Appendix B: Uni and Airframe — History and Relationship

If you've worked in the Scala ecosystem, the ideas in this book may feel familiar. That is not an accident: Uni is a refinement of code that began in the Airframe project. This appendix explains the relationship — useful background if you have existing Airframe code, and reassurance that what you've learned transfers.

Where Uni comes from

Airframe is a broad Scala library and ecosystem built up over many years, spanning dependency wiring, logging, serialization, RPC, and more, across Scala 2 and Scala 3. Uni takes the most-used, most-foundational pieces of that work and re-presents them as a single, cohesive library built for Scala 3 from the start.

The mental models you've built reading this book — Design as object wiring, Rx as a composable stream, Surface for compile-time type information, codecs derived rather than hand-written — all originated in Airframe. They carry over because they are the same ideas, sharpened.

What changed

Uni is a refinement, and the refinements pull in one direction: smaller and more modern.

• Scala 3 only. Uni drops Scala 2 cross-building, so its APIs can use given/using, enum, derives, and indentation syntax directly instead of working around their absence. The syntax notes are the features this buys.
• Minimal dependencies. Uni keeps its dependency surface deliberately small, so adding it to a project doesn't pull in a framework.
• Trimmed scope. Pieces of Airframe that were niche, or better served by a dedicated tool, are left out rather than carried forward. Uni aims to be the essential set, not the exhaustive one.
• A fresh package root. Uni lives under wvlet.uni.*, distinct from Airframe's wvlet.airframe.*.

If you have Airframe code

Because Uni and Airframe occupy different package roots, they are separate libraries and can sit in the same build during a migration — you are not forced to move everything at once. The conceptual translation is usually direct: an Airframe Design and a Uni Design express the same wiring idea, an Airframe codec and a Uni Weaver solve the same serialization problem.

The exact mapping — which APIs are identical, which were renamed, which were dropped — is a moving target better served by the project's migration notes than by a printed table here. Start from the concept you know in Airframe, find its chapter in this book, and you'll usually recognize the Uni shape immediately.

← Appendix A: Scala 3 Syntax Notes | Next → Appendix C: Glossary