👀 Views: 98 💬 Answers: 1 📅 Created: 2025-08-27

scala cats case-class semigroup Scala

I'm relatively new to this, so bear with me. I'm working with a `ClassCastException` when trying to use Cats' `Semigroup` with a custom data type in Scala 2.13. I have defined a case class `Transaction` that contains an `amount: Double` and `description: String`. I want to combine two `Transaction` instances using Cats' `combine` method, but I'm working with this behavior: ``` java.lang.ClassCastException: class Transaction want to be cast to class Transaction ``` Here is my code snippet: ```scala import cats.Semigroup import cats.implicits._ case class Transaction(amount: Double, description: String) implicit val transactionSemigroup: Semigroup[Transaction] = new Semigroup[Transaction] { def combine(x: Transaction, y: Transaction): Transaction = Transaction(x.amount + y.amount, x.description + ", " + y.description) } val t1 = Transaction(100.0, "Payment") val t2 = Transaction(50.0, "Refund") val combined = Semigroup[Transaction].combine(t1, t2) println(combined) ``` When I run this code, I get the `ClassCastException`. I thought that defining the implicit `Semigroup` would handle the type correctly. I have also tried different versions of Cats, and this scenario continues. I'm not sure what I'm missing here. Can someone guide to troubleshoot this? I need to properly combine instances of my case class without running into this behavior. Any insights would be greatly appreciated! For context: I'm using Scala on Windows 10. My development environment is Ubuntu 20.04. Has anyone else encountered this?