shapless example (Show)

a.scala

package com.example

import shapeless._
import shapeless.labelled.FieldType

trait Show[A] {
  def show(a: A): String
}

object Show {

  def apply[A](f: A => String): Show[A] = new Show[A] {
    override def show(a: A): String = f(a)
  }

  implicit def string: Show[String] = Show[String](identity)

  implicit def int: Show[Int] = Show[Int](_.toString)

  implicit def list[T](implicit tShow: Show[T]): Show[List[T]] =
    Show[List[T]] {
      _.map(tShow.show).mkString("[", ", ", "]")
    }

  implicit def hnil: Show[HNil] = Show[HNil] { _ =>
    ""
  }

  implicit def hlist[K <: Symbol, H, T <: HList](
      implicit
      witness: Witness.Aux[K],
      hShow: Lazy[Show[H]],
      tShow: Show[T]): Show[FieldType[K, H] :: T] =
    Show[FieldType[K, H] :: T] {
      case h :: HNil =>
        witness.value.name + "=" + hShow.value.show(h)
      case h :: t =>
        witness.value.name + "=" + hShow.value.show(h) + ", " + tShow.show(t)
    }

  implicit def generic[T, U](implicit gen: LabelledGeneric.Aux[T, U],
                             ev: LowPriority,
                             f: Lazy[Show[U]]): Show[T] = Show[T] { a =>
    val cls = a.getClass.getName
    val hash = System.identityHashCode(a)
    val fields = f.value.show(gen.to(a))
    s"$cls@$hash($fields)"
  }

}

object ShowSyntax {
  implicit class showSyntax[A](a: A)(implicit s: Show[A]) {
    def show: String = s.show(a)
  }
}

case class Person(firstName: String, lastName: String, friends: List[Person])

object Main {

  import ShowSyntax._

  def main(args: Array[String]): Unit = {
    val person = Person("John",
                        "Lennon",
                        List(Person("Paul", "McCartney", Nil),
                             Person("George", "Harrison", Nil),
                             Person("Ringo", "Starr", Nil)))

    println(person.show)
    // com.example.Person@400176287(firstName=John, lastName=Lennon, friends=[com.example.Person@1621417118(firstName=Paul, lastName=McCartney, friends=[]), com.example.Person@1245422131(firstName=George, lastName=Harrison, friends=[]), com.example.Person@762439731(firstName=Ringo, lastName=Starr, friends=[])])
  }

}