A fintech company called Mercury is running roughly 2 million lines of Haskell code in production. The company's engineering team published a post this week detailing their experience with the purely functional programming language, which is rarely used at this scale in industry. Haskell is known for its mathematical rigor, type system, and steep learning curve.
The post appears to be a defense of this unconventional choice, explaining the tradeoffs the team has encountered over years of development. Most startups Mercury's size rely on more mainstream languages like Python, JavaScript, or Go.
Whether the Haskell bet has paid off is still an open question, but the company has apparently grown to millions of users. The post joins a small canon of engineering writeups from companies that chose unorthodox technology stacks — with varying degrees of regret.