Factor instance for a two-parameter sample constructor in Scala

Question

I have a class Foowith two parameters, and I'm trying to write a Functor instance for Foo with a fixed first parameter, as follows:

object Scratchpad {

  trait Functor[F[_]] {
    def fmap[A, B](f: A => B): F[A] => F[B]
  }

  case class Foo[X, Y](value: Y)

  implicit def fooInstances[X]: Functor[Foo[X, _]] =
    new Functor[Foo[X, _]] {
      def fmap[A, B](f: A => B): Foo[X, A] => Foo[X, B] =
        foo => Foo[X, B](f(foo.value))
    }
}

But the above code does not compile, generating the following error:

Error:(9, 41) Scratchpad.Foo[X, _] takes no type parameters, expected: one
  implicit def fooInstances[X]: Functor[Foo[X, _]] =

I know that Scalaz does something like this with a type \/, but checking the source code shows an odd ?one that does not compile for me:

implicit def DisjunctionInstances1[L]: Traverse[L \/ ?] with Monad[L \/ ?] with BindRec[L \/ ?] with Cozip[L \/ ?] with Plus[L \/ ?] with Optional[L \/ ?] with MonadError[L \/ ?, L] =

How does Scalaz work ?and how can I write an instance of Functor for Foo?

Answer 1

. , . , . Foo , .

implicit def fooInstances[X]: Functor[({ type T[A] = Foo[X, A] })#T] =
  new Functor[({ type T[A] = Foo[X, A] })#T] {
    // ...
  }

{} , , , - . , .

Answer 2

?,

? kind-projector, Scala, build.sbt

resolvers += Resolver.sonatypeRepo("releases")

addCompilerPlugin("org.spire-math" %% "kind-projector" % "0.9.4")

lambdas :

implicit def fooInstances[X]: Functor[Foo[X, ?]] =
  new Functor[Foo[X, ?]] {
    def fmap[A, B](f: A => B): Foo[X, A] => Foo[X, B] =
      foo => Foo[X, B](f(foo.value))
}

, :

implicit def fooInstances[X] = {
  type PartiallyAppliedFoo[A] = Foo[X, A]
  new Functor[PartiallyAppliedFoo] {
    override def fmap[A, B](f: (A) => B): (PartiallyAppliedFoo[A]) => PartiallyAppliedFoo[B] = foo => Foo[X, B](f(foo.value))
  }
}