The most accessible category theory book I’ve come across is Lawvere and Schanuel’s Conceptual Mathematics: A First Introduction to Categories 2nd ed. The book mixes Articles, which is written like a normal textbook; and Sessions, which is kind of written like a recitation class.

Even in the Article section, Lawvere uses many pages to go over the basic concept compared to other books, which is good for self learners.

Lawvere:

Before giving a precise definition of ‘category’, we should become familiar with one example, the

category of finite sets and maps. Anobjectin this category is a finite set or collection. … You are probably familiar with some notations for finite sets:

```
{ John, Mary, Sam }
```

There are two ways that I can think of to express this in Scala. One is by using a value `a: Set[Person]`

:

```
scala> :paste
// Entering paste mode (ctrl-D to finish)
sealed trait Person {}
case object John extends Person {}
case object Mary extends Person {}
case object Sam extends Person {}
val a: Set[Person] = Set[Person](John, Mary, Sam)
// Exiting paste mode, now interpreting.
defined trait Person
defined object John
defined object Mary
defined object Sam
a: Set[Person] = Set(John, Mary, Sam)
```

Another way of looking at it, is that `Person`

as the type is a finite set already without `Set`

. **Note**: In Lawvere uses the term “map”, but I’m going to change to *arrow* like Mac Lane and others.

A

arrowfin this cateogry consists of three things:

- a set A, called the
domainof the arrow,- a set B, called the
codomainof the arrow,- a rule assigning to each element
ain the domain, an elementbin the codomain. Thisbis denoted byf ∘ a(or sometimes ’f(a)‘), read ’fofa‘.(Other words for arrow are ‘function’, ‘transformation’, ‘operator’, ‘map’, and ‘morphism’.)

Let’s try implementing the favorite breakfast arrow.

```
scala> :paste
// Entering paste mode (ctrl-D to finish)
sealed trait Breakfast {}
case object Eggs extends Breakfast {}
case object Oatmeal extends Breakfast {}
case object Toast extends Breakfast {}
case object Coffee extends Breakfast {}
val favoriteBreakfast: Person => Breakfast = {
case John => Eggs
case Mary => Coffee
case Sam => Coffee
}
// Exiting paste mode, now interpreting.
defined trait Breakfast
defined object Eggs
defined object Oatmeal
defined object Toast
defined object Coffee
favoriteBreakfast: Person => Breakfast = $$Lambda$4903/1998508933@4339df43
```

Note here that an “object” in this category is `Set[Person]`

or `Person`

, but the “arrow” `favoriteBreakfast`

accepts a value whose type is `Person`

. Here’s the *internal diagram* of the arrow.

The important thing is: For each dot in the domain, we have exactly one arrow leaving, and the arrow arrives at some dot in the codomain.

I get that a map can be more general than `Function1[A, B]`

but it’s ok for this category. Here’s the implementation of `favoritePerson`

:

```
scala> val favoritePerson: Person => Person = {
case John => Mary
case Mary => John
case Sam => Mary
}
favoritePerson: Person => Person = $$Lambda$4904/1066379389@1d4868a3
```

An arrow in which the domain and codomain are the same object is called an

endomorphism.

An arrow, in which the domain and codomain are the same set

A, and for each ofainA,f(a)=a, is called anidentity arrow.

The “identity arrow on A” is denoted as 1_{A}.

Again, identity is an arrow, so it works on an element in the set, not the set itself. So in this case we can just use `scala.Predef.identity`

.

```
scala> identity(John)
res0: John.type = John
```

Here are the *external diagrams* corresponding to the three internal diagrams from the above.

This reiterates the point that *in the category of finite sets*, the “objects” translate to types like `Person`

and `Breakfast`

, and arrows translate to functions like `Person => Person`

. The external diagram looks a lot like the type-level signatures like `Person => Person`

.

The final basic ingredient, which is what lends all the dynamics to the notion of category is

composition of arrows, by which two arrows are combined to obtain a third arrow.

We can do this in scala using `scala.Function1`

’s `andThen`

or `compose`

.

```
scala> val favoritePersonsBreakfast = favoriteBreakfast compose favoritePerson
favoritePersonsBreakfast: Person => Breakfast = scala.Function1$$Lambda$2547/1273946735@42971355
```

Here’s the internal diagram:

and the external diagram:

After composition the external diagram becomes as follows:

’

f ∘ g’ is read ’f following g‘, or sometimes ’f of g‘.

*Data* for a category consists of the four ingredients:

- objects:
*A*,*B*,*C*, … - arrows:
*f: A => B* - identity arrows:
*1*_{A}: A => A - composition of arrows

These data must satisfy the following rules:

The identity laws:

- If
*1*, then_{A}: A => A, g: A => B*g ∘ 1*_{A}= g - If
*f: A => B, 1*, then_{B}: B => B*1*_{A}∘ f = f

The associative law:

- If
*f: A => B, g: B => C, h: C => D*, then*h ∘ (g ∘ f) = (h ∘ g) ∘ f*

Lawvere:

One very useful sort of set is a ‘singleton’ set, a set with exactly one element. Fix one of these, say

`{me}`

, and call this set ’1‘.

Definition: Apointof a set X is an arrows1 => X. … (IfAis some familiar set, an arrow fromAtoXis called an ’A-element’ ofX; thus ’1-elements’ are points.) Since a point is an arrow, we can compose it with another arrow, and get a point again.

If I understand what’s going on, it seems like Lawvere is redefining the concept of the element as a special case of arrow. Another name for singleton is unit set, and in Scala it is `(): Unit`

. So it’s analogous to saying that values are sugar for `Unit => X`

.

```
scala> val johnPoint: Unit => Person = { case () => John }
johnPoint: Unit => Person = $$Lambda$4905/135825525@2fdaad68
scala> val johnFav = favoriteBreakfast compose johnPoint
johnFav: Unit => Breakfast = scala.Function1$$Lambda$2547/1273946735@2396b19b
scala> johnFav(())
res1: Breakfast = Eggs
```

Session 2 and 3 contain nice review of Article I, so you should read them if you own the book.

herding cats — Basic category theory