scala - How can I implement concrete class which extends trait defining a method with type by the type parameter's type alias -

i ask advanced scala developers. problem access type alias belonging type parameters of class' parent.

  case class mymodel(foo: string = "bar")    case class mydispatcher()    trait module[m, d] {     type dispatcher = d     type model = m   }    trait myspecificmodule[a <: module[_, _]] {     def dispatcher(): a#dispatcher   }    class moduleclass extends module[mymodel, mydispatcher] {     //...   }    class myspecificmoduleclass extends myspecificmodule[moduleclass] {     override def dispatcher(): mydispatcher = mydispatcher()   } 

so myspecificmodule extends generic trait, , should know type of dispatcher method. in case of myspecificmoduleclass should mydispatcher. when try compile code getting compilation error because type of method, not same defined: a#dispatcher, in reality is.

error:(21, 18) overriding method dispatcher in trait myspecificmodule of type ()_$2;  method dispatcher has incompatible type     override def dispatcher(): mydispatcher = mydispatcher()               ^ 

i appreciate advice suggest. in advance, gabor

resolved

case class mymodel(foo: string = "bar")  case class mydispatcher()  trait abstractmodule {   type dispatcher   type model }  trait module[m, d] extends abstractmodule {   type dispatcher = d   type model = m }  trait myspecificmodule[a <: abstractmodule] {   def dispatcher(): a#dispatcher }  class moduleclass extends module[mymodel, mydispatcher] {   //... }  class myspecificmoduleclass extends myspecificmodule[moduleclass] {   override def dispatcher(): mydispatcher = mydispatcher() } 

i don't understand scala's reasoning here, if rid of type parameters, things start work:

case class mymodel(foo: string = "bar")  case class mydispatcher()  trait module {   type dispatcher   type model }  trait myspecificmodule[a <: module] {   def dispatcher(): a#dispatcher }  class moduleclass extends module {   type model = mymodel   type dispatcher = mydispatcher   //... }  class myspecificmoduleclass extends myspecificmodule[moduleclass] {   override def dispatcher(): mydispatcher = mydispatcher() } 

and if really want have type params, can introduce helper trait:

trait abstractmodule[m, d] extends module {   type model = m   type dispatcher = d }  class moduleclass extends abstractmodule[mymodel,mydispatcher]