Olivier Lacan

Software bricoleur, word wrangler, scientific skeptic, and logic lumberjack.

Better Feedback on Ruby 2.2 Keyword Argument Errors

Written on March 30, 2015 in Paris, France

I was building a class yesterday and I wanted the instantiation process of the class to be self-documenting so I decided to use a keyword argument in the initializer method.

class HallMonitor
  def initialize(user: nil)
    @user = user

As you can see, I also decided to make the user keyword argument optional by making it default to nil.

Later while pairing and refactoring the implementation of the class and the associated specs, I forgot about my fancy keyword argument and simply wrote the following:


This raised the following error:

ArgumentError: wrong number of arguments (1 for 0)

I was confused for a second. I hesitated, then said "it makes no sense" out loud because I remembered making the user argument optional. Of course I had misread the exception as "wrong number of arguments (0 for 1)", which it wasn't.

Then I went back to the class, looked up the method definition and went "aaaahhh!".

But here, Ruby could have been far more helpful to me. It made sense before we had keyword arguments for Ruby to respond with an exception that would just compare the number of provided arguments against the number of required arguments. Nowadays we have keyword arguments, Ruby can tell us exactly what argument is missing and what its name is (if we use keyword arguments of course), so why not throw this exception instead:

ArgumentError: wrong number of arguments (1 for 0), keyword arguments: [user: nil]

This kind of simple error feedback improvements, similar to one proposed by Richard Schneeman earlier this year would probably save a Rubyists time debugging simple mistakes. And that's bound to make them happier, isn't it?

If you agree or disagree, let me know on Twitter or by email. I'll submit a feature suggestion to bugs.ruby-lang.org if nobody objects for a sensible reason.