Choosing between if and a modifier

A little bit of bikeshedding: In Ruby, even outside of guard clauses, I sometimes use modifiers to make the code easier to read out loud.

A mental rule that I use to choose when to use a full if versus a modifier is taking into consideration if the condition has a side effect:

In this case I would choose the explicit if because the post.save has some important information when reading this code, while return Success(post) if post.save focuses on what is returned when skimming the code.