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)

Starting with a variant type (version 1)

First, we need a type to represent the companies, which are internet-powered apps. Based on that type, we can think of writing functions that will help us build our logic in a type-safe manner. Let's see how it goes.

As a first attempt, we start small, defining a variant type for the internet companies we are interested in. As we have seen in previous chapters, we will use pattern matching to show the list of apps each of these companies delivers to their users.

We define the internet company type as follows:

type internetCompany =
| Facebook
| Google
| Twitter;

Now, we define the function that shows the applications, based on the company, as follows:

let apps = (company: internetCompany) : string => {
switch (company) {
| Facebook => "facebook, messenger, ads"
| Google => "gmail, google+, maps...