partially-applied handler example for https://clojurians.slack.com/archives/CKKPVDX53/p1582074246024200

example.clj

(defn my-regular-handler
  [req]
  {:status 200 :headers {} :body []})

(defn my-resource-handler
  [datasource req]
  (let [foo (do-something-with-datasource datasource req)]
    {:status 200 :headers {} :body foo}))

(defn make-top-level-handler
  [datasource]
  (reitit.ring/ring-handler
   (reitit.ring/router
    [["/" {:get {:handler my-regular-handler}}]
     ["/get-thing" {:get {:handler (partial my-resource-handler datasource)}}]])
   (reitit.ring/routes
    (reitit.ring/create-resource-handler {:path "/"})
    (reitit.ring/create-default-handler))
   {:middleware [,,,]}))

(defmethod ig/init-key ::app
  [_ {:keys [datasource]}]
  (make-top-level-handler datasource))