Scala implicit type class dependency injection

Question

I'd like some help sorting out this scenario. I have an Akka actor where I want to inject a dependency, in this case RemoteFetcher, which I would also like mock in my tests. Like so:

main/src/scala/mypackage/Services.scala

package mypackage
import RemoteFetcherFileSystem._

trait RemoteFetcher {
  def fetch( path:String ): Future[Stream[String]]
}

class MyRemoteResourceActor extends Actor with ActorLogging {
  def fetchRemote( path:String ) = implicitly[RemoteFetcher].fetch( path )
  def receive = {
     case FetchRemoteResource( path ) => fetchRemote( path ).map( _.foreach( sender ! _ ) )
  }
}

For this to work I have an implicit object that I import into the file above. Would look something like this:

implicit object RemoteFetcherFileSystem extends RemoteFetcher {
   def fetchRemote( path:String ) = Future[Stream[String]] { ... reading from file system ... }
}

Now in my tests I have TestActor from the akka-testkit. Here I want to instead import my mock dependency:

implicit object RemoteFetcherMock extends RemoteFetcher {
   def fetchRemote( path:String ) = Future[Stream[String]] { ... mock implementation ... }
}

My problem is that to compile Services.scala I need to import the implicit object. But how do I go about to shadow/override this in my test-files. The reason I'm not using implicit arguments is that I want to avoid having to modify all my actors constructor arguments.

I when looking around and reading up on the type class dependency injection pattern and I get it to work according to the tutorials, but I don't get it to work when I want to test and override like in my example.

Alex DiCarlo Alex DiCarlo · Accepted Answer · 2013-01-15T01:45:54

I'm not sure how to do it with implicits, but typically one could inject instead like so:

trait RemoteFetcherComponent {
  def remoteFetcher: RemoteFetcher
  trait RemoteFetcher {
    def fetch(path: String): Future[Stream[String]]
  }
}

trait RemoteFetcherFileSystemComponent extends RemoteFetcherComponent {
   val remoteFetcher = RemoteFetcherFileSystem
   object RemoteFetcherFileSystem extends RemoteFetcher {
     def fetch(path: String): Future[Stream[String]] = ???
   }
}

class MyRemoteResourceActor extends Actor with ActorLogging with RemoteFetcherFileSystemComponent {
  def fetchRemote(path: String) = remoteFetcher.fetch(path)
  def receive = {
     case FetchRemoteResource(path) => fetchRemote(path).map( _.foreach(sender ! _))
  }
}

val myRemoteResourceActor = new MyRemoteResourceActor()

And then a test value would be defined like so:

trait RemoteFetcherMockComponent extends RemoteFetcherComponent {
  def remoteFetcher = RemoteFetcherMock
  object RemoteFetcherMock extends RemoteFetcher {
    def fetch(path: String): Future[Stream[String]] = ???
  }
}

val myMockedResourceActor = new MyRemoteResourceActor with RemoteFetcherMockComponent {
  override val remoteFetcher = super[RemoteFetcherMockComponent].remoteFetcher
}

The reason you are having an issue with implicits is because the way you're using it is no different from simply using def fetchRemote(path: String) = RemoteFetcherFileSystem.fetch(path). With the import, you've defined the implementation, rather than allowed it to be injected later.

Scala implicit type class dependency injection

2 Answers