Backpack and multiple public library interop

[ekmett]

Haddock gets horribly confused by the use of backpack today.

A quick list of rough edges I've cut myself on recently.

• It seems to have a hard time finding backpacked modules. Does cabal need to feed haddock more information about mixins or something?

• Anything renamed on import causes documentation cross-linking to fail.

• If a backpack package supplies an instance, even if I import that backpacked module and export the class, then it won't show up on the instance list for my class.

• reexported-modules don't show up in the module list at all, forcing use of stubs.

• When a package supplies multiple public libraries each gets separate haddocks, but then hackage doesn't understand anything about that idea.

When working with cabal 3+ and multiple public libraries all of this yields a situation where the haddocks are nigh unreadable.

[Mikolaj]

I've just tested haddock branch 9.0.1 and 9.2.0 and I can confirm it stills overwrites the tar file containing Hackage haddocks for each private library defined in the cabal file. The results is that only the documentation from the last defined library (whether private or not, I suppose) is visible.

Perhaps it's not so hard to fix, because the documentation for each library is generated fine. It should just all be linked from a single page (in the most straightforward manner, for a start) and all tarred into a single archive. That's assuming we want to show documentation of private libraries (perhaps with a flag, similar as with executables?). Otherwise, consider only the public ones.

Related issue: #1196

[Mikolaj]

FYI: the just merged #1277 seems to be a step towards (and a possible stop-gap solution for) haddocks for multiple internal libraries (not sure about backpack).