Cats Effect IO (Scala)

Quick start

Treat every side effect and source of non-determinism as an effect value: return IO[A], SyncIO[A], or F[A] with the smallest needed Cats Effect capability.
Suspend each side-effectful call, not a precomputed result. Instant.now(), random/UUID generation, environment reads, and mutable allocation such as new Array[...] belong inside IO(...), Sync[F].delay, Clock, Random, or an effectful factory.
Initialize shared mutable state through IO/F/Resource factories and pass the resulting reference or service around. Do not hide effects in constructors, top-level vals, or default arguments.
Wrap Java blocking use-phase calls with IO.interruptible by default (or Sync[F].interruptible); use blocking for acquisition, finalizers, and operations that must not be interrupted.
Use Resource to acquire/release resources and IOApp for program entry points.
Prefer structured concurrency (parTraverse, parMapN, background, Supervisor) over manual fiber management.
Do not use unsafeRun* (unsafeRunSync, unsafeRunAndForget, etc.) in app code or tests; for interop with non-Cats-Effect callback APIs, use Dispatcher.
Read references/cats-effect-io.md for concepts, recipes, FAQ guidance, and verified samples.
For deeper Resource guidance, use the cats-effect-resource skill (install: npx skills add https://github.com/alexandru/skills --skill cats-effect-resource).