🔗 Differences between Elixir's protocols and behaviours

I second guess myself a lot about what I know, or what I think I know. It’s even worse when it comes to the names of people I don’t normally interact with: “I’m almost positive his name is Dave, but I don’t want to get it wrong, so I’ll just wait until someone else says his name.” It’s a solution, but I’d rather be certain about their names.

In my previous article, Crawling the Web with Elixir’s Broadway and Wallaby, I suggested that it might be “beneficial to use either a Protocol or Behaviour to reduce code duplication,” but when considering which to use for what I’m working on, it left me second-guessing myself yet again. What’s the difference? When would you use one over the other? How do they actually help?

Because I don’t think I’m alone in my confusion, I’m going to try to answer these questions by first explaining what I thought I understood about both, talking about what I got wrong, showing the differences, and then giving a real-world—if contrived and incomplete—example for when you might use both.