[Haskell-cafe] Windows, doctest and auxiliary FFI wrappers?

[Viktor Dukhovni]

The Network.DNS library contains a bit of helper C-code for determining
the default DNS servers on Windows.  This code is linked in by
Cabal, when building object code for the library on Windows.

The library also has 140 "doctest" examples in the haddock comments,
which I'd like to test on Windows via AppVeyor.

I'm not having much luck making the dependency on the cbits visible
to the GHCi invocations executed by "Test.Doctest".  Does anyone
have some experience getting this sort of thing to work?

What I've tried:

    1.  Add "-liphlpapi" into the "doctest" command argument
	list, this proved necessary, but not sufficient.

    2.  Build a separate internal library (supported in Cabal
	2.0 and later), which is a dependency of the main library
	and links in the C glue code, but this proved to be a
	significant rabbit hole...

In fact approach 2 seems like the more promising one, but it runs
into a substantial obstacles:

    a.  Building internal libraries requires a "Simple" Cabal build,
	but the cabal-doctest package which automates compiling
	doctests, requires a "Custom" build type. :-(

    b.  A Simple build, means abandoning cabal-doctest, and using
	"doctest" directly, but the new sandbox builds place package
	artefacts in locations that the command-line invoked by
	"Test.DocTest.doctest" does not know how to find.

	It appears that the "Custom" build from cabal-doctest is
	there for a good reason: it hooks into Cabal's build process,
	and has access to enough metatata, to generate wrapper code
	that invokes doctest with an argument list of the form:

	    -XCPP
	    -i
	    -i<DISTDIR>/build/<ARCHDIR>/<GHCVER>/<PKG>-<VER>/build/autogen
	    -i<DISTDIR>/build/<ARCHDIR>/<GHCVER>/<PKG>-<VER>/build
	    -i<SRCDIR>
	    -package-env=-
	    -hide-all-packages
	    -no-user-package-db
	    -package-db=<CABALGLOBALSTORE>/<GHCVER>/package.db
	    -package-db=<DISTDIR>/packagedb/<GHCVER>
	    -package-db=<DISTDIR>/build/<ARCHDIR>/<GHCVER>/<PKG>-<VER>/package.conf.inplace
	    -optP-include
	    -optP<DISTDIR>/build/<ARCHDIR>/<GHCVER>/<PKG>-<VER>/build/autogen/cabal_macros.h
	    ...visible packages...
	    ...modules to doctest...

	which are needed for the extracted doctest fragments to find
	their dependencies.

Hard-coding such arguments into the "Simple" build seems to work,
but is hardly maintainable.

I've not yet found an interface in the Cabal library that would
divulge these magic strings as a function of the "pkg.cabal" file,
something of the shape:

    cabalMagicGhcArgs :: FilePath -> IO [String]

would make it possible to compute the arguments at runtime (rather
than build-time) in a Simple build, without having to use a Custom
build to machine-generate wrapper code.

Other than the above, I'm out of ideas for now, any help much
appreciated.

-- 
	Viktor.