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Because PHMs implement clojure.lang.IKVReduce and IPersistentMap, they have nondeterministic dispatch through the protocol that backs reduce-kv (clojure.core.protocols/IKVReduce).

A potential way to solve this is to add an instance check for clojure.lang.IKVReduce inside reduce-kv (This is similar to how reduce checks for IReduceInit)

Another approach is to extend the protocol to PersistentHashMap and PersistentArrayMap directly (in addition to preserving the existing extensions)

3 Answers

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Comment made by: bronsa

CLJ-1807 offers a generic solution for this class of problems

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_Comment made by: gshayban_

Repro code

(def C (atom 0))

;; reextend for instrumentation
(extend-protocol clojure.core.protocols/IKVReduce
 ;;slow path default
   [amap f init]
   (swap! C inc)
   (reduce (fn [ret [k v]] (f ret k v)) init amap)))

(reduce-kv (fn [_ _ _]) nil (array-map 1 2 3 4))
(println "slowpaths" @C)
(reduce-kv (fn [_ _ _]) nil (hash-map 1 2 3 4))
(println "slowpaths" @C)

Repro output

➜  dev /usr/lib/jvm/java-8-openjdk/bin/java -cp $(clojure -Spath) clojure.main kvfastpath.clj
slowpaths 1
slowpaths 2
➜  dev /usr/lib/jvm/java-11-openjdk/bin/java -cp $(clojure -Spath) clojure.main kvfastpath.clj
slowpaths 0
slowpaths 0
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Reference: https://clojure.atlassian.net/browse/CLJ-1879 (reported by gshayban)