List of classes implementing a certain typeclass

Question

I would like to define a List of elements implementing a common type class. E.g.

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


        

Answer 1

An alternative way of handling this would be to use the shapeless library. I would really reccommend this book which explains shapeless in a clear and concise manner.

Shapeless provides two things that I think will help you in this case:

1. Heterogeneous lists (HList)
2. Polymorphic functions to enable the HList mapping operation.

First import the required libraries (shapeless):

import shapeless.{HNil, Poly1, ::}

Create a heterogeneous list of whatever types you require. Note the type annotation is only there for clarity.

val data : Int :: String :: HNil = 1 :: "hello" :: HNil

Create a polymorphic function defining an implicit value for every type you require.

object Show extends Poly1 {
    implicit def atT[T: Show] = at[T] (implicitly[Show[T]].show)
}

Shapeless provides an extension method for map on a HList to enable applying the show function to every element in the list

val result : String :: String :: HNil = data.map(Show)

Edited: thanks to @dk14 for the suggested improvement to the definition of the Show polymorphic function.