What does an argument with a colon but no default value do?

Question

I've seen something like this a couple times with ruby/rails code:

def self.cool_function(a, b:, c:)
   ...
end

But I am having trouble figuring out what it means

Answer 1

It means it's a required (keyword) argument.

You can see this by trying to call the method without one of those arguments:

cool_function(1)
#=> ArgumentError: missing keywords: :b, :c

So to correctly call the function, you must include those keywords, e.g.:

cool_function(1, b: 2, c: 3)

There is lots of documentation and blog posts around this core language feature. For example, you may find this post helpful to dig into the topic in much more detail.