Functional Graph API in Scala, using edge list as representation

Graph.scala

/**
* A functional Graph object in Scala, for learning purposes.
*/
class Graph[V,W](val vertices: List[V], val edges: List[(V,V,W)]) {

	override def toString(): String = vertices.toString() + ", " + edges.toString()


	def addVertex(v: V): Graph[V,W] = Graph( v :: this.vertices, this.edges )
	def addEdge(e: (V,V,W)): Graph[V,W] = Graph(this.vertices, e :: this.edges )


	def neighbors( v: V ): List[V] = {
		for((v1, v2, _) <- edges; if v == v1 ) yield v2
	}


	def weight( e: (V,V) ): W = {
		val l = edges.filter( x => x._1 == e._1 && x._2 == e._2 )
		l.head._3
	}

	def dfs( root: V ): List[V] ={
		def dfs1_( v: V, discovered: Set[V]): List[V] = {
			val dn = discovered + v
			val n = neighbors(v).filter( ! discovered.contains(_))

			n.flatMap( dfs1_( _ , dn) ) ::: List(v)
		}

		 dfs1_( root, Set() ).distinct
	}
}


object Graph{
	def apply[V,W](vertices: List[V], edges: List[(V, V, W)]): Graph[V,W] = new Graph(vertices, edges)

	def apply[V,W](vertices: List[V], edges: List[(V, V)], default: W): Graph[V,W]
		= new Graph(vertices, for ( (a,b) <- edges ) yield (a, b, default))
}

object Main extends App{

	val g = Graph[String, Double](
		List( "S", "A", "B", "C", "D", "E", "F", "G" ),
		List(
			("S", "A"),
			("S", "B"),
			("S", "C"),
			("A", "D"),
			("B", "E"),
			("C", "F"),
			("D", "G"),
			("E", "G"),
			("F", "G")
		), 1.0
	)

	g.dfs( "S" ).foreach( x => println( "Visited: " + x))
}