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 intCanShow: Show[Int] = new Show[Int] {
      def show(int: Int): String = s"int $int"
  }
  implicit val stringCanShow: Show[String] = new Show[String] {
    def show(str: String): String = str
  }

The problem is, how to define a list = List(1, "abc") such that it is guaranteed that a Show instance for these values is in scope? I would then like to map this list over show like list map {_.show}.

Answer 1

I will first sketch a solution, and then explain why the naive approach with List[Any](1, "abc") cannot work.

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What you can do

Define a wrapper class that can hold instances of type A together with instances of Show[A]:

case class Showable[A](a: A, showInst: Show[A]) {
  def show: String = showInst.show(a)
}

Define your list as List[Showable[_]]:

var showableList: List[Showable[_]] = Nil

Maybe define a separate method to fill this list (consider packing the list itself and the builder-method in a class):

def addShowable[A: Show](a: A): Unit = {
  showableList ::= Showable[A](a, implicitly[Show[A]])
}

Alternatively, you can carefully add a (very tightly scoped) implicit conversion:

implicit def asShowable[A](a: A)(implicit s: Show[A]): Showable[A] = 
  Showable(a, s)

and then costruct your list as follows (note the explicit type ascription):

val showableList = List[Showable[_]](1, "abc")

Now you can go through the list and call show:

showableList.map(_.show)

to obtain a list of String.

---

What you cannot do

You cannot simply define

val list: List[Any] = List(1, "abc", <showable3>, ..., <showableN>)

and then expect to be able to call show, because in order to call Show.show, you need actual Show instances. These things are not some type-hints that can be erased at runtime, they are actual objects, and they must be supplied by the compiler. Once you have created a List[Any], all is lost, because all the types are merged into an unexpressive upper bound Any, and the compiler has no way to inject all the necessary implicits Show[T_1],..., Show[T_N]. The argument is very similar to the third section "Dealing with implicits when defining interpreter for the Free monad" of this lengthy answer of mine.

Answer 2

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.

Answer 3

The core problem here is that you want to create a heterogenous list, something like List[Int, String] instead of List[Any]. This means you need a different structure that would preserve Int and String types, but still would be "mappable" like List. The one structure in scala-library that can contain heterogenous types is Tuple:

val tuple = (1, "abc")
val result = List(implicitly[Show[Int]].show(tuple._1), implicitly[Show[Int]].show(tuple._2))

However, scala-library can't map over tuples - you might want some syntax sugar for better readability.

So the obvious solution is HList from Shapeless: Int :: String :: HNil (or you can use tuple ops and stay with (Int, String))

import shapeless._
import poly._


//show is a polymorphic function
//think of it as `T => String` or even `(Show[T], T) => String` 
object show extends Poly1 {
  implicit def atT[T: Show] = at[T](implicitly[Show[T]].show)
}

@ (1 :: "aaaa" :: HNil) map show
res8: String :: String :: HNil = "int 1" :: "aaaa" :: HNil

Or you could use at[Int]/at[String] instead of type-classes, like in @Steve Robinson's answer.

P.S. The lib could be found here. They also provide one-liner to get Ammonite REPL with shapeless integrated, so you could try my example out using:

curl -s https://raw.githubusercontent.com/milessabin/shapeless/master/scripts/try-shapeless.sh | bash    

---

Notes:

1. Practically Shapeless solution requires as same amount of maintenance as Tuple-based one. This is because you have to keep track of your Int and String types anyways - you can never forget about those (unlike in homogenous List[T] case). All Shapeless does for you is nicer syntax and sometimes better type inference.

2. If you go with tuples - you can improve readability by using implicit class instead of Haskell-like style, or if you still want Haskell-like, there is a Simulacrum macro for better type-class syntax.

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Given that other scala-library-only alternatives just capture type class instances inside some regular class, you could be better off with a regular OOP wrapper class:

trait Showable[T]{def value: T; def show: String}
class IntShow(val value: Int) extends Showable[Int]{..}
class StringShow(val value: String) extends Showable[String] {..}

val showables: List[Showable[_]] = List(new Showable(5), new Showable("aaa"))
showables.map(_.show)

Looks cleaner and more readable to me :)

If you like to rewrite dynamic dispatching in FP-style:

sealed trait Showable
final case class ShowableInt(i: Int) extends Showable
final case class ShowableString(s: String) extends Showable

implicit class ShowableDispatch(s: Showable){
  def show = s match{ //use `-Xfatal-warnings` scalac option or http://www.wartremover.org/ to guarantee totality of this function
    case ShowableInt(i) => ...
    case ShowableString(s) => ...
  }
}

List(ShowableInt(5), ShowableString("aaa")).map(_.show)

---

If you really want static dispatching (or ad-hoc polymorphism), given that other solutions introduce Showable[_] which is practically Showable[Any]:

case class Showable[T](v: T, show: String)
def showable(i: Int) = Showable(i, s"int $i") 
def showable(s: String) = Showable(i, s) 
List(showable(5), showable("aaa")).map(_.show)