Nix / NixOS

edit: for the solution, see my comment below

I need/want to build aeson and its subproject attoparsec-aeson from source (it's a fork of the "official" aeson), but I'm stuck... can you help out?

The sources of attoparsec-aeson live in a subdirectory of the aeson ones, so I have the sources:

aeson-src = fetchFromGitHub {
  ...
};

and the "main" aeson library:

aeson = haskellPackages.mkDerivation {
  pname = "aeson";
  src = aeson-src;
  ...
};

When I get to attoparsec-aeson however I run into a wall: I tried to follow the documentation about sourceRoot:

attoparsec-aeson = haskellPackages.mkDerivation {
  pname = "attoparsec-aeson";
  src = aeson-src;
  sourceRoot = "./attoparsec-aeson"; # maybe this should be "${aeson-src}/attoparsec-aeson"?
                                     # (it doesn't work either way)
  ...
};

but I get

 error: function 'anonymous lambda' called with unexpected argument 'sourceRoot'

Did I fail to spot some major blunder (I am nowhere near an expert)? Does sourceRoot not apply to haskellPackages.mkDerivation? What should I do to make it work?

BTW:

IDK if this may cause issues, but the attoparsec-aeson sources include symlinks to files in the "main" attoparsec sources:

~/git-clone-of-attoparsec-sources $ tree attoparsec-aeson/
attoparsec-aeson/
├── src
│   └── Data
│       └── Aeson
│           ├── Internal
│           │   ├── ByteString.hs -> ../../../../../src/Data/Aeson/Internal/ByteString.hs
│           │   ├── Text.hs -> ../../../../../src/Data/Aeson/Internal/Text.hs
│           │   └── Word8.hs -> ../../../../../src/Data/Aeson/Internal/Word8.hs
│           ├── Parser
│           │   └── Internal.hs
│           └── Parser.hs
├── attoparsec-aeson.cabal
└── LICENSE