(ns emptyhead.newlib.delegate
  "Delegation: treat one message as though it were another, enabling behaviour sharing and polymorphism."
  (:require [emptyhead.thought.crud :as thought]
            [emptyhead.idea.crud :as idea]
            [emptyhead.thought.define :as def]
            [emptyhead.thought.eval :as eval]
            [emptyhead.thought.extend :as extend]
            [emptyhead.newlib.core]))

(defn remove-delegate
  "Undelegate `del` from `msg`."
  [msg del]
  (let [remover (thought/register-thought! [:EH :CORE :NOP] :ext-stages [[:EH :MSG :REMOVE-DELEGATE msg del]])]
    (eval/execute! remover)))


;; Forget idea stored in `thought`'s data. Used to make destructors.
(def/define! [:EH :IDEA :FORGET]
  (fn  [thought parent]
    (idea/forget-idea! (thought/data thought))
    [parent nil]))

;; FIXME Delegate order is currently undefined! Give this a proper order.
;; FIXME needs to be called with a list as second arg??
(defn add-delegate
  "Register `del` as a delegate for `msg`: implementations defined for `del` will now run trigger on `msg` as well.
  For instance, if message =(:foo :bar)= is defined and we =(add-delegate :quux :bar)= then =(:foo :quux)= will alias to =(:foo :bar)=.
  Note that this also works on the left hand side."
  [msg del]
  (remove-delegate msg del)
  (let [delegator (thought/register-thought!
                   [:EH :IO :RETURN]
                   :data del
                   :transient false)
        remover (thought/register-thought! [:EH :IDEA :FORGET] :data delegator)]
    (extend/register-extension! delegator [:EH :MSG :DELEGATE msg])
    (extend/register-extension! remover [:EH :MSG :REMOVE-DELEGATE msg del])))

(defn get-delegates
  "Get possible delegates for `msg`."
  [msg]
  (let [stage [:EH :MSG :DELEGATE msg]
        dels (-> (thought/register-thought! [:EH :CORE :CONTAINER] :ext-stages [stage])
                 eval/execute! thought/empty-stack second)]
    (mapcat #(cons % (get-delegates %)) dels)))