Ian Lynagh mentioned this problem to me a few weeks ago and Alex's thread reminded me of it. This doesn't really relate to Alex's proposal, so I'm starting a new thread. Let's say that from GHC's perspective, package A is exposed and package B is hidden. Angela Author creates package C which build-depends on both A and B. She incorrectly lists the build-depends on B, but not on A. While building, she doesn't notice, since A is an exposed package. This kinda sucks, since it would be nice to check build-dependencies at build time. This is very similar to the situation in Debian where a packager doesn't realize that there's a build-dependency on A, since she's already got it installed, but when the auto-builders get to it, the build fails since they don't have A installed. I don't know how to solve this; is there a way to require explicit -package flags for all packages in ghc 6.4? If there were, say -fhide-all-packages, then cabal could use that while building and Angela would catch the error of her ways much earlier. I think the situation for "run-time" dependencies is similar. peace, isaac