When using :exists, adding a binding variable changes the behaviour of the rule, making it trigger more times. For example (only difference being ?i <- in more):

(cr/defrule more
  [?p <- Player (= id ?id)]
  [:exists [?i <- Item (= id ?id)]]
  =>
  (println "more" ?p))

(cr/defrule less
  [?p <- Player (= id ?id)]
  [:exists [Item (= id ?id)]]
  =>
  (println "less" ?p))

more #claraz.rules.Player{:id 2, :y 2}
more #claraz.rules.Player{:id 1, :y 1}
more #claraz.rules.Player{:id 2, :y 2}
more #claraz.rules.Player{:id 1, :y 1}
more #claraz.rules.Player{:id 1, :y 1}
less #claraz.rules.Player{:id 2, :y 2}
less #claraz.rules.Player{:id 1, :y 1}

@oskarkv I think it’s invalid to do a “fact binding” on an :exists

Now that I see that syntax, makes me wonder why it works

So basically, I think more is invalid - but I guess it isn’t being checked well

I don’t understand how it could have any semantically meaningful compilation result

:exists in general does not allow you to access the fact that “exists”

because it isn’t meaningful directly - there could be more than one

if you need to have an actual fact - and I suppose any fact will do for you - you should use an accumulator

[?items <- (acc/all) :from [Item ...]]
=>
;; choose one here
... (first ?items) ...

In your case, you may be thinking that in your domain there is exactly 1 match or no matches. However, the rules do not know this

I do find the situation where the syntax is allowing this fact binding to be put inside the :exists condition to be something to be addressed though