clojure.core/merge-with

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Type: function Added: Clojure 1.0 Examples: 11 Runnable: 8

([f & maps])

Returns a map that consists of the rest of the maps conj-ed onto the first. If a key occurs in more than one map, the mapping(s) from the latter (left-to-right) will be combined with the mapping in the result by calling (f val-in-result val-in-latter).

Examples

by nipra on 2010-07-13 ▶

(merge-with into {"Lisp" ["Common Lisp" "Clojure"] "ML" ["Caml" "Objective Caml"]} {"Lisp" ["Scheme"] "ML" ["Standard ML"]}) ;;=> {"Lisp" ["Common Lisp" "Clojure" "Scheme"], "ML" ["Caml" "Objective Caml" "Standard ML"]}

by fogus on 2010-07-13 ▶

;; merge two maps using the addition function (merge-with + {:a 1 :b 2} {:a 9 :b 98 :c 0}) ;;=> {:c 0, :a 10, :b 100}

by fogus on 2010-07-13 ▶

;; 'merge-with' works with an arbitrary number of maps: (merge-with + {:a 1 :b 2} {:a 9 :b 98 :c 0} {:a 10 :b 100 :c 10} {:a 5} {:c 5 :d 42}) ;;=> {:d 42, :c 15, :a 25, :b 200}

by jaley on 2012-01-06 . May have evaluation errors. ▶

;; Use union to merge sets of elements (use 'clojure.set) (merge-with union {:a #{1 2 3}, :b #{4 5 6}} {:a #{2 3 7 8}, :c #{1 2 3}}) ;;=> {:c #{1 2 3}, :a #{1 2 3 7 8}, :b #{4 5 6}}

by Dimagog on 2015-02-03 ▶

;; Demonstrating difference between merge and merge-with ;; For merge the value from the right-most map wins: (merge {:a 1} {:a 2} {:a 3}) ;;=> {:a 3} ;; while for merge-with values are merged (with function + in this example): (merge-with + {:a 1} {:a 2} {:a 3}) ;;=> {:a 6}

by dmos62 on 2015-03-29 ▶

;; Use merge-with and merge to merge values that are one level deep maps. (merge-with merge {:x {:y 1}} {:x {:z 2}}) ;;=> {:x {:z 2, :y 1}} ;; Deeper maps are not merged: (merge-with merge {:x {:y {:a 1}}} {:x {:y {:b 2}}}) ;;=>{:x {:y {:b 2}}}

by mlanza on 2015-11-23 ▶

;;Use into to avoid losing the shape (i.e. a vector) of the original data: (merge-with into {"Lisp" ["Common Lisp" "Clojure"] "ML" ["Caml" "Objective Caml"]} {"Lisp" ["Scheme"] "ML" ["Standard ML"]}) ;;=> {"Lisp" ["Common Lisp" "Clojure" "Scheme"], "ML" ["Caml" "Objective Caml" "Standard ML"]} ;;No need to use type-specific verbs such as union: (merge-with into {:a #{1 2 3}, :b #{4 5 6}} {:a #{2 3 7 8}, :c #{1 2 3}}) ;;=> {:c #{1 2 3}, :a #{1 2 3 7 8}, :b #{4 5 6}}

by timgilbert on 2017-02-28 . May have evaluation errors. ▶

;; Note that merge-with is fundamentally additive, which can have unintuitive ;; consequences if you are using a subtractive operation. (require '[clojure.set :as set]) ;; Subtract members of one set from another with the same keys: (merge-with set/difference {:a #{1 2 3}} {:a #{1}}) ;;=> {:a #{3 2}} (merge-with set/difference {:a #{1 2 3} :b #{2}} {:a #{1} :b #{4}}) ;;=> {:a #{3 2}, :b #{2}} ;; If a key in the second map doesn't occur in the first, (merge-with) will ;; simply copy it as in (merge), and the passed-in function will not be called: (merge-with set/difference {:a #{1 2 3}} {:a #{1} :z #{4}}) ;;=> {:a #{3 2}, :z #{4}} ;; The solution in this case is to ensure that both maps have the same keys: (merge-with set/difference {:a #{1 2 3} :z #{}} {:a #{1} :z #{4}}) ;;=> {:a #{3 2}, :z #{}}

