MTG: Arena uses a rules engine CLIPS (a s-expr expert system based on the RETE engine), which an acquaintance wrote a course for: https://ryjo.codes/tour-of-clips.html and even a declarative chat server: https://ryjo.codes/articles/a-simple-tcp-server-written-in-g...

    (defrule connection
      (connection ?id)
      =>
      (println "User " ?id " connected")
      (printout ?id "Welcome to the chatroom from CLIPS!" crlf)
      (do-for-all-facts ((?f connection)) (neq ?id (nth$ 1 ?f:implied))
          (printout (nth$ 1 ?f:implied) "User " ?id " connected" crlf)))
    
    (defrule say
      (connection ?id)
      ?f <- (message-buffered ?id)
      ?ff <- (message ?id ~/me ?message)
      =>
      (retract ?f ?ff)
      (printout ?id "You: " ?message crlf)
      (do-for-all-facts ((?f connection)) (neq ?id (nth$ 1 ?f:implied))
        (printout (nth$ 1 ?f:implied)
         ?id ": " ?message crlf)))