Haskell nix integration

There are many guides and tools out there for using Nix to build Haskell projects, but none of them are Invented Here.

My one can be used as follows:

git submodule add https://github.com/duairc/nix
nix/setup

Overview

1. Simple setup.
2. Zero maintenance; all derivations are built dynamically from .cabal file(s) and cabal.project file (if present).
3. You get a working "root" default.nix and shell.nix for your entire project, and individual "leaf" default.nix and shell.nix for each subproject.
4. The "root" shell.nix includes the transitive build-dependencies of all subprojects, minus the subprojects themselves. This allows cabal's to do most of the work when you're doing incremental development across multiple subprojects at a time.
5. It uses a pinned nixpkgs for maximum reproducibility. The setup script by default uses the latest nixpkgs-unstable commit, but you can use whatever commit you want by editing the nixpkgs.json file that gets created.
6. All nix-shells include a Hoogle with the dependencies of that project indexed. This can be configured either in options.nix or on the command-line by passing --arg hoogle false to nix-shell.
7. You can use configure the version of GHC you use, either in options.nix or on the command-line by passing --argstr ghc ghc865 to nix-shell.
8. A config.nix which is read and passed to nixpkgs; you can use this to set options like allowUnfree or allowBroken as required.
9. An overrides.nix, where you can can say things like generic-lens = dontCheck super.generic-lens_1_2_0_1.
10. The setup script is idempotent, it won't clobber any changes you've made. However you might want to run it again if you add new packages to your cabal.project and you want to generate default.nix and shell.nix files for them.