Recursive value needs type error in Scala 3 implicit parameter

Question

I'm trying to compile the following code in Scala 3 (worked on Scala 2.13):

import scala.concurrent.duration._

@main
def main(): Unit = {
  case class AAA(d: FiniteDuration)

  val duration1 = 5.seconds
  implicit val what = AAA(duration1)
}

But getting the following error:

Recursive value duration1 needs type implicit val what = AAA(duration1)

Making the parameter non implicit fixed the error. Why?

What do you mean by "Making the parameter non implicit" - what "parameter"? — Andrey Tyukin
– Andrey Tyukin, Commented Jan 16, 2024 at 18:31

Andrey Tyukin · Accepted Answer · 2024-01-16 18:27:53Z

2

Try

import scala.concurrent.duration._

@main
def main(): Unit = {
  case class AAA(d: FiniteDuration)

  val duration1 = 5.seconds
  implicit val what: AAA = AAA(duration1)
}

or

import scala.concurrent.duration._

@main
def main(): Unit = {
  case class AAA(d: FiniteDuration)

  val duration1 = 5.seconds
  given AAA = AAA(duration1)
}

As a general guideline: always explicitly ascribe the type of all givens / implicits. It's an assignment for a compile-time type-level computation, it's better to specify what type you are assigning the given value to.

answered Jan 16, 2024 at 18:27

Andrey Tyukin

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2 Comments

galbarm Over a year ago

thank you, took your advice further and replaced "implicit val what = AAA(duration1)" with "given AAA(duration1)" and the error is gone. still wonder why implicit val with no explicit type declaration gave the cryptic compiler error.

galbarm Over a year ago

from the migration guide: "The type of implicit definitions (val or def) needs to be given explicitly in Scala 3. They cannot be inferred anymore." I wonder just why the compiler error is so not related.

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