deriving instances for "hidden" types

[Simon Peyton-Jones]

| I've often felt a tinge of disappointment not being able to=20
| derive classes over data structures that are defined in the=20
| libraries (DeepSeq FiniteMap, for instance).  I was wondering=20
| if any of the tools (drift, etc.) or GHCs built-in meta=20
| programming would allow you to say something like:
|=20
| derive (DeepSeq a, DeepSeq b) =3D> DeepSeq (FiniteMap a b)

I think Hal was only suggesting doing this in case where
the representation of the data structure was visible to the library
client.  (As others have said, it's not a good idea if the rep is=20
abstract.)

Template Haskell will be able to do this.  Heart transplant operation
is in progress.  Blood on floor, but patient still alive.

Simon