Generic method arguments in scala -

i'm not sure title describing question best let's give shot.

i have background job execution application resembles simple pipeline processing. there command objects calculation , return output, , worker receive output input , can return result object model looks this:

type output <: anyref trait command[output] {   def dosomething(): output } sealed trait worker[in <: anyref, out <: result] {   def work(input: in): out }  case class worka() extends worker[string, longresult] {   override def work(input: string): longresult = longresult(long.maxvalue) }  case class workb() extends worker[long, stringresult] {   override def work(input: long): stringresult = stringresult(input.tostring) } 

there few problems approach:

when mapping on collection of worker can't make sure worker accepts same output input.

unless i'm mapping list of command, code not compile because of type erasure - expects _$1 receives long or string (everything accepted output)

val workers = list(   new worka(),   new workb() )  val asimplecommand = new command[long] {   override def dosomething() = 123123123l } // doesn't compile. workers.map(worker => worker.work(asimplecommand.dosomething())) 

i'm looking right scala mechanism disallow @ compile time. how can map on worker support output - , in case, workb

if want @ compile time can use shapeless hlists, preserving type of list way through, , using poly handle cases:

val myworkers: worka :: workb :: workb :: hnil =   worka() :: workb() :: workb() :: hnil  object dowork extends poly1 {   implicit def caselong[a] = at[worker[long, a]] {     w => w.work(asimplecommand.dosomething())}   implicit def casestring = //whatever want string worker }  myworkers map dowork 

for less safe example doesn't need shapeless, can match cases long have concrete types:

val myworkers: list[worker[_, _]] = ... myworkers collect {   case wa: worka => //works   case lw: worker[long, _] => //doesn't work }