Allow project to be developed using Nix

[srid]

Some Haskell developers like myself use Nix to manage their environment. This change makes it easier to use Nix to get the project up and running. The nix-shell --run ghcid command for example will pull the dependencies (including ghc itself!), and launch ghcid which is extremely handy for iterative compilation feedback from edits.

I have also increased the version (referring to a yet to be published version) in default.nix so that nix does not accidentally pick the published version from hackage.

Ref:

• https://nixos.org/nixpkgs/manual/#how-to-create-a-development-environment
• https://twitter.com/puffnfresh/status/990154797494943744?lang=en

[snowleopard]

Thanks @srid! I'm unsure if it's worth committing this file to the repository, because it's completely generic, i.e. has no relation to the library itself. What about adding it to .gitignore instead? It would allow you and other Nix users build the library with Nix by just creating this file.

Bumping the version to 0.4 is a good idea, let's do that.

[srid]

Sure, that's fine (the default.nix can grow depending on custom dependencies used though).

[snowleopard]

the default.nix can grow depending on custom dependencies used though

Thanks, I see. In this case I'll be happy to add it!

[srid]

Yea. Here's an example, where my app is using your library but using a local checkout (with changes) with tests disabled:

let
  pkgs = import <nixpkgs> { };
  H = pkgs.haskellPackages;
  dontCheck = pkgs.haskell.lib.dontCheck;
in H.developPackage {
  root = ./.;
  overrides = self: super: {
    algebraic-graphs = dontCheck (H.callCabal2nix "algebraic-graphs" dep/algebraic-graphs {});
  };
}

I could also have it point to a fork (instead of local directory) by using fetchFromGithub (example here).

All of this is only useful if a dev is using Nix for the Haskell workflow.

[snowleopard]

My worry is that I won't be able to maintain any sophisticated Nix scripts, because I'm not a Nix user. It's probably better to not have any Nix scripts at all than to have a Nix script that's occasionally out-of-date.