Let's get a big picture understanding of what the members of the MapToDouble companion object look like:
object MapToDouble { type Aux[L <: HList, LR <: HList] = MapToDouble[L] { type Result = LR } def apply[L <: HList](implicit m: MapToDouble[L]) = m implicit def hcons[H, T <: HList, TR <: HList](implicit td: ToDouble[H] , md: MapToDouble.Aux[T, TR] ): Aux[H ::: T, Double ::: TR] implicit def hnil[H <: HNil]: MapToDouble.Aux[H, HNil]}
So as you can see, we have an Aux type, and apply function definitions, which are common for the type class pattern. Also, we have two implicit members, which define implementations for the MapToDouble type class for hlist. Since there are two possible instances of hlist, we have ...
