"S. Alexander Jacobson"
Things I'd like to know.
1. Is there a programmatic way to detect what packages are installed or do I have to shell out to ghc-pkg?
Yes. Cabal talks to ghc-pkg, you can use the Distribution.Simple.Configure module for this, I think. Look around for a function called "installedPackages" or something.
2. Is the name in the cabal file guaranteed to match the name exposed by ghc-pkg?
The package name in the cabal file is the same as the package name for GHC.
3. Can I access the source of all exposed modules in an installed package to enumerate its external module dependencies and import chase?
Not the source of binary packages, but the .hi files have this information, I believe.
4. If you have an unpacked package in a directory, is there a path from the cabal file to the relative paths of each file in the package? (so you can point at a repository rather than a tarball and access it via http).
Yeah. You can derive it from the module name plus the hs-source-dirs.
5. Given that module names are absolute, if you have another way to find external modules required by a package (e.g. SearchPath or perhaps hackage reverse lookup), can you ignore build-depends?
If you have an orthogonal system to do the same thing, of course you can. peace, isaac