Hm, I wrote a reply yesterday figuring it would show up today (seeing as I
accidentally double posted last time because it did not show up at once),
but that reply seems to be lost.
I see what you mean now, I think. Seeing as you will have to build your
hierarchies yourself you might try something like the following:
(defrecord Label [text])
(defrecord TextField [text])
(defmacro c-mix [orig o-args mixin m-args]
`(with-meta
(new ~orig ~o-args)
{:mixin (new ~mixin ~m-args)}))
;; Extracting the mixin instance
(defn mixin [o]
(:mixin (meta o)))
(defmulti render (fn [x] (or (class (:mixin (meta x)))
(class x))))
(defmethod render TextField [x] (:text x))
(defmethod render Label [x] (str (:text (mixin x)) ": " (render (with-meta
x {}))))
(def labelledtextfield (c-mix TextField "Foo" Label "Name"))
(def textfield (TextField. "Foo"))
(render textfield) ;=> "Foo"
(render labelledtextfield) ;=> "Name: Foo"
This will allow you to emulate dynamic mixin composition by using the c-mix
macro which is like a constructor call that attaches an instance of the
mixin object as meta data. I know you would like to avoid macros, but since
it is not possible to instantiate an object from a function parameter I
don't think you can avoid them entirely.
Ad-hoc hierarchies using derive might also combine well with this solution,
but then you would have to stay away from defrecord and define your own
types I think.
Regards,
Matthias
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