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Typeclass in Scala

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Walter Chang 張瑋修

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weihsiu@gmail.com

+weihsiu

@weihsiu

What we will cover today

• A little history
• Why typeclass
• Relevant features
• Alternatives
• A little terminology
• Our first typeclass
• First use case
• Second use case
• The M word
• Higher-kinded type
• Questions.map(Answer(_))

A little history

• Invented by Philip Wadler for Haskell
• A language feature in Haskell
• A design pattern for Scala
• A little like the strategy pattern
• Separation between data and behavior
• Type class vs. Typeclass

Why typeclass

• Polymorphism (allows values of different data types to be handled using a uniform interface)
• Types of polymorphism
• Subtype
• Parametric
• Allows a function or data type to be written generically, so that it can handle values identically without depending on their types.
• Ad-hoc (overloading)
• Functions which can be applied to arguments of different types, but which behave differently depending on the type of the argument to which they are applied.
• Decoupling
• Retroactive extension
• Binary Resilience

Relevant features

• Traits trait Logger[A] { def info(msg: String) }
• Companion objects object Logger { ... }
• Implicits definitions implicit val defaultLogger = ...
• Curry functions def add(x: Int)(y: Int) = x + y // Int => Int => Int
• Implicit parameter def info[A](msg: String)(implicit logger: Logger[A])
• Generic types type IntList = List[Int]
• Type constructors A[_]

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Alternatives

• Method overloading
• Pattern matching
• Inheritance (subtyping)
• Example: Serializable.scala

serialize(123)

serialize("hello world")

serialize(List(1, 2, 3))

serialize(Map('a -> 1, 'b -> 2, 'c -> 3))

A little terminology

• Typeclasses

trait Serializable[A] {

def serialize(x: A): String

}

• Typeclass instances

object Serializable {

implicit val itemSerializable = new Serializable[Item] {

def serialize(x: Item) = ???

}

}

• Functions/methods that use typeclasses

def serialize[A](x: A)(implicit s: Serializable[A]): String = s.serialize(x)

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Our first typeclass

Ords.scala

First use case

Coerces.scala

Second use case

ShowReads.scala

The M word

Monoids.scala

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Higher-kinded type

Foldables.scala

Questions.map(Answer(_))

Source code:

https://github.com/weihsiu/typeclasses

Avoid method overloading:

http://stackoverflow.com/questions/2510108/why-avoid-method-overloading

Kind-projector:

https://github.com/non/kind-projector

Simulacrum:

https://github.com/mpilquist/simulacrum

Scalaz:

https://github.com/scalaz/scalaz

Cats:

https://github.com/non/cats