> I still wonder why Haskell sans IO isn't a thing

Isn't that just Haskell without the use of IO... ? Which is already a thing...

Simple Haskell could be a thing, there have been some efforts in the community but none really crystalized. Frankly, I don't think the problem is so much difficult I/O, but rather too many language extensions, competing toolchains, and some missing libraries.

At some point you would need to see the results of the rules applied, thus, you'd need IO.

The suggestion is to use pure Haskell for the rules DSL only. The surrounding system that applies any side effects would presumably not be written in that language.

It's a good idea, and Facebook actually does something like this for their spam filtering rules: https://engineering.fb.com/2015/06/26/security/fighting-spam...

Looks like it has become abandonware, or at least the component they open sourced. Oh well, such is life in the Haskell ecosystem. https://github.com/facebook/Haxl

> great way to define/implement rules for a rules engine

There are more ways to implement rule engines and rules than to learn Haskell. One of them could be to use Prolog.

Haha, that's exactly what I'm trying to avoid. We're looking at adopting openpolicyagent. Which frankly, a modern JSON-oriented datalog may be ideal. But this whole time I keep thinking, this would be so much easier in Haskell. Granted, the rest of my team would probably find it equally confusing. Maybe even moreso? IDK, it'd be an interesting experiment. If such a thing as Haskell-based policy agent existed.