How to reference project relative nix files in nix_file_content?

[philderbeast]

With overrides and overlays, external .nix files are often referenced.

https://nixos.org/nixpkgs/manual/#how-to-create-nix-builds-for-your-own-private-haskell-packages
https://nixos.org/nixpkgs/manual/#sec-pkg-override

How would I do that with rules_haskell in a WORKSPACE file?

nixpkgs_package(
    name = "ghc",
    build_file = "@io_tweag_rules_haskell//haskell:ghc.BUILD",
    nix_file_content = """
  let pkgs = import <nixpkgs> {
          overlays = [ (import ./nix/overlays.nix) ];
      }; in
  pkgs.haskell.packages.ghc822.ghcWithPackages (p: with p;
    [newtype scientific aeson cassava template-haskell]
  )
  """,
    repository = "@nixpkgs",
)

It seems that bazel is referencing files relative to a temporary build folder and not relative to the project that I'm building.

flare-timing-bazel> bazel build //...
INFO: Build options have changed, discarding analysis cache.
error: getting status of '/private/var/tmp/_bazel_pdejoux/729fadfaa2051173dd27a953445384ed/external/ghc/nix/overlays.nix': No such file or directory
ERROR: /.../detour-via-sci/BUILD.bazel:6:1: no such package '@ghc//': Cannot build Nix attribute ghc. and referenced by '//detour-via-sci:ghc-impl'
ERROR: Analysis of target '//detour-via-sci:ghc-impl' failed; build aborted: no such package '@ghc//': Cannot build Nix attribute ghc.
INFO: Elapsed time: 0.337s
INFO: 0 processes.
FAILED: Build did NOT complete successfully (5 packages loaded)

I'm trying to avoid uom-plugin-0.2.0.1 that is brought in with ghc822.ghcWithPackages and depend instead on uom-plugin-0.3.0.0.

[mboes]

This is rules_nixpkgs specific. Does the nix_file_deps attribute help (see tweag/rules_nixpkgs#18)? If not, this might be a dupe of tweag/rules_nixpkgs#21.

[philderbeast]

Yes thanks, the nix_file_deps does help.

nixpkgs_package(
    name = "ghc",
    build_file = "@io_tweag_rules_haskell//haskell:ghc.BUILD",
    nix_file_deps = [
            "//nix:overlays.nix",
            "//nix:uom-plugin.nix",
        ],
    nix_file_content = """
  let pkgs = import <nixpkgs> {
          overlays = [ (import //nix:overlays.nix) ];
      }; in
  pkgs.haskell.packages.ghc822.ghcWithPackages (p: with p;
    [newtype scientific aeson cassava template-haskell]
  )
  """