Scala 101 Algebraic Data Types and Typeclasses

Scala Refresher

Traits trait Employee trait Developer extends Employee trait Manager extends Employee { def delegate(d: Developer): Unit = println(d) } Can be used like interfaces Can be mixed in with classes and objects Supports multiple inheritance using trait linearization

Classes class Person(name: String, age: Int) extends Employee case class Person(name: String, age: Int) extends Employee { def hashCode(): Int = name.map(c => c.toInt).sum } Functions def printPerson(p: Person): Unit = { println(p.name, p.age) } def generateId(p: Person): Int = p.hashCode()

Objects object HelloWorld extends App { println("Hello World") } case class Country(name: String) object Country { val codes: Map[Int, String] = ??? def apply(code: Int) = Country(codes(code)) def printCountry(c: Country) = println(c) } object CountryKeys extends Enumeration { val IN, US = Value val UK = Value("uk") }

The tools we use have a profound (and devious!) in uence on our thinking habits, and, therefore, on our thinking abilities -- Edsger Dijkstra, How do we tell truths that might hurt? “ “ Don't you hate code that's not properly indented? Making it part of the syntax guarantees that all code is properly indented! -- Guido van Rossum “ “

Types type Distance = Double type Weight = Double def addWeightWithoutTypes(a: Double, b: Double) = ??? def addWeightWithTypes(a: Weight, b: Weight) = ??? val (d1: Double, w1: Double) = (2.2, 87) // (distance, weight) val (d2: Distance, w2: Weight) = (2.2, 87) // weight addWeightWithoutTypes(d, w) // compiles and runs but is incorrect behavior addWeightWithTypes(d, w) // compile time error preventing incorrect usage Catches bugs at compile time Readable code

Scala Types Source

Product types This and That type Person = (String, Int) case class Person(name: String, age: Int) Every domain class can be considered a Product type Typically is the type that is passed around to functions

Sum types This or That sealed trait Direction case object North extends Direction case object East extends Direction case object West extends Direction case object South extends Direction De nes a base type that can refer to any of its implementations Typically functions would take elements of base type as argument and gure out underlying implementation inside it

Algebraic Data Type sealed trait Tree[T] case class Leaf[T](v: T) extends Tree[T] case class Node[T](l: Tree[T], r: Tree[T]) extends Tree[T] Can be thought of as a combination of Product and Sum types Can de ne bounds and ensure typesafety for generic inputs

Typeclasses An interface that de nes behavior Takes a type as an argument Not the same as extending an interface

Example trait MakesNoise[T] { def makeNoise(t: T): String } case class Car { def horn() = "beep beep" } case class Dog { def bark() = "bow bow" }

Example object Implicits { implicit val carNoise = new MakesNoise[Car] { def makeNoise(t: Car) = t.horn() } implicit val dogNoise = new MakesNoise[Dog] { def makeNoise(t: Dog) = t.bark() } } object KickNoisyObject { import Implicits._ def kickObject[T](t: T)(implicit makesNoise: MakesNoise[T]): String = { makesNoise.makeNoise(t) } }

Demo Problem Statement Build a DSL for a basic calculator Can parse and evaluate input like 3 plus 4 into 7 Use ADTs for optimizations and other functions Source code of demo project

Give someone a program, you frustrate them for a day; teach them how to program, you frustrate them for a lifetime -- David Leinweber, Nerds of Wall Street “ “

For further reading 1. Scala School by Twitter - Good starting point to get familiar with the syntax 2. Scala Exercises - Practise and learn Scala 3. Scala Levels - Conceptual milestones postulated by Martin Odersky 4. Learn you a Haskell for Great Good! - The most fun resource for indepth understanding of functional programming 5. Functional Programming in Scala - If you're someone who prefers a structured way of learning 6. Scala Cookbook - Highly useful if you want to get shit done in scala

References 1. Algebraic Data Type 2. Sum vs Product types in Scala 3. Functional programming with Typeclasses 4. cats - a modular and extensible library for functional programming 5. Uncle Bob's Bowling Game Kata 6. Publishing an SBT project to Bintray 7. Marp, Markdown PPT editor 8. Source code of demo project