What is this hidden package?
You build a package and get a message like:
Could not find module `Data.Map': it is a member of package containers-0.1.0.0, which is hidden.
This is because the package has not been updated for ghc-6.8 which has split the base package into lots of smaller packages. The package needs to be updated to say that it depends on these new split base packages, like containers, process and several others.
If you just want to get the package to build, add the missing package names to the build-depends: line in the .cabal file. For example given the above error message we would add the 'containers' package to the build-depends.
Developers of packages who want to know how to update their package properly so that it will continue to work with old and new compilers should see Upgrading_packages.