May 2, 2026
Banking on nerd magic
A Couple Million Lines of Haskell: Production Engineering at Mercury
The internet can’t decide if this is genius, luck, or just really good management
TLDR: Mercury says it runs huge parts of its banking platform on about 2 million lines of Haskell and has handled massive growth without melting down. Commenters are split between calling that a triumph for safer software design and saying the real reason is simpler: Mercury may just be an unusually well-run company.
Mercury just casually revealed it runs a roughly 2 million-line codebase in Haskell — a famously brainy programming language — while handling eye-watering amounts of money for more than 300,000 businesses. That includes surviving the Silicon Valley Bank chaos, when $2 billion in deposits reportedly flooded in over five days. On paper, commenters felt like this should read like a horror story. Instead, the mood was more like: wait… this actually works?
And that’s where the comment section turned into the real show. One camp was practically cheering from the balcony, arguing that Haskell’s superpower is forcing engineers to make dangerous money-moving steps harder to mess up. A happy Mercury customer even floated the spicy theory that picking Haskell may have been part of the company’s secret sauce all along. But the skeptics were not having a full fairy tale ending. One former Haskell user dropped the bluntest reality check: after years in the language, they still felt twice as productive in Rust, another safer systems language. Ouch.
Then came the classic tech-world subplot: is the tool actually special, or is Mercury just well run? One commenter with a friend at the company basically said the quiet part out loud — maybe great culture deserves more credit than the language itself. Even the side chatter had drama: readers peeking at functional programming through OCaml were intrigued but torn, like they’d just discovered a glamorous new lifestyle with a terrifying learning curve. In other words, the internet’s verdict is deliciously messy: Haskell might be brilliant, but nobody agrees whether it’s magic or just disciplined adults doing their jobs.
Key Points
- •The article launches the Haskell Blog’s “Haskellers from the trenches” series focused on real-world production experience.
- •Ian Duncan says Mercury operates a roughly 2 million-line Haskell codebase, excluding comments.
- •Mercury is described as serving more than 300,000 businesses, processing $248 billion in transaction volume in 2025, and generating $650 million in annualized revenue.
- •The company had around 1,500 employees at the time of writing and was pursuing a U.S. national bank charter from the OCC.
- •Duncan says Mercury’s Haskell systems remained effective through hypergrowth, regulatory examinations, and the SVB crisis, when the company received $2 billion in new deposits over five days.