How to bind a class that extends a Trait with a monadic type parameter using Scala Guice?

Question

I need to bind the implementation of this trait:

trait ClientRepository[F[_]] {
  def list(): F[Iterable[ClientDTO]]
}

to this implementation:

import cats.effect.IO

@Singleton
class ClientRepositoryImpl @Inject()(db: OldDataBase, c: IOContextShift)
    extends ClientRepository[IO] {

  override def list(): IO[Iterable[ClientDTO]] = ???
}

I'm using Scala Play! v2.7.2 and Scala v2.12.8, with scala-guice v4.2.1. In order to bind the trait to its implementation I would like to do something like that in my Module.scala :

class Module(environment: Environment, configuration: Configuration)
    extends AbstractModule
    with ScalaModule {

  override def configure() = {

    bind[ClientRepository].to[ClientRepositoryImpl[IO]].in[Singleton]

  }
}

And the error I get is:

[error] app/Module.scala:37:9: kinds of the type arguments (ClientRepository) do not conform to the expected kinds of the type parameters (type T).
[error] ClientRepository's type parameters do not match type T's expected parameters:
[error] trait ClientRepository has one type parameter, but type T has none
[error]     bind[ClientRepository].to[ClientRepositoryImpl[IO]].in[Singleton]
[error]         ^
[error] app/Module.scala:37:31: ClientRepositoryImpl does not take type parameters
[error]     bind[ClientRepository].to[ClientRepositoryImpl[IO]].in[Singleton]
[error]                               ^
[error]

I've also tried:

bind[ClientRepository[IO]].to[ClientRepositoryImpl].in[Singleton]

Module.scala:37:9: kinds of the type arguments (cats.effect.IO) do not conform to the expected kinds of the type parameters (type T).
[error] cats.effect.IO's type parameters do not match type T's expected parameters:
[error] class IO has one type parameter, but type T has none
[error]     bind[ClientRepository[IO]].to[ClientRepositoryImpl].in[Singleton]
[error]         ^

and bind[ClientRepository[IO[_]]].to[ClientRepositoryImpl].in[Singleton]

Module.scala:37:27: cats.effect.IO[_] takes no type parameters, expected: one
[error]     bind[ClientRepository[IO[_]]].to[ClientRepositoryImpl].in[Singleton]
[error]                           ^

What's the correct way to fix this?

Answer 1

I found the proper solution using Guice's TypeLiteral , after reading this SO answer and this one .

The working solution is:

    // In Module.scala configure()
    bind(new TypeLiteral[ClientRepository[IO]] {}).to(classOf[ClientRepositoryImpl])

because we must provide a class that can be instantiated (with a type parameter, that in our case is IO ). TypeLiteral , which is a special class that enables you to specify a full parameterized type, can be used to create the actual binding to a particular implementation of our Repo[F[_]] . A class with a generic parameter cannot be instantiated but we can force Guice to pick up a specific ClientRepository that has been constructed with the type parameter cats.effect.IO .

Last but not least whenever you have to inject the trait ClientRepository you have to specify the type parameter as well. For instance:

class ClientResourceHandler @Inject()(
    routerProvider: Provider[ClientRouter],
    clientRepository: ClientRepository[IO]
)

the ClientResourceHandler needs to call the repo, so we're injecting it using the trait ClientRepository[IO] (not just ClientRepository ).