Scala implicit logic for companion objects

Question

object Test extends App {

  def print(s: String)(implicit p: Prefixer) = {
    println(p.prefix + s)
  }

  print("test")

}

case class Prefixer(prefix: String)

object Prefixer {
  implicit val p = Prefixer("***")
}

The above code does not compile, because the compiler is not able to find an implicit value for Prefixer. However, if I put the case class Prefixer and the companion object in another file it works. Why is that?

Duplicate of http://stackoverflow.com/questions/25468583/why-cant-scala-find-my-typeclass-instance-defined-implicitly-in-the-companion-o — som-snytt, Aug 31 '14 at 11:46

Ravi Kiran · Answer 1 · 2014-08-31T11:28:59.080

5

It is to do with the order of declaration. It also works if you just move the Prefixer and its companion above the main object. When the class is in another file, the compiler can scan that file first & then come to the file implementing the App.

edited Aug 31 '14 at 11:28

answered Aug 31 '14 at 06:00

Ravi Kiran

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1

The best solution is to give an explicit type to your implicit val, though. Declaring implicits without type is not recommended. – sjrd Aug 31 '14 at 07:46
@sjrd why is it not recommended? – Ionuț G. Stan Aug 31 '14 at 11:36

Scala implicit logic for companion objects

1 Answers1