# Functoriality

Last time we worked through some basic examples of universal properties, specifically singling out quotients, products, and coproducts. There are many many more universal properties that we will mention as we encounter them, but there is one crucial topic in category theory that we have only hinted at: functoriality.

As we’ve repeatedly stressed, the meat of category theory is in the morphisms. One natural question one might ask is, what notion of morphism is there between categories themselves? Indeed, the most straightforward way to see category theoretic concepts in classical mathematics is in a clever choice of functor. For example (and this example isn’t necessary for the rest of the article) one can “associate” to each topological space a group, called the homology group, in such a way that continuous functions on topological spaces translate to group homomorphisms. Moreover, this translation is functorial in the following sense: the group homomorphism associated to a composition is the composition of the associated group homomorphisms. If we denote the association by a subscripted asterisk, then we get the following common formula.

$\displaystyle (fg)_* = f_* g_*$

This is the crucial property that maintains the structure of morphisms. Again, this should reinforce the idea that the crucial ingredient of every definition in category theory is its effect on morphisms.

## Functors: a Definition

In complete generality, a functor is a mapping between two categories which preserves the structure of morphisms. Formally,

Definition: Let $\mathbf{C}, \mathbf{D}$ be categories. A functor $\mathscr{F}$ consists of two parts:

• For each object $C \in \mathbf{C}$ an associated object $\mathscr{F}(C) \in \mathbf{D}$.
• For each morphism $f \in \textup{Hom}_{\mathbf{C}}(A,B)$ a corresponding morphism $\mathscr{F}(f) \in \textup{Hom}_{\mathbf{D}}(\mathscr{F}(A), \mathscr{F}(B))$. Specifically, for each $A,B$ we have a set-function $\textup{Hom}_{\mathbf{C}}(A,B) \to \textup{Hom}_{\mathbf{C}}(\mathscr{F}(A), \mathscr{F}(B))$.

There are two properties that a functor needs to satisfy to “preserve structure.” The first is that the identity morphisms are preserved for every object; that is, $\mathscr{F}(1_A) = 1_{\mathscr{F}(A)}$ for every object $A$. Second, composition must be preserved. That is, if $f \in \textup{Hom}_{\mathbf{C}}(A,B)$ and $g \in \textup{Hom}_{\mathbf{C}}(B,C)$, we have

$\displaystyle \mathscr{F}(gf) = \mathscr{F}(g) \mathscr{F}(f)$

We often denote a functor as we would a function $\mathscr{F}: \mathbf{C} \to \mathbf{D}$, and use the function application notation as if everything were with sets.

Let’s look at a few simple examples.

Let $\mathbf{FiniteSet}$ be the poset category of finite sets with subsets as morphisms, and let $\mathbf{Int}$ be the category whose objects are integers where there is a unique morphism from $x \to y$ if $x \leq y$. Then the size function is a functor $\mathbf{Set} \to \mathbf{Int}$. Continuing with $\mathbf{Int}$, remember that $\mathbf{Int}$ forms a group under addition (also known as $\mathbb{Z}$). And so by its very definition any group homomorphism $\mathbb{Z} \to \mathbb{Z}$ is a functor from $\mathbf{Int} \to \mathbf{Int}$. A functor from a category to itself is often called an endofunctor.

There are many examples of functors from the category $\mathbf{Top}$ of topological spaces to the category $\mathbf{Grp}$ of groups. These include some examples we’ve seen on this blog, such as the fundamental group and homology groups.

One trivial example of a functor is called the forgetful functor. Let $\mathbf{C}$ be a category whose objects are sets and whose morphisms are set-maps with additional structure, for example the category of groups. Define a functor $\mathbf{C} \to \mathbf{Set}$ which acts as the identity on both sets and functions. This functor simply “forgets” the structure in $\mathbf{C}$. In the realm of programs and types (this author is thinking Java), one can imagine this as a ‘type-cast’ from String to Object.  In the same vein, one could define an “identity” endofunctor $\mathbf{C} \to \mathbf{C}$ which does absolutely nothing.

