handlers.clj

;;; Handlers.
;; Approximates effect handlers without unwinding the stack.
;; Captures thread bindings so as to avoid "open recursion" between handlers.
;; Also binds `super` to bubble effect up to the next handler.

(def ^:dynamic super)

(defn bind-handler [var f]
  (let [bindings (assoc (get-thread-bindings) #'super @var)]
    (fn [& args]
      (apply with-bindings* bindings f args))))

(defn with-handler* [handler f]
  (let [h (into {} (for [[var f] handler]
                     [var (bind-handler var f)]))]
    (with-bindings* h f)))

(defmacro with-handler [handler & body]
  `(with-handler* ~handler (fn [] ~@body)))

(defmacro handler [& impls]
  (let [impls (conform! (s/* (s/cat :var symbol?
                                    :methods (s/* seq?)))
                        impls)]
    (into {}
      (for [{:keys [var methods]} impls]
        [(list 'var var) `(fn ~@methods)]))))

(defmacro defhandler [name & impls]
  `(def ~name (handler ~@impls)))