[tikhonj]

You can do a great job of navigating that as long as you have some experienced engineers with taste building the core pieces. You can't have it all, but you can have a lot.

I interned at Jane Street years ago and they seemed to do a great job of walking that line (in OCaml rather than Haskell, but same difference). They moved remarkably quickly despite working in an area with a lot of inherent complexity and where reliability and correctness are an existential concern to the business. (Which, perhaps surprisingly, is massively more the case for a trading firm than for a Mercury-like neobank...) In hindsight, a key thing Jane Street did was hire some experienced OCaml programmers with great taste (like Stephen Weeks, the author of MLton) and let them build the core libraries and guide the whole codebase from the beginning.

Unfortunately, this is one of the things that Mercury didn't do anywhere near as well.

[tome]

Any particular stories illustrating that that you can share?