defrule.clj

(ns yvern.defrule)

(def clara-inserts
  "The forms we care about, capable of inserting facts."
  #{'insert! 'insert-all!})

(def descend*
  "Transducer to descend into inner forms, looping for those we care about, the ones that insert facts.
  Unfortunately we can't really figure out if someone defines another function that calls `insert!`."
  (comp (map macroexpand)
        (drop 1)
        (filter (some-fn #(some clara-inserts (flatten %))
                         clara-inserts))
        cat))

(defn- check-fact-type
  [inserts]
  (some #{:fact-type} (tree-seq coll? seq inserts)))

(defn- descend
  "Recursively checks forms that contain `insert!`s based on whether the verb/head is:
  `insert!`s (exiting),
  `let` forms where the body needs to be checked,
  or any other symbol, meaning a macro/function call around an `insert!`, which could change control flow."
  [[verb & args]]
  (case verb
    (insert! insert-all!)
    (when-not (every? check-fact-type args)
      (ex-info "Facts must have the `:fact-type` key" {}))

    (let clojure.core/let let* clojure.core/let* do clojure.core/do) ; we can add further forms here, but to use as a param we need to use `cond` or write a macro
    (recur (sequence descend* args))

    (-> "`%s` or anything that expands to it is not allowed around `insert!` or `insert-all!` inside a `defrule`"
        (format verb)
        (ex-info {}))))

(defmacro defrule
  [form]
  (some->> form
           (sequence (comp (drop-while (complement #{'=>})) descend*))
           descend
           throw))