Implicit value not found from companion object

Question

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.

I think that even if you put the A object in the same package you still have to say import A._ in the main method, but I'm not sure why it works when you put it in the package object. Can you please post the working version with the package object usage? — Aldo Stracquadanio
– Aldo Stracquadanio, Commented Jan 15, 2014 at 12:06
I edited the title of this question. "disjoint type" has a very specific and very different meaning in Scala. — Kevin Wright
– Kevin Wright, Commented Jan 15, 2014 at 12:08
Could you please provide a transcript like the following that shows how to reproduce this error? gist.github.com/retronym/8435310 — retronym
– retronym, Commented Jan 15, 2014 at 12:27
My suggestion: use implicit vals, rather than implicit objects, so that you can annotated the return type. — retronym
– retronym, Commented Jan 15, 2014 at 15:31

0__ · Accepted Answer · 2014-01-16 09:14:36Z

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.

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