I have touched on the subject of category theory in the past, motivated partly by my enthusiasm of working with a mathematical framework that is so simple yet so powerful, and partly by the usefulness of categorical models in software. This essay draws from previous posts on the old blog and from my previous experience with the subject, and I am posting it hoping that it will represent a starting point for other interesting writings.
I am fairly sure that most of the work in this post is in no way original; that is, there are other publications where categorical approaches to software modeling, and matters pertaining to category theory in general, are already explained, and most probably better than they are here. For example Steve Awodey has an excellent book providing an indepth mathematical exploration of category theory^{1}; Robert Harper discusses on the (major) impact of categories on type theory^{2}, computation and computer programs; Brent Yorgey makes a really good overview of the relation between categories and Haskell type classes^{3}. There is much more material on the web and in books, and while you're not required to peruse it in order to read this, I certainly encourage you to have a look.
Category theory: introduction, definitions
While mathematics is an exact "science"^{4}, its methodology differs from that of, say, physics or biology, which have fundamentally different objectives, although the latter very often make use of mathematical means to make sense of the world. Instead, it'd be fairer to find the origins of mathematics in philosophy, which discusses concepts, or ideas, or essences, rather than objective experience.
For the last century or so all mathematicians and philosophers have been in agreement on the fact that mathematics must have a philosophical and logical basis. For quite a long time, that basis was, and to some degree still is, set theory; the limitations of naïve set theory^{5} have been thoroughly explored in the 20th century and the need for a "more complete" theory of mathematics was and is still felt by mathematicians. Even though nowadays we prefer using computers to solve problems requiring mathematics, this has nothing to do with computers themselves, although it has everything to do with the theory of computation.
Category theory was for a while believed to be this new, previously missing, foundation of mathematics. This doesn't seem to be the consensus among mathematicians anymore, but despite that, categories still play an important role in defining the new framework^{6}. Also note that in Harper's Holy Trinity, the categorical approach defines the socalled "universe of reasoning" in terms of mappings and structures, a view that is very much in sync with that of software architecture and software engineering.
What is then a category? According to the definition, any category necessarily consists of the following three: objects, morphisms (or arrows) and a composition law bearing welldefined properties.
Intuitively, any mathematical object could constitute an object in a category. Category theory classes often provide sets as the most intuitive example of objects; that is, any set is an object in the category of sets. Note that the categorical view doesn't necessarily care about how an object is defined, but rather about its properties in relation to the given category's arrows and the overall category's structure. Formally, given a category $mathcal{C}$, we can denote its set of objects as $text{Ob}(mathcal{C})$.
Also intuitively, any mapping between two objects could constitute an arrow in a category. The canonical example here is represented by functions, i.e. mappings between sets, but many other binary relations fit this description. An interesting example is that of partiallyordered sets. Formally, for a given category $mathcal{C}$ and two objects $A, B in text{Ob}({mathcal{C}})$, $text{Hom}_{mathcal{C}}(A, B)$ denotes the set of arrows from $A$ to $B$; however, the function notation $forall f, f : A rightarrow B$ is also often used.
Finally, composition is denoted using the "$circ$" operator or juxtaposition, and it represents a binary operation on two arrows in a category. Intuitively, one may see composition similarly to function composition: given a category $mathcal{C}$, three arbitrary objects $A, B, C in text{Ob}(mathcal{C})$ two arrows $f in text{Hom}{mathcal{C}}(A, B)$ and $g in text{Hom}{mathcal{C}}(B, C)$, then there exists an arrow $h in text{Hom}_{mathcal{C}}(A, C)$, where $h equiv g circ f$. A good intuition is that the "path" from $A$ to $C$ could be represented as another arrow in $mathcal{C}$.
Composition is associative; that is, given $f : A rightarrow B$, $g : B rightarrow C$ and $h : C rightarrow D$, then:
$(h circ g) circ f equiv h circ (g circ f)$
Intuitively, this tells us that composition "paths" are unique and that the order of application of composition doesn't matter.
Additionally, every object has an associated identity arrow; $forall A in text{Ob}(mathcal{C})$, then:
$exists 1_{A} in text{Hom}_{mathcal{C}}(A, A)$
which is invariant under composition. That is, $forall A, B in text{Ob}(mathcal{C})$, $forall f : A rightarrow B$,
$1_{B} circ f = f circ 1_{A} = f$.
These are all the elements defining a category. Intuitively, they naturally apply to sets and functions, giving rise to the category of sets, denoted Set: all sets are objects and all functions are arrows; functions may be composed associatively and every set has an identity function.
There are other examples of categories in the world of mathematics and computer science, which I advise you to explore on your own. The concepts of functors and natural transformations are also fundamental to category theory, but I will skip them for now due to lack of space. I will instead leave the remainder of this essay to a more interesting example and attempt to model version control systems as categories. This, to my knowledge, provides a new perspective on the subject, so I'm hoping it will prove to be interesting and maybe even a bit challenging.
Example: The Git category
Those of you who are coming from software engineering should be familiar with version control systems (VCS). VCS have been devised as collaborative tools between programmers who want to share code and have a means to keep track of changes in the code base of some particular piece of software. They remain crucial to software development, although nowadays technical people are using them to maintain all sorts of other, usually textbased projects such as papers or web sites. The popularity of GitHub has also drawn less technical people to this world of programming, so everyone and their dog can keep a public project nowadays.
One particular case of version control system are distributed version control systems (DVCS). All VCS maintain a repository where code is stored and where the entire history of a project is maintained as a set of commits. In particular, DVCS state that every contributor to a project has their own copy of the repository offline, and they can keep their changes in sync with a remote repository by pushing their local copy. We're not particularly interested in this aspect at the moment, but it's interesting to note that our categorical model should also apply to distributed systems.
