Pattern matching on a simple Shapeless HLIST

Question

I wrote this simple code using the shapeless library

import shapeless.LabelledGeneric
case class Icecream(name: String, numberOfCherries: Int, inCone: Boolean)

object ShapelessRecordEx2 extends App {
   val gen = LabelledGeneric[Icecream]
   val hlist = gen.to(Icecream("vanilla", 2, false))
   hlist match {
      case h :: _ => println(h)
   }
}

But doesn't even compile

Error:(12, 14) constructor cannot be instantiated to expected type;
 found   : scala.collection.immutable.::[B]
 required: shapeless.::[String with shapeless.labelled.KeyTag[Symbol with shapeless.tag.Tagged[String("name")],String],shapeless.::[Int with shapeless.labelled.KeyTag[Symbol with shapeless.tag.Tagged[String("numberOfCherries")],Int],shapeless.::[Boolean with shapeless.labelled.KeyTag[Symbol with shapeless.tag.Tagged[String("inCone")],Boolean],shapeless.HNil]]]
      case h :: _ => println(h)

This code would have been fine if I was working with a normal list.

Answer 1

You just need an import, by default scala.Predef imports the :: operator from scala.collection.immutable.List.

import shapeless.LabelledGeneric
import shapeless.::
case class Icecream(name: String, numberOfCherries: Int, inCone: Boolean)

object ShapelessRecordEx2 extends App {
   val gen = LabelledGeneric[Icecream]
   val hlist = gen.to(Icecream("vanilla", 2, false))
   hlist match {
      case h :: _ => println(h)
   }
}

There's another option, to import ListCompat._.

import shapeless.HList.ListCompat._

object ShapelessRecordEx2 extends App {
  val gen = LabelledGeneric[Icecream]
  val hlist = gen.to(Icecream("vanilla", 2, false))
  hlist match {
    case h #: _ => println(h)
  }
}