Provide hackage and stackage from haskell.nix

[rvl]

• Provide hackage and stackage from haskell.nix

  Providing hackage and stackage here simplifies usage of the new Haskell Infrastructure.

  With this change, the user doesn't need to specify revisions of the external repos, or update them.

  Basically, with hackage.nix and stackage.nix, the latest version is always best, because snapshots and package versions are added on top. So there is no need for users to choose a revision.

• Add the mkStackPkgSet function which is a shortcut for building stack projects.

• Add a test case for stack projects

• Add nix-tools

• Provide a script which updates hackage.nix, pushes the changes, and then updates hackage-src.json. Also provide a script which likewise updates stackage.nix, and a little bit of documentation.

[rvl]

Thanks for the reviews @angerman @ElvishJerricco.

Am I right to say that with hackage.nix and stackage.nix, the latest revision is always best, because snapshots and package versions are effectively added on top?

[angerman]

Thanks for the reviews @angerman @ElvishJerricco.

Am I right to say that with hackage.nix and stackage.nix, the latest revision is always best, because snapshots and package versions are effectively added on top?

Yes. They should both be append only. hackage should only get new packages and revisions for existing packages (which are separate). And stackage should just gain new nightly snapshots and lts versions. I don't think stackage considers snapshots to be mutable. Thus I don't expect them to mutate published snapshots/lts releases.

I am however only certain that the hackage index is append only. For stackage I'm only pretty sure that that is the case. I don't remember any formal statement.

[ElvishJerricco]

Personally, I'd like to see the necessary modifications made to nix-tools before merging this, so that we don't have the code churn in haskell.nix of including the nix-tools expressions and then deleting them once we're able to just import nix-tools.

[angerman]

I'm ok with a two step approach.

@rvl we don't have the update-stackage.nix though right? any particular reason?

[rvl]

I didn't add a script for updating stackage.nix because I didn't know how it was updated. The script is added now.

I made the PR input-output-hk/nix-tools#28 so that we can import nix-tools. After merging this, we will need to make a couple more PRs to fix git revisions.

Once this PR is approved, I plan to squash the rework commits before merging.