Show.scala

package com.tomwadeson.show

import com.tomwadeson.Show.Showable

import scala.language.implicitConversions

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

object Show {
  def apply[A: Show]: Show[A] = implicitly

  def instance[A](f: A => String): Show[A] = (a: A) => f(a)

  def print[A: Show](a: A): Unit = println(Show[A].show(a))

  implicit val showInt: Show[Int] = instance(_.toString)

  implicit val showString: Show[String] = instance(identity)

  implicit def showList[A: Show]: Show[List[A]] =
    instance(as => as.map(Show[A].show).mkString(", "))

  type Showable = TcBox[Show]

  implicit val showShowable: Show[Showable] =
    Show.instance(s => s.evidence.show(s.value))
}

sealed trait TcBox[Tc[_]] {
  type A
  val value: A
  val evidence: Tc[A]
}

object TcBox {
  final case class MkTcBox[Tc[_], B](value: B)(implicit val evidence: Tc[B]) extends TcBox[Tc] { type A = B }

  implicit def mkTcBox[Tc[_], A : Tc](a: A): TcBox[Tc] = MkTcBox(a)
}

object Main extends App {
  Show.print(List(1, 2, "3", 4, 5): List[Showable])
}