hk_sample.scala

scala> def fid[F[_], A](a: F[A]): F[A] = a
warning: there was one feature warning; re-run with -feature for details
fid: [F[_], A](a: F[A])F[A]

scala> fid(List(1))
res0: List[Int] = List(1)

scala> fid(Option(1))
res1: Option[Int] = Some(1)

scala> fid(Right(1))
<console>:9: error: no type parameters for method fid: (a: F[A])F[A] exist so that it can be applied to arguments (scala.util.Right[Nothing,Int])
 --- because ---
argument expression's type is not compatible with formal parameter type;
 found   : scala.util.Right[Nothing,Int]
 required: ?F[?A]
              fid(Right(1))
              ^
<console>:9: error: type mismatch;
 found   : scala.util.Right[Nothing,Int]
 required: F[A]
              fid(Right(1))
                       ^

scala> type FRight[A] = Either[String, A]
defined type alias FRight

scala> fid[FRight, Int](Right(1))
res4: FRight[Int] = Right(1)

scala> fid[({type F[A] = Either[String, A]})#F, Int](Right(1))
res5: scala.util.Either[String,Int] = Right(1)