I like the approach Wouter van Oortmerssen uses in Lobster:

    class Animal:
        alive = true

    class Cat : Animal
        def hello(): print "meow"

    class Dog : Animal
        barked = 0

    def hello(d::Dog):
        print "bark!"
        barked++

    let d = Dog {}
    d.hello()

    let a:Animal = d
    a.hello()

In other words, A.B(C) is just syntactic sugar for B(A, C), and class definitions are just syntactic on top of that. The relation between OO and regular functions and stateful objects is completely transparent (the only real gotcha I can see is that a.hello() still works here because it was assigned a subclass where this method is defined). This makes it easy to store B for later usage, like you wanted to do in your example, and reason about its behavior.

[0] http://strlen.com/lobster/