haskell: improve support

[DavHau]

resolve all TODO's in ./src/subsystems/haskell

[eldritch-cookie]

is this still valid if so where are the TODO's currently?

[DavHau]

This refers to code on the legacy branch, which has been created before the module system based architecture was introduced.

I haven't ported over the haskell code from the legacy branch (yet).

In the mean time, a new haskell module (unrelated to the ones from legacy branch) has been created by @t4ccer.

I could go and port the haskell support from the legacy branch, but I'm not sure if it is worthwhile. It's largely based around doing lookups at eval time at the json DB all-cabal-json which is a custom json representation of what can be found at commercialhaskell/all-cabal-hashes and commercialhaskell/all-cabal-files.

Maintenance of the tooling around the custom json database has been challenging. The tool cabal2json which was invented for this purpose is unmaintained and our update action for the all-cabal-json repo recently stopped working. Also doing all the data processing entirely with nix is complex.

Therefore it might be better to continue development based on the approach started @t4ccer where all required metadata is locked via a python script and then fed into nix in a simplified form instead of accessing a custom json DB with nix.

@eldritch-cookie If you are interested in contributing, maybe have a look at @t4ccer's WIP-haskell-cabal module. You can try out this example (I should probably move this elsewhere as it's still WIP, therefore the permalink). As of now, this module is probably pretty simplistic, so feel free to go ahead and improve it. Also feel free to ask questions.