So, I’ve been working on expert systems for two years. The central pitch (of a rules engine) is that it will allow business people to build their rules without programmer involvement. sounds nice, but is not often nice in practice. rule systems can grow very hard to maintain over time. and you maybe need tooling so you can cope with it by detecting overlapping rules or visualize the program flow. There is also the question, what skills you need to author and maintain rules. Do people who author rules automatically learn quasi programming skills? Is it maybe better to let (carreer) programmers author the rules, and have business-readable notation instead, to still allow to build a rich and deep communication channel between programmers and business/underlying domain? I’ve been reading halfway through “out of the tar pit”, watched half a dozen of lighttable/eve talks with all the eve experiments, and look into vega dataflow code. I think I’ll work in this area more in the upcoming months.

I had no idea people still used expert systems — there was a ton of hype around them in the 80s (IIRC), maybe earlier, but I haven't heard much of them since. Wild.

They're used all over the place actually! A ton of enterprise systems are run off of rules engines. Blaze is probably the leader from a commercial perspective, but it's very old.

Yes, there are quite some systems still uses. One popular Java library is Drools. They also have a section in their docs dedicated for the history of expert system (or knowledge representation and reasoning): https://docs.jboss.org/drools/release/6.5.0.Final/drools-docs/html/ch05.html#d0e4247 Rule engines are even used in games. The HD remake of Age of Empires II for example used them. Here is my copy of the circa 30,000 lines of code rules file: https://gist.github.com/hastebrot/ddac3624aaf7963f59a798f8ad5a0a16 Using Java sometimes means that they neglect the frontend almost completely. I saw systems where they only offered a textarea widget to author the rules, with the assumption, the business analysts will keep the rules simple (less then 10 lines of code) and everything will be fine. I saw rules with 100 lines of code.