Question

How can I let a module to be included by another module look for a constant inside the including module? In other words, how can I make B.foo and D.foo below give the expected results?

module A
    module_function
    public
    def foo; C end
end
module B
    extend A
    C = "foo in B"
end
module D
    extend A
    C = "foo in D"
end

B.foo #=> (Expected) "foo in B"
D.foo #=> (Expected) "foo in D"

Answer 1

You need to tell it to lookup C in the context of where A is extended in:

module A
  def foo
    self::C
  end
end

module B
  extend A
  C = "foo in B"
end

module D
  extend A
  C = "foo in D"
end

B.foo #=> "foo in B"
D.foo #=> "foo in D"