The Proof - Monad as a Monoid in Category of Endofunctors

You have probably seen folllowing Definition somewhere on your FP path.

A monad is just a monoid in the category of endofunctors

If you’re curious, as I am, you’ve probably even googled for some explaination of it. And you probably have found some answer on SO or Quora… the math answer. So now you know the reasoning but probably your level of understanding have stayed on the exact same level, zero.

In the following post I will try to express this Definition in simple Scala code. Little disclaimer: all of the following code could have been written using scalaz or cats libraries, but for clarity I will stick to the pure Scala.

The Monoid

I assume you already have some knowledge from category theory, but we will make short remainder here. First, lets define a Monoid:

trait Monoid[T] {
  def zero: T
  def combine(a1: T, a2: T): T

As we can see, it allows us to combine two instances of type T and gives us zero element, also of type T. An example may be an Int with 0 and + operation.

object IntAddMonoid extends Monoid[Int] {
  def zero: Int = 0
  def combine(a1: Int, a2: Int): Int = a1 + a2

The Category

Ok, that was the easy part. To go further we need to realize, that Definition speaks about monoid in category of …, so we need to know what it means. In fact what we have defined above is a monoid in category of sets where every type is a set. Lets make it more abstract and start with defining a generic MonoidalCategory:

trait MonoidalCategory {
  type Morphism[F, G]
  type MonoidalProduct[F,G]
  type IdentityObject

I won’t go deep into this, but I hope everything will get much clearer when we define our CategoryOfSets

trait CategoryOfSets extends MonoidalCategory {
  type Morphism[F, G] = F => G
  type MonoidalProduct[F,G] = (F, G)
  type IdentityObject = Unit

As we can see, Morphism is just traditional Function, that translates object of type F into object of type G. MonoidalPoruct is just a way to combine two elements of category, so in this case we are just making a Tuple2 out of them. For now, we don’t care much for the IdentityObject, but we can say, that Unit is an IndentityObject because (F, Unit) is equal to F in terms of possible values it can take.

The Monoid in Category

Now we’ve got all the necesseary pieces to define MonoidInCategory and MonoidInCategoryOfSets:

trait MonoidInCategory[T] {
  type Category <: MonoidalCategory
  def zero: Category#Morphism[Category#IdentityObject, T]
  def combine: Category#Morphism[Category#MonoidalProduct[T,T], T]

trait MonoidInCategoryOfSets[T] extends MonoidInCategory[T] {
  type Category = CategoryOfSets
  def zero: Unit => T
  def combine: ((T, T)) => T

So, our zero is function from IdentityObject to T and our combine is function from product of T and T to another instance of T. Can you see that MonoidInCategoryOfSets is almost the same our firs Monoid? If not, here is and example for Int type with add operation:

object IntAddMonoidInCategoryOfSets extends MonoidInCategoryOfSets[Int] {
    def zero: Unit => Int = (a: Unit) => 0 
    def combine: ((Int, Int)) => Int = (t: (Int, Int)) => t._1 + t._2

Not so scary, is it?

The Functor

Now we should define an endofunctor. We will simplify here and use the name Functor where we should use EndofunctorInCategoryOfSets:

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

So F[_] is a Functor because it is a type constructor(kind * -> *) and it allows as to lift any function from A to B into function from F[A] to F[B].

The Category of Endofunctors

Now we would probably like to define a CategoryOfEndofunctors but for this we need few additional steps.That’s because Scala doesn’t provide kind polymorphism, so we cannot put type constructor F[_](kind * -> *) when we require type F(kind *).

trait Product[F[_], G[_]] {
  type Out[T]

trait MonoidalCategoryK2 {
  type Morphism[F[_], G[_]]
  type MonoidalProduct[F[_],G[_]] <: Product[F, G]
  type IndentityObject[T]

Okey, so we migrated our MonoidalCategory from kind * into kind * -> *. One thing that may be not obious here is the Product. When we were acting in kind * the product was just a type function(type constructor) from two types into new type, so it was * -> * -> *, and when aplied with two types(*), it resulted in type(*). But now we are acting in world of type constructors(* -> *) so we need to change * -> * -> * into (* -> *) -> (* -> *) -> (* -> *), so when applied with two type constructors(* -> *) it will result with type constructor(* -> *). If it is not clear yet, we’ll have an example soon. Now lets go back to our mission of defining ` CategoryOfEndofunctors`:

case class Id[T](value: T)

trait NaturalTransformation[-F[_], +G[_]] { self =>
  def apply[A](fa: F[A]): G[A]
type ~~>[-F[_], +G[_]] = NaturalTransformation[F, G]

trait Compose[F[_], G[_]] extends Product[F,G]{
  type Out[T] = F[G[T]]

Okey, so we’ve defined three things here. At first we have simple case class that we will use in a moment. Second goes the NaturalTransformation and it is just a function from F[T] to G[T] for any type T(we also introduce a type alias ~~> that we will use to make things shorter). Then we have an example of a product type that just composes the type constructors. Here is a small example to make it more real:

import scala.util.Try
object TryToOption extends NaturalTransformation[Try, Option] {
  def apply[T](t: Try[T]): Option[T] = t.toOption
def example[T](v: T): Compose[Option, Try]#Out[T] = {
  val result: Option[Try[T]] = Option(Try(v))

And finally, here comes our CategoryOfEndofunctors!

trait CategoryOfEndofunctors extends MonoidalCategoryK2{
  type Morphism[F[_], G[_]] = NaturalTransformation[F, G]
  type MonoidalProduct[F[_],G[_]] = Compose[F,G]
  type IndentityObject[T] = Id[T]

As we can see, it’s just assembled with little elements we’ve already definded.

