Programming with Type Classes

Typically, object-oriented and functional languages don't have a lot in common. Former usually implement polymorphism through class hierachies and inheritance. Haskell, being a prototypical functional language, offers polymorphism through another mechanism: type classes. This is available in Scala as well! With type classes it becomes surprisingly easy to abstract over completely different classes and methods.

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1. Programming with Type Classes Lars Hupel Munich Scala User Group April 22nd, 2013
2. Patterns Type Classes Use cases Goal var total = 0 for (doc <- docs) { total += doc.length } total Lars Hupel Programming with Type Classes April 22nd, 2013 2 / 27
3. Patterns Type Classes Use cases Goal var total = 0 docs.foreach(doc => total += doc.length) total Lars Hupel Programming with Type Classes April 22nd, 2013 2 / 27
4. Patterns Type Classes Use cases Goal docs.foldLeft(0)((accum, current) => accum + current.length ) Lars Hupel Programming with Type Classes April 22nd, 2013 2 / 27
5. Patterns Type Classes Use cases Goal docs.foldLeft(0)(_ + _.length) Lars Hupel Programming with Type Classes April 22nd, 2013 2 / 27
6. Patterns Type Classes Use cases Goal (0 /: docs)(_ + _.length) Lars Hupel Programming with Type Classes April 22nd, 2013 2 / 27
7. Patterns Type Classes Use cases Goal docs.map(_.length).sum Lars Hupel Programming with Type Classes April 22nd, 2013 2 / 27
8. Patterns Type Classes Use cases Goal docs.map(_.length).sum SELECT SUM(docs.length) FROM docs Lars Hupel Programming with Type Classes April 22nd, 2013 2 / 27
9. Patterns Type Classes Use cases Goal docs.foldMap(_.length) Lars Hupel Programming with Type Classes April 22nd, 2013 2 / 27
10. Patterns Type Classes Use cases . . “ Good mathematicians see analogies. Great mathematicians see analogies between analogies. Stefan Banach . . ” Lars Hupel Programming with Type Classes April 22nd, 2013 3 / 27
11. Patterns Type Classes Use cases Object-oriented patterns Type Classes Use cases Lars Hupel Programming with Type Classes April 22nd, 2013 4 / 27
12. Patterns Type Classes Use cases Factory Pattern Problem Abstract object generation Underlying problem Abstraction over constructors impossible Solution in Java Interface defining abstract factory methods Lars Hupel Programming with Type Classes April 22nd, 2013 5 / 27
13. Patterns Type Classes Use cases Factory Pattern Problem Abstract object generation Underlying problem Abstraction over constructors impossible Solution in Java Interface defining abstract factory methods Lars Hupel Programming with Type Classes April 22nd, 2013 5 / 27
14. Patterns Type Classes Use cases Factory Pattern Problem Abstract object generation Underlying problem Abstraction over constructors impossible Solution in Java Interface defining abstract factory methods Lars Hupel Programming with Type Classes April 22nd, 2013 5 / 27
15. Patterns Type Classes Use cases Factory Pattern in Scala type FictionFactory[T <: Fiction] = (Int, String) => T val sciFi: FictionFactory[SciFi] = new SciFi(true, _, _) Lars Hupel Programming with Type Classes April 22nd, 2013 6 / 27
16. Patterns Type Classes Use cases Factory Pattern in Scala Problem How can factories be organized? Idea Use implicits. But where to put them? Lars Hupel Programming with Type Classes April 22nd, 2013 7 / 27
17. Patterns Type Classes Use cases Factory Pattern in Scala Problem How can factories be organized? Solution Use implicits in companion objects. object SciFi { implicit def factory: FictionFactory[SciFi] = // ... } implicitly[FictionFactory[SciFi]] Lars Hupel Programming with Type Classes April 22nd, 2013 7 / 27
18. Patterns Type Classes Use cases Excursion: Context Bounds Scala pre-2.8 def apply[T](implicit ev: Ordering[T]) = // ... Lars Hupel Programming with Type Classes April 22nd, 2013 8 / 27
19. Patterns Type Classes Use cases Excursion: Context Bounds Scala since 2.8 def apply[T : Ordering] = // ... Looks nicer, doesn’t it? Lars Hupel Programming with Type Classes April 22nd, 2013 8 / 27
20. Patterns Type Classes Use cases Factory Pattern in Scala Usage Java <T> String doSomething( Factory<T> factory, int nTimes ); Scala def doSomething[T : Factory](nTimes: Int) Lars Hupel Programming with Type Classes April 22nd, 2013 9 / 27
21. Patterns Type Classes Use cases Factory Pattern in Scala Advantages ▶ factory = function ▶ composable ▶ less boilerplate code Lars Hupel Programming with Type Classes April 22nd, 2013 10 / 27
22. Patterns Type Classes Use cases Recap What have we done so far? Lars Hupel Programming with Type Classes April 22nd, 2013 11 / 27
23. Patterns Type Classes Use cases Adapter Pattern Problem Making third-party libraries work together with your code Underlying problem Extending existing classes with additional methods difficult Lars Hupel Programming with Type Classes April 22nd, 2013 12 / 27
