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Editorial Team
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Asked: 2026-05-18T19:40:02+00:00

I was reading in Practical Clojure (Chapter 5) that the rseq function operation executes

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I was reading in Practical Clojure (Chapter 5) that the rseq function operation executes in constant time. It seems to me that it should be a linear time operation. Can anyone shed some light on this for me?

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  1. Editorial Team

    Try this:

    (class [1 2 3 4])

    You’ll see:

    clojure.lang.PersistentVector

    Now try this:

    (class (rseq [1 2 3 4]))

    And the sequence implementation is different:

    clojure.lang.APersistentVector$RSeq

    As Roman said, it is a changed interface to a sequence. All the elements are where they were you are just accessing them in a reverse order.

    You can see RSeq class to see how it’s implemented here: https://github.com/clojure/clojure/blob/b578c69d7480f621841ebcafdfa98e33fcb765f6/src/jvm/clojure/lang/APersistentVector.java

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