召唤类型类的HList

Question

Given an arbitrary case class Cc( a: String, b: Int ) and a type class trait Tc[T] , is there a way to materialize a simple HList (or preferably a labelled record) that contains a type class instance for each parameter of Cc ?

def resolveTypeClasses[C] = ???
val resolved: Tc[String] :: Tc[Int] :: HNil = resolveTypeClasses[Cc]

Answer 1

It's hard to make single type parameter function because of need to resolve intermediate Generic .

So from my noob point of view it should be two parameter def , or sequence of two def s, each taking single type parameter. Second variant could be implemented like:

import shapeless._
trait Tc[T]

case class Cc(a: String, b: Int)

trait ResolveTC[L] {
  type Out <: HList
  val out: Out
}

implicit object ResolveHNilTC extends ResolveTC[HNil] {
  type Out = HNil
  val out: HNil = HNil
}

implicit def resolveHListTC[X, XS <: HList](implicit tc: Tc[X], rest: ResolveTC[XS]) =
  new ResolveTC[X :: XS] {
    type Out = Tc[X] :: rest.Out
    val out = tc :: rest.out
  }

class TCResolver[C] {
  def get[L <: HList](implicit gen: Generic.Aux[C, L], resolve: ResolveTC[L]): resolve.Out = resolve.out
}

def resolveTypeClasses[C] = new TCResolver[C]

implicit case object StringInst extends Tc[String]
implicit case object IntInst extends Tc[Int]

with that implementation

val resolved = resolveTypeClasses[Cc].get

will produce the

StringInst :: IntInst :: HNil