zio-reference

ZIO Coding Reference

You are an expert Scala/ZIO developer. Follow these rules strictly when generating ZIO code.

Core Concepts

• ZIO[R, E, A]: Immutable effect description. R = environment (contravariant), E = error (covariant), A = success (covariant). Effects are descriptions, not running computations.
• Fibers: Lightweight virtual threads managed by ZIO runtime. Millions can run concurrently. Fork starts one, join awaits it.
• ZLayer: Recipe for constructing services from dependencies with effectful init/finalize. Automates dependency graph wiring.
• Scope: Resource lifetime manager. Finalizers run on success, failure, or interruption.
• Typed Errors vs Defects: E channel = expected domain failures (compiler-checked). Defects = unexpected bugs captured in Cause, not in E.

Deep dive: references/core-concepts.md

Coding Rules

Error Handling

Deep dive: references/error-handling-deep-dive.md

• Model domain errors as sealed trait ADTs: sealed trait UserError; case class NotFound(id: UUID) extends UserError
• Use ZIO.fail(NotFound(id)) not ZIO.fail(new Exception("not found"))
• Use orDie to shift Throwable to defect channel when unrecoverable: ZIO.attempt(expr).orDie
• Use refineToOrDie[SpecificError] to keep one error type, defect the rest
• Never swallow defects: effect.tapDefect(c => ZIO.logErrorCause("Unexpected", c)) not effect.catchAllDefect(_ => ZIO.unit)
• Use mapError to unify error hierarchies across layers
• Use catchAll for recovery, foldZIO for transforming both paths
• Low-level: broad errors (IOException). High-level: refined domain ADTs or defects.
• Use Either only when errors are unrelated and you want to avoid widening E to Any

Composition

• For-comprehensions for sequential composition:

  for { user <- getUser(id); _ <- notify(user) } yield ()
  

• *> (zipRight) / <* (zipLeft) when discarding one result:

  Console.printLine("Starting") *> runTask
  

Dependencies (Service Pattern)

• Trait defines interface (methods return IO[E, A] with R = Any)
• Case class implements trait, receives dependencies via constructor
• Companion object provides ZLayer

trait Greeter { def sayHello(name: String): UIO[Unit] }

case class GreeterLive(console: Console) extends Greeter {
  def sayHello(name: String): UIO[Unit] = console.printLine(s"Hello, $name!").orDie
}

object Greeter {
  val layer: URLayer[Console, Greeter] = ZLayer.derive[GreeterLive]
}

Resources

• Always use ZIO.acquireRelease or ZIO.acquireReleaseWith, never ensuring for resource cleanup
• Use ZIO.scoped for narrowest possible resource lifetime:

  ZIO.scoped { ZIO.acquireRelease(open)(r => ZIO.succeed(r.close())).flatMap(use) }
  

Concurrency

Deep dive: references/concurrency-and-fiber.md

• Prefer foreachPar, zipPar, race over manual fork/join
• Always join or interrupt any manually forked fiber
• Use ZIO.attemptBlocking or ZIO.attemptBlockingIO for blocking I/O, never ZIO.attempt
• Use ZIO.uninterruptibleMask(restore => setup *> restore(task) *> cleanup) for custom operators
• Use Ref for single atomic state, STM/TRef for multi-variable transactional updates
• Don't wrap monolithic loops in ZIO.succeed — runtime can't interrupt inside them

Anti-Patterns (Never Do These)

Deep dive: references/anti-patterns.md

Anti-Pattern	Correct
Task[A] for business logic	IO[DomainError, A] with sealed trait ADT
val f = api(); ZIO.fromFuture(_ => f)	ZIO.fromFuture(ec => api()(ec))
trait Svc { def get: ZIO[Database, E, A] }	case class LiveSvc(db: Database) extends Svc { def get: IO[E, A] }
acquire.flatMap(r => use(r).ensuring(release(r)))	ZIO.acquireReleaseWith(acquire)(release)(use)
ZIO.attempt(blockingIOCall())	ZIO.attemptBlockingIO(blockingIOCall())
effect.catchAllDefect(_ => ZIO.unit)	effect.tapDefect(c => ZIO.logErrorCause("err", c))
effect.fork *> ZIO.unit	effect.fork.flatMap(_.join) or use structured concurrency
ZIO.succeed(while(true) { ... })	Recursive ZIO effect or .forever
object Svc { def method(a: A): ZIO[Svc, E, B] = ZIO.serviceWithZIO(_.method(a)) }	Call ZIO.serviceWithZIO[Svc](_.method(a)) directly at call sites

Decision Guide: Which Abstraction?

Deep dive: references/when-to-use-what.md

Need	Use	Not
Service wiring + lifecycle	ZLayer	Manual instance passing
Single shared mutable state	Ref	var, AtomicReference
Multi-variable atomic updates	STM / TRef	Multiple Ref updates
Work distribution (one consumer per item)	Queue	Hub
Broadcast (all consumers get every item)	Hub	Queue
Retry with backoff / repeat on schedule	Schedule	Manual recursion
One-shot synchronization between fibers	Promise	Queue, Ref
Limit concurrent access	Semaphore	Manual counting
Incremental / infinite data processing	ZStream	ZIO returning List
Resource with finalization	Scope + acquireRelease	ensuring
Low-level fiber control	Fiber + fork	Default; prefer foreachPar/zipPar

Key Patterns

More patterns: references/minimal-working-patterns.md | references/idioms.md

Service Pattern (Repository variant)

trait UserRepo { def findById(id: UUID): IO[RepoError, Option[User]] }

case class PostgresUserRepo(ds: DataSource) extends UserRepo {
  def findById(id: UUID): IO[RepoError, Option[User]] =
    ZIO.attemptBlocking(query(ds, id)).mapError(DbError(_))
}

object UserRepo {
  val layer: ZLayer[DataSource, Nothing, UserRepo] = ZLayer.derive[PostgresUserRepo]
}

Background Worker

def startWorker(queue: Queue[Job]): ZIO[Any, Nothing, Fiber.Runtime[Nothing, Nothing]] =
  queue.take.flatMap(job => process(job).ignoreLogged).forever.forkDaemon

Retry with Backoff

val policy = Schedule.exponential(1.second) && Schedule.recurs(5)
ZIO.attempt(unstableCall()).retry(policy)

Parallel with Limit

ZIO.foreachPar(urls)(fetch).withParallelism(10)

HTTP Route (ZIO HTTP)

val route: Route[UserRepo, Nothing] =
  Method.GET / "users" / uuid("id") -> handler { (id: UUID, req: Request) =>
    ZIO.serviceWithZIO[UserRepo](_.findById(id)).map {
      case Some(user) => Response.json(user.toJson)
      case None       => Response.notFound
    }.orDie
  }