Add right and left functor to BiFunctor

core/src/main/scala/cats/functor/Bifunctor.scala

@@ -24,6 +24,12 @@ import simulacrum.typeclass
    */
   def bimap[A, B, C, D](fab: F[A, B])(f: A => C, g: B => D): F[C, D]
 
+  def rightFunctor[X]: Functor[F[X, ?]] =
+    new RightFunctor[F, X] {val F = self}
+
+  def leftFunctor[X]: Functor[F[?, X]] =
+    new LeftFunctor[F, X] {val F = self}
+
   // derived methods
   /**
    * apply a function to the "left" functor
@@ -61,3 +67,17 @@ private[cats] trait ComposedBifunctor[F[_, _], G[_, _]]
     F.bimap(fab)(innerBimap, innerBimap)
   }
 }
+
+private trait LeftFunctor[F[_, _], X] extends Functor[F[?, X]] {
+  implicit val F: Bifunctor[F]
+
+  override def map[A, C](fax: F[A, X])(f: A => C): F[C, X] =
+    F.bimap(fax)(f, identity)
+}
+
+private trait RightFunctor[F[_, _], X] extends Functor[F[X, ?]] {
+  implicit val F: Bifunctor[F]
+
+  override def map[A, C](fxa: F[X, A])(f: A => C): F[X, C] =
+    F.bimap(fxa)(identity, f)
+}