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    <p>This isn't a direct answer to your question, but one reason why a function like <code>update-in</code> could exist would be for efficiency&mdash;not just convenience&mdash;if it were able to update the value in the map "in-place". That is, rather than</p> <ul> <li>seeking the key in the map,</li> <li>finding the corresponding key-value tuple,</li> <li>extracting the value,</li> <li>computing a new value based on the current value,</li> <li>seeking the key in the map,</li> <li>finding the corresponding key-value tuple,</li> <li>and overwriting the value in the tuple or replacing the tuple with a new one</li> </ul> <p>one can instead imagine an algorithm that would omit the second search for the key:</p> <ul> <li>seek the key in the map,</li> <li>find the corresponding key-value tuple,</li> <li>extract the value,</li> <li>compute a new value based on the current value,</li> <li>and overwrite the value in the tuple</li> </ul> <p>Unfortunately, <a href="http://clojuredocs.org/clojure_core/clojure.core/update-in" rel="noreferrer">the current implementation of <code>update-in</code></a> does not do this "in-place" update. It uses <a href="http://clojuredocs.org/clojure_core/clojure.core/get" rel="noreferrer"><code>get</code></a> for the extraction and <a href="http://clojuredocs.org/clojure_core/clojure.core/assoc" rel="noreferrer"><code>assoc</code></a> for the replacement. Unless <code>assoc</code> is using some caching of the last looked up key and the corresponding key-value tuple, the call to <code>assoc</code> winds up having to seek the key again.</p>
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    1. POHow to use "Update-in" in Clojure?
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    1. COYou make it sound as if the only difference between your in-place variant and the actual implementation is that the in-place variant only looks up the keys once (which implies that `update-in` could be changed in later versions of clojure to use your in-place variant without breaking any existing code). However a much more important difference is that the in-place variant would necessarily have to mutate the map, while `update-in` does not (and, given that mutating methods usually have a `!` in their name, should not).
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    2. COWell, even a non-destructive version could take advantage of knowing the "hit" position; even `assoc` needs to figure which part of the original map it's *not* going to copy and instead provide the updated value.
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    3. CONice, +1 for a concrete performance improvement suggestion to the core Clojure library!
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