Mercury scales to 2 million lines of Haskell in production fintech
Original: A couple million lines of Haskell: Production engineering at Mercury
Why This Matters
Demonstrates large-scale Haskell adoption in critical financial infrastructure
Fintech company Mercury operates 2 million lines of Haskell code in production, serving 300,000+ businesses with $248 billion transaction volume in 2025. Despite hiring generalists who learn Haskell on the job, the system has successfully handled hypergrowth and financial crises.
Mercury, a US fintech providing banking services to over 300,000 businesses, has built its production system on roughly 2 million lines of Haskell code. The company processed $248 billion in transaction volume in 2025 on $650 million annualized revenue and is pursuing a national bank charter. With 1,500 employees, Mercury's engineering team primarily hires generalists who learn Haskell on the job. The system successfully weathered the SVB crisis, handling $2 billion in new deposits over five days, and continues operating through regulatory examinations and hypergrowth. Engineer Ian Duncan argues that Haskell's value lies not just in elegance but in practical operational benefits: packing operational knowledge into APIs, securing dangerous operations behind tight boundaries, and making safe paths the easiest to follow.