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For those of you playing along at home, this is because of Clojure's reducers framework [1]. If we were to extend 'clojure.core.protocols.CollReduce' to ArrayList, we'd get similar performance, but the important point is that efficient iteration across immutable collections is very much possible.

http://clojure.com/blog/2012/05/08/reducers-a-library-and-mo...

EDIT: and my response is just a little redundant now

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Are you sure? Reducers framework has its own version of major functions (map, reduce, etc.) and has to be called explicitly by use or require. Last time I checked, in version 1.5, if you just call normal reduce, it does not use reducers framework.

there were changes to the core data structures in order to support reducers, namely the addition of polymorphic reduction by the collection. See: https://github.com/clojure/clojure/commit/5b281880571573c591...

It was a pretty transparent change.

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