Scala type projection -

i have collection of models, each of exposes next(...) method moves model forward discrete step. each next() method has parameter given abstract type t.

i want able wrap each model class, wrapper inherit type t (which different each model), provide additional logic. trivial letting wrapper extend model, not feasible internal logic of actual model implementations.

my solution use type projections so:

trait model {   type t // works fine when concrete implementation, cannot overridden   def next(input : t) = println(input) }  abstract class parent {   type s <: model   type t = s#t   val model : s   def next(input : t) = model.next(input)  } 

this fails compiler error: type mismatch; found : input.type (with underlying type parent.this.t) required: parent.this.model.t

note in concrete implementation of parent, parent.this.t should equal parent.this.model.t.

so far workaround abandon using type system, , creating unique parent classes each model (ie duplicating of other logic parent may expose). correct way this?

credits go @senia. setting type t = model.t works fine:

abstract class parent {   type s <: model   type t = model.t   val model : s   def next(input : t) = model.next(input)  }  class simpleparent[gt <: model](val model: gt) extends parent {   type s = gt }  trait stringmodel extends model { type t = string } trait intmodel    extends model { type t = int }  object test {   new simpleparent(new stringmodel {})   new simpleparent(new intmodel {}) } 

have @ here, see why type t = s#t doesn't work. (short version: have model.type <: s , s#t not concrete, hence s#t incompatible model.t).