Book Image

Professional Scala

By : Mads Hartmann, Ruslan Shevchenko
Book Image

Professional Scala

By: Mads Hartmann, Ruslan Shevchenko

Overview of this book

This book teaches you how to build and contribute to Scala programs, recognizing common patterns and techniques used with the language. You’ll learn how to write concise, functional code with Scala. After an introduction to core concepts, syntax, and writing example applications with scalac, you’ll learn about the Scala Collections API and how the language handles type safety via static types out-of-the-box. You’ll then learn about advanced functional programming patterns, and how you can write your own Domain Specific Languages (DSLs). By the end of the book, you’ll be equipped with the skills you need to successfully build smart, efficient applications in Scala that can be compiled to the JVM.
Table of Contents (12 chapters)


Variance provides a way to constrain parameterized types. It defines a subtyping relationship between parameterized types based on the subtyping relationship of their component types.

Imagine that you have the following class hierarchy:

class Tool
class HandTool extends Tool
class PowerTool extends Tool
class Hammer extends HandTool
class Screwdriver extends HandTool
class Driller extends PowerTool
If we define a generic box:
trait Box[T] {
  def get: T

How can Box of Tools relate to one another? Scala provides three ways:

  • Covariant: Box[Hammer] <: Box[Tool] if Hammer <: Tool

  • Contravariant: Box[Tool] <: Box[Hammer] if Tool <: Hammer

  • Invariant: There's no subtyping relationship between Box[Tool] and Box[Hammer] independently of the subtyping relationship of Tool and Hammer


Let's assume that we want to define a function called isSuitable, which takes a Box[HandTool] and tests if the box is suitable to accommodate the tool it attempts to box: