CODEWITHSUNDEEP
scala
maks
maks

Reputation: 6006

Implicit value not found from companion object

I have a function foo declared in Bar object:

package test
object Bar{
 def foo[T : A](b: T) = println(b)
 def main(args: Array[String]) {
    foo(3)
 }
}

where A is declared in the same package:

sealed trait A[T]
object A {
  implicit object AInt extends A[Int]
  implicit object AString extends A[String]
}

in this configuration Bar object doesn't compile and produces this error message:

could not find implicit value for evidence parameter of type test.A[Int]
    foo(3)
       ^

But when I place trait A and its companion object into package object test all work fine. Why?

EDIT

The problem is in order of declaration. If I place A typeclass declaration before Bar declaration everything works fine.

Upvotes: 2

Views: 1081

Answers (1)

0__
0__

Reputation: 67330

So the two solutions seem to be:

(1) Declare A first:

sealed trait A[T]
object A {
  implicit object AInt    extends A[Int   ]
  implicit object AString extends A[String]
}

object Bar {
  def foo[T: A](b: T) = println(b)
  def main(args: Array[String]): Unit = foo(3)
}

(2) Use def or val with given return type:

object Bar {
  def foo[T: A](b: T) = println(b)
  def main(args: Array[String]): Unit = foo(3)
}

sealed trait A[T]
object A {
  implicit def aInt   : A[Int   ] = AInt
  implicit def aString: A[String] = AString

  private object AInt    extends A[Int   ]
  private object AString extends A[String]
}

I would consider this is a compiler bug, but at least you can work around it.

Upvotes: 3

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