constraint-on-hlist-check-for-single-occurrence-of-a-type

TATB.scala

/**
 * http://stackoverflow.com/questions/39226147/constraint-on-hlist-check-for-single-occurrence-of-a-type/39226608#39226608
 */

import shapeless._, ops.hlist.{ToList}

trait T
case class TA() extends T
case class TB() extends T

trait UniqueTB[L <: HList] extends DepFn1[L] {
  type Out = TB
  def apply(l: L): TB
}

object UniqueTB {
  def apply[L <: HList](implicit utb: UniqueTB[L]): UniqueTB[L] = utb
  //def getTB[L <: HList](l: L)(implicit utb: UniqueTB[L]): TB = utb(l)

  implicit def firstTB[T <: HList](implicit tl: ToList[T, TA]): UniqueTB[TB :: T] = new UniqueTB[TB :: T] {
    def apply(l: TB :: T): TB = l.head
  }

  implicit def afterTB[T <: HList](implicit utb: UniqueTB[T]): UniqueTB[TA :: T] = new UniqueTB[TA :: T] {
    def apply(l: TA :: T): TB = utb(l.tail)
  }
}

object Main extends App {
  // it should contain any arbitrary number of elements of type TA (from 0 to N);
  // it should contain one and only one element of type TB.
  UniqueTB[TB :: HNil]
  UniqueTB[TA :: TB :: HNil]
  UniqueTB[TA :: TB :: TA :: HNil]
  UniqueTB[TB :: HNil]

  import shapeless.test._
  illTyped("UniqueTB[TA :: HNil]")
  illTyped("UniqueTB[HNil]")
  illTyped("UniqueTB[TB :: TB :: HNil]")
}