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This question is about a limitation of Scala's implicit resolution system that I've run into a few times when using Scalaz and that doesn't make a lot of sense to me. I've distilled the problem to a Scalaz-less version below, but I'm happy to provide more information about motivation if needed. Suppose I have a couple of type classes that witness something about a type constructor: import scala.language.higherKinds trait Foo[F[_]] trait Bar[F[_], A] Now also suppose that if I have a Foo instance for some F , I know I also have a Foo instance for Bar[F, _] : implicit def barFoo[F[_]: Foo] = new Foo[({type L[X] = Bar[F, X]})#L] {} I've also got instances for List and the right side of Either : implicit object listFoo extends Foo[List] implicit def eitherFoo[A] = new Foo[({type L[X] = Either[A, X]})#L] {} Now it's pretty clear that I should be able to write the following: type BarList[X] = Bar[List, X] implicitly[Foo[BarList]] Or, equivalently: implicitly[Foo[({type L[X] = Bar[List, X]})#L]] And indeed, both work exactly as expected. So I try the following: type StringOr[X] = Either[String, X] type BarStringOr[X] = Bar[StringOr, X] And then: scala> implicitly[Foo[BarStringOr]] res2: Foo[BarStringOr] = $anon$1@39a6c855 Again, no surprises here. But then I try: implicitly[Foo[({type L[X] = Bar[StringOr, X]})#L]] And I get the following: <console>:15: error: could not find implicit value for parameter e: Foo[[X]Bar[[X]scala.util.Either[String,X],X]] implicitly[Foo[({type L[X] = Bar[StringOr, X]})#L]] ^ Note that I have no problem i

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