[Locutus_]

I've used Haskell several times for implementing isolated 'maths business logic units in commercial backend applications.

In one such system I built had the main (REST API exposing) backend implemented in Kotlin with a separate application in Haskell doing a complex set of maths driven business rules against GIS data to calculate area specific prices.

The amount of IO on the Haskell side was fairly minimum and abstracted away quite nicely.

Haskell allowed expressing all complexity in a way that was easy to audit and translate from business/data analyst requirements.

Would do again :-) But only with the correct amount isolation so you can lean into Haskell's strong sides.