cabal.project to Nix

[rvl]

There is no project-to-nix tool for cabal new-style projects.

Unfortunately the whole project parsing logic is in cabal-install, so it can't easily be re-used.

[ElvishJerricco]

Yea the fact that the parsing logic is in cabal-install is the main reason I haven't tried to do anything like this even for the nixpkgs Haskell infra. I'd really like it if cabal-install would export a few key data types and parsers in a library component.

[angerman]

Just for the record. @hvr pointed out that https://github.com/haskell-CI/haskell-ci/blob/master/src/HaskellCI/Project.hs should be sufficient to parse the cabal.project files; and give us a similar level of generic interface as we have to the stack.yaml

[angerman]

@rvl I think this might actually be done. The current plan-to-nix Behaves almost identical to stack-to-nix.

[rvl]

@angerman So I can go to my source tree containing cabal.project, follow the documentation, then have a working multi-project Nix build?

[rvl]

This is done, and documented in #165