👀 Views: 259 💬 Answers: 1 📅 Created: 2025-06-29

scala typeclasses implicits scala3 Scala

I've been struggling with this for a few days now and could really use some help... I've encountered a strange issue with I'm working with unexpected behavior when using implicit conversions in Scala 3.1.2 with my custom type classes. I have defined a type class for `Show` that formats an object as a string, but sometimes the implicit resolution fails in contexts where I expect it to succeed. Here's the implementation of my type class and its instances: ```scala trait Show[A] { def show(a: A): String } object Show { implicit val stringShow: Show[String] = new Show[String] { def show(a: String): String = a } implicit val intShow: Show[Int] = new Show[Int] { def show(a: Int): String = a.toString } implicit def listShow[A](implicit ev: Show[A]): Show[List[A]] = new Show[List[A]] { def show(a: List[A]): String = a.map(ev.show).mkString("[", ", ", "]") } } ``` And here’s how I’m trying to use it: ```scala def printShow[A](value: A)(implicit showInstance: Show[A]): Unit = { println(showInstance.show(value)) } printShow("Hello, world!") printShow(List(1, 2, 3)) ``` When I call `printShow(List(1, 2, 3))`, I get the following compilation behavior: ``` behavior: could not find implicit value for parameter showInstance: Show[List[Int]] ``` I've double-checked that the `Show[Int]` instance is correctly defined and available in scope. I've also tried explicitly importing the `Show` companion object to ensure that the implicits are properly visible. However, the scenario continues. Is there something I'm missing about implicit conversions and type classes in Scala 3? How can I resolve this behavior and ensure that my `Show` type class instances are correctly resolved in all cases? I appreciate any help you can offer! I'd really appreciate any guidance on this. For reference, this is a production CLI tool.