user=> (merge-with concat
{"Lisp" ["Common Lisp" "Clojure"]
"ML" ["Caml" "Objective Caml"]}
{"Lisp" ["Scheme"]
"ML" ["Standard ML"]})
{"Lisp" ("Common Lisp" "Clojure" "Scheme"), "ML" ("Caml" "Objective Caml" "Standard ML")}
user=> (clojure.pprint/pp)
{"Lisp" ("Common Lisp" "Clojure" "Scheme"), "ML" ("Caml" "Objective Caml" "Standard ML")}
nil
;; 'merge-with' works with an arbitrary number of maps:
user=> (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}
;; Use union to merge sets of elements
user=> (use 'clojure.set)
user=> (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}}
Source
clojure/core.clj:2277
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(defn merge-with
"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)."
{:added "1.0"}
[f & maps]
(when (some identity maps)
(let [merge-entry (fn [m e]
(let [k (key e) v (val e)]
(if (contains? m k)
(assoc m k (f (get m k) v))
(assoc m k v))))
merge2 (fn [m1 m2]
(reduce merge-entry (or m1 {}) (seq m2)))]
(reduce merge2 maps))))
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