Starting Type-Driven Development

In this book, we are exploring the techniques and idioms available in type-driven development. Some people also refer to type-driven development as type-level programming. Static types offer several benefits, including:

• Preventing incorrect code from getting a chance to run
• Documenting the current codebase
• Helping to correctly refactor the codebase by pointing out any parts of code you may have missed
• Offering richer IDE support, for example, auto-completion
• Better performance when the compiler knows types and can optimize code accordingly

Type-driven development is the practice of using static types to restrict what your code can do. Normally, your programming language gives you enough power to represent any computation. With type-driven development, you are essentially trying to make it impossible for your code to do undesirable things.

In this chapter, we will do some basic critical analysis of a piece of code and look at the possible errors it may contain. We'll also introduce ReasonML, the language we will use to learn type-driven development and compare it with JavaScript. We'll get started with a basic Reason project and then introduce Reason, as well as its related communities and ecosystems.

In this chapter, we will cover the following topics:

• The main idea and benefits of type-driven development
• Dynamically typed code versus its statically typed ReasonML equivalent
• The Reason language, ecosystem, and related projects
• How to set up a basic Reason project, which we will use throughout this book
• The Try Reason online playground