A Couple Million Lines of Haskell: Production Engineering at Mercury
Mercury, a fintech company serving over 300,000 businesses, runs a production system built on approximately 2 million lines of Haskell code. Despite conventional skepticism about large-scale Haskell use with a team of mostly Haskell-naive engineers, the system has proven reliable through rapid growth and financial crises. The company emphasizes operational pragmatism, focusing on system resilience and maintainability rather than purely theoretical language benefits.
- ▪Mercury processes banking services for over 300,000 businesses and handled $248 billion in transaction volume in 2025.
- ▪The company's codebase consists of roughly 2 million lines of Haskell, maintained by engineers many of whom learned Haskell on the job.
- ▪Mercury has maintained system reliability through hypergrowth, the SVB crisis, and regulatory scrutiny using disciplined engineering practices.
- ▪The company prioritizes system resilience and operational knowledge embedded in APIs to manage complexity and ensure continuity.
- ▪Mercury is in the process of obtaining a national bank charter from the U.S. Office of the Comptroller of the Currency (OCC).
Opening excerpt (first ~120 words) tap to expand
A Couple Million Lines of Haskell: Production Engineering at Mercury Ian Duncan March 30, 2026 [Haskellers from the trenches] #Production #Mercury The editors of the Haskell Blog are happy to announce a new series of articles called "Haskellers from the trenches", where we invite experienced engineers to talk about their subjects of expertise, best practices, and production tales. Engineering rigour and artistic creativity are a fantastic combination, and this series aims to be the synthesis of these two aspects within the Haskell world.
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