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)

Polymorphic variant types

While Reason's variant types are conceptually simple, much of their power comes from the ability to use them with pattern matching and exhaustivity checking. Reason also provides a more powerful, but also more complex sum type, called polymorphic variants. As the name suggests, these variants are more flexible than regular variants. Here are a few things that we can do with polymorphic variants, but not with regular variants:

  • Create values without defining the types beforehand, letting the compiler infer the type
  • Compose multiple sets of variant cases together
  • Define functions that handle at least a set of variant cases as input
  • Define functions that output at most a set of variant cases as output

In a sense, we can think of polymorphic variants as being related to regular variants in the same way that objects are related to records. They are,...