Version: 2.0.x

Defining Polymorphic Services in ZIO

As we discussed here, the ZEnvironment, which is the underlying data type used by ZLayer, is backed by a type-level mapping from types of services to implementations of those services. This functionality is backed by izumi.reflect.Tag, which captures a type as a value.

We just need to know what is the type of service when we put it in the ZEnvironment because ZEnvironment is essentially a map from service types (interfaces) to implementation of those interfaces. To implement the map, the ZEnvironment needs a type tag for the new service, and also needs a way to remove the old service from the type level map. So we should have service type information at the runtime.

We can think of Tag[A] as like a TypeTag[A] or ClassTag[A] from the Scala standard library but available on a cross-version and cross-platform basis. Basically, it carries information about a certain type into runtime that was available at compile time. Methods that construct ZEnvironment values generally require a tag for the value being included in the “bundle of services”.

As a user, we should not normally interact with Tag except where we define polymorphic services. In general, a Tag should always be available whenever we have a concrete type. The only time we should have to use it is when we have a polymorphic service. If we are using polymorphic code, we need to provide implicit evidence that a tag exists for that type (implicit tag: Tag[A]) or as a context-bound for that type parameter: (A: Tag).

Let's try to write a polymorphic service. Assume we have the following service interface:

trait KeyValueStore[K, V, E, F[_, _]] {
  def get(key: K): F[E, V]

  def set(key: K, value: V): F[E, V]

  def remove(key: K): F[E, Unit]
}

In the next step, we are going to write its accessors. We might end up with the following snippet code:

import zio._

object KeyValueStore {
  def get[K, V, E](key: K): ZIO[KeyValueStore[K, V, E, IO], E, V] =
    ZIO.serviceWithZIO[KeyValueStore[K, V, E, IO]](_.get(key))

  def set[K, V, E](key: K, value: V): ZIO[KeyValueStore[K, V, E, IO], E, V] =
    ZIO.serviceWithZIO[KeyValueStore[K, V, E, IO]](_.set(key, value))

  def remove[K, V, E](key: K): ZIO[KeyValueStore[K, V, E, IO], E, Unit] =
    ZIO.serviceWithZIO(_.remove(key))
}

// error: could not find implicit value for izumi.reflect.Tag[K]. Did you forget to put on a Tag, TagK or TagKK context bound on one of the parameters in K? e.g. def x[T: Tag, F[_]: TagK] = ...
// 
// 
// <trace>: 
//   deriving Tag for K, dealiased: K:
//   could not find implicit value for Tag[K]: K is a type parameter without an implicit Tag!
//     ZIO.serviceWithZIO[KeyValueStore[K, V, E, IO]](_.get(key))
//                                                   ^
// error: could not find implicit value for izumi.reflect.Tag[K]. Did you forget to put on a Tag, TagK or TagKK context bound on one of the parameters in K? e.g. def x[T: Tag, F[_]: TagK] = ...
// 
// 
// <trace>: 
//   deriving Tag for K, dealiased: K:
//   could not find implicit value for Tag[K]: K is a type parameter without an implicit Tag!
//     ZIO.serviceWithZIO[KeyValueStore[K, V, E, IO]](_.set(key, value))
//                                                   ^
// error: could not find implicit value for izumi.reflect.Tag[K]. Did you forget to put on a Tag, TagK or TagKK context bound on one of the parameters in K? e.g. def x[T: Tag, F[_]: TagK] = ...
// 
// 
// <trace>: 
//   deriving Tag for K, dealiased: K:
//   could not find implicit value for Tag[K]: K is a type parameter without an implicit Tag!
//     ZIO.serviceWithZIO(_.remove(key))
//                       ^

The compiler generates the following errors:

could not find implicit value for izumi.reflect.Tag[K]. Did you forget to put on a Tag, TagK or TagKK context bound on one of the parameters in K? e.g. def x[T: Tag, F[_]: TagK] = ...


<trace>: 
  deriving Tag for K, dealiased: K:
  could not find implicit value for Tag[K]: K is a type parameter without an implicit Tag!
    ZIO.serviceWithZIO[KeyValueStore[K, V, E, IO]](_.get(key))

As the compiler says, we should put Tag as a context-bound for K, V, and E type parameters:

import zio._

object KeyValueStore {
  def get[K: Tag, V: Tag, E: Tag](key: K): ZIO[KeyValueStore[K, V, E, IO], E, V] =
    ZIO.serviceWithZIO[KeyValueStore[K, V, E, IO]](_.get(key))

  def set[K: Tag, V: Tag, E: Tag](key: K, value: V): ZIO[KeyValueStore[K, V, E, IO], E, V] =
    ZIO.serviceWithZIO[KeyValueStore[K, V, E, IO]](_.set(key, value))

  def remove[K: Tag, V: Tag, E: Tag](key: K): ZIO[KeyValueStore[K, V, E, IO], E, Unit] =
    ZIO.serviceWithZIO(_.remove(key))
}

Now, we can continue and implement the in-memory version of this key-value store:

case class InmemoryKeyValueStore(map: Ref[Map[String, Int]])
  extends KeyValueStore[String, Int, String, IO] {

  override def get(key: String): IO[String, Int] =
    map.get.map(_.get(key)).someOrFail(s"$key not found")

  override def set(key: String, value: Int): IO[String, Int] =
    map.update(_.updated(key, value)).map(_ => value)

  override def remove(key: String): IO[String, Unit] =
    map.update(_.removed(key))
}

object InmemoryKeyValueStore {
  def layer: ULayer[KeyValueStore[String, Int, String, IO]] =
    ZLayer {
      Ref.make(Map[String, Int]()).map(InmemoryKeyValueStore.apply)
    }
}

The last step is to use the service in a ZIO application:

import zio._

object MainApp extends ZIOAppDefault {

  val myApp: ZIO[KeyValueStore[String, Int, String, IO], String, Unit] =
    for {
      _ <- KeyValueStore.set[String, Int, String]("key1", 3).debug
      _ <- KeyValueStore.get[String, Int, String]("key1").debug
      _ <- KeyValueStore.remove[String, Int, String]("key1")
      _ <- KeyValueStore.get[String, Int, String]("key1").either.debug
    } yield ()

  def run = myApp.provide(InmemoryKeyValueStore.layer)
  
}

// Output:
// 3
// 3
// not found

Note that in the above example, one might want to write accessors more polymorphic. So in this case we should add TagKK as a context-bound of the F type parameter:

object KeyValueStore {
  def get[K: Tag, V: Tag, E: Tag, F[_, _] : TagKK](key: K): ZIO[KeyValueStore[K, V, E, F], Nothing, F[E, V]] =
    ZIO.serviceWith[KeyValueStore[K, V, E, F]](_.get(key))

  def set[K: Tag, V: Tag, E: Tag, F[_, _] : TagKK](key: K, value: V): ZIO[KeyValueStore[K, V, E, F], Nothing, F[E, V]] =
    ZIO.serviceWith[KeyValueStore[K, V, E, F]](_.set(key, value))

  def remove[K: Tag, V: Tag, E: Tag, F[_, _] : TagKK](key: K): ZIO[KeyValueStore[K, V, E, F], E, Unit] =
    ZIO.serviceWith(_.remove(key))
}

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