OptionSelector.sc

import shapeless._; import syntax.singleton._; import record._

object os {
	
	trait OptionSelector[L <: HList, K] {
		type Out
		def apply(l : L): Out
	}
	
	trait LowPriorityOptionSelector {
		type Aux[L <: HList, K, Out0] = OptionSelector[L, K] { type Out = Out0 }
		implicit def hnilSelect[K] = new OptionSelector[HNil, K] {
			type Out = None.type
			def apply(l: HNil) = None
		}
		implicit def hlistSelect[H, T <: HList, K]
			(implicit st : OptionSelector[T, K]): Aux[H :: T, K, st.Out] =
				new OptionSelector[H :: T, K] {
					type Out = st.Out
					def apply(l : H :: T): Out = st(l.tail)
				}
	}

	object OptionSelector extends LowPriorityOptionSelector {
		implicit def hlistSelect1[K, V, T <: HList]: Aux[FieldType[K, V] :: T, K, Some[V]] =
			new OptionSelector[FieldType[K, V] :: T, K] {
				type Out = Some[V]
				def apply(l : FieldType[K, V] :: T): Out = Some(l.head)
			}
	}
	
}
import os._

def getOption[L <: HList, K](l: L, k: Witness)(implicit
	s: OptionSelector[L,k.T]
) = s(l)

val l = "a" ->> 1 :: HNil


val i: Option[Int] = getOption(l,"a")
val n: None.type = getOption(l,"b")