Functions can implement protocols too

fn_protocols.clj

(defprotocol TwoPersonFn
  (operate [this a b]))

(defn shorter? [a b])
(defn marry! [a b])
(defn same-age? [a b])

(extend-type (class shorter?) TwoPersonFn
  (operate [this a b] (this a b)))
(extend-type (class marry!) TwoPersonFn
  (operate [this a b] (this a b)))
(extend-type (class same-age?) TwoPersonFn
  (operate [this a b] (this a b)))

(satisfies? TwoPersonFn shorter?) ; => true
(satisfies? TwoPersonFn marry!) ; => true
(satisfies? TwoPersonFn same-age?) ; => true

(operate shorter? {:height 1} {:height 2}) ;or whatever