To update the Stackage snapshot you need to clone the hazel repository into a separate directory:
$ git clone https://github.com/tweag/rules_haskell.git
Change into the hazel directory.
$ cd rules_haskell/hazel
Then execute the following command to update to the specified Stackage
snapshot, where $PROJECT points to the root of this repository:
(Requires stack)
$ Stackage.hs lts-12.4 "$PROJECT/hazel/packages.bzl"
On NixOS you may need to modify Stackage.hs to append the following flag to
the list of stack intrepreter flags: --nix-packages zlib.
This will take a while.
To update the nixpkgs revision, go to the nix/nixpkgs directory in
this repository and run nix-update-src-json ./nixos-* to update to
the latest release in the current NixOS channel. To upgrade to a new
channel move nixos-18.09 to nixos-19.03 (adapt to the channels you
want to move from and to), change the branch field in
default.src.json and modify nix/nixpkgs.nix to point to the new
channel.
After upgrading the revision, the easiest solution is usually to open a PR and see what fails on CI (running the builds locally can take quite some time). The most common reason for failures is usually that we have overriden a specific package with certain patches that no longer work. In that case, a good first step is to check if these patches are still necessary and if not try to switch to the unpatched package from nixpkgs.