Create a development-only user Namespace.

While doing interactive development, it's important to have a place to put code that is useful for that purpose, but maybe isn't appropriate for the software you ultimately ship.

• Create a file named dev/user.clj

  Most REPLs will automatically load in any user namespace. This makes it the easiest place to put "development only" code.

  (ns user)
             

• Add a :dev alias to your deps.edn.

  You want to have the dev folder included as an extra path.

  Because user.clj is treated specially, you can run into wacky behavior during distribution if you include it in your normal build. So the alias serves to make sure dev/user.clj is only on the path during development.

  {:paths ["src"]
   :deps {...}
   :aliases {...
             :dev {:extra-paths ["dev"]}}}
               

• Add the dev folder to your :nREPL alias as well.

  Seeing as you will only be using nREPL during development, this is needed. We can merge :dev and :nREPL eventually if they become too much to keep in sync.

  {:paths ["src"]
   :deps {...}
   :aliases {:nREPL
              {:extra-paths ["dev"]
               :extra-deps
                {nrepl/nrepl {:mvn/version "1.2.0"}}}
             :dev {:extra-paths ["dev"]}}}
               

• Put a def at the top assigning system to nil.

  (ns user)
  
  (def system nil)

• Put a start-system! function in user.

  This should call start-system from example.system and set the value of the system var to the return value.

  If a system is already started - which you can tell by checking is system is nil - you can print a message and exit early.

  (ns user
    (:require [example.system :as system]))
  
  (def system nil)
  
  (defn start-system!
    []
    (if system
      (println "Already Started")
      (alter-var-root #'system (constantly (system/start-system)))))

• Put a stop-system! function in user.

  If no system has been started this should be a noop.

  (ns user
    (:require [example.system :as system]))
  
  (def system nil)
  
  (defn start-system!
    []
    (if system
      (println "Already Started")
      (alter-var-root #'system (constantly (system/start-system)))))
  
  (defn stop-system!
    []
    (when system
      (system/stop-system system)
      (alter-var-root #'system (constantly nil))))

• Put a restart-system function in user

  When you want a quick way to turn your system off and back on again, it's nice to have such a function available.

  (ns user
    (:require [example.system :as system]))
  
  (def system nil)
  
  (defn start-system!
    []
    (if system
      (println "Already Started")
      (alter-var-root #'system (constantly (system/start-system)))))
  
  (defn stop-system!
    []
    (when system
      (system/stop-system system)
      (alter-var-root #'system (constantly nil))))
  
  (defn restart-system!
    []
    (stop-system!)
    (start-system!))
  
  (defn server
    []
    (::system/server system))

• Put a server function in user.

  The purpose of this will be to let code access the running server.

  (ns user
    (:require [example.system :as system]))
  
  (def system nil)
  
  (defn start-system!
    []
    (if system
      (println "Already Started")
      (alter-var-root #'system (constantly (system/start-system)))))
  
  (defn stop-system!
    []
    (when system
      (system/stop-system system)))
  
  (defn server
    []
    (::system/server system))

• Configure your editor

  Now that we have a :dev alias which includes your development only code, you need to make sure your editor uses that alias when you are developing.

  Again I will defer to the official documentation of those tools. For both Calva and Cursive its simply a button to click in the UI.