It's a double-edged sword. Two million lines is a major feat. It's also represents a significant maintenance burden.

The advantages to Haskell are theoretically obvious. The downsides are harder to intuit.

The temptation is to model _everything_ as types. The codebase itseld becomes a _business specification_, not an application. Every policy change is a major refactor (some of which are shockingly high-touch thanks to Haskell safety).

The lesson is you cannot have your cake and eat it too. Eventually you become trapped by your types.

Haskell is really impressive and powerful, perhaps especially at this scale. However it brings its own unique problems. The temptation to model business logic as types leads to rigid structures. And the safety these structures bring can blind you to other classes of risk.