Configure REPL on Startup

A Clojure REPL starts in the user namespace by default. Clojure will automatically load code from a user.clj file into the REPL to carry out common startup tasks.

• load project code into the REPL by requiring namespaces
• call functions to run an application or service
• start components (i.e for mount, component, integrant)

Create a dev/user.clj file and :env/dev alias

Create a dev/user.clj file with a namespace called user.

user.clj should include a namespace definition

(ns user)

practicalli/clojure-deps-edn includes a :env/dev alias which adds the dev directory to the project classpath. Alternatively, edit the deps.edn file and add the following code:

:env/dev
  {:extra-paths ["dev"]}

Running a Clojure REPL with the -A:env/dev alias will make the dev/user.clj file available to be loaded by the REPL.

In this example the dev/ path is added to the project and then the REPL is run using rebel readline.

clojure -M:env/dev:repl/rebel

Requiring namespaces

By requiring a namespace in the dev/user.clj file, the code defined in that namespace will be loaded into the REPL once started.

Add a require expression to the namespace definition in dev/user.clj

(ns user
  (:require [practicalli.project-namespace]))

Require loads all the expressions into the REPL, so functions are immediately available.

Calling functions

Functions from the required namespace can be called, to start the application for example.

(ns user
  (:require [practicalli.project-namespace]))

(practicalli.project-namespace/-main)

Fuzzy searching for library dependencies - deps.edn

The find-deps project fuzzy searches Maven Central and Clojars for dependencies when given a name.

Add the find-deps project to the env/dev alias as an :extra-deps (:env/dev is available in practicalli/clojure-deps-edn)

:env/dev
  {:extra-paths ["dev"]
   :extra-deps  {find-deps/find-deps
                 {:git/url "https://github.com/hagmonk/find-deps"
                  :sha     "6fc73813aafdd2288260abb2160ce0d4cdbac8be"}}}

Require the find-deps.core namespace in the dev/user.clj file to use its deps and print-deps functions

(ns user
  (:require [find-deps.core :as find-deps]))

Start a REPL using the :env/dev alias.

To start a Rebel REPL with :env/dev use the following command in a terminal

clojure -A:env/dev:repl/rebel

In the REPL, call the (find-deps/deps "library-name") to return a map of the matching dependency, or (find-deps/print-deps "library name") to print dependencies in a table.

Starting Component Life-cycle Services

Clojure has several library to manage the life-cycle of components that make up the application, especially those components with state. Components can be started and stopped in a specific order.

Example component life-cycle libraries included

• mount
• integrant
• component

In Clojure it is idiomatic to define the component life-cycle services in a namespace called dev. In the dev/user.clj file, add the following ns declaration to require the dev namespace and change to that namespace with in-ns

(ns user
  (:require [dev]))

(dev/go)

Now define code in the dev/dev.clj file that controls the component life-cycle services library for the project.

Example project with component life-cycle

• Mount
• Integrant
• Component

(ns user
  :require [mount.core :refer [defstate]]
           [practicalli.app.main])

(defn start []
  (with-logging-status)
  (mount/start #'practicalli.app.conf/environment
               #'practicalli.app.db/connection
               #'practicalli.app.www/business-app
               #'practicalli.app.service/nrepl))

(defn go
  "Start all states defined by defstate"
  []
  (start)
  :ready)

(defn stop [] (mount/stop))

(defn reset
  "Stop all states defined by defstate.
  Reload modified source files and restart all states"
  []
  (stop)
  (namespace/refresh :after 'dev/go))

Reference

• Mount project on GitHub
• Mount - collection of Clojure/Script mount apps
• Component
• A tutorial to Stuart Sierra's Component
• Refactoring to Components - Walmart Labs Lacinia
• Integrant
• Compojure and Integrant
• Build a Clojure web app using Duct - CircleCI
• Reloading Woes - Lambda island