Integrant demo: swapping out components using Clojure's key hierarchy

demo.clj

(ns demo
  (:require [integrant.core :as ig]))

(def config
  {::main {:widget (ig/ref ::base-widget)}
   ::prod-widget {:name "Production widget"}})

(derive ::prod-widget ::base-widget)
(derive ::mock-widget ::base-widget)

(defmethod ig/init-key ::prod-widget
  [_ config]
  (str "Prod: " (:name config)))

(defmethod ig/init-key ::mock-widget
  [_ config]
  (str "Mock: " (:type config)))

(defmethod ig/init-key ::main
  [_ {:keys [widget]}]
  (println "Widget:" widget))

(defn prod-system
  [_]
  (println "Production system:" (ig/init config)))

(defn test-system
  [_]
  (let [test-config (-> config
                        (dissoc ::prod-widget)
                        (assoc ::mock-widget {:type "my mock"}))]
    (println "Test system with mock component:" (ig/init test-config))))