Safely convert final tagless-style algebras implemented in Future to cats-effect Async

Async Utilities

Dwolla/async-utils CI license GitHub release (latest SemVer)

Use ReaderT for safely converting Future-based traits to cats-effect

You have a higher-kinded trait like this:

trait FooService[F[_]] {
  def foo(i: Int): F[Unit]

and an implementation in Future:

class FutureFoo extends FooService[Future] {
  def foo(i: Int): Future[Unit] = Future(println(i))

(Perhaps the implementation was automatically generated by a tool like Twitter Scrooge.)

With cats-tagless, you can auto-derive a FunctorK[FooService] instance and an implementation in ReaderT[Future, FooService[Future], *]:

object FooService {
  import cats.tagless._

  implicit def FooServiceReaderT[F[_]]: FooService[ReaderT[F, FooService[F], *]] = 
    Derive.readerT[FooService, F]
  implicit val FooServiceFunctorK: FunctorK[FooService] = Derive.functorK

These implicit instances can be automatically added to the code generated by Scrooge by running the AddCatsTaglessInstances scalafix rule! See below for more detail.

Now you can safely convert your Future-based implementation into one in F[_] : Async:

import cats.tagless.syntax.all._
import com.dwolla.util.async.stdlib._

val futureFoo: FooService[Future] = new FutureFoo

val fooService: FooService[IO] = futureFoo.asyncMapK[IO]

Twitter Futures

The same structure works for Twitter Futures. Just replace import com.dwolla.util.async.stdlib._ with com.dwolla.util.async.twitter._ in the example above.

import cats.tagless.{Derive, FunctorK}
import com.twitter.util.Closable

// generated by twitter-scrooge
trait FooScroogeService[F[_]] {
  def foo(i: Int): F[Unit]

  def asClosable: Closable = Closable.nop

object FooScroogeService {
  // Let Scalafix generate these instances for you! 
  // Follow the instructions in the `Scalafix Rule` section below.
  implicit def FooScroogeServiceReaderT[F[_]]: FooScroogeService[ReaderT[F, FooScroogeService[F], *]] =
    Derive.readerT[FooScroogeService, F]
  implicit val FooScroogeServiceFunctorK: FunctorK[FooScroogeService] = Derive.functorK[FooScroogeService]

Finagle Clients

Safely create a Finagle client in IO from an implementation in Twitter Future:

import com.dwolla.util.async.finagle.ThriftClient
import com.dwolla.util.async.twitter._

val fooClient: Resource[IO, FooScroogeService[IO]] = ThriftClient[FooScroogeService]("destination")

Finagle Servers

Safely create a Finagle server in Twitter Future from an implementation in IO:

import com.dwolla.util.async.finagle.ThriftServer
import com.dwolla.util.async.twitter._

val fooImpl: FooScroogeService[IO] = new FooScroogeService[IO] {
  def foo(i: Int): IO[Unit] = IO(println(i))

val thriftServer: IO[Nothing] = ThriftServer("address", fooImpl)

Scalafix Rule

Add Scalafix to your project's build by following the instructions:

  1. Add the Scalafix plugin to the project by adding this to project/plugins.sbt:

    addSbtPlugin("ch.epfl.scala" % "sbt-scalafix" % "0.9.34")
  2. Enable SemanticDB by adding this to build.sbt:

    ThisBuild / semanticdbEnabled := true
    ThisBuild / semanticdbVersion := scalafixSemanticdb.revision
    ThisBuild / scalafixScalaBinaryVersion := CrossVersion.binaryScalaVersion(scalaVersion.value)
    ThisBuild / scalafixDependencies += "com.dwolla" %% "finagle-tagless-scalafix" % "0.0.7"
  3. Run the Scalafix rule automatically after generating the Thrift sources by adding this to build.sbt:

    Compile / scalafix / unmanagedSources := (Compile / sources).value
    Compile / compile := Def.taskDyn {
      val compileOutput = (Compile / compile).value
      Def.task {
        (Compile / scalafix).toTask(" AddCatsTaglessInstances").value
    libraryDependencies ++= {
      val catsTaglessV = "0.14.0"
        "org.typelevel" %% "cats-tagless-core" % catsTaglessV,
        "org.typelevel" %% "cats-tagless-macros" % catsTaglessV,


The AddCatsTaglessInstances rule finds generated Thrift service traits and adds implicit instances of ThriftService[Kleisli[F, ThriftService[Future], *]] and FunctorK[ThriftService] to each service's companion object.

Twitter's Scrooge project changed the way it generates code for Thrift services, removing the higher-kinded service trait used by this library, leaving only the MethodPerEndpoint trait that used to extend the higher-kinded service trait, setting the type parameter to com.twitter.util.Future. The AddCatsTaglessInstances rule now addresses this as well, rewriting MethodPerEndpoint to {Name}Service and reintroducing the type parameter. (A new MethodPerEndpoint is also added, going back to how it used to extend {Name}Service[Future].)

This Scalafix rule should be idempotent, so it can be rerun many times.


Because the AddCatsTaglessInstances rewrite rule couldn't easily move the new {Name}Service trait up to the same level as the {Name}Service object, the new traits must be addressed differently. In other words, instead of finding the trait at com.example.ThriftService, it will now be at com.example.ThriftService.ThriftService.

The AdaptHigherKindedThriftCode rule exists to adapt existing code to the new location. It will find references to traits that extend com.twitter.finagle.thrift.ThriftService and have a type parameter of the correct shape, and add the object name before the trait name (i.e., rewriting ThriftService to ThriftService.ThriftService or com.example.ThriftService to com.example.ThriftService.ThriftService).

This rule is not idempotent, but it will typically only be executed once per codebase.

The order in which the rule is executed matters. Follow these steps:

  1. Add Scalafix to your project by following steps 1 and 2 under "Scalafix Rule" above.

  2. Look at your project's sbt project graph. Because the rule is a semantic rule, it depends on the compiler being able to compile the code it will modify. This means the leaves of the project graph need to be updated before the nodes that depend on each leaf.

    For example, run Test/scalafix AdaptHigherKindedThriftCode before running Compile/scalafix AdaptHigherKindedThriftCode.

  3. Only after running the AdaptHigherKindedThriftCode rule should you update the Scrooge and Finagle version being used in the project. Once this is updated, you can run the AddCatsTaglessInstances rule on the updated generated code.


The Group ID for each artifact is "com.dwolla". All artifacts are published to Maven Central.

Artifact Description Cats Effect Version Scala 2.12 Scala 2.13 Scala.js
"async-utils-ce2" Implementation for stdlib Scala Future Cats Effect 2
"async-utils-ce3" Cats Effect 3
"async-utils-twitter-ce2" Implementation for Twitter Future Cats Effect 2 🚫
"async-utils-twitter-ce3" Cats Effect 3
"async-utils-finagle-ce2" Safely create Thrift clients and servers using cats-effect as the effect type Cats Effect 2 🚫
"async-utils-finagle-ce3" Cats Effect 3
"finagle-tagless-scalafix" Automatically adds implicit instances needed by `asyncMapK` to the companion objects of Finagle services generated by Scrooge All N/A
"async-utils-core-ce2" Shared definition of AsyncFunctorK and supporting code Cats Effect 2
"async-utils-core-ce3" Cats Effect 3


Thanks to Georgi Krastev and the cats-tagless project for the idea to use ReaderT in this way.