Ugly nix incremental haskell build

default.nix

# Store the nix-created `dist` in `localSrc/.nix-cache` after build (whether
# successful or not) and re-injects it for subsequent builds.
#
# `./mask` should contain a haskell project with an `out.nix` file. That file
# should describe a typical cabal derivation with the extra field:
#
# preUnpack = ''
#   localSrc=${toString ./.}
# '';
#
#  (this is a hack to pass `localSrc`)

# TODO:
#  * Restore cache before cabal kicks in so that we don't have to delete files
#  * Store .nix-cache is `/nix/cache/<hash>` instead and add support for
#    sandbox
#  * Try with non-haskell projects like Rust, make-based, etc
#
#
let
    pkgs = (import <nixpkgs>) { overlays = [ overlayDirtyDist ]; };
    overlayDirtyDist = self: super:
    {
        haskellPackages = super.haskellPackages.extend (haskellPackagesNew: haskellPackgesOld: rec {
                  foobar =
                    pkgs.haskell.lib.overrideCabal
                      ( haskellPackagesNew.callPackage ./mask/out.nix { } )
                      ( oldDerivation:
                        {
                          postUnpack = ''
                            # Restore timestamps from the original source
                            if [[ -n $localSrc ]]; then
                              pushd "$sourceRoot"
                              rm -rf .nix-cache dist
                              for f in $(find . -type f); do
                                touch -r "$localSrc/$f" "$f"
                              done
                              popd
                            fi
                            '';

                          preBuild = ''
                            save_cache() {
                              rm -rf "$localSrc/.nix-cache"

                              # TODO unpack cache before preBuild to make sure
                              # we don't overwrite cabal stuff
                              #
                              # these files are generated by cabal
                              # rm -rf ./dist/build ./dist/setup-config

                              cp -av ./dist/ "$localSrc/.nix-cache/"
                            }
                            # Load and save cache state
                            if [[ -n $localSrc ]]; then
                              trap "save_cache" EXIT
                              if [[ -d "$localSrc/.nix-cache" ]]; then
                                cp -av "$localSrc/.nix-cache"/* ./dist/
                              fi
                            fi
                            '';
                        }
                      );
                }
            );
    };
  in { foobar = pkgs.haskellPackages.foobar;  }