24. Patterns Type Classes Use cases Adapter Pattern Problem Making third-party libraries work together with your code Underlying problem Extending existing classes with additional methods difficult Lars Hupel Programming with Type Classes April 22nd, 2013 12 / 27
25. Patterns Type Classes Use cases Adapter Pattern Problem Making third-party libraries work together with your code Underlying problem Extending existing classes with additional methods difficult Solution in Ruby/Python/... Monkey-patching! Lars Hupel Programming with Type Classes April 22nd, 2013 12 / 27
26. Patterns Type Classes Use cases Adapter Pattern Problem Making third-party libraries work together with your code Underlying problem Extending existing classes with additional methods difficult Solution in Java ▶ Composition (Wrapping) ▶ (Inheritance) Lars Hupel Programming with Type Classes April 22nd, 2013 12 / 27
27. Patterns Type Classes Use cases Type Classes ▶ not to be confused with “class” in the Java sense ▶ more of a “family” of types offering common functionality Lars Hupel Programming with Type Classes April 22nd, 2013 13 / 27
28. Patterns Type Classes Use cases Type Classes . . “ Polymorphism captures similar structure over different values, while type classes capture similar operations over different structures. Paul Hudak . . ” Lars Hupel Programming with Type Classes April 22nd, 2013 14 / 27
29. Patterns Type Classes Use cases Example: Numeric trait Numeric[A] { def add(a1: A, a2: A): A def neg(a: A): A val zero: A } Looks familiar? It’s a group. Lars Hupel Programming with Type Classes April 22nd, 2013 15 / 27
30. Patterns Type Classes Use cases Example: Numeric Implementations object FloatNumeric extends Numeric[Float] { def add(a1: Float, a2: Float) = a1 + a2 def neg(a: Float) = -a val zero: Float = 0.0f } Lars Hupel Programming with Type Classes April 22nd, 2013 16 / 27
31. Patterns Type Classes Use cases Example: Numeric Implementations object ComplexNumeric extends Numeric[Complex] { def add(a1: Complex, a2: Complex) = new Complex(a1.re + a2.re, a1.im + a2.im) // ... } Lars Hupel Programming with Type Classes April 22nd, 2013 17 / 27
32. Patterns Type Classes Use cases Example: Numeric Usage def solveEquations[T : Numeric]( system: Array[Array[T]] ): Option[Array[T]] = // ... Lars Hupel Programming with Type Classes April 22nd, 2013 18 / 27
33. Patterns Type Classes Use cases Discussion Advantages ▶ less boilerplate code ▶ decoupling ▶ retroactive integration ▶ no “self type” problem Disadvantages ▶ tiny bit of runtime overhead Lars Hupel Programming with Type Classes April 22nd, 2013 19 / 27
34. Patterns Type Classes Use cases Use case: Algebra We already know that one. ▶ semigroups ▶ monoids ▶ groups ▶ semirings ▶ rings ▶ fields ▶ vector spaces ▶ ... Lars Hupel Programming with Type Classes April 22nd, 2013 20 / 27
35. Patterns Type Classes Use cases Monoids trait Monoid[A] { def append(x: A, y: A): A def zero: A } Lars Hupel Programming with Type Classes April 22nd, 2013 21 / 27
36. Patterns Type Classes Use cases Monoids Implementations Lots of things are monoids. ▶ Int, BigInt, ... (obviously) ▶ List[T] ▶ Map[K, V] ▶ Option[T] ▶ ... Lars Hupel Programming with Type Classes April 22nd, 2013 22 / 27
37. Patterns Type Classes Use cases Monoids Implementations Lots of things are monoids. ▶ Int, BigInt, ... (obviously) ▶ List[T] ▶ Map[K, V] ▶ Option[T] ▶ ... Lars Hupel Programming with Type Classes April 22nd, 2013 22 / 27
38. Patterns Type Classes Use cases Monoids Implementations Lots of things are monoids. ▶ Int, BigInt, ... (obviously) ▶ List[T] ▶ Map[K, V] ▶ Option[T] ▶ ... Lars Hupel Programming with Type Classes April 22nd, 2013 22 / 27
39. Patterns Type Classes Use cases Monoids Implementations Lots of things are monoids. ▶ Int, BigInt, ... (obviously) ▶ List[T] ▶ Map[K, V] ▶ Option[T] ▶ ... Lars Hupel Programming with Type Classes April 22nd, 2013 22 / 27
40. Patterns Type Classes Use cases Monoids Implementations Lots of things are monoids. ▶ Int, BigInt, ... (obviously) ▶ List[T] ▶ Map[K, V] ▶ Option[T] ▶ ... Lars Hupel Programming with Type Classes April 22nd, 2013 22 / 27
41. Demo
42. Patterns Type Classes Use cases Serialization Java serialization: bad Why? Reflection, runtime behaviour, ... Lars Hupel Programming with Type Classes April 22nd, 2013 24 / 27
43. Patterns Type Classes Use cases Serialization Java serialization: bad Why? Reflection, runtime behaviour, ... Lars Hupel Programming with Type Classes April 22nd, 2013 24 / 27
44. Patterns Type Classes Use cases Serialization trait Binary[A] { def encode(a: A): Vector[Byte] def decode(bytes: Vector[Byte]): Option[(A, Vector[Byte])] } Lars Hupel Programming with Type Classes April 22nd, 2013 25 / 27
45. Demo
46. Q & A larsrh.github.io @larsr h