When using HList with GADTs I am having to cast using asInstanceOf[H]. Is there a way to avoid the cast? - scala

Question

Given 2 GADT Algebras which know about each other and 2 interpreters that are mutually recursive, I am having issues having to cast from type A to type h <: HList even though in the context of the pattern match, it should be implied that type A is type h.
Is there a way to avoid the asInstanceOf[h] call in the interpreter?
abstract class KvpHList[H<:HList]
object KvpNil extends KvpHList[HNil]
case class KvpCons[H <: A :: T,A, T<:HList](head: KvpValue[A], tail: KvpHList[T])(implicit isHCons: IsHCons.Aux[H,A,T]) extends KvpHList[H] {
val hCons: IsHCons.Aux[H,A,T] = isHCons
}
abstract class KvpValue[A]
case object StringData extends KvpValue[String]
case class HListData[H <:HList](member: KvpHList[H]) extends KvpValue[H]
def hListInterpreter[H<:HList](hList: KvpHList[H]): H => String = {
hList match {
case KvpNil => (hNil: H) => "Nil"
case cons: KvpCons[H,a,t]=> {
implicit val hCons = cons.hCons
(input: H) => {
s"${kvpInterpreter(cons.head)(input.head)} :: ${hListInterpreter(cons.tail)(input.tail)}"
}
}
}
}
def kvpInterpreter[A](kvpValue: KvpValue[A]): A => String = {
kvpValue match {
case StringData => (str: String) => str
case h: HListData[h] => {
(input: A) => { // tried (input: h) as well
val toString: h => String = hListInterpreter(h.member)
toString(input.asInstanceOf[h]) // <--- CASTING :(
}
}
}
}
kvpInterpreter(HListData(KvpCons(StringData, KvpNil))).apply("Hello" :: HNil)

Answer 1

Since H in KvpCons is uniquely determined by A and T, KvpCons can be parametrized with two type parameters rather than three.
Type-level pattern matching is type classes
abstract class KvpHList[H <: HList]
object KvpNil extends KvpHList[HNil]
case class KvpCons[A, T <: HList](head: KvpValue[A], tail: KvpHList[T]) extends KvpHList[A :: T]
abstract class KvpValue[A]
case object StringData extends KvpValue[String]
case class HListData[H <: HList](member: KvpHList[H]) extends KvpValue[H]
trait HListInterpreter[H <: HList] {
def apply(hList: KvpHList[H]): H => String
}
object HListInterpreter {
implicit val nil: HListInterpreter[HNil] = new HListInterpreter[HNil] {
override def apply(hList: KvpHList[HNil]): HNil => String = _ => "Nil"
}
implicit def cons[A, T <: HList](implicit
headKvpInterpreter: KvpInterpreter[A],
tailHListInterpreter: HListInterpreter[T]
): HListInterpreter[A :: T] = new HListInterpreter[A :: T] {
override def apply(hList: KvpHList[A :: T]): A :: T => String = hList match {
case cons: KvpCons[_, _] => input => s"${headKvpInterpreter(cons.head)(input.head)} :: ${tailHListInterpreter(cons.tail)(input.tail)}"
}
}
}
def hListInterpreter[H <: HList](hList: KvpHList[H])(implicit hListInterp: HListInterpreter[H]): H => String = hListInterp(hList)
trait KvpInterpreter[A] {
def apply(kvpValue: KvpValue[A]): A => String
}
object KvpInterpreter {
implicit val string: KvpInterpreter[String] = new KvpInterpreter[String] {
override def apply(kvpValue: KvpValue[String]): String => String = str => str
}
implicit def hList[H <: HList : HListInterpreter]: KvpInterpreter[H] = new KvpInterpreter[H] {
override def apply(kvpValue: KvpValue[H]): H => String = kvpValue match {
case h: HListData[H] => input => {
val toString: H => String = hListInterpreter(h.member)
toString(input)
}
}
}
}
def kvpInterpreter[A](kvpValue: KvpValue[A])(a: A)(implicit kvpInterp: KvpInterpreter[A]): String = kvpInterp(kvpValue)(a)