OPS5 is a rule-based or production system computer language, notable as the first such language to be used in a successful expert system, the R1/XCON system used to configure VAX computers. An OPS5 program consists of a set of production rules. Each rule has a precedent, or set of conditions which govern application of the rule, and an antecedent which define actions to take upon triggering the rule. OPS5 supports both forward-chaining and backward-chaining control models.
(object-class request ^action) (startup (strategy MEA) (make request ^action hello) ) (rule hello (request ^action hello) (write |Hello, world!| (crlf)) )