Replying to myself : )
My "good enough" solution, in case it's of use to others, is
(defn set-hierarchy-root! [x root]
(if-let [p (-> x parents first)]
(set-hierarchy-root! p root)
(derive x root)))
(defn with-root [x root]
(when-not (isa? x root)
(set-hierarchy-root! x root))
x)
Then the dispatch function looks like:
(defmulti combine (fn [x y] [(with-root (:type x) ::shape) (with-root (:type y)
::shape)]))
So, at the cost of an extra call to isa? on every dispatch, any newly seen
values (or their final ancestor already in the hierarchy, if any) are
automatically declared as being derived from ::shape.
Not pretty, but it has the benefit of not needing to know the full set of
possible values in advance.
T
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