Full algebraic data types wouldn't have added much here. Product types are already everywhere, and we didn't need sum or exponential types.

Splitting IO and pure code was just routine refactoring, not a full redesign. Our app logic wasn't strictly pure because it generates pseudorandom numbers and logs events, but practically speaking, splitting the IO and shell from the relatively pure app logic made for much cleaner code.

In retrospect, I consider FCIS a good practice that I first learned with Haskell. It's valuable elsewhere, even when used in a less formal way than Haskell mandates.