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Cobalt REPL

A ClojureScript REPL mixed with the Chrome DevTools Protocol (CDP). Cobalt + Chrome = Cobalt-Chrome, a particularly strong alloy.

Goals

Headless browser based REPL development. I want to start a real browser based REPL without having to interact with a browser window, but while keeping visibility into the browser's state, console logs, network activity, etc.

Inject a ClojureScript runtime into any page. I would like to "REPL in" to any arbitrary web page, without having to touch the hosting environment of the page itself.

Virtual machine access. When my Clojure program misbehaves or becomes opaque, I can introspect the JVM without my fingers leaving the keyboard. I want similar levels of visibility into my browser based ClojureScript code.

Motivation

It started off as a personal project, to learn more about the innards of ClojureScript. I wanted to teach my kids ClojureScript, and decided (foolishly, in hindsight) to ramp up my ClojureScript skills by writing a Grafana plugin. This exposed me to much of the hackery involved in getting ClojureScript actually running inside a browser, connecting a REPL, handling JS modules, and more.

I started to wonder why I couldn't just blast things from the REPL into the browser using Chrome DevTools Protocol. Turns out the awesome Tatut has already written an excellent Clojure library for interop, and so I started hacking ...

Getting started

So far, it's possible to load the cobalt.sample-test namespace and play around with it.

This namespace relies on httpurr, chosen because it also has a foreign-libs dependency on the Bluebird promises library, which forced me to not be lazy and learn how the heck that stuff works (I still have no idea, but it works).

I also listen for CDP runtime and log domain events and relay those to the Clojure logs, allowing you to watch errors on the JS side get logged in REPL.

git clone git@github.com:hagmonk/cobalt.git && cd cobalt
mkdir checkouts
cd checkouts && git clone git@github.com:hagmonk/clj-chrome-devtools
cd .. && lein repl

The clj-chrome-devtools checkout from my fork is required while I'm still in heavy development. I've already merged one set of changes upstream, but there are more PRs to come.

Now, start a repl environment. This will start a non-headless Chrome instance for debug purposes, compile the cobalt/sample_test.cljs file and associated dependencies, and load them. After which you should be left in a ClojureScript REPL.

(cobalt.repl/repl {:filename "cobalt/sample_test.cljs"})

Try evaluating some test ClojureScript. The following function should be available, which will make an XHR call to the github API:

(cobalt.sample-test/query-target)

Bugs / Incomplete functionality

This is a massively incomplete list.

  • define print-fn and friend in cljs on bootstrap
  • eval of whole file makes goog.provide complain about already defined ns.
  • websocket CDP connection to browser keeps timing out?? :(
  • handle errors in IJavaScriptEnv/-evaluate
  • why isn't IJavaScriptEnv/-load being called?
  • pretty printing ain't workin'