On Jan 30, 1:09 pm, Jan Rychter <[email protected]> wrote:
> From what I read, the reasoning is that Clojure provides extremely
> general multimethods where you have to define a dispatch function. On
> the object side, there is metadata and you can do with it whatever you
> want. But this seems to leave a gaping hole saying "fill me with an
> object system". Is the goal for everyone to roll their own?
I think the goal is to provide object-like capabilities without
needing actual objects. The emerging pattern is to use maps, with
a :tag key identifying the type. Types are usually namespace-
qualified keywords. You can create any kind of inheritance hierarchy
with "derive".
(defn make-window [id]
{:tag ::window, :id id})
(defn make-color-window [id color]
(assoc (make-window id)
:tag ::color-window
:color color))
(derive ::color-window ::window)
(defmulti describe-window :tag)
(defmethod describe-window ::window [w]
(println "Window with ID" (:id w)))
(defmethod describe-window ::color-window [w]
(println (:color w) "Window with ID" (:id w)))
(let [w (make-color-window 24 "blue")]
(describe-window w))
;; => prints "blue window with ID 24"
-Stuart Sierra
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