Introduction
Mercury, a fintech company, made a bold decision to build its infrastructure on a two-million-line Haskell codebase. For those unfamiliar with Haskell, a functional language known for its robustness but also its steep learning curve, this choice might seem risky. Yet, Mercury has proven that this gamble was worth it.
Why Haskell?
Robustness and Security
Haskell is renowned for its ability to prevent common errors through its advanced type system. At Mercury, this translates into reduced bugs related to null pointers and enhanced security in handling financial transactions. With over 300,000 business clients and $248 billion processed in 2025, security and reliability are not options but necessities.
Scalability
Scalability is a major challenge for any rapidly growing company. Haskell enables modular code structuring, making updates and new feature additions easier without disrupting the entire system. With approximately 1,500 employees, many of whom had never coded in Haskell before joining Mercury, this modularity is crucial.
The Challenge of Training
Mercury primarily hires generalists, meaning many of their engineers learn Haskell on the job. However, thanks to a company culture focused on learning and knowledge sharing, these newcomers become effective quickly. Internal bootcamps and mentoring play a key role in this learning process.
Innovation at Scale
By obtaining a national banking charter in the United States, Mercury demonstrates how an innovative approach can lead to significant opportunities. Using Haskell has allowed them to build a flexible banking system that adapts to changing market needs while maintaining a robust and secure codebase.
Conclusion
Mercury illustrates how using Haskell can transform a fintech company, offering both security and flexibility. For those looking to innovate in the tech sector, Mercury shows that investing in a functional language can yield significant dividends.
Let's discuss your project in 15 minutes.