Appendix C: Glossary
Short definitions of terms as this book uses them. The intent is to make cross-references precise, not to define every word in Scala. Each entry points to the chapter where the concept is developed.
Binding — One entry in a Design: a rule for how to produce a value of some type. Created with bindSingleton, bindInstance, bindImpl, or bindProvider. See Chapter 3.
Cancelable — The handle returned by Rx.subscribe / Rx.run. Calling cancel tears down the subscription and stops further values. Holding and releasing it is how Rx avoids leaked streams. See Chapter 7.
Circuit breaker — A control-flow primitive that stops calling a backend once its failure rate crosses a threshold, "opening" to fail fast until the backend recovers. See Chapter 8.
Codec — A bidirectional translator between bytes (JSON or MessagePack) and Scala values. In Uni the codec is a Weaver. See Chapter 6.
Cross-project — An sbt module declared once with crossProject and compiled for the JVM, Scala.js, and Scala Native. Shared code lives in src/; platform-specific code in .jvm / .js / .native. See Chapter 11.
Design — An immutable description of how types are wired together — which implementations satisfy which dependencies, and which values live as singletons. A Design is a value: building it runs nothing. See Chapter 3.
Launcher — The annotation-driven CLI argument parser. It populates a plain case class from Array[String] using @option and @argument. See Chapter 2.
LogSupport — The trait a class mixes in to get trace / debug / info / warn / error methods and a logger named after the class. Log messages carry their source (file:line), captured at compile time. See Chapter 5.
Resource — A control primitive (Resource.withResource) that lends you a resource for a block and closes it on exit, on both the normal and exception paths — the loan pattern. See Chapter 8.
Retry — A policy (Retry.withBackOff) that re-runs a failing operation with increasing delays. Safe only for idempotent operations. See Chapter 8.
RPC — Calling a remote service through a shared Scala trait, so the client and server are checked against one contract by the compiler. See Chapter 10.
Rx — Uni's abstraction for a value that arrives later or changes over time: a lazy description of zero or more values that you compose with map / flatMap / filter and consume with subscribe. See Chapter 7.
RxVar — A mutable Rx you can watch: created with Rx.variable, updated with :=, and observed by subscribers. See Chapter 7.
Session — The runtime context Design.build (or withSession) opens: it owns the singletons it constructs and runs their onStart / onShutdown lifecycle hooks. A session has a lifetime; closing it tears the graph down. See Chapter 3.
Surface — Compile-time type reflection. It captures a type's structure (fields, parameters, type arguments) for the codec layer to use, without runtime reflection — which is what lets derivation work on Scala.js and Native. See Chapter 6.
Weaver — Uni's derivation-based codec. One Weaver[A] serializes a type to JSON or MessagePack; you get it with derives Weaver. See Chapter 6.