cabal2arch (the binary) tries to load two files, ghc-provides.txt and library-providers.txt. These two files are installed by archlinux (the package). This means that the PKGBUILD for cabal2arch can't makedepend on archlinux but instead has to depend on it. In other words, since archlinux contain data files the basic assumption that executables only need to makedepend on all libraries is wrong for executables that use archlinux.
I think there are two solutions to this:
I would prefer the second solution.
I should probably add that the two solutions are specific to cabal2arch. In the long run we need to address the more general problem of libraries having datafiles.
As a quick-fix, I've uploaded a new version of cabal2arch to AUR and kiwilight.com that declares a run-time dependency on haskell-archlinux. The PKGBUILD was hand-edited, though, so this is really a hack.
Remy, I think this can be closed, right?