Can't print dependencies of bytestring

[sjakobi]

I'd like to use this project to investigate the potential fallout of a change in bytestring (see haskell/bytestring#140).

As a first step I tried to get a list of its reverse dependencies, but failed:

$ nix-build default.nix --argstr reverseDepsOf bytestring --arg justPrintAllDeps true
error: value is null while a set was expected, at /home/simon/src/nix-reverse-deps-of-haskell-package/default.nix:127:44
(use '--show-trace' to show detailed location information)

The result is the same with e.g. text.

Am I hitting a fundamental limitation of this project or could this be fixed?

[cdepillabout]

Unfortunately I think this is a fundamental limitation.

bytestring and text are GHC boot packages, so they are actually set to null in nixpkgs:

https://github.com/NixOS/nixpkgs/blob/303c94493c36406159c033aa9100e4aac534e366/pkgs/development/haskell-modules/configuration-ghc-8.6.x.nix#L10-L42

That's why you're getting the value is null while a set was expected error message.

One way to work around this might be by adding your own bytestring package to the Haskell package set and overwriting the built-in nulled-out bytestring package. I'm not sure how easy this would be to do.

[sjakobi]

Thanks for the quick response!

I think I'll try to build a custom GHC using the modified bytestring version then.

[cdepillabout]

@sjakobi Oh, I don't mean you have to use a modified GHC. I just mean that in an overlay to haskellPackages, you might be able to use your own bytestring version:

self: super: {
  bytestring = haskell.lib.callCabal2nix "bytestring" ../some/path/to/modified/bytestring-src/ {};
}

I'm not 100% sure this will be easy to get to work, although hopefully it will.

---

Of course, using a modified GHC will almost certainly work, it will just take a very long time to do the initial compile.