Updating Hackage and Stackage Nix expressions

The hackage.nix and stackage.nix repos and corresponding files hackage-src.json and stackage-src.json will be regularly and automatically updated using scripts in this repo.

To run the updater scripts manually, use:

nix-build build.nix -A maintainer-scripts.update-hackage -o update-hackage.sh

nix-build build.nix -A maintainer-scripts.update-stackage -o update-stackage.sh

The scripts will clone the repo, generate the latest data, then attempt to push back to the repo and update the source JSON file.