(.module:
  [library
   [lux {"-" [Module]}
    [abstract
     ["." monad {"+" [do]}]]
    [control
     ["." maybe ("#\." functor)]
     ["." try {"+" [Try]}]
     ["." state]
     ["." function
      ["." memo {"+" [Memo]}]]]
    [data
     ["." text
      ["%" format {"+" [format]}]]
     [collection
      ["." list ("#\." functor mix)]
      ["." dictionary {"+" [Dictionary]}]
      ["." set {"+" [Set]}]]]]]
  [///
   ["." archive {"+" [Output Archive]}
    [key {"+" [Key]}]
    ["." descriptor {"+" [Module Descriptor]}]
    ["." document {"+" [Document]}]]])

(type: Ancestry
  (Set Module))

(def: fresh
  Ancestry
  (set.empty text.hash))

(type: .public Graph
  (Dictionary Module Ancestry))

(def: empty
  Graph
  (dictionary.empty text.hash))

(def: .public modules
  (-> Graph (List Module))
  dictionary.keys)

(type: Dependency
  (Record
   [#module Module
    #imports Ancestry]))

(def: .public graph
  (-> (List Dependency) Graph)
  (list\mix (function (_ [module imports] graph)
              (dictionary.has module imports graph))
            ..empty))

(def: (ancestry archive)
  (-> Archive Graph)
  (let [memo (: (Memo Module Ancestry)
                (function (_ recur module)
                  (do {! state.monad}
                    [.let [parents (case (archive.find module archive)
                                     (#try.Success [descriptor document])
                                     (value@ #descriptor.references descriptor)
                                     
                                     (#try.Failure error)
                                     ..fresh)]
                     ancestors (monad.each ! recur (set.list parents))]
                    (in (list\mix set.union parents ancestors)))))
        ancestry (memo.open memo)]
    (list\mix (function (_ module memory)
                (if (dictionary.key? memory module)
                  memory
                  (let [[memory _] (ancestry [memory module])]
                    memory)))
              ..empty
              (archive.archived archive))))

(def: (dependency? ancestry target source)
  (-> Graph Module Module Bit)
  (let [target_ancestry (|> ancestry
                            (dictionary.value target)
                            (maybe.else ..fresh))]
    (set.member? target_ancestry source)))

(type: .public Order
  (List [Module [archive.ID [Descriptor (Document .Module) Output]]]))

(def: .public (load_order key archive)
  (-> (Key .Module) Archive (Try Order))
  (let [ancestry (..ancestry archive)]
    (|> ancestry
        dictionary.keys
        (list.sorted (..dependency? ancestry))
        (monad.each try.monad
                    (function (_ module)
                      (do try.monad
                        [module_id (archive.id module archive)
                         [descriptor document output] (archive.find module archive)
                         document (document.check key document)]
                        (in [module [module_id [descriptor document output]]])))))))