In this blog post we will explore the consequences of postulating 0 = 1 in an algebraic structure with two binary operations (S, +, 0) and (S, ⋅, 1). Such *united monoids* have a few interesting properties, which are not immediately obvious. For example, we will see that the axiom 0 = 1 is equivalent to a seemingly less extravagant axiom ab = ab + a, which will send us tumbling down the rabbit hole of algebraic graphs and topology.

# Tag Archives: algebra

# Old graphs from new types

After I got back from the holiday that I planned in the previous blog post, I spent the whole January playing with the algebra of graphs and trying to find interesting and useful ways of constructing graphs, focusing on writing polymorphic code that can manipulate graph expressions without turning them into concrete data structures. I’ve put together a small toolbox containing a few quirky types, which I’d like to share with you in this blog post. If you are not familiar with the algebra of graphs, please read the introductory blog post first.

**Update:** This series of blog posts was published as a functional pearl at the Haskell Symposium 2017.

# Graphs in disguise: from todo lists to build systems

In this blog post we will look at an example of using the algebra of graphs for manipulating *sequences* of items, which at first sight might not look like graphs, but are actually *dependency graphs* in disguise. We will develop a tiny DSL for composing todo lists on top of the alga library and will show how it can be used for planning a holiday and, on a more serious note, for writing software build systems. This blog post will partially answer the question about possible applications of the algebra of graphs that was asked in this reddit discussion.

**Update:** This series of blog posts was published as a functional pearl at the Haskell Symposium 2017.

Continue reading Graphs in disguise: from todo lists to build systems

# Graphs à la carte

I received an overwhelming response to the introductory blog post about the algebra of graphs; thank you all for your remarks, questions and suggestions! In the second part of the series I will show that the algebra is not restricted only to directed graphs, but can be extended to axiomatically represent undirected graphs, reachability and dependency graphs (i.e. preorders and partial orders), their various combinations, and even hypergraphs.

**Update:** This series of blog posts was published as a functional pearl at the Haskell Symposium 2017.

# An algebra of graphs

Graph theory is my favourite topic in mathematics and computing science and in this blog post I’ll introduce *an algebra of graphs* that I’ve been working on for a while. The algebra has become my go-to tool for manipulating graphs and I hope you will find it useful too.

The roots of this work can be traced back to my CONCUR’09 conference submission that was rightly rejected. I subsequently published a few application-specific papers gradually improving my understanding of the algebra. The most comprehensive description can be found in ACM TECS (a preprint is available here). Here I’ll give a general introduction to the simplest version of the algebra of graphs and show how it can be implemented in Haskell.

**Update:** This series of blog posts was published as a functional pearl at the Haskell Symposium 2017.