mix.rb

module Numbers
  def foo
    2
  end

  def bar
    foo * 2
  end
end

class Something
  mix Numbers, :foo => :numeric_foo

  def foo
    "foo"
  end
end

# So now…
Something.new.bar #=> 4

# Or
Something.new.bar #=> "foofoo"

The code executes from top to bottom. The mix happens before foo is defined. The mix call will error if it is mixing in a method that already exists, but in your example the foo method doesn't exist yet. I think you would only need to set foo to nil if it was like this:

class Something
  def foo
    "foo"
  end

  mix Numbers, foo:nil
end

# So now…
Something.new.bar #=> "foofoo"

Just my understanding so far. I could very well be wrong.