defrecord+

record+.clj

;; Provides the ability to use :pre and :or options to defrecord via a constructor function.
;; The constructor function is named new-X, where X is the name passed into (defrecord+ X ...)

(defmacro defrecord+
  [name fields & etc]
  (let [[[{:as opts}] etc]
        (if (map? (first etc))
          (split-at 1 etc)
          [nil etc])

        defaults
        (into {}
              (for [[k v] (:or opts)]
                [(keyword k) v]))

        kws (map (comp keyword) fields)
        b-name (symbol (str "base-" name))
        new-name (symbol (str "new-" name))]
    `(do
       (defrecord ~name ~fields ~@etc)
       (def ~b-name (~(symbol (str name \.))
                     ~@((apply juxt kws) defaults)))
       (defn ~new-name
         [& {:keys [~@fields]
             :as ~(symbol "d")
             :or ~(:or opts)}]
         {:pre ~(:pre opts)}
         (merge ~b-name ~(symbol 'd))))))

(defprotocol BRead
  (read [this]))

(do
  (defrecord+ Book [author title]
    {:pre [(string? title)]
     :or {author "n/a" title "n/a"}}
    BRead
    (read [this]
          (str "Reading " (:title this) ", by " (:author this))))
  (read (new-Book :author "F. Scott Fitzgerald" :title "THE GREAT GATSBY")))