Alternative nix

[ajmcmiddlin]

While I was working on #380 I (with the help of some team mates) updated the nix setup. @IvanMalison updated the nix setup before I cleaned up my changes and put them in a PR. I thought I should still raise this version, as it takes a different approach that may be preferable.

The key difference here is that nixpkgs is (by default) pinned to a specific commit, such that this setup should always work regardless of the user's NIX_PATH (where <nixpkgs> comes from).

I've tested this with both nix-build and nix-shell --run "cabal v2-build github".

[colonelpanic8]

@ajmcmiddlin We could easily pin with the setup that I uploaded by pinning in default.nix. I actually deliberately decided NOT to pin (in the overlay would be fine to pin in default.nix) because this is a library that is presumably used by many different people, who might want to pin to different versions of nixpkgs themselves.

The overlay is generally useful enough that I think people will be able to use it with somewhat different versions of nixpkgs, and it automatically plugs in nicely with existing nix setups.

[ajmcmiddlin]

Yeah, that all makes sense. I'll use your setup and maybe raise a PR for taking nixpkgs as an argument in default.nix, defaulting it to a pinned version. I think I'll need that in my current project.