No it's not. The error pattern makes it so that if you pass an error to the function, it just pipes it through and returns the same error. That's different tha

No it's not. The error pattern makes it so that if you pass an error to the function, it just pipes it through and returns the same error. That's different than throwing an error, which is what happens when you call a function with the wrong arity. Which is to say, using the function as defined, if verify_cart() returns {:error, err}, and I pipe it to charge_customer(card: foo), then instead of getting the expected {:error, err} value back out, instead a %FunctionClauseError exception is thrown. On the other hand, if it were defined as def charge_customer({:error, err}, _), do: {:error, err} then I would get the expected {:error, err} back out.