seeking ideas for short lecture on type classes

[Norman Ramsey]

In a fit of madness, I have agreed to deliver a 50-minute lecture
on type classes to an audience of undergraduate students.  These
students will have seen some simple typing rules for F2 and will
have some exposure to Hindley-Milner type inference in the context
of ML.  I am soliciting advice about
  * Cool examples of type classes
  * Papers I could read to explain how to implement type classes,
    especially if I could show the `dictionary translation' which
    is then followed by ordinary Hindley-Milner type inference
  * Any other material on which I might base such a lecture
I'm especially in need of a guide to the literature, as
the `Haskell bookshelf' at haskell.org is silent on the topic
of type classes.

Please send me your recommendations.


Norman Ramsey