Proof System Case Study

This blogpost is the second part of a trilogy of formal verification on computer systems. Here we will take a quick tour of what formal verification means, with a particular focus on verifying systems software and examples of verifying some file system. If you have not read the previous post on system verification, feel free to jump there first for an introduction! Case studies CompCert CompCert1 is probably one of the first and most famous verified compilers (and perhaps also computer systems)....

March 30, 2024 路 16 min 路 Shuntian Liu

System Proof 101

This blogpost is the first part of a trilogy of formal verification on computer systems. Here we will take a quick tour of what formal verification means, with a particular focus on verifying systems software and examples of verifying some file system. What do you mean by formally verified systems? Sometimes we hear people say we have built this formally verified system that is provably correct, and does all sorts of amazing things without bugs....

March 23, 2024 路 13 min 路 Shuntian Liu

Core Dump of S-REPLS 14

This is a very high level summary of the experience attending S-REPLS 14. Feel free to look at the respective website of the speakers for more details. To keep this blog concise, I thought I would just use introductory examples where possible to illustrate the idea rather than going into the actual theory behind them. Graded types and algebraic effects Graded type system is based on the idea of linear types....

March 8, 2024 路 6 min 路 Shuntian Liu

Making sense of adjunctions

When I first encountered the concept of an adjunction, I got quite confused as to what it is, and why it is useful: Just how on earth is a left adjoint of a functor? What鈥檚 the matter of a free and forgetful functor, why is free left to forgetful but not the other way round. That鈥檚 where this blog comes from. I hope this blogpost can help demystify adjunction for you a little bit....

November 3, 2023 路 4 min 路 Shuntian Liu

Monad in programming and category theory

This article is very much my attempt to understand monad as a design pattern in programming languages and why on earth it is useful. There are tons of monad tutorials online, but I found relatively few ones argue for the usefulness of a monad by having side by side code examples that achieve similar functionalties. If you found one, please do let me know! I try to draw connections between monads鈥 original mathematical definition and its actual usage so that this mysterious concept does not come out of the blue....

October 24, 2023 路 11 min 路 Shuntian Liu

Hypermnesia: Eventual Consistency in Mnesia

Motivation Mnesia is a soft real-time embedded Database Management System written for Erlang, a programming language that powers the infrastructures of various organisations such as Cisco, Ericsson and the NHS. Due to Mnesia鈥檚 tight integration with Erlang, it is also impactful in open source projects such as RabbitMQ and ejabberd. However, the development of Mnesia has remained stagnant for years, resulting in the lack of features such as automatic conflict resolution: Mnesia leaves the handling of conflicts after network partitions entirely to the developer....

June 6, 2023 路 9 min 路 Shuntian Liu

Erlog: A Distributed Datalog Engine

Introduction In this blogpost we build a distributed datalog engine that can process datalog queries such as the one below in a distributed fashion. The key idea mimics the usual dataflow programming idea such as MapReduce where we shard our data and create a dataflow-graph to specify the computation we want. link("a","b"). link("b","c"). link("c","c"). link("c","d"). link("d","e"). link("e","f"). reachable(X, Y) :- link(X, Y). reachable(X, Y) :- link(X, Z), reachable(Z, Y). We will build our engine from first principle by looking at how we perform single node evaluation of datalog queries, and then extend it to multiple nodes....

April 16, 2023 路 7 min 路 Shuntian Liu

Equivalence of proof techniques in proving the fixpoint properties

Denotational Semantics is a unique course offered by the Computer Lab at UoC. It can be intimidating at first glance but at the same time bring you lots of fun and frustration at the same time. In this blog post we will look at three proof techniques in domain theory and investigate the connections between them. In this blog we look at three techniques that can be used to prove the fixpoint of a function, namely, Tarski鈥檚 fixpoint theorem, lfp1 + lfp2 and Scott induction....

June 30, 2022 路 8 min 路 Shuntian Liu