scala - trait Enumeratee is invariant in type From when using ADT -

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i trying compose enumeratee.grouped , enumeratee.filter make new enumeratee running variance issue. input , output types of enumeratee adts , following error. 

<console>:24: error: type mismatch;  found   : play.api.libs.iteratee.enumeratee[outputtype,outputtype]  required: play.api.libs.iteratee.enumeratee[product serializable outputtype,outputtype] note: outputtype >: product serializable outputtype, trait enumeratee invariant in type from. may wish define -from instead. (sls 4.5)

i have recreated issue smaller example here (i understand example rewritten enumeratee.collect however, unless there way combine enumeratee.grouped , enumeratee.filter not me.)

import play.api.libs.iteratee.enumeratee import scala.concurrent.executioncontext.implicits.global  sealed abstract class inputtype case class inputa(counter: int) extends inputtype case object inputb extends inputtype  sealed abstract class outputtype case class outputa(msg: string) extends outputtype case object outputb extends outputtype  val e: enumeratee[inputtype, outputtype] = enumeratee.map[inputtype] {    case inputa(counter) => outputa(counter.tostring)   case inputb => outputb } compose enumeratee.filter[outputtype] {    case outputa("4") => false   case _ => true }

i can't redefine from type on enumeratee -from compiler error suggests because play library. limitation of library cannot use adts when composing enumeratees? or there better approach use case (put classtag somewhere, write functionality way, etc...)?

i found solution seems ok. if change enumeratee.map[inputtype] enumeratee.map[inputtype].apply[outputtype] inferencer gets invariant type , compiles fine. 

val e: enumeratee[inputtype, outputtype] = enumeratee.map[inputtype].apply[outputtype] {    case inputa(counter) => outputa(counter.tostring)   case inputb => outputb }

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