(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]))