DI

jafar-is-always-right-redux.clj

(defn lookup
  [dep-graph key]
  (let [{:keys [deps fn]} (dep-graph key)
        context           (map (partial lookup dep-graph) deps) ]
    (apply fn context)))

;###############################################################################
; Define some functions with deps encoded as metadata
(defn bar*
  [a b]
  (+ a b))

(defn foo*
  [bar x y]
  (* (bar x y) (bar x y)))

(defn baz*
  [foo bar pi a b]
  (str "pi -> " pi ", (foo a b) -> " (foo a b) ", (bar a b) -> " (bar a b)))

;################################################################################
; Map names to functions
(def deps {:bar {:deps []
                 :fn   (fn [] bar*)}
           :foo {:deps [:bar]
                 :fn (fn [bar] (partial foo* bar))}
           :pi {:deps [] :fn (constantly 3.14159)}
           :baz {:deps [:foo :bar :pi]
                 :fn (fn [foo bar pi] (partial baz* foo bar pi))}})

; Lookup and call a function
(let [baz (lookup deps :baz)]
  (println (baz 1 2)))
;=> pi -> 3.14159 (foo a b) ->  9 , (bar a b) ->  3

; Inject an alternate dependency impl, and call a function
(let [foo (lookup (assoc deps :bar {:deps [] :fn (constantly (constantly 100))})
                  :foo)]
  (println (foo 4 5)))
;=> 10000