Errore di compilazione su pattern matching con RDD

  def computeContribs(tuples : RDD[(Int,List[Int])], ranks : RDD[(Int, Double)] ) : RDD[(Int, Double)] = {
    val x = tuples.map{case(numpage,list)=>(numpage,(list,1.0/list.size))}
    val joined = x.join(ranks)
    val contribs = joined.flatMap{case(numPage,(pageList,size), rank) => pageList.map( y:Int=>(y,size) )}
    contribs.reduceByKey(_+_)
  }

/*
[error] C:\Users\vital\Documents\spazi_lavoro\spark-examples\src\main\scala\pagerank\PageRankSpark.scala:11: constructor cannot be instantiated to expected type;
[error]  found   : (T1, T2, T3)
[error]  required: (Int, ((List[Int], Double), Double))
[error]     val contribs = joined.flatMap{case(numPage,(pageList,size), rank) => pageList.map( y:Int=>(y,size) )}
*/