Impressions of Fennel with Hammerspoon

A while back I read an introductory book on Lisp programming titled, "The Little Schemer". This book opened my mind to new (to me) programming paradigms, and left me with a strong desire to find a way to incorporate Lisp programming into my every day life. It took some time, but I believe I've found what I've been looking for: Fennel.

The Fennel programming language is a dialect of Lisp that boasts compatibility with Lua, and it just so happens that two of my favorite applications are configured with exactly that language: Hammerspoon, and Neovim.

Initial Observations

To initially explore Fennel, I wanted to start small. My Hammerspoon configuration consists of 7 modules that I use for operations such as: launching applications, managing windows, keeping my computer from going to sleep, and general operating system automation. So the plan is to translate these modules into Fennel, while maintaining without breaking the existing functionality. However, at this point, I wasn't even sure how to embed Fennel into my project...

Integrating Fennel with Hammerspoon

While official documentation exists describing how to embed fennel into your project; it didn't provide me with enough clarity to know my next steps on integrating it with Hammerspoon. I found a few resources online demonstrating how to extend the package.path and package.cpath properties in Lua, but I was unable to get this to work.

• https://blog.exupero.org/hammerspoon-with-fennel/
• https://github.com/Hammerspoon/hammerspoon/issues/2377#issuecomment-636331435
• https://github.com/agzam/spacehammer/blob/master/init.lua

Ultimately, I opted to include the fennel.lua file to my Hammerspoon configuration, and while not ideal, it does make the configuration nicely self-contained. I'll leave it as a future task to include the module installed with LuaRocks.

With Fennel now included in my Hammerspoon configuration, all I need to do is configure the fennel.path to point to the *.fnl files in the .hammerspoon/ directory, and ttranslating these modules can begin!

-- init.lua

local fennel = require('fennel')

fennel.path = package.path .. ";" .. os.getenv("HOME") .. "/.hammerspoon/?.fnl"

table.insert(package.loaders or package.searchers, fennel.searcher)

require 'main'

Translating Lua to Fennel

As a Fennel novice, I was happy to see that the Fennel project provides an online cross-compiler for Lua and Fennel called anti-fennel, and while it can generate some strange-looking Fennel code, it was an extremely useful tool for me to get up-and-running right away. For example, by pasting the simple sleep function from the helpers module into the compiler:

function sleep(ms)
    os.execute("sleep " .. tonumber(ms) / 1000)
end

(fn sleep [ms]
  (os.execute (.. "sleep " (/ (tonumber ms) 1000))))

As another example, here is the output for my caffeine toggle:

hs.hotkey.bind(HYPER, "0", function()
  hs.caffeinate.toggle("displayIdle")
  if hs.caffeinate.get("displayIdle") then
    helpers:show("Caffeine Enabled", nil, helpers.styles.success, helpers.assets.check)
  else
    helpers:show("Caffeine Disabled", nil, helpers.styles.error, helpers.assets.ban)
  end
end)

(hs.hotkey.bind HYPER :0
  (fn [] (hs.caffeinate.toggle :displayIdle)
    (if (hs.caffeinate.get :displayIdle)
      (helpers:show "Caffeine Enabled" nil helpers.styles.success helpers.assets.check)
      (helpers:show "Caffeine Disabled" nil helpers.styles.error helpers.assets.ban))))

This was especially helpful for more gnarly modules like the window module used for window management, and seeing the Lua and Fennel code side-by-side was a kick starter in learning the language!

Next Steps

While my Fennel Hammerspoon configuration now works with parity to its Lua counterpart, I have not yet added new features or modules. I look forward to writing new Fennel code, and deepen my understanding of Lisp and the Fennel programming language.

Additionally, before beginning this endeavor, I was already aware of projects like spacehammer; a wildly impressive Hammerspoon configuration written in Fennel, but, I wanted to start small and learn the integration myself. However, with the basics out of the way, I hope to explore this project further, and seek lessons-learned for the configuration of my own.

The full pull-request for translating my Lua Hammerspoon configuration to Fennel can be found here: https://github.com/cmpadden/dotfiles/pull/19/files