Phil Hagelberg recently won the Lisp Game Jam 2018 with his entry EXO_encounter 667.

What I found most interesting however, was his interactive programming setup.

He programmed his game in (and contributed new features to) a Lisp to Lua compiler called Fennel, and used the game programming library Löve.

With Emacs and some Lua thread magic, he was able to perform runtime changes and introspection to his live running game project. (See below for a demo!)

Based on past experience developing visualization and image processing algorithms, I learned how useful this sort of interactive / runtime programming could be.

Hagelberg wrote up his experience in three great blog posts:

… and he has made the full source code to EXO_encounter 667 available on gitlab, so I had to try the interactive programming setup out for myself.

Due to his great write-ups, this was surprisingly easy.

Below you’ll find a short screencast of the setup in action, the steps I took to get everything running, and finally some information on how he put the interactive programming parts of the game together.

Demonstration

Here is a short video demonstrating a live programming session:

Quickstart

Here are the steps I followed to get everything up and running:

Install löve, lua and fennel

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brew install caskroom/cask/love
brew install lua
luarocks install --server=http://luarocks.org/dev fennel

Install the Emacs fennel-mode

This important piece of code is also by Hagelberg.

Check out fennel-mode where you usually work with github and gitlab checkouts:

mkdir ~/build && cd ~/build
git clone https://gitlab.com/technomancy/fennel-mode.git

Evaluate the following two lines in Emacs using for example M-x eval-region:

(autoload 'fennel-mode (expand-file-name "~/build/fennel-mode/fennel-mode") nil t)
(add-to-list 'auto-mode-alist '("\\.fnl\\'" . fennel-mode))

Get and start playing with EXO_encounter 667

git clone https://gitlab.com/technomancy/exo-encounter-667.git

Start by opening wrap.fnl in the root directory.

Then, as per the instructions, start the Fennel repl using C-u M-x run-lisp. This will ask you which lisp to use. Replace the default fennel –repl with love . (that’s love followed by space and a period)

At this point you will get a repl via which you can enter fennel commands. You can also edit any of the top-level fennel files, and type C-c C-k to reload the whole file, and watch the game change before your eyes.

More detail on how the interactive programming parts work

When you start love . from within Emacs fennel-mode, this runs the game, but starts an extra Lua thread to listen for input from Emacs.

(see the Interactive Development section in the blog post titled “in which a game jam is recounted further“)

Looking at the source, main.lua bootstraps fennel and loads in wrap.fnl which contains the familiar love.load, love.draw and love.update callbacks.

In love.load, it starts the repl, which is loaded from lib.stdio, which is where the extra listener thread is started up.

Emacs fennel-mode does the rest. Once you’ve done run-lisp with love ., you can use all the dynamic commands described on the fennel-mode gitlab page.