Is it possible to make a reasonable rules engine in Ruby rather than a "bolt-on" (separate rules language files)? I think its interesting that its the LISP languages that spawned rules-engines, and as far as I've seen, the only language that can produce a reasonable rules engine as an API library.

Is it possible: yes.

Can you write your own home-grown rules engine in Ruby: yes.

Can you use off-the-shelf gems: yes. Here's a few I poked around in my previous explorations into Ruby Rules Engines:

* durable rules - https://github.com/jruizgit/rules?tab=readme-ov-file#ruby

* wongi - https://github.com/ulfurinn/wongi-engine

* rules - https://github.com/azach/rules

* ruleby - https://github.com/Ruleby/ruleby

  - bonus: video of original ruleby author explaining rules engines: https://www.youtube.com/watch?v=qMh2RDL6aBM

edit: It's also possible to write your own Ruby gem in C so that you can pass the handling of the RHS of your rules to a Ruby script.

Do these Ruby libraries actually implement the Rete optimization in control flow though?

Does it exist for Common Lisp/Scheme? I know that could be implemented in a day, but still...

Edit: https://lisa.sourceforge.net/

The precursor to CLIPS was called ART Inference and was a large commercial & very expensive Expert System development tool, written in Common Lisp. Unfortunately it seems to be lost - I haven't seen anything about it for several decades.

Other than that there are a bunch of rule-based systems in Common Lisp.

For example LispWorks Enterprise includes "KnowledgeWorks", which features a forward chainer based on the RETE algorithm.

https://www.lispworks.com/products/knowledgeworks.html