Local packages

Local Haskell packages are defined in the packages.*.source option under haskellProjects.<name> module. They are automatically detected if you have a single top-level Cabal package, or multiple Cabal packages defined in a cabal.project file, via default settings.

Single package

If your repository has a single top-level .cabal file, haskell-flake will use it by default. There is no need to specify packages.*.source yourself.

Multiple packages

If you have multiple Haskell packages in sub-directories, you can refer to them in a cabal.project file to have haskell-flake automatically use them as local packages by default:

See https://github.com/srid/haskell-multi-nix for example:

$ cat cabal.project
packages:
    ./foo
    ./bar

The cabal.project file must begin with a packages: key, followed by a list of directories containing the cabal files.

$ ls */*.cabal
bar/bar.cabal  foo/foo.cabal

Source filtering

When a local package is in a sub-directory, haskell-flake will create a new store path to avoid changes to parent files (using cleanSourceWith) triggering a rebuild.

When a local package is the only top-level one, however, any file in the repository will by default trigger a rebuild. This is because haskellProjects.<name>.projectRoot is set to self by default.

Avoiding rebuild of top-level package

To avoid rebuilding the top-level package whenever irrelevant files change, you can do one of the following:

Put the top-level package in a sub-directory.

Or, set projectRoot to a subset of your flake root using fileset.toSource. For example:

{
  haskellProjects.default = {
    projectRoot = builtins.toString (lib.fileset.toSource {
      root = ./.;
      fileset = lib.fileset.unions [
        ./src
        ./haskell-template.cabal
      ];
    });
  }
}