The Owlet Project
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Owlet is a collection of web-based self-guided activities designed for middle and high school students to learn technology skills through creative exploration. These slides explain the motivation behind the project.

Generated using Luminus version

To run the app locally:

Run the web server

lein run

Run the front-end development server

lein figwheel

Run the sass watcher

lein auto sassc once

Navigate to http://localhost:3000/



  • Make sure you have a recent version of the Java Development Kit. If not, download the installer from the Oracle downloads page. Select Accept License Agreement at the top of the panel for the latest version, then click the filename for your platform, e.g., jdk-8u112-macosx-x64.dmg. Go to your Downloads folder, open the .dmg file, and follow the instructions. To verify, enter java -version in a terminal. You should see something like the following:

    java version "1.8.0_112"
    Java(TM) SE Runtime Environment (build 1.8.0_112-b16)
    Java HotSpot(TM) 64-Bit Server VM (build 25.112-b16, mixed mode)

    Note how the version — in this case "112" — matches the file you downloaded.

    You'll also need the command-line utilities git, lein, and rlwrap installed on your system. On a Mac, the easiest way to get them is to first install Homebrew, then in a terminal command line, execute this:

    brew install git leiningen rlwrap

    Finally, the SASSC compiler

    brew install sassc

Clone the repo

  • Now, to copy the Owlet code onto your machine, first cd to where you want the Owlet directory to be, then clone this repository:

    git clone
  • Now if you do ls, you should see a new directory, owlet. Go there:

    cd owlet

Extra development tools

Our codebase (mostly) follows this style guide.

Figwheel ClojureScript REPL in Cursive/IntelliJ IDEA

To work on this project, all you really need is a terminal, a browser, and any text editor. However, there is much to be said for using the Cursive plugin in JetBrain's IntelliJ IDEA. Since it understands how the different parts of your code and its dependencies connect together, you can do code refactoring, smart code completion, one-click navigation to a variable definition, instantly view docstrings, and catch arity and spelling errors. Best of all, out of the box you can run a Read-Eval-Print-Loop (REPL) that knows about your code. Quickly experiment with your live, running code in the REPL's command line, load a small change into the REPL, switch namespaces, or run unit tests — all with just a couple keystrokes.

To get started with IntelliJ IDEA and Cursive, follow the Cursive installation instructions. Then Import an existing Leiningen project, namely owlet.

To configure the Cursive REPL connected to our running app, we need to create a Run/Debug Configuration.

  • Open IntelliJ and select menu item Run -> Edit Configurations...

  • Click the + button at the top left and choose Clojure REPL.

  • Choose Remote.

  • Enter a name in the Name field (e.g., Owlet nRepl).

  • Choose the radio button Use Leiningen REPL port.

  • Uncheck the box Activate tool window at the bottom.

  • Click the OK button to save your REPL config.

Now, assuming you have a Clojure nREPL started in the terminal (see Running the application, above), you can hook in to it from Cursive at any time:

  • Go to Run -> Run..., then select your REPL config (called "Owlet nRepl" above). A new Cursive REPL tool window will appear. You should immediately see just this in the REPL window:

    Connecting to remote nREPL server...
    Clojure 1.8.0
  • Now that we're connected to the Clojure nREPL, create a ClojureScript REPL by evaluating the following Clojure code in the text box at the bottom of the REPL tool window:


    You should see something like this output:

    Prompt will show when Figwheel connects to your application
    To quit, type: :cljs/quit
    => nil

Now, since we're just "jacking in" to the same Figwheel server, when you modify and save a .cljs file, Figwheel will notice and automatically reload it. From the REPL, you can control the app as it is running, since you're evaluating code in the context of the live app. Plus, you have access to the Cursive REPL tools that interact with the editor, such as:

  • Switch REPL NS to current file

  • Load file in REPL

  • Send form before caret to REPL

  • Run tests in current NS in REPL

  • Add new REPL Command

For example, a good idea would be to add your own REPL command to evaluate the cljs-repl code, above. Once you have a REPL window, select Tools -> REPL -> Add New REPL Command. Give your command a name, select the Execute radio button, and enter (figwheel-sidecar.repl-api/cljs-repl) as above. Select the Project specific checkbox and OK. Then for easy access, you can define a keyboard shortcut of your choosing in IntelliJ IDEA -> Preferences... -> Keymap.

More ClojureScript REPLs

Once you've run script/, a Clojure nREPL is running, and you can "jack in" to get another Clojure REPL, then another ClojureScript REPL, similar to how we did it in Cursive:

  • Confirm that your nREPL started by script/ is still running.

  • From a terminal, run the following command:

    lein repl :connect

    You should now have a Clojure REPL with prompt, owlet.server=>.

  • As with Cursive, enter the following Clojure code at the prompt:


    You should see output like this:

    To quit, type: :cljs/quit

Debugging with Dirac

With a little extra setup, you can work on Owlet using the amazing Dirac DevTools browser debugging environment. You will still be running the app with Figwheel, so modified files will still compile and load automatically, but the browser REPL will be running in Dirac. The Dirac environment on the browser is actually a Chrome extension consisting of a customized fork of Chrome DevTools, the JavaScript debugging tool built into Chrome. However, it makes use of features only provided by the latest version of Chrome DevTools, which is why the Canary version of Chrome is required.

