functional programming - Dealing with database reads in Clojure -

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i trying 'purify' of clojure functions. point side-effecting code explicitly declared in 1 function. it's easy data @ start , write db @ end , have pure function transforming in between. however, usual situation transformation function requires db read somewhere in middle of logic:

(defn transform-users       [users]       (let [ids (map :id users)             profiles (db/read :profiles ids)]        (profiles->something profiles)))  (->> (db/read :users)      (transform-users)      (db/write :something)

obviously simple example point is, how side-effecting db/read function out of there, how can make transform-users pure (and benefit, testable)?

one thing here dependency-injection-like approach of supplying (potentially) side-effectful function optional parameter, e.g.:

(defn transform-users   [users & {:keys [ids->profiles]             :or {ids->profiles #(db/read :profiles %)}]   (let [ids (map :id users)         profiles (ids->profiles ids)]     (profiles->something profiles)))

this should testable since can mock injected functions without lot of effort. , bonus, supplying default value, you're documenting you're expecting , making function convenient call.


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