Scala Cookbook

Introduction

As you can tell from one look at the Scaladoc for the collections classes, Scala has a powerful type system. However, unless you’re the creator of a library, you can go a long way in Scala without having to go too far down into the depths of Scala types. But once you start creating collections-style APIs for other users, you will need to learn them.

This chapter provides recipes for the most common problems you’ll encounter, but when you need to go deeper, I highly recommend the book, Programming in Scala, by Odersky, Spoon, and Venners. (Martin Odersky is the creator of the Scala programming language, and I think of that book as “the reference” for Scala.)

Scala’s type system uses a collection of symbols to express different generic type concepts, including variance, bounds, and constraints. The most common of these symbols are summarized in the next sections.

Variance

Type variance is a generic type concept, and defines the rules by which parameterized types can be passed into methods. The type variance symbols are briefly summarized in Table 19-1.

Table 19-1. Descriptions of type variance symbols

Symbols	Name	Description
Array[T]	Invariant	Used when elements in the container are mutable. Example: Can only pass Array[String] to a method expecting Array[String].
Seq[+A]	Covariant	Used when elements in the container are immutable. This makes the container more flexible. Example: Can pass a Seq[String] to a method expected Seq[Any].
Foo[-A] Function1[-A, +B]	Contravariant	Contravariance ...