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Final report for GSoC 2018

Haskell.org - Multiple Libraries project - Francesco Gazzetta

Work done

Main "multiple libraries" PR

The majority of my work is available in PR #5526

It implements support for multiple libraries per package, as described in #4206.

There are some TODOs in it. See "Future work" for more info.

EDIT: Since I'm going to do more work on that pr and I'll likely rewrite the git history, I created another branch and pr to record the state of the multiple-libraries-v4 branch at the moment GSoC2018 ended: #5528

Trying it

You can try the multiple libraries functionality by building the example project that can be found here. Follow the instructions in README.md.

Other work: refactors

While working on the main functionality I noticed a few much-needed refactor opportunities.

The Dependency-as-constraint problem

In cabal, Dependency is used in two seemingly related but now incompatible ways:

  • As a dependency specification ("I want this component from this package, version range xy")
  • As a constraint ("Package p must be in version range xy")

During GSoC I repeatedly tried to separate the two concepts. My first early attempt failed due to missing knowledge about the codebase. The second attempt is a rebase of #4265 and can be found on my library-dependency branch. Multilibs took a different direction though (multiple components in a single Dependency), so that patch is again unmergeable and will probably have to be redone. Still, it can be used as a reference.

The pervasive Maybe UnqualComponentName

Maybe UnqualComponentName was used pervasively to mean "a library name", where Nothing is the main library and Just libname a sublibrary. In my main pr I added a new datatype to substitute it: LibraryName.

I wasn't able, though, to replace all uses of Maybe UnqualComponentName without breaking functionality, so this branch is still unmerged.

The full refactor can be found on my maybeunqual-to-libraryname branch.

Future work

The main work is done, but there's still some mostly small things to do and polish.

Cleanups

  • Clean the history of the pr
  • Fix the few tests that still fail (mostly for missing imports and differing output)
  • Make sure roundtripping works
  • Clean up a few functions where I left TODOs. Mostly code that will be removed if BC handling is moved to a single place.
  • I used an empty set wherever I couldn't come up with a sensible Dependency default. I now know better and I should fix it. (it doesn't impact functionality but it could one day)

Missing features

  • A visibility = public | private field to restrict access to sublibraries
  • Version guard the new functionality and forbid the old one in new cabal versions
  • Tests! Possibly lots of them! And docs

Other work

  • Maybe: remove all the bits of code that handle the old syntax (there's lots of them) and do it all in a single spot.
  • Unrelated but really needed in my opinion: Merge the two refactors I mentioned above

Challenges & learnings

...a blog post will follow :)