So there is a compatibility module in the new syb. Unfortunately, that won't tell you about the moves and rationale. Most of the time, you'll want Data.Data (check "ghc -e ':browse Data.Data'" or the Haddock pages, or google for "syb" in the libraries@ archives):
$ ghc-pkg find-module Data.Data c:/ghc/ghc-6.11.20081004\package.conf: base-4.0.0.0
Thanks a lot Claus and José for the info. Since all I use is the Data and Typeable classes (presumably like so many others, which I guess was the reason to keep these in base), it would obviously be better for me to avoid linking to the new syb package when I don't have to.
$ ghc -ignore-dot-ghci -e ':info Data.Data.Data'
Somehow I find this name hilarious. :-)
.. I would prefer to use the new base-4 when possible. The cabal file already includes a conditional "if flag(splitBase)" to handle really old versions, I guess what I'm asking for is something similar for this case. Is there a splitSyb flag or some such?
I was wondering whether there is a way to set user-defined flags depending on whether some package is available. Then I recalled that flags don't work the way I expected - instead Cabal will try all flag settings to find a buildable configuration (a fact I only became aware of when a different kind of flag was added recently that does behave the way I expected;-). Which might be what you want in this case. Duncan: is this correct, and are these subtleties documented somewhere?
While these things would be good to know in general, it seems this is not what I want in this case, since I don't want to use the syb package after all. It seems instead what I want is to simply make a conditional import of either Data.Generics or Data.Data based on which version of base is available. I guess that means more CPP heresy, sigh. Thanks, /Niklas