Links

This is a collection of links I found to be useful when learning about various theoretical computer science and abstract algebra concepts. It’s largely made for myself, but I’ve structured it so that anybody should be able to make use of it. They assume a basic knowledge of functional programming (if you’re at Edi, that implies you’ve taken inf1a-fp at least), but I’ve tried to note where extra learning is necessary.

Category Theory

You’ve probably heard that Haskell is based around Category Theory. CT is a really abstract mathematical field, and isn’t usually taught at undergrad. However, the underlying concepts are often deceptively simple, and with the right frame of mind, you could easily learn enough to massively improve your Haskell skills.

Bartosz Milewski’s Category Theory for Programmers: This was my introduction to Category Theory, and it’s an excellent followup from the FP course. Examples are given throughout in diagrams, Haskell and C++ syntax to best display the concepts being discussed, and it gives excellent insights as to why these concepts matter for programming. Be sure to watch the accompanying video lectures, and if you like it enough, the hardback version is only £25 including shipping (and is absolutely worth it)! I’d recommend checking out his other blog posts too.
An Invitation to Applied Category Theory (by David Spivak): This book uses more examples from mathematics proper, and while technically an introduction to Applied Category Theory, which is a slightly different field, it serves as a very good motivator and gives great intuition if you already have some experience with abstract algebra.
The Catsters: A small, old youtube channel with worryingly energetic hosts. They give really good, if absurdly fast, explanations of most common topics in category theory. I watched these alongside the above video lectures, and it’s really interesting to see the same concepts approached from a pure theoretical-algebra standpoint.
ncatlab: Once you’re deep, and I mean deep into your category theory (I am not yet at this point), you might find this wiki helpful. It’s a very comprehensive manual to many different parts of abstract algebra, mainly focusing on category theory. Be sure to check out their CT Textbook List (you should be able to find most of these through DiscoverEd or your uni’s research portal, and if not, `reverse “negbil”`)

Lambda Calculus and other parts of Theoretical Computer Science

This section focuses mainly on topics related to the theory of computation. Prior knowledge is basic/intermediate Haskell and logic notation (again, if you passed inf1a at Edinburgh you’re ready).

The `fix`point combinator: This blog post is an incredible explanation of exactly why the fixpoint combinator works, and how it’s implemented in Haskell. It’s a really good introduction to the various different combinators and recursion schemes that allow you to write really terse programs.
Catamorphisms and F-algebras: Similar to the above blog post, this article is an excellent walk through the motivating ideas behind catamorphisms, using a simple numeric expression evaluator as an example. Reading the fixpoint combinator article may help as this post relies on it (although it gives a slightly less detailed explanation to what it does). Code examples are in both Haskell and Scala.

Monads etc.

Ah, monads. The “just trust me dude” of Haskell, at least from the perspective of a beginner. While I recommend learning general category theory first (it’s what I did!) to fully understand them, it’s handy to know how to use them regardless.

Nottingham course on advanced FP: A series of youtube lectures from the University of Nottingham. If you’ve taken inf1a, I would skip the first five lectures here. They’re a great introduction to functors, applicatives and monads from a practical viewpoint.
The Extended Functor Family - George Wilson [21:56]: An excellent introduction to contravariant functors and profunctors, and their use in Haskell. The linked talks/posts at the end are worth checking out too.

Practical Haskell

As much as I like messing around with abstract concepts, it’s occasionally useful to compute things. After all, what use is a computer if you can’t use it to compute things? The following is a collection of links that should help you in writing more idiomatic Haskell and in using GHC to your full advantage.

Tweag/Richard Eisenberg’s videos: Richard has made an extensive series of videos that cover many different areas in Haskell programming. Richard (at the time of writing) works on GHC’s internals, and has a wealth of knowledge which he shares through these videos.
forall Type Quantifier: Short github post that explains how the `forall` keyword interacts with various different GHC extensions. Contains some nifty tricks for working with type-level programming.
GHC Wiki article on Code Generation: An article on how GHC code gets generated, with an explanation of the Spineless Tagless G-machine intermediate representation. Critical reading if you wish to understand the execution model used by GHC.

Type Theory

Type theory is a very active area of interest in theoretical CS as of the time of writing. It plays a fundamental role in the theory of computation and can allow us to reason about code in a very rigorous manner (something that is very useful for compiler devs). It’s a natural link between Haskell, Category Theory and the various logic algebras.

Homotopy Type Theory: HoTT is a fairly recent development/discovery, which links together the fields of homotopy and type theory. It can be used as a base set of axioms from which one can derive all of modern mathematics, much like with set theory with ZFC. The advantage is that it maps much more cleanly to programming than set theory + ZFC does, a fact that has led to it being used in most modern theorem provers such as Agda and Coq.

Topology, Homotopy and Homology

These closely related fields of maths deal mainly with “manifolds”, or continuous surfaces in spaces. There’s many interesting results that can be widely applied to different fields (as exemplified by HoTT above). These links should serve as a decent primer into the various topics, and provide good intuition for further study.

Clark Barwick: YouTube channel of a lecturer from UoE! He’s an excellent educator and has a very good (and rigorous) series of lectures on introductory topology.
An Antipodal Abstract Topoligist: The YouTube channel of an aussie Ph.D student studying algebraic topology and homotopy.

Combinatorial Game Theory

HACKENBUSH: a window to a new world of math: An excellent video that walks you through the ideas that give rise to Combinatorial Game Theory, and later on to the surreal number system discovered by John Conway. You can use some of the knowledge here to cheat in gambling ;)
Surreal Numbers (by Donald Knuth): A different take on introducing the surreal numbers, this novella features a couple who discover a clay tablet with strange markings while on an extended retreat…

Fun Things

Sheaves in Minecraft: A blog post explaining categorical sheaves applied to minecraft chunk loading and nearby entity checking.