[Haskell-cafe] Dealing with incoherent instances

[Vyacheslav Akhmechet]

I'm looking at GHC's overlapping instances docs here:
http://web.mit.edu/ghc/www/users_guide/type-extensions.html#instance-decls
and I've ran into the incoherent instances problem.

Basically, I have a "catch all" instance that handles all types in a
generic manner using SYB introspection, and then I have type specific
instances that specialize behavior for certain types. This works with
overlapping instances extension but whenever I take advantage of this
functionality from polymorphic functions I run into the incoherent
instances.

If I enable incoherent instances GHC always picks the general case
which seems like the wrong thing to do. What I want it to do is delay
comitting to an instance until it's processing a specific invocation
of a polymorphic function. All the information is available at compile
time but I found no way to do this.

Is there a way to get around this problem?

Thanks,
 - Slava.