Lua Log #4: Build a (Terrible) Package

I have weird vision that, someday, my Atlas project will be useful to others. This is a particularly weird vision because the project is mostly for me to explore building a web framework. Realistically, I don’t expect any kind of consumer audience for a long time, if ever.

Even if there is no plan on the horizon to have something useful for others, I wanted to learn how to distribute code in the Lua ecosystem. My logic is that building a package now while the framework is small and simple will help ensure that I maintain a package going forward as complexity increases.

In Lua, the most common distribution system that I see is LuaRocks (get it? Lua means “moon” in Portuguese so Lua packages are “moon rocks,” essentially). To build a rock, I had to create a “rockspec” file.

I can create a rockspec, then upload my rock to LuaRocks so that users can run:

luarocks install atlas

This works today! You can see the details about my rock at the LuaRocks's atlas page.

From now on, I’m a rock polisher. :)

Creating a rockspec offers me some benefits beyond easy distribution. Primarily, the rockspec provides tools for managing dependencies. As much as I’m trying to do stuff on my own, there are things that my project depends upon.

  • argparse - For command line flag handling
  • inspect - For pretty printing Lua tables (The default table printing will only print a memory address. That’s not very useful.)
  • lpeg - A PEG parser tool that I use as the backbone of my template engine
  • luv - Lua bindings to the popular libuv project that I use as the basis for the web server

With these packages added to the rockspec, I can run

luarocks make atlas

and get all of my dependencies installed locally with minimal fuss.

So, why is my package terrible as mentioned in the title? Atlas does nothing useful currently. It won’t help you with your next web project. The tools inside of it aren’t even close to finished. I even messed up my handling of ATLAS_CONFIG so that the atlas command will not output help information without setting a non-obvious environment variable.

The nice part about putting out a terrible package is that I can only go up from here. 😜


Using LuaRocks has some quirks that I had to get used to for my project.

Lua lacks a great isolated environment story. When I work with Python, I can create a virtual environment with the venv module and use that environment so that all the packages that I install stay contained to that project. Similarly with JavaScript, if I’m doing some kind of frontend development, npm will prefer to install packages locally in a node_modules directory. With npm, you must choose to install globally.

With LuaRocks, the system prefers to install to a global location by default. In the LuaRocks parlance, rocks are installed in a “tree.” I’m amused by the visual image of trees on the moon or the image of rocks in a tree, but, whatever, that’s the name. To work around this global by default preference, I can use the --tree option on luarocks commands. I’ve added some local aliases to work around this limitation, but that was the first quirk I encountered.

The next quirk was with handling the rockspec file itself. luarocks includes a command named write_rockspec. This command will provide a template of a rockspec that you can fill with details about your package.

One detail that surprised me was the inclusion of a modules list. The modules list was a literal mapping of a Lua module name to the location of the associated source file. Managing this kind of list struck me as a particular kind of insanity. The configuration would look something like:

  modules = {
    ["atlas.main"] = "src/atlas/main.lua",
    ["atlas.templates.code_builder"] = "src/atlas/templates/code_builder.lua",
    ["atlas.templates.environment"] = "src/atlas/templates/environment.lua",
    ["atlas.templates.parser"] = "src/atlas/templates/parser.lua",
    ["atlas.templates.template"] = "src/atlas/templates/template.lua"
    -- And so on.

Who wants to manually update a manifest whenever you create a new file? The process is so susceptible to mistakes that I concluded that there must be another way. As I read LuaRocks docs, I learned that LuaRocks can build the list automatically (much like the template command originally did).

To use this feature, I thought I only needed to remove modules from my rockspec. That’s wrong. The reality is that this auto-modules feature is only available if I declare a newer rockspec_format. For some reason, write_rockspec doesn’t do this by default. After updating my file to include rockspec_format = "3.0", the package built correctly without the explicit modules list.

Another quirk about rockspecs is that they are version specific in the filename. This is different behavior from my packaging experience with Python, Perl, Ruby, and some other languages where I’ve used packaging systems. I’m not sure why LuaRocks uses this methodology, but that was another one that I have to get used to. Mercifully, there is a command called new_version to generate a new rockspec from a previous one.

The final quirk that I struggle with is some command confusion. LuaRocks includes three commands named make, build, and pack. These commands are so close in function with each other that it’s hard to keep track of the differences in behavior. One of the most useful documentation pieces that I discovered was a table comparing these commands. This table leads me to conclude that this confusion is a common problem for LuaRocks.

Other Things This Week

My editor was driving me crazy! I’m using Neovim for this project because Neovim went all in with Lua as its language of choice for configuration. Because of this choice, the Language Server Protocol (LSP) support with Lua is pretty solid. Unless you’re using the Busted test runner.

Busted uses a BDD style. My tests are full of describe, it, and assert.* calls. Unfortunately, these functions are inserted globally into scope by Busted when it runs. Since there is no library to require, I had warning messages all over my test files about undefined fields and functions.

I worked around the biggest ones by adding describe and it to my LSP configuration as globals that it should accept. I could have done this with assert, but assert is one of the built-in functions and Busted adds attributes so that users will call things like assert.equal or assert.truthy.

My epiphany this week is that Busted is doing nothing more than importing my assert table on my behalf. To make my LSP client happy, all I needed to do was require the assert directly.

-- my_module_test.lua
local assert = require "luassert.assert"

describe('Thingy', function()
  it('works', function()
    assert.equal(42, 42)

This revelation made working with my tests far more enjoyable because I no longer have warnings of undefined fields next to all my assert statements. It’s great!

This week I’m going to focus my attention on how to write asynchronous test code. My project will use a lot of async work with coroutines. I need to determine how to control an event loop in my test runs so that the synchronous test runner will play nicely with the asynchronous code. Busted provides an async/done pair of functions where done is supposed to be called when the async work is complete. I probably need to hook into those APIs is some fashion.

I’m going to do this kind of testing so that I can checking that my logger works asynchronously. The challenge will be combining Lua coroutines, the libuv event loop, and Busted’s async API.

I’ll report back when I’ve figured some of this stuff out.

Thanks for reading! Have questions? Let me know on Twitter at @mblayman.