Add nix flakes support

[Myhlamaeus]

This adds a flake.nix which exposes the project's outputs (mainly packages.{system}.run) and defines the dependencies.

niv was removed as nix flakes obviate it.

BREAKING CHANGES: The project.nix module is now exposed as an output
projectModules.pre-commit-hooks on the flake instead of a separate file.

Fixes #57

[Myhlamaeus]

I'll soon create a corresponding PR on hercules-ci/project.nix as this PR breaks compatibility with it.

[domenkozar]

This needs a rebase on top of master that landed 20.09 and other things :)

[Stupremee]

Is this planned to be merged any time soon? (Otherwise, I can open another PR)

[roberth]

We should only offer a cleaned-up interface in the flake. Otherwise people will have to migrate twice - once for flakes and then again for the revised interface. This means implementing some of the ideas in #55 and #61.
This also means we can get rid of some of the inputs and take them from Nixpkgs.
If you enjoy removing unnecessary wiring, I'd be happy to review. Otherwise I can do it, but probably no earlier than a month from now.

[Stupremee]

I don't think I have enough experience with Nix to do it.

[terlar]

Did this result in anything? I see nothing linked here, so I guess not, but just checking as this would be a nice feature to have.

[dhess]

Even just a simple fix for the currentSystem error when evaluating this project in a flake would be really helpful:

error: attribute 'currentSystem' missing

       at /nix/store/smvan4qznfgg79w1s6wjw1zdbszkynav-source/pkgs/top-level/impure.nix:90:39:

           89|                 else (if args ? localSystem then {}
           90|                       else { system = builtins.currentSystem; }) // localSystem;
             |                                       ^
           91| })
(use '--show-trace' to show detailed location information)

[domenkozar]

Even just a simple fix for the currentSystem error when evaluating this project in a flake would be really helpful:

error: attribute 'currentSystem' missing

       at /nix/store/smvan4qznfgg79w1s6wjw1zdbszkynav-source/pkgs/top-level/impure.nix:90:39:

           89|                 else (if args ? localSystem then {}
           90|                       else { system = builtins.currentSystem; }) // localSystem;
             |                                       ^
           91| })
(use '--show-trace' to show detailed location information)

I'm happy to fix this - what's the easiest way to reproduce it?

[dhess]

@domenkozar Thanks for the offer! This public project has a source-code-checks attribute which builds fine using nix-build (via flake-compat) but not using nix build (with flakes support enabled):

https://github.com/hackworthltd/haskell-template/blob/a51b92252120a7fe82bc4d9f70842aae3526cbe8/flake.nix#L82

Beware that it uses haskell.nix so you'll probably want to be using the upstream haskell.nix binary cache. Alternatively, you could just take a look at what I'm doing there and add it to a simple test flake. It's pretty straightforward, except for the part where we override the tools to use the haskell.nix versions, which is irrelevant to this particular issue.

BTW, I thought this was going to be a very easy fix as it looks like you just need to set localSystem.system to system in nixpkgss args, but that still didn't work. If I recall, it has something to do with niv.

[domenkozar]

Sounds like the best way is to add flake support into the project itself.

I'm going to close this and track progress at #57

[domenkozar]

Thanks @Myhlamaeus, for your hard work.

I've incorporated some of your changes and flake support is live now: https://github.com/cachix/pre-commit-hooks.nix#nix-flakes-support