gistfile1.scala

import matryoshka.Recursive
import scalaz.Functor
import matryoshka.implicits._
import matryoshka._

sealed trait Tree
case object Empty extends Tree
case class Node(left: Tree, right: Tree) extends Tree

sealed trait TreeF[+A]
case object EmptyF extends TreeF[Nothing]
case class NodeF[+A](left: A, right: A) extends TreeF[A]
object TreeF {
  implicit val functor: Functor[TreeF] = new Functor[TreeF] {
    override def map[A, B](fa: TreeF[A])(f: A => B): TreeF[B] = fa match {
      case EmptyF => EmptyF
      case NodeF(left, right) => NodeF(f(left), f(right))
    }
  }
}

object Tree {
  implicit val rec: Recursive.Aux[Tree, TreeF] = new Recursive[Tree] {
    override def project(t: Tree)(implicit BF: Functor[TreeF]): TreeF[Tree] = t match {
      case Empty => EmptyF
      case Node(left, right) => NodeF(left, right)
    }

    override type Base[A] = TreeF[A]
  }
}

RecursiveOps[Tree, TreeF](Node(Empty, Empty): Tree)(Tree.rec) // compiles
RecursiveOps             (Node(Empty, Empty): Tree)(Tree.rec) // doesn't compile

// Error:(34, 93) inferred kinds of the type arguments (A$A129.this.Tree.rec.Base[A]) do not conform to the expected kinds of the type parameters (type F).
// A$A129.this.Tree.rec.Base[A]'s type parameters do not match type F's expected parameters:
// type Base has one type parameter, but type F has one
// def get$$instance$$res1 = /* ###worksheet### generated $$end$$ */ RecursiveOps             (Node(Empty, Empty): Tree)(Tree.rec);
                                                                                           ^