;; store
(defn find-by-id [id]
  {})

(defn find-friends-of [id]
  [])

(defn find-last-post-of [id]
  {})

;; core
(defn user [service id]
  (service id))

(defn friends [service id]
  (service id))

(defn last-post-of [service id]
  (service id))

(defn user+friends+last-post [user-service friends-service last-post-service id]
  (let [user (user-service id)
        friends (friends-service id)
        friends+last-posts (map #(assoc % :post (last-post-service (:id %))) friends)]
    (assoc user :friends friends+last-posts)))

;; public
(def user (partial user find-by-id))

(def friends (partial friends find-friends-of))

(def last-post-of (partial last-post-of find-last-post-of))

(def user+friends+last-post
  (partial user+friends+last-post find-by-id find-friends-of find-last-post-of))

;; client
(user 1)
(friends 1)
(last-post-of 2)
(user+friends+last-post 1)