Why Not a Function #12: deep-merge (with)

(defn deep-merge
  [m1 m2 & {:keys [with]}]
  (into {}
    (map (fn [k]
           (let [v1 (get m1 k)
                 v2 (get m2 k)]
             [k (cond
                  (and (map? v1) (map? v2)) (deep-merge v1 v2 :with with)
                  (and with (contains? m1 k) (contains? m2 k)) (with v1 v2)
                  (contains? m2 k) v2
                  :else v1)]))
      (set/union (keys m1) (keys m2)))))

deep-merge was already mentioned on this blog when we discovered a bug in one of the previous versions of it. We showed a fixed version back then as well, but the version above is even more up to date as we have added a new functionality.

The basic deep-merge works just as standard merge but is “deep” meaning that if maps have submaps it will merge these submaps rather than replacing them.

(merge
  {:keyboard {:color "black", :plug "usb"}, :mouse {:make "logitech"}}
  {:keyboard {:plug "ps/2"}})
=> {:keyboard {:plug "ps/2"}, :mouse {:make "logitech"}}

(deep-merge
  {:keyboard {:color "black", :plug "usb"}, :mouse {:make "logitech"}}
  {:keyboard {:plug "ps/2"}})
=> {:keyboard {:plug "ps/2", :color "black"}, :mouse {:make "logitech"}}

We have replaced :plug and retained :color. What if we want to keep both :plug values? We can use the :with option which works similarly to Clojure’s merge-with.

(deep-merge
  {:keyboard {:color "black", :plug "usb"}, :mouse {:make "logitech"}}
  {:keyboard {:plug "ps/2"}}
  :with (fn [a b] (str a "; " b)))
=> {:keyboard {:plug "usb; ps/2", :color "black"}, :mouse {:make "logitech"}}

Written on March 26, 2020