- when recompiling a package with ABI changes, does cabal always update dependent packages?
If Foo depends on Bar and there is a new version of Foo that specifies a newer version of Bar then yes, the newer library being depended on will be build too (out of necessity). OTOH, if you are building a new version of a package on which others depend it won't build all the others. Ex: build a new "containers" package doesn't cause any of the ~1400 packages depending on it to be rebuilt.
It seems "not always" - it didn't update itself, nor refuse the breaking upgrade,
I don't really know what "it" is. Something to do with recompiling Cabal and cabal-install I take it, but I'll refrain from comment seeing as I don't understand what you're doing.
- is there a "specification" of which are the "core" packages?
Are there packages on which the community standardizes? That's the goal of Haskell-Platform [1], but I don't place any special value in a package being in HP yet - I just work with whatever package on Hackage fills my need and am under the impression this is most peoples mode of operation.
While package removal is not supported through cabal, it is sometimes needed
Why? What I see is a need for users to understand ghc-pkg (or whatever package management tool exists for their Haskell compiler). Should "cabal uninstall" provide a unified interface to some number of Haskell compiler packaging systems? It could but doesn't seem like a priority. Cheers, Thomas [1] http://hackage.haskell.org/platform/