One interesting way to think of a functor is as a “realization” of one category inside another. In particular, because the composition structure of $\mathbf{C}$ is preserved by a functor $\mathbf{C} \to \mathbf{D}$, it must be the case that all commutative diagrams are “sent” to commutative diagrams. In addition, isomorphisms are sent to isomorphisms because if $f, g$ are inverses of each other, $1_{\mathscr{F}(A)} = \mathscr{F}(1_A) = \mathscr{F}(gf) = \mathscr{F}(g)\mathscr{F}(f)$ and likewise for the reverse composition. And so if we have a functor $\mathscr{F}$ from a poset category (say, the real numbers with the usual inequality) to some category $\mathbf{C}$, then we can realize the structure of the poset sitting inside of $\mathbf{C}$ (perhaps involving only some of the objects of $\mathbf{C}$). This view comes in handy in a few places we’ll see later in our series on computational topology.

## The Hom Functor

There is a very important and nontrivial example called the “hom functor” which is motivated by the category of vector spaces. We’ll stick to the concrete example of vector spaces, but the generalization to arbitrary categories is straightforward. If the reader knows absolutely nothing about vector spaces, replace “vector space” with “object” and “linear map” with “morphism.” It won’t quite be correct, but it will get the idea across.

To each vector space $V$ one can define a dual vector space of functions $V \to \mathbb{C}$ (or whatever the field of scalars for $V$ is). Following the lead of hom sets, the dual vector space is denoted $\textup{Hom}(V,\mathbb{C})$. Here the morphisms in the set are those from the category of vector spaces (that is, linear maps $V \to \mathbb{C}$). Indeed, this is a vector space: one can add two functions pointwise ($(f+g)(v) = f(v) + g(v)$) and scale them ($(\lambda f)(v) = \lambda f(v)$), and the properties for a vector spaces are trivial to check.

Now the mapping $\textup{Hom}(-, \mathbb{C})$ which takes $V$ and produces $\textup{Hom}(V, \mathbb{C})$ is a functor called the hom functor. But let’s inspect this one more closely. The source category is obviously the category of vector spaces, but what is the target category? The objects are clear: the hom sets $\textup{Hom}(V,\mathbb{C})$ where $V$ is a vector space. The morphisms of the category are particularly awkward. Officially, they are written as

$\displaystyle \textup{Hom}(\textup{Hom}(V, \mathbb{C}), \textup{Hom}(W, \mathbb{C}))$

so a morphism in this category takes as input a linear map $V \to \mathbb{C}$ and produces as output one $W \to \mathbb{C}$. But what are the morphisms in words we can understand? And how can we compose them? Before reading on, think about what a morphism of morphisms should look like.

The morphisms in this category can be thought of as linear maps $W \to V$. More specifically, given a morphism $\varphi: V \to \mathbb{C}$ and a linear map $g: W \to V$, we can construct a linear map $W \to \mathbb{C}$ by composing $\varphi g$.

And so if we apply the $\textup{Hom}$ functor to a morphism $f: W \to V$, we get a morphism in $\textup{Hom}(\textup{Hom}(V, \mathbb{C}), \textup{Hom}(W, \mathbb{C}))$. Let’s denote the application of the hom functor using an asterisk so that $f \mapsto f_*$.

But wait a minute! The mapping here is going in the wrong direction: we took a map in one category going from the $V$ side to the $W$ side, and after applying the functor we got a map going from the $W$ side ($\textup{Hom}(W, \mathbb{C})$) to the $V$ side ($\textup{Hom}(V, \mathbb{C})$). It seems there is no reasonable way to take a map $V \to \mathbb{C}$ and get a map in $W \to \mathbb{C}$ using just $f$, but the other way is obvious. The hom functor “goes backward” in a sense. In other words, the composition property for our “functor” makes the composite $(g f)_*$ to the map taking $\varphi$ to $\varphi g f$. On the other hand, there is no way to compose $g_* f_*$, as they operate on the wrong domains! It must be the other way around:

$\displaystyle (gf)_* = f_* g_*$

We advise the reader to write down the commutative diagram and trace out the compositions to make sure everything works out. But this is a problem, because it makes the hom functor fail the most important requirement. In order to fix this reversal “problem,” we make the following definition:

Definition: A functor $\mathscr{F} : \mathbf{C} \to \mathbf{D}$ is called covariant if it preserves the order of morphism composition, so that $\mathscr{F}(gf) = \mathscr{F}(g) \mathscr{F}(f)$. If it reverses the order, we call it contravariant.

And so the hom functor on vector spaces is a contravariant functor, while all of the other functors we’ve defined in this post are covariant.

