Provide Ormolu via Nix

[aspiwack]

Ormolu has very slow to build dependencies (essentially ghc-lib-parser). This has been an obstacle to contributing, and makes the CI very slow on the first push of a new branch.

Currently, the ./format script build Ormolu itself, via stack build. This is convenient (calling ./format is machine independent). The main reason why we went this route back then, was that the version of Ormolu that was available on Nixpkgs was too old, we needed a recent one. Now, I believe that it should be easy to provide Ormolu via Nixpkgs.

This doesn't prevent contributors who want to provide Ormolu via Stack to call stack build ormolu && stack exec -- ./format. It makes the management of dependencies more package-manager agnostic. And will let us download cached Ormolu when using Nix (in particular on CI).

[endgame]

I happened to want a recent ormolu from nixpkgs in some other place, and I found that I had to do this with ghc-9.2.5 to get a recent enough ghc-lib-parser (which meant rebuilding ghc-lib-parser again):

    ormolu = (prev.haskell.packages.ghc925.override {
      overrides = hfinal: hprev: with prev.haskell.lib; {
        ormolu = hprev.generateOptparseApplicativeCompletions [ "ormolu" ]
          (enableSeparateBinOutput hfinal.ormolu_0_5_0_1);
      };
    }).ormolu.bin;

So ormolu-from-nixpkgs may not dodge the ghc-lib-parser build.

[aspiwack]

Indeed, I've noticed something like this in a project of mine recently.

But for CI at least, we can workaround this by setting up a Cachix cache. Which doesn't have the issue that Github actions seem to have that the first build on a PR doesn't use the cache (or maybe that's an issue with our configuration, but I've checked and it looks good. Obviously I may have missed something and there may be an easy solution, it would be great in the short term).

[mrkkrp]

Why don't you just use https://github.com/mrkkrp/ormolu-action ?

[tbagrel1]

@mrkkrp
AFAIR, we wanted for the same ormolu version to be provided both to the user and to the CI. Which the github action wouldn't help to do, right?

[mrkkrp]

Depends on how badly you want this and whether you want to make a compromise. I usually just use the same ormolu I have on my system for all formatting, and I don't particularly care about its version. I use the GitHub action for all projects on CI. The advantage is that the CI setup is very simple and acts as the definite source of truth, which is all I want. In all my projects it's been really rare, if ever, that my system ormolu would format differently from the CI version. In 99.99% of cases you won't have to reformat, and the benefit is simplicity and reliability.

[tbagrel1]

I'll let @aspiwack decide then :)