The Uni Book

A guided tour of Scala 3 application development with Uni.

This book teaches you to build real applications with Uni on Scala 3, across the JVM, Scala.js, and Scala Native. It is written for developers who already read Scala comfortably and want to understand not only how Uni works but why its pieces are shaped the way they are.

The Book is narrative. It is meant to be read front to back at first, and used as a jumping-off point afterwards. The short-form API documentation under Guide and the module references (e.g. Core, HTTP) stay where they are, and the Book links into them when you need depth.

How to read it

• Work from Part I to the end of Part II in order. Everything that follows assumes the mental model built in those two parts.
• Each part opens with a short intro page; each chapter ends with a "what you have, what comes next" recap. If the recap doesn't feel true yet, re-read the chapter before moving on.
• Type the examples. Reading them is not the same exercise.

Table of contents

Foreword

Part I — Getting Started

• Chapter 1: Getting Started
  • 1.1 Installation
  • 1.2 Hello, Uni!

Part II — Building a CLI Application

• Chapter 2: A URL Fetcher

Part III — Core Concepts

• Chapter 3: Wiring with Design
• Chapter 4: Testing with UniTest
• Chapter 5: Logging That Finds You
• Chapter 6: Data In, Data Out — JSON & MessagePack

Part IV — Async & Control Flow

• Chapter 7: Rx, the Composable Stream
• Chapter 8: Living With Failure — Retry, Circuit Breakers, Resources

Part V — HTTP & RPC

• Chapter 9: HTTP Clients and Servers
• Chapter 10: Typed RPC

Part VI — Cross-Platform Development

• Chapter 11: One Codebase, Three Runtimes

Part VII — Scala.js in the JavaScript Ecosystem

• Chapter 12: Calling NPM Modules from Scala.js
• Chapter 13: Bundling with Vite

Part VIII — Scala Native and the C/Rust World

• Chapter 14: Calling C from Scala Native
• Chapter 15: Exposing Scala Native to C and Rust

Part IX — Capstone

• Chapter 16: A Bookmarks Service

Appendices

• Appendix A: Scala 3 Syntax Notes for This Book
• Appendix B: Uni and Airframe — History and Relationship
• Appendix C: Glossary

Status

All parts are drafted, Foreword through the appendices. The book is best read front to back the first time; afterwards, use it as a jumping-off point into the reference docs.