Mistake on 2/1 of Dep Type Talk

mistake.scala

trait IsFuture[F]{
  type T

  def apply(f: F): Future[T]
}

object IsFuture{
  def apply[F](implicit isf: IsFuture[F]) = isf

  implicit def mk[A] = new IsFuture[Future[A]]{
    type T = A

    def apply(f: Future[A]): Future[A] = f //it really should read as an identity.
  }
}

whythis.scala

//Ok, take #1 was an awful, confused, unhelpful... I need to put more thought into these things when helping people.

//This is a contrived example.

trait Result[+A]{
  def map[B](f: A => B): Result[B]
  
  def recover[B >: A](implicit isf: IsFuture[B]): PartialFunction[Throwable, isf.A] => Result[B]
}

case class Recoverable[+A](value: A) extends Result[A]{
  def recover[B >: A](implicit isf: IsFuture[B]) = f: PartialFunction[Throwable, isf.A] => map(isf(value) recover f)
}

//Better, is the first half of the talk from NE Scala that I gave this year.

trait IsList[LA]{
  type A
  
  def apply(la: LA): List[A]
}

object IsList{
  def apply[LA](implicit isl: IsList[LA]): Aux[LA, isl.A] = isl
  
  type Aux[LA, A0] = IsList[LA]{ type A = A0 }
  
  implicit mk[A0] = new IsList[List[A0]]{
    type A = A0
    
    def apply(la: List[A0]) = la
  }
}

case class Items[A](value: A){
  //Here, this is acting as an implicit guard. You can't call this function unless `A` is a `List`.
  //This is very similar to `sum` from `List` itself where you can only call it if `A` has a `Numeric[A]` type class.
  def fullLength(implicit isl: IsList[A]) = isl(value).length
}

I'm still not seeing how it all fits together. There seems to be a couple of missing pieces:

In the case of Fail, you're passing in a Throwable, however isf.apply expects a type A.

In the case of Pass, you use invariant type param A, so Pass[A] is not a subclass of Pass[B]. You can't change this to covariant because of the implicit IsFuture[A] parameter.

As a result, the above code does not compile.