[plt-scheme] Programming without Objects

Hi,
I'm in the midst of learning Scheme right now, and just need a major
shift in thinking.
What is the basic structure behind Schemes that don't use objects?

ie. in an OO language:

A function that takes a list of objects, and then calls quack() on
each object in turn.

Therefore, any object that implements quack() can be used with that
function.

How would you structure a similar program in Scheme without OO?

Thanks for the help
  -Patrick