by dijonkitchen on 2018-08-24 ▶

(defn deep-merge-with "Like merge-with, but merges maps recursively, applying the given fn only when there's a non-map at a particular level. (deep-merge-with + {:a {:b {:c 1 :d {:x 1 :y 2}} :e 3} :f 4} {:a {:b {:c 2 :d {:z 9} :z 3} :e 100}}) -> {:a {:b {:z 3, :c 3, :d {:z 9, :x 1, :y 2}}, :e 103}, :f 4}" [f & maps] (apply (fn m [& maps] (if (every? map? maps) (apply merge-with m maps) (apply f maps))) maps)) (deep-merge-with + {:a {:b {:c 1 :d {:x 1 :y 2}} :e 3} :f 4} {:a {:b {:c 2 :d {:z 9} :z 3} :e 100}}) ;; {:a {:b {:z 3, :c 3, :d {:z 9, :x 1, :y 2}}, :e 103}, :f 4} (deep-merge-with + {:foo {:bar {:baz 1}}} {:foo {:bar {:baz 6 :qux 42}}}) ;; {:foo {:bar {:baz 7, :qux 42}}} ;; Source: https://clojure.github.io/clojure-contrib/map-utils-api.html#clojure.contrib.map-utils/deep-merge-with

by themustafabasit on 2020-05-22 ▶

;; I will be explaining it From java perspective ;; merge-with is like conflict resolver function that ;; we have in java - map.merge(key, val, (v1, v2)-> ...... ) ;; resolve conflict by discarding second value (defn conflict-resolver [v1 v2] (println "conflict is between these values: " v1 v2 ) v1) (merge-with conflict-resolver {:a 2 :b 4 :c 5} {:a 1} {:a 9 :b 3 :c 10 :d 3}) ;; => conflict is between these values: 2 1 - a,a ;; conflict is between these values: 2 9 - new a, a ;; conflict is between these values: 4 3 - b,b ;; conflict is between these values: 5 10 - c,c ;; {:a 2, :b 4, :c 5, :d 3} ;; example - 2 ;; resolve conflict by adding both values (defn conflict-resolver [v1 v2] (println "conflict is between these values: " v1 v2) (+ v1 v2)) (merge-with conflict-resolver {:a 2 :b 4 :c 5} {:a 1} {:a 9 :b 3 :c 10 :d 3}) ;; => conflict is between these values: 2 1 - a,a ;; conflict is between these values: 3 9 - new a, a ;; conflict is between these values: 4 3 - b,b ;; conflict is between these values: 5 10 - c,c ;; {:a 12, :b 7, :c 15, :d 3}

by joshuamzm on 2021-11-25 . May have evaluation errors. ▶

;; deep-merge version of clojure.core/merge (defn deep-merge [& maps] (letfn [(reconcile-keys [val-in-result val-in-latter] (if (and (map? val-in-result) (map? val-in-latter)) (merge-with reconcile-keys val-in-result val-in-latter) val-in-latter)) (reconcile-maps [result latter] (merge-with reconcile-keys result latter))] (reduce reconcile-maps maps))) (deep-merge {:a 1 :b {:c {:g 99} :d 3}} {:a 10 :b {:c {:g 98} :e 4}} {:a 100 :b {:c {:g 97} :e 40 :f 5}}) ;; => {:a 100, :b {:c {:g 97}, :d 3, :e 40, :f 5}} ;; Take into account processing latter maps with non-maps values "override" ;; previously seen map values. (deep-merge {:deep {:deeep {:a 1}}} {:deep {:deeep {:b 2}}} {:a 34 :deep 8 :v 3}) ;; => {:deep 8, :a 34, :v 3} ;; We decided to define reconcile-maps, which is the then-branch expression ;; of the if within reconcile-keys. This way, using reduce will throw errors ;; as when using merge-with with non-map arguments. (deep-merge 1 {:a 1}) ;; IllegalArgumentException contains? not supported on type: java.lang.Long ... (deep-merge {:a 1} 2) ;; IllegalArgumentException Don't know how to create ISeq from: java.lang.Long ... ;; Reviewed and help received from github.com/datz

See also

• clojure.core/merge (8)

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