Monoids in Clojure

(defn semigroup [append]
  {::append append})

(defn monoid [semigroup empty]
  (assoc semigroup ::empty empty))

(defn append [semigroup a b]
  ((::append semigroup) a b))

(defn concat [monoid xs]
  (reduce (::append monoid) (::empty monoid) xs))

(def semigroup-add
  (semigroup +))

(def monoid-add
  (monoid semigroup-add 0))

(prn (concat monoid-add [1 2 3]))