reader-with-abstract-type.scala

trait R[+A] { self =>
  type In

  def apply(in: In): A

  def map[B](f: A => B): R[B] = new R[B] {
    type In = self.In
    def apply(in: In) = f(self(in))
  }

  def flatMap[B](f: A => R[B] { type In = self.In }): R[B] = new R[B] {
    type In = self.In
    def apply(in: In): B = f(self(in))(in)
  }
}

def foo[I, A](a: A): R[A] = new R[A] {
  type In = I
  def apply(in: In): A = a
}

//
// compiles
//
val a: R[String] = foo[Int, String]("b").map(_ + "a")

// does not compile
//
//  Error:(20, 53) type mismatch;
//  found   : A$A58.this.AA[String]
//  required: A$A58.this.AA[?]{type R = A$A58.this.AA[String]#R}
//  foo[Int, String]("a").flatMap(x => foo[Int, String](x));}
//
val b: R[String] = a.flatMap(x => foo[Int, String](x))