Twins: OOP and FP

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Object-Oriented Programming has well established design principles, such as SOLID. For many developers architecture and functional programming are at odds with each other: they don’t know how their existing tricks of the trade convert into functional design. This problem becomes worse as hybrid languages such as Scala, Java 8 and Ruby become common. I will talk about how functional programming helps you implement the SOLID principles, and how a functional mindset can actually help you achieve the holy grail of OO, encapsulation.

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Twins: OOP and FP

  1. 1. OOP and FP Richard Warburton
  2. 2. What on earth are you talking about? SOLID Principles Design Patterns Anthropology
  3. 3. In Quotes ... "I find OOP technically unsound” - Alexander Stepanov OOP prevents any one programmer from "doing too much damage." - Paul Graham "OOP is to writing a program, what going through airport security is to flying" - Richard Mansfield "Brain explosion is like a traditional pasttime in #haskell" "Some people claim everything is lisp. One time I was eating some spaghetti and someone came by and said: 'Hey, nice lisp dialect you're hacking in there'" "TDD replaces a type checker in Ruby in the same way that a strong drink replaces sorrows." - byorgey
  4. 4. Caveat: some unorthodox definitions may be provided
  5. 5. What on earth are you talking about? SOLID Principles Design Patterns Anthropology
  6. 6. SOLID Principles ● Basic Object Oriented Programming Principles ● Make programs easier to maintain ● Guidelines to remove code smells
  7. 7. Single Responsibility Principle ● Each class/method should have single responsibility ● Responsibility means “reason to change” ● The responsibility should be encapsulated
  8. 8. int countPrimes(int upTo) { int tally = 0; for (int i = 1; i < upTo; i++) { boolean isPrime = true; for (int j = 2; j < i; j++) { if (i % j == 0) { isPrime = false; } } if (isPrime) { tally++; } } return tally; }
  9. 9. int countPrimes(int upTo) { int tally = 0; for (int i = 1; i < upTo; i++) { if (isPrime(i)) { tally++; } } return tally; } boolean isPrime(int number) { for (int i = 2; i < number; i++) { if (number % i == 0) { return false; } } return true; }
  10. 10. long countPrimes(int upTo) { return IntStream.range(1, upTo) .filter(this::isPrime) .count(); } boolean isPrime(int number) { return IntStream.range(2, number) .allMatch(x -> (number % x) != 0); }
  11. 11. Higher Order Functions ● Hard to write single responsibility code in Java before 8 ● Single responsibility requires ability to pass around behaviour ● Not just functions, M&S higher order functions
  12. 12. Open Closed Principle
  13. 13. "software entities should be open for extension, but closed for modification" - Bertrand Meyer
  14. 14. OCP as Polymorphism ● Graphing Metric Data ○ CpuUsage ○ ProcessDiskWrite ○ MachineIO ● GraphDisplay depends upon a TimeSeries rather than each individually ● No need to change GraphDisplay to add SwapTime
  15. 15. OCP by Functions as Arguments // Example creation ThreadLocal<DateFormat> formatter = withInitial(() -> new SimpleDateFormat()); // Usage DateFormat formatter = formatter.get(); // Or ... AtomicInteger threadId = new AtomicInteger(); ThreadLocal<Integer> formatter = withInitial(() -> threadId.getAndIncrement());
  16. 16. OCP as Immutability ● Immutable Object cannot be modified after creation ● Safe to add additional behaviour ● New pure functions can’t break existing functionality because it can’t change state
  17. 17. Liskov Substitution Principle Let q(x) be a property provable about objects x of type T. Then q(y) should be true for objects y of type S where S is a subtype of T. * Excuse the informality
  18. 18. A subclass behaves like its parent. * This is a conscious simplification
  19. 19. 1. Where the parent worked the child should. 2. Where the parent caused an effect then the child should. 3. Where parent always stuck by something then the child should. 4. Don’t change things your parent didn’t.
  20. 20. Functional Perspective ● Inheritance isn’t key to FP ● Lesson: don’t inherit implementation and LSP isn’t an issue! ● Composite Reuse Principle already commonly accepted OOP principle
  21. 21. Interface Segregation Principle "The dependency of one class to another one should depend on the smallest possible interface" - Robert Martin
  22. 22. Factory Example interface Worker { public void goHome(); public void work(); } AssemblyLine requires instances of Worker: AssemblyWorker and Manager but a Robot doesn’t goHome()
  23. 23. Nominal Subtyping ● For Foo to extend Bar you need to see Foo extends Bar in your code. ● Relationship explicit between types based on the name of the type ● Common in Statically Typed, OO languages: Java, C++
  24. 24. class AssemblyWorker implements Worker class Manager implements Worker class Robot implements Worker
  25. 25. public void addWorker(Worker worker) { workers.add(worker); } public static AssemblyLine newLine() { AssemblyLine line = new AssemblyLine(); line.addWorker(new Manager()); line.addWorker(new AssemblyWorker()); line.addWorker(new Robot()); return line; }
  26. 26. Structural Subtyping
  27. 27. Structural Subtyping ● Relationship implicit between types based on the shape/structure of the type ● If you call obj.getFoo() then obj needs a getFoo method ● Common in wacky language: Ocaml, Go, C++ Templates, Ruby (quack quack)
  28. 28. class StructuralWorker { def work(step:ProductionStep) { println( "I'm working on: " + step.getName) } }
