How to use type parameter for Scala implicit class?

Question

I want to implement a retry mechanism for futures.

For example:

myFuture.map { data => println(data) // ... do other stuff }.recover { case e: MyException => logger.error("Something went wrong with XYZ", e) case _ => logger.error("Error!") }.retry(Seq(1.seconds, 10.seconds, 30.seconds))

So the future should be retried after certain intervals.

My implementation looks as follows:

import akka.pattern.after import akka.actor.Scheduler import scala.concurrent.{ExecutionContext, Future} import scala.concurrent.duration.FiniteDuration object FutureExt { implicit class FutureUtils(f: Future[T]) { def retry[T](delays: Seq[FiniteDuration])(implicit ec: ExecutionContext, s: Scheduler): Future[T] = { f recoverWith { case _ if delays.nonEmpty => after(delays.head, s)(f.retry(delays.tail)) } } } }

Unfortunately, the type parameter T cannot be resolved in the implicit class declaration. Any idea what's wrong with that?

danielnixon · Accepted Answer · 2017-03-14 21:09:50Z

Nothing peculiar to implicit classes here. You've specified the type parameter T on the retry method but referred to it earlier than that (in the class parameters). Move the type param to the class itself (FutureUtils[T](f: ...)).

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