Monad transformers with scalaz-streams - scala

Question

In this snippet y.run doesn't typecheck.
object Test {
type StateStringTask[A] = StateStringT[Task, A]
type StateStringT[M[_], A] = StateT[M, String, A]
val x: Process[Task, Unit] = ???
val y: Process[StateStringTask, Unit] = ???
x.run // This typechecks
y.run // This fails
}
The compiler shows this error:
could not find implicit value for parameter C: scalaz.Catchable[[x]Test.StateStringTask[x]]
Do I have to create a Catchable instance for StateStringTask? How do I do that? Or is there an easier way to handle stateful effects when running a Process?

Answer 1

I guess this is suboptimal but I got it by making StateStringTask an instance of Catchable:
implicit val stateStringTaskInstance: Catchable[StateStringTask] =
new Catchable[StateStringTask] {
// `a.attempt` stackoverflows, don't ask me why :)
def attempt[A](a: StateStringTask[A]): StateStringTask[Throwable \/ A] = a >>= (
x => Catchable[Task].attempt(Applicative[Task].pure(x)).liftM[StateStringT]
)
def fail[A](err: Throwable) = Catchable[Task].fail(err).liftM[StateStringT]
}
To hoist the StateT on a Process with aTask as effect. For example:
def received(queue: Queue[Event]): Process[StateStringTask, Event] = {
val toStateStringTask = new (Task ~> StateStringTask) {
def apply[A](t: Task[A]): StateStringTask[A] = t.liftM[StateStringT]
}
// queue.dequeue: Process[Task, Event]
queue.dequeue.translate(toStateStringTask)
}