Typesafe fluent interface hierarchy using type constructors

gistfile1.scala

trait PipeFunction[A, B]
class NullPipeFunction[A, B] extends PipeFunction[A, B]

trait Pipe[S, E]

class Pipeline[S, E, T[S, E]] extends Pipe[S, E] {
  def add[F](pipe: Pipe[E, F]): T[S, F] = 
    throw new UnsupportedOperationException()
}

class FluentPipeline[S, E, T[S, E]] extends Pipeline[S, E, T] {
  def filter1[F](f: PipeFunction[E, F]): T[S, F] = 
    throw new UnsupportedOperationException()
  
  def filter2[F](f: PipeFunction[E, F]): T[S, F] = 
    throw new UnsupportedOperationException()
}

class GremlinPipeline[S, E] extends FluentPipeline[S, E, GremlinPipeline]

object Test {
  def main(args: Array[String]) {
    val long2Float = new NullPipeFunction[Long, Float]
    val float2Double = new NullPipeFunction[Float, Double]
    
    val int2Long = new GremlinPipeline[Int, Long]
    val int2Float = int2Long.filter1(long2Float) //GremlinPipeline[Int, Float] properly inferred
    val int2Double = int2Float.filter2(float2Double) //GremlinPipelin[Int, Double] properly inferred
    
    //GremlinPipeline[Int, Double] properly inferred
    val int2Double2 = new GremlinPipeline[Int, Long].filter1(long2Float).filter2(float2Double)
  }
}