Chapter 7. Subtyping

Now that we’ve covered primitive types, composition, and function types, it’s time to look at another aspect of type systems: relationships between types. In this chapter, we’ll introduce the subtyping relationship. Although you may be familiar with it from object-oriented programming, we will not cover inheritance in this chapter. Instead, we will focus on a different set of applications of subtyping.

First, we’ll talk about what subtyping is and the two ways in which programming languages implement it: structural and nominal. Then we will revisit our Mars Climate Orbiter example and explain the unique symbol trick we used in chapter 4 when discussing type safety.

Because a type can be a subtype of another type, and it can also have other subtypes, we will look at this type hierarchy: we usually have a type that sits at the top of this hierarchy and, sometimes, a type that sits at the bottom. We’ll see how we can use this top type in a scenario such as deserialization, in which we don’t have a lot of typing information readily available. We’ll also see how to use a bottom type as a value for error cases.

7.1. Distinguishing between similar types in TypeScript

7.2. Assigning anything to, assigning to anything

7.3. Allowed substitutions

Summary

Answers to exercises