How to “extract” type parameter to instantiate another class

Question

The following Scala code works:

object ReducerTestMain extends App {

  type MapOutput = KeyVal[String, Int]

  def mapFun(s:String): MapOutput = KeyVal(s, 1         


        

Answer 1

Just write a simple factory:

case class RC[M <: KeyVal[_, _]](){
   def apply[K,V](f: (V,V) => V)(implicit ev: KeyVal[K,V] =:= M) = new ReducerComponent[K,V](f)
}

def plus(x: Double, y: Double) = x + y

scala> RC[KeyVal[Int, Double]].apply(plus)
res12: ReducerComponent[Int,Double] = ReducerComponent@7229d116

scala> RC[KeyVal[Int, Double]]()(plus)
res16: ReducerComponent[Int,Double] = ReducerComponent@389f65fe

As you can see, ReducerComponent has appropriate type. Implicit evidence is used here to catch K and V from your M <: KeyVal[_, _].

P.S. The version above requires to specify parameter types explicitly for your f, like (_: Double) + (_: Double). If you want to avoid this:

case class RC[M <: KeyVal[_, _]](){
   def factory[K,V](implicit ev: KeyVal[K,V] =:= M) = new {
      def apply(f: (V,V) => V) = new ReducerComponent[K,V](f)
   }
}

scala> RC[KeyVal[Int, Double]].factory.apply(_ + _)
res5: ReducerComponent[Int,Double] = ReducerComponent@3dc04400 


scala> val f = RC[KeyVal[Int, Double]].factory
f: AnyRef{def apply(f: (Double, Double) => Double): ReducerComponent[Int,Double]} = RC$$anon$1@19388ff6

scala> f(_ + _)
res13: ReducerComponent[Int,Double] = ReducerComponent@24d8ae83

Update. If you want to generelize keyval - use type function:

type KV[K,V] = KeyVal[K,V] //may be anything, may implement `type KV[K,V]` from some supertrait

case class RC[M <: KV[_, _]](){   
  def factory[K,V](implicit ev: KV[K,V] =:= M) = new {
    def apply(f: (V,V) => V) = new ReducerComponent[K,V](f)
  }
}

But keep in mind that apply from your question still takes KeyVal[K,V].

You can also pass KV into some class:

class Builder[KV[_,_]] {
  case class RC[M <: KV[_, _]](){   
    def factory[K,V](implicit ev: KV[K,V] =:= M) = new {
      def apply(f: (V,V) => V) = new ReducerComponent[K,V](f)
    }
  }
}

scala> val b = new Builder[KeyVal]
scala> val f = b.RC[KeyVal[Int, Double]].factory
scala> f(_ + _)
res2: ReducerComponent[Int,Double] = ReducerComponent@54d9c993