Dirac installation
  • If the script/ process started above is running, then stop it (Control-d).

  • Download and install the desktop application, Google Chrome Canary.

  • If you opened it, quit Chrome Canary.

  • In the terminal, make sure the current working directory is still the one containing this file.

  • At the command line, run


    You'll see an empty Chrome window with location http://localhost:3000/. It is empty because we haven't started up Owlet server yet.

    By the way, this command is how you'll need to start up the browser whenever you work on Owlet with Dirac. See below.

  • Install the Dirac DevTools extension, granting it access to your data. You should see a little green icon to the right of the address bar in the window.

    Since you started Chrome Canary with the script above, the extension will actually be saved in directory .dirac-chrome-profile/, so installing it or changing some settings will not affect (nor be affected by) any existing settings or extensions you may have in Chrome when started normally, say by double-clicking the Chrome or Chrome Canary icon.

Using Dirac

Now that Chrome Canary and the Dirac DevTools extension are installed locally in the Owlet project directory, let's use it with Owlet.

  • In the terminal, make sure the current working directory is still the one containing this file.

  • As above, start the app with Figwheel, but this time using the --dirac option:

    script/ dirac

    When you see the following, the nREPL has started and the Dirac server is waiting for the browser client:

    Dirac Agent v0.8.8
    Connected to nREPL server at nrepl://localhost:8230.
    Agent is accepting connections at ws://localhost:8231.
  • If Chrome Canary isn't already running, start it by running the following in a separate terminal window:


    You should now see the Owlet app running in the window that pops up.

    Once you start up Chrome Canary in this way, you can leave it open, even if you restart the Owlet app and the REPL. As always, you can cleanly reload the app with View -> Force Reload This Page (Command-Shift-R).

  • Click the Dirac DevTools toolbar icon. The Dirac DevTools Console window should appear. Note the instructions there about switching between ClojureScript and JavaScript REPLs (Control-,). If you see the error message, "CLJS DevTools: some custom formatters were not rendered", then just do View -> Force Reload This Page (Command-Shift-R).

    Though you may be in the habit of typing Command-Option-i, don't! Do not open the regular Chrome DevTools.

  • Try out the nice REPL in the Console tab and see how parentheses are automatically balanced, arrow keys take you up and down in the REPL history, symbols are completed as you type, output is colorized EDN data (not obscure JS objects), data structures are presented as collapsible widgets to neatly save space, and more!

  • Try out the debugger too. it works just like the Chrome Devtools debugger, except that source code is both ClojureScript and the JavaScript it compiles to. In the Sources tab, drill down to top -> localhost:4000 -> js/compiled -> out, click on an Owlet .cljs file of interest, then set a breakpoint that will be hit when you do something in the app's GUI. When the app stops at the breakpoint, look at current variables in the Scope section of the debugger. Then back in the Console tab, enter ClojureScript forms into the REPL. They will be evaluated in the breakpoint's context. Click the resume button or key F8 to let the app continue.

Dirac ClojureScript REPL in Cursive/IntelliJ IDEA

With Dirac, you don't have to give up Cursive. Just as we connected with the Figwheel CLJS REPL, above, we can connect with the Dirac REPL.

  • If you have a REPL running in Cursive, stop it by clicking the X in its toolbar.

  • Go to Run -> Run... and select the REPL config we created above, As before, you should immediately see just this in the window:

    Connecting to remote nREPL server...
    Clojure 1.8.0
  • Now, as before, we're connected to the Clojure nREPL, but this time we'll connect to the Dirac ClojureScript REPL. Evaluate the following Clojure code in the text box at the bottom of the REPL tool window:

    (dirac! :join)

    You should see something like this output:

    Your current nREPL session is a joined Dirac session (ClojureScript) which targets 'the most recent Dirac session'
    To quit, type: :cljs/quit
    => nil

As mentioned above, it's a good idea to Add New REPL Command and define a keyboard shortcut to type the (dirac! :join) command for you.

More Dirac ClojureScript REPLs

You can connect with the Dirac REPL, just like we did with the Figwheel REPL, with only a small difference. Of course, first ensure the process you started with script/ dirac is still running, then just follow the directions above, until the last step. Instead, do this one:

  • As with Cursive, enter the following Clojure code at the prompt:

    (dirac! :join)

    You should see output like this:

    To quit, type: :cljs/quit

Dirac REPL Caveat

When you evaluate an expression in the Dirac ClojureScript REPL, the result will be shown after => in the terminal or Cursive REPL window, as expected. However, side effects like printed output or exception stack traces will be shown only in the Dirac DevTools console. This can be confusing, especially if you've inserted a print statement and you see nothing, or you don't realize something broke because you don't see an exception! You need to look in the Dirac DevTools console. The console will mirror the expression you entered, its result, and any printed side effects. So just keep Chrome Canary nearby and the Dirac DevTools window handy.


The ISC License

Copyright (c) Code for Denver and Contributors

Permission to use, copy, modify, and/or distribute this software for any purpose with or without fee is hereby granted, provided that the above copyright notice and this permission notice appear in all copies.