There is another way to describe a contravariant functor as a covariant functor which is often used. It involves the idea of an “opposite” category. For any category $\mathbf{C}$ we can define the opposite category $\mathbf{C}^{\textup{op}}$ to be a category with the same objects as $\mathbf{C}$, but with all morphisms reversed. That is, we define

$\displaystyle \textup{Hom}_{\mathbf{C}^{\textup{op}}}(A,B) = \textup{Hom}_{\mathbf{C}}(B,A)$

We leave it to the reader to verify that this is indeed a category. It is also not hard to see that $(\mathbf{C}^{\textup{op}})^{\textup{op}} = \mathbf{C}$. Opposite categories give us a nice recharacterization of a contrvariant functor. Indeed, because composition in opposite categories is reversed, a contravariant functor $\mathbf{C} \to \mathbf{D}$ is just a covariant functor on the opposite category $\mathbf{C}^{\textup{op}} \to \mathbf{D}$. Or equivalently, one $\mathbf{C} \to \mathbf{D}^{\textup{op}}$. More than anything, opposite categories are syntactical sugar. Composition is only reversed artificially to make domains and codomains line up, but the actual composition is the same as in the original category.

## Functors as Types

Before we move on to some code, let’s take a step back and look at the big picture (we’ve certainly plowed through enough details up to this point). The main thesis is that functoriality is a valuable property for an operation to have, but it’s not entirely clear why. Even the brightest of readers can only assume such properties are useful for mathematical analysis. It seems that the question we started this series out with, “what does category theory allow us to do that we couldn’t do before?” still has the answer, “nothing.” More relevantly, the question of what functoriality allows us to do is unclear. Indeed, once again the answer is “nothing.” Rather, functoriality in a computation allows one to analyze the behavior of a program. It gives the programmer a common abstraction in which to frame operations, and ease in proving the correctness of one’s algorithms.

In this light, the best we can do in implementing functors in programs is to give a type definition and examples. And in this author’s opinion this series is quickly becoming boring (all of the computational examples are relatively lame), so we will skip the examples in favor of the next post which will analyze more meaty programming constructs from a categorical viewpoint.

So recall the ML type definition of a category, a tuple of operations for source, target, identity, and composition:

datatype ('object, 'arrow)Category =
category of ('arrow -> 'object) *
('arrow -> 'object) *
('object -> 'arrow) *
('arrow * 'arrow -> 'arrow)

And so a functor consists of the two categories involved (as types), and the mapping on objects, and the mapping on morphisms.

datatype ('cObject, 'cArrow, 'dObject, 'dArrow)Functor =
aFunctor of ('cObject, 'cArrow)Category *
('cObject -> 'dObject) *
('cArrow -> 'dArrow) *
('dObject -> 'dArrow)Category

We encourage the reader who is uncomfortable with these type definitions to experiment with them by implementing some of our simpler examples (say, the size functor from sets to integers). Insofar as the basic definitions go, functors are not all that interesting. They become much more interesting when additional structure is imposed on them, and in the distant future we will see a glimpse of this in the form of adjointness. We hope to get around to analyzing statements like “syntax and semantics are adjoint functors.” For the next post in this series, we will take the three beloved functions of functional programming (map, foldl(r), and filter), and see what their categorical properties are.

Until then!

# Properties of Morphisms

This post is mainly mathematical. We left it out of our introduction to categories for brevity, but we should lay these definitions down and some examples before continuing on to universal properties and doing more computation. The reader should feel free to skip this post and return to it later when the words “isomorphism,” “monomorphism,” and “epimorphism” come up again. Perhaps the most important part of this post is the description of an isomorphism.

## Isomorphisms, Monomorphisms, and Epimorphisms

Perhaps the most important paradigm shift in category theory is the focus on morphisms as the main object of study. In particular, category theory stipulates that the only knowledge one can gain about an object is in how it relates to other objects. Indeed, this is true in nearly all fields of mathematics: in groups we consider all isomorphic groups to be the same. In topology, homeomorphic spaces are not distinguished. The list goes on. The only way to do determine if two objects are “the same” is by finding a morphism with special properties. Barry Mazur gives a more colorful explanation by considering the meaning of the number 5 in his essay, “When is one thing equal to some other thing?” The point is that categories, more than existing to be a “foundation” for all mathematics as a formal system (though people are working to make such a formal system), exist primarily to “capture the essence” of mathematical discourse, as Mazur puts it. A category defines objects and morphisms, but literally all of the structure of a category lies in its morphisms. And so we study them.

The strongest kind of morphism we can consider is an isomorphism. An isomorphism is the way we say two objects in a category are “the same.” We don’t usually care whether two objects are equal, but rather whether some construction is unique up to isomorphism (more on that when we talk of universal properties). The choices made in defining morphisms in a particular category allow us to strengthen or weaken this idea of “sameness.”

