A Couple Million Lines of Haskell: Production Engineering at Mercury
Summary
A couple million lines of Haskell are run in production at Mercury, a fintech with hundreds of thousands of businesses and multi-billion dollar transaction volume. The article shares practical reliability patterns, including durable workflows with Temporal, boundary-focused design, and instrumentation that surfaces operational insights, while advocating pragmatic tradeoffs over theoretical purity. It also discusses ecosystem stewardship, library design, and how to onboard and enable a team to maintain complex systems at speed.