Why can't Scala find my typeclass instance defined implicitly in the companion object, when the typeclass is not in a dedicated source file?

Question

Please refer to the source code below. All source code is defined in the same package. When I define all of the code within a single source file ShowMain.scala

Answer 1

There's a rule that implicit has to be defined earlier in the compilation unit.

So, move object Show to the top and it compiles.

Alternatively,

object Show {
  //implicit object StringShow extends Show[String] {
  implicit val x: Show[String] = new Show[String] {
    def show(s: String) = s"[String: $s]"
  }
}

see the explanation about the type:

https://stackoverflow.com/a/2731285/1296806