[racket] Code Reuse, Object Oriented vs Functional

On Fri, Jun 21, 2013 at 09:39:12AM -0400, Ray Racine wrote:
...
> 
> 4) The Expression Problem
>    For whatever reason one can code in OO without ever noting the
> expression problem.   After a bit of functional programming you note it,
> even if you don't know what it's been called or the dual nature of the
> issue in OO land.  Once you know of it you'll start noting its dual side in
> OO.
...

Care to describe the Expression Problem  and its dual?  I'm  not 
familiar with the term.

-- hendrik