wip: Add callCabalToNix

[rvl]

It's a bit of a pain to run cabal new-configure ; plan-to-nix ; cabal-to-nix. This should be automated. One way to do this is with IFD.

As currently implemented, this needs the Nix sandbox disabled to work, because it needs to run cabal update. I'm not yet sure how to get a fixed-output derivation for 01-index.tar.gz. This file is always being appended to.

[angerman]

This suffers essentially from the same issues that #120 (cabalPlanToNix) suffers from. We need some form to inject the index-state that is reproducable. And have cabal accept that.

I wonder if we could do the following:

• download the most recent index state from hackage, this will be impure :-/ filter it to the correct timestamp, and
• provide a --file option to new-update that takes the file instead of trying to fetch it from hackage.

[angerman]

Since input-output-hk/nix-tools#52 we have truncate-index which can produce deterministic hackage indices given an index-state time stamp.

In http://chriswarbo.net/projects/nixos/useful_hacks.html, chris lined out how to pre-populate a hackage index directly into ~/.cabal.

The idea now is that we add a lookup of indexState -> hash with day granularity (YYYY-MM-DDT00:00:00Z) from April 1st 2019 to input-output-hk/hackage.nix and update that on a daily basis as we do with the hackage expressions already. This would then allow to pluck a given hackage-index (at day granularity -- or you will have to provide the hash yourself), and compute dependencies from there. This could provide the basis for cabalToNix and projectToNix. We notably have the same similar information with input-output-hk/stackage.nix as well, just with a fixed resolution instead of the option to solve from a larger package universe.

[rvl]

Closed in favour of #122 and #146.