Let's take the Linux kernel as an example: Linux is kept under version control using Git. It has multiple branches and forks (remote copies of a repository) and the code base of the kernel changes as new commits are added to the remote repository. The code is therefore in a particular state at a given point in time and its state changes with each commit, usually by applying a patch, or a diff, which holds as information the "difference", in lines of code (LOC) added or deleted, between the old state and the new one. So far, so good.
Given that there are many possible modifications that could arise from a given state, the code might diverge into multiple branches which will later need to be merged or rebased. I won't go into detail regarding these concepts, but they should nevertheless prove to be interesting from a categorical point of view. For now we assume that the repository goes through a list (as opposed to a graph) of states as it changes, each change, or set of changes, being marked by a diff.
Intuitively, it should be fairly obvious that repository states can be viewed as objects in a category: assuming for example that the commit hashes in a Git repository are unique^{7}, each hash marks the identifier of a "version" of the code in that repository. If we wanted to prove an isomorphism between code revisions and mathematical sets, we would intuitively see each revision as a set comprising arbitrary strings, i.e. the actual code.
Also intuitively enough, we could look at commit diffs in the same way we look at a categorical arrow, each diff providing a mapping between two states in the same way a function provides a mapping between two sets. For example, in git, this difference is provided in terms of lines added and removed from a certain code base^{8}.
This representation gives rise to a small complication. In practice there is usually more than one way to go from one revision to another. Given for example a certain code base upon which various modifications have been made, the developer may choose to either create a big commit containing all the changes, or various smaller commits, each comprising a unit of their work^{9}. For the sake of making our model simpler, we can define a "minimal commit" unit, represented by the removal or addition of a certain line in a code base.
We also note that commit diffs are composable most of the time^{10}. Given two successive commits, one may represent them as a single commit, e.g. by squashing them in Git, or by simply applying gitdiff between two commit hashes. This is fortunate for us, as it allows us to represent a possible commit as a chain of compositions of multiple "minimal commits". The possible compositions are conceptually very similar to a Hasse diagram, which, interestingly enough, provides an analogy between commits and posets.
Finally, we can look at the empty diff, i.e. the diff with no additions and no removals, as the canonical representation of an identity arrow. Git doesn't actually allow empty commits, given that the new generated repository state would be (needlessly) identical to the old one, but we can model them anyway, as we know for sure that a gitdiff between an arbitrary commit hash and itself will always be empty.
From all the above emerges the Git category. The usefulness of this representation is a whole different problem, but I am guessing that various operations, e.g. merging, rebasing, defining submodules or other useful operations that haven't been yet designed into state of the art DVCS, can be represented as monadic actions. This of course would involve answering deeper questions, such as what is an endofunctor in the Git category, but for the sake of brevity we will stop this train of thought here.
Exercise: The Blockchain category, analogy with DVCS
The blockchain is a database design coming from Bitcoin^{11}. Although the idea was conceived specially for implementing a new form of representing money, its uses may theoretically go beyond that, into other distributed systems and applications.
Simply put, the blockchain is a distributed chain of transactions. It is distributed in the sense that all the participants, e.g. in the Bitcoin system, should hold a copy of it. It contains transactions, that is, statements that a certain piece of information, e.g. money in Bitcoin's case, is transferred from one participant to the other, in the broad sense that a "participant" is the same thing as an account. Transactions, and more specifically parent transactions, are identified by their hashes.
There is an immediate analogy between VCS and blockchains. The categorical likeness of the two follows from that directly: in both cases, system states are objects and transitions between states are arrows; in both cases, arrow composition is representable and both allow the existence of a conceptual identity transaction. This shows that the architectural differences between the two are very few.
The design and implementation differences are in the details. Transactions are inserted in the blockchain by a consensus protocol; in Git, the policy for insertion is determined by the computing systems where the bare repositories are stored. Git transactions are independent of their content, containing anything from source code to binary data; blockchain transactions have a more restrictive format, depending on their application.
In theory one could generalize databases^{12} using categories. These examples show that category theory is or could be, among other mathematical abstractions, very useful to defining software both architecturally and at the implementation level. Given that software developers are faced with the pain of building robust and/or resilient systems in a context where software verification and specification doesn't scale, such abstractions are (arguably) needed now more than ever.

Awodey, Steve. Category theory. Vol. 49. Oxford University Press, 2006. ↩

In the broadest sense of the word "science", that coming from its Latin root, where its meaning overlaps with that of "knowledge". ↩

Russell's paradox, for example. ↩

Univalent Foundations Program. Homotopy Type Theory: Univalent Foundations of Mathematics. Univalent Foundations, 2013. ↩

Which, by the way, they aren't. Fortunately the basic properties of the SHA1 hash make collisions highly improbable, and in theory one could devise a (D)VCS commit addressing scheme that completely avoids this problem. ↩

I am deliberately avoiding to see repositories as collections of files, as this would make our definition a lot more complicated. ↩

This is not an easy problem, as seen in Commit Often, Perfect Later, Publish Once. ↩

There is an interesting mention to be made here regarding merge conflicts. In mathematical terms, this only tells us that the "minimal diff" doesn't provide a full mesh of mappings between repository states. ↩

Nakamoto, Satoshi. "Bitcoin: A peertopeer electronic cash system." Consulted 1.2012 (2008): 28. ↩

Transactions are of particular interest to us in this post, but other aspects such as relational algebra could be seen as a particular case of categories. See "Category Theory as a Unifying Database Formalism" for more details. ↩