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dylon/multimethods-under-varying-relationship-hierarchies.clj

Last active December 30, 2015 18:09
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Demonstrates how to dispatch on multimethods with relationship hierarchies other than the global hierarchy. In Clojure, multimethods compare dispatch values using isa?, which operates on a relationship hierarchy that may be overridden via the ":hierarchy" option to defmulti.
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multimethods-under-varying-relationship-hierarchies.clj
(def h (atom (make-hierarchy)))
(swap! h derive ::a ::b)
;-> "(isa? user/a user/b), under the global hierarchy: false"
(println "(isa? user/a user/b), under the global hierarchy:" (isa? ::a ::b))
;-> "(isa? user/a user/b), under the hierarchy, h: true"
(println "(isa? user/a user/b), under the hierarchy, h:" (isa? @h ::a ::b))
(defmulti f identity :hierarchy h :default :unmatched)
(defmethod f ::b [x]
(format "I, %s, isa? user/b under f!" (print-str x)))
(defmethod f :unmatched [x]
(format "I, %s, am sorrowfully-not isa? user/b under f..." (print-str x)))
(println (f ::b)) ;-> "I, user/b, isa? user/b under f!"
(println (f ::a)) ;-> "I, user/a, isa? user/b under f!"
(println (f ::c)) ;-> "I, user/c, am sorrowfully-not isa? user/b under f ..."
(defmulti g identity)
(defmethod g ::b [x]
(format "I, %s, isa? :user/b under g!" (print-str x)))
(defmethod g :default [x]
(format "I, %s, am sorrowfully-not isa? :user/b under g ..." (print-str x)))
(println (g ::b)) ;-> "I, user/b, isa? user/b under g!"
(println (g ::a)) ;-> "I, user/a, am sorrowfully-not isa? user/b under g ..."
(println (g ::c)) ;-> "I, user/c, am sorrowfully-not isa? user/b under g ..."
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