GHC 6.6 Data.Tuple incompatibility

[Simon Peyton-Jones]

| After discussing it with Ian Lynagh, his conclusion was that the
problem
| is that Data.Tuple was not compiled with overlapping instances
support,
| and that it probably ought to be.  He asked me to post here to see if
| everyone else felt the same way.

Another possibility is to loosen the rules, so that you must have
-fallow-overlapping instances when compiling the module with the *more
specific* instance, but not for the less-specific one.  That would make
it easy to override library definitions... but it would make it
impossible to make an instance that you did not want overridden.

I'm open to suggestions here.   The issue is what we want; the
implementation is straightforward now that the machinery is there.

Simon