The Monoid in Category of Endofunctors

As you may feel, we are coming with big steps to the MonoidInCategoryOfEndofunctors but there is this one thing on our way: we need to migrate MonoidInCategory from MonoidalCategory into MonoidalCategoryK2 to make scala compiler happy.

trait MonoidInCategoryK2[T[_]] {
  type Category <: MonoidalCategoryK2
  def zero: Category#Morphism[Category#IndentityObject, T]
  def combine: Category#Morphism[Category#MonoidalProduct[T, T]#Out, T]

And here it comes, THE BIG PLAYER:

trait MonoidInCategoryOfEndofunctors[F[_]] extends MonoidInCategoryK2[F] {
  type Category = CategoryOfEndofunctors
  def zero: Id ~~> F
  def combine: Compose[F,F]#Out ~~> F
  def functor: Functor[F]

Pretty, isn’t it? And not so complicated. Our zero is now a NaturalTransformation(instead of a Function) from Id(instead of an Unit) to F[_]. Our combine is also a NaturalTransformation from F[F[_]] to F[_]. And one more thing, we need an evidence, that F[_] is a Functor so we have this little function called functor. Maybe a little example of defining such a magnificent beeing?

object OptionMonoid extends MonoidInCategoryOfEndofunctors[Option] {
  override def zero: Id ~~> Option = new NaturalTransformation[Id, Option]{
    def apply[T](id: Id[T]): Option[T] = Some(id.value)

  override def combine: Compose[Option,Option]#Out ~~> Option = 
    new NaturalTransformation[Compose[Option,Option]#Out, Option]{
      def apply[T](opt: Option[Option[T]]): Option[T] = opt.flatten

  override def functor: Functor[Option] = new Functor[Option] {
    override def fmap[A, B](f: (A) => B): (Option[A]) => Option[B] = {
      case Some(x) => Some(f(x))
      case None => None

The Monad

Now, when we have dealt with right side of the equasion, lets got back to the left side. Here is a definition of Monad:

trait Monad[M[_]] {
    def unit[T](v: T): M[T]
    def bind[T, U](m: M[T], f: T => M[U]): M[U]

And some example:

object OptionMonad extends Monad[Option] {
  override def unit[T](v: T): Option[T] = Some(v)
  override def bind[T, U](m: Option[T], f: (T) => Option[U]): Option[U] = m.flatMap(f)

The Proof

If the Definition is true, as the math guys say, we should be able to define following functions:

def fromMonoidToMonad[M[_]](monoid: MonoidInCategoryOfEndofunctors[M]): Monad[M]
def fromMonadToMonoid[M[_]](monad: Monad[M[_]]): MonoidInCategoryOfEndofunctors[M]

and in fact we are! Here is the first one:

def fromMonoidToMonad[M[_]](monoid: MonoidInCategoryOfEndofunctors[M]): Monad[M] = {
  new Monad[M]{
    override def unit[A](v: A): M[A] =
    override def bind[A, B](m: M[A], f: (A) => M[B]): M[B] = monoid.combine(monoid.functor.fmap(f)(m))

Pretty short and in fact rather simple. To put value of type A inside M[A] it’s enough to put it inside Id and tranform it with To bind value m of type M[A] with function f from A to M[B] we can lift f with monoid.functor.fmap so now it is M[A] => M[M[B]]. Then we apply it with m and flatten with monoid.combine. No magic attached. And the second one, a bit longer I’ve to admit:

def fromMonadToMonoid[M[_]](monad: Monad[M]): MonoidInCategoryOfEndofunctors[M] = {
  new MonoidInCategoryOfEndofunctors[M] {
    override def zero: Id ~~> M = new (Id ~~> M) {
      override def apply[A](a: Id[A]): M[A] = monad.unit(a.value)
    override def combine: Compose[M, M]#Out ~~> M = new (Compose[M, M]#Out ~~> M) {
      override def apply[A](m: M[M[A]]): M[A] = monad.bind(m, identity[M[A]])
    override def functor: Functor[M] = new Functor[M] {
      override def fmap[A, B](f: (A) => B): (M[A]) => M[B] = (m: M[A]) => {
        monad.bind(m, f.andThen(monad.unit))

Few words of explaination. To transform a, instance of Id[A], into instance of M[A] we just lift the content of Id with monad.unit. To flatten(combine) M[M[A]] it is enough to bind it with identity functions. To create a Functor instance, we define fmap as function that binds it argument with function that is composition of f and monad.unit.

The summary

I hope this article was clear enough, and know you understand what this incomprehensible Definition means. Sadly, I don’t have any formal education in field of category theory, but I hope there are no obvious errors. Because all the code attached compiles, I may be sure that the proof is correct as long as defined types are correct.

One thing I have to confess is that I knowingly ignored all aspects of laws.For example Monad has to obey these three monadic laws, and Monoid has some laws of his own(just as a Functor does). You may treat this as a homework, to prove if they hold :)

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