Fantas, Eel, and Specification 19: Semigroupoid + Category

It’s not goodbye, Fantasists. We’ll have other projects, new memories, more chance encounters. Let’s end on a high: talking about the humble Category, and how we’ve been learning this since the beginning. While it may not be the most immediately useful structure, it’s a gem for the curious.

Before we go any further, let’s get the methods and laws on the table. A Semigroupoid has one method, called compose:

compose :: Semigroupoid c
        => c i j
        ~> c j k
        -> c i k

There’s also one law, which is going to look boringly familiar:

// Associativity..
a.compose(b.compose(c)) ===
  a.compose(b).compose(c)

A Category is a Semigroupoid with one new method:

id :: Category c => () -> c a a

Now, if you’ve been following the series, there are no prizes for guessing the two laws that come with this:

// Left and right identity!
a === C.id().compose(a)
  === a.compose(C.id())

Yep, it’s another Monoid-looking structure. Let’s all pretend to be surprised.


Now, there’s a reason to get excited about these things. You may not directly interact with them every day, but they’re everywhere underneath the surface:

Function.prototype.compose =
  function (that) {
    return x => that(this(x))
  }

Function.id = () => x => x

Yep. Under function composition, our functions form a Category! Do notice that Function#compose is defined the exact same way as Function#map, too. Of course, this might be a bit easier to introduce to your codebase than Function#map:

// Much more colleague-friendly?
const app =
  readInput
  .compose(clean)
  .compose(capitalise)
  .compose(etCetera)

We can think of s a b (for some Semigroupoid s) as “a relationship from a to b” (the set of which, in fancy talk, are called morphisms), and Category types as having “an identity relationship”. When we did the deep dive with Monad, we actually looked at another Category:

//- We need a TypeRep for M to do `id`.
//- Note that, for a `Chain` type, we could
//- only make a `Semigroupoid`, not a
//- `Category`!
const MCompose = M => {
  //+ type MCompose m a b = a -> m b
  const MCompose_ = daggy.tagged('f')

  //+ compose :: Chain m
  //+         => MCompose m a b
  //+         ~> MCompose m b c
  //+         -> MCompose m a c
  MCompose_.prototype.compose =
    function (that) {
      return x => this.f(x).chain(that.f)
    }

  //+ id :: Monad m => () -> MCompose m a a
  MCompose_.id = () => MCompose_(M.of)

  return MCompose_
}

Note that we originally wrote a Monoid for operations a -> m a. With a Category, we can talk about operations a -> m b, and we have much more freedom. Symmetrically, our Comonad friends give us an almost identical Category:

//- Extend for Semigroupoid, Comonad for
//- Category!
//+ type WCompose w a b = w a -> b
const WCompose = daggy.tagged('f')

//+ compose :: Extend w
//+         => (w a -> b)
//+         ~> (w b -> c)
//+         -> (w a -> c)
WCompose.prototype.compose =
  function (that) {
    return x => this.f(x).extend(that.f)
  }

//+ id :: Comonad w => () -> WCompose w a a
WCompose.id = () =>
  WCompose(x => x.extract())

Why limit ourselves? We have composition! Let’s turn the Applicative’s composition into a Category:

//+ Apply for Semigroupoid, Applicative for
//+ Category!

const ApCompose = (A, C) => {
  //+ type ApCompose f c a b = f (c a b)
  const ApCompose_ = daggy.tagged('f')

  //+ compose :: Apply f
  //+         => Semigroupoid s
  //+         => f s a b
  //+         ~> f s b c
  //+         -> f s a c
  ApCompose_.prototype.compose =
    function (that) {
      return that.f.ap(this.f.map(
        x => y => x.compose(y)))
    }

  //+ id :: Applicative f
  //+    => Category s
  //+    => () -> f s a a
  ApCompose_.id = () => A.of(C.id())
}

The really neat thing here is that, instead of limiting ourselves to the Category of functions within Applicative context, we’ve generalised Function to Category! It’s all getting a bit abstract and scary, right? Don’t panic.


With the helpful exception of Function, these examples may all seem a bit impractical. Granted, you might find that MCompose#compose gives you a nice, declarative way to chain together monadic actions, or something to that effect, but this largely seems a bit too… well, abstract!

While this may not be the one you use every day, the Semigroupoid and Category structures form a very important idea. We hinted at this earlier: Category is like a Monoid with more freedom. Well, if everything we’ve seen ended up looking like Semigroup or Monoid… effectively, the base concept been Category all along!

I had originally wanted to show Monoid in terms of Category. Let’s just say the page of required declarations compiled with the help of three other people made me think that, if I couldn’t understand it, I probably shouldn’t put it here!

It’s not crucial that we understand why. Discussion around categories leads me to all sorts of headaches, and to no avail. What is important is that I make a small confession

This entire series has been about a branch of maths called Category Theory: the theory of categories.

Blam! Here in programmer land, using our shiny new structures to write fault-tolerant database systems and reactive event streams, we thought we were safe from maths… but no! The whole purpose of Fantasy Land is to utilise concepts from category theory that help us to write safer code. “Was Functor maths?” I’m afraid so. “Surely our Semigroup was innocent, though?” I wish I could tell you so. “Not Alt though, right? … Right?” All category theory. Please stop asking.

Perhaps you’ll never need to think about Category again, and that’s fine. Function composition is more than valuable enough to justify its existence. However, sooner or later, if you continue down this rabbit hole, you’ll start asking why and how, and I’m sure you’ll find your way back here…

Simply, there’s really not much (immediately practical) to say about Category and Semigroupoid at this level of generality, but it’s a neat little concept, and I thought it was nice to put a name to the pattern we’ve been seeing all the time! Next time you define a weird type of relationship (perhaps one involving Monad?), give a thought to the humble Category we’ve just seen, and see whether you can simplify your code with a more specific compose implementation.


The keen-eyed among you will have noticed that this is the last Fantasy Land structure, and that we’ve really come to the end of the series. Well… don’t panic! I have one more article to publish before we call this whole thing a day. Call it the encore that no one wanted.

On that note, next time, we’ll be talking about bringing concepts together to build some cool little projects, and where you can go next! Otherwise, thank you so much for reading through at least 19 pages of my mindless rambling, and I hope that it has been useful in some small way.

Now, write beautiful JavaScript.

♥ ♥ ♥