Great post.

One more: is_atom(nil) == true as nil is an atom. However, most people mean a defined atom and not ‘undefined’ nil.

Great one! Would have fit well in the current post with all the nil shenanigans but I’ll add it to the list for a follow up!

The chapter How to use constants is a little bit misleading as module a attributes have not been built to be constant in their design, even though we use them for that.

I agree that a new option for Module.register_attribute/3 to enforce it to be constant would be nice, even though I never saw the immediate need for that as if you respect the fact that you define attributes on top before code, this problem goes away.

I mean, the post mentions that they aren’t actually constant and more of a work-around but also linking to the docs saying to use them as constants. I don’t know how I should make it clearer there while keeping some brevity and not making it the entire topic of the post.

Ah my bad, you could also point out that a private function can be used for the same thing.

Already updated with that - or well a public function to keep it accessible. I discussed it a bit with José, there are some things that just don’t work without module attributes or would require other workarounds.

Related list collected a few years ago on this forum: Elixir Gotchas and Common Issues Wiki

Some great ones in there, thanks!

I got caught off guard by:

 [1, 2, 3] -- [1, 2, 3] -- [1, 2, 3]
[1, 2, 3]

After checking, the docs spell it out neatly once I starting looking for an explanation
(-- is right-associative).

Oh wow, that’s also news to me! Thanks!

I think the great thing about Elixir is that pretty much all of these are well spelled out in the docs