Backward compatibility

[Simon Peyton-Jones]

Dear Core Libraries Committee
In addition to seeking your guidance about the comparison-primop question (a decision in the next few days would be most welcome), there are of course other breaking changes in GHC 7.8.
High on the list is Typeable (see http://ghc.haskell.org/trac/ghc/ticket/8132), which has become polykinded, which is SO much better.  Also, to guarantee the type-soundness of Typeable, we want to make it an error to use hand-rolled instances, rather than "deriving( Typeable )".  The latter is very common anyhow, and the hand-rolled ones mostly use Typeable1 which is now heavily deprecated.
There's a compatibility module Data.OldTypeable, but you'd have to import it to get the old behaviour.
It's very difficult to really provide a "bumpless transfer", which is why Michael's idea of a compatibility package might be worth exploring.
Anyway this is just a heads-up.
Simon

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