Collecting all dependencies for haskell nix project

If you are developing a library or a binary with a single cabal file in a repository, nix integration is very straightforward:

devShell = haskellPkgs.shellFor {
  withHoogle = true;
  packages = p: with p; [ your-project-name ];
  nativeBuildInputs = with haskellPkgs; [ cabal-install hpack haskell-language-server ];
};

But what to do if there are several cabal libraries in your repository and you manage dependencies for them separately… What, in that case, you have to put into packages = p: with p; [ ... ]; place?

The naïve answer is – just put all of your libraries there. But doing that can play a bad trick, because libraries does depend on each other. So it you have your-project-lib-a and your-project-lib-b, which depends on your-project-lib-a – then your-project-lib-a will end up in the list of dependencies that shellFor wants to have built with nix in order to provision your nix-shell.

It creates a strange self-dependant loop, in which the very fact of entering nix-shell requires to be built your-project-lib- with nix…

The trick is to use somewhat not-well documented genericBuilderArgsModifier feature of the shellFor in nix – it allows to filter-out some dependencies.

devShell = haskellPkgs.shellFor {
  withHoogle = true;
  packages = p: with p; [ your-project-lib-a your-project-lib-b ];
  genericBuilderArgsModifier = pkgs.lib.mapAttrs (_n: v: if pkgs.lib.isList v then builtins.filter (a: a == null || !(pkgs.lib.hasPrefix "your-project-lib-" a.pname)) v else v);
  nativeBuildInputs = with haskellPkgs; [ cabal-install hpack haskell-language-server ];
};

So when shellFor iterates over [ your-project-lib-a your-project-lib-b ] to discover dependencies of each of them, it will ignore every dependency which starts from your-project-lib- prefix.