Scala typeclass and implicit puzzle

Question

The last statement line below produces an error: "type mismatch; found: TestExpandableWithLibrary.this.library.type (with underlying type org.typeclass.Library) required: V"

This is where I am trying to do an implicit conversion. The preceding line where I simply use the typeclass function does work fine.

Any ideas on how to fix it?

package org.typeclass

///////////////////////////////////////////////////////////////////////////////
// the domain objects

case class Book(bookName: String)

case class Library(libraryName: String, books: Set[Book])

object Library {
  def apply(libraryName: String, bookNames: String*): Library =
    Library(libraryName, bookNames.map(Book(_)).toSet)
}

case class TreeClass(nodeName: String, children: Seq[TreeClass])

///////////////////////////////////////////////////////////////////////////////
// the typeclass definition

trait Expandable[T, V, R] {
  def expandWith(template: T, values: V): R
}

object Expandable {

  def expandWithF[T, V, R](template: T, values: V)(implicit ev: Expandable[T, V, R]): R =
    ev.expandWith(template, values)

  implicit class ExpandableItem[T, V, R](val template: T) extends AnyVal {
    def expandWithM(values: V)(implicit ev: Expandable[T, V, R]): R =
      ev.expandWith(template, values)
  }
}

///////////////////////////////////////////////////////////////////////////////
// a typeclass implementation

object ExpandableImpls {

  implicit object ExpandableTreeClass extends Expandable[TreeClass, Library, TreeClass] {
    def expandWith(template: TreeClass, library: Library): TreeClass = {
      val parentName = s"${template.nodeName}.${library.libraryName}"
      val children = library.books.map(book => TreeClass(s"${parentName}.${book.bookName}", Seq.empty)).toSeq
      TreeClass(parentName, children)
    }
  }

}

//@RunWith(classOf[JUnitRunner])
class TestExpandableWithLibrary /*extends FlatSpec*/ {

  import Expandable._
  import ExpandableImpls._

  val library = Library("test", "black", "white")
  val root = TreeClass("parent", Seq.empty)

  val useF = expandWithF(root, library) // this works

  val useM = root.expandWithM(library) // this doesn't work!

}

Travis Brown · Accepted Answer · 2014-09-10 16:39:52Z

4

The problem is just that you need to put the second two type parameters on the implicit class on the extension method—as it is they'll be inferred to be Nothing, since they're not referred to in the constructor arguments.

implicit class ExpandableItem[T](val template: T) extends AnyVal {
  def expandWithM[V, R](values: V)(implicit ev: Expandable[T, V, R]): R =
    ev.expandWith(template, values)
}

This should work just fine.

answered Sep 10, 2014 at 16:39

Travis Brown

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3 Comments

satyagraha Over a year ago

Not to be churlish, but perhaps somewhere along the line ideally the compiler could have given me some clues as to where I was going adrift. Type inference = Dragons Be Here!

Travis Brown Over a year ago

@satyagraha: I agree that the error could be better. It's a good idea to internalize the rule that if you want a type parameter inferred, it should appear in a non-generic position in the arguments, though.

satyagraha Over a year ago

Spotted this tool which might have helped: blog.cppcabrera.com/posts/scala-wart-remover.html (the offending statement would have needed to be commented out to allow compilation to complete of course)

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