MonadLayer

MonadLayer[M, Inner] has the following functionality: - lifts values from the Monad Inner to the Monad M. - lifts Monad isomorphisms in Inner ((Inner ~> Inner, Inner ~> Inner)) into Monad homomorphisms in M (M ~> M). This allows you to "map" a natural transformation over the Inner inside M, but only if you can provide an inverse of that natural transformation.

MonadLayer has one external law:

def layerRespectsFlatMap[A, B](m: Inner[A])(f: A => Inner[B]) = {
  layer(m).flatMap(f andThen layer) <-> layer(m.flatMap(f))
}