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merge for clojure that doesn't overwrite existing values with nil values
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| (require '[clojure.core.match :refer [match]]) | |
| (defn discard-nils [a b] | |
| (match [a b] | |
| [_ nil] a | |
| :else b)) | |
| (def safe-merge (partial merge-with discard-nils)) | |
| ;; examples | |
| (safe-merge {:a 1 :b 1} {:a 5 :b nil} {:a nil :b 3}) | |
| ;; => {:a 5, :b 3} | |
| (merge {:a 1 :b 1} {:a 5 :b nil} {:a nil :b 3}) | |
| ;; => {:a nil, :b 3} | |
| (safe-merge {:a 1 :b nil} {:a nil :b 4 :c 5 :d nil}) | |
| ;; => {:a 1, :b 4, :c 5, :d nil} | |
| (merge {:a 1 :b nil} {:a nil :b 4 :c 5 :d nil}) | |
| ;; => {:a nil, :b 4, :c 5, :d nil} |
Using core.match looks a bit overengineered here. if/else logic would be enough:
(defn discard-nils
[a b]
(if (nil? b) a b))
(def safe-merge (partial merge-with #(or %2 %1)))
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Use case for this: data collection and parsing projects that rely on maps stored in mutable state.
I'm working on a complex human/computer web data collection project that takes the html from a webpage via a chrome extension, parses it out, with a combination of enlive and regex, into a map, and sends it to a server to be stored in an atom until the human driving the browser answers some questions about it, at which time it's entered into a database.
But the webpage sometimes gets updated by ajax calls, and my chrome extension will scrape that data too and send it to the server to be merged with the existing data.
What happens if a subsequent update of the webpage destroys some data that my parsers found on the initial webpage? If I just merged the maps via the standard
mergefunction in clojure.core, parsing the second version of the page would returnnilfor the destroyed data, and then merge would lose the first version of the page. That's no good.safe-mergeprefers later data to former, but prefers non-nil data to nil data, and the preference for non-nil trumps the preference for later data. Problem solved.