Hi David,
postgresql-libpq clearly depends on the libpq C library (and possibly other things) as listed in the Extra-Libraries field in its cabal file, *as do anything that require postgresql-libpq* which fact does *not* directly show up in their cabal file. Walking the tree and pulling that info is a possibility, but it needs to be done after dependency-resolution in case requirements change version to version.
yes, it's a complicated situation. The information stored in a Cabal file must be "finalized" before Cabal can use it to install the package. According to [1], this process depends on the following parameters: - Cabal flags chosen by the user, - the set of installed (or install-able) packages, - the current architecture and operating system, - the Haskell compiler used to build the package, and - version constraints chosen by the user. Thus, a given package may depend on an external library 'foo' if you compile it with GHC 7.6.3, but it depends on 'bar' if you compile it with GHC 7.4.2, and the same variance occurs recursively in all its dependencies. It would be nice to know the list of external dependencies for any given package, but in fact it's not "the list" -- it's many possible lists, because the outcome depends on the exact environment in which the package is built. A rough estimate of these external dependencies is available from Nix, though. The package postgresql-libpq, for example, has its Hackage home page at [2]. That page links to the NixOS build information at [3], which shows the following dependencies: acl-2.2.52 attr-2.4.47 bash-4.2-p45 binutils-2.23.1 coreutils-8.21 gcc-4.8.2 gcc-wrapper-4.8.2 ghc-7.6.3 glibc-2.18 gmp-5.1.3 gnutar-1.27.1 linux-headers-3.7.1 ncurses-5.9 perl-5.16.3 postgresql-9.2.7 readline-6.2 zlib-1.2.8 That list is not be accurate for all environments, but it's a start. Take care, Peter [1] http://hackage.haskell.org/package/Cabal-1.18.1.2/docs/Distribution-PackageD... [2] http://hackage.haskell.org/package/postgresql-libpq [3] http://hydra.nixos.org/build/9708550#tabs-runtime-deps