Not sure how I can express this, but is there a way to express a left-join type of a rule?

@iku000888 I’m thinking you’d probably write 2 rules in that case

or do some of the joining on the RHS - which I often hesitate to think is a good idea

(r/defrule joins
  [A (= ?id id)]
  [B (= ?id id)]
  =>
  (r/insert! (->C)))

(r/defrule nil-joins
  [A (= ?id id)]
  [:not [B (= ?id id)]]
  =>
  (r/insert! (->C)))

is that the “left join” sort of thing?

A always produce a C, just sometimes there is no B to go with an A

C can hold whatever info you wanted out of the join