  29. 29. def addWorker(worker: {def work(step:ProductionStep)}) { workers += worker } def newLine() = { val line = new AssemblyLine line.addWorker(new Manager()) line.addWorker(new StructuralWorker()) line.addWorker(new Robot()) line }
  30. 30. Hypothetically … def addWorker(worker) { workers += worker } def newLine() = { val line = new AssemblyLine line.addWorker(new Manager()) line.addWorker(new StructuralWorker()) line.addWorker(new Robot()) line }
  31. 31. Functional Interfaces ● An interface with a single abstract method ● By definition the minimal interface! ● Used as the inferred types for lambda expressions in Java 8
  32. 32. Thoughts on ISP ● Structural Subtyping obliviates the need for Interface Segregation Principle ● Functional Interfaces provide a nominalstructural bridge ● ISP != implementing 500 interfaces
  33. 33. Dependency Inversion Principle
  34. 34. Dependency Inversion Principle ● Abstractions should not depend on details, details should depend on abstractions ● Decouple glue code from business logic ● Inversion of Control/Dependency Injection is an implementation of DIP
  35. 35. Streams Library album.getMusicians() .filter(artist -> artist.name().contains(“The”)) .map(artist -> artist.getNationality()) .collect(toList());
  36. 36. Resource Handling & Logic List<String> findHeadings() { try (BufferedReader reader = new BufferedReader(new FileReader(file))) { return reader.lines() .filter(isHeading) .collect(toList()); } catch (IOException e) { throw new HeadingLookupException(e); } }
  37. 37. Business Logic private List<String> findHeadings() { return withLinesOf(file, lines -> lines.filter(isHeading) .collect(toList()), HeadingLookupException::new); }
  38. 38. Resource Handling <T> T withLinesOf(String file, Function<Stream<String>, T> handler, Function<IOException, RuntimeException> error) { try (BufferedReader reader = new BufferedReader(new FileReader(file))) { return handler.apply(reader.lines()); } catch (IOException e) { throw error.apply(e); } }
  39. 39. DIP Summary ● Higher Order Functions also provide Inversion of Control ● Abstraction != interface ● Functional resource handling, eg withFile in haskell
  40. 40. All the solid patterns have a functional equivalent
  41. 41. The same idea expressed in different ways
  42. 42. What on earth are you talking about? SOLID Principles Design Patterns Anthropology
  43. 43. Command Pattern • Receiver - performs the actual work. • Command - encapsulates all the information required to call the receiver. • Invoker - controls the sequencing and execution of one or more commands. • Client - creates concrete command instances
  44. 44. Macro: take something that’s long and make it short
  45. 45. public interface Editor { public void save(); public void open(); public void close(); }
  46. 46. public interface Action { public void perform(); }
  47. 47. public class Open implements Action { private final Editor editor; public Open(Editor editor) { this.editor = editor; } public void perform() { editor.open(); } }
  48. 48. public class Macro { private final List<Action> actions; … public void record(Action action) { actions.add(action); } public void run() { actions.forEach(Action::perform); } }
  49. 49. Macro macro = new Macro(); macro.record(new Open(editor)); macro.record(new Save(editor)); macro.record(new Close(editor)); macro.run();
  50. 50. The Command Object is a Function Macro macro = new Macro(); macro.record(editor::open); macro.record(editor::save); macro.record(editor::close); macro.run();
  51. 51. Observer Pattern
  52. 52. Concrete Example: Profiler public interface ProfileListener { public void accept(Profile profile); }
  53. 53. private final List<ProfileListener> listeners; public void addListener(ProfileListener listener) { listeners.add(listener); } private void accept(Profile profile) { for (ProfileListener listener : listeners) { listener.accept(profile) } }
  54. 54. EVERY time.
  55. 55. Consumer<T> === () → T ProfileListener === () → Profile ActionListener === () → Action
  56. 56. public class Listeners<T> implements Consumer<T> { private final List<Consumer<T>> consumers; public Listeners<T> add(Consumer<T> consumer) { consumers.add(consumer); return this; } @Override public void accept(T value) { consumers.forEach(consumer -> consumer.accept(value)); }
  57. 57. public ProfileListener provide( FlatViewModel flatModel, TreeViewModel treeModel) { Listeners<Profile> listener = new Listeners<Profile>() .of(flatModel::accept) .of(treeModel::accept); return listener::accept; }
  58. 58. Existing Design Patterns don’t need to be thrown away.
  59. 59. Existing Design Patterns can be improved.
  60. 60. What on earth are you talking about? SOLID Principles Design Patterns Anthropology
  61. 61. Popular programming language evolution follows Arnie’s career.
  62. 62. The 1980s were great!
  63. 63. Programming 80s style ● Strongly multiparadigm languages ○ Smalltalk 80 had lambda expressions ○ Common Lisp Object System ● Polyglot Programmers ● Fertile Language Research ● Implementation Progress - GC, JITs, etc.
  64. 64. The 1990s ruined everything
  65. 65. 90s and 2000s Market Convergence ● Huge Java popularity ramp ○ Javaone in 2001 - 28,000 attendees ○ Servlets, J2EE then Spring ● Virtual death of Smalltalk, LISP then Perl ● Object Oriented Dominance
  66. 66. Now everyone is friends
  67. 67. Increasingly Multiparadigm ● Established languages going multiparadigm ○ Java 8 - Generics + Lambdas ○ C++ - Templates, Lambdas ● Newer Languages are multi paradigm ○ F# ○ Ruby/Python/Groovy can be functional ○ New JVM languages: ■ Scala ■ Ceylon ■ Kotlin
  68. 68. Q&A http://is.gd/javalambdas @richardwarburto http://insightfullogic.com

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