Definition: A morphism $f : A \to B$ in a category $\mathbf{C}$ is an isomorphism if there exists a morphism $g: B \to A$ so that both ways to compose $f$ and $g$ give the identity morphisms on the respective objects. That is,

$gf = 1_A$ and $fg = 1_B$.

The most basic (usually obvious, but sometimes very deep) question in approaching a new category is to ask what the isomorphisms are. Let us do this now.

In $\mathbf{Set}$ the morphisms are set-functions, and it is not hard to see that any two sets of equal cardinality have a bijection between them. As all bijections have two-sided inverses, two objects in $\mathbf{Set}$ are isomorphic if and only if they have the same cardinality. For example, all sets of size 10 are isomorphic. This is quite a weak notion of “sameness.” In contrast, there is a wealth of examples of groups of equal cardinality which are not isomorphic (the smallest example has cardinality 4). On the other end of the spectrum, a poset category $\mathbf{Pos}_X$ has no isomorphisms except for the identity morphisms. The poset categories still have useful structure, but (as with objects within a category) the interesting structure is in how a poset category relates to other categories. This will become clearer later when we look at functors, but we just want to dissuade the reader from ruling out poset categories as uninteresting due to a lack of interesting morphisms.

Consider the category $\mathbf{C}$ of ML types with ML functions as morphisms. An isomorphism in this category would be a function which has a two-sided inverse. Can the reader think of such a function?

Let us now move on to other, weaker properties of morphisms.

Definition: A morphism $f: A \to B$ is a monomorphism if for every object $C$ and every pair of morphisms $g,h: C \to A$ the condition $fg = fh$ implies $g = h$.

The reader should parse this notation carefully, but truly think of it in terms of the following commutative diagram:

Whenever this diagram commutes and $f$ is a monomorphism, then we conclude (by definition) that $g=h$. Remember that a diagram commuting just means that all ways to compose morphisms (and arrive at morphisms with matching sources and targets) result in an identical morphism. In this diagram, commuting is the equivalent of claiming that $fg = fh$, since there are only two nontrivial ways to compose.

The idea is that monomorphisms allow one to “cancel” $f$ from the left side of a composition (although, confusingly, this means the cancelled part is on the right hand side of the diagram).

The corresponding property for cancelling on the right is defined identically.

Definition: A morphism $f: A \to B$ is an epimorphism if for every object $C$ and every pair of morphism $g,h: B \to C$ the condition $gf = hf$ implies $g = h$.

Again, the relevant diagram.

Whenever $f$ is an epimorphism and this diagram commutes, we can conclude $g=h$.

Now one of the simplest things one can do when considering a category is to identify the monomorphisms and epimorphisms. Let’s do this for a few important examples.

## Monos and Epis in Various Categories

In the category $\mathbf{Set}$, monomorphisms and epimorphisms correspond to injective and surjective functions, respectively. Lets see why this is the case for monomorphisms. Recall that an injective function $f$ has the property that $f(x) = f(y)$ implies $x=y$. With this property we can show $f$ is a monomorphism because if $f(g(x)) = f(h(x))$ then the injective property gives us immediately that $g(x) = h(x)$. Conversely, if $f$ is a monomorphism and $f(x) = f(y)$, we will construct a set $C$ and two convenient functions $g, h: C \to A$ to help us show that $x=y$. In particular, pick $C$ to be the one point set $\left \{ c \right \}$, and define $g(c) = x, h(c) = y$. Then as functions $fg = fh$. Indeed, there is only one value in the domain, so saying this amounts to saying $f(x) = fg(c) = fh(c) = f(y)$, which we know is true by assumption. By the monomorphism property $g = h$, so $x = g(c) = h(c) = y$.

Now consider epimorphisms. It is clear that a surjective map is an epimorphism, but the converse is a bit trickier. We prove by contraposition. Instead of now picking the “one-point set,” for our $C$, we must choose something which is one element bigger than $B$. In particular, define $g, h : B \to B'$, where $B'$ is $B$ with one additional point $x$ added (which we declare to not already be in $B$). Then if $f$ is not surjective, and there is some $b_0 \in B$ which is missed by $f$, we define $g(b_0) = x$ and $g(b) = b$ otherwise. We can also define $h$ to be the identity on $B$, so that $gf = hf$, but $g \neq h$. So epimorphisms are exactly the surjective set-maps.

