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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); | |
^ |
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