Clojurians Log v2

Probably not relevant and more of an FYI, but if the condition? is simply equality it could be done with a third argument to =.

(r/defrule some-rule
  [Request (= ?resource resource "GET")]
  =>
  (println ?resource))

@souenzzo wait, really? Is there a list of functions that Clara handles specially?

I mean, knows how to join, other than =?

[Request (= resource "GET")]
=>
(prn "GET")

you can also do

[?request <- Request (= resource "GET")]
=>
(prn ?request)

I use = and contanis?. I learned it when I did not understand how clara works then never search for it.

As a newcomer it’s not super clear which functions are built-in. This is definitely an advanced user library :)

Thanks @ethanc ! This is actually something that I did need (for a different rule)

I'm believe that the only functions that Clara handles specially are the equality expressions: https://github.com/cerner/clara-rules/blob/master/src/main/clojure/clara/rules/compiler.clj#L213-L223

Yeah, = is the only special thing

It still means = too, but it can also be used for unification purposes/binding variables

nothing is special about contains? (or anything else)

Agreed with other comments that it is just = that is special when used as a top-level value in the constraint to create/join on bindings (starting with ?)

that has perf benefits in that those joins are done by hashing

otherwise you can use arbitrary functions and they should behave the same as elsewhere

While reading the docs, it goes from straightforward to advanced very quickly. In http://www.clara-rules.org/docs/rules/, I fully understand the free-lunch-with-gizmo because there's no ? binding, you're matching that an item is :gizmo, but then in the sophisticated example:

(defrule grant-discount-months  
  [Purchase (= ?month (get-month date))]
  [DiscountMonth (= ?month month)]
  =>
  (insert! (->GrantDiscount :for-month ?month)))

it has the same meaning in that it requires equality, but clojure.core/= won’t be called

Not quite.

= is unification. So, it requires the expression to be true. If there are variables in it that do not yet have a value, the variables are "assigned" such to make it true. If the variables are already assigned, it works like clojure.core/=

That’s a reasonable way to think about it

but Clara may optimize on it

when unifying on a ? binding

it uses it to build the rete graph

> = is unification This distinction is makes sense!

makes a hash-based match for nodes known to be doing = based bindings

but = is also just = when equating to constant values etc

If there are many possible values for a variable, it can assign each value consecutively.

you can do [Purchase (= ?month month "JAN")] for example

(*and what @mikerod is saying is true, I'm only describing the semantics, it doesn't actually work like that.)

yeah, semantics are most important

just making sure someone may not get carried away and try to print within clojure.core/= or something and wonder why it isn’t always called hah

unlikely

[Purchase (= ?month month "JAN")] binds the value of the field month to ?month, when (= month "JAN") (via clojure.core/=)

Of course in this case, ?month would just be “JAN”

so perhaps something a bit more useful than that 😛

This page on the docs might be of interest: http://www.clara-rules.org/docs/hash_joins/

Woah secret docs

“`=` is unification” => correct. Note that this is for top-level constraints though, when nested inside a constraint that isn’t the case i.e. [A (= ?b c)] vs [A (stuff (= ?b c))]

Hah not secret, just nested inside the “performance optimization” page

Or more precisely on my last, that is true if you’re trying to create new bindings - it’s still unification semantically if you’re using bindings from before

> [A (= ?b c)] vs [A (stuff (= ?b c))] Yes! This is what I ran into immediately, I wanted to use my own equality function, but I got pointed to using :test instead.

BTW thank you all for helping me understand this!

some of the semantics around = can get weird, since it kind of plays 2 roles depending on context

like when how you can’t nest it within another call to do a unification

but most of the time not a big deal

@eraserhd Agreed with @mikerod’s previous comments - Another interesting way that this played out (both myself and Mike spent a while doing perf optimizations on Clara) is that in practice, it turned out that even for large cases (hundreds of thousands of facts/tens of thousands of rules) the constant factors seemed to be most important. Hashing turned out to be a large percentage of work performed for example. A lot of these optimizations (on the Clojure side) are in the memory.cljc namespace with lots of Java interop etc. That said, this was in use-cases where we didn’t really have lots of data that we were just going to end up discarding, and there’s definitely cases where more laziness could be useful.

Is it possible to transform the output of defquery or would I need to do that in a wrapper function? The RHS of a defrule is just Clojure but that doesn't seem to exist in defquerys

@jvtrigueros defquery has no RHS

you can create facts in rules with defrule that do some transformations so there is less to transform once you get it from the query

but the query return val will just be the maps with the binding keys to the values they are bound to in the query

Thanks! In the query docs, it suggests one can write a query like so:

(query session get-by-last-name :last-name “Jones”)

(defquery get-by-last-name
  [:last-name]
  [?person <- Person (= last-name :last-name)])

I'm only able to create defquerys that have ?:

(defquery get-by-last-name
  [:?last-name]
  [?person <- Person (= last-name ?last-name)])