There is one additional fact that makes the category of sets well-behaved: a morphism in $\mathbf{Set}$ is an isomorphism if and only if it is both a monomorphism and an epimorphism. Indeed, isomorphisms are set-functions with two-sided inverses (bijections) and we know from classical set theory that bijections are exactly the simultaneous injections and surjections. A warning to the reader: not all categories are like this! We will see in a moment an example of a nontrivial category in which isomorphisms are not the same thing as simultaneous monomorphisms and epimorphisms.

The category $\mathbf{Grp}$ is very similar to $\mathbf{Set}$ in regards to monomorphisms and epimorphisms. The former are simply injective group homomorphisms, while the latter are surjective group homomorphisms. And again, a morphisms is an isomorphism if and only if it is both a monomorphism and an epimorphism. We invite the reader to peruse the details of the argument above and adapt it to the case of groups. In both cases, the hard decision is in choosing $C$ when necessary. For monomorphisms, the “one-point group” does not work because we are constrained to send the identity to the identity in any group homomorphism. The fortuitous reader will avert their eyes and think about which group would work, and otherwise we suggest trying $C = \mathbb{Z}$. After completing the proof, the reader will see that the trick is to find a $C$ for which only one “choice” can be made. For epimorphisms, the required $C$ is a bit more complex, but we invite the reader to attempt a proof to see the difficulties involved.

Why do these categories have the same properties but they are acquired in such different ways? It turns out that although these proofs seem different in execution, they are the same in nature, and they follow from properties of the category as a whole. In particular, the “one-point object” (a singleton set for $\mathbf{Set}$ and $\mathbb{Z}$ for $\mathbf{Grp}$) is more categorically defined as the “free object on one generator.” We will discuss this more when we get to universal properties, but a “free object on $n$ generators” is roughly speaking an object $A$ for which any morphism with source $A$ must make exactly $n$ “choices” in its definition. With sets that means $n$ choices for the images of elements, for groups that means $n$ consistent choices for images of group elements. On the epimorphism side, the construction is a sort of “disjoint union object” which is correctly termed a “coproduct.” But  momentarily putting aside all of this new and confusing terminology, let us see some more examples of morphisms in various categories.

Our recent primer on rings was well-timed, because the category $\mathbf{Ring}$ of rings (with identity) is an example of a not-so-well-behaved category. As with sets and groups, we do have that monomorphisms are equivalent to injective ring homomorphisms, but the argument is trickier than it was for groups. It is not obvious which “convenient” object $C$ to choose here, since maps $\mathbb{Z} \to R$ yield no choices: 1 maps to 1, 0 maps to 0, and the properties of a ring homomorphism determine everything else (in fact, the abelian group structure and the fact that units are preserved is enough). This makes $\mathbb{Z}$ into what’s called an “initial object” in $\mathbf{Ring}$; more on that when we study universal properties. In fact, we invite the reader to return to this post after we talk about the universal property of polynomial rings. It turns out that $\mathbb{Z}[x]$ is a suitable choice for $C$, and the “choice” made is where to send the indeterminate $x$.

On the other hand, things go awry when trying to apply analogous arguments to epimorphisms. While it is true that every surjective ring homomorphism is an epimorphism (it is already an epimorphism in $\mathbf{Set}$, and the argument there applies), there are ring epimorphisms which are not surjections! Consider the inclusion map of rings $i : \mathbb{Z} \to \mathbb{Q}$. The map $i$ is not surjective, but it is an epimorphism. Suppose $g, h : \mathbb{Q} \to R$ are two parallel ring morphisms, and they agree on $\mathbb{Z}$ (they will always do so, since there is only one ring homomorphism $\mathbb{Z} \to R$). Then $g,h$ must also agree on $\mathbb{Q}$, because if $p,q \in \mathbb{Z}$ with $q \neq 0$, then

$\displaystyle g(p/q) = g(p)g(q^{-1}) = g(p)g(q)^{-1} = h(p)h(q)^{-1} = h(p/q)$

Because the map above is also an injection, the category of rings is a very naturally occurring example of a category which has morphisms that are both epimorphisms and monomorphisms, but not isomorphisms.

There are instances in which monomorphisms and epimorphisms are trivial. Take, for instance any poset category. There is at most one morphism between any two objects, and so the conditions for an epimorphism and monomorphism vacuously hold. This is an extreme example of a time when simultaneous monomorphisms and epimorphisms are not the same thing as isomorphisms! The only isomorphisms in a poset category are the identity morphisms.