Implicit parameter need Type annotation to compile

Question

Here is my test code :

object ImplicitTest {
  import JoesPrefs._
  Greeter.greet("Joe") // could not find implicit value for parameter prompt: 
}
class PreferredPrompt(val preference: String)

object JoesPrefs {
  implicit val prompt = new PreferredPrompt("Yes, master> ")
}
object Greeter {
  def greet(name: String)(implicit prompt: PreferredPrompt) = {
    println("Welcome, " + name + ". The system is ready.")
    println(prompt.preference)
  }
}

I use scala 2.11.12, don't know why this implicit not work until add type annotation to val :

object JoesPrefs {
  implicit val prompt: PreferredPrompt  = new PreferredPrompt("Yes, master> ")
}

Anyways an implicit value should always have an explicit type annotation. — Luis Miguel Mejía Suárez, Jul 20 '20 at 03:53
In some cases, it is really needed. In other cases, it helps. And in a few cases, it is not possible to do. — Luis Miguel Mejía Suárez, Jul 20 '20 at 06:06
Related https://stackoverflow.com/questions/32438750/how-does-scala-use-explicit-types-when-resolving-implicits#comment52747717_32438750 and — Mario Galic, Jul 20 '20 at 08:30
Does this answer your question? [How does Scala use explicit types when resolving implicits?](https://stackoverflow.com/questions/32438750/how-does-scala-use-explicit-types-when-resolving-implicits) — C4stor, Jul 20 '20 at 08:57
Yeah, it's so related I voted to close here ^^ good find :-) — C4stor, Jul 20 '20 at 08:57
I think this question is more simple and direct for people to find the similar problem, though the answer you mention is related to this question. — yuxh, Jul 20 '20 at 09:14

score 4 · Accepted Answer · answered Jul 20 '20 at 08:12

4

So, the exact internal are a bit wild, but basically it boils down to the compilation order of the code. When you add the type annotation, the val is "compiled and put in scope" sooner than when you do not, and it becomes available to resolve in ImplicitTest.

Funnily (at least for me ^^), you can also move ImplicitTest to be on a line of code after the JoesPref object (or move it to its own file), it will compile without the type annotation.

answered Jul 20 '20 at 08:12

C4stor

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yes, it act like you told. Do you know anywhere mentioning about this situation? – yuxh Jul 20 '20 at 08:49
There's this comment by the language creator : https://github.com/scala/bug/issues/801#issuecomment-292352399 – C4stor Jul 20 '20 at 08:55

Implicit parameter need Type annotation to compile

1 Answers1