A Clojurescript REPL inside your Emacs

After trying LightTable and discovering that perhaps the environment of functional programming languages is actually better than object-oriented programming languages because you can eval basically anything, I’ve wanted to replicate the same thing in my new and shiny editor, the old Emacs.

With CIDER (acronym for Clojure Interactive Development that Rocks for Emacs), it is pretty easy to load a REPL for Clojure inside emacs.

But sadly for Clojurescript, as your application is usually run in a browser(there are people doing node.js with clojurescript!), the scene changes. It took a while for me to figure it out it’s not the same thing as a normal Clojure REPL. To have a Clojurescript REPL in your terminal, you need Figwheel.

In the Clojurescript land we don’t really need to ever refresh our browser, it even should keep the last state you left. This is different from the conventional approach that other platforms use which watches the filesystem and refreshs the browser using websockets. Well, enough propaganda.

For the sake of having an example, we will start with a clean figwheel project:

lein figwheel new hello-world

Now, inside project.clj we have a :figwheel key with two commented lines, like this:

;; Start an nREPL server into the running figwheel process
;; :nrepl-port 7888

We now will uncomment this nrepl-port line, so it will now start a nREPL server in that port whenever you run lein figwheel, like this:

;; Start an nREPL server into the running figwheel process
:nrepl-port 7888

And we are done on the configuration on the project’s side. Yes, it’s just as easy as uncommenting one line.

Now on the dark, erm, Emacs side, you need to install CIDER. For instructions please check this.

After Cider is installed, we will connect to a specific nREPL which is the nREPL server we configured in project.clj at localhost:7888 by pressing:

M-x cider-connect

Now specify localhost as the Host and 7888 as the port. You should have a REPL tab now, in case it doesn’t open try pressing M-x cider-switch-to-repl-buffer.

At this point, we still won’t have the lovely, long-awaited Clojurescript REPL we wanted. For that, just run those two lines inside the REPL buffer we’ve got:

user> (use 'figwheel-sidecar.repl-api)
user> (cljs-repl)

(thanks @yogthos)

And now, you have a working Clojurescript REPL ready to build on amazing things.

Having problems with nrepl-refactor?

This is a issue that I’ve had. When connecting with cider I’ve got the message nrepl-refactor middleware not available! Did you remember to install it?.

To fix that, add to your emacs config:

(setq cljr-suppress-middleware-warnings t)

And it should work fine. More on that in this Github issue