Scalaz's Unapply: why doesn't traverseU work with State? Why must traverseS exist?

Question

Usually Scalaz's Unapply does a pretty good job, but it seems to break down here with traverseU:

scala> import scalaz._, Scalaz._, Unapply._
import scalaz._
import Scalaz._
import Unapply._

scala> val stateMonadInstance = unapplyMAB2[Monad, State, Int, Unit](IndexedStateT.stateMonad[Int]).TC
stateMonadInstance: scalaz.Monad[[X]scalaz.IndexedStateT[[+X]X,Int,Int,X]] = scalaz.StateTInstances1$$anon$1@27c591e1

scala> List(1, 2, 3).traverseU((i: Int) => stateMonadInstance.pure(i))
<console>:18: error: Unable to unapply type `scalaz.IndexedStateT[[+X]X,Int,Int,Int]` into a type constructor of kind `M[_]` that is classified by the type class `scalaz.Applicative`
1) Check that the type class is defined by compiling `implicitly[scalaz.Applicative[<type constructor>]]`.
2) Review the implicits in object Unapply, which only cover common type 'shapes'
(implicit not found: scalaz.Unapply[scalaz.Applicative, scalaz.IndexedStateT[[+X]X,Int,Int,Int]])
              List(1, 2, 3).traverseU((i: Int) => stateMonadInstance.pure(i))
                                     ^

The traverseS method seems to have been created as a workaround for this problem, whatever it is:

scala> List(1, 2, 3).traverseS((i: Int) => stateMonadInstance.pure(i))
res11: scalaz.State[Int,List[Int]] = scalaz.package$State$$anon$3@2634d0e2

But I'm trying to write a library which is generic with respect to the monad in question, so that's not a very good fit. Does anyone know what the exact problem is here that's preventing this from working, and if there's a workaround which doesn't require special-casing for State?

Answer 1

Ok, this works:

scala> import scalaz._, Scalaz._, Unapply._
import scalaz._
import Scalaz._
import Unapply._

scala> val unapply = unapplyMAB2[Monad, State, Int, Unit](IndexedStateT.stateMonad[Int])
unapply: scalaz.Unapply[scalaz.Monad,scalaz.State[Int,Unit]]{type M[X] = scalaz.State[Int,X]; type A = Unit} = scalaz.Unapply_0$$anon$13@53a6f572

scala> List(1, 2, 3).traverseU((i: Int) => unapply.TC.pure(i))
res0: scalaz.IndexedStateT[scalaz.Id.Id,Int,Int,List[Int]] = scalaz.IndexedStateT$$anon$10@737c45ee

In fact just regular old traverse works in this case:

scala> List(1, 2, 3).traverse((i: Int) => unapply.TC.pure(i))
res1: unapply.M[List[Int]] = scalaz.IndexedStateT$$anon$10@73c622ec

I suppose what's going on is that I need to have the Unapply instance in scope in order to know what types the TC field is referring to.