It’s just convention. This is how Ruby does it which of course inspired a lot of Elixir conventions. Also, Erlang modules names are conventionally snake case (and at least must start with a lowercase letter). There is no technical reason, but one thing to keep in mind is that file != module in Elixir, ie, you can have multiple modules in a single file. I often have some files that aren’t named after any of the contained modules, like errors.ex. For me (and this is just me), using camel case would suggest that the file is the module whereas snake case more correctly suggests there is no such correlation

Hehe. I once tweeted some “evil” Elixir in which I did defmodule :false do. Alas, someone reported it as a bug and it got fixed

edit: actually, I don’t think I needed the colon, but … details

Nitpick for clarity: module names are aliases which is why they must start with an uppcase letter. And to nitpick myself, of course you can use straight up atoms as well, ie defmodule :foo, do: (). But yes, always atoms.

Excellent point. Hairsplitting may be unwelcome in other arenas but in software development it’s essential!

This is technically what you are doing when you do defmodule Foo do. Foo is like an “auto alias” as it expands to :"Elixir.Foo" so you’re really doing:

iex(1)> defmodule :"Elixir.Foo", do: (def hi, do: "hi")
{:module, Foo,
 <<70, 79, 82, 49, 0, 0, 5, 68, 66, 69, 65, 77, 65, 116, 85, 56, 0, 0, 0, 164,
   0, 0, 0, 17, 10, 69, 108, 105, 120, 105, 114, 46, 70, 111, 111, 8, 95, 95,
   105, 110, 102, 111, 95, 95, 10, 97, 116, ...>>, {:hi, 0}}
iex(2)> Foo.hi()
"hi"

Even though it’s pretty simple, I still get confused and I find it much easier to make the distinction when talking about them. And sometimes the difference is meaningful when a macro expects an (unexpanded) alias, like Phoenix.Router.scope.