Towards the end of Chapter 3 (B2) the via function is implemented as:
def via(id) do
{{:via, Registry, {ProducerConsumerRegistry, id}}, []}
end

However the app fails to start unless you change this to:

def via(id) do
{:via, Registry, {ProducerConsumerRegistry, id}}
end

Here is the code that I entered as specified in the book: GitHub - chriseyre2000/scraper: Sample app from chapter 3 of Concurrent Data Processing In Elixir

I did restart this chapter from B1 just in case something had changed/been fixed.