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 langu

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-with-haskell/ https://engineering.fb.com/2015/06/26/security/fighting-spam...