zio / interop-cats   3.3.0

Apache License 2.0 GitHub

ZIO instances for cats-effect type classes

Scala versions: 3.x 2.13 2.12 2.11
Scala.js versions: 1.x 0.6

ZIO Interop Cats


libraryDependencies += "dev.zio" %% "zio-interop-cats" % "23.0.x"

ZIO Cats Effect 3 instances

ZIO integrates with Typelevel libraries by providing an instance of Concurrent, Temporal and Async for Task as required, for instance, by fs2, doobie and http4s.

For convenience, the ZIO library defines an alias as follows:

type Task[A] = ZIO[Any, Throwable, A]

Therefore, we provide Cats Effect instances based on this specific datatype.


In order to get a Concurrent[Task] or Concurrent[RIO[R, *]] (note * is kind-projector notation) we need to import zio.interop.catz._:

import cats.effect._
import zio._
import zio.interop.catz._

def ceConcurrentForTaskExample = {
  val F: cats.effect.Concurrent[Task] = implicitly
  F.racePair(F.unit, F.unit)


import cats.effect._
import zio._
import zio.interop.catz._

def ceTemporal = {
  val F: cats.effect.Temporal[Task] = implicitly
  F.sleep(1.second) *> F.unit


import cats.effect._
import zio._
import zio.interop.catz._

def ceAsync = {
  val F: cats.effect.Async[Task] = implicitly
  F.racePair(F.unit, F.sleep(1.second) *> F.unit)

Other typeclasses

There are many other typeclasses and useful conversions that this library provides implementations for:

  • See zio/interop/cats.scala file to see all available typeclass implementations for the Cats Effect 3 typeclasses
  • See zio/stream/interop/cats.scala for ZStream typeclass implementations
  • See zio/stream/interop/FS2StreamSyntax.scala for FS2 <-> ZStream conversions

Easier imports (at a cost)

To use ZIO data structures in cats-effect code we may need to bring the Runtime[Any] via the ZIO.runtime combinator. This may not be ideal since everywhere you use these data structures, you will now be required to feed in the Runtime. For example, with ZIO STM TRef:

import cats.effect._
import zio.Runtime
import zio.interop.stm.TRef

def example(implicit rts: Runtime[Any]): IO[TRef[IO, Int]] = {

Rather than requiring the runtime implicit, we can add an import (if we don't mind depending on Runtime.default):

import cats.effect._
import zio.Runtime
import zio.interop.stm.TRef

import zio.interop.catz.implicits._

def example: IO[TRef[IO, Int]] = {

The major downside to doing this is you will rely on Runtime.default which might make testing more difficult in certain scenarios, e.g. with custom execution contexts.


If you only need instances for cats-core typeclasses, not cats-effect import zio.interop.catz.core._:

import zio.interop.catz.core._

Note that this library only has an Optional dependency on cats-effect – if you or your libraries don't depend on it, this library will not add it to the classpath.


The following example shows how to use ZIO with Doobie (a library for JDBC access) and FS2 (a streaming library), which both rely on Cats Effect instances (cats.effect.Async and cats.effect.Temporal):

import zio._
import zio.interop.catz._
import doobie._
import doobie.implicits._
import zio.clock.Clock
import scala.concurrent.duration._

object DoobieH2Example extends App {
  override def run(args: List[String]): URIO[zio.ZEnv, ExitCode] =
    ZIO.runtime.flatMap { implicit r: Runtime[Clock] =>
      val xa: Transactor[Task] =
        Transactor.fromDriverManager[Task]("org.h2.Driver", "jdbc:h2:mem:test;DB_CLOSE_DELAY=-1", "user", "")

      sql"SELECT 42"
        .evalTap(i => ZIO.succeedBlocking(println(s"Data $i"))))


Learn more on the ZIO Interop Cats homepage!


For the general guidelines, see ZIO contributor's guide.

Code of Conduct

See the Code of Conduct


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