Add MFunctor type class for mapping monad morphisms over monad transf…

…ormers

core/src/main/scala/cats/MFunctor.scala

@@ -0,0 +1,16 @@
+package cats
+
+/**
+ * A type class which abstracts over the ability to lift a natural transformation
+ * (monad morphism) `M ~> N` into a MonadTransformer. In other words, a functor
+ * in the category of monads.
+ */
+
+trait MFunctor[MT[_[_]]] {
+
+  /**
+   * Analogous to fmap
+   */
+  def hoist[M[_], N[_]](m: M ~> N): MT[M] => MT[N]
+
+}

core/src/main/scala/cats/data/EitherT.scala

@@ -331,6 +331,12 @@ private[data] abstract class EitherTInstances extends EitherTInstances1 {
         EitherT.liftT(ma)
     }
 
+  implicit def catsDataMFunctorForEither[E, A]: MFunctor[EitherT[?[_], E, A]] =
+    new MFunctor[EitherT[?[_], E, A]] {
+      def hoist[M[_], N[_]](m: M ~> N): EitherT[M, E, A] => EitherT[N, E, A] =
+        e => EitherT(m(e.value))
+    }
+
   implicit def catsMonoidForEitherT[F[_], L, A](implicit F: Monoid[F[Either[L, A]]]): Monoid[EitherT[F, L, A]] =
     new EitherTMonoid[F, L, A] { implicit val F0 = F }
 

core/src/main/scala/cats/data/IdT.scala

@@ -93,6 +93,12 @@ private[data] sealed abstract class IdTInstances0 extends IdTInstances1 {
 
   implicit def catsDataOrderForIdT[F[_], A](implicit F: Order[F[A]]): Order[IdT[F, A]] =
     F.on(_.value)
+
+  implicit def catsDataMFunctorForIdT[F[_], A]: MFunctor[IdT[?[_], A]] =
+    new MFunctor[IdT[?[_], A]] {
+      def hoist[M[_], N[_]](m: M ~> N): IdT[M, A] => IdT[N, A] =
+        i => IdT(m(i.value))
+    }
 }
 
 private[data] sealed abstract class IdTInstances extends IdTInstances0 {

core/src/main/scala/cats/data/OptionT.scala

@@ -215,6 +215,12 @@ private[data] sealed trait OptionTInstances1 extends OptionTInstances2 {
       def liftT[M[_]: Functor, A](ma: M[A]): OptionT[M, A] = OptionT.liftF(ma)
     }
 
+  implicit def catsDataMFunctorForOptionT[A]: MFunctor[OptionT[?[_], A]] =
+    new MFunctor[OptionT[?[_], A]] {
+      def hoist[M[_], N[_]](m: M ~> N): OptionT[M, A] => OptionT[N, A] =
+        o => OptionT(m(o.value))
+    }
+
   implicit def catsDataMonoidKForOptionT[F[_]](implicit F0: Monad[F]): MonoidK[OptionT[F, ?]] =
     new OptionTMonoidK[F] { implicit val F = F0 }
 

core/src/main/scala/cats/data/WriterT.scala

@@ -87,6 +87,12 @@ private[data] sealed abstract class WriterTInstances extends WriterTInstances0 {
         WriterT(Functor[M].map(ma)((W.empty, _)))
     }
 
+  implicit def catsDataMFunctorForWriterT[W, A](implicit W: Monoid[W]): MFunctor[WriterT[?[_], W, A]] =
+    new MFunctor[WriterT[?[_], W, A]] {
+      def hoist[M[_], N[_]](m: M ~> N): WriterT[M, W, A] => WriterT[N, W, A] =
+        w => WriterT(m(w.run))
+    }
+
   implicit def catsDataShowForWriterT[F[_], L, V](implicit F: Show[F[(L, V)]]): Show[WriterT[F, L, V]] = new Show[WriterT[F, L, V]] {
     override def show(f: WriterT[F, L, V]): String = f.show
   }

free/src/main/scala/cats/free/FreeT.scala

@@ -218,6 +218,12 @@ private[free] sealed trait FreeTInstances1 extends FreeTInstances2 {
       override def liftT[M[_]: Functor, A](ma: M[A]): FreeT[S, M, A] =
         FreeT.liftT(ma)
     }
+
+  implicit def catsDataMFunctorForFreeT[S[_], A]: MFunctor[FreeT[S, ?[_], A]] =
+    new MFunctor[FreeT[S, ?[_], A]] {
+      def hoist[M[_], N[_]](m: M ~> N): FreeT[S, M, A] => FreeT[S, N, A] =
+        f => f.hoist(m)
+    }
 }
 
 private[free] sealed trait FreeTInstances0 extends FreeTInstances1 {