Ability to provision custom package database

[HeinrichApfelmus]

Haskell notebooks in the browser via WebAssembly are awesome! 🥳

However, when using Haskell notebooks for real, it is typically necessary to curate and install a custom set of packages, e.g. containers, diagrams-svg, Chart, …

As I understand correctly, xeus-haskell currently offers no way of provisioning a custom package set. This issue is about adding support for that.

[tani]

Hi! thank you @HeinrichApfelmus .
Here is the prototype 😉 I'd like to hear your feedback. Thanks!
https://github.com/jupyter-xeus/xeus-haskell/tree/feat/custom-library?tab=readme-ov-file#start-jupyter-lite-with-your-library-experimental-feature

[gilgamec]

I'm guessing that importing Haskell source means that mhs will have to compile it? Might it be easier to include an mcabal package directory? It's precompiled, but the binary format is a bag of combinators so should be compatible (except maybe for FFI?).

[tani]

@gilgamec compilation phase is done by mhs automatically for the first loading phase. You do not need to compile it manually. However, mcabel would be nice. I am happy to hear the implementation strategy and pull request. Thanks!

[gilgamec]

I got a proof-of-concept system working pretty easily. I built a package directory with mcabal, using the same version of mhs as the jupyter kernel, then mounted it to Jupyter with a XeusAddon.mounts line. On the MHS end, the directory is added using exactly the same mechanism as your MHS_LIBRARY_PATH, but adds the path to the pkgPath flag. The modules from installed packages can be imported and used. I didn't test extensively, but ran some stuff from mtl, containers, and random.

I'm not making a PR because 1) the changes are really trivial, and 2) I'm still having weird problems: the kernel will take 20s to start, or I get unresolved symbols, or mhs can't find an instance of PrintOrRun for things like Int.