scala - Returning a path-dependent type -

how can design method returns path dependent type? in following example, deliberately want vertex path dependent on tree such forbidden mix vertices across trees (and example):

trait tree {   trait vertex   def root: vertex   def addchild(parent: vertex): vertex }  trait treefactory { def make: tree } 

now following cannot constructed:

def test(f: treefactory): (tree, map[tree#vertex, tree#vertex]) = {   val t  = f.make   var sq = indexedseq(t.root)   var m  = map.empty[t.vertex, t.vertex]   for( <- 1 100) {     val p = sq(util.random.nextint(sq.size))     val c = t.addchild(p)     m    += c -> p     sq  :+= c   }   (t, m) } 

because map return should not have keys , values of type tree#vertex of path dependent vertex...

error: type mismatch;  found   : scala.collection.immutable.map[t.vertex,t.vertex]  required: map[tree#vertex,tree#vertex] note: t.vertex <: tree#vertex, trait map invariant in type a. may wish investigate wildcard type such `_ <: tree#vertex`. (sls 3.2.10)            (t, m)                ^ 

if try decouple tree creation , parent-child map build-up:

def test(t: tree): map[t.vertex, t.vertex] = {   var sq = indexedseq(t.root)   var m  = map.empty[t.vertex, t.vertex]   (i <- 1 100) {     val p = sq(util.random.nextint(sq.size))     val c = t.addchild(p)     m    += c -> p     sq  :+= c   }   m } 

this fails reason: "error: illegal dependent method type"

my twisted mind came one. hope there more elegant solution:

trait gagaism {   val tree: tree   val map: map[tree.vertex, tree.vertex] }  def test(f: treefactory) = new gagaism {   val tree = f.make   val map = {     var sq = indexedseq(tree.root)     var m = map.empty[tree.vertex, tree.vertex]     (i <- 1 100) {       val p = sq(util.random.nextint(sq.size))       val c = tree.addchild(p)       m    += c -> p       sq  :+= c     }     m   } }