nREPL – Starting shadow-cljs from lein or clojure

This is a tiny example project for setting up development using nREPL and shadow-cljs. The ClojureScript app is rendered using Reagent.

By default the project uses deps.edn for dependencies. There is a Leiningen setup as well.

Install npm dependencies

npm i

(Or yarn, whatever you fancy.)

Using a Clojure editor

If you are using something like Calva or CIDER, just jack in to the project as a shadow-cljs project type, and hack away.

Starting the project w/o an editor

The following instructions are for working with the project without an editor. You probably wouldn't do that. The point is mostly to demystify what the editors are doing when starting and attaching to a shadow-cljs project. So, you could say it is for educational purposes. Since it's pretty quick, why not try it out?

You can start this project using either shadow-cljs, clojure, or lein. As you are doing this to maybe learn something, please consider trying all three options.

With the shadow-cljs npm executable

npx shadow-cljs -d cider/cider-nrepl:0.28.5 clj-repl
...
shadow.user=>

NB: If you use shadow-cljs it will in turn use clojure to start Clojure and the nREPL server. (Or lein if you edit shadow-cljs.edn and change this.)

With deps/clojure

To start the project and the nREPL server with clojure, in one terminal:

% clojure -Sdeps '{:deps {nrepl/nrepl {:mvn/version,"1.0.0"},cider/cider-nrepl {:mvn/version,"0.28.5"}}}'  -m nrepl.cmdline --middleware "[cider.nrepl/cider-middleware,shadow.cljs.devtools.server.nrepl/middleware]"

Attach to the nREPL server in another terminal:

% clojure -Sdeps '{:deps {reply/reply {:mvn/version "0.5.1"}}}' -M -m reply.main --attach `< .nrepl-port` 
...
user=> 

Or, since the deps.edn file specifies the nrepl dependency:

% clj -M -m nrepl.cmdline -c --port `< .nrepl-port`
...
user=> 

With Leiningen

To start the project and the nREPL server with Leiningen, in one terminal:

lein update-in :dependencies conj '[nrepl,"1.0.0"]' -- update-in :dependencies conj '[cider/cider-nrepl,"0.28.5"]' -- update-in :plugins conj '[cider/cider-nrepl,"0.28.5"]' -- update-in '[:repl-options,:nrepl-middleware]' conj '[cider.nrepl/cider-middleware,shadow.cljs.devtools.server.nrepl/middleware]' -- repl :headless

NB: This doesn't properly inject the shadow-cljs nREPL middleware for some reason. You'll need to add this to project.clj:

  :repl-options {:nrepl-middleware [shadow.cljs.devtools.server.nrepl/middleware]}

Attach to the nREPL server in another terminal:

% lein repl :connect
...
main.server=> 

Starting shadow-cljs

(Not needed if if you started the REPL using the shadow-cljs executable.)

user=> (do (require 'shadow.cljs.devtools.server) (shadow.cljs.devtools.server/start!))
shadow-cljs - HTTP server available at http://localhost:8700
shadow-cljs - server version: 2.19.9 running at http://localhost:9632
shadow-cljs - nREPL server started on port 57770
:shadow.cljs.devtools.server/started
user=> 

(Never mind the second nREPL server started here.)

At this point the shadow-cljs dev browser is running and serving on port 8700, but your ClojureScript app is not running. It is not even compiled. As you'll see if you visit http://localhost:8700 with your browser.

Start the shadow-cljs watcher

(The require is not needed if you started the REPL with the shadow-cljs executable.)

user=> (do (require 'shadow.cljs.devtools.api) (shadow.cljs.devtools.api/watch :app))
[:app] Configuring build.
[:app] Compiling ...
[:app] Build completed. (170 files, 0 compiled, 0 warnings, 1.35s)
:watching
user=> 

Start the ClojureScript app

Open http://localhost:8700 in a web browser. (Or refresh if you opened it in an earlier step.)

Confirm that the shadow-cljs watcher is running by editing some hiccup in src/main/core.cljs and save the file.

Attach to the ClojureScript REPL

(The require is not needed if you started the REPL with the shadow-cljs executable.)

user=> (do (require 'shadow.cljs.devtools.api) (shadow.cljs.devtools.api/nrepl-select :app))
To quit, type: :cljs/quit
[:selected :app]
cljs.user=>

(If you instead of [:selected :app] see :missing-nrepl-middleware, you are probably using Leiningen and missed a note above.)

Confirm that your REPL is attached to the app in the browser:

cljs.user=> (js/alert "BOOM!")

(You need to dismiss the alert in the browser to get the prompt back.)

Something more interesting could be to update the app state, maybe?

cljs.user=> (load-file "src/main/core.cljs")
[]
cljs.user=> (in-ns 'main.core)
nil
main.core=> (swap! app-state update :text str " UPDATED")
{:text "Hello world! UPDATED"}

(Check the app in the browser.)

Notes

If you use this project as a base for something, then edit .gitignore and remove package-lock.json.

The reason the Leiningen project file is not including the shadow nrepl middleware configuration is that I think the failure to inject via the command line is a bug in Leiningen and that it will eventually be fixed.