:exists conditions perform downstream work when their state didn't change

[WilliamParker]

We added support for :exists conditions that only fire once per binding group with #130. Since they use a count accumulator behind the scenes, Clara will consider a change to have taken place each time a match is added or removed, even if it doesn't change whether a match exists. This implementation can be found in extract-exists. For example, suppose we have a condition like

[:exists [Cold]]

and 2 Cold facts in the session. We shouldn't perform any downstream work in the rules network when we retract or add 1 Cold fact since we'd have 1 or 3 Cold facts afterward. That is, we have a Cold fact before and a Cold fact after in these cases, so the answer to the question "Does a Cold fact exist in the session" did not change. However, we will current retract downstream work based on a count of 2 and then perform that work again based on a count of 3, even though the count is only used by a :test condition added by extract-exists that determines whether the count is positive.

We can improve this by adding a :convert-return-fn to the accumulator in this case that returns true if the count is greater than 0 and nil if is 0. Due to the enhancements in #182 if the final result of the accumulator does not change then work will not be performed downstream in the rules network.

[WilliamParker]

PR at #411

[WilliamParker]

Resolved by #411 , closing