: target of super not known at design-time!
    * calls resolve right-to-left
    * selftype annotation: type of this can assume type of class where trait is mixed in!
  • * abstract override: target of super not known at design-time!
    * calls resolve right-to-left
    * selftype annotation: type of this can assume type of class where trait is mixed in!
  • * abstract override: target of super not known at design-time!
    * calls resolve right-to-left
    * selftype annotation: type of this can assume type of class where trait is mixed in!
  • * geen new keyword nodig: case class is class + companion object met apply method!
    * Sealed abstract class: compiler kan checken of alle cases gedekt zijn
  • * geen new keyword nodig: case class is class + companion object met apply method!
    * Sealed abstract class: compiler kan checken of alle cases gedekt zijn
  • * geen new keyword nodig: case class is class + companion object met apply method!
    * Sealed abstract class: compiler kan checken of alle cases gedekt zijn
  • * Twee doelen: selecteren goede case, en binden van variabelen in 1 stap
    * Java kent heeeel beperkte pattern matching: catch-clauses
    * Geen NPE: helaas heeft Scala wel null, vanwege compatibility -> tradeoff
    * voorbeelden van option: Map.get, List.find, parseInt, etc.
  • * Twee doelen: selecteren goede case, en binden van variabelen in 1 stap
    * Java kent heeeel beperkte pattern matching: catch-clauses
    * Geen NPE: helaas heeft Scala wel null, vanwege compatibility -> tradeoff
    * voorbeelden van option: Map.get, List.find, parseInt, etc.
  • * match is an expression too



  • * Of course case objects can be used to implement enums
  • * Of course case objects can be used to implement enums
  • * Of course case objects can be used to implement enums
  • * Of course case objects can be used to implement enums
  • * Of course case objects can be used to implement enums
  • Ook test frameworks

  • Proberen stukje realworld code van Java->Scala als Java programmeur->Idiomatische Scala




  • * TIOBE top-30: beating Groovy, Haskell
  • * TIOBE top-30: beating Groovy, Haskell


  • Java -> Scala voorbeeld: een Scala - methode wordt vertaald naar $minus$.
    Veel conventies voor compiler gegenereerde classes.
  • ×