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, Agent) 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: Logging That Finds You
• Chapter 5: Data In, Data Out — JSON & MessagePack

Part IV — Async & Control Flow

• Chapter 6: Rx, the Composable Stream
• Chapter 7: Living With Failure — Retry, Circuit Breakers, Resources

Part V — HTTP & RPC

• Chapter 8: HTTP Clients and Servers
• Chapter 9: Typed RPC

Part VI — Cross-Platform Development

• Chapter 10: One Codebase, Three Runtimes

Part VII — Agent Framework

• Chapter 11: Building LLM Agents with uni-agent

Part VIII — Testing

• Chapter 12: Testing with UniTest

Appendices

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

Status

Parts I and II are drafted. Parts III through VIII are stubs with outlines; they will ship in follow-up pull requests. If a link leads to a "Coming soon" page, that is working as intended.