scala-guidelineslisted

# Scala Guidelines Shared Scala rubric for agents. Prefer repo-local conventions (Scala 2 vs 3, effect library) when deliberate; push back when code reaches for nulls, exceptions-as-control-flow, or implicit magic. ## Source Of Truth - Scala 3 (Dotty) reference and the official style guide - *Effective Scala* (Twitter); the cats / ZIO docs when the repo uses an effect system - The repo's `build.sbt`/`build.mill` and compiler flags (`-Xfatal-warnings`, `-Wunused`) — these are the contract ## Core Rubric - `val` and immutable collections by default; `var` never in a public API. Prefer pure functions and referential transparency. - Model data with `case class` and sealed-trait/`enum` ADTs; exhaust pattern matches (let `-Xfatal-warnings` catch the rest). - `Option`/`Either`/`Try` over `null` and over throwing for expected failures. Reserve exceptions for truly exceptional cases. - Pattern matching and `for`-comprehensions over nested `flatMap`/`isInstanceOf`/`asInstanceOf`. - Scala 3: `given`/`using` for type classes; Scala 2: implicit instances scoped narrowly. Don't smuggle behavior through broad implicit conversions. - Effects: if the repo uses cats-effect or ZIO, keep effects in `IO`/`F[_]` to the edges; don't run effects in constructors or block inside them. Tagless-final only when it buys real abstraction. - Collections: know strict vs lazy (`View`, `LazyList`); avoid O(n) `head`/`apply` on `List`; pick the structure that matches the access pattern. - Keep type signatu