Book Image

Learn Type-Driven Development

By : Yawar Amin, Kamon Ayeva
Book Image

Learn Type-Driven Development

By: Yawar Amin, Kamon Ayeva

Overview of this book

Type-driven development is an approach that uses a static type system to achieve results including safety and efficiency. Types are used to express relationships and other assumptions directly in the code, and these assumptions are enforced by the compiler before the code is run. Learn Type-Driven Development covers how to use these type systems to check the logical consistency of your code. This book begins with the basic idea behind type-driven development. You’ll learn about values (or terms) and how they contrast with types. As you progress through the chapters, you’ll cover how to combine types and values inside modules and build structured types out of simpler ones. You’ll then understand how to express choices or alternatives directly in the type system using variants, polymorphic variants, and generalized algebraic data types. You’ll also get to grips with sum types, build sophisticated data types from generics, and explore functions that express change in the types of values. In the concluding chapters, you’ll cover advanced techniques for code reuse, such as parametric polymorphism and subtyping. By end of this book, you will have learned how to iterate through a type-driven process of solving coding problems using static types, together with dynamic behavior, to obtain more safety and speed.
Table of Contents (12 chapters)

Record types

We've used record types in several places in the book so far, mostly to build a person type with an ID and a name. Let's examine this simple record type a bit more closely and pick apart what exactly happens when it is created:

type person = {id: int, name: string};

As a whole, this type definition creates a new nominal type called person with two named fields: id and name, with specific int and string types. A nominal type is one that is distinguished by the typechecker from other types solely by name.

This is as opposed to structural types, which are considered by the typechecker to be equal to their constituent types. For example, we saw in the previous chapter that modules are structurally typed. We'll see some more examples in this chapter and the next one.

At any rate, nominal types cannot be used interchangeably, even if they have the exact same...