support simple union types.

e.g. unions where the possibilities are all of type `Value`.

clj/clarity/cljs/clarity.cljs

(defmethod empty-value 'String [{:keys [default]}]
  (or default ""))
(defmethod empty-value 'U [[_ & [[_ v]]]]
  v)
(defmethod empty-value 'HMap [spec]
  (let [entries (nth spec 2)]
    (into {}
                    :value (om/value string)
                    :onChange #(om/update! string (fn [_ n] n) (.. % -target -value))})))
(defmethod make-typed-input 'U [value owner {type :type}]
  (om/component
    (dom/select nil
      (into-array
        (map (fn [[_ v]]
               (dom/option nil (str v)))
             (rest type))))))
(defmethod make-typed-input 'HMap [m owner {type :type}]
  (om/component
    (dom/div nil
  (let [type '(HMap :mandatory
                    {:name {:type String :default "Paul"},
                     :age {:type Number, :default 10},
                     :language (U (Value :en)
                                  (Value :de)
                                  (Value :fr)
                                  (Value :jp))
                     :gender Keyword})]
    (atom
     {:type type