Subvisual | The keyword arguments falacy

Ruby 2.0 came with a feature that I love, that's the keyword arguments. They allow you to make your objects interface clearer, so you can call methods like so:

game.play(no_of_players: 4, buy_in: 1000)

If you know your ruby 1.9.x you are probably saying "wait a minute, I can already do that!". That's true, but the way it was handled on the method was as a hash:

def play(options = {})
  no_of_players = options[:no_of_players]
  buy_in = options[:buy_in]
end

As you can see, this is not as nice and fluent as ruby tries to be. With keyword arguments you can make it much cleaner:

def play(no_of_players: nil, buy_in: nil)
end

If you're wondering about those nil, they are default values for when that key is not present, so if you called game.play, both no_of_players and buy_in would be nil. We can take advantage of that and add some better defaults:

def play(no_of_players: 2, buy_in: 100)
end

So now when you call game.play, the no_of_players is 2 and the buy_in is 100. This also means that we can just code ahead not worried about having nil values, right? We can do something like:

def play(no_of_players: 2, buy_in: 100)
  @house_money = no_of_players * buy_in
end

Well, no. We can't. And this is where it gets tricky. Even though the defaults will be used when the key is not present, they will not be used when an explicit nil is passed in. Therefore, this:

game.play(no_of_players: nil)

will throw an error, because we're trying to multiply nil by 100.

This example is trivial, the nil is easy to spot, but what if it is a variable calculated somewhere else on your application?

You have two options here:

You ensure that variables passed in to the method can never be nil.
You handle the nil inside of the method with something like this:

def play(no_of_players: nil, buy_in: nil)
  no_of_players ||= 2
  buy_in ||= 100
  @house_money = no_of_players * buy_in
end

This approach defeats the purpose of the keyword arguments almost entirely, but rest assured that you won't have to do this often.

Most of the time you won't really care if the arguments are nil, because they either won't be or you'll be checking that someplace else. If you do need to make sure they are not nil, you are left with these options.

The moral of the story is to not assume that just because you have a default, you can't have a nil.