another type mismatch.

[yves]

 
trait Elt[Element <: Elt[Element,Ret],Ret] { self:Element=> }
 
trait EltBuilder[Element<:Elt[Element,Ret],Ret] {
  def apply(parent: Element, childBuilder: EltBuilder[Element,Ret]): Element
}
trait Motor[-Element<:Elt[_,Ret],Ret] {
  def onEnd(self: Element): Ret
}

class Core[R] {
  type Bld    = EltBuilder[Element,Ret]
  type Ret    = R
 
  trait Element extends Elt[Element,Ret] {
    val parent:Element
    val childBuilder:Bld
  }
 
  trait Mot extends Motor[Element,Ret] with Access {
    class Elt(val parent:Element, val childBuilder:Bld) extends Element {
      def onEnd(): Ret = Mot.this.onEnd(this)
    }
    val builder:Bld = new Bld {
      def apply(parent: Element, childBuilder:Bld) = new Elt(parent,childBuilder)
    }
  }
 
  trait Access {
    type Ret     = Core.this.Ret
    type Bld     = Core.this.Bld
    type Element = Core.this.Element
    /** To define! */
    type Elt<:Element
    def builder:Bld
  }
} 

trait ExtCore[R] extends Core[R] {
  type Data
  def getData(parent:Element):Data
 
  trait Element extends super.Element {
    val data:Data
  }
 
  trait Mot extends super.Mot with Motor[Element,Ret] with Access {
    class Elt(parent:Element, childBuilder:Bld, val data:Data)
          extends super.Elt(parent,childBuilder) with ExtCore.this.Element
    override val builder:Bld = new Bld {
      def apply(parent: Element, childBuilder:Bld)  =
          new Elt(parent,childBuilder, getData(parent))
    }
  }
 
}

The line is red shows for the parent parameter:
type mismatch;
found : Mot.this.Element (which expands to) ExtCore.this.Element
required: ExtCore.this.Element

Elt is a generic element type
EltBuilder is a generic builder for Elt
Motor is a processor class for Elt

Core is a simple implementation, containing trait Mot with an almost full implementation
ExtCore is a refinement of Core, which adds some information to the basic Elt defined in Core.

For various reasons, it is difficult to impossible to change variances (i.e. Elt is not covariant)

I'm stuck on this.

Thanks.
Yves

[Jan Vanek]

Yves, you could look at the proposal about encoding of virtual classes in Scala. I found this link:
https://wiki.scala-lang.org/display/SIW/VirtualClasses
but there might be others. I believe understanding of the mechanism will clarify a lot of things for you.

Regards,
Jan

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[yves]

An interesting link! Thanks!
It certainly is relevant to my problem.

Yves