example_ring_middleware_format.clj

(ns …
  (:require [compojure.core :refer [GET POST defroutes]]
            ; …
            [ring.middleware.format :refer [wrap-restful-format]] ; see https://github.com/ngrunwald/ring-middleware-format
            ))

(defroutes handler
  (POST
    "/url"
    req
    (let [params (:params req)]
        {:body (foo params)})) ; `(foo params)` will be Transit-encoded! But NOTE! return value must be map with :body slot!
  …)

(def app (-> #'handler
             ; …
             (wrap-restful-format)))