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Learn Type-Driven Development
Learn Type-Driven Development

Learn Type-Driven Development: Benefit from type systems to build reliable and safe applications using ReasonML 3

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Profile Icon Amin Profile Icon Soumya Mukherjee Profile Icon Kamon Ayeva
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Learn Type-Driven Development

Programming with Types and Values

In the previous chapter, we looked at type-driven development using ReasonML, but what are types exactly? And how do they interact with other parts of your program? How do they help you on a regular basis, and what does it look like to use a strong static type system with type inference?

In this chapter, we will cover the following topics:

  • Setting up an editor workflow
  • Types and values
  • Immutable values and memory
  • Static versus dynamic typing
  • Type erasure
  • Syntax errors
  • Type errors and inference
  • Unification

Workflow

To get the most from this chapter, we will set up a comfortable edit-compile workflow. We recommend placing two windows side-by-side in your editor. VSCode supports this functionality with the View | Split Editor command. On one side, load a Reason source file; on the other side, load the JavaScript output file (once it is initially compiled). Then in a terminal, run the following command:

bsb -w

The preceding command starts a build in watch mode, which automatically recompiles any parts of a project that are affected whenever you change any source code. In fact, watch mode is smart enough to also remove an outdated JavaScript output file whenever its corresponding Reason source file is deleted. The editor will also auto-reload the compiled JavaScript file whenever you save a Reason source file.

In VSCode, you can also open a terminal session directly below the files...

Types and values

Let's set the stage for the rest of the book with a discussion of types and values. At its core, a type is a set of values. Think of the type bool, which is what Reason calls the normal Boolean type. A bool value can be one of two different things: true or false. We say that these values inhabit (live in) the type. Anything else is an error.

This raises an interesting question: what does it mean to say? Anything else is an error' in this context? In fact, why should we care about types at all?

To answer these questions, let's think about what should happen if we try to do the operation "Bob" / 5. What does it mean to divide the string Bob by the number 5?

If you can't think of a good answer, well neither can anyone else. It's kind of a meaningless question. It's like asking, how does the color green taste? (Although this...

Static and dynamic environments

Let's develop a mental model for what happens in a program with types and values. At its core, a program is made up of a series of type and value definitions. For example:

/* src/Ch02/Ch02_Demo.re */
type person = {id: int, name: string};
type company = {id: int, name: string, employees: list(person)};

let bob = {id: 1, name: "Bob"};
let acmeCo = {id: 1, name: "Acme Co.", employees: [bob]};

Here, we're defining person and company types, and then allocating a person (bob) and a company he works for (acmeCo).

Without worrying too much about the syntax (we will introduce this in Chapter 4, Group Values Together in Types), let's think about how the programming environment sees this program.

In a statically typed programming language, the typechecker and runtime environment together make up the static and dynamic environments...

Errors

We mentioned earlier that the compiler will raise errors if it cannot make sense of a piece of code it comes across. There are a few different kinds of compiler errors, and they are as follows

  • Syntax errors
  • Type errors
  • Name errors
  • Stale interface errors (which we'll cover in the next chapter)
  • Compiler bugs (these are rare but shouldn't be discounted)

The two most common types of error that we will deal with are syntax errors and type errors. Name errors are fairly simple to avoid: always start type names with a lowercase letter and ensure that the names you refer to in your code were defined before you refer to them. (Reason supports cyclic references but not forward references; we'll cover cyclic references later on.)

Syntax errors

...

Summary

In this chapter, we covered a lot of ground, including setting up an editorial workflow, learning about types and values, static and dynamic typing, Reason's separation between compile time and runtime and its type erasure, syntax, and type errors, as well as inference and unification.

In future chapters, we will build on this chapter and introduce many more static typing techniques and the potential type errors that we may see from using them.

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Key benefits

  • Reduce code errors with the power of type systems
  • Employ static typechecking and genericity to promote code reuse and consistency
  • Understand functional programming which is the foundation of type-driven development

Description

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.

Who is this book for?

If you’re a programmer working with dynamically typed languages and are looking for ways to mitigate production runtime errors, Learn Type-Driven Development is for you. You’ll also find this book helpful if you’re a programmer working with statically typed languages looking for increased safety and improved performance.

What you will learn

  • Use static types to capture information, making programs safer and faster
  • Learn ReasonML from experienced type-driven developers
  • Enhance safety by simply using basic types
  • Understand the most important type-driven concepts with simple examples
  • Explore a design space using static typing and find the best way to express your system rules
  • Use static types and dynamic runtime in harmony to write even safer and faster code
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Table of Contents

11 Chapters
Starting Type-Driven Development Chevron down icon Chevron up icon
Programming with Types and Values Chevron down icon Chevron up icon
Packaging Types and Values Together Chevron down icon Chevron up icon
Grouping Values Together in Types Chevron down icon Chevron up icon
Putting Alternative Values in Types Chevron down icon Chevron up icon
Making Types That Can Slot into Any Other Type Chevron down icon Chevron up icon
Making Types That Represent Operations Chevron down icon Chevron up icon
Reusing Code with Many Different Types Chevron down icon Chevron up icon
Extending Types with New Behavior Chevron down icon Chevron up icon
Bringing It All Together Chevron down icon Chevron up icon
Other Books You May Enjoy Chevron down icon Chevron up icon

Customer reviews

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Full star icon Full star icon Full star icon Full star icon Full star icon 5
(1 Ratings)
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1 star 0%
Johann Hubert Sonntagbauer Apr 27, 2019
Full star icon Full star icon Full star icon Full star icon Full star icon 5
I always failed to give my data layer the right amount of semantics. This book introduced me in a whole new world of possibilities thanks to the mighty ReasonML/OCaml type system.Decorating arbitrary Strings with semantics and behavior blew my mind, not even mentioning the advanced techniques described in the book. Enjoyed every chapter and loved the examples.Some advanced patterns need to sink in before I could appreciate the value, so I definitely need to revisit some chapters again.
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