Simple, recursive deep-merge in Clojure.

deep-merge-spec.clj

(ns deep-merge-spec
  (:require [midje.sweet :refer :all]
            [util :as u]))

(fact (u/deep-merge {:one 1 :two 2}
                    {:one 1 :two {}})
      => {:one 1 :two {}})

(fact (u/deep-merge {:one 1 :two {:three 3 :four {:five 5}}}
                    {:two {:three {:test true}}})
      => {:one 1 :two {:three {:test true} :four {:five 5}}})

(fact (u/deep-merge {:one {:two {:three 3}}
                     :four {:five {:six 6}}}
                    {:one {:seven 7 :two {:three "three" :nine 9}}
                     :four {:eight 8 :five 5}
                     :ten 10})
      => {:one {:two {:three "three" :nine 9}
                :seven 7}
          :four {:five 5 :eight 8}
          :ten  10})

(fact (u/deep-merge {:one {:two 2 :three 3}}
                    {:one {:four 4 :five 5}})
      => {:one {:two 2 :three 3 :four 4 :five 5}})

(fact (u/deep-merge {:one 1 :two {:three 3}}
                    {:one 2 :two {:three 4}}
                    {:one 3 :two {:three 5}}
                    {:one 4 :two {:three 6}})
      => {:one 4 :two {:three 6}})

deep-merge.clj

(ns util)

(defn deep-merge [v & vs]
  (letfn [(rec-merge [v1 v2]
                     (if (and (map? v1) (map? v2))
                       (merge-with deep-merge v1 v2)
                       v2))]
    (when (some identity vs)
      (reduce #(rec-merge %1 %2) v vs))))

Courtesy of @cgrand

(defn deep-merge
  [a b]
  (if (map? a)
    (into a (for [[k v] b] [k (deep-merge (a k) v)]))
    b))

And @jaihindhreddy

(defn deep-merge [a b]
  (if (map? a)
    (merge-with deep-merge a b)
    b))

and variadic:

(defn deep-merge [a & maps]
  (if (map? a)
    (apply merge-with deep-merge a maps)
    (apply merge-with deep-merge maps)))

Note that the second, variadic version of deep-merge provided here has unusual/incorrect behavior for false-y values. For example:

dev=> (deep-merge {:a nil} {:a false})
{:a nil}

This is due to merge-with checking for at least one "truthy" argument:

(defn merge-with
  "Returns a map ..."
  [f & maps]
  (when (some identity maps)
    (let [merge-entry (fn [m e]
      ...
      (reduce1 merge2 maps))))

Which produces behavior like:

dev=> (merge-with first false nil)
nil

After using deep-merge for a bit, I think I know why it doesn't already exist. It encourages difficult to understand nested maps when flat maps are simpler/better. Perhaps its absence speaks to it as a code smell.

@fl00r Here's the problem (if that makes sense still):

cljs.user=> (defn deep-merge [& maps]
       #_=>   (apply merge-with (fn [& args]
       #_=>                       (if (every? map? args)
       #_=>                         (apply deep-merge args)
       #_=>                         (last args)))
       #_=>          maps))
       #_=> 
#'cljs.user/deep-merge
cljs.user=> (deep-merge {:m {:a 1 :b 2}} {:m nil} {:m {:a 2 :c 3}})
{:m {:a 2, :c 3}}
cljs.user=> (deep-merge {:m {:a 1 :b 2}} {:m  {}} {:m {:a 2 :c 3}})
{:m {:a 2, :b 2, :c 3}}

So I guess sth like this:

(defn deep-merge [& maps]
  (apply merge-with (fn [& args]
                      (if (every? #(or (map? %) (nil? %)) args)
                        (apply deep-merge args)
                        (last args)))
         maps))

@andrewboltachev I would argue that this behavior is an expected one.
nil is a legit value, so (deep-merge {:m {:a 1 :b 2}} {:m nil}) => {:m nil}

cheers

@fl00r well, my argument here is that, when using just merge:

user=> (merge {:a 1 :b 2} nil)
{:a 1, :b 2}

and I think of nil as kinda "absence of value"

@andrewboltachev

but

(merge {:a 1} {:a nil})
{:a nil}

In deep merge I would prefer this semantic.

But that is of course a matter of taste.

And as @dijonkitchen mentioned

After using deep-merge for a bit, I think I know why it doesn't already exist. It encourages difficult to understand nested maps when flat maps are simpler/better. Perhaps its absence speaks to it as a code smell.

Of course you can modify the code to fit your needs.

Just for some shameless self promotion:
https://github.com/JasonStiefel/clojure-deep-merge/blob/master/src/deep/merge.clj

Here's one more version.

This one is a lot less concise, but potentially easier to understand. It might also perform better than some other versions, since I'm using loops and recurs.

(defn merge-entries [value-fn into-map entries]
  (loop [result    into-map
         remaining (seq entries)]
    (if-not remaining
      result
      (let [entry   (first remaining)
            k       (key entry)
            new-val (val entry)
            cur-val (get result k)]
        (recur (if (nil? cur-val)
                 (assoc result k new-val)
                 (assoc result k (value-fn cur-val new-val)))
               (next remaining))))))


(defn merge-maps [value-fn & maps]
  (loop [results   nil
         remaining (seq maps)]
    (if-not remaining
      results
      (recur (merge-entries value-fn (or results {}) (-> remaining first seq))
             (next remaining)))))


(defn deep-merge [& maps]
  (let [value-fn (fn [v1 v2]
                   (if-not (and (map? v1) (map? v2))
                     v2
                     (deep-merge v1 v2)))]
    (apply merge-maps value-fn maps)))


(deep-merge {:m {:a 1 :b 2}} {:m nil} {:m {:a 2 :c 3}})
(deep-merge {:m {:a 1 :b 2}} {:m {}} {:m {:a 2 :c 3}})
(deep-merge {:a {:b true}} {:a {:b false}} {:a {:b nil}})
(deep-merge {:a 1} nil)
(deep-merge {:a {:b true}} {:a {:b false}} {:a {:b nil}})
(deep-merge {:one 1 :two {:three 3}}
            {:one 2 :two {:three 4}}
            {:one 3 :two {:three 5}}
            {:one 4 :two {:three 6}})

{:m {:a 2, :c 3}}
{:m {:a 2, :b 2, :c 3}}
{:a {:b nil}}
{:a 1}
{:a {:b nil}}
{:one 4, :two {:three 6}}