[kdkeyser]

I think what you witnessed is the fact that being an expert in programming language theory or category theory, does not make you a good software engineer.

Haskell is surprisingly well suited for back-end services (e.g REST). Warp (an HTTP server), Servant (a way to define your backend API), and Mio [1] (the Haskell network IO implementation) are amazingly performant and scalable.

Of course, that does not prevent you from messing up on the business logic level. I suspect that they lacked awareness of performance implications of their choices, this is not something Haskell will save you from. On the contrary, parts of the Haskell community do tend to go to over-abstraction, and I believe this does have a negative effect on people writing production software.

[1] https://dl.acm.org/doi/abs/10.1145/2503778.2503790