Invariants Aren't Everything! Property-Based Testing (PBT) in practice...

Virtual  BEAM Meetup GMT - Erlang, Elixir and more!
Virtual BEAM Meetup GMT - Erlang, Elixir and more!
Public group

Erlang Solutions

Suite 2.6, The Loom, Gower's Walk, E1 8PY · London

How to find us

We are a 5 minute walk from Aldgate East and a 10 minute walk from Shadwell Overground station.

Location image of event venue


London BEAMERs...

It's our last meetup of 2018! Come down to the Erlang Solutions offices for a final send-off and another great talk, this time with Joseph Yiasemides.

If you wish to do a lightning talk at this meetup or want to speak at any future meetups in 2019, do get in touch with me. We're always looking for juicy topics spanning across the BEAM.

18:30 - Doors open + beer + pizza
19:00 - Introductions
19:15 - Invariants Aren't Everything!
20:00 - Finish!
20:00 - ESL Pub Ritual

#About the Talk

"Invariants Aren't Everything!" is about stateless Property-Based Testing (PBT). A short introduction, followed by my favourite examples, and few words about PBT in practice.

What is PBT?
What do simple examples look like?
How/when does it get tricky?
PBT is compelling because it can offer more exhaustive testing and test-cases (or properties) are much more like a specification and documentation than traditional approaches. It's more fun than too!

In this talk, we'll open with some rationale, followed by an introduction to Elixir's StreamData, some real-life case-studies, and conclude with how to approach PBT in practice.

# About the Speaker

Joseph writes Erlang, Elixir, & Go. He is an Engineer at Monzo where he writes Go after nearly four years of Erlang and Elixir.

He has spoken about writing software in Erlang/Elixir and likes to teach ( Joseph organises Elixir Kraków ( along with Erlang Solutions and co-organises a conference in Berlin (

In May 2017 he finished sabbatical leave at the Recurse Center ( Find him at and a summary at

# About the Company

Monzo are building a new kind of bank.

A bank that lives on your smartphone and built for the way you live today. By solving your problems, treating you fairly and being totally transparent, we believe we can make banking better.

Since we started we’ve been busy building the best current account in the world. We now have more than one million customers using our debit cards to manage their money and spend around the world.