Scala typeclass and implicit puzzle -

the last statement line below produces error: "type mismatch; found: testexpandablewithlibrary.this.library.type (with underlying type org.typeclass.library) required: v"

this trying implicit conversion. preceding line use typeclass function work fine.

any ideas on how fix it?

package org.typeclass  /////////////////////////////////////////////////////////////////////////////// // 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])  /////////////////////////////////////////////////////////////////////////////// // 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)   } }  /////////////////////////////////////////////////////////////////////////////// // 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) // works    val usem = root.expandwithm(library) // doesn't work!  } 

the problem need put second 2 type parameters on implicit class on extension method—as they'll inferred nothing, since they're not referred in 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 fine.