Add support for building remote imports as FOD to dhall-nixpkgs

[cdepillabout]

This PR adds support for converting remote imports to fixed-output derivations (FOD) to dhall-to-nixpkgs.

This functionality was discussed in #2304 (comment) (and the next few comments).

---

This PR adds a --url-as-fod argument to dhall-to-nixpkgs directory. Passing --url-as-fod causes dhall-to-nixpkgs directory to turn remote imports into FOD.

For instance, the current dhall-to-nixpkgs will output the following for mydhallfile.dhall:

$ dhall-to-nixpkgs directory ./example-dhall-nix --file mydhallfile.dhall
{ buildDhallDirectoryPackage, example-dhall-repo, Prelude }:
  buildDhallDirectoryPackage {
    name = "example-dhall-nix";
    src = ./example-dhall-nix;
    file = "mydhallfile.dhall";
    source = false;
    document = false;
    dependencies = [
      (example-dhall-repo.overridePackage { file = "example1.dhall"; })
      (Prelude.overridePackage { file = "List/map.dhall"; })
      (Prelude.overridePackage { file = "Text/upperASCII.dhall"; })
      ];
    }

With the code from this PR, and using the --url-as-fod argument, the following will be output:

$ dhall-to-nixpkgs directory ./example-dhall-nix --file mydhallfile.dhall --url-as-fod
{ buildDhallDirectoryPackage, buildDhallUrl }:
  buildDhallDirectoryPackage {
    name = "example-dhall-nix";
    src = ./example-dhall-nix;
    file = "mydhallfile.dhall";
    source = false;
    document = false;
    dependencies = [
      (buildDhallUrl {
        url = "https://raw.githubusercontent.com/cdepillabout/example-dhall-repo/c1b0d0327146648dcf8de997b2aa32758f2ed735/example1.dhall";
        hash = "sha256-ZTSiQUXpPbPfPvS8OeK6dDQE6j6NbP27ho1cg9YfENI=";
        dhall-hash = "sha256:6534a24145e93db3df3ef4bc39e2ba743404ea3e8d6cfdbb868d5c83d61f10d2";
        })
      (buildDhallUrl {
        url = "https://raw.githubusercontent.com/dhall-lang/dhall-lang/9758483fcf20baf270dda5eceb10535d0c0aa5a8/Prelude/List/map.dhall";
        hash = "sha256-3YRf+0Vo1AMn8qgX60LRxhOLkpynWNULwzES7zyIVoA=";
        dhall-hash = "sha256:dd845ffb4568d40327f2a817eb42d1c6138b929ca758d50bc33112ef3c885680";
        })
      (buildDhallUrl {
        url = "https://raw.githubusercontent.com/dhall-lang/dhall-lang/9758483fcf20baf270dda5eceb10535d0c0aa5a8/Prelude/Text/upperASCII.dhall";
        hash = "sha256-Ra5PvYFLBHTmXCik7pKyO5eYkvpbtzcwvJlnWueQyik=";
        dhall-hash = "sha256:45ae4fbd814b0474e65c28a4ee92b23b979892fa5bb73730bc99675ae790ca29";
        })
      ];
    }

This uses buildDhallUrl to build all of the remote imports. buildDhallUrl is a function I propose in #2304 (comment). You can find its implementation here. This would likely need to be added to Nixpkgs.

[Gabriella439]

Looks great! Just one minor comment

[Gabriella439]

I'd suggest renaming this to --fixed-output-derivations and not reference URLs specifically. That way if we want to add support for other fixed output derivations (like local path imports or environment variables protected by integrity checks, for whatever reason), then we don't need to rename the option.

[cdepillabout]

That's a good idea.

I rebased this PR on master and added a commit changing the --url-as-fod flag to --fixed-output-derivations: c65b550

[Gabriella439]

@cdepillabout: Thank you for your patience working on this! 🙂

[cdepillabout]

Thanks for merging this in :-)

The next step is to get the Nix functions from https://github.com/cdepillabout/example-dhall-nix/blob/e6a675c72ecd4dd23d254a02aea8181fe875747f/default.nix into Nixpkgs (along with necessary documentation updates).

Since dhall-to-nixpkgs has gotten the flag --fixed-output-derivations in this PR, then the buildDhallUrl function should definitely be upstreamed into Nixpkgs.

Personally I'd also like to get the other functions from that file into Nixpkgs (dhallPackageToNix, generateDhallDirectoryPackage, and dhallDirectoryToNix), since I think these functions are widely(?) useful. But these functions aren't quite as critical as buildDhallUrl.

If this sounds good, I'll start putting together PRs to Nixpkgs adding these functions.

[Gabriella439]

Yeah, go ahead and open a PR to add those utilities to Nixpkgs