Elixir
Documentation
You can't use @doc on private functions
Interestingly you can't use @doc macro for private functions. I understand the argument that documentation shouldn't be generated for private functions, but it is a little surprising that you have to use multiline comments instead of doc strings to document them.
Metaprogramming
Metaprogramming Elixir
I learned about the ability to directly modify your application behavior with metaprogramming in Elixir.