Dummy, no-brainer configuration for pinning nixpkgs over caba2nix for haskell startups.

Assuming you have a haskell project initialized in the monorepo, you can follow the steps below.

Minimal requirements for the setup is to have a cabal file and haskell module, like:

cabal-version: 2.4
name:          foobar
version:       0.1.0.0

library
  exposed-modules: FooBar
  build-depends:   base >= 4 && < 5

and:

module FooBar where

git remote add nix https://github.com/placek/hnix.git
git subtree add --prefix nix nix master --squash

Create a shell.nix file:

import ./nix/shell.nix {
  name = "foobar";
}

If you want to define more tools to be used:

import ./nix/shell.nix {
  name = "foobar";
  devTools = { pkgs }: with pkgs; [
    gawk # add gnu awk
  ];
}

To pin souces to one specific revision add sources.json file and place there some details about the nixpkgs version, like:

{
  "url": "https://github.com/NixOS/nixpkgs.git",
  "rev": "a1a6472993e44c44c437f6b5004e53289bc8399c",
  "ref": "refs/tags/21.11",
  "allRefs": true
}

echo "use nix" > .envrc && direnv allow