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).

Workflow

1. Identify every side effect and non-deterministic value, including time, random values, mutable allocation, and shared-state reads/writes.
2. Choose the effect type: concrete IO or polymorphic F[_] with the smallest capability (Clock, Random, Sync, Async, Temporal, Concurrent, etc.).
3. Wrap side-effectful code using IO(...), IO.interruptible, IO.blocking, IO.async, or the corresponding typeclass operation.
4. Move initialization of stateful services into create/resource factories returning F[A] or Resource[F, A].
5. Compose effects with flatMap/for-comprehensions and collection combinators (traverse, parTraverse).
6. Use concurrency primitives (Ref, Deferred, Queue, Semaphore, Supervisor) and structured concurrency to avoid fiber leaks.
7. Keep effect execution at boundaries (IOApp, framework runtime); for callback-style interop, bridge with Dispatcher.

Side-effect rules (apply to IO, SyncIO, and to F[_]: Sync/Async)

• All side-effectful functions must return results wrapped in IO (or F[_] with Cats Effect typeclasses/capabilities).
• Side effects include all non-determinism (call sites are not referentially transparent):
  • Any I/O (files, sockets, console, databases).
  • Instant.now(), System.currentTimeMillis(), Random.nextInt(), UUID.randomUUID().
  • Reads from environment variables, system properties, clocks, random sources, or shared mutable state.
  • Allocating or returning mutable structures when identity or mutation can escape (Array, mutable.Map, Java collections, AtomicReference, caches).
• Private local mutation is acceptable only when the whole mutable block is suspended and no mutable value escapes.

Blocking I/O rules

• Prefer IO.interruptible/Sync[F].interruptible for blocking use-phase operations. This gives cancellation a chance to interrupt the underlying thread and is the right default for network/socket-style blocking APIs.
• Do not require a proof that every Java API honors interruption before using interruptible; some APIs still ignore interruption, so add a resource-specific cancellation protocol when cancellation responsiveness matters and testing shows interruption is not enough.
• Use IO.blocking/Sync[F].blocking for acquisition, cleanup, and disposal (Closeable#close, AutoCloseable#close), where attempted interruption can leave lifecycle state ambiguous.

Output expectations

• Make side effects explicit in signatures (IO/SyncIO or F[_]: Sync/Async); the guidance here applies equally to concrete IO and polymorphic F[_].
• Use the smallest typeclass constraint that supports the needed operations.
• Keep effects as values; do not execute effects in constructors or top-level vals.
• When adding or changing Scala examples, update and compile the bundled scripts/verify-examples.scala script.

Execution and test rules

• unsafeRun* is forbidden in production and test code, including import cats.effect.unsafe.implicits.global.
• If interop requires running effects from non-Cats-Effect callbacks, use Dispatcher.
• Prefer effect-native test styles (return IO[Assertion]/F[Assertion]) instead of manually running effects.
• Avoid IO.sleep/Thread.sleep in unit tests unless using virtual time with TestControl.

References

• Load references/cats-effect-io.md for documentation summary and patterns.
• For concrete samples, read references/cats-effect-io.md; the representative check is scripts/verify-examples.scala.
• Use the cats-effect-resource skill for Resource-specific workflows and patterns (install: npx skills add https://github.com/alexandru/skills --skill cats-effect-resource).