Question

• Why does Foo.val return nil instead of "foo" before calling Foo.set?
• Is there any mechanism for initializing @val on class evaluation?

• In which scope is @val = "foo" stored into?

  class Foo
    class << self
      @val  = "foo"
      attr_reader :val
  
      def set(val)
        @val = val
      end
    end
  end
  
  p Foo.val # nil
  Foo.set("bar")
  p Foo.val # "bar"
  

Answer 1

You can initialize @val in Foo like this:

class Foo
  @val  = "foo"
  class << self
    attr_reader :val

    def set(val)
      @val = val
    end
  end
end

p Foo.val         #=> "foo"
Foo.set("bar")
p Foo.val         #=> "bar"

Your code initializes @val not on Foo, but on Foo’s metaclass