Type Class Implicits with Context Bound

Question

I am slowly teaching myself Scala type classes. Suppose I have the following example:

object Example extends App { trait Serializer[T] { def serialize(seq: Seq[T]): String } object Serializer { def serialize[T](seq: Seq[T])(implicit serializer: Serializer[T]): Value = serializer.serialize(seq) } implicit object StringSerializer extends Serializer[String] { def serialize(seq: Seq[String]): String = seq.toString() } implicit object IntSerializer extends Serializer[Int] { def serialize(seq: Seq[Int]): String = seq.toString() } case class Data[T: Serializer](x: Seq[T], y: Seq[T], z: Seq[T]) { val series = Data.createString(x, y, z) } object Data { def createString[T : Serializer](x: Seq[T], y: Seq[T], z: Seq[T]) = { val serialize = implicitly[Serializer[T]] List(serialize.serialize(x), serialize.serialize(y)) } } val x = Seq("a", "b", "c") val y = Seq(1, 2, 3, 4) val z = Seq(10, 20, 30) val data = Data(x, y, z) println(data.series) }

Now this fails with

could not find implicit value for evidence parameter of type Example.Serializer[Any] [error] val data = Data(x, y, z)

Can someone please guide me as to how I can make createString method work? From my understanding because I have a Seq[Int] and a Seq[String], the type [T] will get inferred as [Any], which causes the issue.

However, I put a context bound based on my Serializer type class, which I thought would make the compiler look for a String or Int serializer, which it fails to do. I basically want to pass in any sequence for which I have a defined serializer, so in this any combination of Seq[Int] and Seq[String].

Any guidance would be much appreciated!

Luis Miguel Mejía Suárez · Accepted Answer · 2020-06-14 13:49:10Z

Again the problem is that you want different types, but your are using a single type parameter T; so the compiler infers any.
Maybe you want to define your class like this:

final case class Data[A: Serializer, B: Serializer, C: Serializer](x: Seq[A], y: Seq[B], z: Seq[C])

So you can have the three different types.

BTW, I would suggest you to use List or Vector or ArraySeq, i.e. concrete collections, instead of the abstract Seq.

Thanks this works and I now see how to use multiple types for my code! I will shift the code to more concrete collections

Dmytro Mitin · Accepted Answer · 2020-06-14 13:45:38Z

If you want fields of Data to be Seqs of different element types then do this

case class Data[T: Serializer, T1: Serializer, T2: Serializer](x: Seq[T], y: Seq[T1], z: Seq[T2]) { val series = Data.createString(x, y, z) } object Data { def createString[T : Serializer, T1 : Serializer, T2 : Serializer](x: Seq[T], y: Seq[T1], z: Seq[T2]) = { val serialize = implicitly[Serializer[T]] val serialize1 = implicitly[Serializer[T1]] val serialize2 = implicitly[Serializer[T2]] List(serialize.serialize(x), serialize1.serialize(y), serialize2.serialize(z)) } }

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