Make it easier to cache eval-time dependencies.

[TravisWhitaker]

It would be nice to have all of the eval time packages that Haskell.nix requires available in one place. Without this, as far as I can tell one must manually work out the required packages and cache them separately. Things like nix-store -q -R --include-outputs ... don't know about dependencies that come as a consequence of IFD. If you get this wrong, users downstream of Haskell.nix (who might not be writing Haskell at all, merely depending on Haskell packages) will need to wait for e.g. nix-tools to build.

Ideally, one would be able to do something like nix copy --to my-binary-cache $(nix-store -q -R --include-outputs $(nix-instantiate -A haskell-nix.evalDeps)) to get everything needed at eval time on the given platform in their cache. If this already exists and I'm just too dense to find it, a clearer note in the docs might help.

[TravisWhitaker]

To be clear, I'm not talking about materialization or otherwise saving the Nix generated by IFD. I'm totally fine with using IFD, I'd just like to cache all of the exes/libraries needed at eval time.

[hamishmack]

Sorry this is not documented, but please try nix copy --to my-binary-cache $(nix-store -q -R --include-outputs $(nix-instantiate -A haskell-nix.haskellNixRoots)). Make sure defaultCompilerNixName is set as it will only build the roots for that version of ghc. Please let us know if there is anything missing.

[TravisWhitaker]

That's precisely what I was looking for, thanks! I'll leave this open and add a note to the docs.

[michaelpj]

Make sure defaultCompilerNixName is set as it will only build the roots for that version of ghc. Please let us know if there is anything missing.

Damn it, I've been doing this completely wrong (down with defaultCompilerNixName!). Petition to add something like haskellNixRoots to the output of project so we can pin that and get the right thing.

[TravisWhitaker]

Hmm, I think there's something missing for shellFor. With a clean cache I can eval package components without building anything (since haskellNixRoots is cached), but GHC is built all over again when shellFor is evaluated. Trying to pare down to a minimal repro...

[TravisWhitaker]

Narrowed it down to something required by

pkgs.buildPackages.haskell-nix.tool "hoogle" {
        version = "5.0.17.15";
        index-state = pkgs.haskell-nix.internalHackageIndexState;
      }

in builder/default.nix. This is the Hoogle used for hoogleLocal in shellFor.

[TravisWhitaker]

^^^ This specific case looks to be solved by #738