Idiomatic guard clause for checking not nil

[OvermindDL1]

@fmcgeough

As for other uses of not is_nil(val) vs val != nil I’d say just pick one and be consistent within your project. Personally not is_nil(val) is not my favorite.

I’m personally a fan of val != nil because nil is just an atom, so I don’t even know why it has a specialized function like is_nil/1 anyway, especially since the empty list [] is the nil value on the system, those confused me for a bit when I started Elixir (nil should have been [] as the empty list is named nil standardly, not :nil, blah).