I've posted a rough first pass at a chapter about duck typing (and 
other typing issues in Ruby). It covers

- what a type is
- duck typing
- Ruby's built-in protocols (to_str, to_proc, etc)
- coercion and double dispatch

   http://www.pragmaticprogrammer.com/extracts/ducktyping.pdf

It isn't typeset, and I haven't really read it through for typos. I'd 
be interested to see what folks think. Again, all feedback gratefully 
received.

Cheers

Dave