Question

More specifically:

• When do you need to prefix the scope with :: (like ::Foo::Bar)
• When is directly referring to a scoped const ok? (just Foo::Bar)
• Is there a good reason why this behavior is so confusing?

EDIT: I am talking about stuff like this

module Foo
  THING = 'thing'
  module Bar
    puts THING
  end
end
#=> thing

module Foo::Bar
  puts THING
end
#=> NameError: uninitialized constant Foo::Bar::THING

Answer 1

When do you need to prefix the scope
with :: (like ::Foo::Bar)

When there’s another constant with the same name in the current namespace.

When is directly referring to a scoped
const ok? (just Foo::Bar)

When there isn’t another constant with the same name in the current namespace. Ie. when that identifier is unambiguous. Similarly, you could just use Bar to aid readability if it was unambiguous.

Is there a good reason why this
behavior is so confusing?

It’s balancing readability and ease of use against specificity. You don’t always want to have to do ::Foo::Bar::Baz::Boo (the globally unique identifier) when you’re deep down in your namespace.