Where hpack goes so does hpack-dhall.

[philderbeast]

In the same way that we have an --hpack option for taking the package definition from package.yaml with hpack can we also have an --hpack-dhall option for taking the package definition from package.dhall with hpack-dhall?

> cabal2nix --help
cabal2nix converts Cabal files into build instructions for Nix.

Available options:
--hpack                  run hpack before configuring this package (only
                           non-hackage packages)

[philderbeast]

I kept --hpack as an option and added --dhall;

Available options:
  --hpack                  package.yaml --> .cabal (only non-hackage packages)
  --dhall                  package.dhall -> .cabal (only non-hackage packages)

[philderbeast]

> cabal2nix --dhall siggy-chardust
*** found package.dhall. Using hpack-dhall...
...
mkDerivation {
  pname = "siggy-chardust";
  version = "1.0.0";
  src = ./siggy-chardust;
...
  libraryToolDepends = [ hpack-dhall ];
...
  preConfigure = "hpack-dhall";
...
}

cabal2nix --dhall https://github.com/BlockScope/flare-timing --subpath=siggy-chardust
...
*** found package.dhall. Using hpack-dhall...
...
mkDerivation {
  pname = "siggy-chardust";
  version = "1.0.0";
  src = fetchgit {
    url = "https://github.com/BlockScope/flare-timing";
    sha256 = "1f8z45p894m77m80nk0di6x6s7mvdx35d1y70971m661iqybk5z1";
    rev = "5b34dd6c5a8babef45a3beca2af9f04ccf8ed645";
    fetchSubmodules = true;
  };
  postUnpack = "sourceRoot+=/siggy-chardust; echo source root reset to $sourceRoot";
...
  libraryToolDepends = [ hpack-dhall ];
...
  preConfigure = "hpack-dhall";
...
}

[philderbeast]

When package.dhall has an error, in this case I've added a trailing semicolon;

> cabal2nix siggy-chardust
*** found package.dhall. Using hpack-dhall...
*** hpack-dhall error: "\n\ESC[1;31mError\ESC[0m: Invalid input\n\n./siggy-chardust/package.dhall:48:8:\n   |\n48 |       };\n   |        ^\nunexpected ';'\nexpecting \"!=\", \"&&\", \"++\", \"//\", \"//\\\\\", \"/\\\", \"==\", \"||\", \"\8743\", \"\10835\", \"\11005\", '#', '(', '*', '+', '.', ':', '?', built-in expression, double literal, end of input, import, integer literal, label, list literal, natural literal, record type or literal, text literal, the rest of expression, or union type or literal\n". Exiting.

[philderbeast]

@peti you've probably seen cabalism/hpack-dhall#3 where @sol gives some pros and cons of hpack-dhall. I liked using hpack (yaml) for filling in cabal fields for me like other-modules. I like that I can keep those benefits of hpack and yet do more with hpack-dhall.

[sellout]

I think given the discussion on #375, this should probably be closed as wont-fix.

I have opened #628 (also based on the comments on #375), which hopefully captures a better way forward.